[Federal Register Volume 80, Number 206 (Monday, October 26, 2015)]
[Rules and Regulations]
[Pages 65292-65468]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2015-26594]
[[Page 65291]]
Vol. 80
Monday,
No. 206
October 26, 2015
Part II
Environmental Protection Agency
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40 CFR Part 50, 51, 52, et al.
National Ambient Air Quality Standards for Ozone; Final Rule
Federal Register / Vol. 80 , No. 206 / Monday, October 26, 2015 /
Rules and Regulations
[[Page 65292]]
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ENVIRONMENTAL PROTECTION AGENCY
40 CFR Parts 50, 51, 52, 53, and 58
[EPA-HQ-OAR-2008-0699; FRL-9933-18-OAR]
RIN 2060-AP38
National Ambient Air Quality Standards for Ozone
AGENCY: Environmental Protection Agency (EPA).
ACTION: Final rule.
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SUMMARY: Based on its review of the air quality criteria for ozone
(O3) and related photochemical oxidants and national ambient
air quality standards (NAAQS) for O3, the Environmental
Protection Agency (EPA) is revising the primary and secondary NAAQS for
O3 to provide requisite protection of public health and
welfare, respectively. The EPA is revising the levels of both standards
to 0.070 parts per million (ppm), and retaining their indicators
(O3), forms (fourth-highest daily maximum, averaged across
three consecutive years) and averaging times (eight hours). The EPA is
making corresponding revisions in data handling conventions for
O3 and changes to the Air Quality Index (AQI); revising
regulations for the prevention of significant deterioration (PSD)
program to add a transition provision for certain applications; and
establishing exceptional events schedules and providing information
related to implementing the revised standards. The EPA is also revising
the O3 monitoring seasons, the Federal Reference Method
(FRM) for monitoring O3 in the ambient air, Federal
Equivalent Method (FEM) analyzer performance requirements, and the
Photochemical Assessment Monitoring Stations (PAMS) network. Along with
exceptional events schedules related to implementing the revised
O3 standards, the EPA is applying this same schedule
approach to other future new or revised NAAQS and removing obsolete
regulatory language for expired exceptional events deadlines. The EPA
is making minor changes to the procedures and time periods for
evaluating potential FRMs and equivalent methods, including making the
requirements for nitrogen dioxide (NO2) consistent with the
requirements for O3, and removing an obsolete requirement
for the annual submission of Product Manufacturing Checklists by
manufacturers of FRMs and FEMs for monitors of fine and coarse
particulate matter. For a more detailed summary, see the Executive
Summary below.
DATES: The final rule is effective on December 28, 2015.
ADDRESSES: EPA has established a docket for this action (Docket ID No.
EPA-HQ-OAR-2008-0699) and a separate docket, established for the
Integrated Science Assessment (ISA) (Docket No. EPA-HQ-ORD-2011-0050),
which has been incorporated by reference into the rulemaking docket.
All documents in the docket are listed on the www.regulations.gov Web
site. Although listed in the docket index, some information is not
publicly available, e.g., confidential business information or other
information whose disclosure is restricted by statute. Certain other
material, such as copyrighted material, is not placed on the Internet
and may be viewed, with prior arrangement, at the EPA Docket Center.
Publicly available docket materials are available either electronically
in www.regulations.gov or in hard copy at the Air and Radiation Docket
and Information Center, EPA/DC, WJC West Building, Room 3334, 1301
Constitution Ave., NW., Washington, DC. The Public Reading Room is open
from 8:30 a.m. to 4:30 p.m., Monday through Friday, excluding legal
holidays. The telephone number for the Public Reading Room is (202)
566-1744 and the telephone number for the Air and Radiation Docket and
Information Center is (202) 566-1742. For additional information about
EPA's public docket, visit the EPA Docket Center homepage at: http://www.epa.gov/epahome/dockets.htm.
FOR FURTHER INFORMATION CONTACT: Ms. Susan Lyon Stone, Health and
Environmental Impacts Division, Office of Air Quality Planning and
Standards, U.S. Environmental Protection Agency, Mail code C504-06,
Research Triangle Park, NC 27711; telephone: (919) 541-1146; fax: (919)
541-0237; email: [email protected].
SUPPLEMENTARY INFORMATION:
General Information
Availability of Related Information
A number of the documents that are relevant to this action are
available through the EPA's Office of Air Quality Planning and
Standards (OAQPS) Technology Transfer Network (TTN) Web site (http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_index.html). These documents
include the Integrated Science Assessment for Ozone (U.S. EPA, 2013),
available at http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_2008_isa.html; the Health Risk and Exposure Assessment and the
Welfare Risk and Exposure Assessment for Ozone, Final Reports (HREA and
WREA, respectively; U.S. EPA, 2014a, 2014b), available at http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_2008_rea.html; and the
Policy Assessment for the Review of the Ozone National Ambient Air
Quality Standards (PA; U.S. EPA, 2014c), available at http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_2008_pa.html. These and
other related documents are also available for inspection and copying
in the EPA docket identified above.
Table of Contents
The following topics are discussed in this preamble:
Executive Summary
I. Background
A. Legislative Requirements
B. Related Control Programs
C. Review of Air Quality Criteria and Standards for
O3
D. Ozone Air Quality
E. Summary of Proposed Revisions to the O3 Standards
F. Organization and Approach to Decisions in This O3
NAAQS Review
II. Rationale for Decision on the Primary Standard
A. Introduction
1. Overview of Health Effects Evidence
2. Overview of Human Exposure and Health Risk Assessments
B. Need for Revision of the Primary Standard
1. Basis for Proposed Decision
2. Comments on the Need for Revision
3. Administrator's Conclusions on the Need for Revision
C. Conclusions on the Elements of a Revised Primary Standard
1. Indicator
2. Averaging Time
3. Form
4. Level
D. Decision on the Primary Standard
III. Communication of Public Health Information
A. Proposed Revisions to the AQI
B. Comments on Proposed Revisions to the AQI
C. Final Revisions to the AQI
IV. Rationale for Decision on the Secondary Standard
A. Introduction
1. Overview of Welfare Effects Evidence
2. Overview of Welfare Exposure and Risk Assessment
3. Potential Impacts on Public Welfare
B. Need for Revision of the Secondary Standard
1. Basis for Proposed Decision
2. Comments on the Need for Revision
3. Administrator's Conclusions on the Need for Revision
C. Conclusions on Revision of the Secondary Standard
1. Basis for Proposed Revision
2. Comments on Proposed Revision
3. Administrator's Conclusions on Revision
D. Decision on the Secondary Standard
V. Appendix U: Interpretation of the Primary and Secondary NAAQS for
O3
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A. Background
B. Data Selection Requirements
C. Data Reporting and Data Handling Requirements
D. Exceptional Events Information Submission Schedule
VI. Ambient Monitoring Related to O3 Standards
A. Background
B. Revisions to the Length of the Required O3
Monitoring Seasons
1. Proposed Changes to the Length of the Required O3
Monitoring Seasons
2. Comments on the Length of the Required O3
Monitoring Seasons
3. Final Decisions on the Length of the Required O3
Monitoring Seasons
C. Revisions to the PAMS Network Requirements
1. Network Design
2. Speciated VOC Measurements
3. Carbonyl Measurements
4. Nitrogen Oxides Measurements
5. Meteorology Measurements
6. PAMS Season
7. Timing and Other Implementation Issues
D. Addition of a New FRM for O3
1. Proposed Changes to the FRM for O3
2. Comments on the FRM for O3
E. Revisions to the Analyzer Performance Requirements
1. Proposed Changes to the Analyzer Performance Requirements
2. Comments on the Analyzer Performance Requirements
VII. Grandfathering Provision for Certain PSD Permits
A. Summary of the Proposed Grandfathering Provision
B. Comments and Responses
C. Final Action and Rationale
VIII. Implementation of the Revised O3 Standards
A. NAAQS Implementation Plans
1. Cooperative Federalism
2. Additional New Rules and Guidance
3. Background O3
4. Section 110 State Implementation Plans
5. Nonattainment Area Requirements
B. O3 Air Quality Designations
1. Area Designation Process
2. Exceptional Events
C. How do the New Source Review (NSR) requirements apply to the
revised O3 NAAQS?
1. NSR Requirements for Major Stationary Sources for the Revised
O3 NAAQS
2. Prevention of Significant Deterioration (PSD) Program
3. Nonattainment NSR
D. Transportation and General Conformity
1. What are Transportation and General Conformity?
2. When would Transportation and General Conformity apply to
areas designated nonattainment for the revised O3 NAAQS?
3. Impact of a Revised O3 NAAQS on a State's Existing
Transportation and/or General Conformity SIP
E. Regional and International Pollution Transport
1. Interstate Transport
2. International Transport
IX. Statutory and Executive Order Reviews
A. Executive Order 12866: Regulatory Planning and Review and
Executive Order 13563: Improving Regulation and Regulatory Review
B. Paperwork Reduction Act
C. Regulatory Flexibility Act
D. Unfunded Mandates Reform Act
E. Executive Order 13132: Federalism
F. Executive Order 13175: Consultation and Coordination With
Indian Tribal Governments
G. Executive Order 13045: Protection of Children From
Environmental Health & Safety Risks
H. Executive Order 13211: Actions That Significantly Affect
Energy Supply, Distribution, or Use
I. National Technology Transfer and Advancement Act
J. Executive Order 12898: Federal Actions To Address
Environmental Justice in Minority Populations and Low-Income
Populations
K. Congressional Review Act (CRA)
References
Executive Summary
This section summarizes information about the purpose of this
regulatory action, the major provisions of this action, and provisions
related to implementation.
Purpose of This Regulatory Action
Sections 108 and 109 of the Clean Air Act (CAA) govern the
establishment, review, and revision, as appropriate, of the NAAQS to
protect public health and welfare. The CAA requires the EPA to
periodically review the air quality criteria--the science upon which
the standards are based--and the standards themselves. This rulemaking
is being conducted pursuant to these statutory requirements. The
schedule for completing this review is established by a federal court
order, which requires that the EPA make a final determination by
October 1, 2015.
The EPA completed its most recent review of the NAAQS for
O3 in 2008. As a result of that review, EPA took four
principal actions: (1) Revised the level of the 8-hour primary standard
to 0.075 ppm; (2) expressed the standard to three decimal places; (3)
revised the 8-hour secondary standard by making it identical to the
revised primary standard; and (4) made conforming changes to the AQI.
In subsequent litigation, the U.S. Court of Appeals for the
District of Columbia Circuit (DC Circuit) upheld the EPA's 2008 primary
standard but remanded the 2008 secondary standard (Mississippi v. EPA,
744 F. 3d 1334 [D.C. Cir. 2013]). With respect to the primary standard,
the court held that the EPA reasonably determined that the existing
primary standard, set in 1997, did not protect public health with an
adequate margin of safety and required revision. In upholding the EPA's
revised primary standard, the court dismissed arguments that the EPA
should have adopted a more stringent standard. The court remanded the
secondary standard to the EPA after finding that the EPA's
justification for setting the secondary standard identical to the
revised 8-hour primary standard violated the CAA because the EPA had
not adequately explained how that standard provided the required public
welfare protection. In remanding the 2008 secondary standard, the court
did not vacate it. The EPA has addressed the court's remand with this
final action.
This final action reflects the Administrator's conclusions based on
a review of the O3 NAAQS that began in September 2008, and
also concludes the EPA's reconsideration of the 2008 decision that it
initiated in 2009 and subsequently consolidated with the current
review. In conducting this review, the EPA has carefully evaluated the
currently available scientific literature on the health and welfare
effects of O3, focusing particularly on the new literature
available since the conclusion of the previous review in 2008. Between
2008 and 2014, the EPA prepared draft and final versions of the
Integrated Science Assessment, the Health and Welfare Risk and Exposure
Assessments, and the Policy Assessment. Multiple drafts of these
documents were subject to public review and comment, and, as required
by the CAA, were peer-reviewed by the Clean Air Scientific Advisory
Committee (CASAC), an independent scientific advisory committee
established pursuant to the CAA and charged with providing advice to
the Administrator.
The EPA proposed revisions to the primary and secondary
O3 NAAQS on December 17, 2014 (79 FR 75234), and provided a
3-month period for submission of comments from the public. In addition
to written comments submitted to EPA, comments were also provided at
public hearings held in Washington, DC, and Arlington, Texas, on
January 29, 2015, and in Sacramento, California, on February 2, 2015.
After consideration of public comments and the advice from the CASAC,
the EPA has developed this final rulemaking, which is the final step in
the review process.
In this rulemaking, the EPA is revising the suite of standards for
O3 to provide requisite protection of public health and
welfare. In addition, the EPA is updating the AQI, and making changes
in the data handling conventions and ambient air monitoring, reporting,
and network
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design requirements to correspond with the changes to the O3
NAAQS.
Summary of Major Provisions
With regard to the primary standard, the EPA is revising the level
of the standard to 0.070 ppm to provide increased public health
protection against health effects associated with long- and short-term
exposures. The EPA is retaining the indicator (O3),
averaging time (8-hour) and form (annual fourth-highest daily maximum,
averaged over 3 years) of the existing standard. This action provides
increased protection for children, older adults, and people with asthma
or other lung diseases, and other at-risk populations against an array
of adverse health effects that include reduced lung function, increased
respiratory symptoms and pulmonary inflammation; effects that
contribute to emergency department visits or hospital admissions; and
mortality.
The decisions on the adequacy of the current standard and the
appropriate level for the revised standard are based on an integrative
assessment of an extensive body of new scientific evidence, which
substantially strengthens what was known about O3-related
health effects in the last review. The revised standard also reflects
consideration of a quantitative risk assessment that estimates public
health risks likely to remain upon just meeting the current and various
alternative standards. Based on this information, the Administrator
concludes that the current primary O3 standard is not
requisite to protect public health with an adequate margin of safety,
as required by the CAA, and that revision of the level to 0.070 ppm is
warranted to provide the appropriate degree of increased public health
protection for at-risk populations against an array of adverse health
effects. In concluding that a revised primary standard set at a level
of 0.070 ppm is requisite to protect public health with an adequate
margin of safety, the Administrator relies on several key pieces of
information, including: (a) A level of 0.070 ppm is well below the
O3 exposure concentration shown to cause the widest range of
respiratory effects (i.e., 0.080 ppm) and is below the lowest
O3 exposure concentration shown to cause the adverse
combination of decreased lung function and increased respiratory
symptoms (i.e., 0.072 ppm); (b) a level of 0.070 ppm will eliminate, or
nearly eliminate, repeated occurrence of these O3 exposure
concentrations (this is important because the potential for adverse
effects increases with frequency of occurrence); (c) a level of 0.070
ppm will protect the large majority of the population, including
children and people with asthma, from lower exposure concentrations,
which can cause lung function decrements and airway inflammation in
some people (i.e., 0.060 ppm); and (d) a level of 0.070 ppm will result
in important reductions in the risk of O3-induced lung
function decrements as well as the risk of O3-associated
hospital admissions, emergency department visits, and mortality. In
addition, the revised level of the primary standard is within the range
that CASAC advised the Agency to consider.
The EPA is also revising the level of the secondary standard to
0.070 ppm to provide increased protection against vegetation-related
effects on public welfare. The EPA is retaining the indicator
(O3), averaging time (8-hour) and form (annual fourth-
highest daily maximum, averaged over 3 years) of the existing secondary
standard. This action, reducing the level of the standard, provides
increased protection for natural forests in Class I and other similarly
protected areas against an array of vegetation-related effects of
O3. The Administrator is making this decision based on
judgments regarding the currently available welfare effects evidence,
the appropriate degree of public welfare protection for the revised
standard, and currently available air quality information on seasonal
cumulative exposures that may be allowed by such a standard.
In making this decision on the secondary standard, the
Administrator focuses on O3 effects on tree seedling growth
as a proxy for the full array of vegetation-related effects of
O3, ranging from effects on sensitive species to broader
ecosystem-level effects. Using this proxy in judging effects to public
welfare, the Administrator has concluded that the requisite protection
will be provided by a standard that generally limits cumulative
seasonal exposures to 17 ppm-hours (ppm-hrs) or lower, in terms of a 3-
year W126 index. Based on air quality analyses which indicate such
control of cumulative seasonal exposures will be achieved with a
standard set at a level of 0.070 ppm (and the same indicator, averaging
time, and form as the current standard), the Administrator concludes
that a standard revised in this way will provide the requisite
protection. In addition to providing protection of natural forests from
growth-related effects, the revised standard is also expected to
provide increased protection from other effects of potential public
welfare significance, including crop yield loss and visible foliar
injury. Thus, based on all of the information available in this review,
the Administrator concludes that the current secondary O3
standard is not requisite to protect public welfare as required by the
CAA, and that this revision will provide appropriate protection against
known or anticipated adverse effects to the public welfare.
Provisions Related to Implementation
As directed by the CAA, reducing pollution to meet NAAQS always has
been a shared task, one involving the federal government, states,
tribes and local air agencies. This partnership has proved effective
since the EPA first issued O3 standards more than three
decades ago, and is evidenced by significantly lower O3
levels throughout the country. To provide a foundation that helps air
agencies build successful strategies for attaining new O3
standards, the EPA will continue to move forward with federal
regulatory programs, such as the final Tier 3 motor vehicle emissions
standards. To facilitate the development of CAA-compliant
implementation plans and strategies to attain new standards, the EPA
intends to issue timely and appropriate implementation guidance and,
where appropriate and consistent with the law, new rulemakings to
streamline regulatory burdens and provide flexibility in
implementation. Given the regional nature of O3 air
pollution, the EPA will continue to work with states to address
interstate transport of O3 and O3 precursors. The
EPA also intends to work closely with states to identify locations
affected by high background concentrations on high O3 days
due to stratospheric intrusions of O3, wildfire
O3 plumes, or long-range transport of O3 from
sources outside the U.S. and ensure that the appropriate CAA regulatory
mechanisms are employed. To this end, the EPA will be proposing
revisions to the 2007 Exceptional Events Rule and related draft
guidance addressing the effects of wildfires.
In addition to revising the primary and secondary standards, this
action is changing the AQI to reflect the revisions to the primary
standard and also making corresponding revisions in data handling
conventions for O3, extending the O3 monitoring
season in 33 states, revising the requirements for the PAMS network,
and revising regulations for the PSD permitting program to add a
provision grandfathering certain pending permits from certain
requirements with respect to the revised standards. The preamble also
provides schedules and information related to implementing the revised
standards.
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The rule also contains revisions to the schedules associated with
exceptional events demonstration submittals for the revised
O3 standards and other future revised NAAQS, and makes minor
changes related to monitoring for other pollutants.
I. Background
A. Legislative Requirements
Two sections of the CAA govern the establishment and revision of
the NAAQS. Section 108 (42 U.S.C. 7408) directs the Administrator to
identify and list certain air pollutants and then to issue air quality
criteria for those pollutants. The Administrator is to list those air
pollutants that in her ``judgment, cause or contribute to air pollution
which may reasonably be anticipated to endanger public health or
welfare;'' ``the presence of which in the ambient air results from
numerous or diverse mobile or stationary sources;'' and ``for which . .
. [the Administrator] plans to issue air quality criteria . . . .'' Air
quality criteria are intended to ``accurately reflect the latest
scientific knowledge useful in indicating the kind and extent of all
identifiable effects on public health or welfare which may be expected
from the presence of [a] pollutant in the ambient air . . .'' 42 U.S.C.
7408(b). Section 109 (42 U.S.C. 7409) directs the Administrator to
propose and promulgate ``primary'' and ``secondary'' NAAQS for
pollutants for which air quality criteria are issued. Section 109(b)(1)
defines a primary standard as one ``the attainment and maintenance of
which in the judgment of the Administrator, based on such criteria and
allowing an adequate margin of safety, are requisite to protect the
public health.'' \1\ A secondary standard, as defined in section
109(b)(2), must ``specify a level of air quality the attainment and
maintenance of which, in the judgment of the Administrator, based on
such criteria, is requisite to protect the public welfare from any
known or anticipated adverse effects associated with the presence of
[the] pollutant in the ambient air.'' \2\
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\1\ The legislative history of section 109 indicates that a
primary standard is to be set at ``the maximum permissible ambient
air level . . . which will protect the health of any [sensitive]
group of the population,'' and that, for this purpose, ``reference
should be made to a representative sample of persons comprising the
sensitive group rather than to a single person in such a group.'' S.
Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970).
\2\ Welfare effects as defined in section 302(h) (42 U.S.C.
7602(h)) include, but are not limited to, ``effects on soils, water,
crops, vegetation, man-made materials, animals, wildlife, weather,
visibility and climate, damage to and deterioration of property, and
hazards to transportation, as well as effects on economic values and
on personal comfort and well-being.''
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The requirement that primary standards provide an adequate margin
of safety was intended to address uncertainties associated with
inconclusive scientific and technical information available at the time
of standard setting. It was also intended to provide a reasonable
degree of protection against hazards that research has not yet
identified. See Mississippi v. EPA, 744 F. 3d 1334, 1353 (D.C. Cir.
2013); Lead Industries Association v. EPA, 647 F.2d 1130, 1154 (D.C.
Cir 1980); American Petroleum Institute v. Costle, 665 F.2d 1176, 1186
(D.C. Cir. 1981); American Farm Bureau Federation v. EPA, 559 F. 3d
512, 533 (D.C. Cir. 2009); Association of Battery Recyclers v. EPA, 604
F. 3d 613, 617-18 (D.C. Cir. 2010). Both kinds of uncertainties are
components of the risk associated with pollution at levels below those
at which human health effects can be said to occur with reasonable
scientific certainty. Thus, in selecting primary standards that provide
an adequate margin of safety, the Administrator is seeking not only to
prevent pollution levels that have been demonstrated to be harmful but
also to prevent lower pollutant levels that may pose an unacceptable
risk of harm, even if the risk is not precisely identified as to nature
or degree. The CAA does not require the Administrator to establish a
primary NAAQS at a zero-risk level or at background concentrations, see
Lead Industries v. EPA, 647 F.2d at 1156 n.51; Mississippi v. EPA, 744
F. 3d at 1351, but rather at a level that reduces risk sufficiently so
as to protect public health with an adequate margin of safety.
In addressing the requirement for an adequate margin of safety, the
EPA considers such factors as the nature and severity of the health
effects, the size of sensitive population(s) \3\ at risk, and the kind
and degree of the uncertainties that must be addressed. The selection
of any particular approach for providing an adequate margin of safety
is a policy choice left specifically to the Administrator's judgment.
See Lead Industries Association v. EPA, 647 F.2d at 1161-62;
Mississippi, 744 F. 3d at 1353.
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\3\ As used here with regard to human populations, and similarly
throughout this document, the term ``population'' refers to people
having a quality or characteristic in common, including a specific
pre-existing illness or a specific age or lifestage.
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In setting primary and secondary standards that are ``requisite''
to protect public health and welfare, respectively, as provided in
section 109(b), the EPA's task is to establish standards that are
neither more nor less stringent than necessary for these purposes. In
so doing, the EPA may not consider the costs of implementing the
standards. See generally, Whitman v. American Trucking Associations,
531 U.S. 457, 465-472, 475-76 (2001). Likewise, ``[a]ttainability and
technological feasibility are not relevant considerations in the
promulgation of national ambient air quality standards.'' American
Petroleum Institute v. Costle, 665 F. 2d at 1185.
Section 109(d)(1) requires that ``not later than December 31, 1980,
and at 5-year intervals thereafter, the Administrator shall complete a
thorough review of the criteria published under section 108 and the
national ambient air quality standards . . . and shall make such
revisions in such criteria and standards and promulgate such new
standards as may be appropriate . . . .'' Section 109(d)(2) requires
that an independent scientific review committee ``shall complete a
review of the criteria . . . and the national primary and secondary
ambient air quality standards . . . and shall recommend to the
Administrator any new . . . standards and revisions of existing
criteria and standards as may be appropriate . . . .'' Since the early
1980's, the CASAC \4\ has performed this independent review function.
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\4\ Lists of CASAC members and of members of the CASAC Ozone
Review Panel are accessible from: http://yosemite.epa.gov/sab/sabpeople.nsf/WebCommittees/CASAC.
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B. Related Control Programs
States are primarily responsible for ensuring attainment and
maintenance of NAAQS once the EPA has established them. The EPA
performs an oversight function, and as necessary takes actions to
ensure CAA objectives are achieved. Under section 110 of the CAA, and
related provisions, states submit, for the EPA's approval, state
implementation plans (SIPs) that provide for the attainment and
maintenance of such standards through control programs directed to
sources of the relevant pollutants. The states, in conjunction with the
EPA, also administer the PSD program (CAA sections 160 to 169) which is
a pre-construction permit program designed to prevent significant
deterioration in air quality. In addition, federal programs provide for
nationwide reductions in emissions of O3 precursors and
other air pollutants through new source performance standards for
stationary sources under section 111 of the CAA and the federal motor
vehicle and motor vehicle fuel control program under title II of the
CAA (sections 202
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to 250), which involves controls for emissions from mobile sources and
controls for the fuels used by these sources. For some stationary
sources, the national emissions standards for hazardous air pollutants
under section 112 of the CAA may provide ancillary reductions in
O3 precursors.
After the EPA establishes a new or revised NAAQS, the CAA directs
the EPA and the states to take steps to ensure that the new or revised
NAAQS are met. One of the first steps, known as the initial area
designations, involves identifying areas of the country that are not
meeting the new or revised NAAQS along with the nearby areas that
contain emissions sources that contribute to the areas not meeting the
NAAQS. For areas designated ``nonattainment,'' the responsible states
are required to develop SIPs to attain the standards. In developing
their attainment plans, states first take into account projected
emission reductions from federal and state rules that have been already
adopted at the time of plan submittal. A number of significant emission
reduction programs that will lead to reductions of O3
precursors are in place today or are expected to be in place by the
time revised SIPs will be due. Examples of such rules include the
Nitrogen Oxides (NOX) SIP Call and Cross-State Air Pollution
Rule (CSAPR),\5\ regulations controlling on-road and non-road engines
and fuels, hazardous air pollutant rules for utility and industrial
boilers, and various other programs already adopted by states to reduce
emissions from key emissions sources. States will then evaluate the
level of additional emission reductions needed for each nonattainment
area to attain the O3 standards ``as expeditiously as
practicable,'' and adopt new state regulations as appropriate. Section
VIII of this preamble includes additional discussion of designation and
implementation issues associated with the revised O3 NAAQS.
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\5\ The Cross-State Air Pollution Rule was upheld by the Supreme
Court in Environmental Protection Agency v. EME Homer City
Generation, L.P., 134 S. Ct. 1584 (2014), and remanded to the D.C.
Circuit for further proceedings. The D.C. Circuit issued its
decision on remand from the Supreme Court on July 28, 2015,
remanding CSAPR to EPA, without vacating the rule, for EPA to
reconsider certain emission budgets for certain States (EME Homer
City Generation, L.P. v. Environmental Protection Agency, No. 11-
1302, 2015 WL 4528137 [D.C. Cir. July 28, 2015]).
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C. Review of Air Quality Criteria and Standards for O3
The EPA first established primary and secondary NAAQS for
photochemical oxidants in 1971 (36 FR 8186, April 30, 1971). The EPA
set both primary and secondary standards at 0.08 ppm,\6\ as a 1-hour
average of total photochemical oxidants, not to be exceeded more than
one hour per year. The EPA based the standards on scientific
information contained in the 1970 Air Quality Criteria for
Photochemical Oxidants (AQCD; U.S. DHEW, 1970). The EPA initiated the
first periodic review of the NAAQS for photochemical oxidants in 1977.
Based on the 1978 AQCD (U.S. EPA, 1978), the EPA published proposed
revisions to the original NAAQS in 1978 (43 FR 26962, June 22, 1978)
and final revisions in 1979 (44 FR 8202, February 8, 1979). At that
time, the EPA revised the level of the primary and secondary standards
from 0.08 to 0.12 ppm and changed the indicator from photochemical
oxidants to O3, and the form of the standards from a
deterministic (i.e., not to be exceeded more than one hour per year) to
a statistical form. This statistical form defined attainment of the
standards as occurring when the expected number of days per calendar
year with maximum hourly average concentration greater than 0.12 ppm
equaled one or less.
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\6\ Although the level of the 2008 O3 standards are
specified in the units of ppm (i.e., 0.075 ppm), O3
concentrations are described using the units of parts per billion
(ppb) in several sections of this notice (i.e., sections II, III, IV
and VI) for consistency with the common convention for information
discussed in those sections. In ppb, 0.075 ppm is equivalent to 75.
---------------------------------------------------------------------------
Following the EPA's decision in the 1979 review, the city of
Houston challenged the Administrator's decision arguing that the
standard was arbitrary and capricious because natural O3
concentrations and other physical phenomena in the Houston area made
the standard unattainable in that area. The U.S. Court of Appeals for
the District of Columbia Circuit (D.C. Circuit) rejected this argument,
holding (as noted above) that attainability and technological
feasibility are not relevant considerations in the promulgation of the
NAAQS. The court also noted that the EPA need not tailor the NAAQS to
fit each region or locale, pointing out that Congress was aware of the
difficulty in meeting standards in some locations and had addressed
this difficulty through various compliance related provisions in the
CAA. See API v. Costle, 665 F.2d 1176, 1184-6 (D.C. Cir. 1981).
In 1982, the EPA announced plans to revise the 1978 AQCD (47 FR
11561; March 17, 1982), and, in 1983, the EPA initiated the second
periodic review of the O3 NAAQS (48 FR 38009; August 22,
1983). The EPA subsequently published the 1986 AQCD (U.S. EPA, 1986)
and the 1989 Staff Paper (U.S. EPA, 1989). Following publication of the
1986 AQCD, a number of scientific abstracts and articles were published
that appeared to be of sufficient importance concerning potential
health and welfare effects of O3 to warrant preparation of a
Supplement (U.S. EPA, 1992). In August of 1992, under the terms of a
court order, the EPA proposed to retain the existing primary and
secondary standards based on the health and welfare effects information
contained in the 1986 AQCD and its 1992 Supplement (57 FR 35542, August
10, 1992). In March 1993, the EPA announced its decision to conclude
this review by affirming its proposed decision to retain the standards,
without revision (58 FR 13008, March 9, 1993).
In the 1992 notice of its proposed decision in that review, the EPA
announced its intention to proceed as rapidly as possible with the next
review of the air quality criteria and standards for O3 in
light of emerging evidence of health effects related to 6- to 8-hour
O3 exposures (57 FR 35542, August 10, 1992). The EPA
subsequently published the AQCD and Staff Paper for the review (U.S.
EPA, 1996a,b). In December 1996, the EPA proposed revisions to both the
primary and secondary standards (61 FR 65716, December 13, 1996). With
regard to the primary standard, the EPA proposed to replace the then-
existing 1-hour primary standard with an 8-hour standard set at a level
of 0.08 ppm (equivalent to 0.084 ppm based on the proposed data
handling convention) as a 3-year average of the annual third-highest
daily maximum 8-hour concentration. The EPA proposed to revise the
secondary standard either by setting it identical to the proposed new
primary standard or by setting it as a new seasonal standard using a
cumulative form. The EPA completed this review in 1997 by setting the
primary standard at a level of 0.08 ppm, based on the annual fourth-
highest daily maximum 8-hour average concentration, averaged over three
years, and setting the secondary standard identical to the revised
primary standard (62 FR 38856, July 18, 1997). In reaching her decision
on the primary standard, the Administrator identified several reasons
supporting her decision to reject a potential alternate standard set at
0.07 ppm, including first the fact that no CASAC panel member supported
a standard level lower than 0.08 ppm and her consideration of the
scientific uncertainties with regard to the health effects evidence for
exposure concentrations below 0.08 ppm. In addition to those reasons,
the Administrator noted that a standard set
[[Page 65297]]
at a level of 0.07 ppm would be closer to peak background
concentrations that infrequently occur in some areas due to
nonanthropogenic sources of O3 precursors (62 FR 38856,
38868; July 18, 1997).
On May 14, 1999, in response to challenges by industry and others
to the EPA's 1997 decision, the D.C. Circuit remanded the O3
NAAQS to the EPA, finding that section 109 of the CAA, as interpreted
by the EPA, effected an unconstitutional delegation of legislative
authority. American Trucking Assoc. vs. EPA, 175 F.3d 1027, 1034-1040
(D.C. Cir. 1999) (``ATA I''). In addition, the court directed that, in
responding to the remand, the EPA should consider the potential
beneficial health effects of O3 pollution in shielding the
public from the effects of solar ultraviolet (UV) radiation, as well as
adverse health effects. Id. at 1051-53. In 1999, the EPA petitioned for
rehearing en banc on several issues related to that decision. The court
granted the request for rehearing in part and denied it in part, but
declined to review its ruling with regard to the potential beneficial
effects of O3 pollution. 195 F. 3d 4, 10 (D.C Cir., 1999)
(``ATA II''). On January 27, 2000, the EPA petitioned the U.S. Supreme
Court for certiorari on the constitutional issue (and two other
issues), but did not request review of the ruling regarding the
potential beneficial health effects of O3. On February 27,
2001, the U.S. Supreme Court unanimously reversed the judgment of the
D.C. Circuit on the constitutional issue. Whitman v. American Trucking
Assoc., 531 U. S. 457, 472-74 (2001) (holding that section 109 of the
CAA does not delegate legislative power to the EPA in contravention of
the Constitution). The Court remanded the case to the D.C. Circuit to
consider challenges to the O3 NAAQS that had not been
addressed by that court's earlier decisions. On March 26, 2002, the
D.C. Circuit issued its final decision on remand, finding the 1997
O3 NAAQS to be ``neither arbitrary nor capricious,'' and so
denying the remaining petitions for review. American Trucking
Associations, Inc. v. EPA, 283 F.3d 355, 379 (D.C Cir., 2002) (``ATA
III'').
Specifically, in ATA III, the D.C. Circuit upheld the EPA's
decision on the 1997 O3 standard as the product of reasoned
decision making. With regard to the primary standard, the court made
clear that the most important support for EPA's decision to revise the
standard was the health evidence of insufficient protection afforded by
the then-existing standard (``the record is replete with references to
studies demonstrating the inadequacies of the old one-hour standard''),
as well as extensive information supporting the change to an 8-hour
averaging time (283 F. 3d at 378). The court further upheld the EPA's
decision not to select a more stringent level for the primary standard
noting ``the absence of any human clinical studies at ozone
concentrations below 0.08 [ppm]'' which supported the EPA's conclusion
that ``the most serious health effects of ozone are `less certain' at
low concentrations, providing an eminently rational reason to set the
primary standard at a somewhat higher level, at least until additional
studies become available'' (283 F. 3d at 378, internal citations
omitted). The court also pointed to the significant weight that the EPA
properly placed on the advice it received from CASAC (283 F. 3d at
379). In addition, the court noted that ``although relative proximity
to peak background O3 concentrations did not, in itself,
necessitate a level of 0.08 [ppm], the EPA could consider that factor
when choosing among the three alternative levels'' (283 F. 3d at 379).
Independently of the litigation, the EPA responded to the court's
remand to consider the potential beneficial health effects of
O3 pollution in shielding the public from effects of UV
radiation. The EPA provisionally determined that the information
linking changes in patterns of ground-level O3
concentrations to changes in relevant patterns of exposures to UV
radiation of concern to public health was too uncertain, at that time,
to warrant any relaxation in 1997 O3 NAAQS. The EPA also
expressed the view that any plausible changes in UV-B radiation
exposures from changes in patterns of ground-level O3
concentrations would likely be very small from a public health
perspective. In view of these findings, the EPA proposed to leave the
1997 primary standard unchanged (66 FR 57268, Nov. 14, 2001). After
considering public comment on the proposed decision, the EPA published
its final response to this remand in 2003, re-affirming the 8-hour
primary standard set in 1997 (68 FR 614, January 6, 2003).
The EPA initiated the fourth periodic review of the air quality
criteria and standards for O3 with a call for information in
September 2000 (65 FR 57810, September, 26, 2000). The schedule for
completion of that review was ultimately governed by a consent decree
resolving a lawsuit filed in March 2003 by plaintiffs representing
national environmental and public health organizations, who maintained
that the EPA was in breach of a nondiscretionary duty to complete
review of the O3 NAAQS within a statutorily mandated
deadline. In 2007, the EPA proposed to revise the level of the primary
standard within a range of 0.075 to 0.070 ppm (72 FR 37818, July 11,
2007). The EPA proposed to revise the secondary standard either by
setting it identical to the proposed new primary standard or by setting
it as a new seasonal standard using a cumulative form. Documents
supporting these proposed decisions included the 2006 AQCD (U.S. EPA,
2006a) and 2007 Staff Paper (U.S. EPA, 2007) and related technical
support documents. The EPA completed the review in March 2008 by
revising the level of the primary standard from 0.08 ppm to 0.075 ppm,
and revising the secondary standard to be identical to the revised
primary standard (73 FR 16436, March 27, 2008).
In May 2008, state, public health, environmental, and industry
petitioners filed suit challenging the EPA's final decision on the 2008
O3 standards. On September 16, 2009, the EPA announced its
intention to reconsider the 2008 O3 standards, and initiated
a rulemaking to do so. At the EPA's request, the court held the
consolidated cases in abeyance pending the EPA's reconsideration of the
2008 decision.
On January 2010, the EPA issued a notice of proposed rulemaking to
reconsider the 2008 final decision (75 FR 2938, January 19, 2010). In
that notice, the EPA proposed that further revisions of the primary and
secondary standards were necessary to provide a requisite level of
protection to public health and welfare. The EPA proposed to revise the
level of the primary standard from 0.075 ppm to a level within the
range of 0.060 to 0.070 ppm, and to revise the secondary standard to
one with a cumulative, seasonal form. At the EPA's request, the CASAC
reviewed the proposed rule at a public teleconference on January 25,
2010 and provided additional advice in early 2011 (Samet, 2010, 2011).
After considering comments from CASAC and the public, the EPA prepared
a draft final rule, which was submitted for interagency review pursuant
to Executive Order 12866. On September 2, 2011, consistent with the
direction of the President, the Administrator of the Office of
Information and Regulatory Affairs, Office of Management and Budget
(OMB), returned the draft final rule to the EPA for further
consideration. In view of this return and the fact that the Agency's
next periodic review of the O3 NAAQS required under CAA
section 109 had already begun (as announced on September 29, 2008), the
EPA decided to consolidate the
[[Page 65298]]
reconsideration with its statutorily required periodic review.\7\
---------------------------------------------------------------------------
\7\ This rulemaking concludes the reconsideration process. Under
CAA section 109, the EPA is required to base its review of the NAAQS
on the current air quality criteria, and thus the record and
decision for this review also serve for the reconsideration.
---------------------------------------------------------------------------
In light of the EPA's decision to consolidate the reconsideration
with the current review, the D.C. Circuit proceeded with the litigation
on the 2008 final decision. On July 23, 2013, the court upheld the
EPA's 2008 primary O3 standard, but remanded the 2008
secondary standard to the EPA (Mississippi v. EPA, 744 F. 3d 1334).
With respect to the primary standard, the court first held that the EPA
reasonably determined that the existing standard was not requisite to
protect public health with an adequate margin of safety, and
consequently required revision. Specifically, the court noted that
there were ``numerous epidemiologic studies linking health effects to
exposure to ozone levels below 0.08 ppm and clinical human exposure
studies finding a causal relationship between health effects and
exposure to ozone levels at and below 0.08 ppm'' (Mississippi v. EPA,
744 F. 3d at 1345). The court also specifically endorsed the weight of
evidence approach utilized by the EPA in its deliberations (Mississippi
v. EPA, 744 F. 3d at 1344).
The court went on to reject arguments that the EPA should have
adopted a more stringent primary standard. Dismissing arguments that a
clinical study (as properly interpreted by the EPA) showing effects at
0.06 ppm necessitated a standard level lower than that selected, the
court noted that this was a single, limited study (Mississippi v. EPA,
744 F. 3d at 1350). With respect to the epidemiologic evidence, the
court accepted the EPA's argument that there could be legitimate
uncertainty that a causal relationship between O3 and 8-hour
exposures less than 0.075 ppm exists, so that associations at lower
levels reported in epidemiologic studies did not necessitate a more
stringent standard (Mississippi v. EPA, 744 F. 3d at 1351-52).\8\
---------------------------------------------------------------------------
\8\ The court cautioned, however, that ``perhaps more [clinical]
studies like the Adams studies will yet reveal that the 0.060 ppm
level produces significant adverse decrements that simply cannot be
attributed to normal variation in lung function,'' and further
cautioned that ``agencies may not merely recite the terms
`substantial uncertainty' as a justification for their actions.''
Id. at 1350, 1357 (internal citations omitted).
---------------------------------------------------------------------------
The court also rejected arguments that an 8-hour primary standard
of 0.075 ppm failed to provide an adequate margin of safety, noting
that margin of safety considerations involved policy judgments by the
agency, and that by setting a standard ``appreciably below'' the level
of the current standard (0.08 ppm), the agency had made a reasonable
policy choice (Mississippi v. EPA, 744 F. 3d at 1351-52). Finally, the
court rejected arguments that the EPA's decision was inconsistent with
the CASAC's scientific recommendations because the CASAC had been
insufficiently clear in its recommendations whether it was providing
scientific or policy recommendations, and the EPA had reasonably
addressed the CASAC's policy recommendations (Mississippi v. EPA, 744
F. 3d at 1357-58).
With respect to the secondary standard, the court held that the
EPA's justification for setting the secondary standard identical to the
revised 8-hour primary standard violated the CAA because the EPA had
not adequately explained how that standard provided the required public
welfare protection. The court thus remanded the secondary standard to
the EPA (Mississippi v. EPA, 744 F. 3d at 1360-62).
At the time of the court's decision, the EPA had already completed
significant portions of its next statutorily required periodic review
of the O3 NAAQS. This review was formally initiated in 2008
with a call for information in the Federal Register (73 FR 56581, Sept.
29, 2008). On October 28-29, 2008, the EPA held a public workshop to
discuss the policy-relevant science, which informed identification of
key policy issues and questions to frame the review. Based in part on
the workshop discussions, the EPA developed a draft Integrated Review
Plan (IRP) outlining the schedule, process,\9\ and key policy-relevant
questions that would guide the evaluation of the air quality criteria
for O3 and the review of the primary and secondary
O3 NAAQS. A draft of the IRP was released for public review
and comment in September 2009 and was the subject of a consultation
with the CASAC on November 13, 2009 (74 FR 54562; October 22,
2009).\10\ After considering the comments received from that
consultation and from the public, the EPA completed and released the
IRP for the review in 2011 (U.S. EPA, 2011a).
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\9\ As of this review, the document developed in NAAQS reviews
to document the air quality criteria, previously the AQCD, is the
ISA, and the document describing the OAQPS staff evaluation,
previously the Staff Paper, is the PA. These documents are described
in the IRP.
\10\ See http://yosemite.epa.gov/sab/sabproduct.nsf/WebProjectsbyTopicCASAC!OpenView for more information on CASAC
activities related to the current O3 NAAQS review.
---------------------------------------------------------------------------
In preparing the first draft ISA, the EPA's National Center for
Environmental Assessment (NCEA) considered CASAC and public comments on
the IRP, and also comments received from a workshop held on August 6,
2010, to review and discuss preliminary drafts of key ISA sections (75
FR 42085, July 20, 2010). In 2011, the first draft ISA was released for
public comment and for review by CASAC at a public meeting on May 19-
20, 2011 (U.S. EPA, 2011b; 76 FR 10893, February 28, 2011; 76 FR 23809,
April 28, 2011). Based on CASAC and public comments, NCEA prepared a
second draft ISA, which was released for public comment and CASAC
review (U.S. EPA, 2011c; 76 FR 60820, September 30, 2011). The CASAC
reviewed this draft at a January 9-10, 2012, public meeting (76 FR 236,
December 8, 2011). Based on CASAC and public comments, NCEA prepared a
third draft ISA (U.S. EPA, 2012; 77 FR 36534, June 19, 2012), which was
reviewed at a CASAC meeting in September 2012. The EPA released the
final ISA in February 2013 (U.S. EPA, 2013).
The EPA presented its plans for conducting Risk and Exposure
Assessments (REAs) for health risk and exposure (HREA) and welfare risk
and exposure (WREA) in two documents that outlined the scope and
approaches for use in conducting quantitative assessments, as well as
key issues to be addressed as part of the assessments (U.S. EPA, 2011d,
e). The EPA released these documents for public comment in April 2011,
and consulted with CASAC on May 19-20, 2011 (76 FR 23809, April 28,
2011). The EPA considered CASAC advice and public comments in further
planning for the assessments, issuing a memo that described changes to
elements of the REA plans and brief explanations regarding them (Samet,
2011; Wegman, 2012).
In July 2012, the EPA made the first drafts of the Health and
Welfare REAs available for CASAC review and public comment (77 FR
42495, July 19, 2012; 77 FR 51798, August 27, 2012). The first draft PA
was made available for CASAC review and public comment in August 2012
(77 FR 42495, July 19, 2012; 77 FR 51798, August 27, 2012).\11\ The
first
[[Page 65299]]
draft REAs and PA were the focus of a CASAC public meeting in September
2012 (Frey and Samet, 2012a, 2012b). The second draft REAs and PA,
prepared with consideration of CASAC advice and public comments, were
made available for public comment and CASAC review in January 2014 (79
FR 4694, January 29, 2014). These documents were the focus of a CASAC
public meeting on March 25-27, 2014 (Frey, 2014a; Frey, 2014b; Frey,
2014c). The final versions of these documents were developed with
consideration of the comments and recommendations from CASAC, as well
as comments from the public on the draft documents, and were released
in August 2014 (U.S. EPA 2014a; U.S. EPA, 2014b; U.S. EPA, 2014c).
---------------------------------------------------------------------------
\11\ The PA is prepared by the OAQPS staff. Formerly known as
the Staff Paper, it presents a staff evaluation of the policy
implications of the key scientific and technical information in the
ISA and REAs for the EPA's consideration. The PA provides a
transparent evaluation, and staff conclusions, regarding policy
considerations related to reaching judgments about the adequacy of
the current standards, and if revision is considered, what revisions
may be appropriate to consider. The PA is intended to help ``bridge
the gap'' between the agency's scientific assessments presented in
the ISA and REAs, and the judgments required of the EPA
Administrator in determining whether it is appropriate to retain or
revise the NAAQS.
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The proposed decision (henceforth ``proposal'') on this review of
the O3 NAAQS was signed on November 25, 2014, and published
in the Federal Register on December 17, 2014. The EPA held three public
hearings to provide direct opportunity for oral testimony by the public
on the proposal. The hearings were held on January 29, 2015, in
Arlington, Texas, and Washington, DC, and on February 2, 2015, in
Sacramento, California. At these public hearings, the EPA heard
testimony from nearly 500 individuals representing themselves or
specific interested organizations. Transcripts from these hearings and
written testimony provided at the hearings are in the docket for this
review. Additionally, approximately 430,000 written comments were
received from various commenters during the public comment period on
the proposal, approximately 428,000 as part of mass mail campaigns.
Significant issues raised in the public comments are discussed in the
preamble of this final action. A summary of all other significant
comments, along with the EPA's responses, can be found in a separate
document (henceforth ``Response to Comments'') in the docket for this
review.
The schedule for completion of this review is governed by a court
order resolving a lawsuit filed in January 2014 by a group of
plaintiffs who alleged that the EPA had failed to perform its mandatory
duty, under section 109(d)(1), to complete a review of the
O3 NAAQS within the period provided by statute. The court
order that governs this review, entered by the court on April 30, 2014,
provides that the EPA will sign for publication a notice of final
rulemaking concerning its review of the O3 NAAQS no later
than October 1, 2015.
As in prior NAAQS reviews, the EPA is basing its decision in this
review on studies and related information included in the ISA, REAs and
PA, which have undergone CASAC and public review. The studies assessed
in the ISA and PA, and the integration of the scientific evidence
presented in them, have undergone extensive critical review by the EPA,
the CASAC, and the public. The rigor of that review makes these
studies, and their integrative assessment, the most reliable source of
scientific information on which to base decisions on the NAAQS,
decisions that all parties recognize as of great import. NAAQS
decisions can have profound impacts on public health and welfare, and
NAAQS decisions should be based on studies that have been rigorously
assessed in an integrative manner not only by the EPA but also by the
statutorily mandated independent advisory committee, as well as the
public review that accompanies this process. Some commenters have
referred to and discussed individual scientific studies on the health
and welfare effects of O3 that were not included in the ISA
(USEPA, 2013) (`` `new' studies''). In considering and responding to
comments for which such ``new'' studies were cited in support, the EPA
has provisionally considered the cited studies in the context of the
findings of the ISA. The EPA's provisional consideration of these
studies did not and could not provide the kind of in-depth critical
review described above.
The decision to rely on studies and related information included in
the ISA, REAs and PA, which have undergone CASAC and public review, is
consistent with the EPA's practice in prior NAAQS reviews and its
interpretation of the requirements of the CAA. Since the 1970
amendments, the EPA has taken the view that NAAQS decisions are to be
based on scientific studies and related information that have been
assessed as a part of the pertinent air quality criteria, and the EPA
has consistently followed this approach. This longstanding
interpretation was strengthened by new legislative requirements enacted
in 1977, which added section 109(d)(2) of the Act concerning CASAC
review of air quality criteria. See 71 FR 61144, 61148 (October 17,
2006) (final decision on review of NAAQS for particulate matter) for a
detailed discussion of this issue and the EPA's past practice.
As discussed in the EPA's 1993 decision not to revise the NAAQS for
O3, ``new'' studies may sometimes be of such significance
that it is appropriate to delay a decision on revision of a NAAQS and
to supplement the pertinent air quality criteria so the studies can be
taken into account (58 FR at 13013-13014, March 9, 1993). In the
present case, the EPA's provisional consideration of ``new'' studies
concludes that, taken in context, the ``new'' information and findings
do not materially change any of the broad scientific conclusions
regarding the health and welfare effects and exposure pathways of
ambient O3 made in the air quality criteria. For this
reason, reopening the air quality criteria review would not be
warranted even if there were time to do so under the court order
governing the schedule for this rulemaking.
Accordingly, the EPA is basing the final decisions in this review
on the studies and related information included in the O3
air quality criteria that have undergone CASAC and public review. The
EPA will consider the ``new'' studies for purposes of decision making
in the next periodic review of the O3 NAAQS, which the EPA
expects to begin soon after the conclusion of this review and which
will provide the opportunity to fully assess these studies through a
more rigorous review process involving the EPA, CASAC, and the public.
Further discussion of these ``new'' studies can be found in the
Response to Comments document, which is in the docket for this
rulemaking and also available on the web (http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_index.html).
D. Ozone Air Quality
Ozone is formed near the earth's surface due to chemical
interactions involving solar radiation and precursor pollutants
including volatile organic compounds (VOCs) and NOX. Over
longer time periods, methane (CH4) and carbon monoxide (CO)
can also lead to O3 formation at the global scale. The
precursor emissions leading to O3 formation can result from
both man-made sources (e.g., motor vehicles and electric power
generation) and natural sources (e.g., vegetation and wildfires).
Occasionally, O3 that is created naturally in the
stratosphere can also contribute to O3 levels near the
surface. Once formed, O3 near the surface can be transported
by winds before eventually being removed from the atmosphere via
chemical reactions or deposition to surfaces. In sum, O3
concentrations are influenced by complex interactions between precursor
emissions, meteorological conditions, and surface characteristics (U.S.
EPA, 2014a).
[[Page 65300]]
In order to continuously assess O3 air pollution levels,
state and local environmental agencies operate O3 monitors
at various locations and subsequently submit the data to the EPA. At
present, there are approximately 1,400 monitors across the U.S.
reporting hourly O3 averages during the times of the year
when local O3 pollution can be important (U.S. EPA, 2014c,
Section 2.1). Much of this monitoring is focused on urban areas where
precursor emissions tend to be largest, as well as locations directly
downwind of these areas, but there are also over 100 sites in rural
areas where high levels of O3 can also be measured. Based on
data from this national network, the EPA estimates that, in 2013,
approximately 99 million Americans lived in counties where
O3 design values \12\ were above the level of the existing
health-based (primary) NAAQS of 0.075 ppm. High O3 values
can occur almost anywhere within the contiguous 48 states, although the
poorest O3 air quality in the U.S. is typically observed in
California, Texas, and the Northeast Corridor, locations with some of
the most densely populated areas in the country. From a temporal
perspective, the highest daily peak O3 concentrations
generally tend to occur during the afternoon within the warmer months
due to higher solar radiation and other conducive meteorological
conditions during these times. The exceptions to this general rule
include 1) some rural sites where transport of O3 from
upwind areas of regional production can occasionally result in high
nighttime levels of O3, 2) high-elevation sites episodically
influenced by stratospheric intrusions which can occur in other months,
and 3) certain locations in the western U.S. where large quantities of
O3 precursors emissions associated with oil and gas
development can be trapped by strong inversions associated with snow
cover during the colder months and efficiently converted to
O3 (U.S. EPA, 2014c, Section 2.3).
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\12\ A design value is a statistic that describes the air
quality status of a given location relative to the level of the
NAAQS.
---------------------------------------------------------------------------
One of the challenging aspects of developing plans to address high
O3 concentrations is that the response of O3 to
precursor reductions is nonlinear. In particular, NOX
emissions can lead to both increases and decreases of O3.
The net impact of NOX emissions on O3
concentrations depends on the local quantities of NOX, VOC,
and sunlight which interact in a set of complex chemical reactions. In
some areas, such as certain urban centers where NOX
emissions typically are high compared to local VOC emissions,
NOX can suppress O3 locally. This phenomenon is
particularly pronounced under conditions associated with low
O3 concentrations (i.e., during cool, cloudy weather and at
night when photochemical activity is limited or nonexistent). However,
while NOX emissions can initially suppress O3
levels near the emission sources, these same NOX emissions
ultimately react to form higher O3 levels downwind when
conditions are favorable. Photochemical model simulations suggest that,
in general, reductions in NOX emissions in the U.S. will
slightly increase O3 concentrations on days with lower
O3 concentrations in close proximity to NOX
sources (e.g., in urban core areas), while at the same time decreasing
the highest O3 concentrations in downwind areas. See
generally, U.S. EPA, 2014a (section 2.2.1).
At present, both the primary and secondary NAAQS use the annual
fourth-highest daily maximum 8-hour concentration, averaged over 3
years, as the form of the standard. An additional metric, the W126
exposure index, is often used to assess impacts of O3
exposure on ecosystems and vegetation. W126 is a cumulative seasonal
aggregate of weighted hourly O3 values observed between 8
a.m. and 8 p.m. As O3 precursor emissions have decreased
across the U.S., annual fourth-highest 8-hour O3 maxima have
concurrently shown a modest downward trend. The national average change
in annual fourth-highest daily maximum 8-hour O3
concentrations between 2000 and 2013 was an 18% decrease. The national
average change in the annual W126 exposure index over the same period
was a 52% decrease. Air quality model simulations estimate that
O3 air quality will continue to improve over the next decade
as additional reductions in O3 precursors from power plants,
motor vehicles, and other sources are realized.
In addition to being affected by changing emissions, future
O3 concentrations may also be affected by climate change.
Modeling studies in the EPA's Interim Assessment (U.S. EPA, 2009a) that
are cited in support of the 2009 Endangerment Finding under CAA section
202(a) (74 FR 66496, Dec. 15, 2009) as well as a recent assessment of
potential climate change impacts (Fann et al., 2015) project that
climate change may lead to future increases in summer O3
concentrations across the contiguous U.S.\13\ While the projected
impact is not uniform, climate change has the potential to increase
average summertime O3 concentrations by as much as 1-5 ppb
by 2030, if greenhouse gas emissions are not mitigated. Increases in
temperature are expected to be the principal factor in driving any
O3 increases, although increases in stagnation frequency may
also contribute (Jacob and Winner, 2009). If unchecked, climate change
has the potential to offset some of the improvements in O3
air quality, and therefore some of the improvements in public health,
that are expected from reductions in emissions of O3
precursors.
---------------------------------------------------------------------------
\13\ These modeling studies are based on coupled global climate
and regional air quality models and are designed to assess the
sensitivity of U.S. air quality to climate change. A wide range of
future climate scenarios and future years have been modeled and
there can be variations in the expected response in U.S.
O3 by scenario and across models and years, within the
overall signal of higher summer O3 concentrations in a
warmer climate.
---------------------------------------------------------------------------
Another challenging aspect of this air quality issue is the impact
from sources of O3 and its precursors beyond those from
domestic, anthropogenic sources. Modeling analyses indicate that
nationally the majority of O3 exceedances are predominantly
caused by anthropogenic emissions from within the U.S. However,
observational and modeling analyses have concluded that O3
concentrations in some locations in the U.S. on some days can be
substantially influenced by sources that cannot be addressed by
domestic control measures. In particular, certain high-elevation sites
in the western U.S. are impacted by a combination of non-U.S. sources
like international transport, or natural sources such as stratospheric
O3, and O3 originating from wildfire
emissions.\14\ Ambient O3 from these non-U.S. and natural
sources is collectively referred to as background O3. See
generally section 2.4 of the PA (U.S. EPA, 2014c). The analyses suggest
that, at these locations, there can be episodic events with substantial
background contributions where O3 concentrations approach or
exceed the level of the current NAAQS (i.e., 75 ppb). These events are
relatively infrequent, and the EPA has policies that allow for the
exclusion of air quality monitoring data from design value calculations
when they are substantially affected by certain background influences.
---------------------------------------------------------------------------
\14\ Without global greenhouse gas mitigation efforts, climate
change is projected to dramatically increase the area burned by
wildfires across most of the contiguous U.S., especially in the West
(U.S. EPA, 2015 p. 72).
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E. Summary of Proposed Revisions to the O3 Standards
For reasons discussed in the proposal, the Administrator proposed
to revise the
[[Page 65301]]
current primary and secondary standards for O3. With regard
to the primary standard, the Administrator proposed to revise the level
from 75 ppb to a level within a range from 65 to 70 ppb. The EPA
proposed to revise the AQI for O3, consistent with revision
to the primary standard.
With regard to the secondary standard, the Administrator proposed
to revise the level of the current secondary standard to within the
range of 0.065 to 0.070 ppm, which air quality analyses indicate would
provide cumulative, seasonal air quality or exposure values, in terms
of 3-year average W126 index values, at or below a range of 13-17 ppm-
hours.
The EPA also proposed to make corresponding revisions in data
handling conventions for O3; to revise regulations for the
PSD permitting program to add a provision grandfathering certain
pending permits from certain requirements with respect to the proposed
revisions to the standards; and to convey schedules and information
related to implementing any revised standards. In conjunction with
proposing exceptional event schedules related to implementing any
revised O3 standards, the EPA also proposed to extend the
new schedule approach to other future NAAQS revisions and to remove
obsolete regulatory language associated with expired exceptional event
deadlines for historical standards for both O3 and other
pollutants for which NAAQS have been established. The EPA also proposed
to make minor changes to the procedures and time periods for evaluating
potential FRMs and equivalent methods, including making the
requirements for NO2 consistent with the requirements for
O3, and removing an obsolete requirement for the annual
submission of documentation by manufacturers of certain particulate
matter monitors.
F. Organization and Approach to Decisions in This O3 NAAQS Review
This action presents the Administrator's final decisions in the
current review of the primary and secondary O3 standards.
The final decisions addressing standards for O3 are based on
a thorough review in the ISA of scientific information on known and
potential human health and welfare effects associated with exposure to
O3 at levels typically found in the ambient air. These final
decisions also take into account the following: (1) Staff assessments
in the PA of the most policy-relevant information in the ISA as well as
a quantitative health and welfare exposure and risk assessments based
on that information; (2) CASAC advice and recommendations, as reflected
in its letters to the Administrator and its discussions of drafts of
the ISA, REAs, and PA at public meetings; (3) public comments received
during the development of these documents, both in connection with
CASAC meetings and separately; and (4) extensive public comments
received on the proposed rulemaking.
The primary standard is addressed in section II. Corresponding
changes to the AQI are addressed in section III. The secondary standard
is addressed in section IV. Related data handling conventions and
exceptional events are addressed in section V. Updates to the
monitoring regulations are addressed in section VI. Implementation
activities, including PSD-related actions, are addressed in sections
VII and VIII. Section IX addresses applicable statutory and executive
order reviews.
II. Rationale for Decision on the Primary Standard
This section presents the Administrator's final decisions regarding
the need to revise the existing primary O3 standard and the
appropriate revision to the level of that standard. Based on her
consideration of the full body of health effects evidence and exposure/
risk analyses, the Administrator concludes that the current primary
standard for O3 is not requisite to protect public health
with an adequate margin of safety. In order to increase public health
protection, she is revising the level of the primary standard to 70
ppb, in conjunction with retaining the current indicator, averaging
time and form. The Administrator concludes that such a revised standard
will be requisite to protect public health with an adequate margin of
safety. As discussed more fully below, the rationale for these final
decisions draws from the thorough review in the ISA (U.S. EPA, 2013) of
the available scientific evidence, generally published through July
2011, on human health effects associated with the presence of
O3 in the ambient air. This rationale also takes into
account: (1) Analyses of O3 air quality, human exposures to
O3, and O3-associated health risks, as presented
and assessed in the HREA (U.S. EPA, 2014a); (2) the EPA staff
assessment of the most policy-relevant scientific evidence and
exposure/risk information in the PA (U.S. EPA, 2014c); (3) CASAC advice
and recommendations, as reflected in discussions of drafts of the ISA,
REA, and PA at public meetings, in separate written comments, and in
CASAC's letters to the Administrator; (4) public input received during
the development of these documents, either in connection with CASAC
meetings or separately; and (5) public comments on the proposal notice.
Section II.A below summarizes the information presented in the
proposal regarding O3-associated health effects,
O3 exposures, and O3-attributable health risks.
Section II.B presents information related to the adequacy of the
current primary O3 standard, including a summary of the
basis for the Administrator's proposed decision to revise the current
standard, public comments received on the adequacy of the current
standard, and the Administrator's final conclusions regarding the
adequacy of the current standard. Section II.C presents information
related to the elements of a revised primary O3 standard,
including information related to each of the major elements of the
standard (i.e., indicator, averaging time, form, level). Section II.D
summarizes the Administrator's final decisions on the primary
O3 standard.
A. Introduction
As discussed in section II.A of the proposal (79 FR 75243-75246,
December 17, 2014), the EPA's approach to informing decisions on the
primary O3 standard in the current review builds upon the
general approaches used in previous reviews and reflects the broader
body of scientific evidence, updated exposure/risk information, and
advances in O3 air quality modeling now available. This
approach is based most fundamentally on using the EPA's assessment of
the available scientific evidence and associated quantitative analyses
to inform the Administrator's judgments regarding a primary standard
for O3 that is ``requisite'' (i.e., neither more nor less
stringent than necessary) to protect public health with an adequate
margin of safety. Specifically, it is based on consideration of the
available body of scientific evidence assessed in the ISA (U.S. EPA,
2013), exposure and risk analyses presented in the HREA (U.S. EPA,
2014a), evidence- and exposure-/risk-based considerations and
conclusions presented in the PA (U.S. EPA, 2014c), advice and
recommendations received from CASAC (Frey, 2014a, c), and public
comments.
Section II.A.1 below summarizes the information presented in the
proposal regarding O3-associated health effects. Section
II.A.2 summarizes the information presented in the proposal regarding
O3 exposures and O3-attributable health risks.
[[Page 65302]]
1. Overview of Health Effects Evidence
The health effects of O3 are described in detail in the
ISA (U.S. EPA, 2013). Based on its assessment of the health effects
evidence, the ISA determined that a ``causal'' relationship exists
between short-term exposure to O3 in ambient air and effects
on the respiratory system \15\ and that a ``likely to be causal''
relationship exists between long-term exposure to O3 in
ambient air and respiratory effects \16\ (U.S. EPA, 2013, pp. 1-6 to 1-
7). The ISA summarizes the longstanding body of evidence for
O3 respiratory effects as follows (U.S. EPA, 2013, p. 1-5):
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\15\ In determining that a causal relationship exists for
O3 with specific health effects, the EPA has concluded
that ``[e]vidence is sufficient to conclude that there is a causal
relationship with relevant pollutant exposures'' (U.S. EPA, 2013, p.
lxiv).
\16\ In determining a ``likely to be a causal'' relationship
exists for O3 with specific health effects, the EPA has
concluded that ``[e]vidence is sufficient to conclude that a causal
relationship is likely to exist with relevant pollutant exposures,
but important uncertainties remain'' (U.S. EPA, 2013, p. lxiv).
The clearest evidence for health effects associated with
exposure to O3 is provided by studies of respiratory
effects. Collectively, a very large amount of evidence spanning
several decades supports a relationship between exposure to
O3 and a broad range of respiratory effects (see Section
6.2.9 and Section 7.2.8). The majority of this evidence is derived
from studies investigating short-term exposures (i.e., hours to
weeks) to O3, although animal toxicological studies and
recent epidemiologic evidence demonstrate that long-term exposure
---------------------------------------------------------------------------
(i.e., months to years) may also harm the respiratory system.
Additionally, the ISA determined that the relationships between
short-term exposures to O3 in ambient air and both total
mortality and cardiovascular effects are likely to be causal, based on
expanded evidence bases in the current review (U.S. EPA, 2013, pp. 1-7
to 1-8). The ISA determined that the currently available evidence for
additional endpoints is ``suggestive'' of causal relationships with
short-term (central nervous system effects) and long-term exposures
(cardiovascular effects, reproductive and developmental effects,
central nervous system effects and total mortality) to ambient
O3.
Consistent with emphasis in past reviews on O3 health
effects for which the evidence is strongest, in this review the EPA
places the greatest emphasis on studies of health effects that have
been determined in the ISA to be caused by, or likely to be caused by,
O3 exposures (U.S. EPA, 2013, section 2.5.2). This preamble
section summarizes the evidence for health effects attributable to
O3 exposures, with a focus on respiratory morbidity and
mortality effects attributable to short- and long-term exposures, and
cardiovascular system effects (including mortality) and total mortality
attributable to short-term exposures (from section II.B in the
proposal, 79 FR 75246-75271).
The information highlighted here is based on the assessment of the
evidence in the ISA (U.S. EPA, 2013, Chapters 4 to 8) and consideration
of that evidence in the PA (U.S. EPA, 2014c, Chapters 3 and 4) on the
known or potential effects on public health which may be expected from
the presence of O3 in the ambient air. This section
summarizes: (1) Information available on potential mechanisms for
health effects associated with exposure to O3 (II.A.1.a);
(2) the nature of effects that have been associated directly with both
short- and long-term exposure to O3 and indirectly with the
presence of O3 in ambient air (II.A.1.b); (3) considerations
related to the adversity of O3-attributable health effects
(II.A.1.c); and (4) considerations in characterizing the public health
impact of O3, including the identification of ``at risk''
populations (II.A.1.d).
a. Overview of Mechanisms
This section briefly summarizes the characterization of the key
events and pathways that contribute to health effects resulting from
O3 exposures, as discussed in the proposal (79 FR 75247,
section II.B.1) and in the ISA (U.S. EPA, 2013, section 5.3).
Experimental evidence elucidating modes of action and/or mechanisms
contributes to our understanding of the biological plausibility of
adverse O3-related health effects, including respiratory
effects and effects outside the respiratory system (U.S. EPA, 2013,
Chapters 6 and 7). Evidence indicates that the initial key event is the
formation of secondary oxidation products in the respiratory tract
(U.S. EPA, 2013, section 5.3). This mainly involves direct reactions
with components of the extracellular lining fluid (ELF). Although the
ELF has inherent capacity to quench (based on individual antioxidant
capacity), this capacity can be overwhelmed, especially with exposure
to elevated concentrations of O3 (U.S. EPA 2014c, at 3-3, 3-
9). The resulting secondary oxidation products transmit signals to the
epithelium, pain receptive nerve fibers and, if present, immune cells
involved in allergic responses. The available evidence indicates that
the effects of O3 are mediated by components of ELF and by
the multiple cell types in the respiratory tract. Oxidative stress is
an implicit part of this initial key event.
Secondary oxidation products initiate numerous responses at the
cellular, tissue, and whole organ level of the respiratory system.
These responses include the activation of neural reflexes which leads
to lung function decrements; initiation of pulmonary inflammation;
alteration of barrier epithelial function; sensitization of bronchial
smooth muscle; modification of lung host defenses; airways remodeling;
and modulation of autonomic nervous function which may alter cardiac
function (U.S. EPA, 2013, section 5.3, Figure 5-8).
Persistent inflammation and injury, which are observed in animal
models of chronic and quasi-continuous exposure to O3, are
associated with airways remodeling (see section 7.2.3 of the ISA, U.S.
EPA, 2013). Chronic quasi-continuous exposure to O3 has also
been shown to result in effects on the developing lung and immune
system. Systemic inflammation and vascular oxidative/nitrosative stress
are also key events in the toxicity pathway of O3 (U.S. EPA,
2013, section 5.3.8). Extrapulmonary effects of O3 occur in
numerous organ systems, including the cardiovascular, central nervous,
reproductive, and hepatic systems (U.S. EPA, 2013, sections 6.3 to 6.5
and sections 7.3 to 7.5).
Responses to O3 exposure are variable within the
population. Studies have shown a large range of pulmonary function
(i.e., spirometric) responses to O3 among healthy young
adults, while responses within an individual are relatively consistent
over time. Other responses to O3 have also been
characterized by a large degree of interindividual variability,
including airways inflammation. The mechanisms that may underlie the
variability in responses seen among individuals are discussed in the
ISA (U.S. EPA, 2013, section 5.4.2). Certain functional genetic
polymorphisms, pre-existing conditions or diseases, nutritional status,
lifestages, and co-exposures can contribute to altered risk of
O3-induced effects. Experimental evidence for such
O3-induced changes contributes to our understanding of the
biological plausibility of adverse O3-related health
effects, including a range of respiratory effects as well as effects
outside the respiratory system (e.g., cardiovascular effects) (U.S.
EPA, 2013, Chapters 6 and 7).
b. Nature of Effects
This section briefly summarizes the information presented in the
proposal on respiratory effects attributable to short-term exposures
(II.A.1.b.i), respiratory effects attributable to long-
[[Page 65303]]
term exposures (II.A.1.b.ii), cardiovascular effects attributable to
short-term exposures (II.A.1.b.iii), and premature mortality
attributable to short-term exposures (II.A.1.b.iv) (79 FR 75247,
section II.B.2).
i. Respiratory Effects--Short-term Exposure
Controlled human exposure, animal toxicological, and epidemiologic
studies available in the last review provided clear, consistent
evidence of a causal relationship between short-term O3
exposure and respiratory effects (U.S. EPA, 2006a). Recent studies
evaluated since the completion of the 2006 AQCD support and expand upon
the strong body of evidence available in the last review (U.S. EPA,
2013, section 6.2.9).
Key aspects of this evidence are discussed below with regard to (1)
lung function decrements; (2) pulmonary inflammation, injury, and
oxidative stress; (3) airway hyperresponsiveness; (4) respiratory
symptoms and medication use; (5) lung host defense; (6) allergic and
asthma-related responses; (7) hospital admissions and emergency
department visits; and (8) respiratory mortality.\17\
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\17\ CASAC concurred that these were ``the kinds of identifiable
effects on public health that are expected from the presence of
ozone in the ambient air'' (Frey 2014c, p. 3).
---------------------------------------------------------------------------
Lung Function Decrements
Lung function decrements are typically measured by spirometry and
refer to reductions in the maximal amount of air that can be forcefully
exhaled. Forced expiratory volume in 1 second (FEV1) is a
common index used to assess the effect of O3 on lung
function. The ISA summarizes the currently available evidence from
multiple controlled human exposure studies evaluating changes in
FEV1 following 6.6-hour O3 exposures in young,
healthy adults engaged in moderate levels of physical activity \18\
(U.S. EPA, 2013, section 6.2.1.1, Figure 6-1). Exposures to an average
O3 concentration of 60 ppb results in group mean decrements
in FEV1 ranging from 1.8% to 3.6% (Adams, 2002; Adams, 2006;
\19\ Schelegle et al., 2009; \20\ Kim et al., 2011). The weighted
average group mean decrement was 2.7% from these studies. In some
analyses, these group mean decrements in lung function were
statistically significant (Brown et al., 2008; Kim et al., 2011), while
in other analyses they were not (Adams, 2006; Schelegle et al.,
2009).\21\ Prolonged exposure to an average O3 concentration
of 72 ppb results in a statistically significant group mean decrement
in FEV1 of about 6% (Schelegle et al., 2009).\22\ There is a
smooth dose-response curve without evidence of a threshold for
exposures between 40 and 120 ppb O3 (U.S. EPA, 2013, Figure
6-1). When these data are taken together, the ISA concludes that ``mean
FEV1 is clearly decreased by 6.6-hour exposures to 60 ppb
O3 and higher concentrations in [healthy, young adult]
subjects performing moderate exercise'' (U.S. EPA, 2013, p. 6-9).
---------------------------------------------------------------------------
\18\ Table 6-1 of the ISA includes descriptions of the activity
levels evaluated in controlled human exposure studies (U.S. EPA,
2013).
\19\ Adams (2006); (2002) both provide data for an additional
group of 30 healthy subjects that were exposed via facemask to 60
ppb O3 for 6.6 hours with moderate exercise. These
subjects are described on page 133 of Adams (2006) and pages 747 and
761 of Adams (2002). The facemask exposure is not expected to affect
the FEV1 responses relative to a chamber exposure.
\20\ For the 60 ppb target exposure concentration, Schelegle et
al. (2009) reported that the actual mean exposure concentration was
63 ppb.
\21\ Adams (2006) did not find effects on FEV1 at 60
ppb to be statistically significant. In an analysis of the Adams
(2006) data, Brown et al. (2008) addressed the more fundamental
question of whether there were statistically significant differences
in responses before and after the 6.6 hour exposure period and found
the average effect on FEV1 at 60 ppb to be small, but
highly statistically significant using several common statistical
tests, even after removal of potential outliers. Schelegle et al.
(2009) reported that, compared to filtered air, the largest change
in FEV1 for the 60 ppb protocol occurred after the sixth
(and final) exercise period.
\22\ As noted above, for the 70 ppb exposure group, Schelegle et
al. (2009) reported that the actual mean exposure concentration was
72 ppb.
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As described in the proposal (79 FR 75250), the ISA focuses on
individuals with >10% decrements in FEV1 because (1) it is
accepted by the American Thoracic Society (ATS) as an abnormal response
and a reasonable criterion for assessing exercise-induced
bronchoconstriction, and (2) some individuals in the Schelegle et al.
(2009) study experienced 5-10% FEV1 decrements following
exposure to filtered air. The proportion of healthy adults experiencing
FEV1 decrements >10% following prolonged exposures to 80 ppb
O3 while at moderate exertion ranged from 17% to 29% and
following exposures to 60 ppb O3 ranged from 3% to 20%. The
weighted average proportion (i.e., based on numbers of subjects in each
study) of young, healthy adults with >10% FEV1 decrements is
25% following exposure to 80 ppb O3 and 10% following
exposure to 60 ppb O3, for 6.6 hours at moderate exertion
(U.S. EPA, 2013, page 6-18 and 6-19).\23\ Responses within an
individual tend to be reproducible over a period of several months,
reflecting differences in intrinsic responsiveness. Given this, the ISA
concludes that ``[t]hough group mean decrements are biologically small
and generally do not attain statistical significance, a considerable
fraction of exposed individuals [in the clinical studies] experience
clinically meaningful decrements in lung function'' when exposed for
6.6 hours to 60 ppb O3 during quasi-continuous, moderate
exertion (U.S. EPA, 2013, section 6.2.1.1, p. 6-20).
---------------------------------------------------------------------------
\23\ The ISA notes that by considering responses uncorrected for
filtered air exposures, during which lung function typically
improves (which would increase the size of the change, pre-and post-
exposure), 10% is an underestimate of the proportion of healthy
individuals that are likely to experience clinically meaningful
changes in lung function following exposure for 6.6 hours to 60 ppb
O3 during quasi-continuous moderate exertion (U.S. EPA,
2012, section 6.2.1.1).
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This review has marked an advance in the ability to make reliable
quantitative predictions of the potential lung function response to
O3 exposure, and, thus, to reasonably predict the degree of
interindividual response of lung function to that exposure. McDonnell
et al. (2012) and Schelegle et al. (2012) developed models, described
in more detail in the proposal (79 FR 75250), that included
mathematical approaches to simulate the potential protective effect of
antioxidants in the ELF at lower ambient O3 concentrations,
and that included a dose threshold below which changes in lung function
do not occur. The resulting empirical models can estimate the frequency
distribution of individual responses and summary measures of the
distribution such as the mean or median response and the proportions of
individuals with FEV1 decrements >10%, 15%, and 20%.\24\ The
predictions of the models are consistent with the observed results from
the individual controlled human exposure studies of O3-
induced FEV1 decrements (79 FR 75250-51, see also U.S. EPA,
2013, Figures 6-1 and 6-3). CASAC agreed that these models mark a
significant technical advance over the exposure-response modeling
approach used for the lung function risk assessment in the last review
and explicitly found that ``[t]he MSS model to be scientifically and
biologically defensible'' (Frey, 2014a, pp. 8, 2). CASAC also stated
that ``the comparison of the MSS model results to those obtained with
the exposure-response model is of tremendous importance. Typically, the
MSS model gives a result about a factor of three higher . . . for
school-age children, which is expected because the MSS model includes
[[Page 65304]]
responses for a wider range of exposure protocols'' (Frey, 2014a, pp.
8, 2).
---------------------------------------------------------------------------
\24\ One of these models, the McDonnell-Stewart-Smith (MSS)
model (McDonnell et al. 2012) was used to estimate the occurrences
of lung function decrements in the HREA.
---------------------------------------------------------------------------
Epidemiologic studies have consistently linked short-term increases
in ambient O3 concentrations with lung function decrements
in diverse populations and lifestages, including children attending
summer camps, adults exercising or working outdoors, and groups with
pre-existing respiratory diseases such as asthmatic children (U.S. EPA,
2013, section 6.2.1.2). Some of these studies reported O3-
associated lung function decrements accompanied by respiratory symptoms
\25\ in asthmatic children. In contrast, studies of children in the
general population have reported similar O3-associated lung
function decrements but without accompanying respiratory symptoms (79
FR 75251; U.S. EPA, 2013, section 6.2.1.2). As noted in the PA (EPA,
2014c, pp. 4-70 to 4-71), additional research is needed to evaluate
responses of people with asthma and healthy people in the 40 to 70 ppb
range. Further epidemiologic studies and meta-analyses of the effects
of O3 exposure on children will help elucidate the
concentration-response functions for lung function and respiratory
symptom effects at lower O3 concentrations.
---------------------------------------------------------------------------
\25\ Reversible loss of lung function in combination with the
presence of symptoms meets ATS criteria for adversity (ATS, 2000a).
---------------------------------------------------------------------------
Several epidemiologic panel studies \26\ reported statistically
significant associations with lung function decrements at relatively
low ambient O3 concentrations. For outdoor recreation or
exercise, associations were reported in analyses restricted to 1-hour
average O3 concentrations less than 80 ppb, down to less
than 50 ppb. Among outdoor workers, Brauer et al. (1996) found a robust
association with daily 1-hour max O3 concentrations less
than 40 ppb. Ulmer et al. (1997) found a robust association in
schoolchildren with 30-minute maximum O3 concentrations less
than 60 ppb. For 8-hour average O3 concentrations,
associations with lung function decrements in children with asthma were
found to persist at concentrations less than 80 ppb in a U.S. multicity
study (Mortimer et al., 2002) and less than 51 ppb in a study conducted
in the Netherlands (Gielen et al., 1997).
---------------------------------------------------------------------------
\26\ Panel studies include repeated measurements of health
outcomes, such as respiratory symptoms, at the individual level
(U.S. EPA, 2013, p. 1x).
---------------------------------------------------------------------------
As described in the proposal (79 FR 75251), several epidemiologic
panel studies provided information on potential confounding by
copollutants and most O3 effect estimates for lung function
were robust to adjustment for temperature, humidity, and copollutants
such as particulate matter with mass median aerodynamic diameter less
than or equal to 2.5 micrometers (PM2.5), particulate matter
with mass median aerodynamic diameter less than or equal to 10
micrometers (PM10), NO2, or sulfur dioxide
(SO2) (Hoppe et al., 2003; Brunekreef et al., 1994; Hoek et
al. 1993; U.S. EPA, 2013, pp. 6-67 to 6-69). Although examined in only
a few epidemiologic studies, O3 also remained associated
with decreases in lung function with adjustment for pollen or acid
aerosols (79 F 75251; U.S. EPA, 2013, section 6.2.1.2).
Pulmonary Inflammation, Injury and Oxidative Stress
As described in detail in section II.B.2.a.ii of the proposal (79
FR 75252), O3 exposures can result in increased respiratory
tract inflammation and epithelial permeability. Inflammation is a host
response to injury, and the induction of inflammation is evidence that
injury has occurred. Oxidative stress has been shown to play a key role
in initiating and sustaining O3-induced inflammation. As
noted in the ISA (U.S. EPA, 2013, section 6.2.3), O3
exposures can initiate an acute inflammatory response throughout the
respiratory tract that has been reported to persist for at least 18-24
hours after exposure.
Inflammation induced by exposure of humans to O3 can
have several potential outcomes, ranging from resolving entirely
following a single exposure to becoming a chronic inflammatory state,
as described in detail in section II.B.2.a.ii of the proposal (79 FR
75252) and in the ISA (U.S. EPA, 2013, section 6.2.3). Continued
cellular damage due to chronic inflammation ``may alter the structure
and function of pulmonary tissues'' (U.S. EPA, 2013, p. 6-161). Lung
injury and the resulting inflammation provide a mechanism by which
O3 may cause other more serious morbidity effects (e.g.,
asthma exacerbations) (U.S. EPA, 2013, section 6.2.3).\27\
---------------------------------------------------------------------------
\27\ CASAC also addressed this issue: ``The CASAC believes that
these modest changes in FEV1 are usually associated with
inflammatory changes, such as more neutrophils in the
bronchoalveolar lavage fluid. Such changes may be linked to the
pathogenesis of chronic lung disease'' (Frey, 2014a p. 2).
---------------------------------------------------------------------------
Building on the last review, recent studies continue to support the
evidence for airway inflammation and injury with new evidence for such
effects following exposures to lower concentrations than had been
evaluated previously. These studies include recent controlled human
exposure and epidemiologic studies and are discussed more below.
An extensive body of evidence from controlled human exposure
studies, described in section II.B.2.a.ii of the proposal, indicates
that short-term exposures to O3 can cause pulmonary
inflammation and increases in polymorphonuclear leukocyte (PMN) influx
and permeability following 80-600 O3 ppb exposures,
eosinophilic inflammation following exposures at or above 160 ppb, and
O3-induced PMN influx following exposures of healthy adults
to 60 ppb O3, the lowest concentration that has been
evaluated for inflammation. A meta-analysis of 21 controlled human
exposure studies (Mudway and Kelly, 2004) using varied experimental
protocols (80-600 ppb O3 exposures; 1-6.6 hours exposure
duration; light to heavy exercise; bronchoscopy at 0-24 hours post-
O3 exposure) reported that PMN influx in healthy subjects is
linearly associated with total O3 dose.
As with FEV1 responses to O3, inflammatory
responses to O3 are generally reproducible within
individuals, with some individuals experiencing more severe
O3-induced airway inflammation than indicated by group
averages. Unlike O3-induced decrements in lung function,
which are attenuated following repeated exposures over several days,
some markers of O3-induced inflammation and tissue damage
remain elevated during repeated exposures, indicating ongoing damage to
the respiratory system (79 FR 75252). Most controlled human exposure
studies have reported that asthmatics experience larger O3-
induced inflammatory responses than non-asthmatics.\28\
---------------------------------------------------------------------------
\28\ When evaluated, these studies have also reported
O3-induced respiratory symptoms in asthmatics.
Specifically, Scannell et al. (1996), Basha et al. (1994), and
Vagaggini et al. (2001, 2007) reported increased symptoms in
addition to inflammation.
---------------------------------------------------------------------------
In the previous review (U.S. EPA, 2006a), the epidemiologic
evidence of O3-associated changes in airway inflammation and
oxidative stress was limited (79 FR 75253). Since then, as a result of
the development of less invasive test methods, there has been a large
increase in the number of studies assessing ambient O3-
associated changes in airway inflammation and oxidative stress, the
types of biological samples collected, and the types of indicators.
Most of these recent studies have evaluated biomarkers of inflammation
or oxidative stress in exhaled breath, nasal lavage fluid, or induced
sputum (U.S. EPA, 2013, section 6.2.3.2). These recent studies form a
larger database to establish coherence with findings from controlled
human exposure and animal
[[Page 65305]]
studies that have measured the same or related biological markers.
Additionally, results from these studies provide further biological
plausibility for the associations observed between ambient
O3 concentrations and respiratory symptoms and asthma
exacerbations.
Airway Hyperresponsiveness (AHR)
A strong body of controlled human exposure and animal toxicological
studies, most of which were available in the last review of the
O3 NAAQS, report O3-induced AHR after either
acute or repeated exposures (U.S. EPA, 2013, section 6.2.2.2). People
with asthma often exhibit increased airway responsiveness at baseline
relative to healthy control subjects, and asthmatics can experience
further increases in responsiveness following exposures to
O3. Studies reporting increased airway responsiveness after
O3 exposure contribute to a plausible link between ambient
O3 exposures and increased respiratory symptoms in
asthmatics, and increased hospital admissions and emergency department
visits for asthma (section II.B.2.a.iii, 79 FR 75254; U.S. EPA, 2013,
section 6.2.2.2).
Respiratory Symptoms and Medication Use
Respiratory symptoms are associated with adverse outcomes such as
limitations in activity, and are the primary reason for people with
asthma to use quick relief medication and to seek medical care. Studies
evaluating the link between O3 exposures and such symptoms
allow a direct characterization of the clinical and public health
significance of ambient O3 exposure. Controlled human
exposure and toxicological studies have described modes of action
through which short-term O3 exposures may increase
respiratory symptoms by demonstrating O3-induced AHR (U.S.
EPA, 2013, section 6.2.2) and pulmonary inflammation (U.S. EPA, 2013,
section 6.2.3).
The link between subjective respiratory symptoms and O3
exposures has been evaluated in both controlled human exposure and
epidemiologic studies, and the link with medication use has been
evaluated in epidemiologic studies. In the last review, several
controlled human exposure studies reported respiratory symptoms
following exposures to O3 concentrations at or above 80 ppb.
In addition, one study reported such symptoms following exposures to 60
ppb O3, though the increase was not statistically different
from filtered air controls. Epidemiologic studies reported associations
between ambient O3 and respiratory symptoms and medication
use in a variety of locations and populations, including asthmatic
children living in U.S. cities (U.S. EPA, 2013, pp. 6-1 to 6-2). In the
current review, additional controlled human exposure studies have
evaluated respiratory symptoms following exposures to O3
concentrations below 80 ppb and recent epidemiologic studies have
evaluated associations with respiratory symptoms and medication use
(U.S. EPA, 2013, sections 6.2.1, 6.2.4).
As noted in section II.B.2.a.iv in the proposal (79 FR 75255), the
findings for O3-induced respiratory symptoms in controlled
human exposure studies, and the evidence integrated across disciplines
describing underlying modes of action, provide biological plausibility
for epidemiologic associations observed between short-term increases in
ambient O3 concentration and increases in respiratory
symptoms (U.S. EPA, 2013, section 6.2.4).
Most epidemiologic studies of O3 and respiratory
symptoms and medication use have been conducted in children and/or
adults with asthma, with fewer studies, and less consistent results, in
non-asthmatic populations (U.S. EPA, 2013, section 6.2.4). The 2006
AQCD (U.S. EPA, 2006a; U.S. EPA, 2013, section 6.2.4) concluded that
the collective body of epidemiologic evidence indicated that short-term
increases in ambient O3 concentrations are associated with
increases in respiratory symptoms in children with asthma. A large body
of single-city and single-region studies of asthmatic children provides
consistent evidence for associations between short-term increases in
ambient O3 concentrations and increased respiratory symptoms
and asthma medication use in children with asthma (U.S. EPA, 2013,
Figure 6-12, Table 6-20, section 6.2.4.1). Methodological differences,
described in section II.B.2.a.iv of the proposal, among studies make
comparisons across recent multicity studies of respiratory symptoms
difficult.
Available evidence indicates that O3-associated
increases in respiratory symptoms are not confounded by temperature,
pollen, or copollutants (primarily PM) (U.S. EPA, 2013, section
6.2.4.5; Table 6-25). However, identifying the independent effects of
O3 in some studies was complicated due to the high
correlations observed between O3 and PM or different lags
and averaging times examined for copollutants. Nonetheless, the ISA
noted that the robustness of associations in some studies of
individuals with asthma, combined with findings from controlled human
exposure studies for the direct effects of O3 exposure,
provide substantial evidence supporting the independent effects of
short-term ambient O3 exposure on respiratory symptoms (U.S.
EPA, 2013, section 6.2.4.5).
In summary, both controlled human exposure and epidemiologic
studies have reported respiratory symptoms attributable to short-term
O3 exposures. In the last review, the majority of the
evidence from controlled human exposure studies in young, healthy
adults was for symptoms following exposures to O3
concentrations at or above 80 ppb. Although studies that have become
available since the last review have not reported increased respiratory
symptoms in young, healthy adults following exposures with moderate
exertion to 60 ppb, one recent study did report increased symptoms
following exposure to 72 ppb O3. As was concluded in the
last review, the collective body of epidemiologic evidence indicates
that short-term increases in ambient O3 concentration are
associated with increases in respiratory symptoms in children with
asthma (U.S. EPA, 2013, section 6.2.4). Recent studies of respiratory
symptoms and medication use, primarily in asthmatic children, add to
this evidence. In a smaller body of studies, increases in ambient
O3 concentration were associated with increases in
respiratory symptoms in adults with asthma.
Lung Host Defense
The mammalian respiratory tract has a number of closely integrated
defense mechanisms that, when functioning normally, provide protection
from the potential health effects of exposures to a wide variety of
inhaled particles and microbes. Based on toxicological and human
exposure studies, in the last review EPA concluded that available
evidence indicates that short-term O3 exposures have the
potential to impair host defenses in humans, primarily by interfering
with alveolar macrophage function. Any impairment in alveolar
macrophage function may lead to decreased clearance of microorganisms
or nonviable particles. Compromised alveolar macrophage functions in
asthmatics may increase their susceptibility to other O3
effects, the effects of particles, and respiratory infections (U.S.
EPA, 2006a).
Relatively few studies conducted since the last review have
evaluated the effects of O3 exposures on lung host defense.
As presented in section II.B.2.a.v of the proposal (79 FR 75256),
[[Page 65306]]
when the available evidence is taken as a whole, the ISA concludes that
acute O3 exposures impair the host defense capability of
animals, primarily by depressing alveolar macrophage function and
perhaps also by decreasing mucociliary clearance of inhaled particles
and microorganisms. Coupled with limited evidence from controlled human
exposure studies, this suggests that humans exposed to O3
could be predisposed to bacterial infections in the lower respiratory
tract.
Allergic and Asthma Related Responses
Evidence from controlled human exposure and epidemiologic studies
available in the last review indicates that O3 exposure
skews immune responses toward an allergic phenotype and could also make
airborne allergens more allergenic, as discussed in more detail in the
proposal (79 FR 75257). Evidence from controlled human exposure and
animal toxicology studies available in the last review indicates that
O3 may also increase AHR to specific allergen triggers (75
FR 2970, January 19, 2010). When combined with NO2,
O3 has been shown to enhance nitration of common protein
allergens, which may increase their allergenicity (Franze et al.,
2005).
Hospital Admissions and Emergency Department Visits
The 2006 AQCD concluded that ``the overall evidence supports a
causal relationship between acute ambient O3 exposures and
increased respiratory morbidity resulting in increased emergency
department visits and [hospital admissions] during the warm season''
\29\ (U.S. EPA, 2006a). This conclusion was ``strongly supported by the
human clinical, animal toxicologic[al], and epidemiologic evidence for
[O3-induced] lung function decrements, increased respiratory
symptoms, airway inflammation, and airway hyperreactivity'' (U.S. EPA,
2006a).
---------------------------------------------------------------------------
\29\ Epidemiologic associations for O3 are more
robust during the warm season than during cooler months (e.g.,
smaller measurement error, less potential confounding by
copollutants). The rationale for focusing on warm season
epidemiologic studies for O3 can be found at 72 FR 37838-
37840.
---------------------------------------------------------------------------
The results of recent studies largely support the conclusions of
the 2006 AQCD (U.S. EPA, 2013, section 6.2.7). Since the completion of
the 2006 AQCD, relatively fewer studies, conducted in the U.S., Canada,
and Europe, have evaluated associations between short-term
O3 concentrations and respiratory hospital admissions and
emergency department visits, with a growing number of studies conducted
in Asia. This epidemiologic evidence is discussed in detail in the
proposal (79 FR 75258) and in the ISA (U.S. EPA, 2013, section
6.2.7).\30\
---------------------------------------------------------------------------
\30\ The consideration of ambient O3 concentrations
in the locations of these epidemiologic studies are discussed in
sections II.D.1.b and II.E.4.a below, for the current standard and
for alternative standards, respectively.
---------------------------------------------------------------------------
In considering this body of evidence, the ISA focused primarily on
multicity studies because they examine associations with respiratory-
related hospital admissions and emergency department visits over large
geographic areas using consistent statistical methodologies (U.S. EPA,
2013, section 6.2.7.1). The ISA also focused on single-city studies
that encompassed a large number of daily hospital admissions or
emergency department visits, included long study-durations, were
conducted in locations not represented by the larger studies, or
examined population-specific characteristics that may impact the risk
of O3-related health effects but were not evaluated in the
larger studies (U.S. EPA, 2013, section 6.2.7.1). When examining the
association between short-term O3 exposure and respiratory
health effects that require medical attention, the ISA distinguishes
between hospital admissions and emergency department visits because it
is likely that a small percentage of respiratory emergency department
visits will be admitted to the hospital; therefore, respiratory
emergency department visits may represent potentially less serious, but
more common outcomes (U.S. EPA, 2013, section 6.2.7.1).
The collective evidence across studies indicates a mostly
consistent positive association between O3 exposure and
respiratory-related hospital admissions and emergency department
visits. Moreover, the magnitude of these associations may be
underestimated to the extent members of study populations modify their
behavior in response to air quality forecasts, and to the extent such
behavior modification increases exposure misclassification (U.S. EPA,
2013, Section 4.6.6). Studies examining the potential confounding
effects of copollutants have reported that O3 effect
estimates remained relatively robust upon the inclusion of PM and
gaseous pollutants in two-pollutant models (U.S. EPA, 2013, Figure 6-
20, Table 6-29). Additional studies that conducted copollutant
analyses, but did not present quantitative results, also support these
conclusions (Strickland et al., 2010; Tolbert et al., 2007; Medina-
Ramon et al., 2006; U.S. EPA, 2013, section 6.2.7.5).\31\
---------------------------------------------------------------------------
\31\ The ISA concluded that, ``[o]verall, recent studies provide
copollutant results that are consistent with those from the studies
evaluated in the 2006 O3 AQCD [(U.S. EPA, 2006[a]),
Figure 7-12, page 7-80 of the 2006 O3 AQCD], which found
that O3 respiratory hospital admissions risk estimates
remained robust to the inclusion of PM in copollutant models (U.S.
EPA, 2013, pp. 6-152 to 6-153).
---------------------------------------------------------------------------
In the last review, studies had not evaluated the concentration-
response relationship between short-term O3 exposure and
respiratory-related hospital admissions and emergency department
visits. As described in the proposal in section II.B.2.a.vii (79 FR
75257) and in the ISA (U.S. EPA, 2013, section 6.2.7.2), a preliminary
examination of this relationship in studies that have become available
since the last review found no evidence of a deviation from linearity
when examining the association between short-term O3
exposure and asthma hospital admissions (Silverman and Ito, 2010;
Strickland et al., 2010). In addition, an examination of the
concentration-response relationship for O3 exposure and
pediatric asthma emergency department visits found no evidence of a
threshold at O3 concentrations as low as 30 ppb (for daily
maximum 8-hour concentrations) (U.S. EPA, 2013, section 6.2.7.3).
However, in these studies there is uncertainty in the shape of the
concentration-response curve at the lower end of the distribution of
O3 concentrations due to the low density of data in this
range. Further studies at low-level O3 exposures might
reduce this uncertainty.
Respiratory Mortality
Evidence from experimental studies indicates multiple potential
pathways of respiratory effects from short-term O3
exposures, which support the continuum of respiratory effects that
could potentially result in respiratory-related mortality in adults
(U.S. EPA, 2013, section 6.2.8).\32\ The evidence in the last review
was inconsistent for associations between short-term O3
concentrations and respiratory mortality (U.S. EPA, 2006a). New
epidemiologic evidence for respiratory mortality is discussed in detail
in the ISA (U.S. EPA, 2013, section 6.6) and summarized below. The
majority of recent multicity studies have reported positive
associations between short-term O3 exposures and respiratory
mortality, particularly during the summer months (U.S. EPA, 2013,
Figure 6-36).
---------------------------------------------------------------------------
\32\ Premature mortality is discussed in more detail below in
section II.A.1.b.iv.
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[[Page 65307]]
Recent multicity studies from the U.S. (Zanobetti and Schwartz,
2008), Europe (Samoli et al., 2009), Italy (Stafoggia et al., 2010),
and Asia (Wong et al., 2010), as well as a multi-continent study
(Katsouyanni et al., 2009), reported associations between short-term
O3 concentrations and respiratory mortality (U.S. EPA, 2013,
Figure 6-37, page 6-259). With respect to respiratory mortality,
summer-only analyses were consistently positive and most were
statistically significant. All-year analyses had more mixed results,
but most were positive.
Of the studies evaluated, only two studies analyzed the potential
for copollutant confounding of the O3-respiratory mortality
relationship (Katsouyanni et al., (2009); Stafoggia et al., (2010)).
Based on the results of these analyses, the O3 respiratory
mortality risk estimates appear to be moderately to substantially
sensitive (e.g., increased or attenuated) to inclusion of
PM10. However, in the APHENA study (Katsouyanni et al.,
2009), the mostly every-6th-day sampling schedule for PM10
in the Canadian and U.S. datasets greatly reduced their sample size and
limits the interpretation of these results (U.S. EPA, 2013, sections
6.2.8 and 6.2.9).
The evidence for associations between short-term O3
concentrations and respiratory mortality has been strengthened since
the last review, with the addition of several large multicity studies.
The biological plausibility of the associations reported in these
studies is supported by the experimental evidence for respiratory
effects.
ii. Respiratory Effects--Long-Term Exposure
Since the last review, the body of evidence indicating the
occurrence of respiratory effects due to long-term O3
exposure has been strengthened. This evidence is discussed in detail in
the ISA (U.S. EPA, 2013, Chapter 7) and summarized below for new-onset
asthma and asthma prevalence, asthma hospital admissions, pulmonary
structure and function, and respiratory mortality.
Asthma is a heterogeneous disease with a high degree of temporal
variability. The onset, progression, and symptoms can vary within an
individual's lifetime, and the course of asthma may vary markedly in
young children, older children, adolescents, and adults. In the
previous review, longitudinal cohort studies that examined associations
between long-term O3 exposures and the onset of asthma in
adults and children indicated a direct effect of long-term
O3 exposures on asthma risk in adults and effect
modification by O3 in children. Since then, additional
studies have evaluated associations with new onset asthma, further
informing our understanding of the potential gene-environment
interactions, mechanisms, and biological pathways associated with
incident asthma.
In children, the relationship between long-term O3
exposure and new-onset asthma has been extensively studied in the
Children's Health Study (CHS), a long-term study that was initiated in
the early 1990's which has evaluated effects in several cohorts of
children. For this review, recent studies from the CHS provide evidence
for gene-environment interactions in effects on new-onset asthma by
indicating that the lower risks associated with specific genetic
variants are found in children who live in lower O3
communities. Described in detail in the proposal (79 FR 75259) and in
the ISA (U.S. EPA, 2013, section 7.2.1), these studies indicate that
the risk for new-onset asthma is related in part to genetic
susceptibility, as well as behavioral factors and environmental
exposure. Cross-sectional studies by Akinbami et al. (2010) and Hwang
et al. (2005) provide further evidence relating O3 exposures
with asthma prevalence. Gene-environment interactions are discussed in
detail in Section 5.4.2.1 in the ISA (U.S. EPA, 2013).
In the 2006 AQCD (U.S. EPA, 2006a), studies on O3-
related hospital discharges and emergency department visits for asthma
and respiratory disease mainly looked at short-term (daily) metrics.
Recent studies continue to indicate that there is evidence for
increases in both hospital admissions and emergency department visits
in children and adults related to all respiratory outcomes, including
asthma, with stronger associations in the warm months.
In the 2006 AQCD (U.S. EPA, 2006a), few epidemiologic studies had
investigated the effect of chronic O3 exposure on pulmonary
function. As discussed in the proposal, epidemiologic studies of long-
term exposures in both children and adults provide mixed results about
the effects of long-term O3 exposure on pulmonary function
and the growth rate of lung function.
Long-term studies in animals allow for greater insight into the
potential effects of prolonged exposure to O3 that may not
be easily measured in humans, such as structural changes in the
respiratory tract. Despite uncertainties, epidemiologic studies
observing associations of O3 exposure with functional
changes in humans can attain biological plausibility in conjunction
with long-term toxicological studies, particularly O3-
inhalation studies performed in non-human primates whose respiratory
systems most closely resemble that of the human. An important series of
studies, discussed in section 7.2.3.2 of the ISA (U.S. EPA, 2013), have
used nonhuman primates to examine the effect of O3 alone, or
in combination with an inhaled allergen, house dust mite antigen, on
morphology and lung function. Animals exhibit the hallmarks of allergic
asthma defined for humans (NHLBI, 2007). These studies and others have
demonstrated changes in pulmonary function and airway morphology in
adult and infant nonhuman primates repeatedly exposed to
environmentally relevant concentrations of O3 (U.S. EPA,
2013, section 7.2.3.2). As discussed in more detail in the proposal,
the studies provide evidence of an O3-induced change in
airway resistance and responsiveness and provide biological
plausibility of long-term exposure, or repeated short-term exposures,
to O3 contributing to the effects of asthma in children.
Collectively, evidence from animal studies strongly suggests that
chronic O3 exposure is capable of damaging the distal
airways and proximal alveoli, resulting in lung tissue remodeling and
leading to apparent irreversible changes. Potentially, persistent
inflammation and interstitial remodeling play an important role in the
progression and development of chronic lung disease. Further discussion
of the modes of action that lead to O3-induced morphological
changes and the mechanisms involved in lifestage susceptibility and
developmental effects can be found in the ISA (U.S. EPA, 2013, section
5.3.7, section 5.4.2.4). The findings reported in chronic animal
studies offer insight into potential biological mechanisms for the
suggested association between seasonal O3 exposure and
reduced lung function development in children as observed in
epidemiologic studies (U.S. EPA, 2013, section 7.2.3.1). Further
research could help fill in the gaps in our understanding of the
mechanisms involved in lifestage susceptibility and developmental
effects in children of seasonal or long-term exposure to O3.
A limited number of epidemiologic studies have assessed the
relationship between long-term exposure to O3 and mortality
in adults. The 2006 AQCD concluded that an insufficient amount of
evidence existed ``to suggest a causal relationship between chronic
O3 exposure and increased risk for
[[Page 65308]]
mortality in humans'' (U.S. EPA, 2006a). Though total and cardio-
pulmonary mortality were considered in these studies, respiratory
mortality was not specifically considered.
In a recent follow-up analysis of the American Cancer Society
cohort (Jerrett et al., 2009), cardiopulmonary deaths were separately
subdivided into respiratory and cardiovascular deaths, rather than
combined as in the Pope et al. (2002) work. Increased O3
exposure was associated with the risk of death from respiratory causes,
and this effect was robust to the inclusion of PM2.5.
Additionally, a recent multicity time series study (Zanobetti and
Schwartz, 2011), which followed (from 1985 to 2006) four cohorts of
Medicare enrollees with chronic conditions that might predispose to
O3-related effects, observed an association between long-
term (warm season) exposure to O3 and elevated risk of
mortality in the cohort that had previously experienced an emergency
hospital admission due to chronic obstructive pulmonary disease (COPD).
A key limitation of this study is the inability to control for
PM2.5, because data were not available in these cities until
1999.
iii. Cardiovascular Effects--Short-Term Exposure
A relatively small number of studies have examined the potential
effect of short-term O3 exposure on the cardiovascular
system. The 2006 AQCD (U.S. EPA, 2006a, p. 8-77) concluded that
``O3 directly and/or indirectly contributes to
cardiovascular-related morbidity,'' but added that the body of evidence
was limited. This conclusion was based on a controlled human exposure
study that included hypertensive adult males; a few epidemiologic
studies of physiologic effects, heart rate variability, arrhythmias,
myocardial infarctions, and hospital admissions; and toxicological
studies of heart rate, heart rhythm, and blood pressure.
More recently, the body of scientific evidence available that has
examined the effect of O3 on the cardiovascular system has
expanded. There is an emerging body of animal toxicological evidence
demonstrating that short-term exposure to O3 can lead to
autonomic nervous system alterations (in heart rate and/or heart rate
variability) and suggesting that proinflammatory signals may mediate
cardiovascular effects. Interactions of O3 with respiratory
tract components result in secondary oxidation product formation and
subsequent production of inflammatory mediators, which have the
potential to penetrate the epithelial barrier and to initiate toxic
effects systemically. In addition, animal toxicological studies of
long-term exposure to O3 provide evidence of enhanced
atherosclerosis and ischemia/reperfusion (I/R) injury, corresponding
with development of a systemic oxidative, proinflammatory environment.
Recent experimental and epidemiologic studies have investigated
O3-related cardiovascular events and are summarized in the
ISA (U.S. EPA, 2013, section 6.3).
Controlled human exposure studies discussed in previous reviews
have not demonstrated any consistent extrapulmonary effects. In this
review, evidence from controlled human exposure studies suggests
cardiovascular effects in response to short-term O3 exposure
(U.S. EPA, 2013, section 6.3.1) and provides some coherence with
evidence from animal toxicology studies. Controlled human exposure
studies also support the animal toxicological studies by demonstrating
O3-induced effects on blood biomarkers of systemic
inflammation and oxidative stress, as well as changes in biomarkers
that can indicate the potential for increased clotting following
O3 exposures. Increases and decreases in high frequency
heart rate variability (HRV) have been reported. These changes in
cardiac function observed in animal and human studies provide
preliminary evidence for O3-induced modulation of the
autonomic nervous system through the activation of neural reflexes in
the lung (U.S. EPA, 2013, section 5.3.2).
Overall, the ISA concludes that the available body of epidemiologic
evidence examining the relationship between short-term exposures to
O3 concentrations and cardiovascular morbidity is
inconsistent (U.S. EPA, 2013, section 6.3.2.9).
Despite the inconsistent evidence for an association between
O3 concentration and cardiovascular disease (CVD) morbidity,
mortality studies indicate a consistent positive association between
short-term O3 exposure and cardiovascular mortality in
multicity studies and in a multi-continent study. When examining
mortality due to CVD, epidemiologic studies consistently observe
positive associations with short-term exposure to O3.
Additionally, there is some evidence for an association between long-
term exposure to O3 and mortality, although the association
between long-term ambient O3 concentrations and
cardiovascular mortality can be confounded by other pollutants (U.S.
EPA, 2013). The ISA (U.S. EPA, 2013, section 6.3.4) states that taken
together, the overall body of evidence across the animal and human
studies is sufficient to conclude that there is likely to be a causal
relationship between relevant short-term exposures to O3 and
cardiovascular system effects.
iv. Premature Mortality--Short-Term Exposure
The 2006 AQCD concluded that the overall body of evidence was
highly suggestive that short-term exposure to O3 directly or
indirectly contributes to nonaccidental and cardiopulmonary-related
mortality in adults, but additional research was needed to more fully
establish underlying mechanisms by which such effects occur (U.S. EPA,
2006a; U.S. EPA, 2013, p. 2-18). In building on the evidence for
mortality from the last review, the ISA states (U.S. EPA, 2013, p. 6-
261):
The evaluation of new multicity studies that examined the
association between short-term O3 exposures and mortality
found evidence that supports the conclusions of the 2006 AQCD. These
new studies reported consistent positive associations between short-
term O3 exposure and all-cause (nonaccidental) mortality,
with associations persisting or increasing in magnitude during the
warm season, and provide additional support for associations between
O3 exposure and cardiovascular and respiratory mortality.
The 2006 AQCD reviewed a large number of time-series studies of
associations between short-term O3 exposures and total
mortality including single- and multicity studies, and meta-analyses.
Available studies reported some evidence for heterogeneity in
O3 mortality risk estimates across cities and across
studies. Studies that conducted seasonal analyses reported larger
O3 mortality risk estimates during the warm or summer
season. Overall, the 2006 AQCD identified robust associations between
various measures of daily ambient O3 concentrations and all-
cause mortality, which could not be readily explained by confounding
due to time, weather, or copollutants. With regard to cause-specific
mortality, consistent positive associations were reported between
short-term O3 exposure and cardiovascular mortality, with
less consistent evidence for associations with respiratory mortality.
The majority of the evidence for associations between O3 and
cause-specific mortality were from single-city studies, which had small
daily mortality counts and subsequently limited statistical power to
detect associations. The 2006 AQCD concluded that ``the overall body of
evidence is highly suggestive that O3 directly or indirectly
contributes to nonaccidental and cardiopulmonary-related mortality''
(U.S. EPA, 2013, section 6.6.1).
[[Page 65309]]
Recent studies have strengthened the body of evidence that supports
the association between short-term O3 concentrations and
mortality in adults. This evidence includes a number of studies
reporting associations with nonaccidental as well as cause-specific
mortality. Multi-continent and multicity studies have consistently
reported positive and statistically significant associations between
short-term O3 concentrations and all-cause mortality, with
evidence for larger mortality risk estimates during the warm or summer
months (79 FR 75262; U.S. EPA, 2013 Figure 6-27; Table 6-42).
Similarly, evaluations of cause-specific mortality have reported
consistently positive associations with O3, particularly in
analyses restricted to the warm season (79 FR 75262; U.S. EPA, 2013
Fig. 6-37; Table 6-53).
In the previous review, multiple uncertainties remained regarding
the relationship between short-term O3 concentrations and
mortality, including the extent of residual confounding by
copollutants; characterization of the factors that modify the
O3-mortality association; the appropriate lag structure for
identifying O3-mortality effects; and the shape of the
O3-mortality concentration-response function and whether a
threshold exists. Many of the studies, published since the last review,
have attempted to address one or more of these uncertainties and are
described in more detail in the proposal (79 FR 75262 and in the ISA
(U.S. EPA, 2013, section 6.6.2).
In particular, recent studies have evaluated different statistical
approaches to examine the shape of the O3-mortality
concentration-response relationship and to evaluate whether a threshold
exists for O3-related mortality. These studies are detailed
in the proposal (79 FR 75262) and in the ISA (U.S. EPA, 2013, p. 2-32).
The ISA reaches the following overall conclusions that the
epidemiologic studies identified in the ISA indicated a generally
linear C-R function with no indication of a threshold but that there is
a lack of data at lower O3 concentrations and therefore,
less certainty in the shape of the C-R curve at the lower end of the
distribution (U.S. EPA, 2013, p. 2-32).
c. Adversity of Effects
In making judgments as to when various O3-related
effects become regarded as adverse to the health of individuals, in
previous NAAQS reviews, the EPA has relied upon the guidelines
published by the ATS and the advice of CASAC. In 2000, the ATS
published an official statement on ``What Constitutes an Adverse Health
Effect of Air Pollution?'' (ATS, 2000a), which updated and built upon
its earlier guidance (ATS, 1985). The earlier guidance defined adverse
respiratory health effects as ``medically significant physiologic
changes generally evidenced by one or more of the following: (1)
Interference with the normal activity of the affected person or
persons, (2) episodic respiratory illness, (3) incapacitating illness,
(4) permanent respiratory injury, and/or (5) progressive respiratory
dysfunction,'' while recognizing that perceptions of ``medical
significance'' and ``normal activity'' may differ among physicians,
lung physiologists and experimental subjects (ATS, 1985). The more
recent guidance concludes that transient, reversible loss of lung
function in combination with respiratory symptoms should be considered
adverse.\33\ However, the committee also recommended ``that a small,
transient loss of lung function, by itself, should not automatically be
designated as adverse'' (ATS, 2000a, p. 670).
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\33\ ``In drawing the distinction between adverse and nonadverse
reversible effects, this committee recommended that reversible loss
of lung function in combination with the presence of symptoms should
be considered as adverse'' (ATS, 2000a).
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There is also a more specific consideration of population risk in
the 2000 guidance. Specifically, the committee considered that a shift
in the risk factor distribution, and hence the risk profile of the
exposed population, should be considered adverse, even in the absence
of the immediate occurrence of frank illness (ATS, 2000a, p. 668). For
example, a population of asthmatics could have a distribution of lung
function such that no individual has a level associated with clinically
important impairment. Exposure to air pollution could shift the
distribution to lower levels of lung function that still do not bring
any individual to a level that is associated with clinically relevant
effects. However, this would be considered to be adverse because
individuals within the population would already have diminished reserve
function, and therefore would be at increased risk to further
environmental insult (ATS, 2000a, p. 668).
The ATS also concluded in its guidance that elevations of
biomarkers such as cell numbers and types, cytokines, and reactive
oxygen species may signal risk for ongoing injury and more serious
effects or may simply represent transient responses, illustrating the
lack of clear boundaries that separate adverse from nonadverse events.
More subtle health outcomes also may be connected mechanistically to
health effects that are clearly adverse, so that small changes in
physiological measures may not appear clearly adverse when considered
alone, but may be part of a coherent and biologically plausible chain
of related health outcomes that include responses that are clearly
adverse, such as mortality (U.S. EPA, 2014c, section 3.1.2.1).
Application of the ATS guidelines to the least serious category of
effects \34\ related to ambient O3 exposures, which are also
the most numerous and, therefore, are also important from a public
health perspective, involves judgments about which medical experts on
CASAC panels and public commenters have in the past expressed diverse
views. To help frame such judgments, in past reviews, the EPA has
defined gradations of individual functional responses (e.g., decrements
in FEV1 and airway responsiveness) and symptomatic responses
(e.g., cough, chest pain, wheeze), together with judgments as to the
potential impact on individuals experiencing varying degrees of
severity of these responses. These gradations were used by the EPA in
the 1997 O3 NAAQS review and slightly revised in the 2008
review (U.S. EPA, 1996b, p. 59; U.S. EPA, 2007, p. 3-72; 72 FR 37849,
July 11, 2007). These gradations and impacts are summarized in Tables
3-2 and 3-3 in the 2007 O3 Staff Paper (U.S. EPA, 2007, pp.
3-74 to 3-75).
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\34\ These include, for example, the transient and reversible
effects demonstrated in controlled human exposure studies, such as
lung function decrements or respiratory symptoms.
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For the purpose of estimating potentially adverse lung function
decrements in active healthy people, the CASAC panel in the 2008
O3 NAAQS review indicated that a focus on the mid to upper
end of the range of moderate levels of functional responses is most
appropriate (e.g., FEV1 decrements >=15% but <20%)
(Henderson, 2006; U.S. EPA, 2007, p. 3-76). In this review, CASAC
reiterated that the ``[e]stimation of FEV1 decrements of
>=15% is appropriate as a scientifically relevant surrogate for adverse
health outcomes in active healthy adults'' (Frey, 2014c, p. 3).
For the purpose of estimating potentially adverse lung function
decrements in people with lung disease, the CASAC panel in the 2008
O3 NAAQS review indicated that a focus on the lower end of
the range of moderate levels of functional responses is most
appropriate (e.g., FEV1 decrements >=10%) (Henderson, 2006;
U.S. EPA, 2007, p. 3-76). In their letter
[[Page 65310]]
advising the Administrator on the reconsideration of the 2008 final
decision, CASAC stated that ``[a] 10% decrement in FEV1 can
lead to respiratory symptoms, especially in individuals with pre-
existing pulmonary or cardiac disease. For example, people with chronic
obstructive pulmonary disease have decreased ventilatory reserve (i.e.,
decreased baseline FEV1) such that a >= 10% decrement could
lead to moderate to severe respiratory symptoms'' (Samet, 2011). In
this review, CASAC provided similar advice, stating that ``[a]n
FEV1 decrement of >= 10% is a scientifically relevant
surrogate for adverse health outcomes for people with asthma and lung
disease'', and that such decrements ``could be adverse for people with
lung disease'' (Frey, 2014c, pp. 3, 7).
In judging the extent to which these impacts represent effects that
should be regarded as adverse to the health status of individuals, in
previous NAAQS reviews, the EPA has also considered whether effects
were experienced repeatedly during the course of a year or only on a
single occasion (U.S. EPA, 2007). While some experts would judge single
occurrences of moderate responses to be a ``nuisance,'' especially for
healthy individuals, a more general consensus view of the adversity of
such moderate responses emerges as the frequency of occurrence
increases. In particular, not every estimated occurrence of an
O3-induced FEV1 decrement will be adverse.\35\
However, repeated occurrences of moderate responses, even in otherwise
healthy individuals, may be considered to be adverse since they could
set the stage for more serious illness (61 FR 65723). The CASAC panel
in the 1997 NAAQS review expressed a consensus view that these
``criteria for the determination of an adverse physiological response
were reasonable'' (Wolff, 1995). In the review completed in 2008, as in
the current review (II.B, II.C below), estimates of repeated
occurrences continued to be an important public health policy factor in
judging the adversity of moderate lung function decrements in healthy
and asthmatic people (72 FR 37850, July 11, 2007).
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\35\ As noted above, the ATS recommended ``that a small,
transient loss of lung function, by itself, should not automatically
be designated as adverse'' (ATS, 2000a, p. 670).
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d. Ozone-Related Impacts on Public Health
The currently available evidence expands the understanding of
populations that were identified to be at greater risk of
O3-related health effects at the time of the last review
(i.e., people who are active outdoors, people with lung disease,
children and older adults and people with increased responsiveness to
O3) and supports the identification of additional factors
that may lead to increased risk (U.S. EPA, 2006a, section 6.3; U.S.
EPA, 2013, Chapter 8). Populations and lifestages may be at greater
risk for O3-related health effects due to factors that
contribute to their susceptibility and/or vulnerability to
O3. The definitions of susceptibility and vulnerability have
been found to vary across studies, but in most instances
``susceptibility'' refers to biological or intrinsic factors (e.g.,
lifestage, sex, preexisting disease/conditions) while ``vulnerability''
refers to non-biological or extrinsic factors (e.g., socioeconomic
status [SES]) (U.S. EPA, 2013, p. 8-1; U.S. EPA, 2010, 2009b). In some
cases, the terms ``at-risk'' and ``sensitive'' have been used to
encompass these concepts more generally. In the ISA, PA, and proposal,
``at-risk'' is the all-encompassing term used to define groups with
specific factors that increase their risk of O3-related
health effects.
There are multiple avenues by which groups may experience increased
risk for O3-induced health effects. A population or
lifestage \36\ may exhibit greater effects than other populations or
lifestages exposed to the same concentration or dose, or they may be at
greater risk due to increased exposure to an air pollutant (e.g., time
spent outdoors). A group with intrinsically increased risk would have
some factor(s) that increases risk through a biological mechanism and,
in general, would have a steeper concentration-risk relationship,
compared to those not in the group. Factors that are often considered
intrinsic include pre-existing asthma, genetic background, and
lifestage. A group of people could also have extrinsically increased
risk, which would be through an external, non-biological factor, such
as socioeconomic status (SES) and diet. Some groups are at risk of
increased internal dose at a given exposure concentration, for example,
because of breathing patterns. This category would include people who
work or exercise outdoors. Finally, there are those who might be placed
at increased risk for experiencing greater exposures by being exposed
to higher O3 concentrations. This would include, for
example, groups of people with greater exposure to ambient
O3 due to less availability or use of home air conditioners
such that they are more likely to be in locations with open windows on
high O3 days. Some groups may be at increased risk of
O3-related health effects through a combination of factors.
For example, children tend to spend more time outdoors when
O3 levels are high, and at higher levels of activity than
adults, which leads to increased exposure and dose, and they also have
biological, or intrinsic, risk factors (e.g., their lungs are still
developing) (U.S. EPA, 2013, Chapter 8). An at-risk population or
lifestage is more likely to experience adverse health effects related
to O3 exposures and/or, develop more severe effects from
exposure than the general population. The populations and lifestages
identified by the ISA (U.S. EPA, 2013, section 8.5) identified that
have ``adequate'' evidence for increased O3-related health
effects are people with certain genotypes, people with asthma, younger
and older age groups, people with reduced intake of certain nutrients,
and outdoor workers. These at-risk populations and lifestages are
described in more detail in section II.B.4 of the proposal (79 FR
75264-269).
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\36\ Lifestages, which in this case includes childhood and older
adulthood, are experienced by most people over the course of a
lifetime, unlike other factors associated with at-risk populations.
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One consideration in the assessment of potential public health
impacts is the size of various population groups for which there is
adequate evidence of increased risk for health effects associated with
O3-related air pollution exposure (U.S. EPA, 2014c, section
3.1.5.2). The factors for which the ISA judged the evidence to be
``adequate'' with respect to contributing to increased risk of
O3-related effects among various populations and lifestages
included: Asthma; childhood and older adulthood; diets lower in
vitamins C and E; certain genetic variants; and working outdoors (U.S.
EPA, 2013, section 8.5). No statistics are available to estimate the
size of an at-risk population based on nutritional status or genetic
variability.
With regard to asthma, Table 3-7 in the PA (U.S. EPA, 2014c,
section 3.1.5.2) summarizes information on the prevalence of current
asthma by age in the U.S. adult population in 2010 (Schiller et al.
2012; children--Bloom et al., 2011). Individuals with current asthma
constitute a fairly large proportion of the population, including more
than 25 million people. Asthma prevalence tends to be higher in
children than adults. Within the U.S., approximately 8.2% of adults
have reported currently having asthma (Schiller et al., 2012) and 9.5%
of
[[Page 65311]]
children have reported currently having asthma (Bloom et al.,
2011).\37\
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\37\ As noted below (II.C.3.a.ii), asthmatics can experience
larger O3-induced respiratory effects than non-asthmatic,
healthy adults. The responsiveness of asthmatics to O3
exposures could depend on factors that have not been well-evaluated
such as asthma severity, the effectiveness of asthma control, or the
prevalence of medication use.
---------------------------------------------------------------------------
With regard to lifestages, based on U.S. census data from 2010
(Howden and Meyer, 2011), about 74 million people, or 24% of the U.S.
population, are under 18 years of age and more than 40 million people,
or about 13% of the U.S. population, are 65 years of age or older.
Hence, a large proportion of the U.S. population (i.e., more than a
third) is included in age groups that are considered likely to be at
increased risk for health effects from ambient O3 exposure.
With regard to outdoor workers, in 2010, approximately 11.7% of the
total number of people (143 million people) employed, or about 16.8
million people, worked outdoors one or more days per week (based on
worker surveys).\38\ Of these, approximately 7.4% of the workforce, or
about 7.8 million people, worked outdoors three or more days per week.
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\38\ The O*NET program is the nation's primary source of
occupational information. Central to the project is the O*NET
database, containing information on hundreds of standardized and
occupation-specific descriptors. The database, which is available to
the public at no cost, is continually updated by surveying a broad
range of workers from each occupation. http://www.onetcenter.org/overview.html. http://www.onetonline.org/find/descriptor/browse/Work_Context/4.C.2/.
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While it is difficult to estimate the total number of people in
groups that are at greater risk from exposure to O3, due to
the overlap in members of the different at-risk population groups, the
proportion of the total population at greater risk is large. The size
of the at-risk population combined with the estimates of risk of
different health outcomes associated with exposure to O3 can
give an indication of the magnitude of O3 impacts on public
health.
2. Overview of Human Exposure and Health Risk Assessments
To put judgments about health effects into a broader public health
context, the EPA has developed and applied models to estimate human
exposures to O3 and O3-associated health risks.
Exposure and risk estimates that are output from such models are
presented and assessed in the HREA (U.S. EPA, 2014a). Section II.C of
the proposal discusses the quantitative assessments of O3
exposures and O3-related health risks that are presented in
the HREA (79 FR 75270). Summaries of these discussions are provided
below for the approach used to adjust air quality for quantitative
exposure and risk analyses in the HREA (II.A.2.a), the HREA assessment
of exposures to ambient O3 (II.A.2.b), and the HREA
assessments of O3-related health risks (II.A.2.c).
a. Air Quality Adjustment
As discussed in section II.C.1 of the proposal (79 FR 75270), the
HREA uses a photochemical model to estimate sensitivities of
O3 to changes in precursor emissions in order to estimate
ambient O3 concentrations that would just meet the current
and alternative standards (U.S. EPA, 2014a, Chapter 4).\39\ For the 15
urban study areas evaluated in the HREA,\40\ this model-based
adjustment approach estimates hourly O3 concentrations at
each monitor location when modeled U.S. anthropogenic precursor
emissions (i.e., NOX, VOC) \41\ are reduced. The HREA
estimates air quality that just meets the current and alternative
standards for the 2006-2008 and 2008-2010 periods.\42\
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\39\ The HREA uses the Community Multi-scale Air Quality (CMAQ)
photochemical model instrumented with the higher order direct
decoupled method (HDDM) to estimate O3 concentrations
that would occur with the achievement of the current and alternative
O3 standards (U.S. EPA, 2014a, Chapter 4).
\40\ The urban study areas assessed are Atlanta, Baltimore,
Boston, Chicago, Cleveland, Dallas, Denver, Detroit, Houston, Los
Angeles, New York, Philadelphia, Sacramento, St. Louis, and
Washington, DC.
\41\ Exposure and risk analyses for most of the urban study
areas focus on reducing U.S. anthropogenic NOX emissions
alone. The exceptions are Chicago and Denver. Exposure and risk
analyses for Chicago and Denver are based on reductions in emissions
of both NOX and VOC (U.S. EPA, 2014a, section 4.3.3.1;
Appendix 4D).
\42\ These estimates thus reflect design values--8 hour values
using the form of the NAAQS that meet the level of the current or
alternative standards. These simulations are illustrative and do not
reflect any consideration of specific control programs designed to
achieve the reductions in emissions required to meet the specified
standards. Further, these simulations do not represent predictions
of when, whether, or how areas might meet the specified standards.
---------------------------------------------------------------------------
As discussed in Chapter 4 of the HREA (U.S. EPA, 2014a), this
approach to adjusting air quality models the physical and chemical
atmospheric processes that influence ambient O3
concentrations. Compared to the quadratic rollback approach used in
previous reviews, it provides more realistic estimates of the spatial
and temporal responses of O3 to reductions in precursor
emissions. Because ambient NOX can contribute both to the
formation and destruction of O3 (U.S. EPA, 2014a, Chapter
4), the response of ambient O3 concentrations to reductions
in NOX emissions is more variable than indicated by the
quadratic rollback approach. This improved approach to adjusting
O3 air quality is consistent with recommendations from the
National Research Council of the National Academies (NRC, 2008). In
addition, CASAC strongly supported the new approach as an improvement
and endorsed the way it was utilized in the HREA, stating that ``the
quadratic rollback approach has been replaced by a scientifically more
valid Higher-order Decoupled Direct Method (HDDM)'' and that ``[t]he
replacement of the quadratic rollback procedure by the HDDM procedure
is important and supported by the CASAC'' (Frey, 2014a, pp. 1 and 3).
Within urban study areas, the model-based air quality adjustments
show reductions in the O3 levels at the upper ends of
ambient concentrations and increases in the O3 levels at the
lower ends of those distributions (U.S. EPA, 2014a, section 4.3.3.2,
Figures 4-9 and 4-10).\43\ Seasonal means of daily O3
concentrations generally exhibit only modest changes upon model
adjustment, reflecting the seasonal balance between daily decreases in
relatively higher concentrations and increases in relatively lower
concentrations (U.S. EPA, 2014a, Figures 4-9 and 4-10). The resulting
compression in the seasonal distributions of ambient O3
concentrations is evident in all of the urban study areas evaluated,
though the degree of compression varies considerably across areas (U.S.
EPA, 2014a, Figures 4-9 and 4-10).
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\43\ It is important to note that sensitivity analyses in the
HREA indicate that the increases in low O3 concentrations
are smaller when NOX and VOC emissions are reduced than
when only NOX emissions are reduced (U.S. EPA, 2014a,
Appendix 4-D, section 4.7).
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As discussed in the PA (U.S. EPA, 2014c, section 3.2.1), adjusted
patterns of O3 air quality have important implications for
exposure and risk estimates in urban case study areas. Estimates
influenced largely by the upper ends of the distribution of ambient
concentrations (i.e., exposures of concern and lung function risk
estimates, as discussed in sections 3.2.2 and 3.2.3.1 of the PA) will
decrease with model-adjustment to the current and alternative
standards. In contrast, seasonal risk estimates influenced by the full
distribution of ambient O3 concentrations (i.e.,
epidemiology-based risk estimates, as discussed in section 3.2.3.2 of
the PA) either increase or decrease in response to air quality
adjustment, depending on the balance between the daily decreases in
high O3
[[Page 65312]]
concentrations and increases in low O3 concentrations.\44\
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\44\ In addition, because epidemiology-based risk estimates use
``area-wide'' average O3 concentrations, calculated by
averaging concentrations across multiple monitors in urban case
study areas (section 3.2.3.2 below), risk estimates on a given day
depend on the daily balance between increasing and decreasing
O3 concentrations at individual monitors.
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To evaluate uncertainties in air quality adjustments, the HREA
assessed the extent to which the modeled O3 response to
reductions in NOX emissions appropriately represent the
trends observed in monitored ambient O3 following actual
reductions in NOX emissions, and the extent to which the
O3 response to reductions in precursor emissions could
differ with emissions reduction strategies that are different from
those used in HREA to generate risk estimates.
To evaluate the first issue, the HREA conducted a national analysis
evaluating trends in monitored ambient O3 concentrations
during a time period when the U.S. experienced large-scale reductions
in NOX emissions (i.e., 2001 to 2010). Analyses of trends in
monitored O3 indicate that over such a time period, the
upper end of the distribution of monitored O3 concentrations
(i.e., indicated by the 95th percentile) generally decreased in urban
and non-urban locations across the U.S. (U.S. EPA, 2014a, Figure 8-29).
During this same time period, median O3 concentrations
decreased in suburban and rural locations, and in some urban locations.
However, median concentrations increased in some large urban centers
(U.S. EPA, 2014a, Figure 8-28). As discussed in the HREA, these
increases in median concentrations likely reflect the increases in
relatively low O3 concentrations that can occur near
important sources of NOX upon reductions in NOX
emissions (U.S. EPA, 2014a, section 8.2.3.1). These patterns of
monitored O3 during a period when the U.S. experienced large
reductions in NOX emissions are qualitatively consistent
with the modeled responses of O3 to reductions in
NOX emissions.
To evaluate the second issue, the HREA assessed the O3
air quality response to reducing both NOX and VOC emissions
(i.e., in addition to assessing reductions in NOX emissions
alone) for a subset of seven urban study areas. As discussed in the PA
(U.S. EPA, 2014c, section 3.2.1), the addition of VOC reductions
generally resulted in larger decreases in mid-range O3
concentrations (25th to 75th percentiles) (U.S. EPA, 2014a, Appendix
4D, section 4.7).\45\ In addition, in all seven of the urban study
areas evaluated, the increases in low O3 concentrations were
smaller for the NOX/VOC scenarios than the NOX
alone scenarios (U.S. EPA, 2014a, Appendix 4D, section 4.7). This was
most apparent for Denver, Houston, Los Angeles, New York, and
Philadelphia. Given the impacts on total risk estimates of increases in
low O3 concentrations (discussed below), these results
suggest that in some locations optimized emissions reduction strategies
could result in larger reductions in O3-associated mortality
and morbidity than indicated by HREA estimates.
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\45\ This was the case for all of the urban study areas
evaluated, with the exception of New York (U.S. EPA, 2014a, Appendix
4-D, section 4.7). In this analysis, emissions of NOX and
VOC were reduced by equal percentages, a scenario not likely to
reflect the optimal combination for reducing risks. In most of the
urban study areas the inclusion of VOC emissions reductions did not
alter the NOX emissions reductions required to meet the
current or alternative standards. The exceptions are Chicago and
Denver, for which the HREA risk estimates are based on reductions in
both NOX and VOC (U.S. EPA, 2014a, section 4.3.3.1).
---------------------------------------------------------------------------
b. Exposure Assessment
As discussed in section II.C.2 of the proposal, the O3
exposure assessment presented in the HREA (U.S. EPA, 2014a, Chapter 5)
provides estimates of the number and percent of people exposed to
various concentrations of ambient O3 while at specified
exertion levels. The HREA estimates exposures in the 15 urban study
areas for four study groups, all school-age children (ages 5 to 18),
asthmatic school-age children, asthmatic adults (ages 19 to 95), and
all older adults (ages 65 to 95), reflecting the evidence indicating
that these populations are at increased risk for O3-
attributable effects (U.S. EPA, 2013, Chapter 8; II.A.1.d, above). An
important purpose of these exposure estimates is to provide perspective
on the extent to which air quality adjusted to just meet the current
O3 NAAQS could be associated with exposures to O3
concentrations reported to result in respiratory effects.\46\ These
analyses of exposure assessment incorporate behavior patterns,
including estimates of physical exertion, which are critical in
assessing whether ambient concentrations of O3 may pose a
public health risk.\47\ In particular, exposures to ambient or near-
ambient O3 concentrations have only been shown to result in
potentially adverse effects if the ventilation rates of people in the
exposed populations are raised to a sufficient degree (e.g., through
physical exertion) (U.S. EPA, 2013, section 6.2.1.1). Estimates of such
``exposures of concern'' provide perspective on the potential public
health impacts of O3-related effects, including effects that
cannot currently be evaluated in a quantitative risk assessment.\48\
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\46\ In addition, the range of modeled personal exposures to
ambient O3 provide an essential input to the portion of
the health risk assessment based on exposure-response functions (for
lung function decrements) from controlled human exposure studies.
The health risk assessment based on exposure-response information is
discussed below (II.C.3).
\47\ See 79 FR 75269 ``The activity pattern of individuals is an
important determinant of their exposure. Variation in O3
concentrations among various microenvironments means that the amount
of time spent in each location, as well as the level of activity,
will influence an individual's exposure to ambient O3.
Activity patterns vary both among and within individuals, resulting
in corresponding variations in exposure across a population and over
time'' (internal citations omitted).
\48\ In this review, the term ``exposure of concern'' is defined
as a personal exposure, while at moderate or greater exertion, to 8-
hour average ambient O3 concentrations at and above
specific benchmarks levels. As discussed below, these benchmark
levels represent exposure concentrations at which O3-
induced health effects are known to occur, or can reasonably be
anticipated to occur, in some individuals.
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The HREA estimates 8-hour exposures at or above benchmark
concentrations of 60, 70, and 80 ppb for individuals engaged in
moderate or greater exertion (i.e., to approximate conditions in the
controlled human exposure studies on which benchmarks are based).
Benchmarks reflect exposure concentrations at which O3-
induced respiratory effects are known to occur in some healthy adults
engaged in moderate, quasi-continuous exertion, based on evidence from
controlled human exposure studies (U.S. EPA, 2013, section 6.2; U.S.
EPA, 2014c, section 3.1.2.1). The amount of weight to place on the
estimates of exposures at or above specific benchmark concentrations
depends in part on the weight of the scientific evidence concerning
health effects associated with O3 exposures at those
benchmark concentrations. It also depends on judgments about the
importance, from a public health perspective, of the health effects
that are known or can reasonably be inferred to occur as a result of
exposures at benchmark concentrations (U.S. EPA, 2014c, sections 3.1.3,
3.1.5).
In considering estimates of O3 exposures of concern at
or above benchmarks of 60, 70, and 80 ppb, the PA focuses on modeled
exposures for school-age children (ages 5-18), including asthmatic
school-age children, which are key at-risk populations identified in
the ISA (U.S. EPA, 2014c, section 3.1.5). The percentages of children
estimated to experience exposures of concern are considerably larger
than the percentages estimated for adult populations (i.e.,
approximately 3-fold larger across urban
[[Page 65313]]
study areas) \49\ (U.S. EPA, 2014a, section 5.3.2 and Figures 5-5 to 5-
8). The larger exposure estimates for children are due primarily to the
larger percentage of children estimated to spend an extended period of
time being physically active outdoors when O3 concentrations
are elevated (U.S. EPA, 2014a, sections 5.3.2 and 5.4.1).
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\49\ HREA exposure estimates for all children and asthmatic
children are virtually indistinguishable, in terms of the percent
estimated to experience exposures of concern (U.S. EPA, 2014a,
Chapter 5). Consistent with this, HREA analyses indicate that
activity data for people with asthma is generally similar to non-
asthmatic populations (U.S. EPA, 2014a, Appendix 5G, Tables 5G2-to
5G-5).
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Although exposure estimates differ between children and adults, the
patterns of results across the urban study areas and years are similar
among all of the populations evaluated (U.S. EPA, 2014a, Figures 5-5 to
5-8). Therefore, while the PA highlights estimates in children,
including asthmatic school-age children, it also notes that the
patterns of exposures estimated for children represent the patterns
estimated for adult asthmatics and older adults.
Table 1 of the proposal (79 FR 75272 to 75273) summarizes key
results from the exposure assessment. This table is reprinted below.
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\50\ Estimates for each urban case study area were averaged for
the years evaluated in the HREA (2006 to 2010). Ranges reflect the
ranges across urban study areas. Estimates smaller than 0.05% were
rounded downward to zero (from U.S. EPA, 2014a, Tables 5-11 and 5-
12). Numbers in parentheses reflect averages across urban study
areas, as well as over the years evaluated in the HREA.
\51\ Numbers of children exposed in each urban case study area
were averaged over the years 2006 to 2010. These averages were then
summed across urban study areas. Numbers were rounded to nearest
thousand unless otherwise indicated. Estimates smaller than 50 were
rounded downward to zero (from U.S. EPA, 2014a, Appendix 5F Table
5F-5).
\52\ As discussed in section 4.3.3 of the HREA, the model-based
air quality adjustment approach used to estimate exposures and lung
function decrements associated with the current and alternative
standards was unable to estimate the distribution of ambient
O3 concentrations in New York City upon just meeting an
alternative standard with a level of 60 ppb. Therefore, for the 60
ppb standard level, the numbers of children and asthmatic children,
and the ranges of percentages, reflect all of the urban study areas
except New York.
Table 1--Summary of Estimated Exposures of Concern in All School-age Children for the Current and Alternative O3
Standards in Urban Study Areas
----------------------------------------------------------------------------------------------------------------
Average number of
Average % children exposed % Children--worst
Benchmark concentration Standard level children exposed [average number of year and worst
(ppb) \50\ asthmatic children] area
\51\
----------------------------------------------------------------------------------------------------------------
One or more exposures of concern per season
----------------------------------------------------------------------------------------------------------------
>= 80 ppb..................... 75 0-0.3 (0.1) 27,000 [3,000] 1.1
70 0-0.1 (0) 3,700 [300] 0.2
65 0 (0) 300 [0] 0
60 0 (0) 100 \52\ [0] 0
>= 70 ppb..................... 75 0.6-3.3 (1.9) 362,000 [40,000] 8.1
70 0.1-1.2 (0.5) 94,000 [10,000] 3.2
65 0-0.2 (0.1) 14,000 [2,000] 0.5
60 0 (0) 1,400 [200] 0.1
>= 60 ppb..................... 75 9.5-17 (12.2) 2,316,000 [246,000] 25.8
70 3.3-10.2 (6.2) 1,176,000 [126,000] 18.9
65 0-4.2 (2.1) 392,000 [42,000] 9.5
60 0-1.2 (0.4) 70,000 [8,000] 2.2
----------------------------------------------------------------------------------------------------------------
Two or more exposures of concern per season
----------------------------------------------------------------------------------------------------------------
>= 80 ppb..................... 75 0 (0) 600 [100] 0.1
70 0 (0) 0 [0] 0
65 0 (0) 0 [0] 0
60 0 (0) 0 [0] 0
>= 70 ppb..................... 75 0.1-0.6 (0.2) 46,000 [5,000] 2.2
70 0-0.1 (0) 5,400 [600] 0.4
65 0 (0) 300 [100] 0
60 0 (0) 0 [0] 0
>= 60 ppb..................... 75 3.1-7.6 (4.5) 865,000 [93,000] 14.4
70 0.5-3.5 (1.7) 320,000 [35,000] 9.2
65 0-0.8 (0.3) 67,000 [7,500] 2.8
60 0-0.2 (0) 5,100 [700] 0.3
----------------------------------------------------------------------------------------------------------------
Uncertainties in exposure estimates are summarized in section
II.C.2.b of the proposal (79 FR 75273). For example, due to variability
in responsiveness, only a subset of individuals who experience
exposures at or above a benchmark concentration can be expected to
experience health effects.\53\ In addition, not all of these effects
will be adverse. Given the lack of sufficient exposure-response
information for most of the health effects that informed benchmark
concentrations, estimates of the number of people likely to experience
exposures at or above benchmark concentrations generally cannot be
translated into quantitative estimates of the number of people likely
to experience specific health effects.\54\ The PA views health-relevant
exposures as a continuum with greater confidence and less uncertainty
about the existence of adverse health effects at higher O3
exposure concentrations, and less confidence and greater uncertainty as
one considers lower exposure concentrations (e.g., U.S. EPA, 2014c,
[[Page 65314]]
sections 3.1 and 4.6). This view draws from the overall body of
available health evidence, which indicates that as exposure
concentrations increase, the incidence, magnitude, and severity of
effects increases.
---------------------------------------------------------------------------
\53\ As noted below (II.C.3.a.ii), in the case of asthmatics,
responsiveness to O3 could depend on factors that have
not been well-evaluated, such as asthma severity, the effectiveness
of asthma control, or the prevalence of medication use.
\54\ The exception to this is lung function decrements, as
discussed below (and in U.S. EPA, 2014c, section 3.2.3.1).
---------------------------------------------------------------------------
Another important uncertainty is that there is very limited
evidence from controlled human exposure studies, which provided the
basis for health benchmark concentrations for both exposures of concern
and lung function decrements, related to clinical responses in at-risk
populations. Compared to the healthy young adults included in the
controlled human exposure studies, members of at-risk populations could
be more likely to experience adverse effects, could experience larger
and/or more serious effects, and/or could experience effects following
exposures to lower O3 concentrations.\55\
---------------------------------------------------------------------------
\55\ ``The CASAC further notes that clinical studies do not
address sensitive subgroups, such as children with asthma, and that
there is a scientific basis to anticipate that the adverse effects
for such subgroups are likely to be more significant at 60 ppb than
for healthy adults'' (Frey 2014a, p. 7).
---------------------------------------------------------------------------
There are also uncertainties associated with the exposure
modelling. These are described most fully, and their potential impact
characterized, in section 5.5.2 of the HREA (U.S. EPA, 2013, pp. 5-72
to 5-79). These include interpretation of activity patterns set forth
in diaries which do not typically distinguish the basis for activity
patterns and so may reflect averting behavior,\56\ and whether the HREA
underestimates exposures for groups spending especially large
proportion of time being active outdoors during the O3
season (outdoor workers and especially active children).
---------------------------------------------------------------------------
\56\ See EPA 2014a pp. 5-53 to 54 describing EPA's sensitivity
analysis regarding impacts of potential averting behavior for
school-age children on the exposure and lung function decrement
estimate, and see also section B.2.a.i below.
---------------------------------------------------------------------------
c. Quantitative Health Risk Assessments
As discussed in section II.C.3 of the proposal (79 FR 75274), for
some health endpoints, there is sufficient scientific evidence and
information available to support the development of quantitative
estimates of O3-related health risks. In the current review,
for short-term O3 concentrations, the HREA estimates lung
function decrements; respiratory symptoms in asthmatics; hospital
admissions and emergency department visits for respiratory causes; and
all-cause mortality (U.S. EPA, 2014a). For long-term O3
concentrations, the HREA estimates respiratory mortality (U.S. EPA,
2014a).\57\ Estimates of O3-induced lung function decrements
are based on exposure modeling using the MSS model (see section
II.1.b.i.(1) above, and 79 FR 75250), combined with exposure-response
relationships from controlled human exposure studies (U.S. EPA, 2014a,
Chapter 6). Estimates of O3-associated respiratory symptoms,
hospital admissions and emergency department visits, and mortality are
based on concentration-response relationships from epidemiologic
studies (U.S. EPA, 2014a, Chapter 7). As with the exposure assessment
discussed above, O3-associated health risks are estimated
for recent air quality and for ambient concentrations adjusted to just
meet the current and alternative O3 standards, based on
2006-2010 air quality and adjusted precursor emissions. The following
sections summarize the discussions from the proposal on the lung
function risk assessment (II.A.2.c.i) and the epidemiology-based
morbidity and mortality risk assessments (II.A.2.c.ii).
---------------------------------------------------------------------------
\57\ Estimates of O3-associated respiratory mortality
are based on the study by Jerrett et al. (2009). This study used
seasonal averages of 1-hour daily maximum O3
concentrations to estimate long-term concentrations.
---------------------------------------------------------------------------
i. Lung Function Risk Assessment
The HREA estimates risks of lung function decrements in school-aged
children (ages 5 to 18), asthmatic school-aged children, and the
general adult population for the 15 urban study areas. The results
presented in the HREA are based on an updated dose-threshold model that
estimates FEV1 responses for individuals following short-
term exposures to O3 (McDonnell et al., 2012), reflecting
methodological improvements since the last review (II.B.2.a.i (1),
above; U.S. EPA, 2014a, section 6.2.4). The impact of the dose
threshold is that O3-induced FEV1 decrements
result primarily from exposures on days with average ambient
O3 concentrations above about 40 ppb (U.S. EPA, 2014a,
section 6.3.1, Figure 6-9).\58\
---------------------------------------------------------------------------
\58\ Analysis of this issue in the HREA is based on risk
estimates in Los Angeles for 2006 unadjusted air quality. The HREA
shows that more than 90% of daily instances of FEV1
decrements >=10% occur when 8-hr average ambient concentrations are
above 40 ppb for this modeled scenario. The HREA notes that the
distribution of responses will be different for different study
areas, years, and air quality scenarios (U.S. EPA, 2014c, Chapter
6).
---------------------------------------------------------------------------
Table 2 in the proposal (79 FR 75275), and reprinted below,
summarizes key results from the lung function risk assessment. Table 2
presents estimates of the percentages of school-aged children estimated
to experience O3-induced FEV1 decrements
10, 15, or 20% when air quality was adjusted to just meet
the current and alternative 8-hour O3 standards. Table 2
also presents the numbers of children, including children with asthma,
estimated to experience such decrements.
Table 2--Summary of Estimated O3-Induced Lung Function Decrements for the Current and Potential Alternative O3
Standards in Urban Case Study Areas
----------------------------------------------------------------------------------------------------------------
Number of children (5
Alternative Average % to 18 years) [number of % Children worst
Lung function decrement standard level children \59\ asthmatic children] year and area
\60\
----------------------------------------------------------------------------------------------------------------
One or more decrements per season
----------------------------------------------------------------------------------------------------------------
>=10%......................... 75 14-19 3,007,000 [312,000] 22
70 11-17 2,527,000 [261,000] 20
65 3-15 1,896,000 [191,000] 18
60 5-11 \61\1,404,000 [139,000] 13
>=15%......................... 75 3-5 766,000 [80,000] 7
70 2-4 562,000 [58,000] 5
65 0-3 356,000 [36,000] 4
60 1-2 225,000 [22,000] 3
>=20%......................... 75 1-2 285,000 [30,000] 2.8
70 1-2 189,000 [20,000] 2.1
65 0-1 106,000 [11,000] 1.4
60 0-1 57,000 [6,000] 0.9
----------------------------------------------------------------------------------------------------------------
[[Page 65315]]
Two or more decrements per season
----------------------------------------------------------------------------------------------------------------
>=10%......................... 75 7.5-12 1,730,000 [179,000] 14
70 5.5-11 1,414,000 [145,000] 13
65 1.3-8.8 1,023,000 [102,000] 11
60 2.1-6.4 741,000 [73,000] 7.3
>=15%......................... 75 1.7-2.9 391,000 [40,000] 3.8
70 0.9-2.4 276,000 [28,000] 3.1
65 0.1-1.8 168,000 [17,000] 2.3
60 0.2-1.0 101,000 [10,000] 1.4
>=20%......................... 75 0.5-1.1 128,000 [13,000] 1.5
70 0.3-0.8 81,000 [8,000] 1.1
65 0-0.5 43,000 [4,000] 0.8
60 0-0.2 21,000 [2,000] 0.4
----------------------------------------------------------------------------------------------------------------
---------------------------------------------------------------------------
\59\ Estimates in each urban case study area were averaged for
the years evaluated in the HREA (2006 to 2010). Ranges reflect the
ranges across urban study areas.
\60\ Numbers of children estimated to experience decrements in
each study urban case study area were averaged over 2006 to 2010.
These averages were then summed across urban study areas. Numbers
are rounded to nearest thousand unless otherwise indicated.
\61\ As discussed in section 4.3.3 of the HREA, the model-based
air quality adjustment approach used to estimate risks associated
with the current and alternative standards was unable to estimate
the distribution of ambient O3 concentrations in New York
City upon just meeting an alternative standard with a level of 60
ppb. Therefore, for the 60 ppb standard level, the numbers of
children and asthmatic children experiencing decrements, and the
ranges of percentages of such children across study areas, reflect
all of the urban study areas except New York City. Because of this,
in some cases (i.e., when New York City provided the smallest risk
estimate), the lower end of the ranges in Table 2 are higher for a
standard level of 60 ppb than for a level of 65 ppb.
---------------------------------------------------------------------------
Uncertainties in estimates of lung function risks are summarized in
section II.C.3.a.ii of the proposal (79 FR 75275). In addition to the
uncertainties noted for exposure estimates, an uncertainty which
impacts lung function risk estimates stems from the lack of exposure-
response information in children. In the near absence of controlled
human exposure data for children, risk estimates are based on the
assumption that children exhibit the same lung function response
following O3 exposures as healthy 18 year olds (i.e., the
youngest age for which controlled human exposure data is generally
available) (U.S. EPA, 2014a, section 6.5.3). This assumption is
justified in part by the findings of McDonnell et al. (1985), who
reported that children (8-11 years old) experienced FEV1
responses similar to those observed in adults (18-35 years old) (U.S.
EPA, 2014a, p. 3-10). In addition, as discussed in the ISA (U.S. EPA,
2013, section 6.2.1), summer camp studies of school-aged children
reported O3-induced lung function decrements similar in
magnitude to those observed in controlled human exposure studies using
adults. In extending the risk model to children, the HREA thus fixes
the age term in the model at its highest value, the value for age 18.
Notwithstanding the information just summarized supporting this
approach, EPA acknowledges the uncertainty involved, and notes that the
approach could result in either over- or underestimates of
O3-induced lung function decrements in children, depending
on how children compare to the adults used in controlled human exposure
studies (U.S. EPA, 2014a, section 6.5.3).
A related source of uncertainty is that the risk assessment
estimates of O3-induced decrements in asthmatics used the
exposure-response relationship developed from data collected from
healthy individuals. Although the evidence has been mixed (U.S. EPA,
2013, section 6.2.1.1), several studies have reported statistically
larger, or a tendency toward larger, O3-induced lung
function decrements in asthmatics than in non-asthmatics (Kreit et al.,
1989; Horstman et al., 1995; Jorres et al., 1996; Alexis et al., 2000).
On this issue, CASAC noted that ``[a]sthmatic subjects appear to be at
least as sensitive, if not more sensitive, than non-asthmatic subjects
in manifesting O3-induced pulmonary function decrements''
(Frey, 2014c, p. 4). To the extent asthmatics experience larger
O3-induced lung function decrements than the healthy adults
used to develop exposure-response relationships, the HREA could
underestimate the impacts of O3 exposures on lung function
in asthmatics, including asthmatic children. The implications of this
uncertainty for risk estimates remain unknown at this time (U.S. EPA,
2014a, section 6.5.4), and could depend on a variety of factors that
have not been well-evaluated, including the severity of asthma and the
prevalence of medication use. However, the available evidence shows
responses to O3 increase with severity of asthma (Horstman
et al., 1995) and corticosteroid usage does not prevent O3
effects on lung function decrements or respiratory symptoms in people
with asthma (Vagaggini et al., 2001, 2007).
ii. Mortality and Morbidity Risk Assessments
As discussed in section II.C.3.b of the proposal (79 FR 75276), the
HREA estimates O3-associated risks in 12 urban study areas
\62\ using concentration-response relationships drawn from
epidemiologic studies. These concentration-response relationships are
based on ``area-wide'' average O3 concentrations.\63\ The
HREA estimates risks for the years 2007 and 2009 in order to provide
estimates of risk for a year with generally higher O3
[[Page 65316]]
concentrations (2007) and a year with generally lower O3
concentrations (2009) (U.S. EPA, 2014a, section 7.1.1).
---------------------------------------------------------------------------
\62\ The 12 urban areas evaluated are Atlanta, Baltimore,
Boston, Cleveland, Denver, Detroit, Houston, Los Angeles, New York,
Philadelphia, Sacramento, and St. Louis.
\63\ In the epidemiologic studies that provide the health basis
for HREA risk assessments, concentration-response relationships are
based on daytime O3 concentrations, averaged across
multiple monitors within study areas. These daily averages are used
as surrogates for the spatial and temporal patterns of exposures in
study populations. Consistent with this approach, the HREA
epidemiologic-based risk estimates also utilize daytime
O3 concentrations, averaged across monitors, as
surrogates for population exposures. In this notice, we refer to
these averaged concentrations as ``area-wide'' O3
concentrations. Area-wide concentrations are discussed in more
detail in section 3.1.4 of the PA (U.S. EPA, 2014c).
---------------------------------------------------------------------------
In considering the epidemiology-based risk estimates, the proposal
focuses on mortality risks associated with short-term O3
concentrations. The proposal considers estimates of total risk (i.e.,
based on the full distributions of ambient O3
concentrations) and estimates of risk associated with O3
concentrations in the upper portions of ambient distributions. Both
estimates are discussed to provide information that considers risk
estimates based on concentration-response relationships being linear
over the entire distribution of ambient O3 concentrations,
and thus have the greater potential for morbidity and mortality to be
affected by changes in relatively low O3 concentrations, as
well as risk estimates that are associated with O3
concentrations in the upper portions of the ambient distribution, thus
focusing on risk from higher O3 concentrations and placing
greater weight on the uncertainty associated with the shapes of
concentration-response curves for O3 concentrations in the
lower portions of the distribution. These results for O3-
associated mortality risk are summarized in Table 3 in the proposal (79
FR 75277).
Important uncertainties in epidemiology-based risk estimates, based
on their consideration in the HREA and PA, are discussed in section
II.C.3.b.ii of the proposal (79 FR 75277). Compared to estimates of
O3 exposures of concern and estimates of O3-
induced lung function decrements (discussed above), the HREA
conclusions reflect lower confidence in epidemiologic-based risk
estimates (U.S. EPA, 2014a, section 9.6). In particular, the HREA
highlights the heterogeneity in effect estimates between locations, the
potential for exposure measurement errors, and uncertainty in the
interpretation of the shape of concentration-response functions at
lower O3 concentrations (U.S. EPA, 2014a, section 9.6). The
HREA also concludes that lower confidence should be placed in the
results of the assessment of respiratory mortality risks associated
with long-term O3, primarily because that analysis is based
on only one study, though that study is well-designed, and because of
the uncertainty in that study about the existence and identification of
a potential threshold in the concentration-response function (U.S. EPA,
2014a, section 9.6).\64,65\ This section further discusses some of the
key uncertainties in epidemiologic-based risk estimates, as summarized
in the PA (U.S. EPA, 2014c, section 3.2.3.2), with a focus on
uncertainties that can have particularly important implications for the
Administrator's consideration of epidemiology-based risk estimates.
---------------------------------------------------------------------------
\64\ The CASAC also concluded that ``[i]n light of the potential
nonlinearity of the C-R function for long-term exposure reflecting a
threshold of the mortality response, the estimated number of
premature deaths avoidable for long-term exposure reductions for
several levels need to be viewed with caution'' (Frey, 2014a, p. 3).
\65\ There is also uncertainty about the extent to which
mortality estimates based on the long-term metric used in the study
by Jerrett et al. (2009) (i.e., seasonal average of 1-hour daily
maximum concentrations) reflects associations with long-term average
O3 versus repeated occurrences of elevated short-term
concentrations.
---------------------------------------------------------------------------
The PA notes that reducing NOX emissions generally
reduces O3-associated mortality and morbidity risk estimates
in locations and time periods with relatively high ambient
O3 concentrations and increases risk estimates in locations
and time periods with relatively low concentrations (II.A, above). When
evaluating uncertainties in epidemiologic risk estimates, the PA
considered (1) the extent to which the modeled O3 response
to reductions in NOX emissions appropriately represents the
trends observed in monitored ambient O3 following actual
reductions in NOX emissions, (2) the extent to which the
O3 response to reductions in precursor emissions could
differ with emissions reduction strategies that are different from
those used in HREA to generate risk estimates, and (3) the extent to
which estimated changes in risks in urban study areas are
representative of the changes that would be experienced broadly across
the U.S. population. The first two of these issues are discussed in
section II.A.2.c above. The third issue is discussed below.
The HREA conducted national air quality modeling analyses that
estimated the proportion of the U.S. population living in locations
where seasonal averages of daily O3 concentrations are
estimated to decrease in response to reductions in NOX
emissions, and the proportion living in locations where such seasonal
averages are estimated to increase. Given the close relationship
between changes in seasonal averages of daily O3
concentrations and changes in seasonal mortality and morbidity risk
estimates, this analysis informs consideration of the extent to which
the risk results in urban study areas represent the U.S. population as
a whole. This ``representativeness analysis'' indicates that the
majority of the U.S. population lives in locations where reducing
NOX emissions would be expected to result in decreases in
warm season averages of daily maximum 8-hour ambient O3
concentrations. Because the HREA urban study areas tend to
underrepresent the populations living in such areas (e.g., suburban,
smaller urban, and rural areas), risk estimates for the urban study
areas are likely to understate the average reductions in O3-
associated mortality and morbidity risks that would be experienced
across the U.S. population as a whole upon reducing NOX
emissions (U.S. EPA, 2014a, section 8.2.3.2).
Section 7.4 of the HREA also highlights some additional
uncertainties associated with epidemiologic-based risk estimates (U.S.
EPA, 2014a). This section of the HREA identifies and discusses sources
of uncertainty and presents a qualitative evaluation of key parameters
that can introduce uncertainty into risk estimates (U.S. EPA, 2014a,
Table 7-4). For several of these parameters, the HREA also presents
quantitative sensitivity analyses (U.S. EPA, 2014a, sections 7.4.2 and
7.5.3). Of the uncertainties discussed in Chapter 7 of the HREA, those
related to the application of concentration-response functions from
epidemiologic studies can have particularly important implications for
consideration of epidemiology-based risk estimates, as discussed below.
An important uncertainty is the shape of concentration-response
functions at low ambient O3 concentrations (U.S. EPA, 2014a,
Table 7-4).\66\ In recognition of the ISA's conclusion that certainty
in the shape of O3 concentration-response functions
decreases at low ambient concentrations, the HREA provides estimates of
epidemiology-based mortality risks for entire distributions of ambient
O3 concentrations, as well as estimates of total mortality
associated with various ambient O3 concentrations. The PA
considers both types of risk estimates, recognizing greater public
health concern for adverse O3-attributable effects at higher
ambient O3 concentrations (which drive higher exposure
concentrations, section 3.2.2 of the PA (U.S. EPA, 2014c)), as compared
to lower concentrations.
---------------------------------------------------------------------------
\66\ A related uncertainty is the existence, or not, of a
threshold. The HREA addresses this issue for long-term O3
by evaluating risks in models that include potential thresholds
(II.D.2.c).
---------------------------------------------------------------------------
A related consideration is associated with the public health
importance of the increases in relatively low O3
concentrations following air quality adjustment. There is uncertainty
that relates to the assumption that the concentration response function
for O3 is linear, such that total risk estimates are equally
influenced by decreasing
[[Page 65317]]
high concentrations and increasing low concentrations, when the
increases and decreases are of equal magnitude. Even on days with
increases in relatively low area-wide average concentrations, resulting
in increases in estimated risks, some portions of the urban study areas
could experience decreases in high O3 concentrations. To the
extent adverse O3-attributable effects are more strongly
supported for higher ambient concentrations (which, as noted above, are
consistently reduced upon air quality adjustment), the impacts on risk
estimates of increasing low O3 concentrations reflect an
important source of uncertainty. In addition to the uncertainties
discussed above, the proposal also notes uncertainties related to (1)
using concentration-response relationships developed for a particular
population in a particular location to estimate health risks in
different populations and locations; (2) using concentration-response
functions from epidemiologic studies reflecting a particular air
quality distribution to adjusted air quality necessarily reflecting a
different (simulated) air quality distribution; (3) using a national
concentration-response function to estimate respiratory mortality
associated with long-term O3; and (4) unquantified
reductions in risk that could be associated with reductions in the
ambient concentrations of pollutants other than O3,
resulting from control of NOX (79 FR 75277 to 75279).
B. Need for Revision of the Primary Standard
The initial issue to be addressed in the current review of the
primary O3 standard is whether, in view of the advances in
scientific knowledge and additional information, it is appropriate to
revise the existing standard. This section presents the Administrator's
final decision on whether it is ``appropriate'' to revise the current
standard within the meaning of section 109 (d)(1) of the CAA. Section
II.B.1 contains a summary discussion of the basis for the proposed
conclusions on the adequacy of the primary standard. Section II.B.2
discusses comments received on the adequacy of the primary standard.
Section II.B.3 presents the Administrator's final conclusions on the
adequacy of the current primary standard.
1. Basis for Proposed Decision
In evaluating whether it is appropriate to retain or revise the
current standard, the Administrator's considerations build upon those
in the 2008 review, including consideration of the broader body of
scientific evidence and exposure and health risk information now
available, as summarized in sections II.A to II.C (79 FR 75246-75279)
of the proposal and section II.A above.
In developing conclusions on the adequacy of the current primary
O3 standard, the Administrator takes into account both
evidence-based and quantitative exposure- and risk-based
considerations. Evidence-based considerations include the assessment of
evidence from controlled human exposure, animal toxicological, and
epidemiologic studies for a variety of health endpoints. The
Administrator focuses on health endpoints for which the evidence is
strong enough to support a ``causal'' or a ``likely to be causal''
relationship, based on the ISA's integrative synthesis of the entire
body of evidence. The Administrator's consideration of quantitative
exposure and risk information draws from the results of the exposure
and risk assessments presented in the HREA.
The Administrator's consideration of the evidence and exposure/risk
information is informed by the considerations and conclusions presented
in the PA (U.S. EPA, 2014c). The purpose of the PA is to help ``bridge
the gap'' between the scientific and technical information assessed in
the ISA and HREA, and the policy decisions that are required of the
Administrator (U.S. EPA, 2014c, Chapter 1); see also American Farm
Bureau Federation, 559 F. 3d at 516, 521 (``[a]lthough not required by
the statute, in practice EPA staff also develop a Staff Paper, which
discusses the information in the Criteria Document that is most
relevant to the policy judgments the EPA makes when it sets the
NAAQS''). The PA's evidence-based and exposure-/risk-based
considerations and conclusions are briefly summarized below in sections
II.B.1.a (evidence-based considerations), II.B.1.b (exposure- and risk-
based considerations), and II.B.1.c (PA conclusions on the current
standard). Section II.B.1.d summarizes CASAC advice to the
Administrator and public commenter views on the current standard.
Section II.B.1.e presents a summary of the Administrator's proposed
conclusions concerning the adequacy of the public health protection
provided by the current standard, and her proposed decision to revise
that standard.
a. Evidence-Based Considerations From the PA
In considering the available scientific evidence, the PA evaluates
the O3 concentrations in health effects studies (U.S. EPA,
2014c, section 3.1.4). Specifically, the PA characterizes the extent to
which health effects have been reported for the O3 exposure
concentrations evaluated in controlled human exposure studies, and
effects occurring over the distributions of ambient O3
concentrations in locations where epidemiologic studies have been
conducted. These considerations, as they relate to the adequacy of the
current standard, are presented in detail in section 3.1.4 of the PA
(U.S. EPA, 2014c) and are summarized in the proposal (79 FR 75279-
75287). The PA's considerations are summarized briefly below for
controlled human exposure, epidemiologic panel studies, and
epidemiologic population-based studies.
Section II.D.1.a of the proposal discusses the PA's consideration
of the evidence from controlled human exposure and panel studies. This
evidence is assessed in section 6.2 of the ISA (U.S. EPA, 2013) and is
summarized in section 3.1.2 of the PA (U.S. EPA, 2014c). A large number
of controlled human exposure studies have reported lung function
decrements, respiratory symptoms, air inflammation, airway
hyperresponsiveness, and/or impaired lung host defense in young,
healthy adults engaged in moderate quasi-continuous exertion, following
6.6-hour O3 exposures. These studies have consistently
reported such effects following exposures to O3
concentrations of 80 ppb or greater. In addition to lung function
decrements, available studies have evaluated respiratory symptoms or
airway inflammation following exposures to O3 concentrations
below 75 ppb. Table 3-1 in the PA highlights the group mean results of
individual controlled human exposure studies that evaluated exposures
to O3 concentrations below 75 ppb. These studies observe the
combination of lung function decrements and respiratory symptoms
following exposures to O3 concentrations as low as 72 ppb,
and lung function decrements and airway inflammation following
exposures to O3 concentrations as low as 60 ppb (based on
group means).
Based on this evidence, the PA notes that controlled human exposure
studies have reported a variety of respiratory effects in young,
healthy adults following exposures to a wide range of O3
concentrations for 6.6 hours, including exposures to concentrations
below 75 ppb. In particular, the PA further notes that a recent
controlled human exposure study reported the combination of lung
function decrements and respiratory symptoms in healthy adults engaged
in quasi-
[[Page 65318]]
continuous, moderate exertion following 6.6 hour exposures to 72 ppb
O3, a combination of effects that have been classified as
adverse based on ATS guidelines for adversity (ATS, 2000a). In
addition, a recent study has also reported lung function decrements and
pulmonary inflammation following exposure to 60 ppb O3.
Sixty ppb is the lowest exposure concentration for which inflammation
has been evaluated and reported to occur, and corresponds to the lowest
exposure concentration demonstrated to result in lung function
decrements large enough to be judged an abnormal response by ATS (ATS,
2000b). The PA also notes, and CASAC agreed, that these controlled
human exposure studies were conducted in healthy adults, while at-risk
groups (e.g., children, people with asthma) could experience larger
and/or more serious effects. Therefore, the PA concludes that the
evidence from controlled human exposure studies provide support that
the respiratory effects experienced following exposures to
O3 concentrations lower than 75 ppb would be adverse in some
individuals, particularly if experienced by members of at-risk
populations (e.g., people with asthma, children).
The PA also notes consistent results in some panel studies of
O3-associated lung function decrements. In particular, the
PA notes that epidemiologic panel studies in children and adults
consistently indicate O3-associated lung function decrements
when on-site, ambient monitored concentrations were below 75 ppb
(although the evidence becomes less consistent at low O3
concentrations, and the averaging periods involved ranged from 10
minutes to 12 hours (U.S. EPA, 2014c, section 3.2.4.2)).
Section II.D.1.b of the proposal summarizes the PA's analyses of
monitored O3 concentrations in locations of epidemiologic
studies. While the majority of the epidemiologic study areas evaluated
would have violated the current standard during study periods, the PA
makes the following observations with regard to health effect
associations at O3 concentrations likely to have met the
current standard:
(1) A single-city study reported positive and statistically
significant associations with asthma emergency department visits in
children and adults in Seattle, a location that would have met the
current standard over the entire study period (Mar and Koenig, 2009).
(2) Additional single-city studies support associations with
respiratory morbidity at relatively low ambient O3
concentrations, including when virtually all monitored concentrations
were below the level of the current standard (Silverman and Ito, 2010;
Strickland et al., 2010).
(3) Canadian multicity studies reported positive and statistically
significant associations with respiratory morbidity or mortality when
the majority of study cities, though not all study cities, would have
met the current standard over the study period in each of these studies
(Cakmak et al., 2006; Dales et al., 2006; Katsouyanni et al., 2009;
Stieb et al., 2009).
(4) A U.S. multicity study reported positive and statistically
significant associations with mortality when ambient O3
concentrations were restricted to those likely to have met the current
O3 standard (Bell et al., 2006).
The PA also takes into account important uncertainties in these
analyses of air quality in locations of epidemiologic study areas.
These uncertainties are summarized in section II.D.1.b.iii of the
proposal. Briefly, they include the following: (1) Uncertainty in
conclusions about the extent to which multicity effect estimates
reflect associations with air quality meeting the current standard,
versus air quality violating that standard; (2) uncertainty regarding
the potential for thresholds to exist, given that regional
heterogeneity in O3 health effect associations could obscure
the presence of thresholds, should they exist; (3) uncertainty in the
extent to which the PA appropriately recreated the air quality analyses
in the published study by Bell et al. (2006); and (4) uncertainty in
the extent to which reported health effects are caused by exposures to
O3 itself, as opposed to other factors such as co-occurring
pollutants or pollutant mixtures, particularly at low ambient
O3 concentrations.\67\
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\67\ As noted above (section II.A.1.B.i), the ISA concludes that
studies that examined the potential confounding effects of
copollutants found that O3 effect estimates remained
relatively robust upon the inclusion of PM and gaseous pollutants in
two-pollutant models (U.S. EPA, 2013, section 6.2.7.5).
---------------------------------------------------------------------------
In considering the analyses of monitored O3 air quality
in locations of epidemiologic studies, as well as the important
uncertainties in these analyses, the PA concludes that these analyses
provide support for the occurrence of morbidity and mortality
associated with short-term ambient O3 concentrations likely
to meet the current O3 standard.\68\ In considering the
evidence as a whole, the PA concludes that (1) controlled human
exposure studies provide strong support for the occurrence of adverse
respiratory effects following exposures to O3 concentrations
below the level of the current standard and (2) epidemiologic studies
provide support for the occurrence of adverse respiratory effects and
mortality under air quality conditions that would meet the current
standard.
---------------------------------------------------------------------------
\68\ Unlike for the studies of short-term O3, the
available U.S. and Canadian epidemiologic studies evaluating long-
term ambient O3 concentration metrics have not been
conducted in locations likely to have met the current 8-hour
O3 standard during the study period, and have not
reported concentration-response functions that indicate confidence
in health effect associations at O3 concentrations
meeting the current standard (U.S. EPA, 2014c, section 3.1.4.3).
---------------------------------------------------------------------------
b. Exposure- and Risk-Based Considerations in the PA
In order to further inform judgments about the potential public
health implications of the current O3 NAAQS, the PA
considers the exposure and risk assessments presented in the HREA (U.S.
EPA, 2014c, section 3.2). Overviews of these exposure and risk
assessments, including brief summaries of key results and
uncertainties, are provided in section II.A.2 above. Section II.D.2 of
the proposal summarizes key observations from the PA related to the
adequacy of the current O3 NAAQS, based on consideration of
the HREA exposure assessment, lung function risk assessment, and
mortality/morbidity risk assessments (79 FR 75283).
Section II.D.2.a of the proposal summarizes key observations from
the PA regarding estimates of O3 exposures of concern (79 FR
75283). Given the evidence for respiratory effects from controlled
human exposure studies, the PA considers the extent to which the
current standard would be estimated to protect at-risk populations
against exposures of concern to O3 concentrations at or
above the health benchmark concentrations of 60, 70, and 80 ppb (i.e.,
based on HREA estimates of one or more and two or more exposures of
concern). In doing so, the PA notes the CASAC conclusion that (Frey,
2014c, p. 6):
The 80 ppb-8hr benchmark level represents an exposure level for
which there is substantial clinical evidence demonstrating a range
of ozone-related effects including lung inflammation and airway
responsiveness in healthy individuals. The 70 ppb-8hr benchmark
level reflects the fact that in healthy subjects, decreases in lung
function and respiratory symptoms occur at concentrations as low as
72 ppb and that these effects almost certainly occur in some people,
including asthmatics and others with low lung function who are less
tolerant of such effects, at levels of 70 ppb and below. The 60 ppb-
8hr benchmark level represents the lowest exposure level at which
ozone-
[[Page 65319]]
related effects have been observed in clinical studies of healthy
individuals.
For exposures of concern at or above 60 ppb, the proposal
highlights the following key observations for air quality adjusted to
just meet the current standard:
(1) On average over the years 2006 to 2010, the current standard is
estimated to allow approximately 10 to 18% of children in urban study
areas to experience one or more exposures of concern at or above 60
ppb. Summing across urban study areas, these percentages correspond to
almost 2.5 million children experiencing approximately 4 million
exposures of concern at or above 60 ppb during a single O3
season. Of these children, almost 250,000 are asthmatics.\69\
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\69\ As discussed in section II.C.2.b of the proposal, due to
variability in responsiveness, only a subset of individuals who
experience exposures at or above a benchmark concentration can be
expected to experience adverse health effects.
---------------------------------------------------------------------------
(2) On average over the years 2006 to 2010, the current standard is
estimated to allow approximately 3 to 8% of children in urban study
areas to experience two or more exposures of concern to O3
concentrations at or above 60 ppb. Summing across the urban study
areas, these percentages correspond to almost 900,000 children
(including almost 90,000 asthmatic children).
(3) In the worst-case years (i.e., those with the largest exposure
estimates), the current standard is estimated to allow approximately 10
to 25% of children to experience one or more exposures of concern at or
above 60 ppb, and approximately 4 to 14% to experience two or more
exposures of concern at or above 60 ppb.
For exposures of concern at or above 70 ppb, the PA highlights the
following key observations for air quality adjusted to just meet the
current standard:
(1) On average over the years 2006 to 2010, the current standard is
estimated to allow up to approximately 3% of children in urban study
areas to experience one or more exposures of concern at or above 70
ppb. Summing across urban study areas, almost 400,000 children
(including almost 40,000 asthmatic children) are estimated to
experience O3 exposure concentrations at or above 70 ppb
during a single O3 season.
(2) On average over the years 2006 to 2010, the current standard is
estimated to allow less than 1% of children in urban study areas to
experience two or more exposures of concern to O3
concentrations at or above 70 ppb.
(3) In the worst-case location and year, the current standard is
estimated to allow approximately 8% of children to experience one or
more exposures of concern at or above 70 ppb, and approximately 2% to
experience two or more exposures of concern, at or above 70 ppb.
For exposures of concern at or above 80 ppb, the PA highlights the
observation that the current standard is estimated to allow about 1% or
fewer children in urban study areas to experience exposures of concern
at or above 80 ppb, even in years with the highest exposure estimates.
Uncertainties in exposure estimates are summarized in section
II.C.2.b of the proposal (79 FR 75273), and discussed more fully in the
HREA (U.S. EPA, 2014a, section 5.5.2) and the PA (U.S. EPA, 2014c,
section 3.2.2). Key uncertainties include the variability in
responsiveness following O3 exposures, resulting in only a
subset of exposed individuals experiencing health effects, adverse or
otherwise, and the limited evidence from controlled human exposure
studies conducted in at-risk populations. In addition, there are a
number of uncertainties in the exposure modelling approach used in the
HREA, contributing to overall uncertainty in exposure estimates.
Section II.D.2.b of the proposal summarizes key observations from
the PA regarding the estimated risk of O3-induced lung
function decrements (79 FR 75283 to 75284). With respect to the lung
function decrements that have been evaluated in controlled human
exposure studies, the PA considers the extent to which standards with
revised levels would be estimated to protect healthy and at-risk
populations against one or more, and two or more, moderate (i.e.,
FEV1 decrements >=10% and >=15%) and large (i.e.,
FEV1 decrements >=20%) lung function decrements. As
discussed in section 3.1.3 of the PA (U.S. EPA, 2014c), although some
experts would judge single occurrences of moderate responses to be a
nuisance, especially for healthy individuals, a more general consensus
view of the adversity of moderate lung function decrements emerges as
the frequency of occurrence increases.
With regard to decrements >=10%, the PA highlights the following
key observations for air quality adjusted to just meet the current
standard:
(1) On average over the years 2006 to 2010, the current standard is
estimated to allow approximately 14 to 19% of children in urban study
areas to experience one or more lung function decrements >=10%. Summing
across urban study areas, this corresponds to approximately 3 million
children experiencing 15 million O3-induced lung function
decrements >=10% during a single O3 season. Of these
children, about 300,000 are asthmatics.
(2) On average over the years 2006 to 2010, the current standard is
estimated to allow approximately 7 to 12% of children in urban study
areas to experience two or more O3-induced lung function
decrements >=10%. Summing across the urban study areas, this
corresponds to almost 2 million children (including almost 200,000
asthmatic children) estimated to experience two or more O3-
induced lung function decrements greater than 10% during a single
O3 season.
(3) In the worst-case years, the current standard is estimated to
allow approximately 17 to 23% of children in urban study areas to
experience one or more lung function decrements >=10%, and
approximately 10 to 14% to experience two or more O3-induced
lung function decrements >=10%.
With regard to decrements >=15%, the PA highlights the following key
observations for air quality adjusted to just meet the current
standard:
(1) On average over the years 2006 to 2010, the current standard is
estimated to allow approximately 3 to 5% of children in urban study
areas to experience one or more lung function decrements <=15%. Summing
across urban study areas, this corresponds to approximately 800,000
children (including approximately 80,000 asthmatic children) estimated
to experience at least one O3-induced lung function
decrement <=15% during a single O3 season.
(2) On average over the years 2006 to 2010, the current standard is
estimated to allow approximately 2 to 3% of children in urban study
areas to experience two or more O3-induced lung function
decrements <=15%.
(3) In the worst-case years, the current standard is estimated to
allow approximately 4 to 6% of children in urban study areas to
experience one or more lung function decrements <=15%, and
approximately 2 to 4% to experience two or more O3-induced
lung function decrements <=15%.
With regard to decrements <=20%, the PA highlights the following
key observations for air quality adjusted to just meet the current
standard:
(1) On average over the years 2006 to 2010, the current standard is
estimated to allow approximately 1 to 2% of children in urban study
areas to experience one or more lung function decrements >=20%. Summing
across
[[Page 65320]]
urban study areas, this corresponds to approximately 300,000 children
(including approximately 30,000 asthmatic children) estimated to
experience at least one O3-induced lung function decrement
>=20% during a single O3 season.
(2) On average over the years 2006 to 2010, the current standard is
estimated to allow less than 1% of children in urban study areas to
experience two or more O3-induced lung function decrements
>=20%.
(3) In the worst-case years, the current standard is estimated to
allow approximately 2 to 3% of children to experience one or more lung
function decrements >=20%, and less than 2% to experience two or more
O3-induced lung function decrements >=20%.
Uncertainties in lung function risk estimates are summarized in section
II.C.3.a of the proposal, and discussed more fully in the HREA (U.S.
EPA, 2014a, section 6.5) and the PA (U.S. EPA, 2014c, section 3.2.3.1).
In addition to the uncertainties noted above for exposure estimates,
the key uncertainties associated with estimates of O3-
induced lung function decrements include the paucity of exposure-
response information in children and in people with asthma.
Section II.D.2.c of the proposal summarizes key observations from
the PA regarding risk estimates of O3-associated mortality
and morbidity (79 FR 75284 to 75285). With regard to total mortality or
morbidity associated with short-term O3, the PA notes the
following for air quality adjusted to just meet the current standard:
(1) When air quality was adjusted to the current standard for the
2007 model year (the year with generally ``higher'' O3-
associated risks), 10 of 12 urban study areas exhibited either
decreases or virtually no change in estimates of the number of
O3-associated deaths (U.S. EPA, 2014a, Appendix 7B).
Increases were estimated in two of the urban study areas (Houston, Los
Angeles)\70\ (U.S. EPA, 2014a, Appendix 7B).\71\
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\70\ As discussed above (II.C.1), in locations and time periods
when NOX is predominantly contributing to O3
formation (e.g., downwind of important NOX sources, where
the highest O3 concentrations often occur), model-based
adjustment to the current and alternative standards decreases
estimated ambient O3 concentrations compared to recent
monitored concentrations (U.S. EPA, 2014a, section 4.3.3.2). In
contrast, in locations and time periods when NOX is
predominantly contributing to O3 titration (e.g., in
urban centers with high concentrations of NOX emissions,
where ambient O3 concentrations are often suppressed and
are thus relatively low), model-based adjustment increases ambient
O3 concentrations compared to recent monitored
concentrations (U.S. EPA, 2014a, section 4.3.3.2). Changes in
epidemiology-based risk estimates depend on the balance between the
daily decreases in high O3 concentrations and increases
in low O3 concentrations following the model-based air
quality adjustment. Commenting on this issue, CASAC noted that
``controls designed to reduce the peak levels of ozone (e.g., the
fourth-highest annual MDA8) may not be effective at reducing lower
levels of ozone on more typical days and may actually increase ozone
levels on days where ozone concentrations are low'' (Frey 2014a, p.
2). CASAC further noted that risk results ``suggest that the ozone-
related health risks in the urban cores can increase for some of the
cities as ozone NAAQS alternatives become more stringent. This is
because reductions in nitrogen oxides emissions can lead to less
scavenging of ozone and free radicals, resulting in locally higher
levels of ozone'' (Frey 2014c, p. 10).
\71\ For the 2009 adjusted year (i.e., the year with generally
lower O3 concentrations), changes in risk were generally
smaller than in 2007 (i.e., most changes about 2% or smaller).
Increases were estimated for Houston, Los Angeles, and New York
City.
---------------------------------------------------------------------------
(2) In focusing on total risk, the current standard is estimated to
allow thousands of O3-associated deaths per year in the
urban study areas. In focusing on the risks associated with the upper
portions of distributions of ambient concentrations (area-wide
concentrations <= 40, 60 ppb), the current standard is estimated to
allow hundreds to thousands of O3-associated deaths per year
in the urban study areas.
(3) The current standard is estimated to allow tens to thousands of
O3-associated morbidity events per year (i.e., respiratory-
related hospital admissions, emergency department visits, and asthma
exacerbations).
With regard to respiratory mortality associated with long-term
O3, the PA notes the following for air quality adjusted to
just meet the current standard:
(1) Based on a linear concentration-response function, the current
standard is estimated to allow thousands of O3-associated
respiratory deaths per year in the urban study areas.
(2) Based on threshold models, HREA sensitivity analyses indicate
that the number of respiratory deaths associated with long-term
O3 concentrations could potentially be considerably lower
(i.e., by more than 75% if a threshold exists at 40 ppb, and by about
98% if a threshold exists at 56 ppb) (U.S. EPA, 2014a, Figure 7-9).\72\
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\72\ Risk estimates for respiratory mortality associated with
long-term O3 exposures are based on the study by Jerrett
et al. (2009) (U.S. EPA, 2014a, Chapter 7). As discussed above
(II.B.2.b.iv) and in the PA (U.S. EPA, 2014c, section 3.1.4.3),
Jerrett et al. (2009) reported that when seasonal averages of 1-hour
daily maximum O3 concentrations ranged from 33 to 104
ppb, there was no statistical deviation from a linear concentration-
response relationship between O3 and respiratory
mortality across 96 U.S. cities (U.S. EPA, 2013, section 7.7).
However, the authors reported ``limited evidence'' for an effect
threshold at an O3 concentration of 56 ppb (p=0.06). In
communications with EPA staff (Sasser, 2014), the study authors
indicated that it is not clear whether a threshold model is a better
predictor of respiratory mortality than the linear model, and that
``considerable caution should be exercised in accepting any specific
threshold.''
---------------------------------------------------------------------------
Compared to the weight given to HREA estimates of exposures of
concern and lung function risks, and the weight given to the evidence,
the PA places relatively less weight on epidemiologic-based risk
estimates. In doing so, the PA notes that the overall conclusions from
the HREA likewise reflect less confidence in estimates of
epidemiologic-based risks than in estimates of exposures and lung
function risks. The determination to attach less weight to the
epidemiologic-based estimates reflects the uncertainties associated
with mortality and morbidity risk estimates, including the
heterogeneity in effect estimates between locations, the potential for
exposure measurement errors, and uncertainty in the interpretation of
the shape of concentration-response functions at lower O3
concentrations (U.S. EPA, 2014a, section 9.6).
Uncertainty in the shape of concentration-response functions at
lower O3 concentrations is particularly important to
interpreting risk estimates given the approach used to adjust air
quality to just meet the current standard, and potential alternative
standards, and the resulting compression in the air quality
distributions (i.e., decreasing high concentrations and increasing low
concentrations) (II.A.2.a, above). Total risk estimates in the HREA are
based on the assumption that the concentration response function for
O3 is linear, such that total risk estimates are equally
influenced by decreasing high concentrations and increasing low
concentrations, when the increases and decreases are of equal
magnitude. However, consistent with the PA's consideration of risk
estimates, in the proposal the Administrator notes that the overall
body of evidence provides stronger support for the occurrence of
[[Page 65321]]
O3-attributable health effects following exposures to
O3 concentrations corresponding to the upper ends of typical
ambient distributions (II.E.4.d of the proposal). In addition, even on
days with increases in relatively low area-wide average concentrations,
resulting in increases in estimated risks, some portions of the urban
study areas could experience decreases in high O3
concentrations. Therefore, to the extent adverse O3-
attributable effects are more strongly supported for higher ambient
concentrations (which, as noted above, are consistently reduced upon
air quality adjustment), the PA notes that the impacts on risk
estimates of increasing low O3 concentrations reflect an
important source of uncertainty.
c. PA Conclusions on the Current Standard
Section II.D.3 of the proposal summarizes the PA conclusions on the
adequacy of the existing primary O3 standard (79 FR 75285).
As an initial matter, the PA concludes that reducing precursor
emissions to achieve O3 concentrations that meet the current
standard will provide important improvements in public health
protection. This initial conclusion is based on (1) the strong body of
scientific evidence indicating a wide range of adverse health outcomes
attributable to exposures to O3 concentrations commonly
found in the ambient air and (2) estimates indicating decreased
occurrences of O3 exposures of concern and decreased health
risks upon meeting the current standard, compared to recent air
quality.
In particular, the PA concludes that strong support for this
initial conclusion is provided by controlled human exposure studies of
respiratory effects, and by quantitative estimates of exposures of
concern and lung function decrements based on information in these
studies. Analyses in the HREA estimate that the percentages of children
(i.e., all children and children with asthma) in urban study areas
experiencing exposures of concern, or experiencing abnormal and
potentially adverse lung function decrements, are consistently lower
for air quality that just meets the current O3 standard than
for recent air quality. The HREA estimates such reductions consistently
across the urban study areas evaluated and throughout various portions
of individual urban study areas, including in urban cores and the
portions of urban study areas surrounding urban cores. These reductions
in exposures of concern and O3-induced lung function
decrements reflect the consistent decreases in the highest
O3 concentrations following reductions in precursor
emissions to meet the current standard. Thus, populations in both urban
and non-urban areas would be expected to experience important
reductions in O3 exposures and O3-induced lung
function risks upon meeting the current standard.
The PA further concludes that support for this initial conclusion
is also provided by estimates of O3-associated mortality and
morbidity based on application of concentration-response relationships
from epidemiologic studies to air quality adjusted to just meet the
current standard. These estimates are based on the assumption that
concentration-response relationships are linear over entire
distributions of ambient O3 concentrations, an assumption
which has uncertainties that complicate interpretation of these
estimates (II.A.2.c.ii). However, risk estimates for effects associated
with short- and long-term O3 exposures, combined with the
HREA's national analysis of O3 responsiveness to reductions
in precursor emissions and the consistent reductions estimated for the
highest ambient O3 concentrations, suggest that
O3-associated mortality and morbidity would be expected to
decrease nationwide following reductions in precursor emissions to meet
the current O3 standard.
After reaching the initial conclusion that meeting the current
primary O3 standard will provide important improvements in
public health protection, and that it is not appropriate to consider a
standard that is less protective than the current standard, the PA
considers the adequacy of the public health protection that is provided
by the current standard. In considering the available scientific
evidence, exposure/risk information, advice from CASAC (II.B.1.d,
below), and input from the public, the PA reaches the conclusion that
the available evidence and information clearly call into question the
adequacy of public health protection provided by the current primary
standard. In reaching this conclusion, the PA notes that evidence from
controlled human exposure studies provides strong support for the
occurrence of adverse respiratory effects following exposures to
O3 concentrations below the level of the current standard.
Epidemiologic studies provide support for the occurrence of adverse
respiratory effects and mortality under air quality conditions that
would likely meet the current standard. In addition, based on the
analyses in the HREA, the PA concludes that the exposures and risks
projected to remain upon meeting the current standard are indicative of
risks that can reasonably be judged to be important from a public
health perspective. Thus, the PA concludes that the evidence and
information provide strong support for giving consideration to revising
the current primary standard in order to provide increased public
health protection against an array of adverse health effects that range
from decreased lung function and respiratory symptoms to more serious
indicators of morbidity (e.g., including emergency department visits
and hospital admissions), and mortality. In consideration of all of the
above, the PA draws the conclusion that it is appropriate for the
Administrator to consider revision of the current primary O3
standard to provide increased public health protection.
d. CASAC Advice
Section II.D.4 of the proposal summarizes CASAC advice regarding
the adequacy of the existing primary O3 standard. Following
the 2008 decision to revise the primary O3 standard by
setting the level at 0.075 ppm (75 ppb), CASAC strongly questioned
whether the standard met the requirements of the CAA. In September
2009, the EPA announced its intention to reconsider the 2008 standards,
issuing a notice of proposed rulemaking in January 2010 (75 FR 2938).
Soon after, the EPA solicited CASAC review of that proposed rule and in
January 2011, solicited additional advice. This proposal was based on
the scientific and technical record from the 2008 rulemaking, including
public comments and CASAC advice and recommendations. As further
described above (I.D), in the fall of 2011, the EPA did not revise the
standard as part of the reconsideration process but decided to defer
decisions on revisions to the O3 standards to the next
periodic review, which was already underway. Accordingly, in this
section we describe CASAC's advice related to the 2008 final decision
and the subsequent reconsideration, as well as its advice on this
current review of the O3 NAAQS that was initiated in
September 2008.
In April 2008, the members of the CASAC Ozone Review Panel sent a
letter to EPA stating ``[I]n our most-recent letters to you on this
subject--dated October 2006 and March 2007--the CASAC unanimously
recommended selection of an 8-hour average Ozone NAAQS within the range
of 0.060 to 0.070 parts per million [60 to 70 ppb] for the primary
(human health-based) Ozone NAAQS'' (Henderson, 2008). In 2010, in
response to the EPA's solicitation of advice on the EPA's
[[Page 65322]]
proposed rulemaking as part of the reconsideration, CASAC again stated
that the current standard should be revised to provide additional
protection to the public health (Samet, 2010):
CASAC fully supports EPA's proposed range of 0.060-0.070 parts
per million (ppm) for the 8-hour primary ozone standard. CASAC
considers this range to be justified by the scientific evidence as
presented in the Air Quality Criteria for Ozone and Related
Photochemical Oxidants (March 2006) and Review of the National
Ambient Air Quality Standards for Ozone: Policy Assessment of
Scientific and Technical Information, OAQPS Staff Paper (July 2007).
As stated in our letters of October 24, 2006, March 26, 2007 and
April 7, 2008 to former Administrator Stephen L. Johnson, CASAC
unanimously recommended selection of an 8-hour average ozone NAAQS
within the range proposed by EPA (0.060 to 0.070 ppm). In proposing
this range, EPA has recognized the large body of data and risk
analyses demonstrating that retention of the current standard would
leave large numbers of individuals at risk for respiratory effects
and/or other significant health impacts including asthma
exacerbations, emergency room visits, hospital admissions and
mortality.
In response to the EPA's request for additional advice on the
reconsideration in 2011, CASAC reaffirmed their conclusion that ``the
evidence from controlled human and epidemiological studies strongly
supports the selection of a new primary ozone standard within the 60-70
ppb range for an 8-hour averaging time'' (Samet, 2011, p ii). As
requested by the EPA, CASAC's advice and recommendations were based on
the scientific and technical record from the 2008 rulemaking. In
considering the record for the 2008 rulemaking, CASAC stated the
following to summarize the basis for their conclusions (Samet, 2011,
pp. ii to iii):
(1) The evidence available on dose-response for effects of
O3 shows associations extending to levels within the range
of concentrations currently experienced in the United States.
(2) There is scientific certainty that 6.6-hour exposures with
exercise of young, healthy, non-smoking adult volunteers to
concentrations >=80 ppb cause clinically relevant decrements of lung
function.
(3) Some healthy individuals have been shown to have clinically
relevant responses, even at 60 ppb.
(4) Since the majority of clinical studies involve young, healthy
adult populations, less is known about health effects in such
potentially ozone sensitive populations as the elderly, children and
those with cardiopulmonary disease. For these susceptible groups,
decrements in lung function may be greater than in healthy volunteers
and are likely to have a greater clinical significance.
(5) Children and adults with asthma are at increased risk of acute
exacerbations on or shortly after days when elevated O3
concentrations occur, even when exposures do not exceed the NAAQS
concentration of 75 ppb.
(6) Large segments of the population fall into what the EPA terms a
``sensitive population group,'' i.e., those at increased risk because
they are more intrinsically susceptible (children, the elderly, and
individuals with chronic lung disease) and those who are more
vulnerable due to increased exposure because they work outside or live
in areas that are more polluted than the mean levels in their
communities.
With respect to evidence from epidemiologic studies, CASAC stated
``while epidemiological studies are inherently more uncertain as
exposures and risk estimates decrease (due to the greater potential for
biases to dominate small effect estimates), specific evidence in the
literature does not suggest that our confidence on the specific
attribution of the estimated effects of ozone on health outcomes
differs over the proposed range of 60-70 ppb'' (Samet, 2011, p. 10).
Following its review of the second draft PA in the current review,
which considers an updated scientific and technical record since the
2008 rulemaking, CASAC concluded that ``there is clear scientific
support for the need to revise the standard'' (Frey, 2014c, p. ii). In
particular, CASAC noted the following (Frey, 2014c, p. 5):
[T]he scientific evidence provides strong support for the
occurrence of a range of adverse respiratory effects and mortality
under air quality conditions that would meet the current standard.
Therefore, CASAC unanimously recommends that the Administrator
revise the current primary ozone standard to protect public
health.\73\
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\73\ CASAC provided similar advice in their letter to the
Administrator on the HREA, stating that ``The CASAC finds that the
current primary NAAQS for ozone is not protective of human health
and needs to be revised'' (Frey, 2014a, p. 15).
In supporting these conclusions, CASAC judged that the strongest
evidence comes from controlled human exposure studies of respiratory
effects. The Committee specifically noted that ``the combination of
decrements in FEV1 together with the statistically
significant alterations in symptoms in human subjects exposed to 72 ppb
ozone meets the American Thoracic Society's definition of an adverse
health effect'' (Frey, 2014c, p. 5). CASAC further judged that ``if
subjects had been exposed to ozone using the 8-hour averaging period
used in the standard, adverse effects could have occurred at lower
concentration'' and that ``the level at which adverse effects might be
observed would likely be lower for more sensitive subgroups, such as
those with asthma'' (Frey, 2014c, p. 5). With regard to 60 ppb
exposures, CASAC noted that ``a level of 60 ppb corresponds to the
lowest exposure concentration demonstrated to result in lung function
decrements large enough to be judged an abnormal response by ATS and
that could be adverse in individuals with lung disease'' (Frey, 2014c,
p. 7). The CASAC further noted that ``a level of 60 ppb also
corresponds to the lowest exposure concentration at which pulmonary
inflammation has been reported'' (Frey, 2014c, p. 7).
In their advice, CASAC also took note of estimates of O3
exposures of concern and the risk of O3-induced lung
function decrements. With regard to the benchmark concentrations used
in estimating exposures of concern, CASAC stated the following (Frey,
2014c, p. 6):
The 80 ppb-8hr benchmark level represents an exposure level for
which there is substantial clinical evidence demonstrating a range
of ozone-related effects including lung inflammation and airway
responsiveness in healthy individuals. The 70 ppb-8hr benchmark
level reflects the fact that in healthy subjects, decreases in lung
function and respiratory symptoms occur at concentrations as low as
72 ppb and that these effects almost certainly occur in some people,
including asthmatics and others with low lung function who are less
tolerant of such effects, at levels of 70 ppb and below. The 60 ppb-
8hr benchmark level represents the lowest exposure level at which
ozone-related effects have been observed in clinical studies of
healthy individuals. Based on its scientific judgment, the CASAC
finds that the 60 ppb-8hr exposure benchmark is relevant for
consideration with respect to adverse effects on asthmatics.
With regard to lung function risk estimates, CASAC concluded that
``estimation of FEV1 decrements of >=15% is appropriate as a
scientifically relevant surrogate for adverse health outcomes in active
healthy adults, whereas an FEV1 decrement of >=10% is a
scientifically relevant surrogate for adverse health outcomes for
people with asthma and lung disease'' (Frey, 2014c, p. 3). The
Committee further concluded that ``[a]sthmatic subjects appear to be at
least as sensitive, if not more sensitive, than non-asthmatic subjects
in manifesting O3-induced pulmonary function decrements''
(Frey, 2014c, p. 4).
Although CASAC judged that controlled human exposure studies of
respiratory effects provide the strongest
[[Page 65323]]
evidence supporting their conclusion on the current standard, the
Committee judged that there is also ``sufficient scientific evidence
based on epidemiologic studies for mortality and morbidity associated
with short-term exposure to ozone at the level of the current
standard'' (Frey, 2014c, p. 5) and noted that ``[r]ecent animal
toxicological studies support identification of modes of action and,
therefore, the biological plausibility associated with the
epidemiological findings'' (Frey, 2014c, p. 5).
e. Administrator's Proposed Decision
Section II.D.5 in the proposal (79 FR 75287-75291) discusses the
Administrator's proposed conclusions related to the adequacy of the
public health protection provided by the current primary O3
standard, resulting in her proposed decision to revise that standard.
These proposed conclusions and her proposed decision, summarized below,
were based on the Administrator's consideration of the available
scientific evidence, exposure/risk information, the comments and advice
of CASAC, and public input that had been received by the time of
proposal.
As an initial matter, the Administrator concluded that reducing
precursor emissions to achieve O3 concentrations that meet
the current primary O3 standard will provide important
improvements in public health protection, compared to recent air
quality. In reaching this initial conclusion, she noted the discussion
in section 3.4 of the PA (U.S. EPA, 2014c). In particular, the
Administrator noted that this initial conclusion is supported by (1)
the strong body of scientific evidence indicating a wide range of
adverse health outcomes attributable to exposures to O3
concentrations commonly measured in the ambient air and (2) estimates
indicating decreased occurrences of O3 exposures of concern
and decreased O3-associated health risks upon meeting the
current standard, compared to recent air quality. Thus, she concluded
that it would not be appropriate in this review to consider a standard
that is less protective than the current standard.\74\
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\74\ Although the Administrator noted that reductions in
O3 precursor emissions (e.g., NOX; VOC) to
achieve O3 concentrations that meet the current standard
could also increase public health protection by reducing the ambient
concentrations of pollutants other than O3 (e.g.,
PM2.5, NO2), we did not quantitatively analyze
these effects, consistent with CASAC advice (Frey, 2014a, p.10).
However, the Administrator is not setting the standard to address
risks from pollutants other than O3.
---------------------------------------------------------------------------
After reaching the initial conclusion that meeting the current
primary O3 standard will provide important improvements in
public health protection, and that it is not appropriate to consider a
standard that is less protective than the current standard, the
Administrator next considered the adequacy of the public health
protection that is provided by the current standard. In doing so, the
Administrator first noted that studies evaluated since the completion
of the 2006 AQCD support and expand upon the strong body of evidence
that, in the last review, indicated a causal relationship between
short-term O3 exposures and respiratory health effects, the
strongest determination under the ISA's hierarchical system for
classifying weight of evidence for causation. Together, experimental
and epidemiologic studies support conclusions regarding a continuum of
O3 respiratory effects ranging from small reversible changes
in pulmonary function, and pulmonary inflammation, to more serious
effects that can result in respiratory-related emergency department
visits, hospital admissions, and premature mortality. The Administrator
further noted that recent animal toxicology studies support
descriptions of modes of action for these respiratory effects and
provide support for biological plausibility for the role of
O3 in reported effects. With regard to mode of action,
evidence indicates that antioxidant capacity may modify the risk of
respiratory morbidity associated with O3 exposure, and that
the inherent capacity to quench (based on individual antioxidant
capacity) can be overwhelmed, especially with exposure to elevated
concentrations of O3. In addition, based on the consistency
of findings across studies and evidence for the coherence of results
from different scientific disciplines, evidence indicates that certain
populations are at increased risk of experiencing O3-related
effects, including the most severe effects. These include populations
and lifestages identified in previous reviews (i.e., people with
asthma, children, older adults, outdoor workers) and populations
identified since the last review (i.e., people with certain genotypes
related to antioxidant and/or anti-inflammatory status; people with
reduced intake of certain antioxidant nutrients, such as Vitamins C and
E).
The Administrator further noted that evidence for adverse
respiratory health effects attributable to long-term \75\ O3
exposures is much stronger than in previous reviews, and noted the
ISA's conclusion that there is ``likely to be'' a causal relationship
between such O3 exposures and adverse respiratory health
effects (the second strongest causality determination). She noted that
the evidence available in this review includes new epidemiologic
studies using a variety of designs and analysis methods, conducted by
different research groups in different locations, evaluating the
relationships between long-term O3 exposures and measures of
respiratory morbidity and mortality. New evidence supports associations
between long-term O3 exposures and the development of asthma
in children, with several studies reporting interactions between
genetic variants and such O3 exposures. Studies also report
associations between long-term O3 exposures and asthma
prevalence, asthma severity and control, respiratory symptoms among
asthmatics, and respiratory mortality.
---------------------------------------------------------------------------
\75\ Based on the exposure surrogates used in recent
epidemiologic studies of long-term O3 exposure, it is not
possible to distinguish between the impacts of long-term
O3 exposure and exposure to repeated short-term peaks
over an O3 season.
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In considering the O3 exposure concentrations reported
to elicit respiratory effects, the Administrator agreed with the
conclusions of the PA and with the advice of CASAC (Frey, 2014c) that
controlled human exposure studies provide the most certain evidence
indicating the occurrence of health effects in humans following
exposures to specific O3 concentrations. In particular, she
noted that the effects reported in controlled human exposure studies
are due solely to O3 exposures, and interpretation of study
results is not complicated by the presence of co-occurring pollutants
or pollutant mixtures.
In considering the evidence from controlled human exposure studies,
the Administrator first noted that these studies have reported a
variety of respiratory effects in healthy adults following exposures to
O3 concentrations of 60, 72, or 80 ppb, and higher. The
largest respiratory effects, and the broadest range of effects, have
been studied and reported following exposures of healthy adults to 80
ppb O3 or higher, with most exposure studies conducted at
these higher concentrations. She further noted that recent evidence
includes controlled human exposure studies reporting the combination of
lung function decrements and respiratory symptoms in healthy adults
engaged in quasi-continuous, moderate exertion following 6.6 hour
exposures to concentrations as low as 72 ppb, and lung function
decrements and
[[Page 65324]]
pulmonary inflammation following exposures to O3
concentrations as low as 60 ppb. As discussed below, compared to the
evidence available in the last review, the Administrator viewed these
studies as having strengthened support for the occurrence of abnormal
and adverse respiratory effects attributable to short-term exposures to
O3 concentrations below the level of the current standard.
The Administrator stated that such exposures to O3
concentrations below the level of the current standard are potentially
important from a public health perspective, given the following:
(1) The combination of lung function decrements and respiratory
symptoms reported to occur in healthy adults following exposures to 72
ppb O3 or higher, while at moderate exertion, meet ATS
criteria for an adverse response. In specifically considering the 72
ppb exposure concentration, CASAC noted that ``the combination of
decrements in FEV1 together with the statistically
significant alterations in symptoms in human subjects exposed to 72 ppb
ozone meets the American Thoracic Society's definition of an adverse
health effect'' (Frey, 2014c, p. 5).
(2) With regard to 60 ppb O3, CASAC agreed that ``a
level of 60 ppb corresponds to the lowest exposure concentration
demonstrated to result in lung function decrements large enough to be
judged an abnormal response by ATS and that could be adverse in
individuals with lung disease'' (Frey, 2014c, p. 7). CASAC further
noted that ``a level of 60 ppb also corresponds to the lowest exposure
concentration at which pulmonary inflammation has been reported''
(Frey, 2014c, p. 7).
(3) The controlled human exposure studies reporting these
respiratory effects were conducted in healthy adults, while at-risk
groups (e.g., children, people with asthma) could experience larger
and/or more serious effects. In their advice to the Administrator,
CASAC concurred with this reasoning (Frey, 2014a, p. 14; Frey, 2014c,
p. 5).
(4) These respiratory effects are coherent with the serious health
outcomes that have been reported in epidemiologic studies evaluating
exposure to O3 (e.g., respiratory-related hospital
admissions, emergency department visits, and mortality).
As noted above, the Administrator's proposed conclusions regarding
the adequacy of the current primary O3 standard placed a
large amount of weight on the results of controlled human exposure
studies. In particular, given the combination of lung function
decrements and respiratory symptoms following 6.6-hour exposures to
O3 concentrations as low as 72 ppb, and given CASAC advice
regarding effects at 72 ppb, along with ATS adversity criteria, she
concluded that the evidence in this review supports the occurrence of
adverse respiratory effects following exposures to O3
concentrations lower than the level of the current standard.\76\ As
discussed below, the Administrator further considered information from
the broader body of controlled human exposure studies within the
context of quantitative estimates of exposures of concern and
O3-induced FEV1 decrements.
---------------------------------------------------------------------------
\76\ This CASAC advice and ATS recommendations are discussed in
more detail in section II.C.4 below (see also II.A.1.c, above).
---------------------------------------------------------------------------
While putting less weight on information from epidemiologic studies
than on information from controlled human exposure studies, the
Administrator also considered what the available epidemiologic evidence
indicates with regard to the adequacy of the public health protection
provided by the current primary O3 standard. She noted that
recent epidemiologic studies provide support, beyond that available in
the last review, for associations between short-term O3
exposures and a wide range of adverse respiratory outcomes (including
respiratory-related hospital admissions, emergency department visits,
and mortality) and with total mortality. Associations with morbidity
and mortality are stronger during the warm or summer months, and remain
robust after adjustment for copollutants.
In considering information from epidemiologic studies within the
context of her conclusions on the adequacy of the current standard, the
Administrator considered the extent to which available studies support
the occurrence of O3 health effect associations with air
quality likely to be allowed by the current standard. Most of the
epidemiologic studies considered by the Administrator were conducted in
locations likely to have violated the current standard over at least
part of the study period. However, she noted three U.S. single-city
studies that support the occurrence of O3-associated
hospital admissions or emergency department visits at ambient
O3 concentrations below the level of the current standard,
or when virtually all monitored concentrations were below the level of
the current standard (Mar and Koenig, 2009; Silverman and Ito, 2010;
Strickland et al., 2010) (section II.D.1 of the proposal). While the
Administrator acknowledged greater uncertainty in interpreting air
quality for multicity studies, she noted that O3
associations with respiratory morbidity or mortality have been reported
when the majority of study locations (though not all study locations)
would likely have met the current O3 standard. When taken
together, the Administrator reached the initial conclusion at proposal
that single-city epidemiologic studies and associated air quality
information support the occurrence of O3-associated hospital
admissions and emergency department visits for ambient O3
concentrations likely to have met the current standard, and that air
quality analyses in locations of multicity studies provide some support
for this conclusion for a broader range of effects, including
mortality.
Beyond her consideration of the scientific evidence, the
Administrator also considered the results of the HREA exposure and risk
analyses in reaching initial conclusions regarding the adequacy of the
current primary O3 standard. In doing so, as noted above,
she focused primarily on exposure and risk estimates based on
information from controlled human exposure studies (i.e., exposures of
concern and O3-induced lung function decrements) and placed
relatively less weight on epidemiologic-based risk estimates.
With regard to estimates of exposures of concern, the Administrator
considered the extent to which the current standard provides protection
against exposures to O3 concentrations at or above 60, 70,
and 80 ppb. Consistent with CASAC advice (Frey, 2014c), the
Administrator focused on children in these analyses of O3
exposures, noting that estimates for all children and asthmatic
children are virtually indistinguishable, in terms of the percent
estimated to experience exposures of concern.\77\ Though she focused on
children, she also recognized that exposures to O3
concentrations at or above 60 or 70 ppb could be of concern for adults.
As discussed in the HREA and PA (and II.C.2.a of the proposal), the
patterns of exposure estimates across urban study areas, across years,
and across air quality scenarios are similar in adults with asthma,
older adults, all children, and children with asthma, though smaller
percentages of adult populations are estimated to experience exposures
of concern than children and children with asthma. Thus, the
Administrator recognized that the exposure patterns for children across
years, urban study areas, and air
[[Page 65325]]
quality scenarios are indicative of the exposure patterns in a broader
group of at-risk populations that also includes asthmatic adults and
older adults.
---------------------------------------------------------------------------
\77\ As noted above, HREA analyses indicate that activity data
for asthmatics is generally similar to non-asthmatics (U.S. EPA,
2014a, Appendix 5G, Tables 5G2-to 5G-5).
---------------------------------------------------------------------------
She further noted that while single exposures of concern could be
adverse for some people, particularly for the higher benchmark
concentrations (70, 80 ppb) where there is stronger evidence for the
occurrence of adverse effects, she became increasingly concerned about
the potential for adverse responses as the number of occurrences
increases (61 FR 75122).\78\ In particular, she noted that repeated
occurrences of the types of effects shown to occur following exposures
of concern can have potentially adverse outcomes. For example, repeated
occurrences of airway inflammation could potentially result in the
induction of a chronic inflammatory state; altered pulmonary structure
and function, leading to diseases such as asthma; altered lung host
defense response to inhaled microorganisms; and altered lung response
to other agents such as allergens or toxins (U.S. EPA, 2013, section
6.2.3). Thus, the Administrator noted that the types of respiratory
effects shown to occur in some individuals following exposures to
O3 concentrations from 60 to 80 ppb, particularly if
experienced repeatedly, provide a mode of action by which O3
may cause other more serious effects (e.g., asthma exacerbations).
Therefore, the Administrator placed the most weight on estimates of two
or more exposures of concern (i.e., as a surrogate for the occurrence
of repeated exposures), though she also considered estimates of one or
more, particularly for the 70 and 80 ppb benchmarks.\79\
---------------------------------------------------------------------------
\78\ The Administrator noted that not all people who experience
an exposure of concern will experience an adverse effect (even
members of at-risk populations). For most of the endpoints evaluated
in controlled human exposure studies (with the exception of
O3-induced FEV1 decrements, as discussed
below), the number of those experiencing exposures of concern who
will experience adverse effects cannot be reliably quantified.
\79\ The Administrator's considerations related to estimated
O3 exposures of concern, including her views on estimates
of two or more and one or more such exposures, are discussed in more
detail within the context of her consideration of public comments on
the level of the revised standard and her final decision on level
(II.C.4.b and II.C.4.c, below).
---------------------------------------------------------------------------
As illustrated in Table 1 (above), the Administrator noted that if
the 15 urban study areas evaluated in the HREA were to just meet the
current O3 standard, fewer than 1% of children in those
areas would be estimated to experience two or more exposures of concern
at or above 70 ppb, though approximately 3 to 8% of children, including
approximately 3 to 8% of asthmatic children, would be estimated to
experience two or more exposures of concern to O3
concentrations at or above 60 ppb \80\ (based on estimates averaged
over the years of analysis). To provide some perspective on these
percentages, the Administrator noted that they correspond to almost
900,000 children in urban study areas, including about 90,000 asthmatic
children, estimated to experience two or more exposures of concern at
or above 60 ppb. Nationally, if the current standard were to be just
met, the number of children experiencing such exposures would be
larger. In the worst-case year and location (i.e., year and location
with the largest exposure estimates), the Administrator noted that over
2% of children are estimated to experience two or more exposures of
concern at or above 70 ppb and over 14% are estimated to experience two
or more exposures of concern at or above 60 ppb.
---------------------------------------------------------------------------
\80\ Almost no children in those areas would be estimated to
experience two or more exposures of concern at or above 80 ppb.
---------------------------------------------------------------------------
Although, as discussed above and in section II.E.4.d of the
proposal, the Administrator was less concerned about single occurrences
of exposures of concern, she noted that even single occurrences can
cause adverse effects in some people, particularly for the 70 and 80
ppb benchmarks. Therefore, she also considered estimates of one or more
exposures of concern. As illustrated in Table 1 (above), if the 15
urban study areas evaluated in the HREA were to just meet the current
O3 standard, fewer than 1% of children in those areas would
be estimated to experience one or more exposures of concern at or above
80 ppb (based on estimates averaged over the years of analysis).
However, approximately 1 to 3% of children, including 1 to 3% of
asthmatic children, would be estimated to experience one or more
exposures of concern to O3 concentrations at or above 70 ppb
and approximately 10 to 17% would be estimated to experience one or
more exposures of concern to O3 concentrations at or above
60 ppb. In the worst-case year and location, the Administrator noted
that over 1% of children are estimated to experience one or more
exposures of concern at or above 80 ppb, over 8% are estimated to
experience one or more exposures of concern at or above 70 ppb, and
about 26% are estimated to experience one or more exposures of concern
at or above 60 ppb.
In addition to estimated exposures of concern, the Administrator
also considered HREA estimates of the occurrence of O3-
induced lung function decrements. In doing so, she particularly noted
CASAC advice that ``estimation of FEV1 decrements of >=15%
is appropriate as a scientifically relevant surrogate for adverse
health outcomes in active healthy adults, whereas an FEV1
decrement of >=10% is a scientifically relevant surrogate for adverse
health outcomes for people with asthma and lung disease'' (Frey, 2014c,
p. 3). While these surrogates provide perspective on the potential for
the occurrence of adverse respiratory effects following O3
exposures, the Administrator agreed with the conclusion in past reviews
that a more general consensus view of the adversity of moderate
responses emerges as the frequency of occurrence increases (citing to
61 FR 65722-3) (Dec, 13, 1996). Therefore, in the proposal the
Administrator expressed increasing concern about the potential for
adversity as the frequency of occurrences increased and, as a result,
she focused primarily on estimates of two or more O3-induced
FEV1 decrements (i.e., as a surrogate for repeated
exposures).
When averaged over the years evaluated in the HREA, the
Administrator noted that the current standard is estimated to allow
about 1 to 3% of children in the 15 urban study areas (corresponding to
almost 400,000 children) to experience two or more O3-
induced lung function decrements =15%, and to allow about 8
to 12% of children (corresponding to about 180,000 asthmatic children)
to experience two or more O3-induced lung function
decrements =10%. Nationally, larger numbers of children
would be expected to experience such O3-induced decrements
if the current standard were to be just met. The current standard is
also estimated to allow about 3 to 5% of children in the urban study
areas to experience one or more decrements =15% and about 14
to 19% of children to experience one or more decrements
=10%. In the worst-case year and location, the current
standard is estimated to allow 4% of children in the urban study areas
to experience two or more decrements =15% (and 7% to
experience one or more such decrements) and 14% of children to
experience two or more decrements =10% (and 22% to
experience one or more such decrements).\81\
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\81\ As discussed below (II.C.4), in her consideration of
potential alternative standard levels, the Administrator placed less
weight on estimates of the risk of O3-induced
FEV1 decrements. In doing so, she particularly noted
that, unlike exposures of concern, the variability in lung function
risk estimates across urban study areas is often greater than the
differences in risk estimates between various standard levels (Table
2, above). Given this, and the resulting considerable overlap
between the ranges of lung function risk estimates for different
standard levels, although the Administrator noted her confidence in
the lung function risk estimates themselves, she viewed them as
providing a more limited basis than exposures of concern for
distinguishing between the degree of public health protection
provided by alternative standard levels.
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[[Page 65326]]
In further considering the HREA results, the Administrator
considered the epidemiology-based risk estimates. Compared to the
weight given to HREA estimates of exposures of concern and lung
function risks, she placed relatively less weight on epidemiology-based
risk estimates. Consistent with the conclusions in the PA, her
determination to attach less weight to the epidemiologic-based risk
estimates reflected her consideration of key uncertainties, including
the heterogeneity in effect estimates between locations, the potential
for exposure measurement errors, and uncertainty in the interpretation
of the shape of concentration-response functions for O3
concentrations in the lower portions of ambient distributions (U.S.
EPA, 2014a, section 9.6) (section II.D.2 of the proposal).
The Administrator focused on estimates of total mortality risk
associated with short-term O3 exposures.\82\ Given the
decreasing certainty in the shape of concentration-response functions
for area-wide O3 concentrations at the lower ends of warm
season distributions (U.S. EPA, 2013, section 2.5.4.4), the
Administrator focused on estimates of risk associated with
O3 concentrations in the upper portions of ambient
distributions. Even when considering only area-wide O3
concentrations from these upper portions of seasonal distributions, the
Administrator noted that the current standard is estimated to allow
hundreds to thousands of O3-associated deaths per year in
urban study areas (79 FR 75291 citing to section II.C.3 of the
proposal).
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\82\ In doing so, she concluded that lower confidence should be
placed in the results of the assessment of respiratory mortality
risks associated with long-term O3 exposures, primarily
because that analysis is based on only one study (even though that
study is well-designed) and because of the uncertainty in that study
about the existence and identification of a potential threshold in
the concentration-response function (U.S. EPA, 2014a, section 9.6)
(section II.D.2 of the proposal). CASAC also called into question
the extent to which it is appropriate to place confidence in risk
estimates for respiratory mortality (Frey, 2014a, p. 11).
---------------------------------------------------------------------------
In addition to the evidence and exposure/risk information discussed
above, the Administrator took note of the CASAC advice in the current
review and in the 2010 proposed reconsideration of the 2008 decision
establishing the current standard. As discussed in more detail above,
the current CASAC ``finds that the current NAAQS for ozone is not
protective of human health'' and ``unanimously recommends that the
Administrator revise the current primary ozone standard to protect
public health'' (Frey, 2014c, p. 5).
In consideration of all of the above, the Administrator proposed
that the current primary O3 standard is not adequate to
protect public health, and that it should be revised to provide
increased public health protection. This proposed decision was based on
the Administrator's initial conclusions that the available evidence and
exposure and risk information clearly call into question the adequacy
of public health protection provided by the current primary standard
and, therefore, that the current standard is not requisite to protect
public health with an adequate margin of safety. With regard to the
evidence, she specifically noted that (1) controlled human exposure
studies provide support for the occurrence of adverse respiratory
effects following exposures to O3 concentrations below the
level of the current standard (i.e., as low as 72 ppb), and that (2)
single-city epidemiologic studies provide support for the occurrence of
adverse respiratory effects under air quality conditions that would
likely meet the current standard, with multicity studies providing
limited support for this conclusion for a broader range of effects
(i.e., including mortality). In addition, based on the analyses in the
HREA, the Administrator concluded that the exposures and risks
projected to remain upon meeting the current standard can reasonably be
judged to be important from a public health perspective. Thus, she
reached the proposed conclusion that the evidence and information,
together with CASAC advice based on their consideration of that
evidence and information, provide strong support for revising the
current primary standard in order to increase public health protection
against an array of adverse effects that range from decreased lung
function and respiratory symptoms to more serious indicators of
morbidity (e.g., including emergency department visits and hospital
admissions), and mortality.
2. Comments on the Need for Revision
The EPA received a large number of comments, more than 430,000
comments, on the proposed decision to revise the current primary
O3 standard. These comments generally fell into one of two
broad groups that expressed sharply divergent views.
Many commenters asserted that the current primary O3
standard is not sufficient to protect public health, especially the
health of sensitive groups, with an adequate margin of safety. These
commenters agreed with the EPA's proposed decision to revise the
current standard to increase public health protection. Among those
calling for revisions to the current primary standard were medical
groups (e.g., American Academy of Pediatrics (AAP), American Medical
Association, American Lung Association (ALA), American Thoracic
Society, American Heart Association, and the American College of
Occupational and Environmental Medicine); national, state, and local
public health and environmental organizations (e.g., the National
Association of County and City Health Officials, American Public Health
Association, Physicians for Social Responsibility, Sierra Club, Natural
Resources Defense Council, Environmental Defense Fund, Center for
Biological Diversity, and Earthjustice); the majority of state and
local air pollution control authorities that submitted comments (e.g.,
agencies from California Air Resources Board and Office of
Environmental Health Hazard Assessment, Connecticut, Delaware, Iowa,
Illinois, Maryland, Minnesota, New Hampshire, New York, North Dakota,
Oregon, Pennsylvania, Tennessee, and Wisconsin); the National Tribal
Air Association; State organizations (e.g., National Association of
Clean Air Agencies (NACAA), Northeast States for Coordinated Air Use
Management, Ozone Transport Commission). While all of these commenters
agreed with the EPA that the current O3 standard needs to be
revised, many supported a more protective standard than proposed by
EPA, as discussed in more detail below (II.C.4). Many individual
commenters also expressed similar views.
A second group of commenters, representing industry associations,
businesses and some state agencies, opposed the proposed decision to
revise the current primary O3 standard, expressing the view
that the current standard is adequate to protect public health,
including the health of sensitive groups, and to do so with an adequate
margin of safety. Industry and business groups expressing this view
included the American Petroleum Institute (API), the Alliance of
Automobile Manufacturers (AAM), the American Forest and Paper
Association, the Dow Chemical Company, the National Association of
Manufacturers, the
[[Page 65327]]
National Mining Association, the U.S. Chamber of Commerce (in a joint
comment with other industry groups), and the Utility Air Regulatory
Group (UARG). State environmental agencies opposed to revising the
current primary O3 standard included agencies from Arkansas,
Georgia, Louisiana, Kansas, Michigan, Mississippi, Nebraska, North
Carolina, Ohio, Texas, Virginia, and West Virginia.
The following sections discuss comments submitted by these and
other groups, and the EPA's responses to those comments. Comments
dealing with overarching issues that are fundamental to EPA's decision-
making methodology are addressed in section II.B.2.a. Comments on the
health effects evidence, including evidence from controlled human
exposure and epidemiologic studies, are addressed in section II.B.2.b.
Comments on human exposure and health risk assessments are addressed in
section II.B.2.c. Comments on the appropriate indicator, averaging
time, form, or level of a revised primary O3 standard are
addressed below in section II.C. In addition to the comments addressed
in this preamble, the EPA has prepared a Response to Comments document
that addresses other specific comments related to standard setting, as
well as comments on implementation- and/or cost-related factors that
the EPA may not consider as part of the basis for decisions on the
NAAQS. This document is available for review in the docket for this
rulemaking and through the EPA's OAQPS TTN Web site (http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_index.html).
a. Overarching Comments
Some commenters maintained that the proposed rule (and by extension
the final rule) is fundamentally flawed because it does not quantify,
or otherwise define, what level of protection is ``requisite'' to
protect the public health. These commenters asserted that ``EPA has not
explained how far above zero-risk it believes is appropriate or how
close to background is acceptable. EPA has failed to explain how the
current standard is inadequate on this specific basis'' (e.g., UARG, p.
10). These commenters further maintained that the failure to quantify a
requisite level of protection ``drastically reduces the value of public
participation'' since ``the public does not understand what is driving
EPA's decision'' (e.g., UARG, p. 11).
The EPA disagrees with these comments and notes that industry
petitioners made virtually the same argument before the D.C. Circuit in
ATA III, on remand from the Supreme Court, arguing that unless EPA
identifies and quantifies a degree of acceptable risk, it is impossible
to determine if a NAAQS is requisite (i.e., neither too stringent or
insufficiently stringent to protect the public health). The D.C.
Circuit rejected petitioners' argument, holding that ``[a]lthough we
recognize that the Clean Air Act and circuit precedent require EPA
qualitatively to describe the standard governing its selection of
particular NAAQS, we have expressly rejected the notion that the Agency
must `establish a measure of the risk to safety it considers adequate
to protect public health every time it establish a [NAAQS]''' ATA III,
283 F. 3d at 369 (quoting NRDC v. EPA, 902 F.2d 962, 973 (D.C. Cir.
1990)). The court went on to explain that the requirement is only for
EPA to engage in reasoned decision-making, ``not that it definitively
identify pollutant levels below which risks to public health are
negligible.'' ATA III, 283 F. 3d at 370.
Thus, the Administrator is required to exercise her judgment in the
face of scientific uncertainty to establish the NAAQS to provide
appropriate protection against risks to public health, both known and
unknown. As discussed below, in the current review, the Administrator
judges that the existing primary O3 standard is not
requisite to protect public health with an adequate margin of safety, a
judgment that is consistent with CASAC's conclusion that ``there is
clear scientific support for the need to revise the standard'' (Frey,
2014c, p. ii). Further, in section II.C.4 below, the Administrator has
provided a thorough explanation of her rationale for concluding that a
standard with a level of 70 ppb is requisite to protect public health
with an adequate margin of safety, explaining the various scientific
uncertainties which circumscribe the range of potential alternative
standards, and how she exercised her ``judgment'' (per section 109
(b)(1) of the CAA) in selecting a standard from within that range of
scientifically reasonable choices. This ``reasoned decision making'' is
what the Act requires, 283 F. 3d at 370, not the quantification
advocated by these commenters.
The EPA further disagrees with the comment that a failure to
quantify a requisite level of protection impaired or impeded public
notice and comment opportunities. In fact, the EPA clearly gave
adequate notice of the bases both for determining that the current
standard does not afford requisite protection,\83\ and for determining
how the standard should be revised. In particular, the EPA explained in
detail which evidence it considered critical, and the scientific
uncertainties that could cause the Administrator to weight that
evidence in various ways (79 FR 75308-75310). There were robust
comments submitted by commenters from a range of viewpoints on all of
these issues, an indication of the adequacy of notice. The public was
also afforded multiple opportunities to comment to the EPA and to CASAC
during the development of the ISA, REA, and PA. Thus, the EPA does not
agree that lack of quantification of a risk level that is ``requisite''
has deprived commenters of adequate notice and opportunity to comment
in this proceeding.
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\83\ See 79 FR 75287-91 (noting, among other things, that
exposure to ambient O3 concentrations below the level of
the current standard has been associated with diminished lung
function capacity, respiratory symptoms, and respiratory health
effects resulting in emergency room visits or hospital admissions,
and that a single-city epidemiologic study showed associations with
asthma emergency department visits in an area that would have met
the current standard over the entire study period). See also Frey
2014c, p. 5 (CASAC reiterated its conclusion, after multiple public
comment opportunities, that as a matter of science the current
standard ``is not protective of public health'' and provided the
bases for that conclusion).
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Various commenters maintained that it was inappropriate to revise
the current NAAQS based on their view that natural background
concentrations in several states are at or above O3
concentrations associated with meeting a NAAQS set at a level less than
75 ppb (presumably retaining the same indicator, form, and averaging
time), making the NAAQS impossible for those states to attain and
maintain, a result they claim is legally impermissible. In support for
their argument, the commenters cite monitoring and modelling results
from various areas in the intermountain west, state that EPA analyses
provide underestimates of background O3 and conclude that
high concentrations of background O3 \84\ exist
[[Page 65328]]
in many parts of the United States that will ``prevent attainment'' of
a revised standard (NMA, p. 5).
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\84\ Background O3 can be generically defined as the
portion of O3 in ambient air that comes from sources
outside the jurisdiction of an area and can include natural sources
as well as transported O3 of anthropogenic origin. EPA
has identified two specific definitions of background O3
relevant to this discussion: natural background (NB) and United
States background (USB). NB is defined as the O3 that
would exist in the absence of any manmade precursor emissions. USB
is defined as that O3 that would exist in the absence of
any manmade emissions inside the U.S. This includes anthropogenic
emissions outside the U.S. as well as naturally occurring ozone. In
many cases, the comments reference background O3 only in
the generic sense. Unless explicitly noted otherwise, we have
assumed all references to background in the comments are intended to
refer to USB.
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The courts have clearly established that ``[a]ttainability and
technological feasibility are not relevant considerations in the
promulgation of [NAAQS].'' API v. EPA, 665 F. 2d 1176, 1185 (D.C. Cir.
1981). Further, the courts have clarified that the EPA may consider
proximity to background concentrations as a factor in the decision
whether and how to revise the NAAQS only in the context of considering
standard levels within the range of reasonable values supported by the
air quality criteria and judgments of the Administrator. 79 FR 75242-43
(citing ATA III, 283 F. 3d at 379). In this review, the overall body of
scientific evidence and exposure/risk information, as discussed in
Section II.B of this notice, is clear and convincing: The existing
standard is not adequate to protect public health with an adequate
margin of safety and that the standard needs to be revised to reflect a
lower level to provide that protection. The EPA analyses indicate that
there may be infrequent instances in a limited number of rural areas
where background O3 would be appreciable but not the sole
contributor to an exceedance of the revised NAAQS, but do not indicate
U.S. background (USB) O3 concentrations will prevent
attainment of a revised O3 standard with a level of 70 ppb.
USB is defined as that O3 that would exist even in the
absence of any manmade emissions within the United States.
The EPA's estimates of U.S. background ozone concentrations are
based on frequently-utilized, state-of-the-science air quality models
and are considered reasonable and reliable, not underestimates. In
support of their view, the commenters state that monitored (not
modelled) ozone concentrations in remote rural locations include
instances of 8-hour average concentrations very occasionally higher
than 70 ppb. Monitoring data from places like the Grand Canyon and
Yellowstone National Parks, are examples cited in comments. It is
inappropriate to assume that monitored O3 concentrations at
remote sites can be used as a proxy for background O3. Even
at the most remote locations, local O3 concentrations are
impacted by anthropogenic emissions from within the U.S. The EPA
modeling analyses (U.S. EPA, 2014c, Figure 2-18) estimate that, on a
seasonal basis, 10-20% of the O3 at even the most remote
locations in the intermountain western U.S. originates from manmade
emissions from the U.S., and thus is not part of USB. This conclusion
is supported by commenter-submitted recent data analyses of rural
O3 observations in Nevada and Utah (NMA, Appendices D and
H). These analyses conclude that natural sources, international
O3 transport, O3 transported from upwind states,
and O3 transported from urban areas within a state all
contributed to O3 concentrations at rural sites.\85\ Thus,
while O3 in high-altitude, rural portions of the
intermountain western U.S. can, at times, be substantially influenced
by background sources such as wildfires, international transport or the
stratosphere, measured O3 in rural locations are also
influenced by domestic emissions and so cannot, by themselves, be used
to estimate USB concentrations. Accordingly, the fact that 2011-2013
design values in locations like Yellowstone National Park (66 ppb) or
Grand Canyon National Park (72 ppb) approach or exceed 70 ppb, does not
support the conclusion that a standard with a level of 70 ppb is
impossible to attain.
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\85\ The analysis of observations in Utah notes the influence of
domestic emissions--either from Salt Lake City (for two of the
areas) or from Los Angeles and California (for the third of the
areas)--on O3 concentrations at each of the locations
included (NMA comments, Appendix E). Additionally, the analysis of
monitoring data for Nevada also describes the influence of the
monitoring sites by domestic emissions from other western states
(NMA, Appendix H).
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To accurately estimate USB concentrations, it is necessary to use
air quality models which can estimate how much of the O3 at
any given location originates from sources other than manmade emissions
within the U.S. As part of the rulemaking, the EPA has summarized a
variety of modeling-based analyses of background O3 (U.S.
EPA, 2013, Chapter 3) and conducted our own multi-model assessment of
USB concentrations across the U.S. (U.S. EPA, 2014c, Chapter 2). The
EPA analyses, which are consistent with the previously-summarized
studies highlighted by commenters, concluded that seasonal mean daily
maximum 8-hour average concentrations of USB O3 range from
25-50 ppb, with the highest estimates located across the intermountain
western U.S.
Importantly, the modeling analyses also indicate that the highest
O3 days (i.e., the days most relevant to the form of the
NAAQS) generally have similar daily maximum 8-hour average USB
concentrations as the seasonal means of this metric, but have larger
contributions from U.S. anthropogenic sources. As summarized in the PA,
``the highest modeled O3 site-days tend to have background
O3 levels similar to mid-range O3 days . . .
[T]he days with highest O3 levels have similar distributions
(i.e. means, inter-quartile ranges) of background levels as days with
lower values, down to approximately 40 ppb. As a result, the proportion
of total O3 that has background origins is smaller on high O3 days
(e.g. greater than 60 ppb) than on the more common lower O3 days that
tend to drive seasonal means'' (U.S. EPA, 2014c, p. 2-21, emphasis
added). When averaged over the entire U.S., the models estimate that
the mean USB fractional contribution to daily maximum 8-hour average
O3 concentrations above 70 ppb is less than 35 percent. U.S.
anthropogenic emission sources are thus the dominant contributor to the
majority of modeled O3 exceedances across the U.S. (U.S.
EPA, 2014c, Figures 2-14 and 2-15).
As noted in the PA, and as highlighted by the commenters based on
existing modeling, there can be infrequent events where daily maximum
8-hour O3 concentrations approach or exceed 70 ppb largely
due to the influence of USB sources like a wildfire or stratospheric
intrusion. As discussed below in Section V, the statute and EPA
implementing regulations allow for the exclusion of air quality
monitoring data from design value calculations when there are
exceedances caused by certain event-related U.S. background influences
(e.g., wildfires or stratospheric intrusions). As a result, these
``exceptional events'' will not factor into attainability concerns.
In sum, the EPA believes that the commenters have failed to
establish the predicate for their argument. Uncontrollable background
concentrations of O3 are not expected to preclude attainment
of a revised O3 standard with a level of 70 ppb. The EPA
also disagrees with aspects of the specific statements made by the
commenters as support for their view that the EPA analyses have
underestimated background O3.\86\ Thus, even assuming the
commenters are correct that the EPA may use proximity to background as
a justification for not revising a standard that, in the judgment of
the Administrator, is inadequate to protect public health, the
commenters' arguments for the justification and need to do so for this
review are based on a flawed premise.
---------------------------------------------------------------------------
\86\ Specific aspects of the comments on the EPA analyses are
addressed in more detail in the RTC.
---------------------------------------------------------------------------
b. Comments on the Health Effects Evidence
As noted above, comments on the adequacy of the current standard
fell into two broad categories reflecting very
[[Page 65329]]
different views of the available scientific evidence. Commenters who
expressed support for the EPA's proposed decision to revise the current
primary O3 standard generally concluded that the body of
scientific evidence assessed in the ISA is much stronger and more
compelling than in the last review. These commenters also generally
emphasized CASAC's interpretation of the body of available evidence,
which formed an important part of the basis for CASAC's reiterated
recommendations to revise the O3 standard to provide
increased public health protection. In some cases, these commenters
supported their positions by citing studies published since the
completion of the ISA.
The EPA generally agrees with these commenters regarding the need
to revise the current primary O3 standard in order to
increase public health protection though, in many cases, not with their
conclusions about the degree of protection that is appropriate
(II.C.4.b and II.C.4.c, below). The scientific evidence noted by these
commenters was generally the same as that assessed in the ISA (U.S.
EPA, 2013) and the proposal,\87\ and their interpretation of the
evidence was often, though not always, consistent with the conclusions
of the ISA and CASAC. The EPA agrees that the evidence available in
this review provides a strong basis for the conclusion that the current
O3 standard is not adequately protective of public health.
In reaching this conclusion, the EPA places a large amount of weight on
the scientific advice of CASAC, and on CASAC's endorsement of the
assessment of the evidence in the ISA (Frey and Samet, 2012).
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\87\ As discussed in section I.C above, the EPA has
provisionally considered studies that were highlighted by commenters
and that were published after the ISA. These studies are generally
consistent with the evidence assessed in the ISA, and they do not
materially alter our understanding of the scientific evidence or the
Agency's conclusions based on that evidence.
---------------------------------------------------------------------------
In contrast, while commenters who opposed the proposed decision to
revise the primary O3 standard generally focused on many of
the same studies assessed in the ISA, these commenters highlighted
different aspects of these studies and reached substantially different
conclusions about their strength and the extent to which progress has
been made in reducing uncertainties in the evidence since the last
review. These commenters generally concluded that information about the
health effects of concern has not changed significantly since 2008 and
that the uncertainties in the underlying health science have not been
reduced since the 2008 review. In some cases, these commenters
specifically questioned the EPA's approach to assessing the scientific
evidence and to reaching conclusions on the strength of that evidence
in the ISA. For example, several commenters asserted that the EPA's
causal framework, discussed in detail in the ISA, is flawed and that it
has not been applied consistently across health endpoints. Commenters
also noted departures from other published causality frameworks (Samet
and Bodurow, 2008) and from the criteria for judging causality put
forward by Sir Austin Bradford Hill (Hill, 1965).
The EPA disagrees with comments questioning the ISA's approach to
assessing the evidence, the causal framework established in the ISA, or
the consistent application of that framework across health endpoints.
While the EPA acknowledges the ISA's approach departs from assessment
and causality frameworks that have been developed for other purposes,
such departures reflect appropriate adaptations for the NAAQS. As with
other ISAs, the O3 ISA uses a five-level hierarchy that
classifies the weight of evidence for causation. In developing this
hierarchy, the EPA has drawn on the work of previous evaluations, most
prominently the IOM's Improving the Presumptive Disability Decision-
Making Process for Veterans (Samet and Bodurow, 2008), EPA's Guidelines
for Carcinogen Risk Assessment (U.S. EPA, 2005), and the U.S. Surgeon
General's smoking report (CDC, 2004). The ISA's weight of evidence
evaluation is based on the integration of findings from various lines
of evidence from across the health and environmental effects
disciplines. These separate judgments are integrated into a qualitative
statement about the overall weight of the evidence and causality. The
ISA's causal framework has been developed over multiple NAAQS reviews,
based on extensive interactions with CASAC and based on the public
input received as part of the CASAC review process. In the current
review, the causality framework, and the application of that framework
to causality determinations in the O3 ISA, have been
reviewed and endorsed by CASAC (Frey and Samet, 2012).
Given these views on the assessment of the evidence in the ISA, it
is relevant to note that many of the issues and concerns raised by
commenters on the EPA's interpretation of the evidence, and on the
EPA's conclusions regarding the extent to which uncertainties have been
reduced since the 2008 review, are essentially restatements of issues
raised during the development of the ISA, HREA, and/or PA. The CASAC
O3 Panel reviewed the interpretation of the evidence, and
the EPA's use of information from specific studies, in drafts of these
documents. In CASAC's advice to the Administrator, which incorporates
its consideration of many of the issues raised by commenters, CASAC
approved of the scientific content, assessments, and accuracy of the
ISA, REA, and PA, and indicated that these documents provide an
appropriate basis for use in regulatory decision making for the
O3 NAAQS (Frey and Samet, 2012, Frey, 2014a, Frey, 2014c).
Therefore, the EPA's responses to many of the comments on the evidence
rely heavily on the process established in the ISA for assessing the
evidence, which is the product of extensive interactions with CASAC
over a number of different reviews, and on CASAC advice received as
part of this review of the O3 NAAQS.
The remainder of this section discusses public comments and the
EPA's responses, on controlled human exposure studies (II.B.2.b.i);
epidemiologic studies (II.B.2.b.ii); and at-risk populations
(II.B.2.b.iii).
i. Evidence From Controlled Human Exposure Studies
This section discusses major comments on the evidence from
controlled human exposure studies and provides the Agency's responses
to those comments. To support their views on the adequacy of the
current standard, commenters often highlighted specific aspects of the
scientific evidence from controlled human exposure studies. Key themes
discussed by these commenters included the following: (1) The adversity
of effects demonstrated in controlled human exposure studies,
especially studies conducted at exposure concentrations below 80 ppb;
(2) representativeness of different aspects of the controlled human
exposure studies for making inferences to the general population and
at-risk populations; (3) results of additional analyses of the data
from controlled human exposure studies; (4) evaluation of a threshold
for effects; and (5) importance of demonstration of inflammation at 60
ppb. This section discusses these key comment themes, and provides the
EPA's responses. More detailed discussion of individual comments, and
the EPA's responses, is provided in the Response to Comments document.
Adversity
Some commenters who disagreed with the EPA's proposed decision to
revise the current primary O3 standard disputed the Agency's
characterization
[[Page 65330]]
of the adversity of the O3-induced health effects shown to
occur in controlled human exposure studies. Some of these commenters
contended that the proposal does not provide a clear definition of
adversity or that there is confusion concerning what responses the
Administrator considers adverse. The EPA disagrees with these comments,
and notes that section II.E.4.d of the proposal describes the
Administrator's proposed approach to considering the adversity of
effects observed in controlled human exposure studies. Her final
approach to considering the adversity of these effects, and her
conclusions on adversity, are described in detail below (II.C.4.b,
II.C.4.c).
Other commenters disagreed with the EPA's judgments regarding
adversity and expressed the view that the effects observed in
controlled human exposure studies following 6.6-hour exposures to
O3 concentrations below the level of the current standard
(i.e., 75 ppb) are not adverse.\88\ This group of commenters cited
several reasons to support their views, including that: (1) The lung
function decrements and respiratory symptoms observed at 72 ppb in the
study by Schelegle et al. (2009) were not correlated with each other,
and therefore were not adverse; and (2) group mean FEV1
decrements observed following exposures below 75 ppb are small (e.g.,
<10%, as highlighted by some commenters), transient and reversible, do
not interfere with daily activities, and do not result in permanent
respiratory injury or progressive respiratory dysfunction.
---------------------------------------------------------------------------
\88\ Commenters who supported revising the primary O3
standard often concluded that there is clear evidence for adverse
effects following exposures to O3 concentrations at least
as low as 60 ppb, and that such adverse effects support setting the
level of a revised primary O3 standard at 60 ppb. These
comments, and the EPA's responses, are discussed below within the
context of the Administrator's decision on a revised level
(II.C.4.b).
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While the EPA agrees that not all effects reported in controlled
human exposure studies following exposures below 75 ppb can reasonably
be considered to be adverse, the Agency strongly disagrees with
comments asserting that none of these effects can be adverse. As an
initial matter, the Administrator notes that, when considering the
extent to which the current or a revised standard could allow adverse
respiratory effects, based on information from controlled human
exposure studies, she considers not only the effects themselves, but
also quantitative estimates of the extent to which the current or a
revised standard could allow such effects. Quantitative exposure and
risk estimates provide perspective on the extent to which various
standards could allow populations, including at-risk populations such
as children and children with asthma, to experience the types of
O3 exposures that have been shown in controlled human
exposure studies to cause respiratory effects. As discussed further
below (II.B.3, II.C.4.b, II.C.4.c), to the extent at-risk populations
are estimated to experience such exposures repeatedly, the
Administrator becomes increasingly concerned about the potential for
adverse responses in the exposed population. Repeated exposures provide
a plausible mode of action by which O3 may cause other more
serious effects. Thus, even though the Administrator concludes there is
important uncertainty in the adversity of some of the effects observed
in controlled human exposure studies based on the single exposure
periods evaluated in these studies (e.g., FEV1 decrements
observed following exposures to 60 ppb O3, as discussed in
sections II.C.4.b and II.C.4.c below), she judges that the potential
for adverse effects increases as the number of exposures increases.
Contrary to the commenters' views noted above, the Administrator
considers the broader body of available information (i.e., including
quantitative exposure and risk estimates) when considering the extent
to which the current or a revised standard could allow adverse
respiratory effects (II.B.3, II.C.4.b, II.C.4.c, below).
In further considering commenters' views on the potential adversity
of the respiratory effects themselves (i.e., without considering
quantitative estimates), the EPA notes that although the results of
controlled human exposure studies provide a high degree of confidence
regarding the occurrence of health effects following exposures to
O3 concentrations from 60 to 80 ppb, there are no
universally accepted criteria by which to judge the adversity of the
observed effects. Therefore, as in the proposal, the Administrator
relies upon recommendations from the ATS and advice from CASAC to
inform her judgments on adversity.
In particular, the Administrator focuses on the ATS recommendation
that ``reversible loss of lung function in combination with the
presence of symptoms should be considered adverse'' (ATS, 2000a). The
study by Schelegle et al. (2009) reported a statistically significant
decrease in group mean FEV1 and a statistically significant
increase in respiratory symptoms in healthy adults following 6.6-hour
exposures to average O3 concentrations of 72 ppb. In
considering these effects, CASAC noted that ``the combination of
decrements in FEV1 together with the statistically
significant alterations in symptoms in human subjects exposed to 72 ppb
ozone meets the American Thoracic Society's definition of an adverse
health effect'' (Frey, 2014c, p. 5).
As mentioned above, some commenters nonetheless maintained that the
effects observed in Schelegle et al. (2009) following exposure to 72
ppb O3 (average concentration) were not adverse because the
magnitudes of the FEV1 decrements and the increases in
respiratory symptoms (as measured by the total subjective symptoms
score, TSS) were not correlated across individual study subjects. A
commenter submitted an analysis of the individual-level data from the
study by Schelegle et al. (2009) to support their position. This
analysis indicated that, while the majority of study volunteers (66%)
did experience both lung function decrements and increased respiratory
symptoms following 6.6-hour exposures to 72 ppb O3, some
(33%) did not (e.g., Figure 3 in comments from Gradient).\89\ In
addition, the study subjects who experienced relatively large lung
function decrements did not always also experience relatively large
increases in respiratory symptoms. These commenters interpreted the
lack of a statistically significant correlation between the magnitudes
of decrements and symptoms as meaning that the effects reported by
Schelegle et al. (2009) at 72 ppb did not meet the ATS criteria for an
adverse response.
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\89\ The figure provided in comments by Gradient only clearly
illustrated the responses of 30 out of 31 subjects.
---------------------------------------------------------------------------
However, the ATS recommendation that the combination of lung
function decrements and symptomatic responses be considered adverse is
not restricted to effects of a particular magnitude nor a requirement
that individual responses be correlated. Similarly, CASAC made no such
qualifications in its advice on the combination of respiratory symptoms
and lung function decrements (See e.g., Frey, 2014c, p. 5). Therefore,
as in the proposal and consistent with both CASAC advice and ATS
recommendations, the EPA continues to conclude that the finding of both
statistically significant decrements in lung function and significant
increases in respiratory symptoms following 6.6-hour exposures to an
average O3 concentration of 72 ppb provides a strong
indication of the
[[Page 65331]]
potential for exposed individuals to experience this combination of
effects.\90\
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\90\ Indeed, the finding of statistically significant decreases
in lung function and increases in respiratory symptoms in the same
study population indicates that, on average, study volunteers did
experience both effects.
---------------------------------------------------------------------------
In particular, the Administrator notes that lung function provides
an objective measure of the respiratory response to O3
exposure while respiratory symptoms are subjective, and as evaluated by
Schelegle et al. (2009) were based on a TSS score. If an O3
exposure causes increases in both objectively measured lung function
decrements and subjective respiratory symptoms, which indicate that
people may modify their behavior in response to the exposure, then the
effect is properly viewed as adverse. As noted above, the commenter's
analysis shows that the majority of study volunteers exposed to 72 ppb
O3 in the study by Schelegle et al. (2009) did, in fact,
experience both a decrease in lung function and an increase in
respiratory symptoms.
In further considering this comment, the EPA recognizes that,
consistent with commenter's analysis, some individuals may experience
large decrements in lung function with minimal to no respiratory
symptoms (McDonnell et al., 1999), and vice versa. As indicated above
and discussed in the proposal (79 FR 75289), the Administrator
acknowledges such interindividual variability in responsiveness in her
interpretation of estimated exposures of concern. Specifically, she
notes that not everyone who experiences an exposure of concern,
including for the 70 ppb benchmark, is expected to experience an
adverse response. However, she further judges that the likelihood of
adverse effects increases as the number of occurrences of O3
exposures of concern increases. In making this judgment, she notes that
the types of respiratory effects that can occur following exposures of
concern, particularly if experienced repeatedly, provide a plausible
mode of action by which O3 may cause other more serious
effects.\91\ Therefore, her decisions on the primary standard emphasize
the public health importance of limiting the occurrence of repeated
exposures to O3 concentrations at or above those shown to
cause adverse effects in controlled human exposure studies (II.B.3,
II.C.4.b, II.C.4.c). The Administrator views this approach to
considering the evidence from controlled human exposure studies as
being consistent with commenter's analysis indicating that, while the
majority did, not all study volunteers exposed to 72 ppb O3
experienced the adverse combination of lung function decrements and
respiratory symptoms following the single exposure period evaluated by
Schelegle et al. (2009).
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\91\ For example, as discussed in the proposal (79 FR 75252) and
the ISA (p. 6-76), inflammation induced by a single exposure (or
several exposures over the course of a summer) can resolve entirely.
However, repeated occurrences of airway inflammation could
potentially result in the induction of a chronic inflammatory state;
altered pulmonary structure and function, leading to diseases such
as asthma; altered lung host defense response to inhaled
microorganisms; and altered lung response to other agents such as
allergens or toxins (ISA, section 6.2.3).
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Representativeness
A number of commenters raised issues concerning the
representativeness of controlled human exposure studies considered by
the Administrator in this review, based on different aspects of these
studies. These commenters asserted that since the controlled human
exposure studies were not representative of real-world exposures, they
should not be relied upon as a basis for finding that the current
standard is not adequate to protect public health. Some issues
highlighted by commenters include: Small size of the study populations;
unrealistic activity levels used in the studies; unrealistic exposure
scenarios (i.e., triangular exposure protocol) used in some studies,
including Schelegle et al. (2009); and differences in study design that
limit comparability across studies.
Some commenters noted that the controlled human exposure studies
were not designed to have individuals represent portions of any larger
group and that the impacts on a small number of people do not implicate
the health of an entire subpopulation, particularly when the
FEV1 decrements are small, temporary, and reversible. These
commenters also noted that the Administrator failed to provide an
explanation or justification for why the individuals in these studies
can be viewed as representatives of a subpopulation. Further, they
asserted that EPA's use of results from individuals, rather than the
group mean responses, contradicts the intent of CAA section 109 to
protect groups of people, not just the most sensitive individuals in
any group (79 FR 75237).
Consistent with CASAC advice (Frey, 2014c, p. 5), the EPA concludes
that the body of controlled human exposure studies are sufficiently
representative to be relied upon as a basis for finding that the
current standard is not adequate to protect public health. These
studies generally recruit healthy young adult volunteers, and often
expose them to O3 concentrations found in the ambient air
under real-world exposure conditions. As described in more detail above
in section II.A.1.b, the evidence from controlled human exposure
studies to date makes it clear that there is considerable variability
in responses across individuals, even in young healthy adult
volunteers, and that group mean responses are not representative of
more responsive individuals. It is important to look beyond group mean
responses to the responses of these individuals to evaluate the
potential impact on more responsive members of the population.
Moreover, relying on group mean changes to evaluate lung function
responses to O3 exposures would mask the responses of the
most sensitive groups, particularly where, as here, the group mean
reflects responses solely among the healthy young adults who were the
study participants. Thus, the studies of exposures below 80 ppb
O3 show that 10% of young healthy adults experienced
FEV1 decrements >10% following exposures to 60 ppb
O3, and 19% experienced such decrements following exposures
to 72 ppb (under the controlled test conditions involving moderate
exertion for 6.6 hours). These percentages would likely have been
higher had people with asthma or other at-risk populations been exposed
(U.S. EPA, 2013, pp. 6-17 and 6-18; Frey 2014c, p. 7; Frey, 2014a, p.
14).\92\
---------------------------------------------------------------------------
\92\ See also National Environmental Development Associations
Clean Action Project v. EPA, 686 F. 3d 803, 811 (D.C. Cir. 2012)
(EPA drew legitimate inference that serious asthmatics would
experience more serious health effects than clinical test subjects
who did not have this degree of lung function impairment).
---------------------------------------------------------------------------
Moreover, the EPA may legitimately view the individuals in these
studies as representatives of the larger subpopulation of at-risk or
sensitive groups. As stated in the Senate Report to the 1970
legislation establishing the NAAQS statutory provisions, ``the
Committee emphasizes that included among these persons whose health
should be protected by the ambient standard are particularly sensitive
citizens such as bronchial asthmatics and emphysematics who in the
normal course of daily activity are exposed to the ambient environment.
In establishing an ambient standard necessary to protect the health of
these persons, reference should be made to a representative sample of
persons comprising the sensitive group rather than to a single person
in such a group. . . . For purposes of this description, a
statistically related sample is the number of persons necessary to test
in order to detect a deviation in the health of any person within such
sensitive group which is attributable to the condition of the ambient
air.'' S. Rep. No. 11-1196, 91st
[[Page 65332]]
Cong. 2d sess. at 10. As just noted above, 10% of healthy young adults
in these studies experienced >10% FEV1 decrements following
exposure to 60 ppb O3, and the proportion of individuals
experiencing such decrements increases with increasing O3
exposure concentrations. This substantial percentage certainly can be
viewed as ``a representative sample of persons'' and as a sufficient
number to ``detect a deviation in the health of any person within such
sensitive group,'' especially given that it reflects the percentage of
healthy adults who experienced decrements >10%.
These results are consistent with estimates from the MSS model,
which makes reliable quantitative predictions of the lung function
response to O3 exposures, and reasonably predicts the
magnitude of individual lung function responses following such
exposures. As described in section II.A.2.c above, and documented in
the HREA, when the MSS model was used to quantify the risk of
O3-induced FEV1 decrements in 15 urban study
areas, the current standard was estimated to allow about 8 to 12% of
children to experience two or more O3-induced
FEV1 decrements >=10%, and about 2 to 3% to experience two
or more decrements >=15% (Table 2, above). These percentages correspond
to hundreds of thousands of children in urban study areas, and tens of
thousands of asthmatic children. While the Administrator judges that
there is uncertainty with regard to the adversity of these
O3-induced lung function decrements (see II.C.4.b, II.C.4.c,
below), such risk estimates clearly indicate that they are a matter of
public health importance on a broad scale, not isolated effects on
idiosyncratically responding individuals.
Other commenters considered the ventilation rates used in
controlled human exposure studies to be unreasonably high and at the
extreme of prolonged daily activity. Some of these commenters noted
that these scenarios are unrealistic for sensitive populations, such as
asthmatics and people with COPD, whose conditions would likely prevent
them from performing the intensity of exercise, and therefore
experiencing the ventilation rates, required to produce decrements in
lung function observed in experimental settings.
The EPA disagrees with these commenters. The activity levels used
in controlled human exposure studies were summarized in Table 6-1 of
the ISA (U.S. EPA, 2013). The exercise level in the 6.6-hour exposure
studies by Adams (2006), Schelegle et al. (2009), and Kim et al. (2011)
of young healthy adults was moderate and ventilation rates are
typically targeted for 20 L/min-m\2\ BSA.\93\ Following the exposures
to 60 ppb at this activity level, 10% of the individuals had greater
than a 10% decrement in FEV1 (U.S. EPA, 2013, p. 6-18).
Similar 6.6-hour exposure studies of individuals with asthma are not
available to assess either the effects of O3 on their lung
function or their ability to perform the required level of moderate
exercise.
---------------------------------------------------------------------------
\93\ Exercise consisted of alternating periods walking on a
treadmill at a pace of 17-18 minutes per mile inclined to a grade of
4-5% or cycling at a load of about 72 watts. Typical heart rates
during the exercise periods were between 115-130 beats per minute.
This activity level is considered moderate (Table 6-1, U.S. EPA,
2013, p. 6-18).
---------------------------------------------------------------------------
However, referring to Tables 6-9 and 6-10 of the HREA (U.S. EPA,
2014a), between 42% and 45% of FEV1 decrements >= 10% were
estimated to occur at exercise levels of <13 L/min-m\2\ BSA. This
corresponds to light exercise, and this level of exercise has been used
in a 7.6-hour study of healthy people and people with asthma exposed to
160 ppb O3 (Horstman et al., 1995). In that study, people
with asthma exercised with an average minute ventilation of 14.2 L/min-
m\2\ BSA. Adjusted for filtered air responses, an average 19%
FEV1 decrement was seen in the people with asthma versus an
average 10% FEV1 decrement in the healthy people. In
addition, the EPA noted in the HREA that the data underlying the
exposure assessment indicate that ``activity data for asthmatics [is]
generally similar to [that for] non-asthmatics'' (U.S. EPA, 2014a, p.
5-75, Tables 5G-2 and 5G-3). Thus, contrary to the commenters'
assertion, based on both the HREA and the Horstman et al. (1995) study,
people with respiratory disease such as asthma can exercise for a
prolonged period under conditions where they would experience >10%
FEV1 decrements in response to O3 exposure.
Additionally, a number of commenters asserted that the exposure
scenarios in Schelegle et al. (2009), which are based on a so-called
triangular study protocol, where O3 concentrations ramp up
and down as the study is conducted, are not directly generalizable to
most healthy or sensitive populations because of large changes in the
O3 concentrations from one hour to the next. Commenters
stated that although large fluctuations in O3 are possible
in certain locations due to meteorological conditions (e.g., in valleys
on very hot, summer days), they believe that, in general,
concentrations of O3 do not fluctuate by more than 20-30 ppb
from one hour to the next. Thus, commenters suggested the Schelegle et
al. (2009) study design could happen in a ``worst-case'' exposure
scenario, but that the exposure protocol was not reflective of
conditions in most cities and thus not informative with regard to the
adequacy of the current standard.
The EPA disagrees with the comment that these triangular exposure
scenarios are not generalizable because of hour-to-hour fluctuations.
Adams (2002, 2006) showed that FEV1 responses following 6.6
hours of exposure to 60 and 80 ppb average O3 exposures do
not differ between triangular (i.e. ramping concentration up and down)
and square-wave (i.e. constant concentration). Schelegle et al. (2009)
used the 80 ppb triangular protocol and a slightly modified 60 ppb
triangular protocol (concentrations during the third and fourth hours
were reversed) from Adams (2006). Therefore, in considering pre- to
post-exposure changes in lung function, concerns about the hour-by-hour
changes in O3 concentrations at 60 and 80 ppb in the
Schelegle et al. (2009) study are unfounded.
Finally, some commenters also stated that the Kim et al. (2011)
study is missing critical information and its study design makes
comparison to the other studies difficult. That is, the commenter
suggests that data at times other than pre- and post-exposure should
have been provided.
The EPA disagrees with this comment. With regard to providing data
at other time points besides pre- and post-exposure, there is no
standard that suggests an appropriate frequency at which lung function
should be measured in prolonged 6.6-hour exposure studies. The Adams
(2006) study showed that lung function decrements during O3
exposures with moderate exercise become most apparent following the
third hour of exposure. As such, it makes little sense to measure lung
function during the first couple hours of exposure. However, having
data at multiple time points toward the end of an exposure can provide
evidence that the mean post-exposure FEV1 response is not a
single anomalous data point. The FEV1 response data for the
3-, 4.6-, 5.6-, and 6.6-hour time points of the Kim et al. (2011) study
are available in Figure 6 of the McDonnell et al. (2012) paper where
they are plotted with the Adams (2006) data for 60 ppb. Similar to the
Adams (2006) study, the responses at 5.6 hours are only marginally
smaller than the response at 6.6 hours in the Kim et al. (2011) study.
This indicates that the post-exposure FEV1 responses in both
studies are consistent with responses at an earlier time point and thus
not likely to be anomalous data.
[[Page 65333]]
Additional Studies
Several commenters analyzed the data from controlled human exposure
studies, or they commented on the EPA's analysis of the data from some
of these studies (Brown et al., 2008), to come to a different
conclusion than the EPA's interpretation of these studies thereby
questioning the proposed decision that the current standard is not
adequate to protect public health. One commenter submitted an
independent assessment of the scientific evidence and risk, and used
this analysis to assert that there are multiple flaws in the underlying
studies and their interpretation by the EPA. This commenter stated that
the EPA's discussion of the spirometric responses of children and
adolescents and older adults to O3 was misleading. They
claimed that the EPA did not mention that ``the responses of children
and adolescents are equivalent to those of young adults (18-35 years
old; McDonnell et al., 1985) and that this response diminishes in
middle-aged and older adults (Hazucha 1985).'' The EPA notes that the
commenter misrepresented our characterization of the effect of age on
FEV1 responses to O3 and asserted mistakenly that
EPA did not mention diminished responses on older adults. In fact, the
proposal clearly states that, ``Respiratory symptom responses to
O3 exposure appears to increase with age until early
adulthood and then gradually decrease with increasing age (U.S. EPA,
1996b); lung function responses to O3 exposure also decline
from early adulthood (U.S. EPA, 1996b)'' (79 FR 75267) (see also U.S.
EPA, 2014c p. 3-82). With regard to differences between children and
adults, it was clearly stated in the ISA (U.S. EPA, 2013, p. 6-21) that
healthy children exposed to filtered air and 120 ppb O3
experienced similar spirometric responses, but lesser symptoms than
similarly exposed young healthy adults (McDonnell et al., 1985). In
addition, the EPA's approach to modeling the effect of age on responses
to O3 is clearly provided in the HREA (U.S. EPA, 2014a,
Table 6-2).
The commenter also stated that the EPA's treatment of filtered air
responses in the dose-response curve was incorrect. They claimed that
when creating a dose-response curve, it is most appropriate to include
a zero-dose point and not to subtract the filtered air response from
responses to O3. Contrary to this assertion, EPA correctly
adjusted FEV1 responses to O3 by responses
following filtered air, as was also done in the McDonnell et al. (2012)
model. As indicated in the ISA (U.S. EPA, 2013, p. 6-4), the majority
of controlled human exposure studies investigating the effects
O3 are of a randomized, controlled, crossover design in
which subjects were exposed, without knowledge of the exposure
condition and in random order, to clean filtered air and, depending on
the study, to one or more O3 concentrations. The filtered
air control exposure provides an unbiased estimate of the effects of
the experimental procedures on the outcome(s) of interest. Comparison
of responses following this filtered air exposure to those following an
O3 exposure allows for estimation of the effects of
O3 itself on an outcome measurement while controlling for
independent effects of the experimental procedures, such as ventilation
rate. Thus, the commenter's approach does not provide an estimate of
the effects of O3 alone. Furthermore, as illustrated in
these comments, following ``long'' filtered air exposures, there is
about a 1% improvement in FEV1. By not accounting for this
increase in FEV1, the commenter underestimated the
FEV1 decrement due to O3 exposure. The
commenter's approach thus is fundamentally flawed.
The commenter also asserted that the McDonnell et al. (2012) model
and exposure-response (E-R) models incorrectly used only the most
responsive people and that EPA's reliance on data from clinical trials
that use only the most responsive people irrationally ignores large
portions of relevant data. The EPA rejects this assertion that the
McDonnell et al. (2012) model and the E-R analysis ignored large
portions of relevant data. The McDonnell et al. (2012) model was fit to
the FEV1 responses of 741 individuals to O3 and
filtered air (i.e., reflecting all available data for O3-
induced changes in FEV1). The filtered air responses were
subtracted from responses measured during O3 exposures.
Subsequently, as illustrated by the figures in the McDonnell et al.
(2012) paper and described in the text of paper, the model was fit to
all available FEV1 data measured during the course of
O3 exposures, including exposures shorter than 6.6 hours.
Thus, the model predicts temporal dynamics of FEV1 response
to any set of O3 exposure conditions that might reasonably
be experienced in the ambient environment, predicting the mean
responses and the distribution of responses around the mean. For the
HREA (EPA, 2014a), the proportion of individuals, under variable
exposure conditions, predicted to have FEV1 decrements >=10,
15 and 20% was estimated.
Finally, the commenter referenced the exposure-response model on p.
6-18 of the HREA. However, they neglected to note that this was in a
section describing the exposure-response function approach used in
prior reviews (U.S. EPA, 2014a, starting on p. 6-17). Thus, the
commenter confused the exposure-response model used in the last review
with the updated approach used in this review.
The commenter also stated that EPA did not properly consider
O3 dose when interpreting the human clinical data. Ozone
total dose includes three factors: duration of exposure, concentration,
and ventilation rate. The commenter claimed the EPA emphasized only
concentration without properly considering and communicating duration
of exposure and ventilation rate. Further, they asserted that because
people are not exposed to the same dose, they cannot be judged to have
the same exposure and would therefore not be expected to respond
consistently. The EPA rejects the claim that we emphasized only
concentration without properly incorporating the other two factors. As
noted in the ISA, total O3 dose does not describe the
temporal dynamics of FEV1 responses as a function of
concentration, ventilation rate, time and age of the exposed
individuals (U.S. EPA, 2013, p. 6-5). Thus, the use of total
O3 dose is antiquated and the EPA therefore conducted a more
sophisticated analysis of FEV1 response to O3 in
the HREA. In this review, the HREA estimates risks of lung function
decrements in school-aged children (ages 5 to 18), asthmatic school-
aged children, and the general adult population for 15 urban study
areas. A probabilistic model designed to account for the numerous
sources of variability that affect people's exposures was used to
simulate the movement of individuals through time and space and to
estimate their exposure to O3 while occupying indoor,
outdoor, and in-vehicle locations. That information was linked with the
McDonnell et al. (2012) model to estimate FEV1 responses
over time as O3 exposure concentrations and ventilation
rates changed. As noted earlier, CASAC agreed that this approach is
both scientifically valid and a significant improvement over approaches
used in past O3 reviews (Frey, 2014a, p. 2).
Several commenters criticized the EPA analysis published by Brown
et al. (2008). One commenter suggested that the EPA needed to state why
the Brown et al. (2008) analysis was relied on rather than Nicolich
(2007) or Lefohn et
[[Page 65334]]
al. (2010). Further, commenters stated that the analysis of the Adams
(2006) data in Brown et al. (2008) was flawed. Among other reasons, one
commenter expressed the opinion that it was not appropriate for Brown
et al. (2008) to only examine a portion of the Adams (2006) data,
citing comments submitted by Gradient.
The EPA disagrees with these commenters.\94\ As an initial matter,
Nicolich (2007) was a public comment and is not a peer-reviewed
publication that would be used to assess the scientific evidence for
effects of O3 on lung function in the ISA (U.S. EPA, 2013).
The Nicolich (2007) comments were specifically addressed by the EPA on
pp. 24-25 in the Response to Comments Document for the 2007 proposed
rule (U.S. EPA, 2008). On page A-3 of his comments, Dr. Nicolich stated
``that the residuals are not normally distributed and the observations
do not meet the assumptions required for the model'' and that ``the
subject-based errors are not independently, identically and normally
distributed and the subjects do not meet the assumptions required for
the model.'' The EPA reasonably chose not to rely on this analysis:
``Therefore, given that the underlying statistical assumptions required
for his analyses were not met and that significance levels are
questionable, in EPA's judgment the analyses presented by Dr. Nicolich
are ambiguous'' (U.S. EPA, 2008). It is likely that the Lefohn et al.
(2010) analysis of the Adams (2006) data would similarly not meet the
statistical assumptions of the model (e.g., homoscedasticity). In
contrast, recognizing the concerns related to the distribution of
responses, Brown et al. (2008) conservatively used a nonparametric sign
test to obtain a p-value of 0.002 for the comparison responses
following 60 ppb O3 versus filter air. Other common
statistical tests also showed significant effects on lung function. In
addition, the effects of 60 ppb O3 on FEV1
responses in Brown et al. (2008) remained statistically significant
even following the exclusion of three potential outliers.
---------------------------------------------------------------------------
\94\ The DC Circuit has held that EPA reasonably used and
interpreted the Brown (2007) study in the last review. Mississippi,
744 F. 3d at 1347. In this review, there is now additional
corroborative evidence supporting the Brown (2007) analysis, in the
form of further controlled human clinical studies finding health
effects in young, healthy adults at moderate exercise at
O3 concentrations of 60 ppb over a 6.6 hour exposure
period.
---------------------------------------------------------------------------
EPA disagrees with the comment stating that it was not appropriate
for Brown et al. (2008) to only examine a portion of the Adams (2006)
data. In fact, there is no established single manner or protocol
decreeing that data throughout the protocol must be analyzed and
included. Furthermore, Brown et al. (2008) was a peer-reviewed journal
publication. CASAC also expressed favorable comments in their March 30,
2011, letter to Administrator Jackson. With reference to a memorandum
(Brown, 2007) that preceded the Brown et al. (2008) publication, on p.
6 of the CASAC Consensus Responses to Charge Questions CASAC stated,
``The results of the Adams et al. study also have been carefully
reanalyzed by EPA investigators (Brown et. al., [2008]), and this
reanalysis showed a statistically significant group effect on
FEV1 after 60 ppb ozone exposure.'' On p. A-13, a CASAC
panelist and biostatistician stated, ``Thus, from my understanding of
the statistical analyses that have been conducted, I would argue that
the analysis by EPA should be preferred to that of Adams for the
specific comparison of the FEV1 effects of 0.06 ppm exposure
relative to filtered air exposure.'' (Samet 2011, p. a-13)
Threshold
Several commenters used the new McDonnell et al. (2012) and
Schelegle et al. (2012) models to support their views about the
O3 concentrations associated with a threshold for adverse
lung function decrements. For example, one commenter who supported
retaining the current standard noted that McDonnell et al. (2012) found
that the threshold model fit the observed data better than the original
(no-threshold) model, especially at earlier time points and at the
lowest exposure concentrations. The commenter expressed the view that
the threshold model showed that the population mean FEV1
decrement did not reach 10% until exposures were at least 80 ppb,
indicating that O3 exposures of 80 ppb or higher may cause
lung function decrements and other respiratory effects.\95\
---------------------------------------------------------------------------
\95\ Conversely, another group of commenters who supported
revising the standard to a level of 60 ppb noted that the results of
these models are consistent with the results of controlled human
exposure studies finding adverse health effects at 60 ppb. These
comments are discussed below (II.C.4.b), within the context of the
Administrator's decision on a revised standard level.
---------------------------------------------------------------------------
As described above in section II.A.1.b, the McDonnell et al. (2012)
and Schelegle et al. (2012) models represent a significant
technological advance in the exposure-response modeling approach since
the last review, and these models indicate that a dose-threshold model
fits the data better than a non-threshold model. However, the EPA
disagrees that using the predicted group mean response from the
McDonnell model provides support for retaining the current standard. As
discussed above, the group mean responses do not convey information
about interindividual variability, or the proportion of the population
estimated to experience the larger lung function decrements (e.g., 10
or 15% FEV1 decrements) that could be adverse. In fact, it
masks this variability. These variable effects in individuals have been
found to be reproducible. In other words, a person who has a large lung
function response after exposure to O3 will likely have
about the same response if exposed again in a similar manner (raising
health concerns, as noted above). Group mean responses are not
representative of this segment of the population that has much larger
than average responses to O3.
Inflammation
Some commenters asserted that the pulmonary inflammation observed
following exposure to 60 ppb in the controlled human exposure study by
Kim et al. (2011) was small and unlikely to result in airway damage. It
was also suggested that this inflammation is a normal physiological
response in all living organisms to stimuli to which people are
normally exposed.
The EPA recognized in the proposal (79 FR 75252) and the ISA (U.S.
EPA, 2013, p. 6-76) that inflammation induced by a single exposure (or
several exposures over the course of a summer) can resolve entirely.
Thus, the inflammatory response observed following the single exposure
to 60 ppb in the study by Kim et al. (2011) is not necessarily a
concern. However, the EPA notes that it is also important to consider
the potential for continued acute inflammatory responses to evolve into
a chronic inflammatory state and to affect the structure and function
of the lung.\96\ The Administrator considers this possibility through
her consideration of estimated exposures of concern for the 60 ppb
benchmark (II.B.3, II.C.4). As discussed in detail below (II.C.4.b),
while she judges that there is uncertainty in the adversity of the
effects shown to occur following exposures to 60 ppb O3,
including the inflammation reported by Kim et al.
[[Page 65335]]
(2011), she gives some consideration to estimates of two or more
exposures of concern for the 60 ppb benchmark (i.e., as a health-
protective surrogate for repeated exposures of concern at or above 60
ppb), particularly when considering the extent to which the current and
revised standards incorporate a margin of safety.
---------------------------------------------------------------------------
\96\ Inflammation induced by exposure of humans to O3
can have several potential outcomes, ranging from resolving entirely
following a single exposure to becoming a chronic inflammatory state
(U.S. EPA, 2013, section 6.2.3). Lung injury and the resulting
inflammation provide a mechanism by which O3 may cause
other more serious morbidity effects (e.g., asthma exacerbations)
(U.S. EPA, 2013, section 6.2.3). See generally section II.A.1.a
above.
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ii. Evidence Fom epidemiologic studies
This section discusses key comments on the EPA's assessment of the
epidemiologic evidence and provides the Agency's responses to those
comments. The focus in this section is on overarching comments related
to the EPA's approach to assessing and interpreting the epidemiologic
evidence as a whole. Detailed comments on specific studies, or specific
methodological or technical issues, are addressed in the Response to
Comments document. As discussed above, many of the issues and concerns
raised by commenters on the interpretation of the epidemiologic
evidence are essentially restatements of issues raised during the
development of the ISA, HREA, and/or PA, and in many instances were
considered by CASAC in the development of its advice on the current
standard. The EPA's responses to these comments rely heavily on the
process established in the ISA for assessing the evidence, and on CASAC
advice received as part of this review of the O3 NAAQS.
As with evidence from controlled human exposure studies, commenters
expressed sharply divergent views on the evidence from epidemiologic
studies, and on the EPA's interpretation of that evidence. One group of
commenters, representing medical, public health and environmental
organizations, and some states, generally supported the EPA's
interpretation of the epidemiologic evidence with regard to the
consistency of associations, the coherence with other lines of
evidence, and the support provided by epidemiologic studies for the
causality determinations in the ISA. These commenters asserted that the
epidemiologic studies evaluated in the ISA provide valuable information
supporting the need to revise the level of the current primary
O3 standard in order to increase public health protection.
In reaching this conclusion, commenters often cited studies (including
a number from the past review) which they interpreted as showing health
effect associations in locations with O3 air quality
concentrations below the level of the current standard. A second group
of commenters, mostly representing industry associations, businesses,
and states opposed to revising the primary O3 standard,
expressed the general view that while many new epidemiologic studies
have been published since the last review of the O3 NAAQS,
inconsistencies and uncertainties inherent in these studies as a whole,
and in the EPA's assessment of study results, should preclude any
reliance on them as justification for a more stringent primary
O3 standard. To support their views, these commenters often
focused on specific technical or methodological issues that contribute
to uncertainty in epidemiologic studies, including the potential for
exposure error, confounding by copollutants and by other factors (e.g.,
weather, season, disease, day of week, etc.), and heterogeneity in
results across locations.
The EPA agrees with certain aspects of each of these views.
Specifically, while the EPA agrees that epidemiologic studies are an
important part of the broader body of evidence that supports the ISA's
causality determinations, and that these studies provide support for
the decision to revise the current primary O3 standard, the
Agency also acknowledges that there are important uncertainties and
limitations associated with these epidemiologic studies that should be
considered when reaching decisions on the current standard. Thus,
although these studies show consistent associations between
O3 exposures and serious health effects, including morbidity
and mortality, and some of these studies reported such associations
with ambient O3 concentrations below the level of the
current standard, there are also uncertainties regarding the ambient
O3 concentrations in critical studies, such that they lend
only limited support to establishing a specific level for a revised
standard. (See generally, Mississippi, 744 F. 3d at 1351 (noting that
in prior review, EPA reasonably relied on epidemiologic information in
determining to revise the standard but appropriately gave the
information limited weight in determining a level of a revised
standard); see also ATA III, 283 F. 3d at 370 (EPA justified in
revising NAAQS when health effect associations are observed in
epidemiologic studies at levels allowed by the current NAAQS);
Mississippi, 744 F. 3d at 1345 (same)).
Uncertainties in the evidence were considered by the Administrator
in the proposal, and contributed to her decision to place less weight
on information from epidemiologic studies than on information from
controlled human exposure studies when considering the adequacy of the
current primary O3 standard (see 79 FR 75281-83). Despite
receiving less weight in the proposal, the EPA does not agree with
commenters who asserted that uncertainties in the epidemiologic
evidence provide a basis for concluding that the current primary
standard does not need revision. The Administrator specifically
considered the extent to which available studies support the occurrence
of O3 health effect associations with air quality likely to
be allowed by the current standard, while also considering the
implications of important uncertainties, as assessed in the ISA and
discussed in the PA. This consideration is consistent with CASAC
comments on consideration of these studies in the draft PA (Frey,
2014c, p. 5).
Based on analyses of study area air quality in the PA, the EPA
notes that most of the U.S. and Canadian epidemiologic studies
evaluated were conducted in locations likely to have violated the
current standard over at least part of the study period. Although these
studies support the ISA's causality determinations, they provide
limited insight into the adequacy of the public health protection
provided by the current primary O3 standard. However, as
discussed in the proposal, air quality analyses in the locations of
three U.S. single-city studies provide support for the occurrence of
O3-associated hospital admissions or emergency department
visits at ambient O3 concentrations below the level of the
current standard.\97\ Specifically, a U.S. single-city study reported
associations with respiratory emergency department visits in children
and adults in a location that would have met the current O3
standard over the entire study period (Mar and Koenig, 2009). In
addition, for two studies conducted in locations where the current
standard was likely not met (i.e., Silverman and Ito, 2010; Strickland
et al., 2010), PA analyses indicate that reported concentration-
response functions and available air quality data support the
occurrence of O3-health effect associations on subsets of
days with virtually all monitored ambient O3 concentrations
below the level of the current standard (U.S. EPA, 2014c,
[[Page 65336]]
section 3.1.4.2, pp. 3-66 to 67).\98\ Thus, the EPA notes that a small
number of O3 epidemiologic studies provide support for the
conclusion that the current primary standard is not requisite, and that
it should be revised to increase public health protection.
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\97\ As discussed in section II.E.4.d of the proposal, is the
Administrator noted the greater uncertainty in using analyses of
short-term O3 air quality in locations of the multicity
studies in this review to inform decisions on the primary
O3 standard. This is because the health information in
these studies cannot be disaggregated by individual city. Thus, the
multicity effect estimates reported in these studies do not provide
clear indication of the extent to which health effects are
associated with the ambient O3 concentrations in the
study locations that met the current O3 standard, versus
the ambient O3 concentrations in the study locations that
violated the standard.
\98\ Air quality analyses in locations of the studies by
Silverman and Ito (2010) and Strickland et al. (2010) were used in
the PA to inform staff conclusions on the adequacy of the current
primary O3 standard. However, the appropriate
interpretation of these analyses became less clear for standard
levels below 75 ppb, as the number of days increased with monitored
concentrations exceeding the level being evaluated (U.S. EPA, 2014c,
Appendix 3B, Tables 3B-6 and 3B-7). Therefore, these analyses were
not used in the PA to inform conclusions on potential alternative
standard levels lower than 75 ppb (U.S. EPA, 2014c, Chapters 3 and
4).
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As part of a larger set of comments criticizing the EPA's
interpretation of the evidence from time series epidemiologic studies,
some commenters objected to the EPA's reliance on the studies by
Strickland et al. (2010), Silverman and Ito (2010), and Mar and Koenig
(2009). These commenters highlighted what they considered to be key
uncertainties in interpreting these studies, including uncertainties
due to the potential for confounding by co-pollutants, aeroallergens,
or the presence of upper respiratory infections; and uncertainties in
the interpretation of zero-day lag models (i.e., specifically for Mar
and Koenig, 2009).
While the EPA agrees that there are uncertainties associated with
interpreting the O3 epidemiologic evidence, as discussed
above and elsewhere in this preamble, we disagree with commenters'
assertion that these uncertainties should preclude the use of the
O3 epidemiologic evidence in general, or the studies by
Silverman and Ito, Strickland, or Mar and Koenig in particular, as part
of the basis for the Administrator's decision to revise the current
primary standard. As a general point, when considering the potential
importance of uncertainties in epidemiologic studies, we rely on the
broader body of evidence, not restricted to these three studies, and
the ISA conclusions based on this evidence. The evidence, the ISA's
interpretation of specific studies, and the use of information from
these studies in the HREA and PA, was considered by CASAC in its review
of drafts of the ISA, HREA, and PA. Based on the assessment of the
evidence in the ISA, and CASAC's endorsement of the ISA conclusions, as
well as CASAC's endorsement of the approaches to using and considering
information from epidemiologic studies in the HREA and PA (Frey, 2014c,
p. 5), we do not agree with these commenters' conclusions regarding the
usefulness of the epidemiologic studies by Strickland et al. (2010),
Silverman and Ito (2010), and Mar and Koenig (2009).
More specifically, with regard to confounding by co-pollutants, we
note the ISA conclusion that, in studies of O3-associated
hospital admissions and emergency department visits ``O3
effect estimates remained relatively robust upon the inclusion of PM .
. . and gaseous pollutants in two-pollutant models'' (U.S. EPA, 2013,
pp. 6-152 and 6-153). This conclusion was supported by several studies
that evaluated co-pollutant models including, but not limited to, two
of the studies specifically highlighted by commenters (i.e., Silverman
and Ito, 2010; Strickland et al., 2010) (U.S. EPA, 2013, section
6.2.7.5; Figure 6-20 and Table 6-29).
Other potential uncertainties highlighted by commenters have been
evaluated less frequently (e.g., confounding by allergen exposure,
respiratory infections). However, we note that Strickland et al. (2010)
did consider the potential for pollen (a common airborne allergen) to
confound the association between ambient O3 and emergency
department visits. While quantitative results were not presented, the
authors reported that ``estimates for associations between ambient air
pollutant concentrations and pediatric asthma emergency department
visits were similar regardless of whether pollen concentrations were
included in the model as covariates'' (Strickland et al., 2010, p.
309). This suggests a limited impact of aeroallergens on O3
associations with asthma-related emergency department visits and
hospital admissions.
With respect to the comment about epidemiologic studies not
controlling for respiratory infections in the model, the EPA disagrees
with the commenter's assertion. We recognize that asthma is a multi-
etiologic disease and that air pollutants, including O3,
represent only one potential avenue to trigger an asthma exacerbation.
Strickland et al. attempted to further clarify the relationship between
short-term O3 exposures and asthma emergency department
visits by controlling for the possibility that respiratory infections
may lead to an asthma exacerbation. By including the daily count of
upper respiratory visits as a covariate in the model, Strickland et al.
were able to account for the possibility that respiratory infections
contribute to the daily counts of asthma emergency department visits,
and to identify the O3 effect on asthma emergency department
visits. In models that controlled for upper respiratory infection
visits, associations between O3 and emergency department
visits remained statistically significant (Strickland et al., Table 4
in published study), demonstrating a relatively limited influence of
respiratory infections on the association observed between short-term
O3 exposures and asthma emergency department visits,
contrary to the commenter's claim.
In addition, with regard to the criticism of the results reported
by Mar and Koenig, the EPA disagrees with commenters who questioned the
appropriateness of a zero-day lag. These commenters specifically noted
uncertainty in the relative timing of the O3 exposure and
the emergency department visit when they occurred on the same day.
However, based on the broader body of evidence the ISA concludes that
the strongest support is for a relatively immediate respiratory
response following O3 exposures. Specifically, the ISA
states that ``[t]he collective evidence indicates a rather immediate
response within the first few days of O3 exposure (i.e., for
lags days averaged at 0-1, 0-2, and 0-3 days) for hospital admissions
and [emergency department] visits for all respiratory outcomes, asthma,
and chronic obstructive pulmonary disease in all-year and seasonal
analyses'' (U.S. EPA, 2013, p. 2-32). Thus, the use of a zero-day lag
is consistent with the broader body of evidence supporting the
occurrence of O3-associated health effects. In addition,
while Mar and Koenig reported the strongest associations for zero-day
lags, they also reported positive associations for lags ranging from
zero to five days (Mar and Koenig, 2009, Table 5 in the published
study). In considering this study, the ISA stated that Mar and Koenig
(2009) ``found consistent positive associations across individual lag
days'' and that ``[f]or children, consistent positive associations were
observed across all lags . . . with the strongest associations observed
at lag 0 (33.1% [95% CI: 3.0, 68.5]) and lag 3 (36.8% [95% CI: 6.1,
77.2])'' (U.S. EPA, 2013, p. 6-150). Given support for a relatively
immediate response to O3 and given the generally consistent
results in analyses using various lags, we disagree with commenters who
asserted that the use of a zero-day lag represents an important
uncertainty in the interpretation of the study by Mar and Koenig
(2009).
Given all of the above, we do not agree with commenters who
asserted that uncertainties in the epidemiologic evidence in general,
or in specific key studies, should preclude the
[[Page 65337]]
Administrator from relying on those studies to inform her decisions on
the primary O3 standard.
Some commenters also objected to the characterization in the ISA
and the proposal that the results of epidemiologic studies are
consistent. These commenters contended that the purported consistency
of results across epidemiologic studies is the result of inappropriate
selectivity on the part of the EPA in focusing on specific studies and
specific results within those studies. In particular, commenters
contend that EPA favors studies that show positive associations and
selectively ignores certain studies that report null results. They also
cite a study published after the completion of the ISA (Goodman et al.,
2013) suggesting that, in papers where the results of more than one
statistical model are reported, the EPA tends to report the results
with the strongest associations.
The EPA disagrees that it has inappropriately focused on specific
positive studies or specific positive results within individual
studies. The ISA appropriately builds upon the assessment of the
scientific evidence presented in previous AQCDs and ISAs.\99\ When
evaluating new literature, ``[s]election of studies for inclusion in
the ISA is based on the general scientific quality of the study, and
consideration of the extent to which the study is informative and
policy-relevant'' (U.S. EPA, 2013, p. liii). In addition, ``the intent
of the ISA is to provide a concise review, synthesis, and evaluation of
the most policy-relevant science to serve as a scientific foundation
for the review of the NAAQS, not extensive summaries of all health,
ecological and welfare effects studies for a pollutant'' (U.S. EPA,
2013, p. lv). Therefore, not all studies published since the previous
review would be appropriate for inclusion in the ISA.\100\ With regard
to the specific studies that are included in the ISA, and the analyses
focused upon within given studies, the EPA notes that the ISA undergoes
extensive peer review in a public setting by the CASAC. This process
provides ample opportunity for CASAC and the public to comment on
studies not included in the ISA, and on the specific analyses focused
upon within individual studies. In endorsing the final O3
ISA as adequate for rule-making purposes, CASAC agreed with the
selection and presentation of analyses on which to base the ISA's key
conclusions.
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\99\ Cf. Coalition for Responsible Regulation v. EPA, 684 F. 3d
102, 119 (D.C. Cir. 2012) (aff'd in part and rev'd in part on other
grounds sub. nom UARG v. EPA, S Ct. (2014)) (``EPA simply did here
what it and other decision-makers often must do to make a science-
based judgment: it sought out and reviewed existing scientific
evidence to determine whether a particular finding was warranted. It
makes no difference that much of the scientific evidence in large
part consisted of `syntheses' of individual studies and research.
Even individual studies and research papers often synthesize past
work in an area and then build upon it. That is how science
works'').
\100\ See also section II.C.4.b below responding to comments
from environmental interests that EPA inappropriately omitted many
studies which (in their view) support establishing a revised
standard at a level of 60 ppb or lower. Although, as explained
there, the EPA disagrees with these comments, the comments
illustrate that the EPA was even-handed in its consideration of the
epidemiologic evidence, and most certainly did not select merely
studies favorable to the point of view of revising the current
standard.
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iii. Evidence Pertaining to At-Risk Populations and Lifestages
A number of groups submitted comments on the EPA's identification
of at-risk populations and lifestages. Some industry commenters who
opposed revising the current standard disagreed with the EPA's
identification of people with asthma or other respiratory diseases as
an at-risk population for O3-attributable effects, citing
controlled human exposure studies that did not report larger
O3-induced FEV1 decrements in people with asthma
than in people without asthma. In contrast, comments from medical,
environmental, and public health groups generally agreed with the at-
risk populations identified by EPA, and also identified other
populations that they stated should be considered at risk, including
people of lower socio-economic status, people with diabetes or who are
obese, pregnant women (due to reproductive and developmental effects,
and African American, Asian, Hispanic/Latino or tribal communities. As
support for the additional populations, these commenters cited various
studies, including some that were not included in the ISA (which we
have provisionally considered, as described in section I.C above).
With regard to the former group of comments stating that the
evidence does not support the identification of asthmatics as an at-
risk population, we disagree. As summarized in the proposal, the EPA's
identification of populations at risk of O3 effects is based
on a systematic approach that assesses the current scientific evidence
across the relevant scientific disciplines (i.e., exposure sciences,
dosimetry, controlled human exposure, toxicology, and epidemiology),
with a focus on studies that conducted stratified analyses allowing for
an evaluation of different populations exposed to similar O3
concentrations within the same study design (U.S. EPA, 2013, pp. 8-1 to
8-3). Based on this established process and framework, the ISA
identifies individuals with asthma among the populations and lifestages
for which there is ``adequate'' evidence to support the conclusion of
increased risk of O3-related health effects. Other
populations for which the evidence is adequate are individuals with
certain genotypes, younger and older age groups, individuals with
reduced intake of certain nutrients, and outdoor workers. These
conclusions are based on consistency in findings across studies and
evidence of coherence in results from different scientific disciplines.
For example, with regard to people with asthma, the ISA notes a
number of epidemiologic and controlled human exposure studies reporting
larger and/or more serious effects in people with asthma than in people
without asthma or other respiratory diseases. These include
epidemiologic studies of lung function, respiratory symptoms, and
medication use, as well as controlled human exposure studies showing
larger inflammatory responses and markers indicating altered immune
functioning in people with asthma, and also includes evidence from
animal models of asthma that informs the EPA's interpretation of the
other studies. We disagree with the industry commenters' focus solely
on the results of certain studies without an integrated consideration
of the broader body of evidence, and wider range of respiratory
endpoints. It is such an integrated approach that supports EPA's
conclusion that ``there is adequate evidence for asthmatics to be an
at-risk population'' (U.S. EPA, 2013, section 8.2.2).
We also disagree with commenters' misleading reference to various
studies cited to support the claim that asthmatics are not at increased
risk of O3-related health effects. One of the controlled
human studies cited in those comments (Mudway et al. 2001) involved
asthmatic adults who were older than the healthy controls, and it is
well-recognized that responses to O3 decrease with age (U.S.
EPA, 2014c, p. 3-80). Another study (Alexis et al. 2000) used subjects
with mild asthma who are unlikely to be as responsive as people with
more severe disease (Horstman et al., 1995) (EPA 2014c, p. 3-80).
Controlled human exposure studies and epidemiologic studies of adults
and children amply confirm that ``there is adequate evidence for
asthmatics to be an at-risk population'' (U.S. EPA, 2014c, p. 3-81).
[[Page 65338]]
We also do not agree with the latter group of commenters that there
is sufficient evidence to support the identification of additional
populations as at risk of O3-attributable health effects.
Specifically with regard to pregnant women, the ISA concluded that the
``evidence is suggestive of a causal relationship between exposures to
O3 and reproductive and developmental effects'' including
birth outcomes, noting that ``the collective evidence for many of the
birth outcomes examined is generally inconsistent'' (U.S. EPA, 2013,
pp. 7-74 and 7-75). At the time of the completion of the ISA, no
studies had been identified that examined the relationship between
exposure to O3 and the health of pregnant women (e.g.,
studies on pre-eclampsia, gestational hypertension). Due to the
generally inconsistent epidemiologic evidence for effects on birth
outcomes, the lack of studies on the health of pregnant women, and the
lack of studies from other disciplines to provide biological
plausibility for the effects examined in epidemiologic studies,
pregnant women were not considered an at-risk population. Based on the
EPA's provisional consideration of studies published since the
completion of the ISA (I.C, above), recent studies that examine
exposure to O3 and pre-eclampsia and other health effects
experienced by pregnant women are not sufficient to materially change
the ISA's conclusions on at-risk populations (I.C, above). In addition,
as summarized in the proposal, the ISA concluded that the evidence for
other populations was either suggestive of increased risk, with further
investigation needed (e.g., other genetic variants, obesity, sex, and
socioeconomic status), or was inadequate to determine if they were of
increased risk of O3-related health effects (influenza/
infection, COPD, CVD, diabetes, hyperthyroidism, smoking, race/
ethnicity, and air conditioning use) (U.S. EPA, 2013, section 2.5.4.1).
The CASAC has concurred with the ISA conclusions (Frey, 2014c).
c. Comments on Exposure and Risk Assessments
This section discusses major comments on the EPA's quantitative
assessments of O3 exposures and health risks, presented in
the HREA and considered in the PA, and the EPA's responses to those
comments. The focus in this section is on overarching comments related
to the EPA's approach to assessing exposures and risks, and to
interpreting the exposure/risk results within the context of the
adequacy of the current primary O3 standard. More detailed
discussion of comments and Agency responses is provided in the Response
to Comments document. Section II.B.2.c.i discusses comments on
estimates of O3 exposures of concern, section II.B.2.c.ii
discusses comments on estimates of the risk of O3-induced
lung function decrements, and section II.B.2.b.iii discusses comments
on estimates of the risk of O3-associated mortality and
morbidity.
i. O3 Exposures of Concern
The EPA received a number of comments expressing divergent views on
the estimation of, and interpretation of, O3 exposures of
concern. In general, comments from industry, business, and some state
groups opposed to revising the current primary O3 standard
asserted that the approaches and assumptions that went into the HREA
assessment result in overestimates of O3 exposures. These
commenters highlighted several aspects of the assessment, asserting
that the HREA overestimates the proportion of the population expected
to achieve ventilation rates high enough to experience an exposure of
concern; that the use of out-of-date information on activity patterns
results in overestimates of the amount of time people spend being
active outdoors; and that exposure estimates do not account for the
fact that people spend more time indoors on days with bad air quality
(i.e., they engage in averting behavior). In contrast, comments from
medical, public health, and environmental groups that supported
revision of the current standard asserted that the HREA assessment of
exposures of concern, and the EPA's interpretation of exposure
estimates, understates the potential for O3 exposures that
could cause adverse health effects. These commenters claimed that the
EPA's focus on 8-hour exposures understates the O3 impacts
on public health since effects in controlled human exposure studies
were shown following 6.6-hour exposures; that the HREA exposure
estimates do not capture the most highly exposed populations, such as
highly active children and outdoor workers; and that the EPA's
interpretation of estimated exposures of concern impermissibly relies
on the assumption that people stay indoors to avoid dangerous air
pollution (i.e., that they engage in averting behavior).
In considering these comments, the EPA first notes that as
discussed in the HREA, PA, and the proposal, there are aspects of the
exposure assessment that, considered by themselves, can result in
either overestimates or underestimates of the occurrence of
O3 exposures of concern. Commenters tended to highlight the
aspects of the assessment that supported their positions, including
aspects that were discussed in the HREA and/or the PA and that were
considered by CASAC. In contrast, commenters tended to ignore the
aspects of the assessment that did not support their positions. The EPA
has carefully described and assessed the significance of the various
uncertainties in the exposure analysis (U.S. EPA, 2014a, Table 5-10),
noting that, in most instances, the uncertainties could result in
either overestimates or underestimates of exposures and that the
magnitudes of the impacts on exposure results were either ``low,''
``low to moderate,'' or ``moderate'' (U.S. EPA, 2014a, Table 5-10).
Consistent with the characterization of uncertainties in the HREA,
PA, and the proposal, the EPA agrees with some, though not all, aspects
of these commenters' views. For example, the EPA agrees with the
comment by groups opposed to revision that the equivalent ventilation
rate (EVR) used to characterize individuals as at moderate or greater
exertion in the HREA likely leads to overestimates of the number of
individuals experiencing exposures of concern (U.S. EPA, 2014a, Table
5-10, p. 5-79). In addition, we note that other physiological processes
that are incorporated into exposure estimates are also identified in
the HREA as likely leading to overestimates of O3 exposures,
based on comparisons with the available scientific literature (U.S.
EPA, 2014a, Table 5-10, p. 5-79). These aspects of the exposure
assessment are estimated to have either a ``moderate'' (i.e., EVR) or a
``low to moderate'' (i.e., physiological processes) impact on exposure
estimates (U.S. EPA, 2014a, Table 5-10, p. 5-79). Focusing on these
aspects of the assessment, by themselves, could lead to the conclusion
that the HREA overstates the occurrence of O3 exposures of
concern.
However, the EPA notes that there are also aspects of the HREA
exposure assessment that, taken by themselves, could lead to the
conclusion that the HREA understates the occurrence of O3
exposures of concern. For example, as noted above, some medical, public
health, and environmental groups asserted that the exposure assessment
could underestimate O3 exposures for highly active
populations, including outdoor workers and children who spend a large
portion of time outdoors during summer. In support of these assertions,
commenters highlighted sensitivity analyses conducted in the HREA.
However, as noted in the HREA (U.S. EPA, 2014a, Table 5-10), this
[[Page 65339]]
aspect of the assessment is likely to have a ``low to moderate'' impact
on exposure estimates (i.e., a smaller impact than uncertainty
associated with the EVR, and similar in magnitude to uncertainties
related to physiological processes, as noted above). Therefore, when
considered in the context of all of the uncertainties in exposure
estimates, it is unlikely that the HREA's approach to using data on
activity patterns leads to overall underestimates of O3
exposures. The implications of this uncertainty are discussed in more
detail below (II.C.4.b), within the context of the Administrator's
decision on a revised standard level.
In addition, medical, public health, and environmental groups also
pointed out that the controlled human exposures studies that provided
the basis for health effect benchmarks were conducted in healthy
adults, rather than at-risk populations, and these studies evaluated
6.6 hour exposures, rather than the 8-hour exposures evaluated in the
HREA exposure analyses. They concluded that adverse effects would occur
at lower exposure concentrations in at-risk populations, such as people
with asthma, and if people were exposed for 8 hours, rather than 6.6
hours. In its review of the PA, CASAC clearly recognized these
uncertainties, which provided part of the basis for CASAC's advice to
consider exposures of concern for the 60 ppb benchmark. For example,
when considering the results of the study by Schelegle et al. (2009)
for 6.6-hour exposures to an average O3 concentration of 72
ppb, CASAC judged that if subjects had been exposed for eight hours,
the adverse combination of lung function decrements and respiratory
symptoms ``could have occurred'' at lower O3 exposure
concentrations (Frey, 2014c, p. 5). With regard to at-risk populations,
CASAC concluded that ``based on results for clinical studies of healthy
adults, and scientific considerations of differences in responsiveness
of asthmatic children compared to healthy adults, there is scientific
support that 60 ppb is an appropriate exposure of concern for asthmatic
children'' (Frey, 2014c, p. 8). As discussed below (II.B.3, II.C.4.b,
II.C.4.c), based in large part on CASAC advice, the Administrator does
consider exposure results for the 60 ppb benchmark.
Thus, rather than viewing the potential implications of various
aspects of the HREA exposure assessment in isolation, as was done by
many commenters, the EPA considers them together, along with other
issues and uncertainties related to the interpretation of exposure
estimates. As discussed above, CASAC recognized the key uncertainties
in exposure estimates, as well as in the interpretation of those
estimates in the HREA and PA (Frey, 2014a, c). In its review of the 2nd
draft REA, CASAC concluded that ``[t]he discussion of uncertainty and
variability is comprehensive, appropriately listing the major sources
of uncertainty and their potential impacts on the APEX exposure
estimates'' (Frey, 2014a, p. 6). Even considering these and other
uncertainties, CASAC emphasized estimates of O3 exposures of
concern as part of the basis for their recommendations on the primary
O3 NAAQS. In weighing these uncertainties, which can bias
exposure results in different directions but tend to have impacts that
are similar in magnitude (U.S. EPA, 2014a, Table 5-10), and in light of
CASAC's advice based on its review of the HREA and the PA, the EPA
continues to conclude that the approach to considering estimated
exposures of concern in the HREA, PA, and the proposal reflects an
appropriate balance, and provides an appropriate basis for considering
the public health protectiveness of the primary O3 standard.
The EPA disagrees with other aspects of commenters' views on HREA
estimates of exposures of concern. For example, commenters on both
sides of the issue objected to the EPA's handling of averting behavior
in exposure estimates. Some commenters who supported retaining the
current standard claimed that the HREA overstates exposures of concern
because available time-location-activity data do not account for
averting behavior. These commenters noted sensitivity analyses in the
HREA that estimated fewer exposures of concern when averting behavior
was considered. In contrast, commenters supporting revision of the
standard criticized the EPA's estimates of exposures of concern,
claiming that the EPA ``emphasizes the role of averting behavior,
noting that it may result in an overestimation of exposures of concern,
and cites this behavior (essentially staying indoors or not exercising)
in order to reach what it deems an acceptable level of risk'' (e.g.,
ALA et al., p. 120).
The EPA disagrees with both of these comments. In brief, the NAAQS
must ``be established at a level necessary to protect the health of
persons,'' not the health of persons refraining from normal activity or
resorting to medical interventions to ward off adverse effects of poor
air quality (S. Rep. No. 11-1196, 91st Cong. 2d Sess. at 10). On the
other hand, ignoring normal activity patterns for a pollutant like
O3, where adverse responses are critically dependent on
ventilation rates, will result in a standard which provides more
protection than is requisite. This issue is discussed in more detail
below (II.C.4.b), within the context of the Administrator's decision on
a revised standard level.
These commenters also misconstrue the EPA's limited sensitivity
analyses on impacts of averting behavior in the HREA. The purpose of
the HREA sensitivity analyses was to provide perspective on the
potential role of averting behavior in modifying O3
exposures. These sensitivity analyses were limited to a single urban
study area, a 2-day period, and a single air quality adjustment
scenario (U.S. EPA, 2014a, section 5.4.3.3). In addition, the approach
used in the HREA to simulate averting behavior was itself uncertain,
given the lack of actual activity pattern data that explicitly
incorporated this type of behavioral response. In light of these
important limitations, sensitivity analyses focused on averting
behavior were discussed in the proposal within the context of the
discussion of uncertainties in the HREA assessment of exposures of
concern (II.C.2.b in the proposal) and, contrary to the claims of some
commenters, they were not used to support the proposed decision.
Some industry groups also claimed that the time-location-activity
diaries used by APEX to estimate exposures are out-of-date, and do not
represent activity patterns in the current population. These commenters
asserted that the use of out-of-date diary information leads to
overestimates in exposures of concern. This issue was explicitly
addressed in the HREA and the EPA disagrees with commenters'
conclusions. In particular, diary data was updated in this review to
include data from studies published as late as 2010, directly in
response to CASAC concerns. In their review of this data, CASAC stated
that ``[t]he addition of more recent time activity pattern data
addresses a concern raised previously by the CASAC concerning how
activity pattern information should be brought up to date'' (Frey,
2014a, p. 8). As indicated in the HREA (U.S. EPA, 2014a, Appendix 5G,
Figures 5G-7 and Figure 5G-8), the majority of diary days used in
exposure simulations of children originate from the most recently
conducted activity pattern studies (U.S. EPA, 2014a, Table 5-3). In
addition, evaluations included in the HREA indicated that there were
not major systematic differences in time-location-activity patterns
based on information from older diaries versus those collected more
recently (U.S. EPA,
[[Page 65340]]
2014a, Appendix 5G, Figures 5G-1 and 5G-2). Given all of the above, the
EPA does not agree with commenters who claimed that the time-location-
activity diaries used by APEX are out-of-date, and result in
overestimates of exposures of concern.
ii. Risk of O3-Induced FEV1 Decrements
The EPA also received a large number of comments on the
FEV1 risk assessment presented in chapter 6 of the HREA
(U.S. EPA, 2014a) and summarized in the proposal (II.C.3.a in the
proposal). Commenters representing medical, public health, and
environmental groups generally expressed the view that these risk
estimates support the need to revise the current primary O3
standard in order to increase public health protection, though these
groups also questioned some of the assumptions inherent in the EPA's
interpretation of those risk estimates. For example, ALA et al. (p.
127) stated that ``[t]he HREA uses a risk function derived from a
controlled human exposure study of healthy young adults to estimate
lung function decrements in children, including children with asthma.
This assumption could result in an underestimate of risk.'' On this
same issue, commenters representing industry groups opposed to revising
the standard also asserted that assumptions about children's responses
to O3 exposures are highly uncertain. In contrast to medical
and public health groups, these commenters concluded that this
uncertainty, along with others discussed below, call into question the
use of FEV1 risk estimates to support a decision to revise
the current primary O3 standard.
The EPA agrees that an important source of uncertainty is the
approach to estimating the risk of FEV1 decrements in
children and in children with asthma based on data from healthy adults.
However, this issue is discussed at length in the HREA and the PA, and
was considered carefully by CASAC in its review of draft versions of
these documents. The conclusions of the HREA and PA, and the advice of
CASAC, were reflected in the Administrator's interpretation of
FEV1 risk estimates in the proposal, as described below.
Commenters have not provided additional information that changes the
EPA's views on this issue.
As discussed in the proposal (II.C.3.a.ii in the proposal), in the
near absence of controlled human exposure data for children, risk
estimates are based on the assumption that children exhibit the same
lung function response following O3 exposures as healthy 18-
year olds (i.e., the youngest age for which sufficient controlled human
exposure data is available) (U.S. EPA, 2014a, section 6.5.3). As noted
by CASAC (Frey, 2014a, p. 8), this assumption is justified in part by
the findings of McDonnell et al. (1985), who reported that children (8-
11 years old) experienced FEV1 responses similar to those
observed in adults (18-35 years old). The HREA concludes that this
approach could result in either over- or underestimates of
O3-induced lung function decrements in children, depending
on how children compare to the adults used in controlled human exposure
studies (U.S. EPA, 2014a, section 6.5.3). With regard to people with
asthma, although the evidence has been mixed (U.S. EPA, 2013, section
6.2.1.1), several studies have reported statistically larger, or a
tendency for larger, O3-induced lung function decrements in
asthmatics than in non-asthmatics (Kreit et al., 1989; Horstman et al.,
1995; Jorres et al., 1996; Alexis et al., 2000). On this issue, CASAC
noted that ``[a]sthmatic subjects appear to be at least as sensitive,
if not more sensitive, than non-asthmatic subjects in manifesting
O3-induced pulmonary function decrements'' (Frey, 2014c, p.
4). To the extent asthmatics experience larger O3-induced
lung function decrements than the healthy adults used to develop
exposure-response relationships, the HREA could underestimate the
impacts of O3 exposures on lung function in asthmatics,
including asthmatic children (U.S. EPA, 2014a, section 6.5.4). As noted
above, these uncertainties have been considered carefully by the EPA
and by CASAC during the development of the HREA and PA. In addition,
the Administrator has appropriately considered these and other
uncertainties in her interpretation of risk estimates, as discussed
further below (II.B.3, II.C.4.b, II.C.4.c).
Some commenters additionally asserted that the HREA does not
appropriately characterize the uncertainty in risk estimates for
O3-induced lung function decrements. Commenters pointed out
that there is statistical uncertainty in model coefficients that is not
accounted for in risk estimates. One commenter presented an analysis of
this uncertainty, and concluded that there is considerable overlap
between risk estimates for standard levels of 75, 70, and 65 ppb,
undercutting the confidence in estimated risk reductions for standard
levels below 75 ppb.
The Agency recognizes that there are important sources of
uncertainty in the FEV1 risk assessment. In some cases,
these sources of uncertainty can contribute to substantial variability
in risk estimates, complicating the interpretation of those estimates.
For example, as discussed in the proposal, the variability in
FEV1 risk estimates across urban study areas is often
greater than the differences in risk estimates between various standard
levels (Table 2, above and 79 FR 75306 n. 164). Given this, and the
resulting considerable overlap between the ranges of FEV1
risk estimates for different standard levels, in the proposal the
Administrator viewed these risk estimates as providing a more limited
basis than exposures of concern for distinguishing between the degree
of public health protection provided by alternative standard levels.
Thus, although the EPA does not agree with the overall conclusions of
industry commenters, their analysis of statistical uncertainty in risk
estimates, and the resulting overlap between risk estimates for
standard levels of 75, 70, and 65 ppb, tends to reinforce the
Administrator's approach, which places greater weight on estimates of
O3 exposures of concern than on risk estimates for
O3-induced FEV1 decrements.
iii. Risk of O3-Associated Mortality and Morbidity
In the proposal, the Administrator placed the greatest emphasis on
the results of controlled human exposure studies and on quantitative
analyses based on information from these studies, and less weight on
mortality and morbidity risk assessments based on information from
epidemiology studies. The EPA received a number of comments on its
consideration of epidemiology-based risks, with some commenters
expressing support for the Agency's approach and others expressing
opposition.
In general, commenters representing industry organizations or
states opposed to revising the current primary O3 standard
agreed with the Administrator's approach in the proposal to viewing
epidemiology-based risk estimates, though these commenters reached a
different conclusion than the EPA regarding the adequacy of the current
standard. In supporting their views, these commenters highlighted a
number of uncertainties in the underlying epidemiologic studies, and
concluded that risk estimates based on information from such studies do
not provide an appropriate basis for revising the current standard. For
example, commenters noted considerable spatial heterogeneity in health
effect associations; the potential for co-occurring pollutants (e.g.,
PM2.5) to confound O3 health effect associations;
[[Page 65341]]
and the lack of statistically significant O3 health effect
associations in many of the individual cities evaluated as part of
multicity analyses. In contrast, some commenters representing medical,
public health, or environmental organizations placed greater emphasis
than the EPA on epidemiology-based risk estimates. These commenters
asserted that risk estimates provide strong support for a lower
standard level, and pointed to CASAC advice to support their position.
As in the proposal, the EPA continues to place the greatest weight
on the results of controlled human exposure studies and on quantitative
analyses based on information from these studies (particularly
exposures of concern, as discussed below in II.B.3 and II.C.4), and
less weight on risk analyses based on information from epidemiologic
studies. In doing so, the Agency continues to note that controlled
human exposure studies provide the most certain evidence indicating the
occurrence of health effects in humans following specific O3
exposures. In addition, the effects reported in these studies are due
solely to O3 exposures, and interpretation of study results
is not complicated by the presence of co-occurring pollutants or
pollutant mixtures (as is the case in epidemiologic studies). The
Agency further notes the CASAC judgment that ``the scientific evidence
supporting the finding that the current standard is inadequate to
protect public health is strongest based on the controlled human
exposure studies of respiratory effects'' (Frey, 2014c, p. 5).
Consistent with this emphasis, the HREA conclusions reflect relatively
greater confidence in the results of the exposure and risk analyses
based on information from controlled human exposure studies than the
results of epidemiology-based risk analyses. As discussed in the HREA
(U.S. EPA, 2014a, section 9.6), several key uncertainties complicate
the interpretation of these epidemiology-based risk estimates,
including the heterogeneity in O3 effect estimates between
locations, the potential for exposure measurement errors in these
epidemiologic studies, and uncertainty in the interpretation of the
shape of concentration-response functions at lower O3
concentrations. Commenters who opposed the EPA's approach in the
proposal to viewing the results of quantitative analyses tended to
highlight aspects of the evidence and CASAC advice that were considered
by the EPA at the time of proposal and nothing in these commenters'
views has changed those considerations. Therefore, the EPA continues to
place the most emphasis on using the information from controlled human
exposure studies to inform consideration of the adequacy of the primary
O3 standard.
However, while the EPA agrees that there are important
uncertainties in the O3 epidemiology-based risk estimates,
the Agency disagrees with industry commenters that these uncertainties
support a conclusion to retain the current standard. As discussed
below, the decision to revise the current primary O3
standard is based on the EPA's consideration of the broad body of
scientific evidence, quantitative analyses of O3 exposures
and risks, CASAC advice, and public comments. While recognizing
uncertainties in the epidemiology-based risk estimates here, and giving
these uncertainties appropriate consideration, the Agency continues to
conclude that these risk estimates contribute to the broader body of
evidence and information supporting the need to revise the primary
O3 standard.
Some commenters opposed to revising the current O3
standard highlighted the fact that, in a few urban study locations,
larger risks are estimated for standard levels below 75 ppb than for
the current standard with its level of 75 ppb. For example, TCEQ (p. 3)
states that ``differential effects on ozone in urban areas also lead to
the EPA's modeled increases in mortality in Houston and Los Angeles
with decreasing ozone standards.'' These commenters cited such
increases in estimated risk as part of the basis for their conclusion
that the current standard should be retained.
For communities across the U.S. (including in the Houston and Los
Angeles areas), exposure and risk analyses indicate that reducing
emissions of O3 precursors (NOX, VOCs) to meet a
revised standard with a level of 70 ppb will substantially reduce the
occurrence of adverse respiratory effects and mortality risk
attributable to high O3 concentrations (U.S. EPA, 2014a,
Appendix 9A; U.S. EPA, 2014c, sections 4.4.2.1 to 4.4.2.3). However,
because of the complex chemistry governing the formation and
destruction of O3, some NOX control strategies
designed to reduce the highest ambient O3 concentrations can
also result in increases in relatively low ambient O3
concentrations. As a result of the way the EPA's epidemiology-based
risk assessments were conducted (U.S. EPA, 2014a, Chapter 7), increases
estimated in low O3 concentrations impacted mortality and
morbidity risks, leading to the estimated risk increases highlighted by
some commenters. However, while the EPA is confident that reducing the
highest ambient O3 concentrations will result in substantial
improvements in public health, including reducing the risk of
O3-associated mortality, the Agency is far less certain
about the public health implications of the changes in relatively low
ambient O3 concentrations (79 FR at 75278/3, 75291/1, and
75308/2). Therefore, reducing precursor emissions to meet a lower
O3 standard is expected to result in important reductions in
O3 concentrations from the part of the air quality
distribution where the evidence provides the strongest support for
adverse health effects.
Specifically, for area-wide O3 concentrations at or
above 40 ppb,\101\ a revised standard with a level of 70 ppb is
estimated to reduce the number of premature deaths associated with
short-term O3 concentrations by about 10%, compared to the
current standard. In addition, for area-wide concentrations at or above
60 ppb, a revised standard with a level of 70 ppb is estimated to
reduce O3-associated premature deaths by about 50% to
70%.\102\ The EPA views these results, which focus on the portion of
the air quality distribution where the evidence indicates the most
certainty regarding the occurrence of adverse O3-
attributable health effects, not only as supportive of the need to
revise the current standard (II.B.3, below), but also as showing the
benefits of reducing the peak O3 concentrations associated
with air quality distributions meeting the current standard (II.C.4,
below).
---------------------------------------------------------------------------
\101\ The ISA concludes that there is less certainty in the
shape of concentration-response functions for area-wide
O3 concentrations at the lower ends of warm season
distributions (i.e., below about 20 to 40 ppb) (U.S. EPA, 2013,
section 2.5.4.4).
\102\ Available experimental studies provide the strongest
evidence for O3-induced effects following exposures to
O3 concentrations corresponding to the upper portions of
typical ambient distributions. In particular, as discussed above,
controlled human exposure studies showing respiratory effects
following exposures to O3 concentrations at or above 60
ppb.
---------------------------------------------------------------------------
In addition, even considering risk estimates based on the full
distribution of ambient O3 concentrations (i.e., estimates
influenced by decreases in higher concentrations and increases in lower
concentrations), the EPA notes that, compared to the current standard,
standards with lower levels are estimated to result in overall
reductions in mortality risk across the urban study areas evaluated
(U.S. EPA, 2014c, Figure 4-10). As discussed above (II.A.2.a,
II.A.2.c), analyses in the HREA indicate that these overall risk
reductions could understate the actual reductions that
[[Page 65342]]
would be experienced by the U.S. population as a whole.
For example, the HREA's national air quality modeling analyses
indicate that the HREA urban study areas tend to underrepresent the
populations living in areas where reducing NOX emissions
would be expected to result in decreases in warm season averages of
daily maximum 8-hour ambient O3 concentrations.\103\ Given
the strong connection between these warm season average O3
concentrations and risk, risk estimates for the urban study areas are
likely to understate the average reductions in O3-associated
mortality and morbidity risks that would be experienced across the U.S.
population as a whole upon reducing NOX emissions (U.S. EPA,
2014a, section 8.2.3.2).
---------------------------------------------------------------------------
\103\ Specifically, the HREA urban study areas tend to
underrepresent populations living in suburban, smaller urban, and
rural areas, where reducing NOX emissions would be
expected to result in decreases in warm season averages of daily
maximum 8-hour ambient O3 concentrations (U.S. EPA,
2014a, section 8.2.3.2).
---------------------------------------------------------------------------
In addition, in recognizing that the reductions in modeled
NOX emissions used in the HREA's core analyses are meant to
be illustrative, rather than to imply a particular control strategy for
meeting a revised O3 NAAQS, the HREA also conducted
sensitivity analyses in which both NOX and VOC emissions
reductions were evaluated. In all of the urban study areas evaluated in
these analyses, the increases in low O3 concentrations were
smaller for the NOX/VOC emission reduction scenarios than
the NOX only emission reduction scenario (U.S. EPA, 2014a,
Appendix 4D, section 4.7). This was most apparent for Denver, Houston,
Los Angeles, New York, and Philadelphia. These results suggest that in
some locations, optimized emissions reduction strategies could result
in larger reductions in O3-associated mortality and
morbidity than indicated by HREA's core estimates.
Thus, the patterns of estimated mortality and morbidity risks
across various air quality scenarios and locations have been evaluated
and considered extensively in the HREA and the PA, as well as in the
proposal. Epidemiology-based risk estimates have also been considered
by CASAC, and those considerations are reflected in CASAC's advice.
Specifically, in considering epidemiology-based risk estimates in its
review of the REA, CASAC stated that ``[a]lthough these estimates for
short-term exposure impacts are subject to uncertainty, the CASAC is
confident that that the evidence of health effects of O3
presented in the ISA and Second Draft HREA in its totality, indicates
that there are meaningful reductions in mean, absolute, and relative
premature mortality associated with short-term exposures to
O3 levels lower than the current standard'' (Frey, 2014a, p.
3). Commenters' views on this issue are not based on new information,
but on an interpretation of the analyses presented in the HREA that is
different from the EPA's, and CASAC's, interpretation. Given this, the
EPA's considerations and conclusions related to this issue, as
described in the proposal and as summarized briefly above, remain
valid. Therefore, the EPA does not agree with commenters who cited
increases in estimated risk in some locations as supporting a
conclusion that the current standard should be retained.
For risk estimates of respiratory mortality associated with long-
term O3, several industry commenters supported placing more
emphasis on threshold models, and including these models as part of the
core analyses rather than as sensitivity analyses. The EPA agrees with
these commenters that an important uncertainty in risk estimates of
respiratory mortality associated with long-term O3 stems
from the potential for the existence of a threshold. Based on
sensitivity analyses included in the HREA in response to CASAC advice,
the existence of a threshold could substantially reduce estimated
risks. CASAC discussed this issue at length during its review of the
REA and supported the EPA's approach to including a range of threshold
models as sensitivity analyses (Frey, 2014a p. 3). Based in part on
uncertainty in the existence and identification of a threshold, the
HREA concluded that lower confidence should be placed in risk estimates
for respiratory mortality associated with long-term O3
exposures (U.S. EPA, 2014a, section 9.6). This uncertainty was also a
key part of the Administrator's rationale for placing only limited
emphasis on risk estimates for long-term O3 exposures. In
her final decisions, discussed below (II.B.3, II.C.4.b, II.C.4.c), the
Administrator continues to place only limited emphasis on these
estimates. The EPA views this approach to considering risk estimates
for respiratory mortality as generally consistent with the approach
supported by the commenters noted above.
3. Administrator's Conclusions on the Need for Revision
This section discusses the Administrator's conclusions related to
the adequacy of the public health protection provided by the current
primary O3 standard, and her final decision that the current
standard is not requisite to protect public health with an adequate
margin of safety. These conclusions, and her final decision, are based
on the Administrator's consideration of the available scientific
evidence assessed in the ISA (U.S. EPA, 2013), the exposure/risk
information presented and assessed in the HREA (U.S. EPA, 2014a), the
consideration of that evidence and information in the PA (U.S. EPA,
2014c), the advice of CASAC, and public comments received on the
proposal.
As an initial matter, the Administrator concludes that reducing
precursor emissions to achieve O3 concentrations that meet
the current primary O3 standard will provide important
improvements in public health protection, compared to recent air
quality. In reaching this conclusion, she notes the discussion in
section 3.4 of the PA (U.S. EPA, 2014c). In particular, the
Administrator notes that this conclusion is supported by (1) the strong
body of scientific evidence indicating a wide range of adverse health
outcomes attributable to exposures to O3 at concentrations
commonly found in the ambient air and (2) estimates indicating
decreased occurrences of O3 exposures of concern and
decreased O3-associated health risks upon meeting the
current standard, compared to recent air quality. Thus, she concludes
that it would not be appropriate in this review to consider a standard
that is less protective than the current standard.
After reaching the conclusion that meeting the current primary
O3 standard will provide important improvements in public
health protection, and that it is not appropriate to consider a
standard that is less protective than the current standard, the
Administrator next considers the adequacy of the public health
protection that is provided by the current standard. In doing so, the
Administrator first notes that studies evaluated since the completion
of the 2006 AQCD support and expand upon the strong body of evidence
that, in the last review, indicated a causal relationship between
short-term O3 exposures and respiratory morbidity outcomes
(U.S. EPA, 2013, section 2.5). This is the strongest causality finding
possible under the ISA's hierarchical system for classifying weight of
evidence for causation. In addition, the Administrator notes that the
evidence for respiratory health effects attributable to long-term
O3 exposures, including the development of asthma in
children, is much stronger than in previous reviews, and the ISA
concludes that there is ``likely to be'' a causal relationship
[[Page 65343]]
between such O3 exposures and adverse respiratory health
effects (the second strongest causality finding).
Together, experimental and epidemiologic studies support
conclusions regarding a continuum of O3 respiratory effects
ranging from small, reversible changes in pulmonary function, and
pulmonary inflammation, to more serious effects that can result in
respiratory-related emergency department visits, hospital admissions,
and premature mortality. Recent animal toxicology studies support
descriptions of modes of action for these respiratory effects and
augment support for biological plausibility for the role of
O3 in reported effects. With regard to mode of action,
evidence indicates that the initial key event is the formation of
secondary oxidation products in the respiratory tract, that antioxidant
capacity may modify the risk of respiratory morbidity associated with
O3 exposure, and that the inherent capacity to quench (based
on individual antioxidant capacity) can be overwhelmed, especially with
exposure to elevated concentrations of O3.
In addition, based on the consistency of findings across studies
and the coherence of results from different scientific disciplines, the
available evidence indicates that certain populations are at increased
risk of experiencing O3-related effects, including the most
severe effects. These include populations and lifestages identified in
previous reviews (i.e., people with asthma, children, older adults,
outdoor workers) and populations identified since the last review
(i.e., people with certain genotypes related to antioxidant and/or
anti-inflammatory status; people with reduced intake of certain
antioxidant nutrients, such as Vitamins C and E).
In considering the O3 exposure concentrations reported
to elicit respiratory effects, as in the proposal, the Administrator
agrees with the conclusions of the PA that controlled human exposure
studies provide the most certain evidence indicating the occurrence of
health effects in humans following specific O3 exposures. In
particular, she notes that the effects reported in controlled human
exposure studies are due solely to O3 exposures, and
interpretation of study results is not complicated by the presence of
co-occurring pollutants or pollutant mixtures (as is the case in
epidemiologic studies). Therefore, consistent with CASAC advice (Frey,
2014c), she places the most weight on information from controlled human
exposure studies in reaching conclusions on the adequacy of the current
primary O3 standard.
In considering the evidence from controlled human exposure studies,
the Administrator first notes that these studies have reported a
variety of respiratory effects in healthy adults following exposures to
O3 concentrations of 60, 63,\104\ 72,\105\ or 80 ppb, and
higher. The largest respiratory effects, and the broadest range of
effects, have been studied and reported following exposures of healthy
adults to 80 ppb O3 or higher, with most exposure studies
conducted at these higher concentrations. As discussed above (II.A.1),
the Administrator further notes that recent evidence includes
controlled human exposure studies reporting the combination of lung
function decrements and respiratory symptoms in healthy adults engaged
in moderate exertion following 6.6-hour exposures to concentrations as
low as 72 ppb, and lung function decrements and pulmonary inflammation
following exposures to O3 concentrations as low as 60 ppb.
---------------------------------------------------------------------------
\104\ For a 60 ppb target exposure concentration, Schelegle et
al. (2009) reported that the actual 6.6-hour mean exposure
concentration was 63 ppb.
\105\ For a 70 ppb target exposure concentration, Schelegle et
al. (2009) reported that the actual 6.6-hour mean exposure
concentration was 72 ppb.
---------------------------------------------------------------------------
As discussed in her response to public comments above (II.B.2.b.i),
and in detail below (II.C.4.b, II.C.4.c), the Administrator concludes
that these controlled human exposure studies indicate that adverse
effects are likely to occur following exposures to O3
concentrations below the level of the current standard. The effects
observed following such exposures are coherent with the serious health
outcomes that have been reported in O3 epidemiologic studies
(e.g., respiratory-related hospital admissions, emergency department
visits), and the Administrator judges that such effects have the
potential to be important from a public health perspective.
In reaching these conclusions, she particularly notes that the
combination of lung function decrements and respiratory symptoms
reported to occur in healthy adults following exposures to 72 ppb
O3 meets ATS criteria for an adverse response (II.B.2.b.i,
above). In specifically considering the 72 ppb exposure concentration,
CASAC noted that ``the combination of decrements in FEV1
together with the statistically significant alterations in symptoms in
human subjects exposed to 72 ppb ozone meets the American Thoracic
Society's definition of an adverse health effect'' (Frey, 2014c, p. 5).
In addition, given that the controlled human exposure study reporting
these results was conducted in healthy adults, CASAC judged that the
adverse combination of lung function decrements and respiratory
symptoms ``almost certainly occur in some people'' (e.g., people with
asthma) following exposures to lower O3 concentrations
(Frey, 2014c, p. 6).
While the Administrator is less certain regarding the adversity of
the lung function decrements and airway inflammation that have been
observed following exposures as low as 60 ppb, as discussed in more
detail elsewhere in this preamble (II.B.2.b.i, II.C.4.b, II.C.4.c), she
judges that these effects also have the potential to be adverse, and to
be of public health importance, particularly if they are experienced
repeatedly. With regard to this judgment, she specifically notes the
ISA conclusion that, while the airway inflammation induced by a single
exposure (or several exposures over the course of a summer) can resolve
entirely, continued inflammation could potentially result in adverse
effects, including the induction of a chronic inflammatory state;
altered pulmonary structure and function, leading to diseases such as
asthma; altered lung host defense response to inhaled microorganisms;
and altered lung response to other agents such as allergens or toxins
(U.S. EPA, 2013, section 6.2.3). Thus, the Administrator becomes
increasingly concerned about the potential for adverse effects at 60
ppb O3 as the number of exposures increases, though she
notes that the available evidence does not indicate a particular number
of occurrences of such exposures that would be required to achieve an
adverse respiratory effect, and that this number is likely to vary
across the population.
In addition to controlled human exposure studies, the Administrator
also considers what the available epidemiologic evidence indicates with
regard to the adequacy of the public health protection provided by the
current primary O3 standard. She notes that recent
epidemiologic studies provide support, beyond that available in the
last review, for associations between short-term O3
exposures and a wide range of adverse respiratory outcomes (including
respiratory-related hospital admissions, emergency department visits,
and mortality) and with total mortality. As discussed above in the EPA
responses to public comments (II.B.2.b.ii), associations with morbidity
and mortality are stronger during the warm or summer months, and remain
robust after adjustment for copollutants (U.S. EPA, 2013, Chapter 6).
[[Page 65344]]
In considering information from epidemiologic studies within the
context of her conclusions on the adequacy of the current standard, the
Administrator specifically considers analyses in the PA that evaluate
the extent to which O3 health effect associations have been
reported for air quality concentrations likely to be allowed by the
current standard. She notes that such analyses can provide insight into
the extent to which the current standard would allow the distributions
of ambient O3 concentrations that provided the basis for
these health effect associations. While the majority of O3
epidemiologic studies evaluated in the PA were conducted in areas that
would have violated the current standard during study periods, as
discussed above (II.B.2.b.ii), the Administrator observes that the
study by Mar and Koenig (2009) reported associations between short-term
O3 concentrations and asthma emergency department visits in
children and adults in a U.S. location that would have met the current
O3 standard over the entire study period.\106\ Based on
this, she notes the conclusion from the PA that the current primary
O3 standard would have allowed the distribution of ambient
O3 concentrations that provided the basis for the
associations with asthma emergency department visits reported by Mar
and Koenig (2009) (U.S. EPA, 2014c, section 3.1.4.2).
---------------------------------------------------------------------------
\106\ The large majority of locations evaluated in U.S.
epidemiologic studies of long-term O3 would have violated
the current standard during study periods, thus providing limited
insight into the adequacy of the current standard (U.S. EPA, 2014c,
section 3.1.4.3).
---------------------------------------------------------------------------
In addition, even in some single-city study locations where the
current standard was violated (i.e., those evaluated in Silverman and
Ito, 2010; Strickland et al., 2010), the Administrator notes that PA
analyses of reported concentration-response functions and available air
quality data support the occurrence of O3-attributable
hospital admissions and emergency department visits on subsets of days
with virtually all ambient O3 concentrations below the level
of the current standard. PA analyses of study area air quality further
support the conclusion that exposures to the ambient O3
concentrations present in the locations evaluated by Strickland et al.
(2010) and Silverman and Ito (2010) could have plausibly resulted in
the respiratory-related emergency department visits and hospital
admissions reported in these studies (U.S. EPA, 2014c, section
3.1.4.2). The Administrator agrees with the PA conclusion that these
analyses indicate a relatively high degree of confidence in reported
statistical associations with respiratory health outcomes on days when
virtually all monitored 8-hour O3 concentrations were 75 ppb
or below. She further agrees with the PA conclusion that although these
analyses do not identify true design values, the presence of
O3-associated respiratory effects on such days provides
insight into the types of health effects that could occur in locations
with maximum ambient O3 concentrations below the level of
the current standard.
Compared to the single-city epidemiologic studies discussed above,
the Administrator notes additional uncertainty in interpreting the
relationships between short-term O3 air quality in
individual study cities and reported O3 multicity effect
estimates. In particular, she judges that the available multicity
effect estimates in studies of short-term O3 do not provide
a basis for considering the extent to which reported O3
health effect associations are influenced by individual locations with
ambient O3 concentrations low enough to meet the current
O3 standard, versus locations with O3
concentrations that violate this standard.\107\ While such
uncertainties limit the extent to which the Administrator bases her
conclusions on air quality in locations of multicity epidemiologic
studies, she does note that O3 associations with respiratory
morbidity or premature mortality have been reported in several
multicity studies when the majority of study locations (though not all
study locations) would have met the current O3 standard
(U.S. EPA, 2014c, section 3.1.4.2).
---------------------------------------------------------------------------
\107\ As noted in the proposal (II.E.4.d), this uncertainty
applies specifically to interpreting air quality analyses within the
context of multicity effect estimates for short-term O3
concentrations, where effect estimates for individual study cities
are not presented (as is the case for the key O3 studies
analyzed in the PA, with the exception of the study by Stieb et al.
(2009) where none of the city-specific effect estimates for asthma
emergency department visits were statistically significant). This
specific uncertainty does not apply to multicity epidemiologic
studies of long-term O3 concentrations, where multicity
effect estimates are based on comparisons across cities. For
example, see discussion of study by Jerrett et al. (2009) in the PA
(U.S. EPA, 2014c, section 3.1.4.3).
---------------------------------------------------------------------------
Looking across the body of epidemiologic evidence, the
Administrator thus reaches the conclusion that analyses of air quality
in study locations support the occurrence of adverse O3-
associated effects at ambient O3 concentrations that met, or
are likely to have met, the current standard. She further concludes
that the strongest support for this conclusion comes from single-city
studies of respiratory-related hospital admissions and emergency
department visits associated with short-term O3
concentrations, with some support also from multicity studies of
morbidity or mortality.
Taken together, the Administrator concludes that the scientific
evidence from controlled human exposure and epidemiologic studies calls
into question the adequacy of the public health protection provided by
the current standard. In reaching this conclusion, she particularly
notes that the current standard level is higher than the lowest
O3 exposure concentration shown to result in the adverse
combination of lung function decrements and respiratory symptoms (i.e.,
72 ppb), and that CASAC concluded that such effects ``almost certainly
occur in some people'' following exposures to O3
concentrations below 72 ppb (Frey, 2014c, p. 6). While she also notes
that the current standard level is well-above the lowest O3
exposure concentration shown to cause respiratory effects (i.e., 60
ppb), she has less confidence that the effects observed at 60 ppb are
adverse (discussed in II.B.2.b.i, II.C.4.b, II.C.4.c). She further
considers these effects, and the extent to which the current primary
O3 standard could protect against them, within the context
of quantitative analyses of O3 exposures (discussed below).
With regard to the available epidemiologic evidence, the Administrator
notes PA analyses of O3 air quality indicating that, while
most O3 epidemiologic studies reported health effect
associations with ambient O3 concentrations that violated
the current standard, a small number of single-city U.S. studies
support the occurrence of asthma-related hospital admissions and
emergency department visits at ambient O3 concentrations
below the level of the current standard, including one study with air
quality that would have met the current standard during the study
period. Some support for such O3 associations is also
provided by multicity studies of morbidity or mortality. The
Administrator further judges that the biological plausibility of
associations with clearly adverse morbidity effects is supported by the
evidence noted above from controlled human exposure studies conducted
at, or in some cases below, typical warm-season ambient O3
concentrations.
Beyond her consideration of the scientific evidence, the
Administrator also considers the results of the HREA exposure and risk
analyses in reaching final conclusions regarding the adequacy of the
current primary O3 standard. In doing so, consistent with
[[Page 65345]]
her consideration of the evidence, she focuses primarily on
quantitative analyses based on information from controlled human
exposure studies (i.e., exposures of concern and risk of O3-
induced FEV1 decrements). Consistent with the considerations
in the PA, and with CASAC advice (Frey, 2014c), she particularly
focuses on exposure and risk estimates in children.\108\ As discussed
in the HREA and PA (and II.B, above), the patterns of exposure and risk
estimates across urban study areas, across years, and across air
quality scenarios are similar in children and adults though, because
children spend more time being physically active outdoors and are more
likely to experience the types of O3 exposures shown to
cause respiratory effects, larger percentages of children are estimated
to experience exposures of concern and O3-induced
FEV1 decrements. Children also have intrinsic risk factors
that make them particularly susceptible to O3-related
effects (e.g., higher ventilation rates relative to lung volume) (U.S.
EPA, 2013, section 8.3.1.1; see section II.A.1.d above). In focusing on
exposure and risk estimates in children, the Administrator recognizes
that the exposure patterns for children across years, urban study
areas, and air quality scenarios are indicative of the exposure
patterns in a broader group of at-risk populations that also includes
asthmatic adults and older adults. She judges that, to the extent the
primary O3 standard provides appropriate protection for
children, it will also do so for adult populations,\109\ given the
larger exposures and intrinsic risk factors in children.
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\108\ She focuses on estimates for all children and estimates
for children with asthma, noting that exposure and risk estimates
for these groups are virtually indistinguishable in terms of the
percent estimated to experience exposures of concern or
O3-induced FEV1 decrements (U.S. EPA, 2014c,
sections 3.2 and 4.4.2).
\109\ As noted below (II.C.4.2), this includes populations of
highly active adults, such as outdoor workers. Limited sensitivity
analyses in the HREA indicate that when diaries were selected to
mimic exposures that could be experienced by outdoor workers, the
percentages of modeled individuals estimated to experience exposures
of concern were generally similar to the percentages estimated for
children (i.e., using the full database of diary profiles) in the
urban study areas and years with the largest exposure estimates
(U.S. EPA, 2014, section 5.4.3.2, Figure 5-14).
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In first considering estimates of exposures of concern, the
Administrator considers the extent to which estimates indicate that the
current standard limits population exposures to the broader range of
O3 concentrations shown in controlled human exposure studies
to cause respiratory effects. In doing so, she focuses on estimates of
O3 exposures of concern at or above the benchmark
concentrations of 60, 70, and 80 ppb. She notes that the current
O3 standard can provide some protection against exposures of
concern to a range of O3 concentrations, including
concentrations below the standard level, given that (1) with the
current fourth-high form, most days will have concentrations below the
standard level and that (2) exposures of concern depend on both the
presence of relatively high ambient O3 concentrations and on
activity patterns in the population that result in exposures to such
high concentrations while at an elevated ventilation rate (discussed in
detail below, II.C.4.b and II.C.4.c).
In considering estimates of O3 exposures of concern
allowed by the current standard, she notes that while single exposures
of concern could be adverse for some people, particularly for the
higher benchmark concentrations (70, 80 ppb) where there is stronger
evidence for the occurrence of adverse effects (II.B.2.b.i, II.C.4.b,
II.C.4.c, below), she becomes increasingly concerned about the
potential for adverse responses as the number of occurrences
increases.\110\ In particular, as discussed above with regard to
inflammation, she notes that the types of lung injury shown to occur
following exposures to O3 concentrations from 60 to 80 ppb,
particularly if experienced repeatedly, provide a mode of action by
which O3 may cause other more serious effects (e.g., asthma
exacerbations). Therefore, the Administrator places the most weight on
estimates of two or more exposures of concern (i.e., as a surrogate for
the occurrence of repeated exposures), though she also considers
estimates of one or more exposures for the 70 and 80 ppb benchmarks.
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\110\ Not all people who experience an exposure of concern will
experience an adverse effect (even members of at-risk populations).
For the endpoints evaluated in controlled human exposure studies,
the number of those experiencing exposures of concern who will
experience adverse effects cannot be reliably quantified.
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In considering estimates of exposures of concern, the Administrator
first notes that if the 15 urban study areas evaluated in the HREA were
to just meet the current O3 standard, fewer than 1% of
children in those areas would be estimated to experience two or more
exposures of concern at or above 70 ppb, based on exposure estimates
averaged over the years of analysis, though up to about 2% would be
estimated to experience such exposures in the worst-case year and
location (i.e., year and location with the largest exposure
estimates).\111\ Although the Administrator is less concerned about
single occurrences of exposures of concern, she notes that even single
occurrences could cause adverse effects in some people, particularly
for the 70 and 80 ppb benchmarks.\112\ As illustrated in Table 1
(above), the current standard could allow up to about 3% of children to
experience one or more exposures of concern at or above 70 ppb,
averaged over the years of analysis, and up to about 8% in the worst-
case year and location. In addition, in the worst-case year and
location, the current standard could allow about 1% of children to
experience at least one exposure of concern at or above 80 ppb, the
highest benchmark evaluated.
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\111\ Virtually no children in those areas would be estimated to
experience two or more exposures of concern at or above 80 ppb.
\112\ That is, adverse effects are a possible outcome of single
exposures of concern at/above 70 or 80 ppb, though the available
information is not sufficient to estimate the likelihood of such
effects.
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While the Administrator has less confidence in the adversity of the
effects observed following exposures to 60 ppb O3
(II.B.2.b.i, II.C.4.b, II.C.4.c), particularly for single exposures,
she judges that the potential for adverse effects increases as the
number of exposures of concern increases. With regard to the 60 ppb
benchmark, she particularly notes that the current standard is
estimated to allow approximately 3 to 8% of children in urban study
areas, including approximately 3 to 8% of asthmatic children, to
experience two or more exposures of concern to O3
concentrations at or above 60 ppb, based on estimates averaged over the
years of analysis. To provide some perspective on the average
percentages estimated, the Administrator notes that they correspond to
almost 900,000 children in urban study areas, including about 90,000
asthmatic children. Nationally, if the current standard were to be just
met, the number of children experiencing such exposures would be
larger.
Based on her consideration of these estimates within the context of
her judgments on adversity, as discussed in her responses to public
comments (II.B.2.b.i, II.C.4.b), the Administrator concludes that the
exposures projected to remain upon meeting the current standard can
reasonably be judged to be important from a public health perspective.
In particular, given that the average percent of children estimated to
experience two or more exposures of concern for the 60 ppb benchmark
approaches 10% in some areas, even based on estimates averaged over the
[[Page 65346]]
years of the analysis, she concludes that the current standard does not
incorporate an adequate margin of safety against the potentially
adverse effects that can occur following repeated exposures at or above
60 ppb. Although she has less confidence that the effects observed at
60 ppb are adverse, compared to the effects at and above 72 ppb, she
judges that this approach to considering the results for the 60 ppb
benchmark is appropriate given CASAC advice, which clearly focuses the
EPA on considering the effects observed at 60 ppb (Frey, 2014c)
(II.C.4.b, II.C.4.c below).\113\ This approach to considering estimated
exposures of concern is consistent with setting standards that provide
some safeguard against dangers to human health that are not fully
certain (i.e., standards that incorporate an adequate margin of safety)
(See, e.g., State of Mississippi, 744 F. 3d at 1353).
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\113\ Though this advice is less clear regarding the adversity
of effects at 60 ppb than CASAC's advice regarding the adversity of
effects at 72 ppb (II.C.4.b, II.C.4.c).
---------------------------------------------------------------------------
In addition to estimated exposures of concern, the Administrator
also considers HREA estimates of the risk of O3-induced
FEV1 decrements >=10 and 15%. In doing so, she particularly
notes CASAC advice that ``estimation of FEV1 decrements of
>=15% is appropriate as a scientifically relevant surrogate for adverse
health outcomes in active healthy adults, whereas an FEV1
decrement of >=10% is a scientifically relevant surrogate for adverse
health outcomes for people with asthma and lung disease'' (Frey, 2014c,
p. 3). The Administrator notes that while single occurrences of
O3-induced lung function decrements could be adverse for
some people, as discussed above (II.B.1), she agrees with the judgment
in past reviews that a more general consensus view of the potential
adversity of such decrements emerges as the frequency of occurrences
increases. Therefore, as in the proposal, the Administrator focuses
primarily on the estimates of two or more O3-induced lung
function decrements. When averaged over the years evaluated in the
HREA, the Administrator notes that the current standard is estimated to
allow about 1 to 3% of children in the 15 urban study areas
(corresponding to almost 400,000 children) to experience two or more
O3-induced lung function decrements >=15%, and to allow
about 8 to 12% of children (corresponding to about 180,000 asthmatic
children) to experience two or more O3-induced lung function
decrements >=10%.
In further considering the HREA results, the Administrator
considers the epidemiology-based risk estimates. As discussed in the
proposal, compared to the weight given to HREA estimates of exposures
of concern and lung function risks, she places relatively less weight
on epidemiology-based risk estimates. In giving some consideration to
these risk estimates, as discussed in the proposal and above in the
EPA's responses to public comments (II.B.2.b.iii), the Administrator
focuses on the risks associated with O3 concentrations in
the upper portions of ambient distributions. In doing so, she notes the
increasing uncertainty associated with the shapes of concentration-
response curves for O3 concentrations in the lower portions
of ambient distributions and the evidence from controlled human
exposure studies, which provide the strongest support for
O3-induced effects following exposures to O3
concentrations corresponding to the upper portions of typical ambient
distributions (i.e., 60 ppb and above). Even when considering only
area-wide O3 concentrations from the upper portions of
seasonal distributions (i.e., >=40, 60 ppb, Table 3 in the proposal),
the Administrator notes that the general magnitude of mortality risk
estimates suggests the potential for a substantial number of
O3-associated deaths and adverse respiratory events to occur
nationally, even when the current standard is met (79 FR 75277 and
II.B.2.c.iii above).
In addition to the evidence and exposure/risk information discussed
above, the Administrator also takes note of the CASAC advice in the
current review, in the 2008 review and decision establishing the
current standard, and in the 2010 reconsideration of the 2008 decision.
As discussed in more detail above, the current CASAC ``finds that the
current NAAQS for ozone is not protective of human health'' and
``unanimously recommends that the Administrator revise the current
primary ozone standard to protect public health'' (Frey, 2014c, p. 5).
The prior CASAC O3 Panel likewise recommended revision of
the current standard to one with a lower level due to the lack of
protectiveness of the current standard. This earlier recommendation was
based entirely on the evidence and information in the record for the
2008 standard decision, which, as discussed above, has been
substantially strengthened in the current review (Samet, 2011; Frey and
Samet, 2012).
In consideration of all of the above, the Administrator concludes
that the current primary O3 standard is not requisite to
protect public health with an adequate margin of safety, and that it
should be revised to provide increased public health protection. This
decision is based on the Administrator's conclusions that the available
evidence and exposure and risk information clearly call into question
the adequacy of public health protection provided by the current
primary standard such that it is not appropriate, within the meaning of
section 109(d)(1) of the CAA, to retain the current standard. With
regard to the evidence, she particularly notes that the current
standard level is higher than the lowest O3 exposure
concentration shown to result in the adverse combination of lung
function decrements and respiratory symptoms (i.e., 72 ppb), and also
notes CASAC's advice that at-risk groups (e.g., people with asthma)
could experience adverse effects following exposure to lower
concentrations. In addition, while the Administrator is less certain
about the adversity of the effects that occur following lower exposure
concentrations, she judges that recent controlled human exposure
studies at 60 ppb provide support for a level below 75 ppb in order to
provide an increased margin of safety, compared to the current
standard, against effects with the potential to be adverse,
particularly if they are experienced repeatedly. With regard to
O3 epidemiologic studies, she notes that while most
available studies reported health effect associations with ambient
O3 concentrations that violated the current standard, a
small number provide support for the occurrence of adverse respiratory
effects at ambient O3 concentrations below the level of the
current standard.\114\
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\114\ Courts have repeatedly held that this type of evidence
justifies an Administrator's conclusion that it is ``appropriate''
(within the meaning of section 109 (d)(1) of the CAA) to revise a
primary NAAQS to provide further protection of public health. See
e.g. Mississippi, 744 F. 3d at 1345; American Farm Bureau, 559 F. 3d
at 525-26.
---------------------------------------------------------------------------
Based on the analyses in the HREA, the Administrator concludes that
the exposures and risks projected to remain upon meeting the current
standard can reasonably be judged to be important from a public health
perspective. In particular, this conclusion is based on her judgment
that it is appropriate to set a standard that would be expected to
eliminate, or almost eliminate, exposures of concern at or above 70 and
80 ppb. In addition, given that the average percent of children
estimated to experience two or more exposures of concern for the 60 ppb
benchmark approaches 10% in some urban study areas, the Administrator
concludes that the current standard does not incorporate an adequate
margin of safety
[[Page 65347]]
against the potentially adverse effects that could occur following
repeated exposures at or above 60 ppb. Beyond estimated exposures of
concern, the Administrator concludes that the HREA risk estimates
(FEV1 risk estimates, mortality risk estimates) further
support a conclusion that the O3-associated health effects
estimated to remain upon just meeting the current standard are an issue
of public health importance on a broad national scale. Thus, she
concludes that O3 exposure and risk estimates, when taken
together, support a conclusion that the exposures and health risks
associated with just meeting the current standard can reasonably be
judged important from a public health perspective, such that the
current standard is not sufficiently protective and does not
incorporate an adequate margin of safety.
In the next section, the Administrator considers what revisions are
appropriate in order to set a standard that is requisite to protect
public health with an adequate margin of safety.
C. Conclusions on the Elements of a Revised Primary Standard
Having reached the conclusion that the current O3
standard is not requisite to protect public health with an adequate
margin of safety, based on the currently available scientific evidence
and exposure/risk information, the Administrator next considers the
range of alternative standards supported by that evidence and
information. Consistent with her consideration of the adequacy of the
current standard, the Administrator's conclusions on the elements of
the primary standard are informed by the available scientific evidence
assessed in the ISA, exposure/risk information presented and assessed
in the HREA, the evidence-based and exposure-/risk-based considerations
and conclusions in the PA, CASAC advice, and public comments. The
sections below discuss the evidence and exposure/risk information,
CASAC advice and public input, and the Administrator's proposed
conclusions, for the major elements of the NAAQS: Indicator (II.C.1),
averaging time (II.C.2), form (II.C.3), and level (II.C.4).
1. Indicator
In the 2008 review, the EPA focused on O3 as the most
appropriate indicator for a standard meant to provide protection
against ambient photochemical oxidants. In this review, while the
complex atmospheric chemistry in which O3 plays a key role
has been highlighted, no alternatives to O3 have been
advanced as being a more appropriate indicator for ambient
photochemical oxidants. More specifically, the ISA noted that
O3 is the only photochemical oxidant (other than
NO2) that is routinely monitored and for which a
comprehensive database exists (U.S. EPA, 2013, section 3.6). Data for
other photochemical oxidants (e.g., peroxyacetyl nitrate, hydrogen
peroxide, etc.) typically have been obtained only as part of special
field studies. Consequently, no data on nationwide patterns of
occurrence are available for these other oxidants; nor are extensive
data available on the relationships of concentrations and patterns of
these oxidants to those of O3 (U.S. EPA, 2013, section 3.6).
In its review of the second draft PA, CASAC stated ``The indicator of
ozone is appropriate based on its causal or likely causal associations
with multiple adverse health outcomes and its representation of a class
of pollutants known as photochemical oxidants'' (Frey, 2014c, p. ii).
In addition, the PA notes that meeting an O3 standard
can be expected to provide some degree of protection against potential
health effects that may be independently associated with other
photochemical oxidants, even though such effects are not discernible
from currently available studies indexed by O3 alone (U.S.
EPA, 2014c, section 4.1). That is, since the precursor emissions that
lead to the formation of O3 generally also lead to the
formation of other photochemical oxidants, measures leading to
reductions in population exposures to O3 can generally be
expected to lead to reductions in population exposures to other
photochemical oxidants. In considering this information, and CASAC's
advice, the Administrator reached the proposed conclusion that
O3 remains the most appropriate indicator for a standard
meant to provide protection against photochemical oxidants.\115\
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\115\ The DC Circuit upheld the use of O3 as the
indicator for photochemical oxidants based on these same
considerations. American Petroleum Inst. v. Costle, 665 F. 2d 1176,
1186 (D.C. Cir. 1981).
---------------------------------------------------------------------------
The EPA received very few comments on the indicator of the primary
standard. Those who did comment supported the proposed decision to
retain O3 as the indicator, noting the rationale put forward
in the preamble to the proposed rule. These commenters generally
expressed support for retaining the current indicator in conjunction
with retaining other elements of the current standard, such as the
averaging time and form. After considering the available evidence,
CASAC advice, and public comments, the Administrator concludes that
O3 remains the most appropriate indicator for a standard
meant to provide protection against photochemical oxidants. Therefore,
she is retaining O3 as the indicator for the primary
standard in this final rule.
2. Averaging Time
The EPA established the current 8-hour averaging time \116\ for the
primary O3 NAAQS in 1997 (62 FR 38856). The decision on
averaging time in that review was based on numerous controlled human
exposure and epidemiologic studies reporting associations between
adverse respiratory effects and 6- to 8-hour O3
concentrations (62 FR 38861). The EPA also noted that a standard with a
maximum 8-hour averaging time is likely to provide substantial
protection against respiratory effects associated with 1-hour peak
O3 concentrations. The EPA reached similar conclusions in
the last O3 NAAQS review and thus, the EPA retained the 8-
hour averaging time in 2008.
---------------------------------------------------------------------------
\116\ This 8-hour averaging time reflects daily maximum 8-hour
average O3 concentrations.
---------------------------------------------------------------------------
In reaching a proposed conclusion on averaging time in the current
review, the Administrator considered the extent to which the available
evidence continues to support the appropriateness of a standard with an
8-hour averaging time (79 FR 75292). Specifically, the Administrator
considered the extent to which the available information indicates that
a standard with the current 8-hour averaging time provides appropriate
protection against short- and long-term O3 exposures. These
considerations from the proposal are summarized below in sections
II.C.2.a (short-term) and II.C.2.b (long-term). Section II.C.2.c
summarizes the Administrator's proposed decision on averaging time.
Section II.C.2.d discusses comments received on averaging time. Section
II.C.2.e presents the Administrator's final decision regarding
averaging time.
a. Short-Term
As an initial consideration with respect to the most appropriate
averaging time for the O3 NAAQS, in the proposal the
Administrator noted that the strongest evidence for O3-
associated health effects is for respiratory effects following short-
term exposures. More specifically, the Administrator noted the ISA
conclusion that the evidence is ``sufficient to infer a causal
relationship'' between short-term O3 exposures and
respiratory effects. The ISA also judges that for short-term
O3 exposures, the evidence indicates ``likely to be causal''
relationships with
[[Page 65348]]
both cardiovascular effects and mortality (U.S. EPA, 2013, section
2.5.2). Therefore, as in past reviews, the Administrator noted that the
strength of the available scientific evidence provides strong support
for a standard that protects the public health against short-term
exposures to O3.
In first considering the level of support available for specific
short-term averaging times, the Administrator noted in the proposal the
evidence available from controlled human exposure studies. As discussed
in more detail in Chapter 3 of the PA, substantial health effects
evidence from controlled human exposure studies demonstrates that a
wide range of respiratory effects (e.g., pulmonary function decrements,
increases in respiratory symptoms, lung inflammation, lung
permeability, decreased lung host defense, and airway
hyperresponsiveness) occur in healthy adults following 6.6-hour
exposures to O3 (U.S. EPA, 2013, section 6.2.1.1). Compared
to studies evaluating shorter exposure durations (e.g., 1-hour),
studies evaluating 6.6-hour exposures in healthy adults have reported
respiratory effects at lower O3 exposure concentrations and
at more moderate levels of exertion.
The Administrator also noted in the proposal the strength of
evidence from epidemiologic studies that evaluated a wide variety of
populations (e.g., including at-risk lifestages and populations, such
as children and people with asthma, respectively). A number of
different averaging times have been used in O3 epidemiologic
studies, with the most common being the max 1-hour concentration within
a 24-hour period (1-hour max), the max 8-hour average concentration
within a 24-hour period (8-hour max), and the 24-hour average. These
studies are assessed in detail in Chapter 6 of the ISA (U.S. EPA,
2013). Limited evidence from time-series and panel epidemiologic
studies comparing risk estimates across averaging times does not
indicate that one exposure metric is more consistently or strongly
associated with respiratory health effects or mortality, though the ISA
notes some evidence for ``smaller O3 risk estimates when
using a 24-hour average exposure metric'' (U.S. EPA, 2013, section
2.5.4.2; p. 2-31). For single- and multi-day average O3
concentrations, lung function decrements were associated with 1-hour
max, 8-hour max, and 24-hour average ambient O3
concentrations, with no strong difference in the consistency or
magnitude of association among the averaging times (U.S. EPA, 2013, p.
6-71). Similarly, in studies of short-term exposure to O3
and mortality, Smith et al. (2009) and Darrow et al. (2011) have
reported high correlations between risk estimates calculated using 24-
hour average, 8-hour max, and 1-hour max averaging times (U.S. EPA,
2013, p. 6-253). Thus, the Administrator noted that the epidemiologic
evidence alone does not provide a strong basis for distinguishing
between the appropriateness of 1-hour, 8-hour, and 24-hour averaging
times.
Considering the health information discussed above, in the proposal
the Administrator concluded that an 8-hour averaging time remains
appropriate for addressing health effects associated with short-term
exposures to ambient O3. An 8-hour averaging time is similar
to the exposure periods evaluated in controlled human exposure studies,
including recent studies that provide evidence for respiratory effects
following exposures to O3 concentrations below the level of
the current standard. In addition, epidemiologic studies provide
evidence for health effect associations with 8-hour O3
concentrations, as well as with 1-hour and 24-hour concentrations. As
in previous reviews, the Administrator noted that a standard with an 8-
hour averaging time (combined with an appropriate standard form and
level) would also be expected to provide substantial protection against
health effects attributable to 1-hour and 24-hour exposures (e.g., 62
FR 38861, July 18, 1997). This conclusion is consistent with the advice
received from CASAC that ``the current 8-hour averaging time is
justified by the combined evidence from epidemiologic and clinical
studies'' (Frey, 2014c, p. 6).
b. Long-Term
The ISA concludes that the evidence for long-term O3
exposures indicates that there is ``likely to be a causal
relationship'' with respiratory effects (U.S. EPA, 2013, chapter 7).
Thus, in this review the Administrator also considers the extent to
which currently available evidence and exposure/risk information
suggests that a standard with an 8-hour averaging time can provide
protection against respiratory effects associated with longer term
exposures to ambient O3.
In considering this issue in the 2008 review of the O3
NAAQS, the Staff Paper noted that ``because long-term air quality
patterns would be improved in areas coming into attainment with an 8-hr
standard, the potential risk of health effects associated with long-
term exposures would be reduced in any area meeting an 8-hr standard''
(U.S. EPA, 2007, p. 6-57). In the current review, the PA further
evaluates this issue, with a focus on the long-term O3
metrics reported to be associated with mortality or morbidity in recent
epidemiologic studies. As discussed in section 3.1.3 of the PA (U.S.
EPA, 2014c, section 4.2), much of the recent evidence for such
associations is based on studies that defined long-term O3
in terms of seasonal averages of daily maximum 1-hour or 8-hour
concentrations.
As an initial consideration, in the proposal the Administrator
noted the risk results from the HREA for respiratory mortality
associated with long-term O3 concentrations. These HREA
analyses indicate that as air quality is adjusted to just meet the
current 8-hour standard, most urban study areas are estimated to
experience reductions in respiratory mortality associated with long-
term O3 concentrations based on the seasonal averages of 1-
hour daily maximum O3 concentrations evaluated in the study
by Jerrett et al. (2009) (U.S. EPA, 2014a, chapter 7).\117\ As air
quality is adjusted to meet lower alternative standard levels, for
standards based on 3-year averages of the annual fourth-highest daily
maximum 8-hour O3 concentrations, respiratory mortality
risks are estimated to be reduced further in urban study areas. This
analysis indicates that an O3 standard with an 8-hour
averaging time, when coupled with an appropriate form and level, can
reduce respiratory mortality reported to be associated with long-term
O3 concentrations.
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\117\ Though the Administrator also notes important
uncertainties associated with these risk estimates, as discussed in
section II.C.3.b of the proposal.
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In further considering the study by Jerrett et al. (2009), in the
proposal the Administrator noted the PA comparison of long-term
O3 concentrations following model adjustment in urban study
areas (i.e., adjusted to meet the current and alternative 8-hour
standards) to the concentrations present in study cities that provided
the basis for the positive and statistically significant association
with respiratory mortality. As indicated in Table 4-3 of the PA (U.S.
EPA, 2014c, section 4.2), this comparison suggests that a standard with
an 8-hour averaging time can decrease seasonal averages of 1-hour daily
maximum O3 concentrations, and can maintain those
O3 concentrations below the seasonal average concentration
where the study indicates the most confidence in the reported
concentration-response relationship with respiratory mortality (U.S.
EPA, 2014c, sections 4.2 and 4.4.1).
[[Page 65349]]
The Administrator also noted in the proposal that the HREA
conducted analyses evaluating the impacts of reducing regional
NOX emissions on the seasonal averages of daily maximum 8-
hour O3 concentrations. Seasonal averages of 8-hour daily
max O3 concentrations reflect long-term metrics that have
been reported to be associated with respiratory morbidity effects in
several recent O3 epidemiologic studies (e.g., Islam et al.,
2008; Lin et al., 2008a, 2008b; Salam et al., 2009). The HREA analyses
indicate that the large majority of the U.S. population lives in
locations where reducing NOX emissions would be expected to
result in decreases in seasonal averages of daily max 8-hour ambient
O3 concentrations (U.S. EPA, 2014a, chapter 8). Thus,
consistent with the respiratory mortality risk estimates noted above,
these analyses suggest that reductions in O3 precursor
emissions in order to meet a standard with an 8-hour averaging time
would also be expected to reduce the long-term O3
concentrations that have been reported in recent epidemiologic studies
to be associated with respiratory morbidity.
c. Administrator's Proposed Conclusion on Averaging Time
In the proposal the Administrator noted that, when taken together,
the analyses summarized above indicate that a standard with an 8-hour
averaging time, coupled with the current fourth-high form and an
appropriate level, would be expected to provide appropriate protection
against the short- and long-term O3 concentrations that have
been reported to be associated with respiratory morbidity and
mortality. The CASAC agreed with this conclusion, stating that ``[t]he
current 8-hour averaging time is justified by the combined evidence
from epidemiologic and clinical studies'' and that ``[t]he 8-hour
averaging window also provides protection against the adverse impacts
of long-term ozone exposures, which were found to be ``likely causal''
for respiratory effects and premature mortality'' (Frey, 2014c, p. 6).
Therefore, considering the available evidence and exposure risk
information, and CASAC's advice, the Administrator proposed to retain
the current 8-hour averaging time, and not to set an additional
standard with a different averaging time.
d. Comments on Averaging Time
Most public commenters did not address the issue of whether the EPA
should consider additional or alternative averaging times. Of those who
did address this issue, some commenters representing state agencies or
industry groups agreed with the proposed decision to retain the current
8-hour averaging time, generally noting the supportive evidence
discussed in the preamble to the proposed rule. In contrast, several
medical organizations and environmental groups questioned the degree of
health protection provided by a standard based on an 8-hour averaging
time. For example, one group asserted that ``[a]veraging over any time
period, such as 8 hours, is capable of hiding peaks that may be very
substantial if they are brief enough.''
The EPA agrees with these commenters that an important issue in the
current review is the appropriateness of using a standard with an 8-
hour averaging time to protect against adverse health effects that are
attributable to a wide range of O3 exposure durations,
including those shorter and longer than 8 hours. This is an issue that
has been thoroughly evaluated by the EPA in past reviews, as well as in
the current review.
The 8-hour O3 NAAQS was originally set in 1997, as part
of revising the then-existing standard with its 1-hour averaging time,
and was retained in the review completed in 2008 (73 FR 16472). In both
of these reviews, several lines of evidence and information provided
support for an 8-hour averaging time rather than a shorter averaging
time. For example, substantial health evidence demonstrated
associations between a wide range of respiratory effects and 6- to 8-
hour exposures to relatively low O3 concentrations (i.e.,
below the level of the 1-hour O3 NAAQS in place prior to the
review completed in 1997). A standard with an 8-hour averaging time was
determined to be more directly associated with health effects of
concern at lower O3 concentrations than a standard with a 1-
hour averaging time. In addition, results of quantitative analyses
showed that a standard with an 8-hour averaging time can effectively
limit both 1- and 8-hour exposures of concern, and that an 8-hour
averaging time results in a more uniformly protective national standard
than a 1-hour averaging time. In past reviews, CASAC has agreed that an
8-hour averaging time is appropriate.
In reaching her proposed decision to retain the 8-hour averaging
time in the current review, the Administrator again considered the body
of evidence for adverse effects attributable to a wide range of
O3 exposure durations, including studies specifically
referenced by public commenters who questioned the protectiveness of a
standard with an 8-hour averaging time. For example, as noted above a
substantial body of health effects evidence from controlled human
exposure studies demonstrates that a wide range of respiratory effects
occur in healthy adults following 6.6-hour exposures to O3
(U.S. EPA, 2013, section 6.2.1.1). Compared to studies evaluating
shorter exposure durations (e.g., 1-hour), studies evaluating 6.6-hour
exposures in healthy adults have reported respiratory effects at lower
O3 exposure concentrations and at more moderate levels of
exertion. The Administrator also noted the strength of evidence from
epidemiologic studies that evaluated a number of different averaging
times, with the most common being the maximum 1-hour concentration
within a 24-hour period (1-hour max), the maximum 8-hour average
concentration within a 24-hour period (8-hour max), and the 24-hour
average. Evidence from time-series and panel epidemiologic studies
comparing risk estimates across averaging times does not indicate that
one exposure metric is more consistently or strongly associated with
respiratory health effects or mortality (U.S. EPA, 2013, section
2.5.4.2; p. 2-31). For single- and multi-day average O3
concentrations, lung function decrements were associated with 1-hour
max, 8-hour max, and 24-hour average ambient O3
concentrations, with no strong difference in the consistency or
magnitude of association among the averaging times (U.S. EPA, 2013, p.
6-71). Similarly, in studies of short-term exposure to O3
and mortality, Smith et al. (2009) and Darrow et al. (2011) have
reported high correlations between risk estimates calculated using 24-
hour average, 8-hour max, and 1-hour max averaging times (U.S. EPA,
2013, p. 6-253). Thus, the epidemiologic evidence does not provide a
strong basis for distinguishing between the appropriateness of 1-hour,
8-hour, and 24-hour averaging times.
In addition, quantitative exposure and risk analyses in the HREA
are based on an air quality adjustment approach that estimates hourly
O3 concentrations, and on scientific studies that evaluated
health effects attributable to a wide range of O3 exposure
durations. For example, the risk of lung function decrements is
estimated using a model based on controlled human exposure studies with
exposure durations ranging from 2 to 7.6 hours (U.S. EPA, 2013, section
6.2.1.1). Epidemiology-based risk estimates are based on studies that
reported health effect associations with short-term ambient
O3 concentrations ranging from 1-hour to 24-hours and with
long-term seasonal average concentrations (U.S. EPA, 2014a, Table 7-2).
Thus, the HREA estimated health
[[Page 65350]]
risks associated with a wide range of O3 exposure durations
and the Administrator's conclusions on averaging time in the current
review are based, in part, on consideration of these estimates.
When taken together, the evidence and analyses indicate that a
standard with an 8-hour averaging time, coupled with the current
fourth-high form and an appropriate level, would be expected to provide
appropriate protection against the short- and long-term O3
concentrations that have been reported to be associated with
respiratory morbidity and mortality. The CASAC agreed with this,
stating the following (Frey, 2014c, p. 6):
The current 8-hour averaging time is justified by the combined
evidence from epidemiologic and clinical studies referenced in
Chapter 4. Results from clinical studies, for example, show a wide
range of respiratory effects in healthy adults following 6.6 hours
of exposure to ozone, including pulmonary function decrements,
increases in respiratory symptoms, lung inflammation, lung
permeability, decreased lung host defense, and airway
hyperresponsiveness. These findings are supported by evidence from
epidemiological studies that show causal associations between short-
term exposures of 1, 8 and 24-hours and respiratory effects and
``likely to be causal'' associations for cardiovascular effects and
premature mortality. The 8-hour averaging window also provides
protection against the adverse impacts of long-term ozone exposures,
which were found to be ``likely causal'' for respiratory effects and
premature mortality.
Given all of the above, the EPA disagrees with commenters who question
the protectiveness of an O3 standard with an 8-hour
averaging time, particularly for an 8-hour standard with the revised
level of 70 ppb that is being established in this review, as discussed
below (II.C.4).
e. Administrator's Final Decision Regarding Averaging Time
In considering the evidence and information summarized in the
proposal and discussed in detail in the ISA, HREA, and PA; CASAC's
views; and public comments, the Administrator concludes that a standard
with an 8-hour averaging time can effectively limit health effects
attributable to both short- and long-term O3 exposures. As
was the case in the proposal, this final conclusion is based on (1) the
strong evidence that continues to support the importance of protecting
public health against short-term O3 exposures (e.g., <= 1-
hour to 24-hour) and (2) analyses in the HREA and PA supporting the
conclusion that the current 8-hour averaging time can effectively limit
long-term O3 exposures. Furthermore, the Administrator
observes that the CASAC Panel agreed with the choice of averaging time
(Frey, 2014c). Therefore, in the current review, the Administrator
concludes that it is appropriate to retain the 8-hour averaging time
and to not set a separate standard with a different averaging time in
this final rule.
3. Form
The ``form'' of a standard defines the air quality statistic that
is to be compared to the level of the standard in determining whether
an area attains that standard. The foremost consideration in selecting
a form is the adequacy of the public health protection provided by the
combination of the form and the other elements of the standard. In this
review, the Administrator considers the extent to which the available
evidence and/or information continue to support the appropriateness of
a standard with the current form, defined by the 3-year average of
annual fourth-highest 8-hour daily maximum O3
concentrations. Section II.C.3.a below summarizes the basis for the
current form. Section II.C.3.b discusses the Administrator's proposed
decision to retain the current form. Section II.C.3.c discusses public
comments received on the form of the primary standard. Section II.C.3.d
discusses the Administrator's final decision on form.
a. Basis for the Current Form
The EPA established the current form of the primary O3
NAAQS in 1997 (62 FR 38856). Prior to that time, the standard had a
``1-expected-exceedance'' form.\118\ An advantage of the current
concentration-based form recognized in the 1997 review is that such a
form better reflects the continuum of health effects associated with
increasing ambient O3 concentrations. Unlike an expected
exceedance form, a concentration-based form gives proportionally more
weight to years when 8-hour O3 concentrations are well above
the level of the standard than years when 8-hour O3
concentrations are just above the level of the standard.\119\ The EPA
judged it appropriate to give more weight to higher O3
concentrations, given that available health evidence indicated a
continuum of effects associated with exposures to varying
concentrations of O3, and given that the extent to which
public health is affected by exposure to ambient O3 is
related to the actual magnitude of the O3 concentration, not
just whether the concentration is above a specified level.
---------------------------------------------------------------------------
\118\ For a standard with a 1-expected-exceedance form to be met
at an air quality monitoring site, the fourth-highest air quality
value in 3 years, given adjustments for missing data, must be less
than or equal to the level of the standard.
\119\ As discussed (61 FR 65731), this is because with an
exceedance-based form, days on which the ambient O3
concentration is well above the level of the standard are given
equal weight to those days on which the O3 concentration
is just above the standard (i.e., each day is counted as one
exceedance), even though the public health impact of such days would
be very different. With a concentration-based form, days on which
higher O3 concentrations occur would weigh proportionally
more than days with lower O3 concentrations since the
actual concentrations are used directly to calculate whether the
standard is met or violated.
---------------------------------------------------------------------------
During the 1997 review, the EPA considered a range of alternative
``concentration-based'' forms, including the second-, third-, fourth-
and fifth-highest daily maximum 8-hour concentrations in an
O3 season. The fourth-highest daily maximum was selected,
recognizing that a less restrictive form (e.g., fifth-highest) would
allow a larger percentage of sites to experience O3 peaks
above the level of the standard, and would allow more days on which the
level of the standard may be exceeded when the site attains the
standard (62 FR 38856). The EPA also considered setting a standard with
a form that would provide a margin of safety against possible but
uncertain chronic effects, and would provide greater stability to
ongoing control programs.\120\ A more restrictive form was not
selected, recognizing that the differences in the degree of protection
afforded by the alternatives were not well enough understood to use any
such differences as a basis for choosing the most restrictive forms (62
FR 38856).
---------------------------------------------------------------------------
\120\ See American Trucking Assn's v. EPA, 283 F. 3d at 374-75
(less stable implementation programs may be less effective and would
thereby provide less public health protection; EPA may therefore
legitimately consider programmatic stability in determining the form
of a NAAQS).
---------------------------------------------------------------------------
In the 2008 review, the EPA additionally considered the potential
value of a percentile-based form. In doing so, the EPA recognized that
such a statistic is useful for comparing datasets of varying length
because it samples approximately the same place in the distribution of
air quality values, whether the dataset is several months or several
years long. However, the EPA concluded that a percentile-based
statistic would not be effective in ensuring the same degree of public
health protection across the country. Specifically, a percentile-based
form would allow more days with higher air quality values in locations
with longer O3 seasons relative to locations with shorter
O3 seasons. Thus, in the 2008 review, the EPA concluded that
a form based on the nth-highest maximum O3 concentration
would more effectively ensure that people who live in areas
[[Page 65351]]
with different length O3 seasons receive the same degree of
public health protection.
Based on analyses of forms specified in terms of an nth-highest
concentration (n ranged from 3 to 5), advice from CASAC, and public
comment, the Administrator concluded that a fourth-highest daily
maximum should be retained (73 FR 16465, March 27, 2008). In reaching
this decision, the Administrator recognized that ``there is not a clear
health-based threshold for selecting a particular nth-highest daily
maximum form of the standard'' and that ``the adequacy of the public
health protection provided by the combination of the level and form is
a foremost consideration'' (73 FR 16475, March 27, 2008). Based on
this, the Administrator judged that the existing form (fourth-highest
daily maximum 8-hour average concentration) should be retained,
recognizing the increase in public health protection provided by
combining this form with a lower standard level (i.e., 75 ppb).
The Administrator also recognized that it is important to have a
form that provides stability with regard to implementation of the
standard. In the case of O3, for example, he noted the
importance of a form insulated from the impacts of extreme
meteorological events that are conducive to O3 formation.
Such events could have the effect of reducing public health protection,
to the extent they result in frequent shifts in and out of attainment
due to meteorological conditions. The Administrator noted that such
frequent shifting could disrupt an area's ongoing implementation plans
and associated control programs (73 FR 16474, March 27, 2008). In his
final decision, the Administrator judged that a fourth-high form
``provides a stable target for implementing programs to improve air
quality'' (id. at 16475).
b. Proposed Decision on Form
In the proposal for the current review, the Administrator
considered the extent to which newly available information provides
support for the current form (79 FR 75293). In so doing, she took note
of the conclusions of prior reviews summarized above. She recognized
the value of an nth-high statistic over that of an expected exceedance
or percentile-based form in the case of the O3 standard, for
the reasons summarized above. The Administrator additionally took note
of the importance of stability in implementation to achieving the level
of protection specified by the NAAQS. Specifically, she noted that to
the extent areas engaged in implementing the O3 NAAQS
frequently shift from meeting the standard to violating the standard,
it is possible that ongoing implementation plans and associated control
programs could be disrupted, thereby reducing public health protection.
In light of this, while giving foremost consideration to the
adequacy of public health protection provided by the combination of all
elements of the standard, including the form, the Administrator
considered particularly the findings from prior reviews with regard to
the use of the nth-high metric. As noted above, the EPA selected the
fourth-highest daily maximum, recognizing the public health protection
provided by this form, when coupled with an appropriate averaging time
and level, and recognizing that such a form can provide stability for
implementation programs. In the proposal the Administrator concluded
that the currently available evidence and information do not call into
question these conclusions from previous reviews. In reaching this
initial conclusion, the Administrator noted that CASAC concurred that
the O3 standard should be based on the fourth-highest, daily
maximum 8-hour average value (averaged over 3 years), stating that this
form ``provides health protection while allowing for atypical
meteorological conditions that can lead to abnormally high ambient
ozone concentrations which, in turn, provides programmatic stability''
(Frey, 2014c, p. 6). Thus, a standard with the current fourth-high
form, coupled with a level lower than 75 ppb as discussed below, would
be expected to increase public health protection relative to the
current standard while continuing to provide stability for
implementation programs. Therefore, the Administrator proposed to
retain the current fourth-highest daily maximum form for an
O3 standard with an 8-hour averaging time and a revised
level.
c. Public Comments on Form
Several commenters focused on the stability of the standard to
support their positions regarding form. Some industry associations and
state agencies support changing to a form that would allow a larger
number of exceedances of the standard level than are allowed by the
current fourth-high form. In some cases, these commenters argued that a
standard allowing a greater number of exceedances would provide the
same degree of public health protection as the current standard. Some
commenters advocated a percentile-based form, such as the 98th
percentile. These commenters cited a desire for consistency with short-
term standards for other criteria pollutants (e.g., PM2.5,
NO2), as well as a desire to allow a greater number of
exceedances of the standard level, thus making the standard less
sensitive to fluctuations in background O3 concentrations
and to extreme meteorological events.
Other commenters submitted analyses purporting to indicate that a
fourth-high form provides only a small increase in stability, relative
to forms that allow fewer exceedances of the standard level (i.e.,
first-high, second-high). These commenters also called into question
the degree of health protection achieved by a standard with a fourth-
high form and a level in the proposed range (i.e., 65 to 70 ppb). They
pointed out that a fourth-high form will, by definition, allow 3 days
per year, on average, with 8-hour O3 concentrations above
the level of the standard. Commenters further stated that ``[i]f ozone
levels on these peak days are appreciably higher than on the fourth-
highest day, given EPA's acknowledged concerns regarding single or
multiple (defined by EPA as 2 or more) exposures to elevated ozone
concentrations, EPA must account for the degree of under-protection in
setting the level of the NAAQS'' (e.g., ALA et al., p. 138).
For the reasons discussed in the proposal, and summarized above,
the EPA disagrees with commenters who supported a percentile-based
form, such as the 98th percentile, for the O3 NAAQS. As
noted above, a percentile-based statistic would not be effective in
ensuring the same degree of public health protection across the
country. Rather, a percentile-based form would allow more days with
higher air quality values in locations with longer O3
seasons relative to locations with shorter O3 seasons. Thus,
as in the 2008 review, in the current review the EPA concludes that a
form based on the nth-highest maximum O3 concentration would
more effectively ensure that people who live in areas with different
length O3 seasons receive the same degree of public health
protection.
In considering various nth-high values, as in past reviews (e.g.,
73 FR 16475, March 27, 2008), the EPA recognizes that there is not a
clear health-based threshold for selecting a particular nth-highest
daily maximum form. Rather, the primary consideration is the adequacy
of the public health protection provided by the combination of all of
the elements of the standard, including the form. Environmental and
public health commenters are correct that a standard with the current
fourth-high form will allow 3 days per year, on average, with 8-hour
O3 concentrations higher than the standard level. However,
the EPA disagrees with these
[[Page 65352]]
commenters' assertion that using a fourth-high form results in a
standard that is under-protective. The O3 exposure and risk
estimates that informed the Administrator's consideration of the degree
of public health protection provided by various standard levels were
based on air quality that ``just meets'' various standards with the
current 8-hour averaging time and fourth-high, 3-year average form
(U.S. EPA, 2014a, section 4.3.3). Therefore, air quality adjusted to
meet various levels of the standard with the current form and averaging
time will include days with concentrations above the level of the
standard, and these days contribute to exposure and risk estimates. In
this way, the Administrator has reasonably considered the public health
protection provided by the combination of all of the elements of the
standard, including the fourth-high form.
In past reviews, EPA selected the fourth-highest daily maximum form
in recognition of the public health protection provided by this form,
when coupled with an appropriate averaging time and level, and
recognizing that such a form can provide stability for ongoing
implementation programs. As noted above, some commenters submitted
analyses suggesting that a fourth-high form provides only a small
increase in stability, relative to a first- or second-high form. The
EPA has conducted analyses of ambient O3 monitoring data to
further consider these commenters' assertions regarding stability. The
EPA's analyses of nth-high concentrations ranging from first-high to
fifth-high have been summarized in a memo to the docket (Wells, 2015a).
Consistent with commenters' analyses, Wells (2015a) indicates a
progressive decrease in the variability of O3
concentrations, and an increase in the stability of those
concentrations, as ``n'' increases. Based on these analyses, there is
no clear threshold for selecting a particular nth-high form based on
stability alone. Rather, as in past reviews, the decision on form in
this review focuses first and foremost on the Administrator's judgments
on public health protection, with judgments regarding stability of the
standard being a legitimate, but secondary consideration. The
Administrator's final decision on form is discussed below.
d. Administrator's Final Decision Regarding Form
In reaching a final decision on the form of the primary
O3 standard, as described in the proposal and above, the
Administrator recognizes that there is not a clear health-based
rationale for selecting a particular nth-highest daily maximum form.
Her foremost consideration is the adequacy of the public health
protection provided by the combination of all of the elements of the
standard, including the form. In this regard, the Administrator
recognizes the support from analyses in previous reviews, and from the
CASAC in the current review, for the conclusion that the current
fourth-high form of the standard, when combined with a revised level as
discussed below, provides an appropriate balance between public health
protection and a stable target for implementing programs to improve air
quality. In particular, she notes that the CASAC concurred that the
O3 standard should be based on the fourth-highest, daily
maximum 8-hour average value (averaged over 3 years), stating that this
form ``provides health protection while allowing for atypical
meteorological conditions that can lead to abnormally high ambient
ozone concentrations which, in turn, provides programmatic stability''
(Frey, 2014c, p. 6). Based on these considerations, and on
consideration of public comments on form as discussed above, the
Administrator judges it appropriate to retain the current fourth-high
form (fourth-highest daily maximum 8-hour O3 concentration,
averaged over 3 years) in this final rule.
4. Level
This section summarizes the basis for the Administrator's proposed
decision to revise the current standard level (II.C.4.a); discusses
public comments, and the EPA's responses, on that proposed decision
(II.C.4.b); and presents the Administrator's final decision regarding
the level of the primary O3 standard (II.C.4.c).
a. Basis for the Administrator's Proposed Decision on Level
In conjunction with her proposed decisions to retain the current
indicator, averaging time, and form (II.C.1 to II.C.3, above), the
Administrator proposed to revise the level of the primary O3
standard to within the range of 65 to 70 ppb. In proposing this range
of standard levels, as discussed in section II.E.4 of the proposal, the
Administrator carefully considered the scientific evidence assessed in
the ISA (U.S. EPA, 2013); the results of the exposure and risk
assessments in the HREA (U.S. EPA, 2014a); the evidence-based and
exposure-/risk-based considerations and conclusions in the PA (U.S.
EPA, 2014c); CASAC advice and recommendations, as reflected in CASAC's
letters to the Administrator and in public discussions of drafts of the
ISA, HREA, and PA (Frey and Samet, 2012; Frey, 2014 a, c); and public
input received during the development of these documents.
The Administrator's proposal to revise the standard level built
upon her proposed conclusion that the overall body of scientific
evidence and exposure/risk information calls into question the adequacy
of public health protection afforded by the current primary
O3 standard, particularly for at-risk populations and
lifestages. In reaching proposed conclusions on alternative levels for
the primary O3 standard, the Administrator considered the
extent to which various alternatives would be expected to protect the
public, including at-risk populations, against the wide range of
adverse health effects that have been linked with short- or long-term
O3 exposures.
As was the case for her consideration of the adequacy of the
current primary O3 standard (II.B.3, above), the
Administrator placed the greatest weight on the results of controlled
human exposure studies and on exposure and risk analyses based on
information from these studies. In doing so, she noted that controlled
human exposure studies provide the most certain evidence indicating the
occurrence of health effects in humans following exposures to specific
O3 concentrations. The effects reported in these studies are
due solely to O3 exposures, and interpretation of study
results is not complicated by the presence of co-occurring pollutants
or pollutant mixtures (as is the case in epidemiologic studies). She
further noted the CASAC judgment that ``the scientific evidence
supporting the finding that the current standard is inadequate to
protect public health is strongest based on the controlled human
exposure studies of respiratory effects'' (Frey, 2014c, p. 5).
In considering the evidence from controlled human exposure studies,
the Administrator first noted that the largest respiratory effects, and
the broadest range of effects, have been studied and reported following
exposures to 80 ppb O3 or higher, with most exposure studies
conducted at these higher concentrations. Exposures of healthy adults
to O3 concentrations of 80 ppb or higher have been reported
to decrease lung function, increase airway inflammation, increase
respiratory symptoms, result in airway hyperresponsiveness, and
decrease lung host defenses. The Administrator further noted that
O3 exposure concentrations as low as 72 ppb have been shown
to both decrease lung function and increase respiratory
[[Page 65353]]
symptoms (Schelegle et al., 2009),\121\ a combination that meets the
ATS criteria for an adverse response, and that exposures as low as 60
ppb have been reported to decrease lung function and increase airway
inflammation.
---------------------------------------------------------------------------
\121\ As noted above, for the 70 ppb target exposure
concentration, Schelegle et al. (2009) reported that the actual mean
exposure concentration was 72 ppb.
---------------------------------------------------------------------------
Based on this evidence, the Administrator reached the initial
conclusion that the results of controlled human exposure studies
strongly support setting the level of a revised O3 standard
no higher than 70 ppb. In reaching this conclusion, she placed a large
amount of weight on the importance of setting the level of the standard
well below 80 ppb, the exposure concentration at which the broadest
range of effects have been studied and reported, and below 72 ppb, the
lowest exposure concentration shown to result in the adverse
combination of lung function decrements and respiratory symptoms. She
placed significant weight on this combination of effects, as did CASAC,
in making judgments regarding the potential for adverse responses.
In further considering the potential public health implications of
a standard with a level of 70 ppb, the Administrator also considered
quantitative estimates of the extent to which such a standard would be
expected to limit population exposures to the broader range of
O3 concentrations shown in controlled human exposure studies
to cause respiratory effects. In doing so, she focused on estimates of
O3 exposures of concern at or above the benchmark
concentrations of 60, 70, and 80 ppb. The Administrator judged that the
evidence supporting the occurrence of adverse respiratory effects is
strongest for exposures at or above the 70 and 80 ppb benchmarks.
Therefore, she placed a large amount of emphasis on the importance of
setting a standard that limits exposures of concern at or above these
benchmarks.
The Administrator expressed less confidence that adverse effects
will occur following exposures to O3 concentrations as low
as 60 ppb. In reaching this conclusion, she highlighted the fact that
statistically significant increases in respiratory symptoms, combined
with lung function decrements, have not been reported following
exposures to 60 or 63 ppb O3, though several studies have
evaluated the potential for such effects (Kim et al., 2011; Schelegle
et al., 2009; Adams, 2006).\122\ The proposal specifically stated that
``[t]he Administrator has decreasing confidence that adverse effects
will occur following exposures to O3 concentrations below 72
ppb. In particular, compared to O3 exposure concentrations
at or above 72 ppb, she has less confidence that adverse effects will
occur following exposures to O3 concentrations as low as 60
ppb'' (79 FR 73304-05).
---------------------------------------------------------------------------
\122\ In the study by Schelegle, for the 60 ppb target exposure
concentration, study authors reported that the actual mean exposure
concentration was 63 ppb.
---------------------------------------------------------------------------
However, she noted the possibility for adverse effects following
such exposures given that: (1) CASAC judged the adverse combination of
lung function decrements and respiratory symptoms ``almost certainly
occur in some people'' following exposures to O3
concentrations below 72 ppb (though CASAC did not specify or otherwise
indicate how far below) (Frey, 2014c, p. 6); (2) CASAC indicated the
moderate lung function decrements (i.e., FEV1 decrements >=
10%) that occur in some healthy adults following exposures to 60 ppb
O3 could be adverse to people with lung disease; and (3)
airway inflammation has been reported following exposures as low as 60
ppb O3. She also took note of CASAC advice that the
occurrence of exposures of concern at or above 60 ppb is an appropriate
consideration for people with asthma (Frey, 2014c, p. 6). Therefore,
while the Administrator expressed less confidence that adverse effects
will occur following exposures to O3 concentrations as low
as 60 ppb, compared to 70 ppb and above, based on the evidence and
CASAC advice she also gave some consideration to exposures of concern
for the 60 ppb benchmark.
Due to interindividual variability in responsiveness, the
Administrator further noted that not every occurrence of an exposure of
concern will result in an adverse effect, and that repeated occurrences
of some of the effects demonstrated following exposures of concern
could increase the likelihood of adversity (U.S. EPA, 2013, section
6.2.3). Therefore, the Administrator was most concerned about
protecting at-risk populations against repeated occurrences of
exposures of concern. Based on the above considerations, the
Administrator focused on the extent to which a revised standard with a
level of 70 ppb would be expected to protect populations from
experiencing two or more O3 exposures of concern (i.e., as a
surrogate for repeated exposures).
As illustrated in Table 1 in the proposal (and Table 1 above), the
Administrator noted that, in urban study areas, a revised standard with
a level of 70 ppb is estimated to eliminate the occurrence of two or
more exposures of concern to O3 concentrations at and above
80 ppb and to virtually eliminate the occurrence of two or more
exposures of concern to O3 concentrations at and above 70
ppb, even in the worst-case urban study area and year evaluated. Though
the Administrator acknowledged greater uncertainty with regard to the
occurrence of adverse effects following exposures to 60 ppb, she noted
that a revised standard with a level of 70 ppb would also be expected
to protect the large majority of children in the urban study areas
(i.e., about 96% to more than 99% of children in individual urban study
areas) from experiencing two or more exposures of concern at or above
the 60 ppb benchmark. Compared to the current standard, this represents
a reduction of more than 60%.\123\
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\123\ The Administrator judged that the evidence is less
compelling, and indicates greater uncertainty, with regard to the
potential for adverse effects following single occurrences of
O3 exposures of concern. While acknowledging this greater
uncertainty, she noted that a standard with a level of 70 ppb would
also be expected to virtually eliminate all occurrences (including
single occurrences) of exposures of concern at or above 80 ppb, even
in the worst-case year and location. She also judged that such a
standard will achieve important reductions, compared to the current
standard, in the occurrence of one or more exposures of concern at
or above 70 and 60 ppb.
---------------------------------------------------------------------------
In further evaluating the potential public health impacts of a
standard with a level of 70 ppb, the Administrator also considered the
HREA estimates of O3-induced lung function decrements. To
inform her consideration of these decrements, the Administrator took
note of CASAC advice that ``estimation of FEV1 decrements of
>= 15% is appropriate as a scientifically relevant surrogate for
adverse health outcomes in active healthy adults, whereas an
FEV1 decrement of >= 10% is a scientifically relevant
surrogate for adverse health outcomes for people with asthma and lung
disease'' (Frey, 2014c, p. 3).
Although these FEV1 decrements provide perspective on
the potential for the occurrence of adverse respiratory effects
following O3 exposures, the Administrator agreed with the
conclusion in past reviews that a more general consensus view of the
adversity of moderate responses emerges as the frequency of occurrence
increases (61 FR 65722-3, Dec, 13, 1996). Specifically, she judged that
not every estimated occurrence of an O3-induced
FEV1 decrement will be adverse and
[[Page 65354]]
that repeated occurrences of moderate responses could lead to more
serious illness. Therefore, the Administrator noted increasing concern
about the potential for adversity as the number of occurrences
increases and, as a result, she focused primarily on estimates of two
or more O3-induced FEV1 decrements (i.e., as a
surrogate for repeated exposures).\124\
---------------------------------------------------------------------------
\124\ In the proposal, the Administrator further judged that it
would not be appropriate to set a standard that is intended to
eliminate all O3-induced FEV1 decrements. She
noted that this is consistent with CASAC advice, which did not
include a recommendation to set the standard level low enough to
eliminate all O3-induced FEV1 decrements
= 10 or 15% (Frey, 2014c).
---------------------------------------------------------------------------
The Administrator noted that a revised O3 standard with
a level of 70 ppb is estimated to protect about 98 to 99% of children
in urban study areas from experiencing two or more O3-
induced FEV1 decrements =15%, and about 89 to 94%
from experiencing two or more decrements =10%. She judged
that these estimates reflect important risk reductions, compared to the
current standard. Given these estimates, as well as estimates of one or
more decrements per season (about which she was less concerned (79 FR
75290, December 17, 2014)), the Administrator concluded that a revised
standard with a level of 70 ppb would be expected to provide
substantial protection against the risk of O3-induced lung
function decrements, and would be expected to result in important
reductions in such risks, compared to the current standard. The
Administrator further noted, however, that the variability in lung
function risk estimates across urban study areas is often greater than
the differences in risk estimates between various standard levels
(Table 2, above). Given this, and the resulting considerable overlap
between the ranges of lung function risk estimates for different
standard levels, in the proposal the Administrator viewed lung function
risk estimates as providing a more limited basis than exposures of
concern for distinguishing between the degrees of public health
protection provided by alternative standard levels (79 FR 75306 n.
164).
In next considering the additional protection that would be
expected from standard levels below 70 ppb, the Administrator evaluated
the extent to which a standard with a level of 65 ppb would be expected
to further limit O3 exposures of concern and O3-
induced lung function decrements. In addition to eliminating almost all
exposures of concern to O3 concentrations at or above 80 and
70 ppb, even in the worst-case years and locations, the Administrator
noted that a revised standard with a level of 65 ppb would be expected
to protect more than 99% of children in urban study areas from
experiencing two or more exposures of concern at or above 60 ppb and to
substantially reduce the occurrence of one or more such exposures,
compared to the current standard. With regard to O3-induced
lung function decrements, an O3 standard with a level of 65
ppb is estimated to protect about 98% to more than 99% of children from
experiencing two or more O3-induced FEV1
decrements =15% and about 91 to 99% from experiencing two or
more decrements =10%.\125\
---------------------------------------------------------------------------
\125\ Although the Administrator was less concerned about the
public health implications of single O3-induced lung
function decrements, she also noted that a revised standard with a
level of 65 ppb is estimated to reduce the risk of one or more
O3-induced decrements per season, compared to the current
standard.
---------------------------------------------------------------------------
Taken together, the Administrator concluded that the evidence from
controlled human exposure studies, and the information from
quantitative analyses that draw upon these studies, provide strong
support for standard levels from 65 to 70 ppb. In particular, she based
this conclusion on the fact that such standard levels would be well
below the O3 exposure concentration shown to result in the
widest range of respiratory effects (i.e., 80 ppb),\126\ and below the
lowest O3 exposure concentration shown to result in the
adverse combination of lung function decrements and respiratory
symptoms (i.e., 72 ppb). A standard with a level from 65 to 70 ppb
would also be expected to result in important reductions, compared to
the current standard, in the occurrence of O3 exposures of
concern for all of the benchmarks evaluated (i.e., 60, 70, and 80 ppb)
and in the risk of O3-induced lung function decrements
=10 and 15%.
---------------------------------------------------------------------------
\126\ Although the widest range of effects have been evaluated
following exposures to 80 ppb O3, there is no evidence
that 80 ppb is a threshold for these effects.
---------------------------------------------------------------------------
In further considering the evidence and exposure/risk information,
the Administrator considered the extent to which the epidemiologic
evidence also provides support for standard levels from 65 to 70 ppb.
In particular, the Administrator noted analyses in the PA (U.S. EPA,
2014c, section 4.4.1) indicating that a revised standard with a level
of 65 or 70 ppb would be expected to maintain distributions of short-
term ambient O3 concentrations below those present in the
locations of all the single-city epidemiologic studies of hospital
admissions or emergency department visits analyzed. She concluded that
a revised standard with a level at least as low as 70 ppb would result
in improvements in public health, beyond the protection provided by the
current standard, in the locations of the single-city epidemiologic
studies that reported significant health effect associations.\127\
---------------------------------------------------------------------------
\127\ The Administrator also concluded that analyses in the HREA
and PA indicate that a standard with an 8-hour averaging time,
coupled with the current fourth-high form and a level from 65 to 70
ppb, would be expected to provide increased protection, compared to
the current standard, against the long-term O3
concentrations that have been reported to be associated with
respiratory morbidity or mortality (79 FR 75293; 75308).
---------------------------------------------------------------------------
The Administrator noted additional uncertainty in interpreting air
quality in locations of multicity epidemiologic studies of short-term
O3 for the purpose of evaluating alternative standard levels
(II.D.1 and U.S. EPA, 2014c, section 4.4.1). While acknowledging this
uncertainty, and therefore placing less emphasis on these analyses of
study location air quality, she noted that PA analyses suggest that
standard levels of 65 or 70 ppb would require reductions, beyond those
required by the current standard, in ambient O3
concentrations present in several of the locations that provided the
basis for statistically significant O3 health effect
associations in multicity studies.
In further evaluating information from epidemiologic studies, the
Administrator considered the HREA's epidemiology-based risk estimates
for O3-associated morbidity or mortality (U.S. EPA, 2014a,
Chapter 7). Compared to the weight given to the evidence from
controlled human exposure studies, and to HREA estimates of exposures
of concern and lung function risks, she placed relatively less weight
on epidemiology-based risk estimates. In doing so, she noted that the
overall conclusions from the HREA likewise reflect relatively less
confidence in estimates of epidemiology-based risks than in estimates
of exposures of concern and lung function risks.
In considering epidemiology-based risk estimates, the Administrator
focused on risks associated with O3 concentrations in the
upper portions of ambient distributions, given the greater uncertainty
associated with the shapes of concentration-response curves for
O3 concentrations in the lower portions of ambient
distributions (i.e., below about 20 to 40 ppb depending on the
O3 metric, health endpoint, and study population) (U.S. EPA,
2013, section 2.5.4.4). The Administrator further noted that
experimental studies provide the strongest evidence for O3-
induced effects following exposures to O3 concentrations
corresponding to the upper portions of typical ambient
[[Page 65355]]
distributions. In particular, as discussed above, she noted controlled
human exposure studies showing respiratory effects following exposures
to O3 concentrations at or above 60 ppb (79 FR 75308,
December 17, 2014). Therefore, in considering risks associated with
O3 concentrations in the upper portions of ambient
distributions, the Administrator focused on the extent to which revised
standards with levels of 70 or 65 ppb are estimated to reduce the risk
of premature deaths associated with area-wide O3
concentrations at or above 40 ppb and 60 ppb.
Given all of the above evidence, exposure/risk information, and
advice from CASAC, the Administrator proposed to revise the level of
the current primary O3 standard to within the range of 65 to
70 ppb. In considering CASAC advice on the range of standard levels,
the Administrator placed a large amount of weight on CASAC's conclusion
that there is adequate scientific evidence to consider a range of
levels for a primary standard that includes an upper end at 70 ppb. She
also noted that although CASAC expressed concern about the margin of
safety at a level of 70 ppb, it further acknowledged that the choice of
a level within the range recommended based on scientific evidence is a
policy judgment (Frey, 2014c, p. ii). While she agreed with CASAC that
it is appropriate to consider levels below 70 ppb, as reflected in her
range of proposed levels from 65 to 70 ppb, for the reasons discussed
above she also concluded that a standard level as high as 70 ppb, which
CASAC concluded could be supported by the scientific evidence, could
reasonably be judged to be requisite to protect public health with an
adequate margin of safety.
In considering the appropriateness of standard levels below 65 ppb,
the Administrator noted the conclusions of the PA and the advice of
CASAC that it would be appropriate for her to consider standard levels
as low as 60 ppb. In making the decision to not propose levels below 65
ppb, she focused on CASAC's rationale for a level of 60 ppb, which
focused on the importance of limiting exposures to O3
concentrations as low as 60 ppb (Frey, 2014c, p. 7). As discussed
above, the Administrator agreed that it is appropriate to consider the
implications of a revised standard level for estimated exposures of
concern at or above 60 ppb. She noted that standards within the
proposed range of 65 to 70 ppb would be expected to substantially limit
the occurrence of exposures of concern to O3 concentrations
at or above 60 ppb, particularly the occurrence of two or more
exposures. When she further considered that not all exposures of
concern lead to adverse effects, and that the NAAQS are not meant to be
zero-risk or background standards, the Administrator judged that
alternative standard levels below 65 ppb are not needed to further
reduce such exposures.
b. Comments on Level
A number of groups representing medical, public health, or
environmental organizations; some state agencies; and many individuals
submitted comments on the appropriate level of a revised primary
O3 standard.\128\ Virtually all of these commenters
supported setting the standard level within the range recommended by
CASAC (i.e., 60 to 70). Some expressed support for the overall CASAC
range, without specifying a particular level within that range, while
others expressed a preference for the lower part of the CASAC range,
often emphasizing support for a level of 60 ppb. Some of these
commenters stated that if the EPA does not set the level at 60 ppb,
then the level should be set no higher than 65 ppb (i.e., the lower
bound of the proposed range of standard levels).
---------------------------------------------------------------------------
\128\ In general, commenters who expressed the view that the EPA
should retain the current O3 NAAQS (i.e., commenters
representing industry and business groups, and some states) did not
provide comments on alternative standard levels. As a result, this
section focuses primarily on comments from commenters who expressed
support for the proposed decision to revise the current primary
O3 standard.
---------------------------------------------------------------------------
To support their views on the level of a revised standard, some
commenters focused on overarching issues related to the statutory
requirements for the NAAQS. For example, some commenters maintained
that the primary NAAQS must be set at a level at which there is an
absence of adverse effects in sensitive populations. While this
argument has some support in the case law and in the legislative
history to the 1970 CAA (see Lead Industries Ass'n v. EPA, 647 F. 2d
1147, 1153 (D.C. Cir. 1980)), it is well established that the NAAQS are
not meant to be zero risk standards. See Lead Industries v. EPA, 647
F.2d at 1156 n.51; Mississippi v. EPA, 744 F. 3d at 1351. From the
inception of the NAAQS standard-setting process, the EPA and the courts
have acknowledged that scientific uncertainties in general, and the
lack of clear thresholds in pollutant effects in particular, preclude
any such definitive determinations. Lead Industries, 647 F. 2d at 1156
(setting standard at a level which would remove most but not all sub-
clinical effects). Likewise, the House report to the 1977 amendments
addresses this question (H. Rep. 95-294, 95th Cong. 1st sess. 127):
\129\
---------------------------------------------------------------------------
\129\ Similarly, Senator Muskie remarked during the floor
debates on the 1977 Amendments that ``there is no such thing as a
threshold for health effects. Even at the national primary standard
level, which is the health standard, there are health effects that
are not protected against''. 123 Cong. Rec. S9423 (daily ed. June
10, 1977).
Some have suggested that since the standards are to protect
against all known or anticipated effects and since no safe threshold
can be established, the ambient standards should be set at zero or
background levels. Obviously, this no-risk philosophy ignores all
economic and social consequences and is impractical. This is
particularly true in light of the legal requirement for mandatory
---------------------------------------------------------------------------
attainment of the national primary standards within 3 years.
Thus, post-1970 jurisprudence makes clear the impossibility, and
lack of legal necessity, for NAAQS removing all health risk. See ATA
III, 283 F. 3d at 360 (``[t]he lack of a threshold concentration below
which these pollutants are known to be harmless makes the task of
setting primary NAAQS difficult, as EPA must select standard levels
that reduce risks sufficiently to protect public health even while
recognizing that a zero-risk standard is not possible''); Mississippi,
744 F. 3d at 1351 (same); see also id. at 1343 (``[d]etermining what is
`requisite' to protect the `public health' with an `adequate' margin of
safety may indeed require a contextual assessment of acceptable risk.
See Whitman, 531 U.S. at 494-95 (Breyer J. concurring)'').
In this review, EPA is setting a standard based on a careful
weighing of available evidence, including a weighing of the strengths
and limitations of the evidence and underlying scientific uncertainties
therein. The Administrator's choice of standard level is rooted in her
evaluation of the evidence, which reflects her legitimate uncertainty
as to the O3 concentrations at which the public would
experience adverse health effects. This is a legitimate, and well
recognized, exercise of ``reasoned decision-making.'' ATA III. 283 F.
3d at 370; see also id. at 370 (``EPA's inability to guarantee the
accuracy or increase the precision of the . . . NAAQS in no way
undermines the standards' validity. Rather, these limitations indicate
only that significant scientific uncertainty remains about the health
effects of fine particulate matter at low atmospheric concentration. .
. .''); Mississippi, 744 F. 3d at 1352-53 (appropriate for EPA to
balance scientific uncertainties in determining level of revised
O3 NAAQS).
[[Page 65356]]
In an additional overarching comment, some commenters also
fundamentally objected to the EPA's consideration of exposure estimates
in reaching conclusions on the primary O3 standard. These
commenters' general assertion was that NAAQS must be established so as
to be protective, with an adequate margin of safety, regardless of the
activity patterns that feed into exposure estimates. They contended
that ``[a]ir quality standards cannot rely on avoidance behavior in
order to protect the public health and sensitive groups'' and that
``[i]t would be unlawful for EPA to set the standard at a level that is
contingent upon people spending most of their time indoors'' (e.g., ALA
et al., p. 124). To support these comments, for example, ALA et al.
analyzed ambient monitoring data from Core-Based Statistical Areas
(CBSAs) with design values between 66-70 ppb (Table 17, pp. 145-151 in
ALA et al.) and 62-65 ppb (Table 18, pp. 153-154 in ALA et al.) and
pointed out that there are many more days with ambient concentrations
above the benchmark levels than were estimated in the EPA's exposure
analysis (i.e., at and above the benchmark level of 60, 70 and 80 ppb).
The EPA disagrees with these commenters' conclusions regarding the
appropriateness of considering exposure estimates, and notes that NAAQS
must be ``requisite'' (i.e., ``sufficient, but not more than
necessary'' (Whitman, 531 U.S. at 473)) to protect the ``public
health'' (``the health of the public'' (Whitman, 531 U.S. at 465)).
Estimating exposure patterns based on extensive available data \130\ is
a reasonable means of ascertaining that standards are neither under-
nor over-protective, and that standards address issues of public health
rather than health issues pertaining only to isolated individuals.\131\
Behavior patterns are critical in assessing whether ambient
concentrations of O3 may pose a public health risk.\132\
Exposures to ambient or near-ambient O3 concentrations have
only been shown to result in potentially adverse effects if the
ventilation rates of people in the exposed populations are raised to a
sufficient degree (e.g., through physical exertion) (U.S. EPA, 2013,
section 6.2.1.1).\133\ Ignoring whether such elevated ventilation rates
are actually occurring, as advocated by these commenters, would not
provide an accurate assessment of whether the public health is at risk.
Indeed, a standard established without regard to behavior of the public
would likely lead to a standard which is more stringent than necessary
to protect the public health.
---------------------------------------------------------------------------
\130\ The CHAD database used in the HREA's exposure assessment
contains over 53,000 individual daily diaries including time-
location-activity patterns for individuals of both sexes across a
wide range of ages (U.S. EPA, 2014a, Chapter 5).
\131\ CASAC generally agreed with the EPA's methodology for
characterizing exposures of concern (Frey, 2014a, pp. 5-6).
\132\ See 79 FR 75269 (``The activity pattern of individuals is
an important determinant of their exposure. Variation in
O3 concentrations among various microenvironments means
that the amount of time spent in each location, as well as the level
of activity, will influence an individual's exposure to ambient
O3. Activity patterns vary both among and within
individuals, resulting in corresponding variations in exposure
across a population and over time'' (internal citations omitted).
\133\ For healthy young adults exposed at rest for 2 hours, 500
ppb is the lowest O3 concentration reported to produce a
statistically significant O3-induced group mean
FEV1 decrement (U.S. EPA, 2013, section 6.2.1.1).
---------------------------------------------------------------------------
While setting the primary O3 standard based only on
ambient concentrations, without consideration of activity patterns and
ventilation rates, would likely result in a standard that is over-
protective, the EPA also concludes that setting a standard based on the
assumption that people will adjust their activities to avoid exposures
on high-pollution days would likely result in a standard that is under-
protective. The HREA's exposure assessment does not make this latter
assumption.\134\ The time-location-activity diaries that provided the
basis for exposure estimates reflect actual variability in human
activities. While some diary days may reflect individuals spending less
time outdoors than would be typical for them, it is similarly likely
that some days reflect individuals spending more time outdoors than
would be typical. Considering the actual variability in time-location-
activity patterns is at the least a permissible way of identifying
standards that are neither over- nor under-protective.\135\
---------------------------------------------------------------------------
\134\ The EPA was aware of the possibility of averting behavior
during the development of the HREA, and that document includes
sensitivity analyses to provide perspective on the potential role of
averting behavior in modifying O3 exposures. As discussed
further above (II.B.2.c), these sensitivity analyses were limited
and the results were discussed in the proposal within the context of
uncertainties in the HREA assessment of exposures of concern.
\135\ See Mississippi, 744 F. 3d at 1343 (``[d]etermining what
is `requisite' to protect the `public health' with an `adequate'
margin of safety may indeed require a contextual assessment of
acceptable risk. See Whitman, 531 U.S. at 494-95 (Breyer, J.
concurring . . .))''
---------------------------------------------------------------------------
Further, the EPA sees nothing in the CAA that prohibits
consideration of the O3 exposures that could result in
effects of public health concern. While a number of judicial opinions
have upheld the EPA's decisions in other NAAQS reviews to place little
weight on particular risk or exposure analyses (i.e., because of
scientific uncertainties in those analyses), none of these opinions
have suggested that such analyses are irrelevant because actual
exposure patterns do not matter. See, e.g. Mississippi, 744 F. 3d at
1352-53; ATA III, 283 F. 3d at 373-74. Therefore, because behavior
patterns are critical in assessing whether ambient concentrations of
O3 may pose a public health risk, the EPA disagrees with the
views expressed by these commenters objecting to the consideration of
O3 exposures in reaching decisions on the primary
O3 standard.
In addition to these overarching comments, a number of commenters
supported their views on standard level by highlighting specific
aspects of the scientific evidence, exposure/risk information, and/or
CASAC advice. Key themes expressed by these commenters included the
following: (1) Controlled human exposure studies provide strong
evidence of adverse lung function decrements and airway inflammation in
healthy adults following exposures to O3 concentrations as
low as 60 ppb, and at-risk populations would be likely to experience
more serious effects or effects at even lower concentrations; (2)
epidemiologic studies provide strong evidence for associations with
mortality and morbidity in locations with ambient O3
concentrations below 70 ppb, and in many cases in locations with
concentrations near and below 60 ppb; (3) quantitative analyses in the
HREA are biased such that they understate O3 exposures and
risks, and the EPA's interpretation of lung function risk estimates is
not appropriate and not consistent with other NAAQS; and (4) the EPA
must give deference to CASAC advice, particularly CASAC's policy advice
to set the standard level below 70 ppb. The next sections discuss
comments related to each of these points, and provide the EPA's
responses to those comments. More detailed discussion of individual
comments, and the EPA's responses, is provided in the Response to
Comments document.
i. Effects in Controlled Human Exposure Studies
Some commenters who advocated for a level of 60 ppb (or absent
that, for 65 ppb) asserted that controlled human exposure studies have
reported adverse respiratory effects in healthy adults following
exposures to O3 concentrations as low as 60 ppb. These
commenters generally based their conclusions on the demonstration of
FEV1 decrements >= 10% and increased airway inflammation
following exposures of healthy adults to 60 ppb O3. They
concluded that even more serious effects would occur in at-risk
[[Page 65357]]
populations exposed to 60 ppb O3, and that such populations
would experience adverse effects following exposures to O3
concentrations below 60 ppb.
While the EPA agrees that information from controlled human
exposure studies conducted at 60 ppb can help to inform the
Administrator's decision on the standard level, the Agency does not
agree that this information necessitates a level below 70 ppb. In fact,
as discussed in the proposal, a revised O3 standard with a
level of 70 ppb can be expected to provide substantial protection
against the effects shown to occur following various O3
exposure concentrations, including those observed following exposures
to 60 ppb. This is because the degree of protection provided by any
NAAQS is due to the combination of all of the elements of the standard
(i.e., indicator, averaging time, form, level). In the case of the
fourth-high form of the O3 NAAQS, which the Administrator is
retaining in the current review (II.C.3), the large majority of days in
areas that meet the standard will have 8-hour O3
concentrations below the level of the standard, with most days well
below the level. Therefore, as discussed in the proposal, in
considering the degree of protection provided by an O3
standard with a particular level, it is important to consider the
extent to which that standard would be expected to limit population
exposures of concern to the broader range of O3 exposure
concentrations shown in controlled human exposure studies to result in
health effects. The Administrator's consideration of such exposures of
concern is discussed below (II.C.4.c).
Another important part of the Administrator's consideration of
exposure estimates is the extent to which she judges that adverse
effects could occur following specific O3 exposures. While
controlled human exposure studies provide a high degree of confidence
regarding the extent to which specific health effects occur following
exposures to O3 concentrations from 60 to 80 ppb, the
Administrator notes that there are no universally accepted criteria by
which to judge the adversity of the observed effects. Therefore, in
making judgments about the extent to which the effects observed in
controlled human exposure studies have the potential to be adverse, the
Administrator considers the recommendations of ATS and advice from
CASAC (II.A.1.c, above).
As an initial matter, with regard to the effects shown in
controlled human exposure studies following O3 exposures,
the Administrator notes the following:
1. The largest respiratory effects, and the broadest range of
effects, have been studied and reported following exposures to 80 ppb
O3 or higher, with most exposure studies conducted at these
higher concentrations. Specifically, 6.6-hour exposures of healthy
young adults to 80 ppb O3, while engaged in quasi-
continuous, moderate exertion, can decrease lung function, increase
airway inflammation, increase respiratory symptoms, result in airway
hyperresponsiveness, and decrease lung host defenses.
2. Exposures of healthy young adults for 6.6 hours to O3
concentrations as low as 72 ppb, while engaged in quasi-continuous,
moderate exertion, have been shown to both decrease lung function and
result in respiratory symptoms.
3. Exposures of healthy young adults for 6.6 hours to O3
concentrations as low as 60 ppb, while engaged in quasi-continuous,
moderate exertion, have been shown to decrease lung function and to
increase airway inflammation.
To inform her judgments on the potential adversity to public health
of these effects reported in controlled human exposure studies, as in
the proposal, the Administrator considers the ATS recommendation that
``reversible loss of lung function in combination with the presence of
symptoms should be considered adverse'' (ATS, 2000a). She notes that
this combination of effects has been shown to occur following 6.6-hour
exposures to O3 concentrations at or above 72 ppb. In
considering these effects, CASAC observed that ``the combination of
decrements in FEV1 together with the statistically
significant alterations in symptoms in human subjects exposed to 72 ppb
ozone meets the American Thoracic Society's definition of an adverse
health effect'' (Frey, 2014c, p. 5).
Regarding the potential for adverse effects following exposures to
lower concentrations, the Administrator notes the CASAC judgment that
the adverse combination of lung function decrements and respiratory
symptoms ``almost certainly occur in some people'' following exposures
to O3 concentrations below 72 ppb (Frey, 2014c, p. 6). In
particular, when commenting on the extent to which the study by
Schelegle et al. (2009) suggests the potential for adverse effects
following O3 exposures below 72 ppb, CASAC judged that:
[I]f subjects had been exposed to ozone using the 8-hour
averaging period used in the standard [rather than the 6.6-hour
exposures evaluated in the study], adverse effects could have
occurred at lower concentration. Further, in our judgment, the level
at which adverse effects might be observed would likely be lower for
more sensitive subgroups, such as those with asthma (Frey, 2014c, p.
5).
Though CASAC did not provide advice as to how far below 72 ppb
adverse effects would likely occur, the Administrator agrees that such
effects could occur following exposures at least somewhat below 72 ppb.
The Administrator notes that while adverse effects could occur
following exposures at least somewhat below 72 ppb, the combination of
statistically significant increases in respiratory symptoms and
decrements in lung function has not been reported following 6.6-hour
exposures to average O3 concentrations of 60 ppb or 63 ppb,
though studies have evaluated the potential for such effects (Adams,
2006; Schelegle et al., 2009; Kim et al., 2011). In the absence of this
combination, the Administrator looks to additional ATS recommendations
and CASAC advice in order to inform her judgments regarding the
potential adversity of the effects that have been observed following
O3 exposures as low as 60 ppb.
With regard to ATS, she first notes the recommendations that ``a
small, transient loss of lung function, by itself, should not
automatically be designated as adverse'' and that ``[f]ew . . .
biomarkers have been validated sufficiently that their responses can be
used with confidence to define the point at which a response should be
equated to an adverse effect warranting preventive measures'' (ATS,
2000a).\136\ Based on these recommendations, compared to effects
following exposures at or above 72 ppb, the Administrator has less
confidence in the adversity of the respiratory effects that have been
observed following exposures to 60 or 63 ppb.
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\136\ With regard to this latter recommendation, as discussed
above (II.A.1.c), the ATS concluded that elevations of biomarkers
such as cell numbers and types, cytokines, and reactive oxygen
species may signal risk for ongoing injury and more serious effects
or may simply represent transient responses, illustrating the lack
of clear boundaries that separate adverse from nonadverse events.
---------------------------------------------------------------------------
She further notes that some commenters who advocated for a level of
60 ppb also focused on ATS recommendations regarding population-level
risks. These commenters specifically stated that lung function
decrements ``may be adverse in terms of `population risk,' where
exposure to air pollution increases the risk to the population even
though it might not harm lung function to a degree that is, on its own,
`clinically important' to an individual'' (e.g., ALA et al., p. 118).
These commenters asserted that the EPA
[[Page 65358]]
has not appropriately considered the potential for such population-
level risk. Contrary to the views expressed by these commenters, the
Administrator carefully considers the potential for population risk,
particularly within the context of the ATS recommendation that ``a
shift in the risk factor distribution, and hence the risk profile of
the exposed population, should be considered adverse, even in the
absence of the immediate occurrence of frank illness'' (ATS, 2000a).
Given that exposures to 60 ppb O3 have been shown in
controlled human exposure studies to cause transient and reversible
decreases in group mean lung function, the Administrator notes the
potential for such exposures to result in similarly transient and
reversible shifts in the risk profile of an exposed population.
However, in contrast to commenters who advocated for a level of 60 ppb,
the Administrator also notes that the available evidence does not
provide information on the extent to which a short-term, transient
decrease in lung function in a population, as opposed to a longer-term
or permanent decrease, could affect the risk of other, more serious
respiratory effects (i.e., change the risk profile of the population).
This uncertainty, together with the additional ATS recommendations
noted above, indicates to the Administrator that her judgment that
there is uncertainty in the adversity of the effects shown to occur at
60 ppb is consistent with ATS recommendations.\137\
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\137\ ATS provided additional recommendations to help inform
judgments regarding the adversity of air pollution-related effects
(e.g., related to ``quality of life''), though it is not clear
whether, or how, such recommendations should be applied to the
respiratory effects observed in controlled human exposure studies
following 6.6-hour O3 exposures (ATS, 200a, p. 672).
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With regard to CASAC advice, the Administrator notes that, while
CASAC clearly advised the EPA to consider the health effects shown to
occur following exposures to 60 ppb O3, its advice regarding
the adversity of those effects is less clear. In particular, she notes
that CASAC was conditional about whether the lung function decrements
observed in some people at 60 ppb (i.e., FEV1 decrements >=
10%) are adverse. Specifically, CASAC stated that these decrements
``could be adverse in individuals with lung disease'' (Frey, 2014c, p.
7, emphasis added) and that they provide a ``surrogate for adverse
health outcomes for people with asthma and lung disease'' (Frey, 2014c,
p. 3, emphasis added). Further, CASAC did not recommend considering
standard levels low enough to eliminate O3-induced
FEV1 decrements >= 10% (Frey, 2014c). With regard to the
full range of effects shown to occur at 60 ppb (i.e., FEV1
decrements, airway inflammation), CASAC stated that exposures of
concern for the 60 ppb benchmark are ``relevant for consideration''
with respect to people with asthma (Frey, 2014c, p. 6, italics added).
In addition, ``[t]he CASAC concurs with EPA staff regarding the finding
based on scientific evidence that a level of 60 ppb corresponds to the
lowest exposure concentration demonstrated to result in lung function
decrements large enough to be judged an abnormal response by ATS and
that could be adverse in individuals with lung disease'' (Frey, 2014c,
p. 7, italics added). The Administrator contrasts these statements with
CASAC's clear advice that ``the combination of decrements in
FEV1 together with the statistically significant alterations
in symptoms in human subjects exposed to 72 ppb ozone meets the
American Thoracic Society's definition of an adverse health effect''
(Frey, 2014c, p. 5).
Based on her consideration of all of the above recommendations and
advice noted above, the Administrator judges that, compared to exposure
concentrations at and above 72 ppb, there is greater uncertainty with
regard to the adversity of effects shown to occur following
O3 exposures as low as 60 ppb. However, based on the effects
that have been shown to occur at 60 ppb (i.e., lung function
decrements, airway inflammation), and CASAC advice indicating the
importance of considering these effects (though its advice regarding
the adversity of effects at 60 ppb is less clear), she concludes that
it is appropriate to give some consideration to the extent to which a
revised standard could allow such effects.
In considering estimates of exposures of concern for the 60, 70,
and 80 ppb benchmarks within the context of her judgments on adversity,
the Administrator notes that, due to interindividual variability in
responsiveness, not every occurrence of an exposure of concern will
result in an adverse effect. As discussed above (II.B.2.b.i), this
point was highlighted by some commenters who opposed revision of the
current standard, based on their analysis of effects shown to occur
following exposures to 72 ppb O3. This point was also
highlighted by some commenters who advocated for a level of 60 ppb,
based on the discussion of O3-induced inflammation in the
proposal. In particular, this latter group of commenters highlighted
discussion from the proposal indicating that ``[i]nflammation induced
by a single O3 exposure can resolve entirely but, as noted
in the ISA (U.S. EPA, 2013, p. 6-76), `continued acute inflammation can
evolve into a chronic inflammatory state''' (e.g., ALA et al., p. 48).
Consistent with these comments, and with her consideration of estimated
exposurs of concern in the proposal, the Administrator judges that the
types of respiratory effects that can occur following exposures of
concern, particularly if experienced repeatedly, provide a plausible
mode of action by which O3 may cause other more serious
effects. Because of this, as in the proposal, the Administrator is most
concerned about protecting against repeated occurrences of exposures of
concern.
The Administrator's consideration of estimated exposures of concern
is discussed in more detail below (II.C.4.b.iv, II.C.4.c). In summary,
contrary to the conclusions of commenters who advocated for a level of
60 ppb, the Administrator judges that a revised standard with a level
of 70 ppb will effectively limit the occurrence of the O3
exposures for which she is most confident in the adversity of the
resulting effects (i.e., based on estimates for the 70 and 80 ppb
benchmarks). She further concludes that such a standard will provide
substantial protection against the occurrence of O3
exposures for which there is greater uncertainty in the adversity of
effects (i.e., based on estimates for the 60 ppb benchmark).
As noted above, commenters also pointed out that benchmark
concentrations are based on studies conducted in healthy adults,
whereas at-risk populations are likely to experience more serious
effects and effects at lower O3 exposure concentrations. In
considering this issue, the EPA notes CASAC's endorsement of 60 ppb as
the lower end of the range of benchmarks for evaluation, and its advice
that ``the 60 ppb-8hr exposure benchmark is relevant for consideration
with respect to adverse effects on asthmatics'' (Frey, 2014c, p. 6). As
discussed in detail below (II.C.4.c), the Administrator has carefully
considered estimated exposures of concern for the 60 ppb benchmark. In
addition, though the available information does not support the
identification of specific benchmarks below 60 ppb that could be
appropriate for consideration for at-risk populations, and though CASAC
did not recommend consideration of any such benchmarks, the EPA expects
that a revised standard with a level of 70 ppb will also reduce the
occurrence of exposures to O3 concentrations at least
somewhat below 60 ppb (U.S. EPA,
[[Page 65359]]
2014a, Figures 4-9 and 4-10).\138\ Thus, even if some members of at-
risk populations may experience effects following exposures to
O3 concentrations somewhat below 60 ppb, a revised level of
70 ppb would be expected to reduce the occurrence of such
exposures.\139\ Therefore, the EPA has considered O3
exposures that could be relevant for at-risk populations such as
children and people with asthma, and does not agree that controlled
human exposure studies reporting respiratory effects in healthy adults
following exposures to 60 ppb O3 necessitate a standard
level below 70 ppb.
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\138\ Air quality analyses in the HREA indicate that reducing
the level of the primary standard from 75 ppb to 70 ppb will result
in reductions in the O3 concentrations in the upper
portions of ambient distributions. This includes 8-hour ambient
O3 concentrations at, and somewhat below, 60 ppb (U.S.
EPA, 2014a, Figures 4-9 and 4-10).
\139\ The uncertainty associated with the potential adversity of
any such effects would be even greater than that discussed above for
the 60 ppb benchmark.
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ii. Epidemiologic Studies
Commenters representing environmental and public health
organizations also highlighted epidemiologic studies that, in their
view, provide strong evidence for associations with mortality and
morbidity in locations with ambient O3 concentrations near
and below 60 ppb. These commenters focused both on the epidemiologic
studies evaluated in the PA's analyses of study location air quality
(U.S. EPA, 2014c, Chapter 4) and on studies that were not explicitly
analyzed in the PA, and in some cases on studies that were not included
in the ISA.
The EPA agrees that epidemiologic studies can provide perspective
on the degree to which O3-associated health effects have
been identified in areas with air quality likely to have met various
standards. However, as discussed below, we do not agree with the
specific conclusions drawn by these commenters regarding the
implications of epidemiologic studies for the standard level. As an
initial matter in considering epidemiologic studies, the EPA notes its
decision, consistent with CASAC advice, to place the most emphasis on
information from controlled human exposure studies (II.B.2 and II.B.3,
above). This decision reflects the greater certainty in using
information from controlled human exposure studies to link specific
O3 exposures with health effects, compared to using air
quality information from epidemiologic studies of O3 for
this purpose.
While being aware of the uncertainties discussed above
(II.B.2.b.ii), in considering what epidemiologic studies can tell us,
the EPA notes analyses in the PA (U.S. EPA, 2014c, section 4.4.1)
indicating that a revised standard with a level at or below 70 ppb
would be expected to maintain distributions of short-term ambient
O3 concentrations below those present in the locations of
all of the single-city epidemiologic studies analyzed. As discussed in
the PA (U.S. EPA, 2014c, section 4.4.1), this includes several single-
city studies conducted in locations that would have violated the
current standard, and the study by Mar and Koenig (2009) that reported
positive and statistically significant associations with respiratory
emergency department visits with children and adults in a location that
would have met the current standard over the entire study period, but
would have violated a standard with a level of 70 ppb.\140\ While these
analyses provide support for a level at least as low as 70 ppb, the
Administrator judges that they do not provide a compelling basis for
distinguishing between the appropriateness of 70 ppb and lower standard
levels.
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\140\ As noted above (II.B.2.b.ii and II.B.3), the studies by
Silverman and Ito (2010) and Strickland et al. (2010) provided
support for the Administrator's decision to revise the current
primary O3 standard, but do not provide insight into the
appropriateness of specific standard levels below 75 ppb.
---------------------------------------------------------------------------
As in the proposal, the EPA acknowledges additional uncertainty in
interpreting air quality in locations of multicity epidemiologic
studies of short-term O3 for the purpose of evaluating
alternative standard levels (U.S. EPA, 2014c, sections 3.1.4.2, 4.4.1).
In particular, the PA concludes that interpretation of such air quality
information is complicated by uncertainties in the extent to which
multicity effect estimates (i.e., which are based on combining
estimates from multiple study locations) can be attributed to ambient
O3 in the subset of study locations that would have met a
particular standard, versus O3 in the study locations that
would have violated the standard. While giving only limited weight to
air quality analyses in these study areas because of this uncertainty,
the EPA also notes PA analyses indicating that a standard level at or
below 70 ppb would require additional reductions, beyond those required
by the current standard, in the ambient O3 concentrations
that provided the basis for statistically significant O3
health effect associations in multicity epidemiologic studies. As was
the case for the single-city studies, and contrary to the views
expressed by the commenters noted above, the Administrator judges that
these studies do not provide a compelling basis for distinguishing
between the appropriateness of alternative standard levels at or below
70 ppb.
In some cases, commenters highlighted studies that were assessed in
the 2008 review of the O3 NAAQS, but were not included in
the ISA in the current review. These commenters asserted that such
studies support the occurrence of O3 health effect
associations in locations with air quality near or, in some cases,
below 60 ppb. Specifically, commenters highlighted a number of studies
included in the 2007 Staff Paper that were not included in the ISA,
claiming that these studies support a standard level below 70 ppb, and
as low as 60 ppb.
As an initial matter with regard to these studies, the EPA notes
that the focus of the ISA is on assessing the most policy-relevant
scientific evidence. In the current review, the ISA considered over
1,000 new studies that have been published since the last review. Thus,
it is not surprising that, as the body of evidence has been
strengthened since the last review, some of the studies considered in
the last review are no longer among the most policy relevant. However,
based on the information included in the 2007 Staff Paper, the EPA does
not agree that the studies highlighted by commenters provide compelling
support for a level below 70 ppb. In fact, as discussed in the Staff
Paper in the last review (U.S. EPA, 2007, p. 6-9; Appendix 3B), the
O3 concentrations reported for these studies, and the
concentrations highlighted by commenters, were based on averaging
across multiple monitors in study areas. Given that the highest monitor
in an area is used to determine whether that area meets or violates the
NAAQS, the averaged concentrations reported in the Staff Paper are thus
not appropriate for direct comparison to the level of the O3
standard. When the Staff Paper considered the O3
concentrations measured at individual monitors for the subset of these
study areas with particularly low concentrations, they were almost
universally found to be above, and in many cases well above, even the
current standard level of 75 ppb.\141\ Based on the above
[[Page 65360]]
considerations, and consistent with the Administrator's overall
decision to place less emphasis on air quality in locations of
epidemiologic studies to select a standard level, the EPA disagrees
with commenters who asserted that epidemiologic studies included in the
last review, but not cited in the ISA or PA in this review, necessitate
a level below 70 ppb. In fact, the EPA notes that these studies are
consistent with the majority of the U.S. studies evaluated in the PA in
the current review, in that most were conducted in locations that would
have violated the current O3 NAAQS over at least part of the
study periods.
---------------------------------------------------------------------------
\141\ For one study conducted in Vancouver, where data from
individual monitors did indicate ambient concentrations below the
level of the current standard (Vedal et al., 2003), the Staff Paper
noted that the study authors questioned whether O3, other
gaseous pollutants, and PM in this study may be acting as surrogate
markers of pollutant mixes that contain more toxic compounds,
``since the low measured concentrations were unlikely, in their
opinion, to cause the observed effects'' (U.S. EPA, 2007, p. 6-16).
The Staff Paper further noted that another study conducted in
Vancouver failed to find statistically significant associations with
O3 (Villeneuve et al., 2003).
---------------------------------------------------------------------------
iii. Exposure and Risk Assessments
Some commenters supporting levels below 70 ppb also asserted that
quantitative analyses in the HREA are biased such that they understate
O3 exposures of concern and risks of O3-induced
FEV1 decrements. Many of these comments are discussed above
within the context of the adequacy of the current standard
(II.B.2.b.i), including comments pointing out that exposure and risk
estimates are based on information from healthy adults rather than at-
risk populations; comments noting that the exposure assessment
evaluates 8-hour O3 exposures rather than the 6.6-hour
exposures used in controlled human exposure studies; and comments
asserting that the EPA's exposure and risk analyses rely on people
staying indoors on high pollution days (i.e., averting behavior).
As discussed in section II.B.2.b.i above, while the EPA agrees with
certain aspects of these commenters' assertions, we do not agree with
their overall conclusions. In particular, there are aspects of the
HREA's quantitative analyses that, if viewed in isolation, would tend
to either overstate or understate O3 exposures and/or health
risks. While commenters tended to focus on those aspects of the
assessments that support their position, they tended to ignore aspects
of the assessments that do not support their position (points that were
often raised by commenters on the other side of the issue). Rather than
viewing the potential implications of these aspects of the HREA
assessments in isolation, the EPA considers them together, along with
other issues and uncertainties related to the interpretation of
exposure and risk estimates.
For example, some commenters who advocated for a level below 70 ppb
asserted that the exposure assessment could underestimate O3
exposures for highly active populations, including outdoor workers and
children who spend a large portion of time outdoors during summer. In
support of these assertions, commenters highlighted sensitivity
analyses conducted in the HREA. However, as noted in the HREA (U.S.
EPA, 2014a, Table 5-10), this aspect of the assessment is likely to
have only a ``low to moderate'' impact on the magnitude of exposure
estimates. To put this magnitude in perspective, HREA sensitivity
analyses conducted in a single urban study area indicate that,
regardless of whether exposure estimates for children are based on all
available diaries or on a subset of diaries restricted to simulate
highly exposed children, a revised standard with a level of 70 ppb is
estimated to protect more than 99% of children from experiencing two or
more exposures of concern at or above 70 ppb (U.S. EPA, 2014a, Chapter
5 Appendices, Figure 5G-9).142 143 In contrast to the focus
of commenters who supported a level below 70 ppb, other aspects of
quantitative assessments, some of which were highlighted by commenters
who opposed revising the current standard (II.B.2), tend to result in
overestimates of O3 exposures. These aspects are
characterized in the HREA as having either a ``low,'' a ``low-to-
moderate,'' or a ``moderate'' impact on the magnitudes of exposure
estimates.
---------------------------------------------------------------------------
\142\ More specifically, based on all children's diaries, just
under 0.1% of children are estimated to experience two or more
exposures of concern at or above 70 ppb. Based on simulated profiles
of highly exposed children, this estimate increased to just over
0.1% (U.S. EPA, 2014a, Chapter 5 Appendices, Figure 5G-9).
\143\ In addition, when diaries were selected to mimic exposures
that could be experienced by outdoor workers, the percentages of
modeled individuals estimated to experience exposures of concern
were generally similar to the percentages estimated for children
(i.e., using the full database of diary profiles) in the worst-case
cities and years (i.e., cities and years with the highest exposure
estimates) (U.S. EPA, 2014, section 5.4.3.2, Figure 5-14).
---------------------------------------------------------------------------
In its reviews of the HREA and PA, CASAC recognized many of the
uncertainties and issues highlighted by commenters. Even considering
these uncertainties, CASAC endorsed the approaches adopted by the EPA
to assess O3 exposures and health risks, and CASAC used
exposure and risk estimates as part of the basis for their
recommendations on the primary O3 NAAQS (Frey, 2014c). Thus,
as discussed in section II.B.2.b.i above, the EPA disagrees with
commenters who claim that the aspects of the quantitative assessments
that they highlight lead to overall underestimates of exposures or
health risks.\144\
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\144\ As discussed in II.B.2.b above, in weighing the various
uncertainties, which can bias exposure results in different
directions but tend to have impacts that are similar in magnitude
(U.S. EPA, 2014a, Table 5-10), and in light of CASAC's advice based
on its review of the HREA and the PA, the EPA continues to conclude
that the approach to considering estimated exposures of concern in
the HREA, PA, and the proposal reflects an appropriate balance, and
provides an appropriate basis for considering the public health
protectiveness of the primary O3 standard.
---------------------------------------------------------------------------
Some commenters further contended that the level of the primary
O3 standard should be set below 70 ppb in order to
compensate for the use of a form that allows multiple days with
concentrations higher than the standard level. These groups submitted
air quality analyses to support their point that the current fourth-
high form allows multiple days per year with ambient O3
concentrations above the level of the standard. While the EPA does not
dispute the air quality analyses submitted by these commenters, and
agrees that fourth-high form allows multiple days per year with ambient
O3 concentrations above the level of the standard (3 days
per year, on average over a 3-year period), the Agency disagrees with
commenters' assertion that, because of this, the level of the primary
O3 standard should be set below 70 ppb. As discussed above
(II.A.2), the quantitative assessments that informed the
Administrator's proposed decision, presented in the HREA and considered
in the PA and by CASAC, estimated O3 exposures and health
risks associated with air quality that ``just meets'' various standards
with the current 8-hour averaging time and fourth-high, 3-year average
form. Thus, in considering the degree of public health protection
appropriate for the primary O3 standard, the Administrator
has considered quantitative exposure and risk estimates that are based
a fourth-high form, and therefore on a standard that, as these
commenters point out, allows multiple days per year with ambient
O3 concentrations above the level of the standard.
iv. CASAC Advice
Many commenters, including those representing major medical, public
health, or environmental groups; some state agencies; and a large
number of individual commenters, focused on CASAC advice in their
rationale supporting levels below 70 ppb, and as low as 60 ppb. These
commenters generally asserted that the EPA must
[[Page 65361]]
give deference to CASAC. In some cases, these commenters expressed
strong objections to a level of 70 ppb, noting CASAC policy advice that
such a level would provide little margin of safety.
The EPA agrees that CASAC advice is an important consideration in
reaching a decision on the standard level (see e.g. CAA section 307
(d)(3)),\145\ though not with commenters' conclusion that CASAC advice
necessitates a standard level below 70 ppb. As discussed above
(II.C.4.a), the Administrator carefully considered CASAC advice in the
proposal, and she judged that her proposed decision to revise the level
to within the range of 65 to 70 ppb was consistent with CASAC advice,
based on the available science.
---------------------------------------------------------------------------
\145\ The EPA notes, of course, that the CAA places the
responsibility for judging what standard is requisite with the
Administrator and only requires that, if her decision differs in
important ways from CASAC's advice, she explain her reasoning for
differing.
---------------------------------------------------------------------------
As in the proposal, in her final decision on level the
Administrator notes CASAC's overall conclusion that ``based on the
scientific evidence from clinical studies, epidemiologic studies,
animal toxicology studies, as summarized in the ISA, the findings from
the exposure and risk assessments as summarized in the HREA, and the
interpretation of the implications of all of these sources of
information as given in the Second Draft PA . . . there is adequate
scientific evidence to recommend a range of levels for a revised
primary ozone standard from 70 ppb to 60 ppb'' (Frey, 2014c, p. 8).
Thus, CASAC used the health evidence and exposure/risk information to
inform its range of recommended standard levels, a range that included
an upper bound of 70 ppb based on the scientific evidence, and it did
not use the evidence and information to recommend setting the primary
O3 standard at any specific level within the range of 70 to
60 ppb. In addition, CASAC further stated that ``the choice of a level
within the range recommended based on scientific evidence [i.e., 70 to
60 ppb] is a policy judgment under the statutory mandate of the Clean
Air Act'' (Frey, 2014c, p. ii).
In addition to its advice based on the scientific evidence, CASAC
offered the ``policy advice'' to set the level below 70 ppb, stating
that a standard level of 70 ppb ``may not meet the statutory
requirement to protect public health with an adequate margin of
safety'' (Frey, 2014c, p. ii). In supporting its policy advice to set
the level below 70 ppb, CASAC noted the respiratory effects that have
been shown to occur in controlled human exposure studies following
exposures from 60 to 80 ppb O3, and the extent to which
various standard levels are estimated to allow the occurrence of
population exposures that can result in such effects (Frey, 2014c, pp.
7-8).
The EPA agrees that an important consideration when reaching a
decision on level is the extent to which a revised standard is
estimated to allow the types of exposures shown in controlled human
exposure studies to cause respiratory effects. In reaching her final
decision that a level of 70 ppb is requisite to protect public health
with an adequate margin of safety (II.C.4.c, below), the Administrator
carefully considers the potential for such exposures and effects. In
doing so, she emphasizes the importance of setting a standard that
limits the occurrence of the exposures about which she is most
concerned (i.e., those for which she has the most confidence in the
adversity of the resulting effects, which are repeated exposures of
concern at or above 70 or 80 ppb, as discussed above in II.C.4.b.i).
Based on her consideration of information from controlled human
exposure studies in light of CASAC advice and ATS recommendations, the
Administrator additionally judges that there is important uncertainty
in the extent to which the effects shown to occur following exposures
to 60 ppb O3 are adverse to public health (discussed above,
II.C.4.b.i and II.C.4.b.iii). However, based on the effects that have
been shown to occur, CASAC advice indicating the importance of
considering these effects, and ATS recommendations indicating the
potential for adverse population-level effects (II.C.4.b.i,
II.C.4.b.iii), she concludes that it is appropriate to give some
consideration to the extent to which a revised standard could allow the
respiratory effects that have been observed following exposures to 60
ppb O3.
When considering the extent to which a revised standard could allow
O3 exposures that have been shown in controlled human
exposures studies to result in respiratory effects, the Administrator
is most concerned about protecting the public, including at-risk
populations, against repeated occurrences of such exposures of concern
(II.C.4.b.i, above). In considering the appropriate metric for
evaluating repeated occurrences of exposures of concern, the
Administrator acknowledges that it is not clear from the evidence, or
from the ATS recommendations, CASAC advice, or public comments, how
particular numbers of exposures of concern could impact the seriousness
of the resulting effects, especially at lower exposure concentrations.
Therefore, the Administrator judges that focusing on HREA estimates of
two or more exposures of concern provides a health-protective approach
to considering the potential for repeated occurrences of exposures of
concern that could result in adverse effects. She notes that other
possible metrics for considering repeated occurrences of exposures of
concern (e.g., 3 or more, 4 or more, etc.) would result in smaller
exposure estimates.
As discussed further below (II.C.4.c), the Administrator notes that
a revised standard with a level of 70 ppb is estimated to eliminate the
occurrence of two or more exposures of concern to O3
concentrations at or above 80 ppb and to virtually eliminate the
occurrence of two or more exposures of concern to O3
concentrations at or above 70 ppb (Table 1, above). For the 70 ppb
benchmark, this reflects about a 90% reduction in the number of
children estimated to experience two or more exposures of concern,
compared to the current standard.\146\ Even considering the worst-case
urban study area and worst-case year evaluated in the HREA, a standard
with a level of 70 ppb is estimated to protect more than 99% of
children from experiencing two or more exposures of concern to
O3 concentrations at or above 70 ppb (Table 1).
---------------------------------------------------------------------------
\146\ Percent reductions in this section refer to reductions in
the number of children in HREA urban study areas (averaged over the
years evaluated in the HREA) estimated to experience exposures of
concern, based on the information in Table 1 above.
---------------------------------------------------------------------------
Though the Administrator judges that there is greater uncertainty
with regard to the occurrence of adverse effects following exposures as
low as 60 ppb, she notes that a revised standard with a level of 70 ppb
is estimated to protect the vast majority of children in urban study
areas (i.e., about 96% to more than 99% in individual areas) from
experiencing two or more exposures of concern at or above 60 ppb.
Compared to the current standard, this represents a reduction of more
than 60% in exposures of concern for the 60 ppb benchmark (Table 1).
Given the Administrator's uncertainty regarding the adversity of the
effects following exposures to 60 ppb O3, and her health-
protective approach to considering repeated occurrences of exposures of
concern, the Administrator judges that this degree of protection is
appropriate and that it reflects substantial protection against the
occurrence of O3-induced effects, including effects for
which she judges the adversity to public health is uncertain.
[[Page 65362]]
While being less concerned about single occurrences of exposures of
concern, especially at lower exposure concentrations, the Administrator
also notes that a standard with a level of 70 ppb is estimated to (1)
virtually eliminate all occurrences of exposures of concern at or above
80 ppb; (2) protect >= about 99% of children in urban study areas from
experiencing any exposures of concern at or above 70 ppb; and (3) to
achieve substantial reductions (i.e., about 50%), compared to the
current standard, in the occurrence of one or more exposures of concern
at or above 60 ppb (Table 1).
Given the information and advice noted above (and in II.C.4.b.i,
II.C.4.b.iii), the Administrator judges that a revised standard with a
level of 70 ppb will effectively limit the occurrence of the
O3 exposures for which she has the most confidence in the
adversity of the resulting effects (i.e., based on estimates for the 70
and 80 ppb benchmarks). She further judges that such a standard will
provide a large degree of protection against O3 exposures
for which there is greater uncertainty in the adversity of effects
(i.e., those observed following exposures to 60 ppb O3),
contributing to the margin of safety of the standard. See Mississippi,
744 F. 3d at 1353 (``By requiring an `adequate margin of safety',
Congress was directing EPA to build a buffer to protect against
uncertain and unknown dangers to human health''). Given the
considerable protection provided against repeated exposures of concern
for all of the benchmarks evaluated, including the 60 ppb benchmark,
the Administrator judges that a standard with a level of 70 ppb will
provide an adequate margin of safety against the adverse O3-
induced effects shown to occur following exposures at or above 72 ppb,
and judged by CASAC likely to occur following exposures somewhat below
72 ppb.\147\
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\147\ As discussed above (II.C.4.b.i), when commenting on the
extent to which the study by Schelegle et al. (2009) suggests the
potential for adverse effects following O3 exposures
below 72 ppb, CASAC stated the following: ``[I]f subjects had been
exposed to ozone using the 8-hour averaging period used in the
standard [rather than the 6.6-hour exposures evaluated in the
study], adverse effects could have occurred at lower concentration.
Further, in our judgment, the level at which adverse effects might
be observed would likely be lower for more sensitive subgroups, such
as those with asthma'' (Frey, 2014c, p. 5).
---------------------------------------------------------------------------
Contrary to the conclusions of commenters who advocated for a level
below 70 ppb, the Administrator notes that her final decision is
consistent with CASAC's advice, based on the scientific evidence, and
with CASAC's focus on setting a revised standard to further limit the
occurrence of the respiratory effects observed in controlled human
exposure studies, including effects observed following exposures to 60
ppb O3. Given her judgments and conclusions discussed above,
and given that the CAA reserves the choice of the standard that is
requisite to protect public health with an adequate margin of safety
for the judgment of the EPA Administrator, she disagrees with
commenters who asserted that CASAC advice necessitates a level below 70
ppb, and as low as 60 ppb. The Administrator's final conclusions on
level are discussed in more detail below (II.C.4.c).
c. Administrator's Final Decision Regarding Level
Having carefully considered the public comments on the appropriate
level of the primary O3 standard, as discussed above and in
the Response to Comments document, the Administrator believes her
scientific and policy judgments in the proposal remain valid. In
conjunction with her decisions to retain the current indicator,
averaging time, and form (II.C.1 to II.C.3, above), the Administrator
is revising the level of the primary O3 standard to 70 ppb.
In doing so, she is selecting a primary O3 standard that is
requisite to protect public health with an adequate margin of safety,
in light of her judgments based on an interpretation of the scientific
evidence and exposure/risk information that neither overstates nor
understates the strengths and limitations of that evidence and
information and the appropriate inferences to be drawn therefrom.
The Administrator's decision to revise the level of the primary
O3 standard to 70 ppb builds upon her conclusion that the
overall body of scientific evidence and exposure/risk information calls
into question the adequacy of public health protection afforded by the
current standard, particularly for at-risk populations and lifestages
(II.B.3).\148\ Consistent with the proposal, her decision on level
places the greatest emphasis on the results of controlled human
exposure studies and on quantitative analyses based on information from
these studies, particularly analyses of O3 exposures of
concern. As in the proposal, and as discussed further below, she views
the results of the lung function risk assessment, analyses of
O3 air quality in locations of epidemiologic studies, and
epidemiology-based quantitative health risk assessments as providing
information in support of her decision to revise the current standard,
but a more limited basis for selecting a particular standard level
among a range of options. See Mississippi, 744 F. 3d at 1351-52
(studies can legitimately support a decision to revise the standard,
but not provide sufficient information to justify their use in setting
the level of a revised standard).
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\148\ At-risk populations include people with asthma; children
and older adults; people who are active outdoors, including outdoor
workers; people with certain genetic variants; and people with
reduced intake of certain nutrients.
---------------------------------------------------------------------------
Given her consideration of the evidence, exposure/risk information,
advice from CASAC, and public comments, the Administrator judges that a
standard with a level of 70 ppb is requisite to protect public health
with an adequate margin of safety. She notes that the determination of
what constitutes an adequate margin of safety is expressly left to the
judgment of the EPA Administrator. See Lead Industries Association v.
EPA, 647 F.2d at 1161-62; Mississippi, 744 F. 3d at 1353. She further
notes that in evaluating how particular standards address the
requirement to provide an adequate margin of safety, it is appropriate
to consider such factors as the nature and severity of the health
effects, the size of sensitive population(s) at risk, and the kind and
degree of the uncertainties present (I.B, above). Consistent with past
practice and long-standing judicial precedent, the Administrator takes
the need for an adequate margin of safety into account as an integral
part of her decision-making on the appropriate level, averaging time,
form, and indicator of the standard.\149\
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\149\ See, e.g. NRDC v. EPA, 902 F. 2d 962, 973-74 (D.C. Cir.
1990).
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In considering the need for an adequate margin of safety, the
Administrator notes that a standard with a level of 70 ppb
O3 would be expected to provide substantial improvements in
public health, including for at-risk groups such as children and people
with asthma. The following paragraphs summarize the basis for the
Administrator's conclusion that a revised primary O3
standard with a level of 70 ppb is requisite to protect the public
health with an adequate margin of safety.
As an initial matter, consistent with her conclusions on the need
for revision of the current standard (II.B.3), in reaching a decision
on level the Administrator places the most weight on information from
controlled human exposure studies. In doing so, she notes that
controlled human exposure studies provide the most certain evidence
indicating the occurrence of health
[[Page 65363]]
effects in humans following specific O3 exposures. In
particular, she notes that the effects reported in controlled human
exposure studies are due solely to O3 exposures, and
interpretation of study results is not complicated by the presence of
co-occurring pollutants or pollutant mixtures (as is the case in
epidemiologic studies). The Administrator also observes that her
emphasis on information from controlled human exposure studies is
consistent with CASAC's advice and interpretation of the scientific
evidence (Frey, 2014c).
With regard to the effects shown in controlled human exposure
studies following specific O3 exposures, as discussed in
more detail above (II.B, II.C.4.b.i), the Administrator notes that (1)
the largest respiratory effects, and the broadest range of effects,
have been studied and reported following exposures to 80 ppb
O3 or higher (i.e., decreased lung function, increased
airway inflammation, increased respiratory symptoms, AHR, and decreased
lung host defense); (2) exposures to O3 concentrations as
low as 72 ppb have been shown to both decrease lung function and result
in respiratory symptoms; and (3) exposures to O3
concentrations as low as 60 ppb have been shown to decrease lung
function and to increase airway inflammation.
While such controlled human exposure studies provide a high degree
of confidence regarding the occurrence of health effects following
exposures to O3 concentrations from 60 to 80 ppb, there are
no universally accepted criteria by which to judge the adversity of the
observed effects. To inform her judgments on the potential adversity to
public health of effects reported in controlled human exposure studies,
the Administrator considers ATS recommendations and CASAC advice, as
described in detail above (II.B.2, II.C.4.b.i, II.C.4.b.iii,
II.C.4.b.iv). Based on her consideration of such recommendations and
advice, the Administrator is confident that the respiratory effects
that have been observed following exposures to 72 ppb O3 or
above can be adverse. In addition, she judges that adverse effects are
likely to occur following exposures somewhat below 72 ppb (II.C.4.b.i).
However, as described above (II.C.4.b.i, II.C.4.b.iii, II.C.4.b.iv),
the Administrator is notably less confident in the adversity to public
health of the respiratory effects that have been observed following
exposures to O3 concentrations as low as 60 ppb, given her
consideration of the following: (1) ATS recommendations indicating
uncertainty in judging adversity based on lung function decrements
alone; (2) uncertainty in the extent to which a short-term, transient
population-level decrease in FEV1 would increase the risk of
other, more serious respiratory effects in that population (i.e., per
ATS recommendations on population-level risk); and (3) compared to 72
ppb, CASAC advice is less clear regarding the potential adversity of
effects at 60 ppb.
Taken together, the Administrator concludes that the evidence from
controlled human exposure studies provides strong support for her
conclusion that a revised standard with a level of 70 ppb is requisite
to protect the public health with an adequate margin of safety. She
bases this conclusion, in part, on the fact that such a standard level
would be well below the O3 exposure concentration shown to
result in the widest range of respiratory effects (i.e., 80 ppb), and
below the lowest O3 exposure concentration shown to result
in the adverse combination of lung function decrements and respiratory
symptoms (i.e., 72 ppb). See Lead Industries, 647 F. 2d at 1160
(setting NAAQS at level well below the level where the clearest adverse
effects occur, and at a level eliminating most ``sub-clinical effects''
provides an adequate margin of safety).
As discussed above (II.C.4.b.i), the Administrator also notes that
a revised O3 standard with a level of 70 ppb can provide
substantial protection against the broader range of O3
exposure concentrations that have been shown in controlled human
exposure studies to result in respiratory effects, including exposure
concentrations below 70 ppb. The degree of protection provided by any
NAAQS is due to the combination of all of the elements of the standard
(i.e., indicator, averaging time, form, level) and, in the case of the
fourth-high form of the revised primary O3 standard
(II.C.3), the large majority of days in areas that meet the revised
standard will have 8-hour O3 concentrations below 70 ppb,
with most days having 8-hour O3 concentrations well below
this level. In addition, the degree of protection provided by the
O3 NAAQS is also dependent on the extent to which people
experience health-relevant O3 exposures in locations meeting
the NAAQS. As discussed above, for a pollutant like O3 where
adverse responses are critically dependent on ventilation rates, the
Administrator notes that it is important to consider activity patterns
in the exposed population. Not considering activity patterns, and
corresponding ventilation rates, can result in a standard that provides
more protection than is requisite. Therefore, as discussed in the
proposal, in considering the degree of protection provided by a revised
primary O3 standard, the Administrator considers the extent
to which that standard would be expected to limit population exposures
of concern (i.e., which take into account activity patterns and
estimated ventilation rates) to the broader range of O3
exposure concentrations shown to result in health effects.
Due to interindividual variability in responsiveness, the
Administrator notes that not every occurrence of an exposure of concern
will result in an adverse effect (II.C.4.b.i). Moreover, repeated
occurrences of some of the effects demonstrated following exposures of
concern could increase the likelihood of adversity (U.S. EPA, 2013,
Section 6.2.3, p. 6-76). In particular, she notes that the types of
respiratory effects that can occur following exposures of concern,
particularly if experienced repeatedly, provide a plausible mode of
action by which O3 may cause other more serious effects.
Therefore, as in the proposal, the Administrator is most concerned
about protecting at-risk populations against repeated occurrences of
exposures of concern. In considering the appropriate metric for
evaluating repeated occurrences of exposures of concern, the
Administrator acknowledges that it is not clear from the evidence, or
from the ATS recommendations, CASAC advice, or public comments, how
particular numbers of exposures of concern could impact the seriousness
of the resulting effects, especially at lower exposure concentrations.
Therefore, the Administrator judges that focusing on HREA estimates of
two or more exposures of concern provides a health-protective approach
to considering the potential for repeated occurrences of exposures of
concern that could result in adverse effects.
Based on her consideration of adversity discussed above, the
Administrator places the most emphasis on setting a standard that
appropriately limits repeated occurrences of exposures of concern at or
above the 70 and 80 ppb benchmarks. She notes that a revised standard
with a level of 70 ppb is estimated to eliminate the occurrence of two
or more exposures of concern to O3 concentrations at or
above 80 ppb and to virtually eliminate the occurrence of two or more
exposures of concern to O3 concentrations at or above 70 ppb
for all children and children with asthma, even in the worst-case year
and location evaluated.
While she is less confident that adverse effects will occur
following exposures to O3 concentrations as low as 60 ppb,
as discussed above, the
[[Page 65364]]
Administrator judges that it is also appropriate to consider estimates
of exposures of concern for the 60 ppb benchmark. Consistent with this
judgment, although CASAC advice regarding the potential adversity of
effects at 60 ppb was less definitive than for effects at 72 ppb, CASAC
did clearly advise the EPA to consider the extent to which a revised
standard is estimated to limit the effects observed following 60 ppb
exposures (Frey, 2014c). Therefore, the Administrator considers
estimated exposures of concern for the 60 ppb benchmark, particularly
considering the extent to which the health protection provided by a
revised standard includes a margin of safety against the occurrence of
adverse O3-induced effects. The Administrator notes that a
revised standard with a level of 70 ppb is estimated to protect the
vast majority of children in urban study areas (i.e., about 96% to more
than 99% of children in individual areas) from experiencing two or more
exposures of concern at or above 60 ppb. Compared to the current
standard, this represents a reduction of more than 60%.
Given the considerable protection provided against repeated
exposures of concern for all of the benchmarks evaluated, including the
60 ppb benchmark, the Administrator judges that a standard with a level
of 70 ppb will incorporate a margin of safety against the adverse
O3-induced effects shown to occur following exposures at or
above 72 ppb, and judged likely to occur following exposures somewhat
below 72 ppb.
While the Administrator is less concerned about single occurrences
of O3 exposures of concern, especially for the 60 ppb
benchmark, she judges that estimates of one or more exposures of
concern can provide further insight into the margin of safety provided
by a revised standard. In this regard, she notes that a standard with a
level of 70 ppb is estimated to (1) virtually eliminate all occurrences
of exposures of concern at or above 80 ppb; (2) protect the vast
majority of children in urban study areas from experiencing any
exposures of concern at or above 70 ppb (i.e., >= about 99%, based on
mean estimates; Table 1); and (3) to achieve substantial reductions,
compared to the current standard, in the occurrence of one or more
exposures of concern at or above 60 ppb (i.e., about a 50% reduction;
Table 1). The Administrator judges that these results provide further
support for her conclusion that a standard with a level of 70 ppb will
incorporate an adequate margin of safety against the occurrence of
O3 exposures that can result in effects that are adverse to
public health.
The Administrator additionally judges that a standard with a level
of 70 ppb would be expected to result in important reductions, compared
to the current standard, in the population-level risk of O3-
induced lung function decrements (>=10%, >=15%) in children, including
children with asthma. Specifically, a revised standard with a level of
70 ppb is estimated to reduce the risk of two or more O3-
induced decrements by about 30% and 20% for decrements >=15 and 10%,
respectively (Table 2, above). However, as discussed above
(II.C.4.b.i), the Administrator judges that there are important
uncertainties in using lung function risk estimates as a basis for
considering the occurrence of adverse effects in the population given
(1) the ATS recommendation that ``a small, transient loss of lung
function, by itself, should not automatically be designated as
adverse'' (ATS, 2000a); (2) uncertainty in the extent to which a
transient population-level decrease in FEV1 would increase
the risk of other, more serious respiratory effects in that population
(i.e., per ATS recommendations on population-level risk); and (3) that
CASAC did not advise considering a standard that would be estimated to
eliminate O3-induced lung function decrements >=10 or 15%
(Frey, 2014c). Moreover, as at proposal, the Administrator notes that
the variability in lung function risk estimates across urban study
areas is often greater than the differences in risk estimates between
various standard levels (Table 2, above).\150\ Given this, and the
resulting considerable overlap between the ranges of lung function risk
estimates for different standard levels, the Administrator puts limited
weight on the lung function risk estimates for distinguishing between
the degrees of public health protection provided by alternative
standard levels. Therefore, the Administrator judges that while a
standard with a level of 70 ppb would be expected to result in
important reductions, compared to the current standard, in the
population-level risk of O3-induced lung function decrements
(10%, 15%) in children, including children with asthma, she
also judges that estimated risks of O3-induced lung function
decrements provide a more limited basis than exposures of concern for
distinguishing between the appropriateness of the health protection
afforded by a standard level of 70 ppb versus lower levels.
---------------------------------------------------------------------------
\150\ For example, the average percentage of children estimated
to experience two or more decrements >=10% ranges from approximately
6 to 11% for a standard level of 70 ppb, up to about 9% for a level
of 65 ppb, and up to about 6% for a level of 60 ppb (Table 2,
above).
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The Administrator also considers the epidemiologic evidence and the
quantitative risk estimates based on information from epidemiologic
studies. As discussed in the proposal, and above in the EPA's responses
to significant comments, although the Administrator acknowledges the
important uncertainties in using the O3 epidemiologic
studies as a basis for selecting a standard level, she notes that these
studies can provide perspective on the degree to which O3-
associated health effects have been identified in areas with air
quality likely to have met various standards. Specifically, the
Administrator notes analyses in the PA (U.S. EPA, 2014c, section 4.4.1)
indicating that a revised standard with a level of 70 ppb would be
expected to require additional reductions, beyond those required by the
current standard, in the short- and long-term ambient O3
concentrations that provided the basis for statistically significant
O3 health effect associations in both the single-city and
multicity epidemiologic studies evaluated. As discussed above in the
response to comments, while the Administrator concludes that these
analyses support a level at least as low as 70 ppb, based on a study
reporting health effect associations in a location that met the current
standard over the entire study period but that would have violated a
revised standard with a level of 70 ppb,\151\ she further judges that
they are of more limited utility for distinguishing between the
appropriateness of the health protection estimated for a standard level
of 70 ppb and the protection estimated for lower levels. Thus, the
Administrator notes that a revised standard with a level of 70 ppb will
provide additional public health protection, beyond that provided by
the current standard, against the clearly adverse effects reported in
[[Page 65365]]
epidemiologic studies. She judges that a standard with a level of 70
ppb strikes an appropriate balance between setting the level to require
reductions in the ambient O3 concentrations associated with
statistically significant health effects in epidemiologic studies,
while not being more protective than necessary in light of her
considerable uncertainty in the extent to which studies clearly show
O3-attributable effects at lower ambient O3
concentrations. This judgment is consistent with the Administrator's
conclusions based on information from controlled human exposure
studies, as discussed above.
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\151\ As discussed above (II.B.2.c.ii and II.B.3), the study by
Mar and Koenig (2009) reported positive and statistically
significant associations with respiratory emergency department
visits in a location that would have met the current standard over
the entire study period, but violated a standard with a level of 70
ppb. In addition, air quality analyses in the locations of two
additional studies highlighted in sections II.B.2 and II.B.3
(Silverman and Ito, 2010; Strickland et al., 2010) were used in the
PA to inform staff conclusions on the adequacy of the current
primary O3 standard. However, they did not provide
insight into the appropriateness of standard levels below 75 ppb
and, therefore, these analyses were not used to inform conclusions
on potential alternative standard levels lower than 75 ppb (U.S.
EPA, 2014c, Chapters 3 and 4). See Mississippi, 744 F. 3d at 1352-53
(study appropriate for determining causation may not be probative
for determining level of a revised standard).
---------------------------------------------------------------------------
With regard to epidemiology-based risk estimates, the Administrator
takes note of the CASAC conclusion that ``[a]lthough the estimates for
short-term exposure impacts are subject to uncertainty, the data
supports a conclusion that there are meaningful reductions in mean
premature mortality associated with ozone levels lower than the current
standard'' (Frey, 2014a, p. 10). While she concludes that epidemiology-
based risk analyses provide only limited support for any specific
standard level, consistent with CASAC advice the Administrator judges
that, compared to the current standard, a revised standard with a level
of 70 ppb will result in meaningful reductions in the mortality and
respiratory morbidity risk that is associated with short-or long-term
ambient O3 concentrations.
Given all of the evidence and information discussed above, the
Administrator judges that a standard with a level of 70 ppb is
requisite to protect public health with an adequate margin of safety,
and that a level below 70 ppb would be more than ``requisite'' to
protect the public health. In reaching this conclusion, she notes that
a decision to set a lower level would place a large amount of emphasis
on the potential public health importance of (1) further reducing the
occurrence of O3 exposures of concern, though the exposures
about which she is most concerned are estimated to be almost eliminated
with a level of 70 ppb, and lower levels would be expected to achieve
virtually no additional reductions in these exposures (see Table 1,
above); (2) further reducing the risk of O3-induced lung
function decrements 10 and 15%, despite having less
confidence in judging the potential adversity of lung function
decrements alone and the considerable overlap between risk estimates
for various standard levels that make it difficult to distinguish
between the risk reductions achieved; (3) further reducing ambient
O3 concentrations, relative to those in locations of
epidemiologic studies, though associations have not been reported for
air quality that would have met a standard with a level of 70 ppb
across all study locations and over entire study periods, and despite
her consequent judgment that air quality analyses in epidemiologic
study locations are not informative regarding the additional degree of
public health protection that would be afforded by a standard set at a
level below 70 ppb; and (4) further reducing epidemiology-based risk
estimates, despite the important uncertainties in those estimates. As
discussed in this section and in the responses to significant comments
above, the Administrator does not agree that it is appropriate to place
significant weight on these factors or to use them to support the
appropriateness of standard levels below 70 ppb O3. Compared
to an O3 standard level of 70 ppb, the Administrator
concludes that the extent to which lower standard levels could result
in further public health improvements becomes notably less certain.
Thus, having carefully considered the evidence, information, CASAC
advice, and public comments relevant to her decision on the level of
the primary O3 standard, as discussed above and in the
Response to Comments document, the Administrator is revising the level
of the primary O3 standard to 70 ppb. She is mindful that
the selection of a primary O3 standard that is requisite to
protect public health with an adequate margin of safety requires
judgments based on an interpretation of the scientific evidence and
exposure/risk information that neither overstate nor understate the
strengths and limitations of that evidence and information and the
appropriate inferences to be drawn therefrom. Her decision places the
greatest emphasis on the results of controlled human exposure studies
and on quantitative analyses based on information from these studies,
particularly analyses of O3 exposures of concern. As in the
proposal, and as discussed above, she views the results of the lung
function risk assessment, analyses of O3 air quality in
locations of epidemiologic studies, and epidemiology-based quantitative
health risk assessments as providing information in support of her
decision to revise the current standard, but a more limited basis for
selecting a particular standard level among a range of options.
In making her decision to revise the level of the primary
O3 standard to 70 ppb, the Administrator judges that a
revised standard with a level of 70 ppb strikes the appropriate balance
between limiting the O3 exposures about which she is most
concerned and not going beyond what would be required to effectively
limit such exposures. Specifically, the Administrator judges it
appropriate to set a standard estimated to eliminate, or almost
eliminate, repeated occurrences of exposures of concern for the 70 and
80 ppb benchmarks. She further judges that a lower standard level would
not be appropriate given that lower levels would be expected to achieve
virtually no additional reductions in repeated occurrences of exposures
of concern for these benchmarks. For the 60 ppb benchmark, a level of
70 ppb is estimated to protect the vast majority of children (including
children with asthma) in urban study areas from experiencing two or
more exposures of concern, reflecting important reductions in such
exposures compared to the current standard and indicating that the
revised primary O3 standard provides an adequate margin of
safety. Given these results, including the considerable protection
provided against repeated exposures of concern for the 60 ppb
benchmark, the Administrator judges that a standard with a level of 70
ppb incorporates an adequate margin of safety against the occurrence of
adverse O3-induced effects.
For all of the above reasons, the Administrator concludes that a
primary O3 standard with an 8-hour averaging time; a 3-year
average, fourth-high form; and a level of 70 ppb is requisite to
protect public health, including the health of at-risk populations,
with an adequate margin of safety. Therefore, in this final rule she is
setting the level of the primary O3 standard at 70 ppb.
D. Decision on the Primary Standard
For the reasons discussed above, and taking into account
information and assessments presented in the ISA, HREA, and PA, the
advice and recommendations of the CASAC Panel, and the public comments,
the Administrator has decided to revise the existing 8-hour primary
O3 standard. Specifically, the Administrator is revising the
level of the primary O3 standard to 70 ppb. The revised 8-
hour primary standard, with a level of 70 ppb, would be met at an
ambient air monitoring site when the 3-year average of the annual
fourth-highest daily maximum 8-hour average O3 concentration
is less than or equal to 70 ppb. Data handling conventions are
specified in the new Appendix U that is adopted, as discussed in
section V below.
[[Page 65366]]
At this time, EPA is also promulgating revisions to the Air Quality
Index (AQI) for O3 to be consistent with the revisions to
the primary O3 standard and the health information evaluated
in this review of the standards. These revisions are discussed below in
section III.
III. Communication of Public Health Information
Information on the public health implications of ambient
concentrations of criteria pollutants is currently made available
primarily through EPA's AQI program. The AQI has been in use since its
inception in 1999 (64 FR 42530). It provides accurate, timely, and
easily understandable information about daily levels of pollution. It
is designed to tell individual members of the public how clean or
unhealthy their air is, whether health effects might be a concern, and,
if so, measures individuals can take to reduce their exposure to air
pollution.\152\ See CAA section 127. The AQI focuses on health effects
individuals may experience within a few hours or days after breathing
unhealthy air. The AQI establishes a nationally uniform system of
indexing pollution concentrations for O3, CO,
NO2, PM and SO2. The AQI converts pollutant
concentrations in a community's air to a number on a scale from 0 to
500. Reported AQI values enable the public to know whether air
pollution concentrations in a particular location are characterized as
good (0-50), moderate (51-100), unhealthy for sensitive groups (101-
150), unhealthy (151-200), very unhealthy (201-300), or hazardous (301-
500). The AQI index value of 100 typically corresponds to the level of
the short-term NAAQS for each pollutant. For the 2008 O3
NAAQS, an 8-hour average concentration of 75 ppb corresponds to an AQI
value of 100. An AQI value greater than 100 means that a pollutant is
in one of the unhealthy categories (i.e., unhealthy for sensitive
groups, unhealthy, very unhealthy, or hazardous) on a given day; an AQI
value at or below 100 means that a pollutant concentration is in one of
the satisfactory categories (i.e., moderate or good). An additional
consideration in selecting breakpoints is for each category to span at
least a 15 ppb range to allow for more accurate air pollution
forecasting. Decisions about the pollutant concentrations at which to
set the various AQI breakpoints, that delineate the various AQI
categories, draw directly from the underlying health information that
supports the NAAQS review.
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\152\ EPA issued the AQI in 1999, updating the previous
Pollutant Standards Index (PSI) to send ``a clear and consistent
message to the public by providing nationally uniform information on
air quality.'' The rule requires metropolitan areas of 350,000 and
larger to report the AQI [and associated health effects] daily; all
other AQI-related activities--including real-time ozone and particle
pollution reporting, next-day air quality forecasting and action
days--are voluntary and are carried out at the discretion of state,
local and tribal air agencies. In the 1999 rule, we acknowledged
these other programs, noting, for example, that while states
primarily use the AQI ``to provide general information to the public
about air quality and its relationship to public health,'' some
state, local or tribal agencies use the index to call ``action
days.'' Action days encourage additional steps, usually voluntary,
that the public, business or industry could take to reduce emissions
when higher levels of pollution are forecast to occur. As the 1999
rule notes, agencies may have several motivations for calling action
days, including: providing health information to the public;
attaining or maintaining NAAQS attainment status; meeting specific
emission reduction targets; and managing or reducing traffic
congestion. State, local and tribal agencies should consider whether
non-voluntary emissions or activity curtailments are necessary (as
opposed to a suite of voluntary measures) for days when the AQI is
forecasted to be on the lower end of the moderate category.
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A. Proposed Revisions to the AQI
Recognizing the importance of revising the AQI in a timely manner
to be consistent with any revisions to the NAAQS, EPA proposed
conforming changes to the AQI, in connection with the Agency's proposed
decision on revisions to the O3 NAAQS. These conforming
changes included setting the 100 level of the AQI at the same level as
the revised primary O3 NAAQS and also making adjustments
based on health information from this NAAQS review to AQI breakpoints
at the lower end of each range (i.e., AQI values of 50, 150, 200 and
300). The EPA did not propose to change the level at the top of the
index (i.e., AQI value of 500) that typically is set equal to the
Significant Harm Level (40 CFR 51.16), which would apply to state
contingency plans.
The EPA proposed to revise the AQI for O3 by setting an
AQI value of 100 equal to the level of the revised O3
standard (65-70 ppb). The EPA also proposed to revise the following
breakpoints: an AQI value of 50 to within a range from 49-54 ppb; an
AQI value of 150 to 85 ppb; an AQI value of 200 to 105 ppb, and an AQI
value of 300 to 200 ppb. All these levels are averaged over 8 hours.
The EPA proposed to set an AQI value of 50, the breakpoint between the
good and moderate categories, at 15 ppb below the value of the proposed
standard, i.e. to within a range from 49 to 54 ppb. The EPA took
comment on what level within this range to select, recognizing that
there is no health message for either at-risk or healthy populations in
the good category. Thus, the level selected should be below the lowest
concentration (i.e., 60 ppb) that has been shown in controlled human
exposure studies of young, healthy adults exposed to O3
while engaged in quasi-continuous moderate exercise for 6.6 hours to
cause moderate lung function decrements (i.e., FEV1
decrements >= 10%, which could be adverse to people with lung disease)
and airway inflammation.\153\ The EPA proposed to set an AQI value of
150, the breakpoint between the unhealthy for sensitive groups and
unhealthy categories, at 85 ppb. At this level, controlled human
exposure studies of young, healthy adults indicate that up to 25% of
exposed people are likely to have moderate lung function decrements
(i.e., 25% have FEV1 decrements >= 10%; 12% have
FEV1 decrements >= 15%) and up to 7% are likely to have
large lung function decrements (i.e., FEV1 decrements >=
20%) (McDonnell et al., 2012; Figure 7). Large lung function decrements
would likely interfere with normal activity for many healthy people.
For most people with lung disease, large lung function decrements would
not only interfere with normal activity but would increase the
likelihood that they would seek medical treatment (72 FR 37850, July
11, 2007). The EPA proposed to set an AQI value of 200, the breakpoint
between the unhealthy and very unhealthy categories, at 105 ppb. At
this level, controlled human exposure studies of young, healthy adults
indicate that up to 38% of exposed people are likely to have moderate
lung function decrements (i.e., 38% have FEV1 decrements >=
10%; 22% have FEV1 decrements >= 15%) and up to 13% are
likely to have large lung function decrements (i.e., FEV1
decrements >= 20%). The EPA proposed to set an AQI value of 300, the
breakpoint between the very unhealthy and hazardous categories, at 200
ppb. At this level, controlled human exposure studies of healthy adults
indicate that up to 25% of exposed individuals are likely to have large
lung function decrements (i.e., FEV1 decrements >= 20%),
which would interfere with daily activities for many of them and likely
cause people with lung disease to seek medical attention.
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\153\ Exposures to 50 ppb have not been evaluated
experimentally, but are estimated to potentially affect only a small
proportion of healthy adults and with only a half to a third of the
moderate to large lung function decrements observed at 60 ppb
(McDonnell et al., 2012; Figure 7).
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EPA stated that the proposed breakpoints reflect an appropriate
balance between reflecting the health evidence that is the basis for
the proposed primary O3 standard and providing category
ranges that are large enough to be forecasted accurately, so
[[Page 65367]]
that the new AQI for O3 can be implemented more easily in
the public forum for which the AQI ultimately exists. However, the EPA
recognized alternative approaches to viewing the evidence and
information and solicited comment on the proposed revisions to the AQI.
With respect to reporting requirements (40 CFR part 58, section
58.50), EPA proposed to revise 40 CFR part 58, section 58.50 (c) to
determine the areas subject to AQI reporting requirements based on the
latest available census figures, rather than the most recent decennial
U.S. census.\154\ This change is consistent with our current practice
of using the latest population figures to make monitoring requirements
more responsive to changes in population.
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\154\ Under 40 CFR 58.50, any MSA with a population exceeding
350,000 is required to report AQI data.
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B. Comments on Proposed Revisions to the AQI
EPA received many comments on the proposed changes to the AQI.
Three issues came up in the comments, including: (1) Whether the AQI
should be revised at all, even if the primary standard is revised; (2)
whether an AQI value of 100 should be set equal to the level of the
primary standard and the other breakpoints adjusted accordingly; and,
(3) whether the AQI reporting requirements should be based on the
latest available census figures rather than the most recent decennial
census.
With respect to the first issue, some industry commenters stated
that the AQI should not be revised at all, even if the level of the
primary O3 standard is revised. In support of this position,
these commenters stated that the proposed conforming changes to the AQI
would lower O3 levels in each category, and would mean that
air quality that is actually improving would be reported as less
healthy. According to commenters, the revised AQI would fail to capture
these improvements and potentially mislead the public into thinking
that air quality has degraded and that EPA and state regulators are not
doing their jobs. These commenters noted that there is no requirement
to revise the AQI, and that the CAA does not tie the AQI to the
standards, stating that the purpose of section 319(a) of the CAA is to
provide a consistent, uniform means of gauging air quality. These
commenters further asserted that EPA's proposed changes run counter to
that uniformity by changing the air quality significance of a given
index value and category and that retention of the current AQI
breakpoints would allow continued uniform information on air quality.
Commenters stated that it is important that the EPA clearly
communicates that the immediate increases in moderate rated days are
due to AQI breakpoint adjustment and not due to a sudden decline in air
quality. One commenter estimated the increased proportion of days in
the moderate category and above in 10 metropolitan areas for 2013 and
also for 2025 for 4 cities from the original 10 that were estimated to
attain a standard below 70 ppb, to compare with 2013. This commenter
noted that the change in the proposed AQI breakpoint between ``good''
and ``moderate'' would result in a larger number of days that did not
meet the ``good'' criteria. They went further to claim that the change
in breakpoints would result in fewer ``good'' days in the year 2025
(using the new breakpoint) than occurred in 2013 (using the old
breakpoints) despite substantial improvement in air quality over that
time period.
On the other hand, state and local agencies and their
organizations, environmental and medical groups, and members of the
public overwhelmingly supported revising the AQI when the level of the
standard is revised. Even state agencies that did not support revising
the standard, expressed support for revising the AQI at the same time
as the standard, if the standard is revised.
Recognizing the importance of the AQI as a communication tool that
allows members of the public to take exposure reduction measures when
air quality poses health risks, the EPA agrees with these comments
about revising the AQI at the same time as the primary standard. The
EPA agrees with state and local agency commenters that its historical
approach of setting an AQI value of 100 equal to the level of the
revised 8-hour primary O3 standard is appropriate, both from
a public health and a communication perspective.
EPA disagrees with commenters who stated that the AQI should not be
linked to the primary standards. As noted in the August 4, 1999,
rulemaking (64 FR 149, 42531) that established the current AQI, the EPA
established the nationally uniform air quality index, called the
Pollutant Standards Index (PSI), in 1976 to meet the needs of state and
local agencies with the following advantages: It sends a clear and
consistent message to the public by providing nationally uniform
information on air quality; it is keyed as appropriate to the NAAQS and
the Significant Harm Level which have a scientific basis relating air
quality and public health; it is simple and easily understood by the
public; it provides a framework for reflecting changes to the NAAQS;
and it can be forecasted to provide advance information on air quality.
Both the PSI and AQI have historically been normalized across
pollutants by defining an index value of 100 as the numerical level of
the short-term (i.e., averaging time of 24-hours or less) primary NAAQS
for each pollutant. Moreover, this approach does not mislead the
public. Since the establishment of the AQI, the EPA and state and local
air agencies and organizations have developed experience in educating
the public about changes in the standards and, concurrently, related
changes to AQI breakpoints and advisories. When the standards change,
EPA and state and local agencies have tried to help the public
understand that air quality is not getting worse, it's that the health
evidence underlying the standards and the AQI has changed. EPA's Air
Quality System (AQS), the primary repository for air quality monitoring
data, is also adjusted to reflect the revised breakpoints.
Specifically, all historical AQI values in AQS are recomputed with the
revised breakpoints, so that all data queries and reports downstream of
AQS will show appropriate trends in AQI values over time.\155\
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\155\ Although we do not contest the assertion that the new AQI
breakpoints will lead to fewer green days in the near future, we do
not agree that commenters' analysis sufficiently demonstrates that
there would be fewer green days in 2025 than in 2013. In their
analysis, they compared observed 2013 data with modeled 2025 data
without doing any model performance evaluation for AQI categories or
comparison of current year modeled and observed data. The current
year observations are not directly comparable to the future-year
modeling data without some such evaluation and, as such, we cannot
support their quantitative conclusions.
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In general, commenters who supported revising the AQI when the
standard is revised, also supported setting an AQI value of 100 equal
to the level of the 8-hour primary O3 standard. The EPA
agrees with these commenters. With respect to an AQI value of 100, the
EPA is taking final action to set an AQI value of 100 equal to the
level of the 8-hour primary standard at 70 ppb O3.
With respect to proposed changes to other AQI breakpoints, some
state and local agency commenters expressed general support for all the
changes in O3 breakpoints (in Table 2 of Appendix G). In
addition, we received a few comments specifically about the breakpoint
between the good and moderate categories. One state expressed the view
that forecasting the AQI for O3 is not an exact science, so
it is important to provide a range large enough to reasonably predict
O3
[[Page 65368]]
concentrations for the following day (>= 20 ppb). Although not
supporting revision of the standard, this state recommended that if the
primary standard was revised to 70 ppb, the lower end of moderate
category should be set at 50 ppb to allow for a 20 ppb spread in that
category. Several commenters recommending a breakpoint between the good
and moderate categories of no higher than 50 ppb stated that this
breakpoint should be set on health information, pointing to
epidemiologic data and the World Health organization guidelines. The
Agency agrees that AQI breakpoints should take into consideration
health information when possible, and also that it is important for AQI
categories to span ranges large enough to support accurate forecasting.
The EPA is setting the breakpoint at the lower end of the moderate
category at 55 ppb, which is 15 ppb below the level of the standard of
70 ppb. This is consistent with past practice of making a proportional
adjustment to this AQI breakpoint, relative to an AQI value of 100
(i.e., 70 ppb), and also retains the current practice of providing a 15
ppb range in the moderate category to allow for accurate forecasting.
This level is below the lowest concentration (i.e., 60 ppb) that has
been shown in controlled human exposure studies of healthy adults to
cause moderate lung function decrements (i.e., FEV1
decrements >= 10%, which could be adverse to people with lung disease),
large lung function decrements (i.e., FEV1 decrements >=
20%) in a small proportion of people, and airway inflammation,
notwithstanding the Administrator's judgment that there is uncertainty
in the adversity of the effects shown to occur at 60 ppb.
We received fewer comments on proposed changes to the AQI values of
150, 200 and 300. Again, some state and local agency commenters
expressed general support for proposed changes to the AQI. Some states
specifically supported these breakpoints. However, a commenter
suggested setting an AQI value at the lower end of the unhealthy
category, at a level much lower than 85 ppb, since they state that it
is a key threshold that is often used in air quality action day
programs as a trigger to encourage specific behavior modifications or
reduce emissions of O3 precursors (e.g., by taking public
transportation to work). This commenter stated that setting the
breakpoint at 85 ppb would, in the Agency's own rationale, not require
the triggering of these pollution reduction measures until air quality
threatened to impact 25% of people exposed. We disagree with this
commenter because EPA does not have any requirements for voluntary
programs. State and local air agencies have discretion to set the
trigger for voluntary action programs at whatever level they choose,
and they are currently set at different levels, not just at the
unhealthy breakpoint specified in the comment. For example, Houston,
Galveston and Brazoria TX metropolitan area calls ozone action days
when air quality reaches the unhealthy for sensitive groups category.
For more information about action days programs across the U.S. see the
AirNow Web site (www.airnow.gov) and click on the link to AirNow Action
Days. The unhealthy category represents air quality where there are
general population-level effects. We believe that setting the
breakpoint between the unhealthy for sensitive groups and unhealthy
categories, at 85 ppb where, as discussed in section IIIA above,
controlled human exposure studies of young, healthy adults exposed to
O3 while engaged in quasi-continuous moderate exercise for
6.6 hours indicate that up to 25% of exposed people are likely to have
moderate lung function decrements and up to 7% are likely to have large
lung function decrements (McDonnell et al., 2012; Figure 7) is
appropriate. A smaller proportion of inactive or less active
individuals would be expected to experience lung function decrements at
85 ppb. Moreover, a breakpoint at 85 ppb allows for category ranges
large enough for accurate forecasting. Accordingly, the EPA is adopting
the proposed revisions to the AQI values of 150, 200 and 300.
As noted earlier, the EPA proposed to revise 40 CFR part 58,
section 58.50(c) to determine the areas subject to AQI reporting
requirements based on the latest available census figures, rather than
the most recent decennial U.S. census.
A total of five state air monitoring agencies provided comments on
this proposed change. Four agencies supported the proposal. One state
commenter did not support the proposal, noting that the change would
unnecessarily complicate AQI reporting and possibly increase reporting
burdens in an unpredictable manner.
The EPA notes that the majority of monitoring network minimum
requirements listed in Appendix D to Part 58 include a reference to
``latest available census figures.'' Minimum network requirements for
O3, PM2.5, SO2, and NO2 all
include this language in the regulatory text and monitoring agencies
have successfully adopted these processes into their planning
activities and the subsequent revision of their annual monitoring
network plans which are posted for public review. Annual population
estimates are easily obtainable from the U.S. Census Bureau and the EPA
does not believe the burden in tracking these annual estimates is
excessive or complicated.\156\ Although the changes in year to year
estimates are typically modest, there are MSAs that are approaching (or
have recently exceeded) the 350,000 population AQI reporting limit and
there is great value in having the AQI reported for these areas when
the population threshold is exceeded versus waiting potentially up to
10 years for a revision to the decennial census. Accordingly, the EPA
is finalizing the proposed revision to 40 CFR part 58, section 58.50(c)
to require the AQI reporting requirements to be based on the latest
available census figures.
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\156\ http://www.census.gov/popest/data/metro/totals/2014/CBSA-EST2014-alldata.html.
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One state requested additional guidance on the frequency of
updating the AQI reporting threshold, and recommended linking the AQI
reporting requirement evaluation with the annual air monitoring network
plan requirements, and recommended requiring AQI reporting to begin no
later than January 1 of the following year. The EPA notes that the
census bureau estimates appear to be released around July 1 of each
year which would not provide sufficient time for monitoring agencies to
incorporate AQI reporting in their annual plans for that year, which
are also due by July 1 each year. EPA believes that it should be
unnecessary for monitoring agencies to wait until the implementation of
the following year's annual plan (i.e., approximately 18 months later)
to begin AQI reporting. Accordingly, EPA is not at this time including
a specific deadline for commencement of AQI reporting for newly-subject
areas in 40 CFR part 58, but will work with agencies to implement
additional AQI reporting as needed to ensure that information is being
disseminated in a timely fashion.
C. Final Revisions to the AQI
For the reasons discussed above, the EPA is revising the AQI for
O3 by setting an AQI value of 100 equal to 70 ppb, 8-hour
average, the level of the revised primary O3 standard. The
EPA is also revising the following breakpoints: An AQI value of 50 is
set at 54 ppb; an AQI value of 150 is set at 85 ppb; an AQI value of
200 is set at 105 ppb; and an AQI value of 300 is set at 200 ppb. All
of these levels are averaged over 8 hours. The revisions to all of the
[[Page 65369]]
breakpoints are based on estimated health outcomes at relevant ambient
concentrations and to allow for each category to span at least a 15-20
ppb category range to allow for more accurate air pollution
forecasting. The EPA believes that the revised breakpoints provide a
balance between adjustments to reflect the health information
supporting the revised O3 standard and providing category
ranges that are large enough to be forecasted accurately, so that the
AQI can be implemented more easily in the public forum for which the
AQI ultimately exists. With respect to AQI reporting requirements (40
CFR part 58, section 58.50), the EPA is revising 40 CFR part 58,
section 58.50(c) to make the AQI reporting requirements based on the
latest available census figures, rather than the most recent decennial
U.S. census. This change is consistent with our current practice of
using the latest population figures to make monitoring requirements
more responsive to changes in population.
IV. Rationale for Decision on the Secondary Standard
A. Introduction
This section (IV) presents the rationale for the Administrator's
decisions regarding the need to revise the current secondary standard
for O3, and the appropriate revision. Based on her
consideration of the full body of welfare effects evidence and related
analyses, including the evidence of effects associated with cumulative
seasonal exposures of the magnitudes allowed by the current standard,
the Administrator has concluded that the current secondary standard for
O3 does not provide the requisite protection of public
welfare from known or anticipated adverse effects. She has decided to
revise the level of the current secondary standard to 0.070 ppm, in
conjunction with retaining the current indicator, averaging time and
form.
The Administrator has made this decision based on judgments
regarding the currently available welfare effects evidence, the
appropriate degree of public welfare protection for the revised
standard, and currently available air quality information on seasonal
cumulative exposures that may be allowed by such a standard. In so
doing, she has focused on O3 effects on tree seedling growth
as a proxy for the full array of vegetation-related effects of
O3, ranging from effects on sensitive species to broader
ecosystem-level effects. Using this proxy in judging effects to public
welfare, the Administrator has concluded that the requisite protection
from adverse effects to public welfare will be provided by a standard
that limits cumulative seasonal exposures to 17 ppm-hrs or lower, in
terms of a 3-year W126 index, in nearly all instances, and she has also
concluded that such control of cumulative seasonal exposures may be
achieved by revising the level of the current standard to 70 ppb. Based
on all of these considerations, the Administrator has decided that a
secondary standard with a level of 0.070 ppm, and the current form and
averaging time, will provide the requisite protection of public welfare
from known or anticipated adverse effects.
As discussed more fully below, this decision is based on a thorough
review, in the ISA, of the latest scientific information on
O3-induced environmental effects. This decision also takes
into account (1) staff assessments in the PA of the most policy-
relevant information in the ISA regarding evidence of adverse effects
of O3 to vegetation and ecosystems, information on
biologically-relevant exposure metrics, WREA analyses of air quality,
exposure, and ecological risks and associated ecosystem services, and
staff analyses of relationships between levels of a W126-based metric
and a metric based on the form and averaging time of the current
standard summarized in the PA and in the proposal notice; (2) CASAC
advice and recommendations; and (3) public comments received during the
development of these documents, either in connection with CASAC
meetings or separately, and on the proposal notice.
This decision draws on the ISA's integrative synthesis of the
entire body of evidence, generally published through July 2011, on
environmental effects associated with the presence of O3 and
related photochemical oxidants in the ambient air (U.S. EPA, 2013, ISA
chapters 9-10), and includes more than four hundred new studies that
build on the extensive evidence base from the last review. In addition
to reviewing the most recent scientific information as required by the
CAA, this rulemaking incorporates the EPA's response to the judicial
remand of the 2008 secondary O3 standard in State of
Mississippi v. EPA, 744 F. 3d 1334 (D.C. Cir. 2013) and, in accordance
with the court's decision in that case, fully explains the
Administrator's conclusions as to the level of air quality that
provides the requisite protection of public welfare from known or
anticipated adverse effects. In drawing conclusions on the secondary
standard, the decision described in this rulemaking is a public welfare
policy judgment made by the Administrator. The Administrator's decision
draws upon the available scientific evidence for O3-
attributable welfare effects and on analyses of exposures and public
welfare risks based on impacts to vegetation, ecosystems and their
associated services, as well as judgments about the appropriate weight
to place on the range of uncertainties inherent in the evidence and
analyses. As described in sections IV.B.3 and IV.C.3 below, such
judgments in the context of this review include judgments on the weight
to place on the evidence of specific vegetation-related effects
estimated to result across a range of cumulative seasonal
concentration-weighted O3 exposures; on the weight to give
associated uncertainties, including those related to the variability in
occurrence of such effects in areas of the U.S., especially areas of
particular public welfare significance; and on the extent to which such
effects in such areas may be considered adverse to public welfare.
Information related to vegetation and ecosystem effects,
biologically relevant exposure indices, and vegetation exposure and
risk assessments were summarized in sections IV.A through IV.C of the
proposal (79 FR at 75314-75329), respectively, and key observations
from the proposal are briefly outlined in sections IV.A.1 to IV.A.3
below. Subsequent sections of this preamble provide a more complete
discussion of the Administrator's rationale, in light of key issues
raised in public comments, for concluding that the current standard is
not requisite to protect public welfare from known or anticipated
adverse effects (section IV.B), and that it is appropriate to revise
the current secondary standard to provide additional public welfare
protection by revising the level while retaining the current indicator,
form and averaging time (section IV.C). A summary of the final
decisions on revisions to the secondary standard is presented in
section IV.D.
1. Overview of Welfare Effects Evidence
a. Nature of Effects
In the more than fifty years that have followed identification of
O3's phytotoxic effects, extensive research has been
conducted both in and outside of the U.S. to examine the impacts of
O3 on plants and their associated ecosystems (U.S. EPA,
1978, 1986, 1996a, 2006a, 2013). As was established in prior reviews,
O3 can interfere with carbon gain (photosynthesis) and
allocation of carbon within the plant, making fewer carbohydrates
available
[[Page 65370]]
for plant growth, reproduction, and/or yield. For seed-bearing plants,
these reproductive effects will culminate in reduced seed production or
yield (U.S. EPA, 1996a, pp. 5-28 and 5-29). Recent studies, assessed in
the ISA, together with this longstanding and well-established
literature on O3-related vegetation effects, further
contribute to the coherence and consistency of the vegetation effects
evidence (U.S. EPA, 2013, chapter 9).
The strongest evidence for effects from O3 exposure on
vegetation is from controlled exposure studies, which ``have clearly
shown that exposure to O3 is causally linked to visible
foliar injury, decreased photosynthesis, changes in reproduction, and
decreased growth'' in many species of vegetation (U.S. EPA, 2013, p. 1-
15). Such effects at the plant scale can also be linked to an array of
effects at larger spatial scales, with the currently available evidence
indicating that ``ambient O3 exposures can affect ecosystem
productivity, crop yield, water cycling, and ecosystem community
composition'' (U.S. EPA, 2013, p. 1-15; Chapter 9, section 9.4). The
current body of O3 welfare effects evidence confirms and
strengthens support for the conclusions reached in the last review on
the nature of O3-induced welfare effects and is summarized
in the ISA as follows (U.S. EPA, 2013, p. 1-8).
The welfare effects of O3 can be observed across
spatial scales, starting at the subcellular and cellular level, then
the whole plant and finally, ecosystem-level processes. Ozone
effects at small spatial scales, such as the leaf of an individual
plant, can result in effects along a continuum of larger spatial
scales. These effects include altered rates of leaf gas exchange,
growth, and reproduction at the individual plant level, and can
result in broad changes in ecosystems, such as productivity, carbon
storage, water cycling, nutrient cycling, and community composition.
Based on assessment of this extensive body of science, the EPA has
determined that, with respect to vegetation and ecosystems, a causal
relationship exists between exposure to O3 in ambient air
and visible foliar injury effects on vegetation, reduced vegetation
growth, reduced productivity in terrestrial ecosystems, reduced yield
and quality of agricultural crops and alteration of below-ground
biogeochemical cycles (U.S. EPA, 2013, Table 1-2). In consideration of
the evidence of O3 exposure and alterations in stomatal
performance, ``which may affect plant and stand transpiration and
therefore possibly affecting hydrological cycling,'' the ISA concludes
that ``[a]lthough the direction of the response differed among
studies,'' the evidence is sufficient to conclude a likely causal
relationship between O3 exposure and the alteration of
ecosystem water cycling (U.S. EPA, 2013, section 2.6.3). The evidence
is also sufficient to conclude a likely causal relationship between
O3 exposure and the alteration of community composition of
some terrestrial ecosystems (U.S. EPA, 2013, section 2.6.5). Related to
the effects on vegetation growth, productivity and, to some extent,
below-ground biogeochemical cycles, the EPA has additionally determined
that a likely causal relationship exists between exposures to
O3 in ambient air and reduced carbon sequestration (also
termed carbon storage) in terrestrial ecosystems (U.S. EPA, 2013, p. 1-
10 and section 2.6.2). Modeling studies available in this review
consistently found negative impacts of O3 on carbon
sequestration, although the severity of impact was influenced by
``multiple interactions of biological and environmental factors'' (U.S.
EPA, 2013, p. 2-39).
Ozone in the troposphere is also a major greenhouse gas and
radiative forcing agent,\157\ with the ISA formally concluding that
``the evidence supports a causal relationship between changes in
tropospheric O3 concentrations and radiative forcing'' (U.S.
EPA, 2013, p. 1-13 and section 2.7.1). While tropospheric O3
has been ranked third in importance after carbon dioxide and methane,
there are ``large uncertainties in the magnitude of the radiative
forcing estimate attributed to tropospheric O3, making the
impact of tropospheric O3 on climate more uncertain than the
effect of the longer-lived greenhouse gases'' (U.S. EPA, 2013, p. 2-
47). The ISA notes that ``[e]ven with these uncertainties, global
climate models indicate that tropospheric O3 has contributed
to observed changes in global mean and regional surface temperatures''
and concludes that ``[a]s a result of such evidence presented in
climate modeling studies, there is likely to be a causal relationship
between changes in tropospheric O3 concentrations and
effects on climate'' (U.S. EPA, 2013, p. 2-47).\158\ The ISA
additionally states that ``[i]mportant uncertainties remain regarding
the effect of tropospheric O3 on future climate change''
(U.S. EPA, 2013, p. 10-31).
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\157\ As described in the ISA, ``[r]adiative forcing by a
greenhouse gas or aerosol is a metric used to quantify the change in
balance between radiation coming into and going out of the
atmosphere caused by the presence of that substance'' (U.S. EPA,
2013, p. 1-13).
\158\ Climate responses, including increased surface
temperature, have downstream climate-related ecosystem effects (U.S.
EPA, 2013, p. 10-7). As noted in section I.D above, such effects may
include an increase in the area burned by wildfires, which, in turn,
are sources of O3 precursor emissions.
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b. Vegetation Effects
Given the strong evidence base and the findings of causal or likely
causal relationships with O3 in ambient air, including the
quantitative assessments of relationships between O3
exposure and occurrence and magnitude of effects, this review has given
primary consideration to three main kinds of vegetation effects, some
of which contribute to effects at scales beyond the plant level, such
as at the ecosystem level and on ecosystem services. The three kinds of
effects are addressed below in the following order: 1) Visible foliar
injury, 2) impacts on tree growth, productivity and carbon storage, and
3) crop yield loss.
Visible foliar injury resulting from exposure to O3 has
been well characterized and documented over several decades of research
on many tree, shrub, herbaceous, and crop species (U.S. EPA, 2013, p.
1-10; U.S. EPA, 2006a, 1996a, 1986, 1978). Ozone-induced visible foliar
injury symptoms on certain plant species, such as black cherry, yellow-
poplar and common milkweed, are considered diagnostic of exposure to
O3 based on the consistent association established with
experimental evidence (U.S. EPA, 2013, p. 1-10). The evidence has found
that visible foliar injury occurs only when sensitive plants are
exposed to elevated O3 concentrations in a predisposing
environment; a major modifying factor is the amount of available soil
moisture during the year (U.S. EPA, 2013, section 9.4.2).
The significance of O3 injury at the leaf and whole
plant levels depends on an array of factors, and therefore, it is
difficult to quantitatively relate visible foliar injury symptoms to
vegetation effects such as individual tree growth, or effects at
population or ecosystem levels (U.S. EPA, 2013, p. 9-39). The ISA notes
that visible foliar injury ``is not always a reliable indicator of
other negative effects on vegetation'' (U.S. EPA, 2013, p. 9-39).
Factors that influence the significance to the leaf and whole plant
include the amount of total leaf area affected, age of plant, size,
developmental stage, and degree of functional redundancy among the
existing leaf area (U.S. EPA, 2013, section 9.4.2). Although there
remains a lack of robust exposure-response functions that would allow
prediction of visible foliar injury severity and incidence under
varying air quality and environmental conditions, ``[e]xperimental
evidence has clearly
[[Page 65371]]
established a consistent association of visible injury with
O3 exposure, with greater exposure often resulting in
greater and more prevalent injury'' (U.S. EPA, 2013, section 9.4.2, p.
9-41).
By far the most extensive field-based dataset of visible foliar
injury incidence is that obtained by the U.S. Forest Service Forest
Health Monitoring/Forest Inventory and Analysis (USFS FHM/FIA)
biomonitoring network program (U.S. EPA, 2013, section 9.4.2.1; Smith,
2012; Coulston et al., 2007). A recently published trend analysis of
data from the sites located in 24 states of the northeast and north
central U.S. for the 16-year period from 1994 through 2009 (Smith,
2012) describes evidence of visible foliar injury occurrence in the
field as well as some insight into the influence of changes in air
quality and soil moisture on visible foliar injury and the difficulty
inherent in predicting foliar injury response under different air
quality and soil moisture scenarios (Smith, 2012; U.S. EPA, 2013,
section 9.4.2.1). Study results showed that incidence and severity of
foliar injury were dependent on local site conditions for soil moisture
availability and O3 exposure (U.S. EPA, 2013, p. 9-41).
Although the study indicated that moderate O3 exposures
continued to cause visible foliar injury at sites throughout the study
area, there was an overall declining trend in the incidence of visible
foliar injury as peak O3 concentrations declined (U.S. EPA,
2013, p. 9-40).
Ozone has been shown to affect a number of important U.S. tree
species with respect to growth, productivity, and carbon storage.
Ambient O3 concentrations have long been known to cause
decreases in photosynthetic rates and plant growth. As discussed in the
ISA, research published since the 2006 AQCD substantiates prior
conclusions regarding O3-related effects on forest tree
growth, productivity and carbon storage, and further strengthens the
support for those conclusions. A variety of factors in natural
environments can either mitigate or exacerbate predicted O3-
plant interactions and are recognized sources of uncertainty and
variability. Such factors include multiple genetically influenced
determinants of O3 sensitivity, changing sensitivity to
O3 across vegetative growth stages, co-occurring stressors
and/or modifying environmental factors (U.S. EPA, 2013, section 9.4.8).
In considering of the available evidence, the ISA states, ``previous
O3 AQCDs concluded that there is strong evidence that
exposure to O3 decreases photosynthesis and growth in
numerous plant species'' and that ``[s]tudies published since the 2008
review support those conclusions'' (U.S. EPA, 2013, p. 9-42). The
available studies come from a variety of different study types that
cover an array of different species, effects endpoints, levels of
biological organization and exposure methods and durations. The
O3-induced effects at the scale of the whole plant may
translate to the ecosystem scale, with changes in productivity and
carbon storage. As stated in the ISA, ``[s]tudies conducted during the
past four decades have demonstrated unequivocally that O3
alters biomass allocation and plant reproduction'' (U.S. EPA, 2013, p.
1-10).
The strong evidence of O3 impacts on trees includes
robust exposure-response (E-R) functions for reduced growth, termed
relative biomass loss (RBL),\159\ in seedlings of 11 species. These
functions were developed under the National Health and Environmental
Effects Research Laboratory-Western Ecology Division program, a series
of experiments that used open top chambers (OTCs) to investigate
seedling growth response for a single growing season under a variety of
O3 exposures (ranging from near background to well above
current ambient concentrations) and growing conditions (U.S. EPA, 2013,
section 9.6.2; Lee and Hogsett, 1996). The evidence from these studies
shows that there is a wide range in sensitivity across the studied
species in the seedling growth stage over the course of a single
growing season, with some species being extremely sensitive and others
being very insensitive over the range of cumulative O3
exposures studied (U.S. EPA, 2014c, Figure 5-1). At the other end of
the organizational spectrum, field-based studies of species growing in
natural stands have compared observed plant responses across a number
of different sites and/or years when exposed to varying ambient
O3 exposure conditions. For example, a study conducted in
forest stands in the southern Appalachian Mountains during a period
when O3 concentrations exceeded the current standard found
that the cumulative effects of O3 decreased seasonal stem
growth (measured as a change in circumference) by 30-50 percent for
most of the examined tree species (i.e., tulip poplar, black cherry,
red maple, sugar maple) in a high-O3 year in comparison to a
low-O3 year (U.S. EPA, 2013, section 9.4.3.1; McLaughlin et
al., 2007a). The study also reported that high ambient O3
concentrations can increase whole-tree water use and in turn reduce
late-season streamflow (McLaughlin et al., 2007b; U.S. EPA, 2013, p. 9-
43).
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\159\ These functions for RBL estimate reduction in a year's
growth as a percentage of that expected in the absence of
O3 (U.S. EPA, 2013, section 9.6.2; U.S. EPA, 2014b,
section 6.2).
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The magnitude of O3 impact on ecosystem productivity and
on forest composition can vary among plant communities based on several
factors, including the type of stand or community in which the
sensitive species occurs (e.g., single species versus mixed canopy),
the role or position of the species in the stand (e.g., dominant, sub-
dominant, canopy, understory), and the sensitivity of co-occurring
species and environmental factors (e.g., drought and other factors).
For example, recent studies found O3 to have little impact
on white fir, but to greatly reduce growth of ponderosa pine in
southern California locations, with associated reductions in ponderosa
pine abundance in the community, and to cause decreased net primary
production of most forest types in the mid-Atlantic region, with only
small impacts on spruce-fir forest (U.S. EPA, 2013, section 9.4.3.4).
There is previously and newly available evidence of the potential
for O3 to alter biomass allocation and plant reproduction in
seasons subsequent to exposure (U.S. EPA, 2013, section 9.4.3). For
example, several studies published since the 2006 AQCD further
demonstrate that O3 can alter the timing of flowering and
the number of flowers, fruits and seeds in herbaceous and woody plant
species (U.S. EPA, 2013, section 9.4.3.3). Further, limited evidence in
previous reviews reported that vegetation effects from a single year of
exposure to elevated O3 could be observed in the following
year. For example, growth affected by a reduction in carbohydrate
storage in one year may result in the limitation of growth in the
following year. Such ``carry-over'' effects have been documented in the
growth of some tree seedlings and in roots (U.S. EPA, 2013, section
9.4.8; Andersen et al., 1997). In the current review, additional field-
based evidence expands the EPA's understanding of the consequences of
single and multi-year O3 exposures in subsequent years.
A number of studies were conducted at a planted forest at the Aspen
free-air carbon-dioxide and ozone enrichment (FACE) experiment site in
Wisconsin. These studies, which occurred in a field setting (more
similar to natural forest stands than OTC studies), observed tree
growth responses when grown in single or two species stands within 30-m
diameter rings and exposed over a period of ten years to existing
ambient conditions and elevated O3
[[Page 65372]]
concentrations. Some studies indicate the potential for carry-over
effects, such as those showing that the effects of O3 on
birch seeds (reduced weight, germination, and starch levels) could lead
to a negative impact on species regeneration in subsequent years, and
that the O3-attributable effect of reduced aspen bud size
might have been related to the observed delay in spring leaf
development. These effects suggest that elevated O3
exposures have the potential to alter carbon metabolism of
overwintering buds, which may have subsequent effects in the following
year (Darbah, et al., 2008, 2007; Riikonen et al., 2008; U.S. EPA,
2013, section 9.4.3). Other studies found that, in addition to
affecting tree heights, diameters, and main stem volumes in the aspen
community, elevated O3 over a 7-year study period was
reported to increase the rate of conversion from a mixed aspen-birch
community to a community dominated by the more tolerant birch, leading
the authors to conclude that elevated O3 may alter intra-
and inter-species competition within a forest stand (U.S. EPA, 2013,
section 9.4.3; Kubiske et al., 2006; Kubiske et al., 2007). These
studies confirm earlier FACE results of aspen growth reductions from
exposure to elevated O3 during the first seven years of
stand growth and of cumulative biomass impacts associated with changes
in annual production in studied tree communities (U.S. EPA, 2013,
section 9.4.3; King et al., 2005).
Robust and well-established E-R functions for RBL are available for
11 tree species: black cherry, Douglas fir, loblolly pine, ponderosa
pine, quaking aspen, red alder, red maple, sugar maple, tulip poplar,
Virginia pine, and white pine (U.S. EPA, 2013; U.S. EPA, 2014c). While
these 11 species represent only a small fraction (0.8 percent) of the
total number of native tree species in the contiguous U.S. (1,497),
this small subset includes eastern and western species, deciduous and
coniferous species, and species that grow in a variety of ecosystems
and represent a range of tolerance to O3 (U.S. EPA, 2013,
section 9.6.2; U.S. EPA, 2014b, section 6.2, Figure 6-2, Table 6-1).
Supporting the E-R functions for each of these species are studies in
OTCs, with most species studied multiple times under a wide range of
exposure and/or growing conditions, with separate E-R functions
developed for each combination of species, exposure condition and
growing condition scenario (U.S. EPA, 2013, section 9.6.1). Based on
these separate E-R functions, species-specific composite E-R functions
have been developed and successfully used to predict the biomass loss
response from tree seedling species over a range of cumulative exposure
conditions (U.S. EPA, 2013, section 9.6.2). These 11 composite
functions, as well as the E-R function for eastern cottonwood (derived
from a field study in which O3 and climate conditions were
not controlled),\160\ are described in the ISA and graphed in the WREA
to illustrate the predicted responses of these species over a wide
range of cumulative exposures (U.S. EPA, 2014b, section 6.2, Table 6-1
and Figure 6-2; U.S. EPA, 2013, section 9.6.2). For some of these
species, the E-R function is based on a single study (e.g., red maple),
while for other species there were as many as 11 studies available
(e.g., ponderosa pine). In total, the E-R functions developed for these
12 species (the 11 with robust composite E-R functions plus eastern
cottonwood) reflect 52 tree seedling studies. A stochastic analysis in
the WREA, summarized in section IV.C of the proposal, indicates the
potential for within-species variability in these relationships for
each species. Consideration of biomass loss estimates in the PA and in
discussions below, however, is based on conventional methods and
focuses on estimates for the 11 species for which the robust datasets
from OTC experiments are available, in consideration of CASAC advice.
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\160\ The CASAC cautioned the EPA against placing too much
emphasis on the eastern cottonwood data. In comments on the draft
PA, the CASAC stated that the eastern cottonwood response data from
a single study ``receive too much emphasis,'' explaining that these
``results are from a gradient study that did not control for ozone
and climatic conditions and show extreme sensitivity to ozone
compared to other studies'' and that ``[a]lthough they are important
results, they are not as strong as those from other experiments that
developed E-R functions based on controlled ozone exposure'' (Frey,
2014c, p. 10).
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The ``detrimental effect of O3 on crop production has
been recognized since the 1960s'' (U.S. EPA, 2013, p. 1-10, section
9.4.4). On the whole, the newly available evidence supports and
strengthens previous conclusions that exposure to O3 reduces
growth and yield of crops. The ISA describes average crop yield loss
reported across a number of recently published meta-analyses and
identifies several new exposure studies that support prior findings for
a variety of crops of decreased yield and biomass with increased
O3 exposure (U.S. EPA, 2013, section 9.4.4.1, Table 9-17).
Studies have also ``linked increasing O3 concentration to
decreased photosynthetic rates and accelerated aging in leaves, which
are related to yield'' and described effects of O3 on crop
quality, such as nutritive quality of grasses, macro- and micronutrient
concentrations in fruits and vegetable crops and cotton fiber quality
(U.S. EPA, 2013, p. 1-10, section 9.4.4). The findings of the newly
available studies do not change the basic understanding of
O3-related crop yield loss since the last review and little
additional information is available in this review on factors that
influence associations between O3 levels and crop yield loss
(U.S. EPA, 2013, section 9.4.4.). However, the evidence available in
this review continues to support the conclusion that O3 in
ambient air can reduce the yield of major commodity crops in the U.S.
Further, the recent evidence increases our confidence in the use of
crop E-R functions based on OTC experiments to characterize the
quantitative relationship between ambient O3 concentrations
and yield loss (U.S. EPA, 2013, section 9.4.4).
The new evidence has strengthened support for previously
established E-R functions for 10 crops (barley, field corn, cotton,
kidney bean, lettuce, peanut, potato, grain sorghum, soybean and winter
wheat), reducing two important areas of uncertainty, especially for
soybean, as summarized in more detail in section IV.A of the proposal.
The established E-R functions for relative yield loss (RYL)\161\ were
developed from OTC-type experiments from the National Crop Loss
Assessment Network (NCLAN) (U.S. EPA, 2013, section 9.6.3; U.S. EPA,
2014b, section 6.2; U.S. EPA, 2014c, Figure 5-4 and section 6.3). With
regard to the first area of uncertainty reduced, evaluations in the ISA
found that yield loss in soybean from O3 exposure at the
SoyFACE (Soybean Free Air Concentration Enrichment) field experiment
was reliably predicted by soybean E-R functions developed from NCLAN
data (U.S. EPA, 2013, section 9.6.3.1),\162\ demonstrating a robustness
of the NCLAN-based E-R functions for predicting relative yield loss
from O3 exposure. A second area of uncertainty that was
reduced is that regarding the
[[Page 65373]]
application of the NCLAN E-R functions to more recent cultivars
currently growing in the field. Recent studies, especially those
focused on soybean, provide little evidence that crops are becoming
more tolerant of O3 (U.S. EPA, 2006a; U.S. EPA, 2013,
sections 9.6.3.1 and 9.6.3.4 and p. 9-59). The ISA comparisons of NCLAN
and SoyFACE data referenced above also ``confirm that the response of
soybean yield to O3 exposure has not changed in current
cultivars'' (U.S. EPA, 2013, p. 9-59; section 9.6.3.1). Additionally, a
recent assessment of the relationship between soybean yield loss and
O3 in ambient air over the contiguous area of Illinois,
Iowa, and Indiana found a relationship that correlates well with
previous results from FACE- and OTC-type experiments (U.S. EPA, 2013,
section 9.4.4.1).
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\161\ These functions for RYL estimate reduction in a year's
growth as a percentage of that expected in the absence of
O3 (U.S. EPA, 2013, section 9.6.2; U.S. EPA, 2014b,
section 6.2).
\162\ The NCLAN program, which was undertaken in the early to
mid-1980s, assessed multiple U.S. crops, locations, and
O3 exposure levels, using consistent methods, to provide
the largest, most uniform database on the effects of O3
on agricultural crop yields (U.S. EPA 1996a; U.S. EPA, 2006a; U.S.
EPA, 2013, sections 9.2, 9.4, and 9.6, Frey, 2014c, p. 9). The
SoyFACE experiment was a chamberless (or free-air) field-based
exposure study conducted in Illinois from 2001--2009 (U.S. EPA,
2013, section 9.2.4).
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c. Biologically Relevant Exposure Metric
In assessing biologically based indices of exposure pertinent to
O3 effects on vegetation, the ISA states the following (U.S.
EPA, 2013, p. 2-44).
The main conclusions from the 1996 and 2006 O3 AQCDs
[Air Quality Criteria Documents] regarding indices based on ambient
exposure remain valid. These key conclusions can be restated as
follows: ozone effects in plants are cumulative; higher
O3 concentrations appear to be more important than lower
concentrations in eliciting a response; plant sensitivity to
O3 varies with time of day and plant development stage;
[and] quantifying exposure with indices that cumulate hourly
O3 concentrations and preferentially weight the higher
concentrations improves the explanatory power of exposure/response
models for growth and yield, over using indices based on mean and
peak exposure values.
The long-standing body of available evidence upon which these
conclusions are based includes a wealth of information on aspects of
O3 exposure that are important in influencing plant response
(U.S. EPA, 1996a; U.S. EPA, 2006a; U.S. EPA, 2013). Specifically, a
variety of ``factors with known or suspected bearing on the exposure-
response relationship, including concentration, time of day, respite
time, frequency of peak occurrence, plant phenology, predisposition,
etc.,'' have been identified (U.S. EPA, 2013, section 9.5.2). In
addition, the importance of the duration of the exposure and the
relatively greater importance of higher concentrations over lower
concentrations in determining plant response to O3 have been
consistently well documented (U.S. EPA, 2013, section 9.5.3). Based on
improved understanding of the biological basis for plant response to
O3 exposure, a large number of ``mathematical approaches for
summarizing ambient air quality information in biologically meaningful
forms for O3 vegetation effects assessment purposes'' have
been developed (U.S. EPA, 2013, section 9.5.3), including those that
cumulate exposures over some specified period while weighting higher
concentrations more than lower (U.S. EPA, 2013, section 9.5.2). As with
any summary statistic, these exposure indices retain information on
some, but not all, characteristics of the original observations.
Based on extensive review of the published literature on different
types of exposure-response metrics, including comparisons between
metrics, the EPA has focused on cumulative, concentration-weighted
indices, recognizing them as the most appropriate biologically based
metrics to consider in this context (U.S. EPA, 1996a; U.S. EPA, 1996b;
U.S. EPA, 2006a; U.S. EPA, 2013). In the last two reviews of the
O3 NAAQS, the EPA concluded that the risk to vegetation
comes primarily from cumulative exposures to O3 over a
season or seasons \163\ and focused on metrics intended to characterize
such exposures: SUM06 \164\ in the 1997 review (61 FR 65716, December
13, 1996) and W126 in the 2008 review (72 FR 37818, July 11, 2007).
Although in both reviews the policy decision was made not to revise the
form and averaging time of the secondary standard, the Administrator,
in both cases, also concluded, consistent with CASAC advice, that a
cumulative, seasonal index was the most biologically relevant way to
relate exposure to plant growth response (62 FR 38856, July 18, 1997;
73 FR 16436, March 27, 2008). This approach for characterizing
O3 exposure concentrations that are biologically relevant
with regard to potential vegetation effects received strong support
from CASAC in the last review and again in this review, including
strong support for use of such a metric as the form for the secondary
standard (Henderson, 2006, 2008; Samet, 2010; Frey, 2014c).
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\163\ In describing the form as ``seasonal,'' the EPA is
referring generally to the growing season of O3-sensitive
vegetation, not to the seasons of the year (i.e., spring, summer,
fall, winter).
\164\ The SUM06 index is a threshold-based approach described as
the sum of all hourly O3 concentrations greater or equal
to 0.06 ppm observed during a specified daily and seasonal time
window (U.S. EPA, 2013, section 9.5.2). The W126 index is a non-
threshold approach, described more fully below.
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Alternative methods for characterizing O3 exposure to
predict plant response have, in recent years, included flux models,
which some researchers have claimed may ``better predict vegetation
responses to O3 than exposure-based approaches'' because
they estimate the ambient O3 concentration that actually
enters the leaf (i.e., flux or deposition). However, the ISA notes that
``[f]lux calculations are data intensive and must be carefully
implemented'' (U.S. EPA, 2013, p. 9-114). Further, the ISA states,
``[t]his uptake-based approach to quantify the vegetation impact of
O3 requires inclusion of those factors that control the
diurnal and seasonal O3 flux to vegetation (e.g., climate
patterns, species and/or vegetation-type factors and site-specific
factors)'' (U.S. EPA, 2013, p. 9-114). In addition to these data
requirements, each species has different amounts of internal
detoxification potential that may protect species to differing degrees.
The lack of detailed species- and site-specific data required for flux
modeling in the U.S. and the lack of understanding of detoxification
processes have continued to make this technique less viable for use in
vulnerability and risk assessments at the national scale in the U.S.
(U.S. EPA, 2013, section 9.5.4).
Therefore, consistent with the ISA conclusions regarding the
appropriateness of considering cumulative exposure indices that
preferentially weight higher concentrations over lower for predicting
O3 effects of concern based on the well-established
conclusions and supporting evidence described above, and in light of
continued CASAC support, we continue to focus on cumulative
concentration-weighted indices as the most biologically relevant
metrics for consideration of O3 exposures eliciting
vegetation-related effects. Quantifying exposure in this way ``improves
the explanatory power of exposure/response models for growth and yield
over using indices based on mean and peak exposure values'' (U.S. EPA,
2013, section 2.6.6.1, p. 2-44). In this review, as in the last review,
we use the W126-based cumulative, seasonal metric (U.S. EPA, 2013,
sections 2.6.6.1 and 9.5.2) for consideration of the effects evidence
and in the exposure and risk analyses in the WREA.
This metric, commonly called the W126 index, is a non-threshold
approach described as the sigmoidally weighted sum of all hourly
O3 concentrations observed during a specified daily and
seasonal time window, where each hourly O3 concentration is
given a weight that increases from zero to one with increasing
concentration (U.S. EPA, 2014c, p. 5-6; U.S. EPA 2013, p. 9-101).
[[Page 65374]]
The first step in calculating the seasonal W126 index, as described and
considered in this review, is to sum the weighted ambient O3
concentrations during daylight hours (defined as 8:00 a.m. to 8:00
p.m.) within each calendar month, resulting in monthly index values
(U.S. EPA, 2014b, pp. 4-5 to 4-6). As more completely described in the
WREA, the monthly W126 index values are calculated from hourly
O3 concentrations as follows:
[GRAPHIC] [TIFF OMITTED] TR26OC15.000
where N is the number of days in the month, d is the day of the month
(d = 1, 2, . . ., N), h is the hour of the day (h = 0, 1, . . ., 23),
and Cdh is the hourly O3 concentration observed
on day d, hour h, in parts per million. The seasonal W126 index value
for a specific year is the maximum sum of the monthly index values for
three consecutive months. Three-year W126 index values are calculated
by taking the average of seasonal W126 index values for three
consecutive years (U.S. EPA, 2014b, pp. 4-5 to 4-6; Wells, 2014a).
2. Overview of Welfare Exposure and Risk Assessment
This section outlines the information presented in section IV.C of
the proposal regarding the WREA conducted for this review, which built
upon similar analyses performed in the last review. The WREA focuses
primarily on analyses related to two types of effects on vegetation:
Reduced growth (biomass loss) in both trees and agricultural crops, and
foliar injury. The assessments of O3-associated reduced
growth in native trees and crops (specifically, RBL and RYL,
respectively) include analysis of associated changes in related
ecosystem services, including pollution removal, carbon sequestration
or storage, and hydrology, as well as economic impacts on the forestry
and agriculture sectors of the economy. The foliar injury assessments
include cumulative analyses of the proportion of USFS biosite index
scores \165\ above zero (or five, in a separate set of analyses) with
increasing W126 exposure index estimates, with and without
consideration of soil moisture conditions. The implications of visible
foliar injury in national parks were considered in a screening level
assessment and three case studies.\166\
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\165\ Sampling sites in the FIA/FHM O3 biomonitoring
program, called ``biosites'', are plots of land on which data are
collected regarding the incidence and severity of visible foliar
injury on a variety of O3-sensitive plant species.
Biosite index scores are derived from these data (U.S. EPA, 2014b,
section 7.2.1).
\166\ All of the analyses are described in detail in the WREA
and summarized in the PA and in section IV.C of the proposal (U.S.
EPA, 2014a; U.S. EPA, 2014b; 79 FR 75324-75329, December 17, 2014).
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Growth-related effects were assessed for W126-based exposure
estimates in five scenarios of national-scale \167\ air quality: Recent
conditions (2006 to 2008), the existing secondary standard, and W126
index values of 15 ppm-hrs, 11 ppm-hrs, and 7 ppm-hrs, using 3-year
averages (U.S. EPA, 2014b, chapter 4). For each of these scenarios, 3-
year average W126 exposure index values were estimated for 12 kilometer
(km) by 12 km grid cells in a national-scale spatial surface. The
method for creating these grid cell estimates generally involved two
steps (summarized in Table 5-4 of the PA).
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\167\ Although the scenarios and the grid cell O3
concentrations on which they are based were limited to the
contiguous U.S., we have generally used the phrase ``national-
scale'' in reference to the WREA scenarios and surfaces.
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The first step in creating the grid cell estimates for each
scenario was calculation of the average W126 index value (across the
three years) at each monitor location. For the recent conditions
scenario, this value was based on unadjusted O3
concentrations from monitoring data. For the other four scenarios, the
W126 index value for each monitor location was calculated from model-
adjusted hourly O3 concentrations. The adjusted
concentrations were based on model-predicted relationships between
O3 at each monitor location and reductions in
NOX. Adjustments were applied independently for each of the
nine U.S. regions (see U.S. EPA, 2014b, section 4.3.4.1).\168\ The
existing standard scenario was created first, with the result being a
national dataset for which the highest monitor location in each U.S.
region had a design value equal to the level of the current
standard.\169\ The W126 scenarios were created from the hourly
concentrations used to create the existing standard scenario, with
model-based adjustments made at all monitor sites in those regions with
a site not already at or below the target W126 value for that scenario
(U.S. EPA, 2014b, section 4.3.4.1).\170\
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\168\ The U.S. regions referenced here and in section IV.C below
are NOAA climate regions, as shown in Figure 2B-1 of the PA.
\169\ The adjustment results in broad regional reductions in
O3 and includes reductions in O3 at some
monitors that were already at or below the target level. These
reductions do not represent an optimized control scenario, but
rather characterize one potential distribution of air quality across
a region that meets the scenario target (U.S. EPA, 2014b, sections
4.3.4.2 and 4.4).
\170\ In regions where the air quality adjustment was applied,
it was based on emissions reductions determined necessary for the
highest monitor in that region to just equal the existing standard
or the W126 target for the scenario. Concentrations at all other
monitor locations in the region were also adjusted based on the same
emissions reductions assumptions.
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After completing step one for all the scenarios, the second step
involved creating the national-scale spatial surfaces (composed of 3-
year W126 index values at grid cell centroids). These were created by
applying the Voronoi Neighbor Averaging (VNA) spatial interpolation
technique to the monitor-location, 3-year W126 index values (described
in step 1).\171\ This step of creating the gridded spatial surfaces
resulted in further reduction of the highest values in each modeling
region, as demonstrated by comparing the W126 index values from steps
one and two for the existing standard scenario. After the step-one
adjustment of the monitor location concentrations such that the highest
location in each NOAA region just met the existing standard (using
relationships mentioned above), the maximum 3-year average W126 values
in the nine regions ranged from 18.9 ppm-hrs in the West region to 2.6
ppm-hrs in the Northeast region (U.S. EPA, 2014b, Table 4-3). After
application of the VNA technique in the second step, however, the
highest 3-year average W126 values across the national surface grid
cells, which were in the Southwest region, were below 15 ppm-hrs (U.S.
EPA, 2014b, Figure 4-7).\172\
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\171\ The VNA technique is described in the WREA (U.S. EPA,
2014b, Appendix 4A).
\172\ Thus, it can be seen that application of the VNA
interpolation method to estimate W126 index values at the centroid
of every 12 km x 12 km grid cell rather than only at each monitor
location results in a lowering of the highest values in each region.
---------------------------------------------------------------------------
All of the assessments based on growth impacts relied on the W126
index estimates from the national-scale spatial surfaces (created from
the 3-year average monitor location values as described above). Among
the analyses related to visible foliar injury, a small component of the
screening-level
[[Page 65375]]
national park assessment and also the three national park case studies
involved summarizing 3-year W126 index estimates from the four air
quality scenarios. However, the visible foliar injury cumulative
proportion analyses and a component of the national park screening-
level assessment relied on national-scale spatial surfaces of single-
year, unadjusted W126 index values created for each year from 2006
through 2010 using the VNA interpolation technique applied to the
monitor location index values for these years (U.S. EPA, 2014b, section
4.3.2, Appendix 4A).
Because the W126 estimates generated for the different air quality
scenarios assessed are inputs to the vegetation risk analyses for tree
biomass and crop yield loss, and also used in some components of the
visible foliar injury assessments, limitations and uncertainties in the
air quality analyses, which are discussed in detail in the WREA and
some of which are mentioned here, are propagated into those analyses
(U.S. EPA, 2014b, chapters 4 and 8 and section 8.5, Table 4-5). An
important uncertainty in the analyses is the application of regionally
determined emissions reductions to meet the existing standard (U.S.
EPA, 2014b, section 8.5.1). The model adjustments are based on
emissions reductions in NOx and characterize only one potential
distribution of air quality across a region when all monitor locations
meet the standard, as well as for the W126 scenarios (U.S. EPA, 2014b,
section 4.3.4.2).\173\
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\173\ The adjustment is applied to all monitor locations in each
region. In this way, the adjustment results in broad regional
reductions in O3 and includes reductions in O3
at some monitors that were already meeting or below the target
level. Thus, the adjustments performed to develop a scenario meeting
a target level at the highest monitor in each region did result in
substantial reduction below the target level in some areas of the
region. This result at the monitors already well below the target
indicates an uncertainty with regard to air quality expected from
specific control strategies that might be implemented to meet a
particular target level.
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An additional uncertainty related to the W126 index estimates in
the national surfaces for each air quality scenario, and to the
estimates for the single-year surfaces used in the visible foliar
injury cumulative analysis, comes with the creation of the national-
scale spatial surfaces of grid cells from the monitor-location
O3 data.\174\ In general, spatial interpolation techniques
perform better in areas where the O3 monitoring network is
denser. Therefore, the W126 index values estimated using this technique
in rural areas in the West, Northwest, Southwest, and West North
Central regions where there are few or no monitors (U.S. EPA, 2014b,
Figure 2-1) are more uncertain than those estimated for areas with
denser monitoring. Further, as described above, this interpolation
method generally underpredicts the highest W126 exposure index values.
Due to the important influence of higher exposures in determining risks
to plants, the potential for the VNA interpolation approach to dampen
peak W126 index values could result in an underestimation of risks to
vegetation in some areas.\175\
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\174\ Some uncertainty is inherent in any approach to
characterizing O3 air quality over broad geographic areas
based on concentrations at monitor locations.
\175\ In the visible foliar injury dataset used for the
cumulative analysis, underestimation of W126 index values at sites
with injury would contribute to overestimates of the cumulative
proportion of sites with injury plotted for the lower W126 values.
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The vegetation analyses performed in the WREA, along with key
observations, insights, uncertainties and limitations were summarized
in sections IV.C.2 through IV.C.3 of the proposal. Highlights for the
three categories of biomass loss and foliar injury assessments are
summarized here.
a. Tree Growth, Productivity and Carbon Storage
These assessments rely on the species-specific E-R functions
described in section IV.A.1.b above. For the air quality scenarios
described above, the WREA applied the species-specific E-R functions to
develop estimates of O3-associated RBL and associated
effects on productivity, carbon storage and associated ecosystem
services (U.S. EPA, 2014b, Chapter 6). More specifically, the WREA
derived species-specific and weighted RBL estimates for grid cells
across the continental U.S. and summarized the estimates by counties
and national parks. Additional WREA case study analyses focused on
selected urban areas. The WREA estimates indicate substantial
heterogeneity in plant responses to O3, both within species
(e.g., study-specific variation), between species, and across regions
of the U.S. National variability in the estimates (e.g., eastern vs
western U.S.) is influenced by there being different sets of resident
species (with different E-R functions) in different areas of the U.S.,
as well as differences in number of national parks and O3
monitors. For example, the eastern U.S. has different resident species
compared to the western U.S., and the eastern U.S. has far more such
species. Additionally, there are more national parks in the western
than the eastern U.S., yet fewer O3 monitors (U.S. EPA,
2014b, chapter 8).
Relative biomass loss nationally (across all of the air quality
surface grid cells) was estimated for each of the 12 studied species
from the composite E-R functions for each species described above and
information on the distribution of those species across the U.S. (U.S.
EPA, 2014b, section 6.2.1.3 and Appendix 6A). In consideration of CASAC
advice (summarized in section IV.A.1.b above), the WREA derived RBL and
weighted RBL (wRBL) estimates separately, both with and without the
eastern cottonwood, and the PA and proposal gave primary focus to
analyses that exclude cottonwood. These analyses provided estimates of
per-species and cross-species RBL in the different air quality
scenarios. Air quality scenario estimates were also developed in terms
of proportion of basal area affected at different magnitudes of RBL.
The wRBL analysis integrated the species-specific estimates, providing
an indication of potential magnitude of ecological effect possible in
some ecosystems. The county analyses also included analyses focused on
the median species response. The WREA also used the E-R functions to
estimate RBL across tree lifespans and the resulting changes in
consumer and producer/farmer economic surplus in the forestry and
agriculture sectors of the economy. Case studies in five urban areas
provided comparisons across air quality scenarios of estimates for
urban tree pollutant removal and carbon storage or sequestration.
The array of uncertainties associated with estimates from these
tree RBL analyses are summarized in the proposal and described in
detail in the WREA, including the potential for the air quality
scenarios to underestimate the higher W126 index values and associated
implications for the RBL-related estimates, as referenced above.
b. Crop Yield Loss
These assessments rely on the species-specific E-R functions
described in section IV.A.1.b above. For the different air quality
scenarios, the WREA applied the species-specific E-R functions to
develop estimates of O3 impacts related to crop yield,
including annual yield losses estimated for 10 commodity crops grown in
the U.S. and how these losses affect producer and consumer economic
surpluses (U.S. EPA, 2014b, sections 6.2, 6.5). The WREA derived
estimates of crop RYL nationally and in a county-specific analysis,
relying on information regarding crop distribution (U.S. EPA, 2014b,
section 6.5). As with the tree analyses described above, the county
analysis included estimates based on
[[Page 65376]]
the median O3 response across the studied crop species (U.S.
EPA, 2014b, section 6.5.1, Appendix 6B).
Overall effects on agricultural yields and producer and consumer
surplus depend on the ability of producers/farmers to substitute other
crops that are less O3 sensitive, and the responsiveness, or
elasticity, of demand and supply (U.S. EPA, 2014b, section 6.5). The
WREA discusses multiple areas of uncertainty associated with the crop
yield loss estimates, including those associated with the model-based
adjustment methodology as well as those associated with the projection
of yield loss using the Forest and Agriculture Sector Optimization
Model (with greenhouse gases) at the estimated O3
concentrations (U.S. EPA, 2014b, Table 6-27, section 8.5). Because the
W126 index estimates generated in the air quality scenarios are inputs
to the vegetation risk analyses for crop yield loss, any uncertainties
in the air quality scenario estimation of W126 index values are
propagated into those analyses (U.S. EPA, 2014b, Table 6-27, section
8.5). Therefore, the air quality scenarios in the crop yield analyses
have the same uncertainties and limitations as in the biomass loss
analyses (summarized above), including those associated with the model-
based adjustment methodology (U.S. EPA, 2014b, section 8.5).
c. Visible Foliar Injury
The WREA presents a number of analyses of O3-related
visible foliar injury and associated ecosystem services impacts (U.S.
EPA, 2014b, Chapter 7). In the initial analysis, the WREA used the
biomonitoring site data from the USFS FHM/FIA Network (USFS,
2011),\176\ associated soil moisture data during the sample years, and
national surfaces of ambient air O3 concentrations based on
spatial interpolation of monitoring data from 2006 to 2010 in a
cumulative analysis of the proportion of biosite records with any
visible foliar injury, as indicated by a nonzero biosite index score
(U.S. EPA, 2014b, section 7.2). This analysis was done for all records
together, and also for subsets based on soil moisture conditions
(normal, wet or dry).
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\176\ Data were not available for several western states
(Montana, Idaho, Wyoming, Nevada, Utah, Colorado, Arizona, New
Mexico, Oklahoma, and portions of Texas).
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In each cumulative analysis, the biosite records were ordered by
W126 index and then, moving from low to high W126 index, the records
were cumulated into a progressively larger dataset. With the addition
of each new data point (composed of biosite index score and W126 index
value for a biosite and year combination) to the cumulative dataset,
the percentage of sites with a nonzero biosite index score was derived
and plotted versus the W126 index estimate for the just added data
point. The cumulative analysis for all sites indicates that (1) as the
cumulative set of sites grows with addition of sites with progressively
higher W126 index values, the proportion of the dataset for which no
foliar injury was recorded changes (increases) noticeably prior to
about 10 ppm-hrs (10.46 ppm-hrs), and (2) as the cumulative dataset
grows still larger with the addition of records for higher W126 index
estimates, the proportion of the cumulative dataset with no foliar
injury remains relatively constant (U.S. EPA, 2014b, Figure 7-10). The
data for normal moisture years are very similar to the dataset as a
whole, with an overall proportion of about 18 percent for presence of
any foliar injury. The data for relatively wet years have a much higher
proportion of biosites showing injury, approximately 25% when all data
are included, and a proportion of approximately 20% when data for W126
index estimates up to about 5-8 ppm-hrs are included (U.S. EPA, 2014b,
Figure 7-10).\177\ The overall proportion showing injury for the subset
for relatively dry conditions is much lower, less than 15% for the
subset (U.S. EPA, 2014b, section 7.2.3, Figures 7-10). While these
analyses indicate the potential for foliar injury to occur under
conditions that meet the current standard, the extent of foliar injury
that might be expected under different exposure conditions is unclear
from these analyses.
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\177\ As discussed in section IV.C.2 below, as the cumulative
set increases, with increasing W126 values, the overall prevalence
of visible foliar injury in the cumulative set is more and more
influenced by data for the lower W126 values. Accordingly, the
``leveling off'' observed above ~10 ppm-hrs in the `all sites'
analysis likely reflects the counterbalancing of visible foliar
injury occurrence at the relatively fewer higher O3 sites
by the larger representation within the subset of the lower W126
conditions associated with which there is lower occurrence or extent
of foliar injury.
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Criteria derived from the cumulative analyses were then used in two
additional analyses. The national-scale screening-level assessment
compared W126 index values estimated within 214 national parks using
the VNA technique described above for the individual years from 2006 to
2010 with benchmark criteria developed from the biosite data analysis
(U.S. EPA, 2014b, Appendix 7A and section 7.3). Separate case study
analyses described visits, as well as visitor uses and expenditures for
three national parks, and the 3-year W126 index estimates in those
parks for the four air quality scenarios (U.S. EPA, 2014b, section
7.4). Uncertainties associated with these analyses, included those
associated with the W126 index estimates, are discussed in the WREA,
sections 7.5 and 8.5.3, and in WREA Table 7-24, and also summarized in
the PA (e.g., U.S. EPA, 2014c, section 6.3).
3. Potential Impacts on Public Welfare
As provided in the CAA, section 109(b)(2), the secondary standard
is to ``specify a level of air quality the attainment and maintenance
of which in the judgment of the Administrator . . . is requisite to
protect the public welfare from any known or anticipated adverse
effects associated with the presence of such air pollutant in the
ambient air.'' Effects on welfare include, but are not limited to,
``effects on soils, water, crops, vegetation, man-made materials,
animals, wildlife, weather, visibility, and climate, damage to and
deterioration of property, and hazards to transportation, as well as
effects on economic values and on personal comfort and well-being''
(CAA section 302(h)). The secondary standard is not meant to protect
against all known or anticipated O3-related effects, but
rather those that are judged to be adverse to the public welfare, and a
bright-line determination of adversity is not required in judging what
is requisite (78 FR 8312, January 15, 2013; see also 73 FR 16496, March
27, 2008). Thus, the level of protection from known or anticipated
adverse effects to public welfare that is requisite for the secondary
standard is a public welfare policy judgment to be made by the
Administrator. In the current review, the Administrator's judgment is
informed by conclusions drawn with regard to adversity of effects to
public welfare in decisions on secondary O3 standards in
past reviews.
As indicated by the Administrator in the 2008 decision, the degree
to which O3 effects on vegetation should be considered to be
adverse to the public welfare depends on the intended use of the
vegetation and the significance of the vegetation to the public welfare
(73 FR 16496, March 27, 2008). Such judgments regarding public welfare
significance in the last O3 NAAQS decision gave particular
consideration to O3 effects in areas with special federal
protections, and lands set aside by states, tribes and public interest
groups to provide similar benefits to the public welfare (73 FR 16496,
March 27, 2008). For example, in reaching his conclusion regarding the
need for revision of the secondary standard in the 2008 review, the
Administrator took
[[Page 65377]]
note of ``a number of actions taken by Congress to establish public
lands that are set aside for specific uses that are intended to provide
benefits to the public welfare, including lands that are to be
protected so as to conserve the scenic value and the natural vegetation
and wildlife within such areas, and to leave them unimpaired for the
enjoyment of future generations'' (73 FR 16496, March 27, 2008). As
further recognized in the 2008 notice, ``[s]uch public lands that are
protected areas of national interest include national parks and
forests, wildlife refuges, and wilderness areas'' (73 FR 16496, March
27, 2008).\178\ \179\ Such areas include Class I areas\180\ which are
federally mandated to preserve certain air quality related values.
Additionally, as the Administrator recognized, ``States, Tribes and
public interest groups also set aside areas that are intended to
provide similar benefits to the public welfare, for residents on State
and Tribal lands, as well as for visitors to those areas'' (73 FR
16496, March 27, 2008). The Administrator took note of the ``clear
public interest in and value of maintaining these areas in a condition
that does not impair their intended use and the fact that many of these
lands contain O3-sensitive species'' (73 FR 16496, March 27,
2008).
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\178\ For example, the National Park Service Organic Act of 1916
established the National Park Service (NPS) and, in describing the
role of the NPS with regard to ``Federal areas known as national
parks, monuments, and reservations'', stated that the ``fundamental
purpose'' for these federal areas ``is to conserve the scenery and
the natural and historic objects and the wild life therein and to
provide for the enjoyment of the same in such manner and by such
means as will leave them unimpaired for the enjoyment of future
generations.'' 16 U.S.C. 1.
\179\ As a second example, the Wilderness Act of 1964 defines
designated ``wilderness areas'' in part as areas ``protected and
managed so as to preserve [their] natural conditions'' and requires
that these areas ``shall be administered for the use and enjoyment
of the American people in such manner as will leave them unimpaired
for future use and enjoyment as wilderness, and so as to provide for
the protection of these areas, [and] the preservation of their
wilderness character . . .'' 16 U.S.C. 1131 (a).
\180\ Areas designated as Class I include all international
parks, national wilderness areas which exceed 5,000 acres in size,
national memorial parks which exceed 5,000 acres in size, and
national parks which exceed six thousand acres in size, provided the
park or wilderness area was in existence on August 7, 1977. Other
areas may also be Class I if designated as Class I consistent with
the CAA.
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The concept described in the 2008 notice regarding the degree to
which effects on vegetation in specially protected areas, such as those
identified above, may be judged adverse also applies beyond the species
level to the ecosystem level, such that judgments can depend on the
intended use\181\ for, or service (and value) of, the affected
vegetation, ecological receptors, ecosystems and resources and the
significance of that use to the public welfare (73 FR 16496, March 27,
2008). Uses or services provided by areas that have been afforded
special protection can flow in part or entirely from the vegetation
that grows there. Aesthetic value and outdoor recreation depend, at
least in part, on the perceived scenic beauty of the environment (U.S.
EPA, 2014b, chapters 5 and 7). Further, analyses have reported that the
American public values--in monetary as well as nonmonetary ways--the
protection of forests from air pollution damage. In fact, studies that
have assessed willingness-to-pay for spruce-fir forest protection in
the southeastern U.S. from air pollution and insect damage have found
that values held by the survey respondents for the more abstract
services (existence, option and bequest)\182\ were greater than those
for recreation or other services (U.S. EPA, 2014b, Table 5-6; Haefele
et al., 1991; Holmes and Kramer, 1995).
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\181\ Ecosystem services have been defined as ``the benefits
that people obtain from ecosystems'' (U.S. EPA, 2013, Preamble, p.
1xxii; UNEP, 2003) and thus are an aspect of the use of a type of
vegetation or ecosystem. Similarly, a definition used for the
purposes of the EPA benefits assessments states that ecological
goods and services are the ``outputs of ecological functions or
processes that directly or indirectly contribute to social welfare
or have the potential to do so in the future'' and that ``[s]ome
outputs may be bought and sold, but most are not marketed'' (U.S.
EPA, 2006b). Ecosystem services analyses were one of the tools used
in the last review of the secondary standards for oxides of nitrogen
and sulfur to inform the decisions made with regard to adequacy and
as such, were used in conjunction with other considerations in the
discussion of adversity to public welfare (77 FR 20241, April 3,
2012).
\182\ Public surveys have indicated that Americans rank as very
important the existence of resources, the option or availability of
the resource and the ability to bequest or pass it on to future
generations (Cordell et al., 2008).
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The spatial, temporal and social dimensions of public welfare
impacts are also influenced by the type of service affected. For
example, a national park can provide direct recreational services to
the thousands of visitors that come each year, but also provide an
indirect value to the millions who may not visit but receive
satisfaction from knowing it exists and is preserved for the future
(U.S. EPA, 2014b, chapter 5, section 5.5.1). Similarly, ecosystem
services can be realized over a range of temporal scales. An evaluation
of adversity to the public welfare might also consider the likelihood,
type, and magnitude of the effect, as well as the potential for
recovery and any uncertainties relating to these conditions, as stated
in the preamble of the 2012 final notice of rulemaking on the secondary
standards for oxides of nitrogen and sulfur (77 FR 20232, April 3,
2012).
The three main categories of effects on vegetation discussed in
section IV.A.1.b above differ with regard to aspects important to
judging their public welfare significance. Judgments regarding crop
yield loss, for example, depend on considerations related to the heavy
management of agriculture in the U.S., while judgments regarding the
other categories of effects generally relate to considerations
regarding forested areas. For example, while both tree growth-related
effects and visible foliar injury have the potential to be significant
to the public welfare through impacts in Class I and other protected
areas, they differ in how they might be significant and with regard to
the clarity of the data that describe the relationship between the
effect and the services potentially affected.
With regard to effects on tree growth, reduced growth is associated
with effects on an array of ecosystem services including reduced
productivity, altered forest and forest community (plant, insect and
microbe) composition, reduced carbon storage and altered water cycling
(U.S. EPA, 2013, Figure 9-1, sections 9.4.1.1 and 9.4.1.2; U.S. EPA,
2014b, section 6.1). For example, forest or forest community
composition can be affected through O3 effects on growth and
reproductive success of sensitive species in the community, with the
extent of compositional changes dependent on factors such as
competitive interactions (U.S. EPA, 2013, sections 9.4.3 and 9.4.3.1).
Depending on the type and location of the affected ecosystem, services
benefitting the public in other ways can be affected as well. For
example, other services valued by people that can be affected by
reduced tree growth, productivity and carbon storage include aesthetic
value, food, fiber, timber, other forest products, habitat,
recreational opportunities, climate and water regulation, erosion
control, air pollution removal, and desired fire regimes (U.S. EPA
2013, sections 9.4.1.1 and 9.4.1.2; U.S. EPA, 2014b, section 6.1,
Figure 6-1, section 6.4, Table 6-13). Further, impacts on some of these
services (e.g., forest or forest community composition) may be
considered of greater public welfare significance when occurring in
Class I or other protected areas.
Consideration of the magnitude of tree growth effects that might
cause or contribute to adverse effects for trees, forests, forested
ecosystems or the public welfare is complicated by aspects
[[Page 65378]]
of, or limitations in, the available information. For example, the
evidence on tree seedling growth effects, deriving from the E-R
functions for 11 species (described in section IV.A.1 above), provides
no clear threshold or breakpoint in the response to O3
exposure. Additionally, there are no established relationships between
magnitude of tree seedling growth reduction and forest ecosystem
impacts and, as noted in section IV.A.1.b above, other factors can
influence the degree to which O3-induced growth effects in a
sensitive species affect forest and forest community composition and
other ecosystem service flows from forested ecosystems. These include
(1) the type of stand or community in which the sensitive species is
found (i.e., single species versus mixed canopy); (2) the role or
position the species has in the stand (i.e., dominant, sub-dominant,
canopy, understory); (3) the O3 sensitivity of the other co-
occurring species (O3 sensitive or tolerant); and (4)
environmental factors, such as soil moisture and others. The lack of
such established relationships complicates judgments as to the extent
to which different estimates of impacts on tree seedling growth would
indicate significance to the public welfare and thus be an important
consideration in the level of protection for the secondary standard.
During the 1997 review of the secondary standard, views related to
this issue were provided by a 1996 workshop of 16 leading scientists in
the context of discussing their views for a secondary O3
standard (Heck and Cowling, 1997). In their consideration of tree
growth effects as an indicator for forest ecosystems and crop yield
reduction as an indicator of agricultural systems, the workshop
participants identified annual percentages, of RBL for forest tree
seedlings and RYL for agricultural crops, considered important to their
judgments on the standard. With regard to forest ecosystems and
seedling growth effects as an indicator, the participants selected a
range of 1-2% RBL per year ``to avoid cumulative effects of yearly
reductions of 2%.'' With regard to crops, they indicated an interest in
protecting against crop yield reductions of 5% RYL yet noted
uncertainties surrounding such a percentage which led them to
identifying 10% RYL for the crop yield endpoint (Heck and Cowling,
1997). The workshop report provides no explicit rationale for the
percentages identified (1-2% RBL and 5% or 10% RYL); nor does it
describe their connection to ecosystem impacts of a specific magnitude
or type, nor to judgments on significance of the identified effects for
public welfare, e.g., taking into consideration the intended use and
significance of the affected vegetation (Heck and Cowling, 1997). In
recognition of the complexity of assessing the adversity of tree growth
effects and effects on crop yield in the broader context of public
welfare, the EPA's consideration of those effects in both the 1997 and
2008 reviews extended beyond the consideration of various benchmark
responses for the studied species, and, with regard to crops,
additionally took note of their extensive management (62 FR 38856, July
18, 1997; 73 FR 16436, March 27, 2008).
While, as noted above, public welfare benefits of forested lands
can be particular to the type of area in which the forest occurs, some
of the potential public welfare benefits associated with forest
ecosystems are not location dependent. A potentially extremely valuable
ecosystem service provided by forested lands is carbon storage, a
regulating service that is ``of paramount importance for human
society'' (U.S. EPA, 2013, section 2.6.2.1 and p. 9-37). As noted
above, the EPA has concluded that this ecosystem service has a likely
causal relationship with O3 in ambient air. The service of
carbon storage is potentially important to the public welfare no matter
in what location the sensitive trees are growing or what their intended
current or future use. In other words, the benefit exists as long as
the tree is growing, regardless of what additional functions and
services it provides. Another example of locations potentially
vulnerable to O3-related impacts but not necessarily
identified for such protection might be forested lands, both public and
private, where trees are grown for timber production. Forests in
urbanized areas also provide a number of services that are important to
the public in those areas, such as air pollution removal, cooling, and
beautification. There are also many other tree species, such as species
identified by the USFS and various ornamental and agricultural species
(e.g., Christmas trees, fruit and nut trees), that provide ecosystem
services that may be judged important to the public welfare but whose
vulnerability to O3 impacts has not been quantitatively
characterized (U.S. EPA, 2014b, Chapter 6).
As noted above, in addition to tree growth-related effects,
O3-induced visible foliar injury also has the potential to
be significant to the public welfare through impacts in Class I and
other similarly protected areas. Visible foliar injury is a visible
bioindicator of O3 exposure in species sensitive to this
effect, with the injury affecting the physical appearance of the plant.
Accordingly visible foliar injury surveys are used by federal land
managers as tools in assessing potential air quality impacts in Class I
areas. These surveys may focus on plant species that have been
identified as potentially sensitive air quality related values (AQRVs)
due to their sensitivity to O3-induced foliar injury (USFS,
NPS, FWS, 2010). An AQRV is defined by the National Park Service as a
``resource, as identified by the [federal land manager] for one or more
Federal areas that may be adversely affected by a change in air
quality,'' and the resource ``may include visibility or a specific
scenic, cultural, physical, biological, ecological, or recreational
resource identified by the [federal land manager] for a particular
area'' (USFS, NPS, USFWS, 2010).\183\ No criteria have been
established, however, regarding a level or prevalence of visible foliar
injury considered to be adverse to the affected vegetation, and, as
noted in section IV.A.1.b above, there is not a clear relationship
between visible foliar injury and other effects, such as reduced growth
and productivity.\184\ Thus, key considerations with regard to public
welfare significance of this endpoint
[[Page 65379]]
have related to qualitative consideration of the plant's aesthetic
value in protected forested areas. Depending on the extent and
severity, O3-induced visible foliar injury might be expected
to have the potential to impact the public welfare in scenic and/or
recreational areas during the growing season, particularly in areas
with special protection, such as Class I areas.
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\183\ The identification, monitoring and assessment of AQRVs
with regard to an adverse effect is an approach used for assessing
the potential for air pollution impacts in Class I areas from
pending permit actions (USFS, NPS, USFWS, 2010). An adverse impact
is recognized by the National Park Service as one that results in
diminishment of the Class I area's national significance or the
impairment of the ecosystem structure or functioning, as well as
impairment of the quality of the visitor experience (USFS, NPS,
USFWS, 2010). Federal land managers make such adverse impact
determinations on a case-by-case basis, using technical and other
information that they provide for consideration by permitting
authorities. The National Park Service has developed a document
describing an overview of approaches related to assessing projects
under the National Environmental Policy Act and other planning
initiatives affecting the National Park System (http://www.nature.nps.gov/air/Pubs/pdf/AQGuidance_2011-01-14.pdf).
\184\ The National Park Service identifies various ranges of
W126 index values in providing approaches for assessing air quality-
related impacts of various development projects which appear to be
based on the 1996 workshop report (Heck and Cowling, 1997), and may,
at the low end, relate to a benchmark derived for the highly
sensitive species, black cherry, for growth effects (10% RBL),
rather than visible foliar injury (Kohut, 2007; Lefohn et al.,
1997). As noted in section IV.A.1.b above, visible foliar injury is
not always a reliable indicator of other negative effects on
vegetation (U.S. EPA, 2013, p. 9-39). We also note that the USFS
biomonitoring analyses of visible foliar injury biomonitoring data
commonly make use of a set of biosite index categories for which
risk assumptions have been assigned, providing a relative scale of
possible impacts (Campbell et al, 2007); however, little information
is available on the studies, effects and judgments on which these
categories are based.
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The ecosystem services most likely to be affected by O3-
induced visible foliar injury (some of which are also recognized above
for tree growth-related effects) are cultural services, including
aesthetic value and outdoor recreation. In addition, several tribes
have indicated that many of the species identified as O3
sensitive (including bioindicator species) are culturally significant
(U.S. EPA, 2014c, Table 5-1). The geographic extent of protected areas
that may be vulnerable to such public welfare effects of O3
is potentially appreciable. Sixty-six plant species that occur on U.S.
National Park Service (NPS) and U.S. Fish and Wildlife Service lands
\185\ have been identified as sensitive to O3-induced
visible foliar injury, and some also have particular cultural
importance to some tribes (U.S. EPA, 2014c, Table 5-1 and Appendix 5-A;
U.S. EPA, 2014b, section 6.4.2). Not all species are equally sensitive
to O3, however, and quantitative E-R relationships for
O3 exposure and other important effects, such as seedling
growth reduction, are only available for a subset of 12 of the 66, as
summarized in section IV.A.1.b above. A diverse array of ecosystem
services has been identified for these twelve species (U.S. EPA, 2014c,
Table 5-1). Two species in this group that are slightly more sensitive
than the median for the group with regard to effects on growth are the
ponderosa pine and quaking aspen (U.S. EPA, 2014b, section 6.2), the
ranges for which overlap with many lands that are protected or
preserved for enjoyment of current and future generations (consistent
with the discussion above on Class I and other protected areas),
including such lands located in the west and southwest regions of the
U.S. where ambient O3 concentrations and associated
cumulative seasonal exposures can be highest (U.S. EPA, 2014c, Appendix
2B).\186\
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\185\ See http://www2.nature.nps.gov/air/Pubs/pdf/flag/NPSozonesensppFLAG06.pdf.
\186\ Basal area for resident species in national forests and
parks are available in files accessible at: http://www.fs.fed.us/foresthealth/technology/nidrm2012.shtml. Basal area is generally
described as the area of ground covered by trees.
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With regard to agriculture-related effects, the EPA has recognized
other complexities, stating that the degree to which O3
impacts on vegetation that could occur in areas and on species that are
already heavily managed to obtain a particular output (such as
commodity crops or commercial timber production) would impair the
intended use at a level that might be judged adverse to the public
welfare has been less clear (73 FR 16497, March 27, 2008). As noted in
section IV.B.2 of the proposal, while having sufficient crop yields is
of high public welfare value, important commodity crops are typically
heavily managed to produce optimum yields. Moreover, based on the
economic theory of supply and demand, increases in crop yields would be
expected to result in lower prices for affected crops and their
associated goods, which would primarily benefit consumers. These
competing impacts on producers and consumers complicate consideration
of these effects in terms of potential adversity to the public welfare
(U.S. EPA, 2014c, sections 5.3.2 and 5.7). When agricultural impacts or
vegetation effects in other areas are contrasted with the emphasis on
forest ecosystem effects in Class I and similarly protected areas, it
can be seen that the Administrator has in past reviews judged the
significance to the public welfare of O3-induced effects on
sensitive vegetation growing within the U.S. to differ depending on the
nature of the effect, the intended use of the sensitive plants or
ecosystems, and the types of environments in which the sensitive
vegetation and ecosystems are located, with greater significance
ascribed to areas identified for specific uses and benefits to the
public welfare, such as Class I areas, than to areas for which such
uses have not been established (FR 73 16496-16497, March 27, 2008).
In summary, several considerations are recognized as important to
judgments on the public welfare significance of the array of effects of
different O3 exposure conditions on vegetation. While there
are complexities associated with the consideration of the magnitude of
key vegetation effects that might be concluded to be adverse to
ecosystems and associated services, there are numerous locations where
O3-sensitive tree species are present that may be vulnerable
to impacts from O3 on tree growth, productivity and carbon
storage and their associated ecosystems and services. Cumulative
exposures that may elicit effects and the significance of the effects
in specific situations can vary due to differences in exposed species
sensitivity, the importance of the observed or predicted O3-
induced effect, the role that the species plays in the ecosystem, the
intended use of the affected species and its associated ecosystem and
services, the presence of other co-occurring predisposing or mitigating
factors, and associated uncertainties and limitations. These factors
contribute to the complexity of the Administrator's judgments regarding
the adversity of known and anticipated effects to the public welfare.
B. Need for Revision of the Secondary Standard
The initial issue to be addressed in this review of the secondary
standard for O3 is whether, in view of the currently
available scientific evidence, exposure and risk information and air
quality analyses, as reflected in the record, the standard should be
retained or revised. In drawing conclusions on adequacy of the current
O3 secondary standard, the Administrator has taken into
account both evidence-based and quantitative exposure- and risk-based
considerations, as well as advice from CASAC and public comment.
Evidence-based considerations draw upon the EPA's assessment and
integrated synthesis of the scientific evidence from experimental and
field studies evaluating welfare effects related to O3
exposure, with a focus on policy-relevant considerations, as discussed
in the PA. Air quality analyses inform these considerations with regard
to cumulative, seasonal exposures occurring in areas of the U.S. that
meet the current standard. Exposure- and risk-based considerations draw
upon the EPA assessments of risk of key welfare effects, including
O3 effects on forest growth, productivity, carbon storage,
crop yield and visible foliar injury, expected to occur in model-based
scenarios for the current standard, with appropriate consideration of
associated uncertainties.
In evaluating whether it is appropriate to revise the current
standard, the Administrator's considerations build on the general
approach used in the last review, as summarized in section IV.A of the
proposal, and reflect the body of evidence and information available
during this review. The approach used is based on an integration of the
information on vegetation effects associated with exposure to
O3 in ambient air, as well as policy judgments on the
adversity of such effects to public welfare and on when the standard is
requisite to protect public welfare from known or anticipated adverse
effects. Such judgments are informed by air quality and related
analyses, quantitative assessments, when available, and qualitative
assessment of impacts that could not be quantified. The Administrator
has taken into
[[Page 65380]]
account both evidence of effects on vegetation and ecosystems and
public uses of these entities that may be important to the public
welfare. The decision on adequacy of the protection provided by the
current standard has also considered the 2013 remand of the secondary
standard by the D.C. Circuit such that this decision incorporates the
EPA's response to this remand.
Section IV.B.1 below summarizes the basis for the proposed decision
by the Administrator that the current secondary standard should be
revised. Significant comments received from the public on the proposal
are discussed in section IV.B.2 and the Administrator's final decision
is described in section IV.B.3.
1. Basis for Proposed Decision
In evaluating whether it was appropriate to propose to retain or
revise the current standard, as discussed in section IV.D of the
proposal, the Administrator carefully considered the assessment of the
current evidence in the ISA, findings of the WREA, including associated
limitations and uncertainties, considerations and staff conclusions and
associated rationales presented in the PA, views expressed by CASAC,
and public comments that had been offered up to that point. In the
paragraphs below, we summarize the proposal presentation of the PA
considerations with regard to adequacy of the current secondary
standard, advice from the CASAC, and the Administrator's proposed
conclusions, drawing from section IV.D of the proposal, where a fuller
discussion is presented.
a. Considerations and Conclusions in the PA
The PA evaluation is based on the longstanding evidence for
O3 effects and the associated conclusions in the current
review of causal and likely causal relationships between O3
in ambient air and an array of welfare effects at a range of biological
and ecological scales of organization, as summarized in section IV.A.1
above (and described in detail in the ISA). Drawing from the ISA and
CASAC advice, the PA emphasizes the strong support in the evidence for
the conclusion that effects on vegetation are attributable to
cumulative seasonal O3 exposures, taking note of the
improved ``explanatory power'' (for effects on vegetation) of the W126
index over other exposure metrics, as summarized in section IV.A.1.c
above. The PA further recognizes the strong basis in the evidence for
the conclusion that it is appropriate to use a cumulative seasonal
exposure metric, such as the W126 index, to judge impacts of
O3 on vegetation; related effects on ecosystems and
services, such as carbon storage; and the level of public welfare
protection achieved for such effects (U.S. EPA, 2014c, p. 5-78). As a
result, based on the strong support in the evidence and advice from
CASAC in the current and past reviews, the PA concludes that the most
appropriate and biologically relevant way to relate O3
exposure to plant growth, and to determine what would be adequate
protection for public welfare effects attributable to the presence of
O3 in ambient air, is to characterize exposures in terms of
a cumulative seasonal form, and in particular the W126 metric (U.S.
EPA, 2014c, pp. 5-7 and 5-78). Accordingly, in considering the evidence
with regard to level of protection provided by the current secondary
standard, the PA considers air quality data and exposure-response
relationships for vegetation effects, particularly those related to
forest tree growth, productivity and carbon storage, in terms of the
W126 index (U.S. EPA, 2014c, section 5.2; 79 FR 75330-75333, December
17, 2014).
In considering the extent to which such growth-related effects
might be expected to occur under conditions that meet the current
secondary standard, the PA focused particularly on tree seedling RBL
estimates for the 11 species for which robust E-R functions have been
developed, noting the CASAC concurrence with use of O3-
related tree biomass loss as a surrogate for related effects extending
to the ecosystem scale (U.S. EPA, 2014c, p. 5-80, Frey, 2014c, p. 10).
The PA evaluation relied on RBL estimates for these 11 species derived
using the robust OTC-based E-R functions, noting that analyses newly
performed in this review have reduced the uncertainty associated with
using OTC E-R functions to predict tree growth effects in the field
(U.S. EPA, 2014c, section 5.2.1; U.S. EPA, 2013, section 9.6.3.2).
In considering the RBL estimates for different O3
conditions associated with the current standard, the PA focused
primarily on the median of the species-specific (composite) E-R
functions. In so doing, in the context of considering the adequacy of
protection afforded by the current standard, the PA takes note of
CASAC's view regarding a 6% median RBL (Frey, 2014c, p. 12). Based on
the summary of RBL estimates in the PA, the PA notes that the median
species RBL estimate, across the 11 estimates derived from the robust
species-specific E-R functions, is at or above 6% for W126 index values
of 19 ppm-hrs and higher (U.S. EPA, 2014c, Tables 6-1 and 5C-3).
In recognition of the potential significance to public welfare of
vegetation effects in Class I areas, the proposal described in detail
findings of the PA analysis of the occurrence of O3
concentrations associated with the potential for RBL estimates above
benchmarks of interest in Class I areas that meet the current standard,
focusing on 22 Class I areas for which air quality data indicated the
current standard was met and cumulative seasonal exposures, in terms of
a 3-year average W126 index, were at or above 15 ppm-hrs (79 FR 75331-
75332, Table 7, December 17, 2014; U.S. EPA, 2014c, Table 5-2). The PA
noted that W126 index values (both annual and 3-year average values) in
many such areas, distributed across multiple states and NOAA climatic
regions, were above 19 ppm-hrs. The highest 3-year average value was
over 22 ppm-hrs and the highest annual value was over 27 ppm-hrs,
exposure values for which the corresponding median species RBL
estimates markedly exceed 6%, which CASAC has termed ``unacceptably
high'' (U.S. EPA, 2014c, section 5.2). The PA additionally considered
the species-specific RBL estimates for two tree species (quaking aspen
and ponderosa pine) that are found in many of these Class I areas and
that have a sensitivity to O3 exposure that places them
slightly more sensitive than the median of the group for which robust
E-R functions have been established (U.S. EPA, 2014c, sections 5.2 and
5.7). As further summarized in the proposal, the PA describes the
results of this analysis, particularly in light of advice from CASAC
regarding the significance of the 6% RBL benchmark, as evidence of the
occurrence in Class I areas, during periods when the current standard
is met, of cumulative seasonal O3 exposures of a magnitude
for which the tree growth impacts indicated by the associated RBL
estimates might reasonably be concluded to be important to public
welfare (79 FR 75332; U.S. EPA, 2014c, sections 5.2.1 and 5.7).
The proposal also noted that the PA additionally considered
findings of the WREA analyses of O3 effects on tree growth
and an array of ecosystem services provided by forests, including
timber production, carbon storage and air pollution removal (79 FR
75332-75333; U.S. EPA, 2014b, sections 6.2-6.8; U.S. EPA, 2014c,
section 5.2). While recognizing that these analyses provide
quantitative estimates of impacts on tree growth and associated
services for several different air quality scenarios,
[[Page 65381]]
the PA takes note of the large uncertainties associated with these
analyses (see U.S. EPA, 2014b, Table 6-27) and the potential for these
findings to underestimate the response at the national scale. While
noting the potential usefulness of considering predicted and
anticipated impacts to these services in assessing the extent to which
the current information supports or calls into question the adequacy of
the protection afforded by the current standard, the PA also recognizes
significant uncertainties associated with the absolute magnitude of the
estimates for these ecosystem service endpoints which limited the
weight staff placed on these results (U.S. EPA, 2014c, sections 5.2 and
5.7).
As described in the proposal, the PA also considered O3
effects on crops, taking note of the extensive and long-standing
evidence of the detrimental effect of O3 on crop production,
which continues to be confirmed by evidence newly available in this
review (79 FR 75333; U.S. 2014c, sections 5.3 and 5.7). With regard to
consideration of the quantitative impacts of O3 exposures
under exposure conditions associated with the current standard, the PA
focused on RYL estimates that had strong support in the current
evidence (as characterized in the ISA, section 9.6) in light of CASAC
comments regarding RYL benchmarks (Frey, 2014c, pp. iii and 14). In
considering such evidence-based analyses, as well as the exposure/risk-
based information for crops, the PA notes the CASAC comments regarding
the use of crop yields as a surrogate for consideration of public
welfare impacts, which noted that ``[c]rops provide food and fiber
services to humans'' and that ``[e]valuation of market-based welfare
effects of O3 exposure in forestry and agricultural sectors
is an appropriate approach to take into account damage that is adverse
to public welfare'' (Frey, 2014c, p. 10; U.S. EPA, 2014c, section 5.7).
The PA additionally notes, however, as recognized in section IV.A.3
above that the determination of the point at which O3-
induced crop yield loss becomes adverse to the public welfare is still
unclear, given that crops are heavily managed (e.g., with fertilizer,
irrigation) for optimum yields, have their own associated markets and
that benefits can be unevenly distributed between producers and
consumers (79 FR 75322; U.S. EPA, 2014c, sections 5.3 and 5.7).
With regard to visible foliar injury, as summarized in the
proposal, the PA recognizes the long-standing evidence that has
established that O3 causes diagnostic visible foliar injury
symptoms on studied bioindicator species and also recognizes that such
O3-induced impacts have the potential to impact the public
welfare in scenic and/or recreational areas, with visible foliar injury
associated with important cultural and recreational ecosystem services
to the public, such as scenic viewing, wildlife watching, hiking, and
camping, that are of significance to the public welfare and enjoyed by
millions of Americans every year, generating millions of dollars in
economic value (U.S. EPA, 2014b, section 7.1). In addition, several
tribes have indicated that many of the O3-sensitive species
(including bioindicator species) are culturally significant (U.S. EPA,
2014c, Table 5-1). Similarly, the PA notes CASAC comments that
``visible foliar injury can impact public welfare by damaging or
impairing the intended use or service of a resource,'' including
through ``visible damage to ornamental or leafy crops that affects
their economic value, yield, or usability; visible damage to plants
with special cultural significance; and visible damage to species
occurring in natural settings valued for scenic beauty or recreational
appeal'' (Frey, 2014c, p. 10). Given the above, and taking note of
CASAC views, the PA recognizes visible foliar injury as an important
O3 effect which, depending on severity and spatial extent,
may reasonably be concluded to be of public welfare significance,
especially when occurring in nationally protected areas, such as
national parks and other Class I areas.
As summarized in the proposal, the PA additionally takes note of
the evidence described in the ISA regarding the role of soil moisture
conditions that can decrease the incidence and severity of visible
foliar injury under dry conditions (U.S. EPA, 2014c, sections 5.4 and
5.7). As recognized in the PA, this area of uncertainty complicates
characterization of the potential for visible foliar injury and its
severity or extent of occurrence for given air quality conditions and
thus complicates identification of air quality conditions that might be
expected to provide a specific level of protection from this effect
(U.S. EPA, 2014c, sections 5.4 and 5.7). While noting the uncertainties
associated with describing the potential for visible foliar injury and
its severity or extent of occurrence for any given air quality
conditions, the PA notes the occurrence of O3-induced
visible foliar injury in areas, including federally protected Class I
areas that meet the current standard, and suggests it may be
appropriate to consider revising the standard for greater protection.
In so doing, however, the PA recognizes that the degree to which
O3-induced visible foliar injury would be judged important
and potentially adverse to public welfare is uncertain (U.S. EPA,
2014c, section 5.7).
As noted in the proposal, with regard to other welfare effects, for
which the ISA determined a causal or likely causal relationships with
O3 in ambient air, such as alteration of ecosystem water
cycling and changes in climate, the PA concludes there are limitations
in the available information that affect our ability to consider
potential impacts of air quality conditions associated with the current
standard.
Based on the considerations described in the PA, summarized in the
proposal and outlined here, the PA concludes that the currently
available evidence and exposure/risk information call into question the
adequacy of the public welfare protection provided by the current
standard and provide support for considering potential alternative
standards to provide increased public welfare protection, especially
for sensitive vegetation and ecosystems in federally protected Class I
and similarly protected areas. In this conclusion, staff gives
particular weight to the evidence indicating the occurrence in Class I
areas that meet the current standard of cumulative seasonal
O3 exposures associated with estimates of tree growth
impacts of a magnitude that may reasonably be considered important to
public welfare.
b. CASAC Advice
The proposal also summarized advice offered by the CASAC in the
current review, based on the updated scientific and technical record
since the 2008 rulemaking. The CASAC stated that it ``[supports] the
conclusion in the Second Draft PA that the current secondary standard
is not adequate to protect against current and anticipated welfare
effects of ozone on vegetation'' (Frey, 2014c, p. iii) and that the PA
``clearly demonstrates that ozone-induced injury may occur in areas
that meet the current standard'' (Frey, 2014c, p. 12). The CASAC
further stated ``[w]e support the EPA's continued emphasis on Class I
and other protected areas'' (Frey, 2014c, p. 9). Additionally, the
CASAC indicated support for the concept of ecosystem services ``as part
of the scope of characterizing damage that is adverse to public
welfare'' and ``concur[red] that trees are important from a public
welfare perspective because they provide valued services to humans,
including aesthetic value, food, fiber, timber, other forest products,
habitat, recreational opportunities, climate regulation, erosion
control, air
[[Page 65382]]
pollution removal, and hydrologic and fire regime stabilization''
(Frey, 2014c, p. 9). Similar to comments from CASAC in the last review,
and comments on the proposed reconsideration, the current CASAC also
endorsed the PA discussions and conclusions on biologically relevant
exposure metrics and the focus on the W126 index accumulated over a 12-
hour period (8 a.m.-8 p.m.) over the 3-month summation period of a year
resulting in the maximum value (Frey, 2014c, p. iii).
In addition, CASAC stated that ``relative biomass loss for tree
species, crop yield loss, and visible foliar injury are appropriate
surrogates for a wide range of damage that is adverse to public
welfare,'' listing an array of related ecosystem services (Frey, 2014c,
p. 10). With respect to RBL for tree species, CASAC states that it is
appropriate to identify in the PA ``a range of levels of alternative
W126-based standards that include levels that aim for not greater than
2% RBL for the median tree species'' and that a median tree species RBL
of 6% is ``unacceptably high'' (Frey, 2014c, pp. 13 and 14). With
respect to crop yield loss, CASAC points to a benchmark of 5%, stating
that a crop RYL for median species over 5% is ``unacceptably high'' and
described crop yield as a surrogate for related services (Frey, 2014c,
p. 13).
c. Administrator's Proposed Conclusions
At the time of proposal, the Administrator took into account the
information available in the current review with regard to the nature
of O3-related effects on vegetation and the adequacy of
protection provided by the current secondary standard. The
Administrator recognized the appropriateness and usefulness of the W126
metric in evaluating O3 exposures of potential concern for
vegetation effects, additionally noting support conveyed by CASAC for
such a use for this metric. Further, the Administrator took particular
note of (1) the PA analysis of the magnitude of tree seedling growth
effects (biomass loss) estimated for different cumulative, seasonal,
concentration-weighted exposures in terms of the W126 metric; (2) the
monitoring analysis in the PA of cumulative exposures (in terms of W126
index) occurring in locations where the current standard is met,
including those locations in or near Class I areas, and associated
estimates of tree seedling growth effects; and (3) the analyses in the
WREA illustrating the geographic distribution of tree species for which
E-R functions are available and estimates of O3-related
growth impacts for different air quality scenarios, taking into account
the identified potential for the WREA's existing standard scenario to
underestimate the highest W126-based O3 values that would be
expected to occur.
With regard to considering the adequacy of public welfare
protection provided by the current secondary standard at the time of
proposal, the Administrator focused first on welfare effects related to
reduced native plant growth and productivity in terrestrial systems,
taking note of the following: (a) The ISA conclusion of a causal
relationship between O3 in the ambient air and these welfare
effects, and supporting evidence related to O3 effects on
vegetation growth and productivity, including the evidence from OTC
studies of tree seedling growth that support robust E-R functions for
11 species; (b) the evidence, described in section IV.D.1 of the
proposal and summarized above, of the occurrence of cumulative seasonal
O3 exposures for which median species RBL estimates are of a
magnitude that CASAC has termed ``unacceptably high'' in Class I areas
during periods where the current standard is met; (c) actions taken by
Congress to establish public lands that are set aside for specific uses
intended to provide benefits to the public welfare, including lands
that are to be protected so as to conserve the scenic value and the
natural vegetation and wildlife within such areas for the enjoyment of
future generations, such as national parks and forests, wildlife
refuges, and wilderness areas (many of which have been designated Class
I areas); and (d) PA conclusions that the current information calls
into question the adequacy of the current standard, based particularly
on impacts on tree growth (and the potential for associated ecosystem
effects), estimated for Class I area conditions meeting the current
standard, that are reasonably concluded to be important from a public
welfare standpoint in terms of both the magnitude of the vegetation
effects and the significance to public welfare of such effects in such
areas.
At the time of proposal, the Administrator also recognized the
causal relationships between O3 in the ambient air and
visible foliar injury, reduced yield and quality of agricultural crops,
and alteration of below-ground biogeochemical cycles associated with
effects on growth and productivity. As to visible foliar injury, she
took note of the complexities and limitations in the evidence base
regarding characterizing air quality conditions with respect to the
magnitude and extent of risk for visible foliar injury, and she
additionally recognized the challenges of associated judgments with
regard to adversity of such effects to public welfare. In taking note
of the conclusions with regard to crops, she recognized the complexity
of considering adverse O3 impacts to public welfare due to
the heavy management common for achieving optimum yields and market
factors that influence associated services and additionally took note
of the PA conclusions that placing emphasis on the protection afforded
to trees inherently also recognizes a level of protection afforded for
crops.
Based on her consideration of the conclusions in the PA, and with
particular weight given to PA findings pertaining to tree growth-
related effects, as well as with consideration of CASAC's conclusion
that the current standard is not adequate, the Administrator proposed
to conclude that the current standard is not requisite to protect
public welfare from known or anticipated adverse effects and that
revision is needed to provide the requisite public welfare protection,
especially for sensitive vegetation and ecosystems in federally
protected Class I areas and in other areas providing similar public
welfare benefits. The Administrator further concluded that the
scientific evidence and quantitative analyses on tree growth-related
effects provide strong support for consideration of alternative
standards that would provide increased public welfare protection beyond
that afforded by the current O3 secondary standard. She
further noted that a revised standard would provide increased
protection for other growth-related effects, including for carbon
storage and for areas for which it is more difficult to determine
public welfare significance, as recognized in section IV.B.2 of the
proposal, as well as other welfare effects of O3, including
visible foliar injury and crop yield loss.
2. Comments on the Need for Revision
In considering comments on the need for revision, we first note the
advice and recommendations from CASAC with regard to the adequacy of
the current standard. In its review of the second draft PA, CASAC
stated that it ``supports the scientific conclusion in the Second Draft
PA that the current secondary standard is not adequate to protect
against current and anticipated welfare effects of ozone on
vegetation'' (Frey, 2014c).
General comments received from the public on the proposal that are
based on relevant factors and either supported or opposed the proposed
decision to revise
[[Page 65383]]
the current O3 secondary standard are addressed in this
section. Comments on specific issues or information that relate to
consideration of the appropriate elements of a revised secondary
standard are addressed below in section IV.C. Other specific comments
related to standard setting, as well as general comments based on
implementation-related factors that are not a permissible basis for
considering the need to revise the current standard, are addressed in
the Response to Comments document.
Public comments on the proposal were divided with regard to support
for the Administrator's proposed decision to revise the current
secondary standard. Many state and local environmental agencies or
government bodies, tribal agencies and organizations, and environmental
organizations agreed with the EPA's proposed conclusion on the need to
revise the current standard, stating that the available scientific
information shows that O3-induced vegetation and ecosystem
effects are occurring under air quality conditions allowed by the
current standard and, therefore, provides a strong basis and support
for the conclusion that the current secondary standard is not adequate.
In support of their view, these commenters relied on the entire body of
evidence available for consideration in this review, including evidence
assessed previously in the 2008 review. These commenters variously
pointed to the information and analyses in the PA and the conclusions
and recommendations of CASAC as providing a clear basis for concluding
that the current standard does not provide adequate protection of
public welfare from O3-related effects. Many of these
commenters generally noted their agreement with the rationale provided
in the proposal with regard to the Administrator's proposed conclusion
on adequacy of the current standard, and some gave additional emphasis
to several aspects of that rationale, including the appropriateness of
the EPA's attention to sensitive vegetation and ecosystems in Class I
areas and other public lands that provide similar public welfare
benefits and of the EPA's reliance on the strong evidence of impacts to
tree growth and growth-related effects.
Comments from tribal organizations additionally noted that many
Class I areas are of sacred value to tribes or provide treaty-protected
benefits to tribes, including the exercise of gathering rights. Tribal
organizations also noted the presence in Class I areas of large numbers
of culturally important plant species, which they indicate to be
impacted by air quality conditions allowed by the current standard. The
impacts described include visible foliar injury, loss in forest growth
and crop yield loss, which these groups describe as especially
concerning when occurring on lands set aside for the benefit of the
public or that are of sacred value to tribes or provide treaty-
protected benefits to tribes.
As described in section IV.B.3 below, the EPA generally agrees with
the view of these commenters regarding the need for revision of the
current secondary standard and with CASAC that the evidence provides
support for the conclusions that the current secondary standard is not
adequate to protect public welfare from known or anticipated adverse
effects, particularly with respect to effects on vegetation.
A number of industries, industry associations, or industry
consultants, as well as some state governors, attorneys general and
environmental agencies, disagreed with the EPA's proposed conclusion on
the adequacy of the current standard and recommended against revision.
In support of their position, these commenters variously stated that
the available evidence is little changed from that available at the
time of the 2008 decision, and that the evidence is too uncertain,
including with regard to growth-related effects and visible foliar
injury, to support revision, and does not demonstrate adverse effects
to public welfare for conditions associated with the current standard,
with some commenters stating particularly that the EPA analysis of
Class I areas did not document adverse effects to public welfare. They
also cited the WREA modeling analyses as indicating that any welfare
improvements associated with a revised standard would be marginal; in
particular, compared to the benefits of achieving the current standard.
Further, they state that, because of long-range transport of
O3 and precursors, it is not appropriate for the EPA to draw
conclusions about the level of protection offered by the current
standard based on current air quality conditions; in support of this
view, these commenters point to different modeling analyses as
demonstrating that under conditions where the current standard is met
throughout the U.S., the associated W126 values would all be below the
upper end of the range proposed as providing requisite public welfare
protection and nearly all below the lower end of 13 ppm-hrs.
As an initial matter, we note that, as noted in sections I.C and
IV.A above, the EPA's 2008 decision on the secondary standard was
remanded back to the Agency because in setting the 2008 secondary
standard, the EPA failed to specify what level of air quality was
requisite to protect public welfare from known or anticipated adverse
effects or explain why any such level would be requisite. So, in
addressing the court remand, the EPA has more explicitly considered the
extent to which protection is provided from known or anticipated
effects that the Administrator may judge to be adverse to public
welfare, and has described how the air quality associated with the
revised standard would provide requisite public welfare protection,
consistent with CAA section 109(b)(2) and the court's decision
remanding the 2008 secondary standard. In undertaking this review,
consistent with the direction of the CAA, the EPA has considered the
current air quality criteria.
While we recognize, as stated in the proposal, that the evidence
newly available in this review is largely consistent with the evidence
available at the time of the last review (completed in 2008) with
regard to the welfare effects of O3, we disagree with the
commenters' interpretations of the evidence and analyses available in
this review and with their views on the associated uncertainties. As
summarized in section IV.A above, the ISA has determined causal
relationships to exist between several vegetation and ecosystem
endpoints and O3 in ambient air (U.S. 2013, section 9.7).
The ISA characterized the newly available evidence as largely
consistent with and supportive of prior conclusions, as summarized in
section IV.A above. This is not to say, however, that there is no newly
available evidence and information in this review or that it is
identical to that available in the last review. In some respects, the
newly available evidence has strengthened the evidence available in the
last review and reduced important uncertainties. As summarized in
section IV.A.1.b above, newly available field studies confirm the
cumulative effects and effects on forest community composition over
multiple seasons. Additionally, among the newly available evidence for
this review are analyses documented in the ISA that evaluate the RBL
and RYL E-R functions for aspen and soybean, respectively, with
experimental datasets that were not used in the derivation of the
functions (U.S. 2013, section 9.6.3). These evaluations confirm the
pertinence of the tree seedling RBL estimates for aspen, a species with
sensitivity roughly midway in the range of sensitivities for the
studied species, across multiple years in older trees.
[[Page 65384]]
With regard to crops, the ISA evaluations demonstrate a robustness of
the E-R functions to predict O3-attributable RYL and confirm
the relevance of the crop RYL estimates for more recent cultivars
currently growing in the field. Together, the information newly
available in this review confirms the basis for the E-R functions and
strengthens our confidence in interpretations drawn from their use in
other analyses newly available in this review that have been described
in the WREA and PA.
With regard to comments on uncertainties associated with estimates
of RBL, we first note that these established, robust E-R functions,
which the EPA gave particular emphasis in this review, are available
for seedling growth for 11 tree species native to the U.S., as
summarized in section IV.A.1.b above and described in the proposal.
These E-R functions are based on studies of multiple genotypes of 11
tree species grown for up to three years in multiple locations across
the U.S. (U.S. EPA, 2013, section 9.6.1). We have recognized the
uncertainty regarding the extent to which the studied species encompass
the O3 sensitive species in the U.S. and also the extent to
which they represent U.S. vegetation as a whole (U.S. EPA, 2014b,
section 6.9). However, the studied species include both deciduous and
coniferous trees with a wide range of sensitivities and species native
to every region across the U.S. and in most cases are resident across
multiple states and NOAA climatic regions (U.S. EPA, 2014b, Appendix
6A). While the CASAC stated that there is ``considerable uncertainty in
extrapolating from the [studied] forest tree species to all forest tree
species in the U.S.,'' it additionally expressed the view that it
should be anticipated that there are highly sensitive vegetation
species for which we do not have E-R functions and others that are
insensitive.\187\ In so doing, the CASAC stated that it ``should not be
assumed that species of unknown sensitivity are tolerant to ozone'' and
``[i]t is more appropriate to assume that the sensitivity of species
without E-R functions might be similar to the range of sensitivity for
those species with E-R functions'' (Frey, 2014c, p. 11). Accordingly,
we disagree with commenters' view that effects on these species are not
appropriate considerations for evaluation of the adequacy of the
current standard.
---------------------------------------------------------------------------
\187\ Use of RBL estimates in the proposal, and in this final
decision, focuses on the RBL for the studied species as a surrogate
for a broad array of growth-related effects of potential public
welfare significance, consistent with the CASAC advice.
---------------------------------------------------------------------------
In support of their view that RBL estimates are too uncertain to
inform a conclusion that the current standard is not adequately
protective of public welfare, some commenters state that some of the 11
E-R functions are based on as few as one study. The EPA agrees that
there are two species for which there is only one study supporting the
E-R function (Virginia pine and red maple). We also note, however, that
those two species are appreciably less sensitive than the median (Lee
and Hogsett, 1996; U.S. EPA, 2014c, Table 5C-1). Thus, in the relevant
analyses, they tend to influence the median toward a relatively less
(rather than more) sensitive response. Further, there are four species
for which the E-R functions are based on more than five studies,\188\
contrary to the commenters' claims of there being no functions
supported by that many studies. That said, the EPA has noted the
relatively greater uncertainty in the species for which fewer studies
are available, and it is in consideration of such uncertainties that
the EPA focused in the proposal on the median E-R function across the
11 species, rather than a function for a species much more (or less)
sensitive than the median. The EPA additionally notes that it gave less
emphasis to the E-R function available for one species, eastern
cottonwood, based on CASAC advice that the study results supporting
that E-R function were not as strong as the results of the other
experiments that support the other, robust E-R functions and that the
eastern cottonwood study results showed extreme sensitivity to
O3 compared to other studies (Frey, 2014c, p. 10).
Accordingly, the EPA has appropriately considered the strength of the
scientific evidence and the associated uncertainties in considering
revision of the secondary standard.
---------------------------------------------------------------------------
\188\ These four species, aspen, Douglas fir, ponderosa pine and
red alder, range broadly in sensitivities that fall above, below and
at the median for the 11 species (Lee and Hogsett, 1996; U.S. EPA,
2014c, Table 5C-1).
---------------------------------------------------------------------------
Other commenters stated that the scientific evidence does not
support revising the NAAQS, pointing to uncertainty related to
interpretation of the RBL estimates (based on tree seedling studies)
with regard to effects on older tree lifestages. Some of these
commenters' claim that mature canopy trees experience reduced
O3 effects. The EPA agrees that the quantitative information
for O3 growth effects on older tree lifestages is available
for a more limited set of species than that available for tree
seedlings. We note, however, that this is an area for which there is
information newly available in this review. A detailed analysis of
study data for seedlings and older lifestages of aspen shows close
agreement between the O3-attributable reduced growth
observed in the older trees and reductions predicted from the seedling
E-R function (U.S. EPA, 2013, section 9.6.3.2; discussed in the PA,
section 5.2.1 as noted in the proposal, p. 75330). This finding, newly
available in this review and documenting impacts on mature trees,
improves our confidence in conclusions drawn with regard to the
significance of RBL estimates for this species, which is prevalent
across multiple regions of the U.S.\189\ It is also noteworthy that
this species is generally more sensitive to O3 effects on
growth than the median of the 11 species with robust E-R functions (as
shown in U.S. EPA 2014c, Table 5C-1). Other newly available studies,
summarized in section IV.A.1.b above and section IV.B.1.b of the
proposal, provide additional evidence of O3 impacts on
mature trees, including a meta-analysis reporting older trees to be
more affected by O3 than younger trees (U.S. EPA, 2013, p.
9-42; Wittig et al., 2007). We additionally note that CASAC
``concur[red] that biomass loss in trees is a relevant surrogate for
damage to tree growth that affects ecosystem services such as habitat
provision for wildlife, carbon storage, provision of food and fiber,
and pollution removal'' additionally stating that ``[b]iomass loss may
also have indirect process-related effects such as on nutrient and
hydrologic cycles'' leading them to conclude that ``[t]herefore,
biomass loss is a scientifically valid surrogate of a variety of
adverse effects to public welfare'' (Frey, 2014c, p. 10).
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\189\ The WREA notes a few additional, limited analyses using
modeling tools and data from previous publications that indicate
there may be species-specific differences in the extent of
similarities between seedling and adult growth response to
O3, with some species showing greater and some lesser
response for seedlings as compared to mature tree, but a general
comparability (U.S. EPA 2014b, section 6.2.1.1 and p. 6-67).
---------------------------------------------------------------------------
As noted in section IV.A above and discussed below, the
Administrator's final decision on the adequacy of the current standard
draws upon, among other things, the available evidence and quantitative
analyses as well as judgments about the appropriate weight to place on
the range of uncertainties inherent in the evidence and analyses. The
strengthening in this review, as compared with the last review, of the
basis for the robust E-R functions for tree seedling RBL, as well as
other newly available quantitative analyses,
[[Page 65385]]
will, accordingly, contribute to judgments made by the Administrator
with regard to these effects in reaching her final decisions in this
review.
Amongst the newly available information in this review is a new
analysis describing W126-based exposures occurring in counties
containing Class I areas for which monitoring data indicated compliance
with the current standard. The PA gave particular attention to this
analysis in consideration of the adequacy of the current standard, and
this analysis was also described in the proposal (U.S. EPA, 2014c,
Appendix 5B and pp. 5-27 to 5-29; 79 FR 75331-75332, December 17,
2014). Some of the commenters who disagreed with the EPA's conclusion
on adequacy of the current standard variously stated that this analysis
does not demonstrate growth effects are occurring in Class I areas and
that the analysis is too uncertain for reliance on by the Administrator
in her judgment on adequacy of the current standard. While the EPA
agrees with commenters that data on the occurrence of growth effects in
the areas and time periods identified are not part of this analysis, we
note that this is because such data have not been collected and
consequently cannot be included. As a result, the EPA has utilized
measurements of O3 in or near these areas in combination
with the established E-R functions to estimate the potential for growth
impacts in these areas under conditions where the current standard is
met. The EPA additionally notes that species for which E-R functions
have been developed have been documented to occur within these areas
(see Table 3).
The EPA disagrees with commenters regarding the appropriateness of
this analysis for the Administrator's consideration. This analysis
documents the occurrence of cumulative growing season exposures in
these ecosystems which the EPA and CASAC have interpreted, through the
use of the established E-R functions for tree seedling growth effects
summarized in section IV.A.1.b above (and described in the ISA, PA and
proposal), as indicating the potential for growth effects of
significance in these protected areas. To the extent that these
comments imply that the Administrator may only consider welfare effects
that are certain in judging the adequacy of the current standard, we
note that section 109(b)(2) of the CAA plainly provides for
consideration of both known and anticipated adverse effects in
establishing or revising secondary NAAQS.
In support of some commenters' view that this analysis is too
uncertain to provide a basis for the Administrator's proposed
conclusion that the current standard is not adequate, one commenter
observed that the O3 monitors used for six of the 22 Class I
areas in the analysis, although in the same county, were sited outside
of the Class I areas. This was the case due to the analysis being
focused on the highest monitor in the county that met the current
standard. To clarify the presentation, however, we have refocused the
presentation, restricting it to data for monitors sited in or within 15
kilometers of a Class I area,\190\ and note that the results are little
changed, continuing to call into question the adequacy of the current
standard. As shown in Table 3, the dataset in the refocused
presentation, which now spans 1998 up through 2013, includes 17 Class I
areas for which monitors were identified in this manner. For context,
we note that this represents nearly a quarter of the Class I areas for
which there are O3 monitors within 15 km.\191\
---------------------------------------------------------------------------
\190\ The 15 km distance was selected as a natural breakpoint in
distance of O3 monitoring sites from Class I areas and as
still providing similar surroundings to those occurring in the Class
I area. We note that given the strict restrictions on structures and
access within some of these areas, it is common for monitors
intended to collect data pertaining to air quality in these types of
areas to be sited outside their boundaries.
\191\ There is an O3 monitor within fewer than 15% of
all Class I areas, and fewer than half of all Class I areas have a
monitor within 15 km.
---------------------------------------------------------------------------
In recognition of the influence that other environmental factors
can exert in the natural environment on the relationship between
ambient O3 exposures and RBL, potentially modifying the
impact predicted by the E-R functions, the PA and proposal took
particular note of the occurrence of 3-year average W126 index values
at or above 19 ppm-hrs. In the re-focused analysis in Table 3, there
are 11 areas, distributed across four states in two NOAA climatic
regions, for which the 3-year W126 exposure index values ranged at or
above 19 ppm-hrs, a value for which the corresponding median species
RBL estimate for a growing season's exposure is 6%, a magnitude termed
``unacceptably high'' by CASAC (Frey, 2014c, p. 13). The highest 3-year
W126 index values in these 11 areas ranged from 19.0 up to 22.2 ppm-
hrs, a cumulative seasonal exposure for which the median species RBL
estimate is 9% for a single growing season. The annual W126 index
values range above 19 ppm-hrs in 15 of the areas in the re-focused
table provided here; these areas are distributed across six states (AZ,
CA, CO, KY, SD, UT) and four regions (West, Southwest, West North
Central and Central).\192\ The highest index values in the areas with
annual index values above 19 ppm-hrs range from 19.1 to 26.9 ppm-hrs.
As is to be expected from the focus on a smaller dataset, the number of
states with 1-year W126 index values above 19 ppm-hrs is smaller in the
refocused analysis (15 as compared to 20), although the number of
regions affected is the same. More importantly, however, the number of
areas with 3-year W126 index values at or above 19 ppm-hrs is the same,
11 Class I areas across two regions, supporting the prior conclusions.
---------------------------------------------------------------------------
\192\ This compares to 20 areas in eight states and four regions
in the earlier analysis.
Table 3--O3 Concentrations for Class I Areas During Period From 1998 to 2013 That Met the Current Standard and Where 3-Year Average W126 Index Value Was
at or Above 15 ppm-hrs
--------------------------------------------------------------------------------------------------------------------------------------------------------
Design 3-Year average W126 (ppm- Number of 3-
Class I area (distance away, if monitor is State/ County value hrs)* (# >= 19 ppm-hrs, Annual W126 (ppm-hrs)* year
not at/within boundaries) (ppb)* range) (# >= 19 ppm-hrs, range) periods
--------------------------------------------------------------------------------------------------------------------------------------------------------
Bridger Wilderness Area \QA, DF\ (8.9 km)... WY/Sublette................... 70-72 16.2-17.0 13.9-18.8 4
Canyonlands National Park \QA, DF, PP\...... UT/San Juan................... 70-73 15.4-19.5 (2, 19.1-19.5) 9.6-23.6 (4, 19.2-23.6) 8
Chiricahua National Monument \DF, PP\ (12 AZ/Cochise.................... 69-73 15.2-19.8 (1, 19.8) 11.7-21.9 (2, 19.8-21.9) 10
km).
Grand Canyon National Park \QA, DF, PP\..... AZ/Coconino................... 68-74 15.3-22.2 (7, 19.1-22.2) 10.1-26.9 (6, 19.8-26.9) 12
Desolation Wilderness \PP\ (3.9 km)......... CA/El Dorado.................. 75 19.8 (1, 19.8) 15.6-22.9 (2, 21.0-22.9) 1
[[Page 65386]]
Lassen Volcanic National Park \DF, PP\...... CA/Shasta..................... 72-74 15.3-15.6 11.5-19.1 (1, 19.1) 2
Mammoth Cave National Park \BC, C, LP, RM, KY/Edmonson................... 74 15.7 12.3-22.0 (1, 22.0) 1
SM, VP, YP\ (0.1 km).
Maroon Bells-Snowmass Wilderness Area \QA, CO/Gunnison................... 68-73 15.6-20.2 (1, 20.2) 13.0-23.8 (3, 21.3-23.8) 8
DF\ (0.8 km).
Mazatzal Wilderness \DF, PP\ (10.9 km)...... AZ/Maricopa................... 74-75 17.8-19.9 (1, 19.9) 10.3-26.2 (3, 19.7-26.2) 2
Mesa Verde National Park \DF\............... CO/Montezuma.................. 67-73 15.4-20.7 (1, 20.7) 10.7-23.4 (4, 19.5-23.4) 11
Petrified Forest National Park \C\.......... AZ/Navajo..................... 70 15.4-16.9 12.7-18.6 2
Rocky Mountain National Park \QA, DF, PP\ CO/Larimer.................... 73-74 15.3-18.4 8.3-26.2 (4, 19.4-26.2) 5
(0.9 km).
Saguaro National Park \DF, PP\ (0.1 km)**... AZ/Pima....................... 69-74 15.4-19.0 (1, 19.0) 7.3-22.9 (3, 19.6-22.9) 6
AZ/Gila....................... 72-75 16.6-20.9 (2, 19.0-20.9) 13.8-25.5 (4, 19.0-25.5) 5
Superstition Wilderness Area \PP\ (6.3, 14.9 AZ/Maricopa................... 70-75 15-20.2 (1, 20.2) 6.3-23.9 (4, 19.6-23.9) 4
km and 7.2 km)**.
AZ/Pinal...................... 72-75 15.3-21.1 (1, 21.1) 10.2-24.7 (4, 21.4-24.7) 7
Weminuche Wilderness Area \QA, DF, PP\ (14.9 CO/La Plata................... 70-74 15.1-19.1 (1, 19.1) 10.8-21.0 (2, 20.8-21.0) 6
km).
Wind Cave National Park \QA, PP\............ SD/Custer..................... 70 15.4 12.3-20.5 (1, 20.5) 1
Zion National Park \QA, DF, PP\ (3.6 km).... UT/Washington................. 70-73 17.0-20.1 (2, 19.4-20.1) 14.2-23.2 (3, 19.8-23.2) 6
--------------------------------------------------------------------------------------------------------------------------------------------------------
* Based on hourly O\3\ concentration data retrieved from AQS on June 25, 2014, and additional CASTNET data downloaded from http://java.epa.gov/castnet/epa_jsp/prepackageddata.jsp on June 25, 2014. Design values shown above are derived in accordance with Appendix P to 40 CFR Part 50. Annual W126 index
values are derived as described in section IV.A.1 above; three consecutive year annual values are averaged for 3-year averages. Prior to presentation,
both types of W126 index values are rounded to one decimal place. The full list of monitoring site identifiers and individual statistics is available
in the docket for this rulemaking.
** No monitor was sited within these Areas and multiple monitors were sited within 15 km. Data for the closest monitor per county are presented.
Superscript letters refer to species present for which E-R functions have been developed. QA=Quaking Aspen, BC=Black Cherry, C=Cottonwood, DF=Douglas
Fir, LP=Loblolly Pine, PP=Ponderosa Pine, RM=Red Maple, SM=Sugar Maple, VP=Virginia Pine, YP=Yellow (Tulip) Poplar. Sources include USDA-NRCS (2014,
http://plants.usda.gov), USDA-FS (2014, http://www.fs.fed.us/foresthealth/technology/nidrm2012.shtml) UM-CFCWI (2014, http://www.wilderness.net/printFactSheet.cfm?WID=583), NPS (http://www.nps.gov/pefo/planyourvisit/upload/Common-Plants-Site-Bulletin-sb-2013.pdf) and Phillips and Comus (2000).
As support for their view that the Class I area analysis is too
uncertain to provide a basis for the Administrator's proposed
conclusion that the current standard is not adequate, some commenters
stated that forests in Class I areas were composed of mature trees and
that the tree seedling E-R functions do not predict growth impacts in
mature forests. The EPA disagrees with the commenters' statement that
Class I areas are only made up of mature trees. Seedlings exist
throughout forests as part of the natural process of replacing aging
trees and overstory trees affected by periodic disturbances.\193\
Seedlings also tend to occur in areas affected by natural disturbances,
such as fires, insect infestations and flooding, and such disturbances
are common in many natural forests. As noted above, information newly
available in this review strengthens our understanding regarding
O3 effects on mature trees for aspen, an important and
O3-sensitive species (U.S. EPA, 2013, section 9.6.3.2).
---------------------------------------------------------------------------
\193\ Basic information on forest processes, including the role
of seedlings is available at: http://www.na.fs.fed.us/stewardship/pubs/NE_forest_regeneration_handbook_revision_130829_desktop.pdf.
---------------------------------------------------------------------------
One commenter additionally stated that the EPA has not shown
reduced biomass to be adverse to public welfare, variously citing
individual studies, most of which are not considering O3, as
support for their view that such an effect of O3 may not
occur in the environment and may be of no significance if it does. With
regard to the occurrence of O3-related reduced growth in the
field, we note the strength of the evidence from field OTC studies on
which the E-R functions are based, and evidence from comparative
studies with open-air chamberless control treatments suggests that
characteristics particular to the OTC did not significantly affect
plant response (U.S. EPA, 2013, p. 9-5). Thus, we view the OTC systems
as combining aspects of controlled exposure systems with field
conditions to facilitate a study providing data that represent the role
of the studied pollutant in a natural system.
Further, we disagree with the commenters on the significance of
O3-attributable reduced growth in natural ecosystems. Even
in the circumstances cited by the commenter (e.g., subsequent to large-
scale disturbances, nutrient limited system, multigeneration exposure),
O3 can affect growth of seedlings and older trees, with the
potential for effects on ecosystem productivity, handicapping the
sensitive species and affecting community dynamics and associated
community composition, as well as ecosystem hydrologic cycles (U.S.
EPA, 2013, p. 1-8). For example, two recent studies report on the role
of O3 exposure in affecting water use in a mixed deciduous
forest and indicated that O3 increased water use in the
forest and also reduced growth rate (U.S. EPA, 2013, p. 9-43,
McLaughlin, 2007a, 2007b). Contrary to the lesser effects implied by
the commenters, the authors of these two studies noted implications of
their findings with regard to the potential for effects to be amplified
under conditions of increased temperature and associated reduced water
availability (McLaughlin, 2007a). We additionally note comments from
[[Page 65387]]
the CASAC, summarized above, in which it concurs with a focus on
biomass loss and the use of RBL estimates, calling biomass loss in
trees a ``relevant surrogate for damage to tree growth'' that affects
an array of ecosystem services (Frey, 2014c, p. 10), and identifies 6%
RBL as ``unacceptably high'' (Frey, 2014c, p. 13). The evidence we
presented includes evidence related to RBL estimates above that
benchmark. Thus, while we agree that some reductions in tree growth may
not be concluded to be adverse to public welfare, we disagree with
commenters that we have not presented the evidence, which includes RBL
estimates well above the 6% magnitude identified by CASAC, that
supports the Administrator's judgments on adversity that may be
indicated by such estimates and her conclusion that adequate protection
is not provided by the current standard, as described in section IV.B.3
below.
Some commenters disagree with the EPA's consideration of the Class
I areas analysis, stating that it is not appropriate for the EPA to
evaluate the level of protection offered by the current primary
O3 standard under current conditions due to the long-range
transport of O3 and O3 precursors to Class I
areas from upwind non-attainment areas. It is the view of these
commenters that once the upwind areas make emissions reductions to
attain the current standard, downwind areas will see improvements in
air quality and decreasing W126 levels. In support of this view,
commenters point to several modeling analyses. Some commenters point to
air quality modeling conducted by an environmental consultant that
projects all sites to have W126 index values below 13 ppm-hrs when
emissions are adjusted such that all upwind monitors are modeled to
meet the current standard. Detailed methodology, results and references
for the commenter's modeling analysis were not provided, precluding a
thorough evaluation and comparison to the EPA's modeling. While the EPA
agrees that transport of O3 and O3 precursors can
affect downwind monitors, we disagree with commenters regarding the
conclusions that are appropriate to draw from modeling simulations for
the reasons noted below.
As support for their view that the current standard provides
adequate protection, some commenters pointed to estimates drawn from
the EPA's air quality modeling performed for the RIA, stating that this
modeling for an alternative standard level of 70 ppb indicates ``only a
handful'' of monitoring sites approaching as high as 13 ppm-hrs as a 3-
year average (e.g., UARG, p. 76). These commenters further point to the
WREA modeling, noting that those estimates project that attainment of
the current standard would result in only 5 sites above 15 ppm-hrs.
Based on these statements, these commenters state that the current
standard is likely to provide conditions with no site having a monitor
over 17 ppm-hrs and a ``minimal number'' likely exceeding 13 ppm-hrs
(e.g., UARG, p. 77). We disagree with commenters' interpretation of the
modeling information from the two different assessments. As we
summarized in section IV.C.1 of the proposal with regard to the WREA
modeling, the modeling estimates are each based on a single set of
precursor emissions reductions that are estimated to achieve the
desired target conditions, which is also the case for the RIA
modeling\194\ (U.S. EPA, 2014c, pp. 5-40 to 5-41; see also section
1.2.2 of the 2014 RIA).
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\194\ Although commenters cite to both analyses as if providing
the same information, there are many differences in specific aspects
of the RIA approach from that of the WREA, which derive, at least in
part, from their very different purposes. The RIA is not developed
for consideration in the NAAQS review. Rather, it is intended to
provide insights and analysis of an illustrative control strategy
that states might adopt to meet the revised standard. The EPA does
not consider this analysis informative to consideration of the
protection provided by the current standard, and the results of the
RIA have not been considered in the EPA's decisions on the
O3 standards.
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As noted in section IV.A.2 above, and in the proposal, the model-
adjusted air quality in the WREA scenario for the current standard does
not represent an optimized control scenario that just meets the current
standard, but rather characterizes one potential distribution of air
quality across a region when all monitor locations meet the standard
(79 FR 75322; U.S. EPA, 2014b, section 4.3.4.2). Alternate precursor
emissions reductions would be expected to produce different patterns of
O3 concentrations and associated differences in W126 index
values. Specifically, the precursor emissions reductions scenarios
examined in the WREA focuses on regional reductions over broad areas
rather than localized cuts that may focus more narrowly on areas
violating the current standard (U.S. EPA, 2014b, p. 4-35). The
assumption of regionally determined across-the-board emissions
reductions is a source of potential uncertainty with the potential to
overestimate W126 scenario benefits (U.S. EPA, 2014b, Table 4-5 [row
G]). The application of emissions reductions to all locations in each
region to bring down the highest monitor in the region to meet the
current standard could potentially lead to W126 index underestimates at
some locations, as noted in the WREA: ``[w]hile the scenarios
implemented in this analysis show that [] bringing down the highest
monitor in a region would lead to reductions below the targeted level
through the rest of the region, to the extent that the regional
reductions from on-the-books controls are supplemented with more local
controls the additional benefit may be overestimated'' (U.S. EPA,
2014b, p. 4-36; U.S. EPA, 2014c, pp. 5-40 to 5-41). This point was
emphasized by CASAC in their comments on the 2nd draft WREA. CASAC
noted that, ``[m]eeting a target level at the highest monitor requires
substantial reductions below the targeted level through the rest of the
region'' and stated that ``[t]his artificial simulation does not
represent an actual control strategy and may conflate differences in
control strategies required to meet different standards'' (Frey, 2014b,
p. 2).
Due to the uncertainty about what actual future emissions control
strategies might be and their associated emissions reductions, and the
impact such uncertainty might have on modeling estimates involving
reductions from recent conditions, we believe it is important to place
weight on ambient air monitoring data for recent conditions in drawing
conclusions regarding W126 index values that would be expected in areas
that meet the current standard. The analysis of air quality data for
Class I areas described in the proposal, and updated in Table 3 above
(1998-2013), indicates the occurrence of 3-year W126 exposure index
values well above 19 ppm-hrs, a cumulative exposure value for which
CASAC termed the associated median RBL estimate ``unacceptably high,''
in multiple Class I areas that meet the current standard (79 FR 75312,
December 17, 2014, Table 7; updated in Table 3 above). Additionally,
analysis of recent air quality data (2011-2013) for all locations
across the U.S. indicates 10 monitor locations distributed across two
NOAA climatic regions that meet the current standard and at which 3-
year W126 index values are above 19 ppm-hrs, with the highest values
extending up to 23 ppm-hrs (Wells, 2015b).
In support of their view that the EPA's modeling supports the
conclusion that W126 index values of interest are achieved under the
current secondary standard, some commenters additionally state that the
W126 values in the WREA are overestimated in unmonitored rural areas
due to the much greater prevalence of urban monitors across the U.S.
The EPA
[[Page 65388]]
disagrees with this conclusion. In order to estimate O3
concentrations in grid cells across a national-scale spatial surface,
the WREA applied the VNA spatial interpolation technique after applying
the HDDM technique to adjust O3 concentrations at monitoring
sites based on the emissions reductions necessary to just meet the
current standard. In estimating concentrations in unmonitored areas,
the VNA method considers only the ``neighboring'' monitors, using an
inverse distance squared weighting formula, which assigns the greatest
influence to the nearest neighboring monitor (U.S. EPA, 2014b, p. 4A-
6). By this approach, monitors in less-densely monitored areas
contribute to the concentration estimates over much larger areas than
do monitors in more-densely monitored areas. In an urban area,
neighboring monitors may be quite close to one another, such that any
one monitor may only be influencing concentration estimates for a
handful of spatial grid cells in the immediate vicinity. By contrast,
monitors in rural areas may influence hundreds of grid cells. A
specific example of this is the monitor in Great Basin National Park in
eastern Nevada. The VNA algorithm assigns very high weights to this
monitor for all of the grid cells covering a 100 km radius around it,
simply because there are no other monitors in that area and it is the
closest. On the other hand, a monitor near downtown Las Vegas may only
get a high weight for, and thus exert influence on the concentration
estimate in, the one grid cell containing it. We agree with the
commenter that urban monitors may influence the spatial surface for
some distance away from the urban areas, although the influence wanes
with increasing distance from that area and decreasing distance to the
next closest monitor. As we lack data for the intervening locations,
however, we have no reason to conclude that the VNA surface is
overestimating the W126 index values. Further, as was summarized in
section IV.A.2 above, and in the WREA, the PA and the proposal (U.S.
EPA, 2014b, Table 6-27, section 8.5; U.S. EPA, 2014c, p. 5-49; 79 FR
75323, December 17, 2014), the VNA approach results in a lowering of
the highest W126 index values at monitoring sites, which contributes to
underestimates of the highest W126 index values in each region.
In support of their view that the current standard is adequate,
some industry commenters additionally cite WREA analyses for the
current standard scenario, including the W126 index estimates in
national parks, as showing that the current standard provides more than
adequate protection, with alternative scenarios providing only marginal
and increasingly uncertain benefits. As we noted in the proposal and
section IV.A.2 above, there are an array of uncertainties associated
with the W126 index estimates, in the current standard scenario and in
the other scenarios, which, as they are inputs to the vegetation risk
analyses, are propagated into those analyses (79 FR 75323; December 17,
2014). As a result, consistent with the approach in the proposal, the
Administrator has not based her decision with regard to adequacy of the
current standard in this review on these air quality scenario analyses.
In support of their view that the current standard provides
adequate protection and should not be revised, some commenters
described their concerns with any consideration of visible foliar
injury in the decision regarding the secondary standard. These
commenters variously stated that visible foliar injury cannot be
reliably evaluated for adversity given lack of available information,
is not an adverse effect on public welfare that must be addressed
through a secondary standard, and is not directly relatable to growth
suppression (and the EPA's use of RBL captures that effect anyway).
Additionally, some state that any associated ecosystem services effects
are not quantifiable. In sum, the view of these commenters is that it
is not appropriate for the Administrator to place any weight on this
O3 effect in determining the adequacy of the current
standard. As an initial matter, the EPA agrees with the comment that
the current evidence does not include an approach for relating visible
foliar injury to growth suppression,\195\ as recognized in section
IV.A.1.b above. Further, we note that, similar to decisions in past
O3 reviews, the Administrator's proposed decision in this
review recognized the ``complexities and limitations in the evidence
base regarding characterizing air quality conditions with respect to
the magnitude and extent of risk for visible foliar injury'' and the
``challenges of associated judgments with regard to adversity of such
effects to public welfare'' (79 FR 75336; December 17, 2014). Contrary
to the implications of the commenters, although the Administrator took
into consideration the potential for adverse effects on public welfare
from visible foliar injury, she placed weight primarily on growth-
related effects of O3, both in her proposed decision on
adequacy and with regard to proposed judgments on what revisions would
be appropriate. Although visible foliar injury may impact the public
welfare and accordingly has the potential to be adverse to the public
welfare (as noted in section IV.B.2 of the proposal), the Administrator
placed less weight on visible foliar injury considerations in
identifying what revisions to the standard would be appropriate to
propose. In considering these effects for this purpose, she recognized
``significant challenges'' in light of ``the variability and the lack
of clear quantitative relationship with other effects on vegetation, as
well as the lack of established criteria or objectives that might
inform consideration of potential public welfare impacts related to
this vegetation effect'' (79 FR 75349; December 17, 2014). As
summarized in section IV.A.1.a above, the evidence demonstrates a
causal relationship of O3 with visible foliar injury.
Accordingly, we note that the uncertainty associated with visible
foliar injury is not with regard to whether O3 causes
visible foliar injury. Rather, the uncertainty is, as discussed in
sections IV.A.1.b and IV.A.3 above, with the lack of established,
quantitative exposure-response functions that document visible foliar
injury severity and incidence under varying air quality and
environmental conditions and information to support associated
judgments on the significance of such responses with regard to
associated public welfare impacts. As with the Administrator's proposed
decisions on the standard, such considerations also informed her final
decisions, described in sections IV.B.3 and IV.C.3 below.
---------------------------------------------------------------------------
\195\ The current evidence indicates that``[t]he significance of
O3 injury at the leaf and whole plant levels depends on
how much of the total leaf area of the plant has been affected, as
well as the plant's age, size, developmental stage, and degree of
functional redundancy among the existing leaf area'' and ``in some
cases, visible foliar symptoms have been correlated with decreased
vegetative growth . . . and with impaired reproductive function''
(U.S. EPA, 2013, p. 9-39). The ISA concludes, however, ``it is not
presently possible to determine, with consistency across species and
environments, what degree of injury at the leaf level has
significance to the vigor of the whole plant'' (U.S. EPA, 2013, p.
9-39).
---------------------------------------------------------------------------
In support of their view that the current standard should be
retained, some commenters note the WREA finding for the current
standard scenario of no U.S. counties with RYL estimates at or above
5%, the RYL value emphasized by CASAC and state that policy reasons
provide support for not focusing on crops in the decision; other
commenters state that additional studies on crops and air quality are
needed. As
[[Page 65389]]
described previously in this section, and in section IV.A.2 above, an
aspect of uncertainties associated with the WREA air quality scenarios,
including the current standard scenario, is underestimation of the
highest W126 index values, contributing to underestimates in the
effects associated with the current standard scenario. The EPA agrees
with commenters that additional studies on crops and air quality will
be useful to future reviews. Additionally, however, as noted above, the
Administrator's proposed conclusion on adequacy of the current
standard, as well as her final decision described in section IV.B.3
below, gives less weight to consideration of effects on agricultural
crops in recognition of the complicating role of heavy management in
that area.
Lastly, we note that many commenters cited the costs of compliance
as supporting their view that the standard should not be revised,
although as we have described in section I.B above, the EPA may not
consider the costs of compliance in determining what standard is
requisite to protect public welfare from known or anticipated adverse
effects.
3. Administrator's Conclusions on the Need for Revision
Having carefully considered the advice from CASAC and public
comments, as discussed above, the Administrator believes that the
fundamental scientific conclusions on the welfare effects of
O3 in ambient air reached in the ISA and summarized in the
PA and in section IV.B of the proposal remain valid. Additionally, the
Administrator believes the judgments she reached in the proposal
(section IV.D.3) with regard to consideration of the evidence and
quantitative assessments and advice from CASAC remain appropriate.
Thus, as described below, the Administrator concludes that the current
secondary standard is not requisite to protect public welfare from
known and anticipated adverse effects associated with the presence of
O3 in the ambient air and that revision is needed to provide
additional protection.
In considering the adequacy of the current secondary O3
standard, the Administrator has carefully considered the available
evidence, analyses and conclusions contained in the ISA, including
information newly available in this review; the information,
quantitative assessments, considerations and conclusions presented in
the PA; the advice and recommendations from CASAC; and public comments.
The Administrator gives primary consideration to the evidence of growth
effects in well-studied tree species and information, presented in the
PA and represented with a narrower focus in section IV.B.2 above, on
cumulative exposures occurring in Class I areas when the current
standard is met. This information indicates the occurrence of exposures
associated with Class I areas during periods when the current standard
is met for which associated estimates of growth effects, in terms of
the tree seedling RBL in the median species for which E-R functions
have been established, extend above a magnitude considered to be
``unacceptably high'' by CASAC. This analysis estimated such cumulative
exposures occurring under the current standard for nearly a dozen
areas, distributed across two NOAA climatic regions of the U.S. The
Administrator gives particular weight to this analysis, given its focus
in Class I areas. Such an emphasis on lands afforded special government
protections, such as national parks and forests, wildlife refuges, and
wilderness areas, some of which are designated Class I areas under the
CAA, is consistent with such emphasis in the 2008 revision of the
secondary standard (73 FR 16485, March 27, 2008). As noted in section
IV.A above, Congress has set such lands aside for specific uses that
are intended to provide benefits to the public welfare, including lands
that are to be protected so as to conserve the scenic value and the
natural vegetation and wildlife within such areas, and to leave them
unimpaired for the enjoyment of future generations. The Administrator
additionally recognizes that states, tribes and public interest groups
also set aside areas that are intended to provide similar benefits to
the public welfare for residents on those lands, as well as for
visitors to those areas.
As noted in prior reviews, judgments regarding effects that are
adverse to public welfare consider the intended use of the ecological
receptors, resources and ecosystems affected. Thus, the Administrator
recognizes that the median RBL estimate for the studied species is a
quantitative tool within a larger framework of considerations
pertaining to the public welfare significance of O3 effects
on the public welfare. Such considerations include effects that are
associated with effects on growth and that the ISA has determined to be
causally or likely causally related to O3 in ambient air,
yet for which there are greater uncertainties affecting our estimates
of impacts on public welfare. These other effects include reduced
productivity in terrestrial ecosystems, reduced carbon sequestration in
terrestrial ecosystems, alteration of terrestrial community
composition, alteration of below-grown biogeochemical cycles, and
alteration of terrestrial ecosystem water cycles, as summarized in
section IV.A.1. Thus, in her attention to CASAC's characterization of a
6% estimate for tree seedling RBL in the median studied species as
``unacceptably high'', the Administrator, while mindful of
uncertainties with regard to the magnitude of growth impact that might
be expected in mature trees, is also mindful of related, broader,
ecosystem-level effects for which our tools for quantitative estimates
are more uncertain and those for which the policy foundation for
consideration of public welfare impacts is less well established. She
finds her consideration of tree growth effects consistent with CASAC
advice regarding consideration of O3-related biomass loss as
a surrogate for the broader array of O3 effects at the plant
and ecosystem levels.
The Administrator also recognizes that O3-related
effects on sensitive vegetation can occur in other areas that have not
been afforded special federal protections, including effects on
vegetation growing in managed city parks and residential or commercial
settings, such as ornamentals used in urban/suburban landscaping or
vegetation grown in land use categories that are heavily managed for
commercial production of commodities such as timber. In her
consideration of the evidence and quantitative information of
O3 effects on crops, the Administrator recognizes the
complexity of considering adverse O3 impacts to public
welfare due to the heavy management common for achieving optimum yields
and market factors that influence associated services. In so doing, she
notes that her judgments that place emphasis on the protection of
forested ecosystems inherently also recognize a level of protection for
crops. Additionally, for vegetation used for residential or commercial
ornamental purposes, the Administrator believes that there is not
adequate information specific to vegetation used for those purposes,
but notes that a secondary standard revised to provide protection for
sensitive natural vegetation and ecosystems would likely also provide
some degree of protection for such vegetation.
The Administrator also takes note of the long-established evidence
of consistent association of the presence of visible foliar injury with
O3 exposure and the currently available information that
indicates the occurrence of visible foliar injury in sensitive species
of
[[Page 65390]]
vegetation during recent air quality in public forests across the U.S.
She additionally notes the PA conclusions regarding difficulties in
quantitatively relating visible foliar injury symptoms to vegetation
effects such as growth or related ecosystem effects. As at the time of
the last review, the Administrator believes that the degree to which
such effects should be considered to be adverse depends on the intended
use of the vegetation and its significance. The Administrator also
believes that the significance of O3-induced visible foliar
injury depends on the extent and severity of the injury and takes note
of studies in the evidence base documenting increased severity and/or
prevalence with higher O3 exposures. However, the
Administrator takes note of limitations in the available information
with regard to judging the extent to which the extent and severity of
visible foliar injury occurrence associated with conditions allowed by
the current standard may be considered adverse to public welfare.
Based on these considerations, and taking into consideration the
advice and recommendations of CASAC, the Administrator concludes that
the protection afforded by the current secondary O3 standard
is not sufficient and that the standard needs to be revised to provide
additional protection from known and anticipated adverse effects to
public welfare, related to effects on sensitive vegetation and
ecosystems, most particularly those occurring in Class I areas. The
Administrator additionally recognizes that states, tribes and public
interest groups also set aside areas that are intended to provide
similar benefits to the public welfare for residents on those lands, as
well as for visitors to those areas. Given the clear public interest in
and value of maintaining these areas in a condition that does not
impair their intended use, and the fact that many of these areas
contain O3-sensitive vegetation, the Administrator further
concludes that it is appropriate to revise the secondary standard in
part to provide increased protection against O3-caused
impairment to vegetation and ecosystems in such areas, which have been
specially protected to provide public welfare benefits. She further
notes that a revised standard would provide increased protection for
other growth-related effects, including for crop yield loss, reduced
carbon storage and for areas for which it is more difficult to
determine public welfare significance, as recognized in section IV.A.3
above, as well other welfare effects of O3, such as visible
foliar injury.
C. Conclusions on Revision of the Secondary Standard
The elements of the standard--indicator, averaging time, form, and
level--serve to define the standard and are considered collectively in
evaluating the welfare protection afforded by the secondary standard.
Section IV.C.1 below summarizes the basis for the proposed revision.
Significant comments received from the public on the proposal are
discussed in section IV.C.2 and the Administrator's final decision on
revisions to the secondary standard is described in section IV.C.3.
1. Basis for Proposed Revision
At the time of proposal, in considering what revisions to the
secondary standard would be appropriate, the Administrator considered
the ISA conclusions regarding the weight of the evidence for a range of
welfare effects associated with O3 in ambient air and
associated areas of uncertainty; quantitative risk and exposure
analyses in the WREA for different adjusted air quality scenarios and
associated limitations and uncertainties; staff evaluations of the
evidence, exposure/risk information and air quality information in the
PA; additional air quality analyses of relationships between air
quality metrics based on form and averaging time of the current
standards and a cumulative seasonal exposure index; CASAC advice; and
public comments received as of that date in the review. In the
paragraphs below, we summarize the proposal presentation with regard to
key aspects of the PA considerations, advice from the CASAC, air
quality analyses of different air quality metrics and the
Administrator's proposed conclusions, drawing from section IV.E of the
proposal.
a. Considerations and Conclusions in the PA
As summarized in the proposal, in identifying alternative secondary
standards appropriate to consider in this review, the PA focused on
standards based on a cumulative, seasonal, concentration-weighted form
consistent with the CASAC advice in the current and last review. Based
on conclusions of the ISA, as also summarized in section IV.A above,
the PA considered a cumulative, seasonal, concentration-weighted
exposure index to provide the most scientifically defensible approach
for characterizing vegetation response to ambient O3 and
comparing study findings, as well as for defining indices for
vegetation protection, as summarized in the proposal section IV.E.2.a.
With regard to the appropriate index, the PA considered the evidence
for a number of different such indices, as described in the proposal,
and noted the ISA conclusion that the W126 index has some important
advantages over other similarly weighted indices. The PA additionally
considered the appropriate diurnal and seasonal exposure periods in a
given year by which to define the seasonal W126 index and based on the
evidence in the ISA and CASAC advice, as summarized in the proposal,
decided on the 12-hour daylight window (8:00 a.m. to 8:00 p.m.) and the
3-consecutive-month period providing the maximum W126 index value.
Based on these considerations, the PA concluded it to be
appropriate to retain the current indicator of O3 and to
consider a secondary standard form that is an average of the seasonal
W126 index values (derived as described in section IV.A.1.c above)
across three consecutive years (U.S. EPA, 2014c, section 6.6). In so
doing, the PA recognized that there is limited information to discern
differences in the level of protection afforded for cumulative growth-
related effects by potential alternative W126-based standards of a
single-year form as compared to a 3-year form (U.S. EPA, 2014c, pp. 6-
30). The PA concluded a 3-year form to be appropriate for a standard
intended to provide the desired level of protection from longer-term
effects, including those associated with potential compounding, and
that such a form might be concluded to contribute to greater stability
in air quality management programs, and thus, greater effectiveness in
achieving the desired level of public welfare protection than might
result from a single-year form. (U.S. EPA, 2014c, section 6.6).
As summarized in the proposal, the PA noted that, due to the
variability in the importance of the associated ecosystem services
provided by different species at different exposures and in different
locations, as well as differences in associated uncertainties and
limitations, it is essential to consider the species present and their
public welfare significance, together with the magnitude of the ambient
concentrations in drawing conclusions regarding the significance or
magnitude of public welfare impacts. Therefore, in development of the
PA conclusions, staff took note of the complexity of judgments to be
made by the Administrator regarding the adversity of known and
anticipated effects to the
[[Page 65391]]
public welfare and recognized that the Administrator's ultimate
judgments on the secondary standard will most appropriately reflect an
interpretation of the available scientific evidence and exposure/risk
information that neither overstates nor understates the strengths and
limitations of that evidence and information. In considering an
appropriate range of levels to consider for an alternative standard,
the PA primarily considered tree growth, crop yield loss, and visible
foliar injury, as well as impacts on the associated ecosystem services,
while noting key uncertainties and limitations.
In specifically evaluating exposure levels, in terms of the W126
index, as to their appropriateness for consideration in this review
with regard to providing the desired level of vegetation protection for
a revised secondary standard, the PA focused particularly on RBL
estimates for the median across the 11 tree species for which robust E-
R functions are available. Table 4 below presents these estimates (U.S.
EPA, 2014c, Appendix 5C, Table 5C-3; also summarized in Table 8 of the
proposal). In so doing and recognizing the longstanding, strong
evidence base supporting these relationships, the PA also noted
uncertainties regarding inter-study variability for some species, as
well as with regard to the extent to which tree seedling E-R functions
can be used to represent mature trees. As summarized in the proposal,
the PA conclusions on a range of W126 levels appropriate to consider
are based on specific advice from CASAC with regard to median tree
seedling RBL estimates that might be considered unacceptably high (6%),
as well as its judgment on a RBL benchmark (2%) for identification of
the lower end of a W126 index value range for consideration that might
give more emphasis to the more sensitive tree seedlings (Frey, 2014c,
p. 14).\196\
---------------------------------------------------------------------------
\196\ The CASAC provided several comments related to 2% RBL for
tree seedlings both with regard to its use in summarizing WREA
results and with regard to consideration of the potential
significance of vegetation effects, as summarized in sections IV.D.2
and IV.E.3 of the proposal.
Table 4--Tree Seedling Biomass Loss and Crop Yield Loss Estimated for O3 Exposure Over a Season
----------------------------------------------------------------------------------------------------------------
Tree seedling biomass loss A Crop yield loss B
W126 index value for exposure -------------------------------------------------------------------------------
period Median value Individual species Median value Individual species
----------------------------------------------------------------------------------------------------------------
23 ppm-hrs...................... Median species w. <= 2% loss: 3/11 Median species w. <= 5% loss: 4/10
7.6% loss. species. 8.8% loss. species
<= 5% loss: 4/11 >5,<10% loss: 1/10
species. species
<=10% loss: 8/11 >10,<20% loss: 4/
species. 10 species
<=15% loss: 10/11 >20: 1/10 species
species.
>40% loss: 1/11
species.
22 ppm-hrs...................... Median species w. <= 2% loss: 3/11 Median species w. <= 5% loss: 4/10
7.2% loss. species. 8.2% loss. species
<= 5% loss: 4/11 >5,<10% loss: 1/10
species. species
<=10% loss: 7/11 >10,<20% loss: 4/
species. 10 species
<=15% loss: 10/11 >20: 1/10 species
species.
>40% loss: 1/11
species.
21 ppm-hrs...................... Median species w. <= 2% loss: 3/11 Median species w. <= 5% loss: 4/10
6.8% loss. species. 7.7% loss. species
<= 5% loss: 4/11 >5,<10% loss: 3/10
species. species
<=10% loss: 7/11 >10,<20% loss: 3/
species. 10 species
<=15% loss: 10/11
species.
>40% loss: 1/11
species.
20 ppm-hrs...................... Median species w. <= 2% loss: 3/11 Median species w. <= 5% loss: 5/10
6.4% loss. species. 7.1% loss. species
<= 5% loss: 5/11 >5,<10% loss: 3/10
species. species
<=10% loss: 7/11 >10,<20% loss: 2/
species. 10 species
<=15% loss: 10/11
species.
>40% loss: 1/11
species.
19 ppm-hrs...................... Median species w. <= 2% loss: 3/11 Median species w. <= 5% loss: 5/10
6.0% loss. species. 6.4% loss. species
<=5% loss: 5/11 >5, <10% loss: 3/
species. 10 species
<=10% loss: 7/11 >10,<20% loss: 2/
species. 10 species
<=15% loss: 10/11
species.
>30% loss: 1/11
species.
18 ppm-hrs...................... Median species w. <= 2% loss: 5/11 Median species w. <= 5% loss: 5/10
5.7% loss. species. 5.7% loss. species
<= 5% loss: 5/11 >5,<10% loss: 3/10
species. species
<=10% loss: 7/11 >10,<20% loss: 2/
species. 10 species
<=15% loss: 10/11
species.
>30% loss: 1/11
species.
17 ppm-hrs...................... Median species w. <= 2% loss: 5/11 Median species w. <= 5% loss: 5/10
5.3% loss. species. 5.1% loss. species
<=5% loss: 5/11 >5, <10% loss: 3/
species. 10 species
<=10% loss: 9/11 >10,<20% loss: 2/
species. 10 species
<=15% loss: 10/11
species.
>30% loss: 1/11
species.
16 ppm-hrs...................... Median species w. <= 2% loss: 5/11 Median species w. <= 5% loss: 5/10
4.9% loss. species. <=5.0% loss. species
<= 5% loss: 6/11 >5,<10% loss: 4/10
species. species
<=10% loss: 10/11 >10,<20% loss: 1/
species. 10 species
>30% loss: 1/11
species.
15 ppm-hrs...................... Median species w. <= 2% loss: 5/11 Median species w. <= 5% loss: 6/10
4.5% loss. species. <=5.0% loss. species
<=5% loss: 6/11 >5, <10% loss: 4/
species. 10 species
<=10% loss: 10/11
species.
>30% loss: 1/11
species.
14 ppm-hrs...................... Median species w. <= 2% loss: 5/11 Median species w. <= 5% loss: 6/10
4.2% loss. species. <=5.0% loss. species
<= 5% loss: 6/11 >5,<10% loss: 4/10
species. species
<=10% loss: 10/11
species.
>30% loss: 1/11
species.
13 ppm-hrs...................... Median species w. <= 2% loss: 5/11 Median species w. <= 5% loss: 6/10
3.8% loss. species. <=5.0% loss. species
<5% loss: 7/11 >5, <10% loss: 4/
species. 10 species
<10% loss: 10/11
species.
>20% loss: 1/11
species.
[[Page 65392]]
12 ppm-hrs...................... Median species w. <= 2% loss: 5/11 Median species w. <= 5% loss: 8/10
3.5% loss. species. <=5.0% loss. species
<= 5% loss: 8/11 >5,<10% loss: 2/10
species. species
<=10% loss: 10/11
species.
>20% loss: 1/11
species.
11 ppm-hrs...................... Median species w. <= 2% loss: 5/11 Median species w. <= 5% loss: 9/10
3.1% loss. species. <=5.0% loss. species
<=5% loss: 8/11 >5, <10% loss: 1/
species. 10 species
<=10% loss: 10/11
species.
>20% loss: 1/11
species.
10 ppm-hrs...................... Median species w. <= 2% loss: 5/11 Median species w. <= 5% loss: 9/10
2.8% loss. species. <=5.0% loss. species
<= 5% loss: 9/11 >5,<10% loss: 1/10
species. species
<10% loss: 10/11
species.
>20% loss: 1/11
species.
9 ppm-hrs....................... Median species w. <= 2% loss: 5/11 Median species w. <= 5% loss: all
2.4% loss. species. <=5.0% loss. species
<= 5% loss: 10/11
species.
>20% loss: 1/11
species.
8 ppm-hrs....................... Median species w. <= 2% loss: 5/11 Median species w. <= 5% loss: all
2.0% loss. species. <=5.0% loss. species
<= 5% loss: 10/11
species.
>15% loss: 1/11
species.
7 ppm-hrs....................... Median species w. <= 2% loss: 7/11 Median species w. <= 5% loss: all
<2.0% loss. species. <=5.0% loss. species
<=5% loss: 10/11
species.
>15% loss: 1/11
species.
----------------------------------------------------------------------------------------------------------------
\A\ Estimates here are based on the E-R functions for 11 species described in the WREA, section 6.2 and
discussed in the PA, section 5.2.1. The cottonwood was excluded to address CASAC comments (Frey, 2014c; U.S.
EPA, 2014b, U.S. EPA, 2014c, Appendix 6F). The median is the median of the 11 composite E-R functions (U.S.
EPA, 2014c, Appendix 5C).
\B\ Estimates here are based on the 10 E-R functions for crops described in the WREA, section 6.2 and discussed
in the PA, section 5.3.1. The median is the median of the 10 composite E-R functions (U.S. EPA, 2014b; U.S.
EPA, 2014c, Appendix 5C).
With regard to secondary standard revisions appropriate to consider
in this review, as summarized in the proposal, the PA concluded it to
be appropriate to consider a W126-based secondary standard with index
values within the range of 7 to 17 ppm-hrs and a form averaged over 3
years (U.S. EPA, 2014c, section 6.7). The PA additionally recognized
the role of policy judgments required of the Administrator with regard
to the public welfare significance of identified effects, the
appropriate weight to assign the range of uncertainties inherent in the
evidence and analyses, and ultimately, in identifying the requisite
protection for the secondary O3 standard.
The PA additionally recognized that to the extent the Administrator
finds it useful to consider the public welfare protection that might be
afforded by revising the level of the current standard, this is
appropriately judged by evaluating the impact of associated
O3 exposures in terms of the cumulative seasonal W126-based
index, an exposure metric considered appropriate for evaluating impacts
on vegetation (U.S. EPA, 2014c, section 6.7). Accordingly, the PA
included several air quality data analyses that might inform such
consideration (U.S. EPA, 2014c, section 6.4). Additional air quality
analyses were performed subsequent to the PA, described in the proposal
and are summarized below.
b. CASAC Advice
Advice received from the CASAC during the current review, similar
to that in the last review, recommended retaining O3 as the
indicator, while also recommending consideration of a secondary
standard with a revised form and averaging time based on the W126 index
(Frey, 2014c, p. iii). The CASAC concurred with the 12-hour period (8
a.m. to 8 p.m.) and 3-month summation period resulting in the maximum
W126 index value, as described in the PA, while recommending a somewhat
narrower range of levels from 7 ppm-hrs to 15 ppm-hrs. While the CASAC
recommended a W126 index limited to a single year, in contrast with the
PA's conclusion that it was appropriate to consider the W126 index
averaged across three years, it also noted that the Administrator may
prefer, as a policy matter, to base the secondary standard on a 3-year
averaging period. In such a case, the CASAC recommended revising
downward the level for such a metric to avoid a seasonal W126 index
value above a level in their recommended range in any given year of the
3-year period, indicating an upper end of 13 ppm-hrs as an example for
such a 3-year average W126 index range (Frey, 2014c, p. iii and iv).
c. Air Quality Analyses
The proposal additionally summarized several analyses of air
quality that considered relationships between metrics based on a 3-year
W126 index and based on the form and averaging time of the current
standard, the ``fourth-high'' metric (U.S. EPA, 2014c, Chapter 2,
Appendix 2B and section 6.4; Wells, 2014a), as well as describing the
uncertainties and limitations associated with these analyses. The
proposal concluded that these analyses suggest that, depending on the
level, a standard of the current averaging time and form can be
expected to control cumulative seasonal O3 exposures to such
that they may meet specific 3-year average W126 index values. The
fourth-high and W126 metrics, and changes in the two metrics over the
past decade, were found to be highly correlated (U.S. EPA, 2014c,
section 6.4 and Appendix 2B; Wells, 2014a). From these analyses, it was
concluded that future control programs designed to help meet a standard
based on the fourth-high metric are also expected to result in
reductions in values of the W126 metric (Wells, 2014a). Further, the
second analysis also found that the Southwest and West NOAA climatic
regions, which showed the greatest potential for sites to measure
elevated cumulative, seasonal O3 exposures without the
occurrence of elevated daily maximum 8-hour average O3
concentrations, exhibited the greatest reduction in W126 metric value
per unit reduction in fourth-high metric (Wells, 2014a, Figures 5b and
12 and Table 6).
[[Page 65393]]
Analyses of the most recent periods studied in the two analyses
(2009-2011 and 2011-2013) had similar findings regarding the highest
W126 metric values occurring at monitoring sites that meet alternative
levels of the fourth-high metric (U.S. EPA, 2014c, section 6.4; Wells,
2014a). In both analyses, the highest W126 metric values were in the
Southwest and West NOAA climatic regions. In both analyses, no
monitoring sites for which the fourth-high metric was at or below 70
ppb had a W126 metric value above 17 ppm-hrs (U.S. EPA, 2014c, Figure
2B-3b; Wells, 2014a, Table 4). All U.S. regions were represented in
these subsets. In the 2011-2013 subset of sites for which the fourth-
high metric was at or below a potential alternative primary standard
level of 65 ppb, no monitoring sites had W126 metric values above 11
ppm-hrs (Wells, 2014a, Table 4).
d. Administrator's Proposed Conclusions
At the time of proposal, the Administrator concluded it to be
appropriate to continue to use O3 as the indicator for a
secondary standard that is intended to address effects associated with
exposure to O3 alone and in combination with related
photochemical oxidants. While the complex atmospheric chemistry in
which O3 plays a key role has been highlighted in this
review, no alternatives to O3 have been advanced as being a
more appropriate surrogate for ambient photochemical oxidants and their
effects on vegetation. The CASAC agreed that O3 should be
retained as the indicator for the standard (Frey, 2014c, p. iii). In
proposing to retain O3 as the indicator, the Administrator
recognized that measures leading to reductions in ecosystem exposures
to O3 would also be expected to reduce exposures to other
photochemical oxidants.
The Administrator proposed to retain the current averaging time and
form and to revise the level of the current secondary standard to a
level within the range of 0.065 to 0.070 ppm. She based this proposal
on her provisional conclusions regarding the level of cumulative
seasonal O3 exposures that would provide the requisite
protection against known or anticipated adverse effects to the public
welfare and on a policy option that would provide this level of
protection. With regard to the former, the Administrator concluded that
in judging the extent of public welfare protection that might be
afforded by a revised standard and whether it meets the appropriate
level of protection, it is appropriate to use a cumulative, seasonal
concentration-weighted exposure metric. For this purpose, the
Administrator concluded it to be appropriate to use the W126 index
value, averaged across three years, with each year's value identified
as that for the 3-month period yielding the highest seasonal value and
with daily O3 exposures within a 3-month period cumulated
for the 12-hour period from 8:00 a.m. to 8:00 p.m.
To identify the range of cumulative seasonal exposures, in terms of
the W126 index, expected to be associated with the appropriate degree
of public welfare protection, the Administrator gave primary
consideration to growth-related impacts, using tree seedling RBL
estimates for a range of W126 exposure index values and CASAC advice
regarding such estimates. Additionally taking into account judgments on
important uncertainties and limitations inherent in the current
available scientific evidence and quantitative assessments, and
judgments regarding the extent to which different RBL estimates might
be considered indicative of effects adverse to public welfare, the
Administrator proposed that ambient O3 concentrations
resulting in cumulative seasonal O3 exposures of a level
within the range from 13 ppm-hrs to 17 ppm-hrs, in terms of a W126
index averaged across three consecutive years, would provide the
requisite protection against known or anticipated adverse effects to
the public welfare. In identifying policy options for a revised
secondary standard that would control exposures to such an extent, the
Administrator considered the results of air quality analyses that
examined the responsiveness of cumulative exposures (in terms of the
W126 index) to O3 reductions in response to the current and
prior standard for which the form and averaging time are summarized as
a fourth-high metric, and also examined the extent to which cumulative
exposures (in terms of the W126 index) may be limited by alternative
levels of a metric based on the current standard averaging time and
form. Based on the results of these analyses, she proposed that
revision of the level of the current secondary standard to within the
range of 0.065 to 0.070 ppm would be expected to provide the requisite
public welfare protection, depending on final judgments concerning such
requisite protection.
2. Comments on Proposed Revision
Significant comments from the public regarding revisions to the
secondary standard are addressed in the subsections below. We first
discuss comments related to our consideration of growth-related effects
and visible foliar injury in identifying appropriate revisions to the
standard (sections IV.C.2.a and IV.C.2.b). Next, we address comments
related to the use of the W126 metric in evaluating vegetation effects
and public welfare protection and comments related to the form and
averaging time for the revised standard (sections IV.C.2.c and
IV.C.2.d). Comments on revisions to the level of the standard are
described in section IV.C.2.e, and those related to the way in which
today's rulemaking addresses the 2013 court remand are addressed in
section IV.C.2.f. Other significant comments related to consideration
of a revised secondary standard, and that are based on relevant
factors, are addressed in the Response to Comments document.
a. Consideration of Growth-Related Effects
In considering public comments received on the consideration of
growth-related effects of O3 in the context of the proposed
decision on a revised secondary standard, we first note related advice
and comments from the CASAC provided during development of the PA,
stating, as summarized in section IV.B.1.b above, that ``relative
biomass loss for tree species, crop yield loss, and visible foliar
injury are appropriate surrogates for a wide range of damage that is
adverse to public welfare'' (Frey, 2014c, p. 10). Additionally, in the
context of different standard levels they considered appropriate for
the EPA to consider, CASAC stated that it is appropriate to ``include[]
levels that aim for not greater than 2% RBL for the median tree
species'' and that a median tree species RBL of 6% is ``unacceptably
high'' (Frey, 2014c, p. 14).\197\ With respect to crop yield loss,
CASAC points to a benchmark of 5%, stating that a crop RYL for median
species over 5% is ``unacceptably high'' (Frey, 2014c, p. 13).
---------------------------------------------------------------------------
\197\ The CASAC made this comment while focusing on Table 6-1 in
the second draft PA and the entry for 17 ppm-hrs (Frey, 2014c, p.
14). That table was revised for inclusion in the final PA in
consideration of CASAC comments on the E-R function for eastern
cottonwood, and after that revision, the median RBL estimate for 17
ppm-hrs in the final table (see Table 4 above) is below the value of
6% that CASAC described in this way.
---------------------------------------------------------------------------
In addition, regarding consideration of RBL benchmarks for tree
seedlings, the CASAC stated that ``[a] 2% biomass loss is an
appropriate scientifically based value to consider as a benchmark of
adverse impact for long-lived perennial species such as trees, because
effects are cumulative over multiple
[[Page 65394]]
years'' (Frey, 2014c, p. 14).\198\ With regard to this benchmark, the
CASAC also commented that ``it is appropriate to identify a range of
levels of alternative W126-based standards that includes levels that
aim for not greater than 2% RBL for the median tree species'' in the PA
(Frey, 2014c, p. 14). The CASAC noted that the ``level of 7 ppm-hrs is
the only level analyzed for which the relative biomass loss for the
median tree species is less than or equal to 2 percent,'' indicating
that 7 ppm was appropriate as a lower bound for the recommended range
(Frey, 2014c, p. 14).\199\
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\198\ The CASAC provided several comments related to 2% RBL for
tree seedlings both with regard to its use in summarizing WREA
results and with regard to consideration of the potential
significance of vegetation effects, as summarized in sections IV.D.2
and IV.E.3 of the proposal.
\199\ The CASAC made this comment while focusing on Table 6-1 in
the second draft PA, which included odd-numbered W126 index values
and in which the median RBL values were based on 12 species. That
table was revised for inclusion in the final PA in consideration of
CASAC comments on the E-R function for eastern cottonwood, such that
the median RBL species estimate for both 7 ppm-hrs and 8 ppm-hrs are
less than or equal to 2.0% in the final table (see Table 4 above and
Table 5C-3 of the final PA).
---------------------------------------------------------------------------
With regard to consideration of effects on crops, in addition to
their comments regarding a median species RYL over 5% yield loss, noted
above (Frey, 2014c, p. 13), the CASAC further noted that ``[c]rop loss
appears to be less sensitive than these other indicators, largely
because of the CASAC judgment that a 5% yield loss represents an
adverse impact, and in part due to more opportunities to alter
management of annual crops'' (Frey, 2014c, p. 14).
Comments from the public with regard to how the EPA considered
growth-related effects in the proposed decision on a revised secondary
standard varied. Generally, those commenters who recommended against
revision of the standard expressed the view that RBL estimates based on
the established E-R functions for the 11 studied species, and their
pertinence to mature trees, were too uncertain to serve as a basis for
judgments regarding public welfare protection afforded by the secondary
standard. The EPA generally disagrees with this view, as discussed in
section IV.B.2 above, and addressed in more detail in the Response to
Comments document.
Some commenters also took note of the unclear basis for CASAC's 2%
benchmark, stating that the CASAC advice on this point is ``not wholly
scientific,'' given that it referenced the 1996 workshop, which
provided little specificity as to scientific basis for such a
benchmark; based on this, the commenters described this CASAC advice as
a policy judgment and described the important role of the EPA's
judgment in such instances. As noted in section IV.E.3 of the proposal,
we generally agree with these commenters regarding the unclear
scientific basis for the 2% value. Consistent with this advice from
CASAC, however, the range of levels for a revised secondary standard
that the PA concluded was appropriate for the Administrator to consider
did include a level for which the estimated median RBL across the 11
studied tree species would be 2%, as well as a level for which the
median RBL would be below 2% (U.S. EPA, 2014c, section 6.7 and Tables
6-1 and 5C-3), and, as described in the proposal, the Administrator
considered the conclusions of the PA in reaching her proposed decision
that it was appropriate to consider a range for the revised secondary
standard that did not focus on this benchmark. The Administrator has
further considered and explained any differences from CASAC's
recommendations on this point in her final decision, as described in
section IV.C.3 below.
Some of the state and local environmental agencies and
organizations and environmental groups that supported the EPA's
proposed decision to revise the secondary standard additionally
indicated their view that the EPA should give more weight to growth-
related effects by setting the standard at a level for which the
estimated RBL would be at or below 2% in the median studied species. In
support of this recommendation, the commenters cited the CASAC advice
and stated that the EPA's rationale deviates from that advice with
regard to consideration of RBL. In so doing, the commenters implied
incorrectly that the EPA's proposal did not put the most weight on the
median RBL. In fact, in considering RBL as a metric for growth effects,
the Administrator's proposed conclusions focused solely on the median
RBL estimates, indicating that appreciable weight was given to growth-
related effects and on the median RBL. Additionally, the commenters
implied that the EPA misconstrued the CASAC comment on 6% RBL to
indicate that it was acceptable. Yet, the proposal notes CASAC's view
that a 6% RBL is ``unacceptably high'' nine times, and, in section
IV.B.3 above, the Administrator takes note of this view in reaching the
decision that the current standard should be revised. The EPA considers
this statement from CASAC, provided in the context of considering
effects related to different W126 index values, to be of a different
nature than CASAC advice discussed above that options for the EPA
consideration ``include'' a level that aims for median RBL at or below
2%.
The comments that state that the standard should control cumulative
exposures to levels for which the estimated median species RBL is at or
below 2% provided little rationale beyond citing to CASAC advice. We
note, however, that the CASAC did not specify that the revised
secondary standard be set to limit cumulative exposures to that extent.
Nor, in identifying a range of alternatives for the EPA to consider,
did CASAC recommend that the EPA consider only W126 index levels
associated with median RBL estimates at or below 2%. Rather, the CASAC
stated that ``it is appropriate to identify a range of levels of
alternative W126-based standards that includes {emphasis added{time}
levels that aim for not greater than 2% RBL for the median tree
species'' (Frey, 2014c, p. 14) and seven of the nine levels in the
CASAC-recommended range of W126 index levels were associated with
higher RBL estimates (as shown in Table 4 above).
In citing to CASAC advice, commenters quoted the CASAC
characterization of a 2% RBL as ``an appropriate scientifically based
value to consider as a benchmark of adverse impact for long-lived
perennial species such as trees, because effects are cumulative over
multiple years'' (Frey, 2014, p. 14). Presumably to indicate reasoning
for this statement, the subsequent sentence in the same CASAC letter
referenced findings for biomass loss in aspen exposed to elevated
O3 over seven years, citing Wittig et al., 2009. As noted in
the proposal, however, the way in which these findings would provide a
basis for CASAC's view with regard to 2% is unclear, as the original
publication that is the source for the 7-year biomass loss value (King,
et al., 2005) and which is cited in Wittig et al. (2009) indicates
yearly RBL values during this 7-year exposure that are each well above
2%, and, in fact, are all above 20% (King, et al., 2005). In the same
paragraph, the CASAC letter additionally referenced the report of the
1996 workshop sponsored by the Southern Oxidants Study group (Heck and
Cowling, 1997, noted in section IV.A.3 above). The workshop report
identified 1-2% per year growth reduction (based on a stated interest
in avoiding 2% cumulative effects) as an appropriate endpoint for
consideration of growth effects in trees, although an explicit
rationale for the identified percentages is not provided
[[Page 65395]]
(Frey, 2014c, p. 14).\200\ Like the 1996 workshop, the CASAC describes
2% RBL as providing the basis for consideration of 7 ppm-hrs, the lower
end of their recommended W126 range (Frey, 2014c, p. 14). As a result,
the specific scientific basis for judging a value of 2% RBL in the
median studied species as an appropriate benchmark of adverse impact
for trees and other long-lived perennials is not clear, which, as
described in the proposal, contributed to the Administrator noting the
greater uncertainty regarding the extent to which estimates of benefits
in terms of ecosystem services and reduced effects on vegetation at
O3 exposures below her identified range of 13 to 17 ppm-hrs
might be judged significant to the public welfare.
---------------------------------------------------------------------------
\200\ The report of the 1996 workshop provides no more explicit
rationale for the percentages identified or specification with
regard to number or proportion of species for which such percentages
should be met (Heck and Cowling, 1997).
---------------------------------------------------------------------------
Some commenters recommended revision of the standard to 7 ppm-hrs
as a W126 form stating that such a change is needed to protect against
climate change. In so doing, one commenter expressed the view that the
relatively lesser weight the EPA placed on the WREA estimates of carbon
storage (in terms of CO2) in consideration of a proposed
revision to the secondary standard is inconsistent with the emphasis
that the EPA placed on CO2 emissions reductions estimated
for the proposed Clean Power Plan (79 FR 34830, 34931-33). As support
for this view of inconsistency, the commenter compared the WREA 30-year
estimate of the amount of CO2 removed from the air and
stored in vegetation with estimated reductions in CO2
emissions from power plants over a 4-year period. We note, however,
some key distinctions between the two types of estimates which
appropriately lead to different levels of emphasis by the EPA in the
two actions. First, we note that the lengths of time pertaining to the
two estimates that the commenter states to be ``roughly equal'' (e.g.,
ALA et al., p. 211) differ by more than a factor of seven (4 years
compared to 30). Second, the CPP estimates are for reductions in
CO2 produced and emitted from power plants, while the WREA
estimates are for amounts of CO2 removed from the air and
stored in vegetation as a result of plant photosynthesis occurring
across the U.S. This leads to two important differences. The first is
whether a ton of additional carbon uptake by plants is equal to a ton
of reduced emissions from fossil fuels. This is still an active area of
discussion due in part to the potentially transient nature of the
carbon storage in vegetation. The second is that there are much larger
uncertainties involved in attempting to quantify the additional carbon
uptake by plants which requires complex modeling of biological and
ecological processes and their associated sources of uncertainty.
Therefore, as summarized in section IV.C.3 below, the Administrator is
judging, as at the time of proposal, that the quantitative
uncertainties are too great to support identification of a revised
standard based specifically on the WREA quantitative estimates of
carbon storage benefits to climate. In so doing, she notes that a
revised standard, established primarily based on other effects for
which our quantitative estimates are less uncertain, can be expected to
also provide increased protection in terms of carbon storage.
b. Consideration of Visible Foliar Injury
In considering public comments received on the EPA's consideration
of visible foliar injury in its decision on a revised secondary
standard, the EPA first notes related advice and comments from the
CASAC received during development of the PA. The CASAC stated that
``[w]ith respect to the secondary standard, the CASAC concurs with the
EPA's identification of adverse welfare effects related to . . . damage
to resource use from foliar injury'' (Frey, 2014, p. iii). In its
comments on levels of a W126-based standard, the CASAC, seemingly in
reference to the WREA visible foliar injury analyses, additionally
stated that ``[a] level below 10 ppm-hrs is required to reduce foliar
injury'' (Frey, 2014, pp. iii and 15), with ``W126 values below 10 ppm-
hr required to reduce the number of sites showing visible foliar
injury'' (Frey, 2014, p. 14).
Public comments were generally split between two views, either that
visible foliar injury was not appropriate to consider in decisions
regarding the standard, based on variously identified reasons, or that
it should be considered and it would lead the EPA to focus on a W126
value below approximately 10 ppm-hrs. Comments of the former type are
discussed in section IV.B.2 above, with, in some cases, additional
detail in the Response to Comments document. Commenters expressing the
latter view variously cite CASAC advice and figures from the WREA
cumulative analysis of USFS biosite data with WREA W126 index value
estimates. The EPA disagrees that only a reduction in cumulative
exposures to W126 index values below 10 ppm-hrs will affect the
occurrence or extent of visible foliar injury. In so doing, we note
that the extensive evidence, which is summarized in the ISA (including
studies of the USFS biomonitoring program), analyses in the 2007 Staff
Paper and also observations based on the WREA dataset do not support
this conclusion.
The evidence regarding visible foliar injury as an indicator of
O3 exposure is well established and generally documents a
greater extent and severity of visible foliar injury with higher
O3 exposures and a modifying role of soil moisture
conditions (U.S. EPA, 2013, section 9.4.2). As stated in the ISA,
``[v]isible foliar injury resulting from exposure to O3 has
been well characterized and documented over several decades of research
on many tree, shrub, herbaceous and crop species'' and ``[o]zone-
induced visible foliar injury symptoms on certain bioindicator plant
species are considered diagnostic as they have been verified
experimentally'' (U.S. EPA, 2013, p. 9-41). Further, a recent study
highlighted in the ISA, which analyzed trends in the incidence and
severity of foliar injury, reported a declining trend in the incidence
of foliar injury as peak O3 concentrations declined (U.S.
EPA, 2013, p. 9-40; Smith, 2012). Another study available in this
review that focused on O3-induced visible foliar injury in
forests of west coast states observed that both percentage of biosites
with injury and average biosite index were higher for sites with
average cumulative O3 concentrations above 25 ppm-hrs in
terms of SUM06 (may correspond to W126 of approximately 21 ppm-hrs
[U.S. EPA, 2007, p. 8-26, Appendix 7B]) as compared to groups of sites
with lower average cumulative exposure concentrations, with much less
clear differences between the two lower exposure groups (Campbell et
al., 2007, Figures 27 and 28 and p. 30). A similar finding was reported
in the 2007 Staff Paper which reported on an analysis that showed a
smaller percentage of injured sites among the group of sites with
O3 exposures below a SUM06 metric of 15 ppm-hrs or a fourth-
high metric of 74 ppb as compared to larger groups that also included
sites with SUM06 values up to 25 ppm-hrs or fourth-high metric up to 84
ppb, respectively (U.S. EPA 2007, pp. 7-63 to 7-64).
With regard to the comments referencing the WREA cumulative
analysis of USFS FHM/FIA biosite data or related CASAC comments, we
note some clarification of this analysis. This analysis does not show,
as implied by the comments, that at W126 index values above 10 ppm-hrs,
there is little change with increasing W126 index in
[[Page 65396]]
the proportion of records with any visible foliar injury (biosite index
above 0). As the analysis is a cumulative analysis, each point graphed
in the analysis includes the records for the same and lower W126 index
values, so the analysis does not compare results for groups of records
with differing, non-overlapping W126 index values. Rather, the points
represent groups with records (and W126 index values) in common and the
number of records in the groups is greater for higher W126 index values
(U.S. EPA, 2014b, section 7.2). Additionally, we note that the pattern
observed in the cumulative analysis is substantially influenced by the
large number of records for which the W126 index estimates are at or
below 11 ppm-hrs, more than two thirds of the dataset (Smith and
Murphy, 2015, Table 1).
To more fully address the comments related to this WREA analysis,
we have drawn several additional observations from the WREA dataset,
re-presenting the same data in a different format in a technical
memorandum to the docket (Smith and Murphy, 2015). Contrary to the
implication of the statements from the commenters and CASAC that no
reduction in the occurrence of visible foliar injury can be achieved
with exposures above 10 ppm-hrs, both the proportion of records with
injury and the average biosite index are lower for groups of records
with W126 index estimates at or below 17 ppm-hrs compared to the group
for the highest W126 index range. This is true when considered
regardless of soil moisture conditions (all records), as well as for
dry, normal and wet records, separately (Smith and Murphy, 2015, Table
2). The pattern of the two measures across record groups with lower
W126 index values differs with moisture level, with the wetter than
normal records generally showing decreasing proportions of injured
sites and decreasing average biosite index with lower W126 index
values, while little difference in these measures is seen among the
middle W126 values although they are lower than the highest W126 index
group and higher than the lowest W126 index group (Smith and Murphy,
2015, Table 2). In summary, the EPA disagrees with commenters, noting
that the available information, including additional observations from
the WREA dataset, indicate declines in the occurrence of visible foliar
injury across decreasing W126 index values that are higher than 10 ppm-
hrs.
c. Use of W126 Metric in Evaluating Vegetation Effects and Public
Welfare Protection
In considering public comments received on the EPA's use of the
W126 exposure index in its decision on a revised secondary standard,
the EPA first notes related advice and comments from the CASAC received
during development of the PA. Although we recognize that CASAC's
comments on the W126 index were provided in the context of its
recommendation for a secondary standard of that form, we find them to
also relate to our use of the W126 metric in evaluating the magnitude
and extent of vegetation effects that might be expected and conversely
the level of protection that might be provided under different air
quality conditions. In comments on the first draft PA, the CASAC stated
that ``discussions and conclusions on biologically relevant exposure
metrics are clear and compelling and the focus on the W126 form is
appropriate'' (Frey and Samet, 2012a). With regard to specific aspects
of the W126 index, the CASAC concurred with the second draft PA focus
on ``the biologically-relevant W126 index accumulated over a 12-hour
period (8 a.m.-8 p.m.) over the 3-month summation period of a single
year resulting in the maximum value of W126'' (Frey, 2014c, p. iii).
The CASAC advice on levels of the W126 index on which to focus for
public welfare protection recommended a level within the range of 7
ppm-hrs to 15 ppm-hrs (Frey, 2014c, p. iii). We note, however, as
summarized in section IV.E.3 of the proposal, that this advice was
provided in the context of the CASAC review of the second draft PA,
which concluded that a range from 7 to 17 ppm-hrs was appropriate to
consider. In considering the upper end of this range, the CASAC
consulted Table 6-1 of the second draft PA which indicated for a W126
index value of 17 ppm-hrs an RBL estimate of 6%, a magnitude that CASAC
described as ``unacceptably high'' and that contributed to a lack CASAC
support for W126 exposures values higher than 15 ppm-hrs (Frey, 2014c,
p. 14; U.S. EPA 2014d, Table 6-1). As noted in section IV.E.3 of the
proposal, revisions to the RBL estimate table in the final PA, which
were made in consideration of other CASAC comments, have resulted in
changes to the median species RBL estimate associated with each W126
index value, such that the median species RBL estimate for a W126 index
value of 17 ppm-hrs in this table in the final PA was 5.3%, rather than
the ``unacceptably high'' value of 6% (U.S. EPA, 2014c, Table 6-1; U.S.
EPA, 2014d, Table 6-1; Frey, 2014c, p. 14).\201\ Additionally, the
CASAC recognized that the Administrator may, as a policy matter, prefer
to use a 3-year average, and stated that in that case, the range of
levels should be revised downward (Frey, 2014c, p. iii-iv).
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\201\ We additionally note that the median species RBL estimate
for 17 ppm-hrs in the final PA is nearly identical to the estimate
for 15 ppm-hrs (the value corresponding to the upper end of the
CASAC-identified range) that was in the second draft PA (5.2%) which
was the subject of the CASAC review (U.S. EPA, 2014c, Table 6-1;
U.S. EPA, 2014d, Table 6-1).
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The majority of comments on the W126 index concurred with its use
for assessing O3 exposures, while some commenters
additionally expressed the view that this index should be used as the
form of the secondary standard (as discussed in section IV.C.2.d
below). Most submissions from state and local environmental agencies or
governments, as well as organizations of state agencies, that provided
comments on the magnitude of cumulative exposure, in terms of the W126
index, appropriate to consider for a revised secondary standard,
recommended that the EPA focus on an index value within the EPA's
proposed range of 13 to 17 ppm-hrs, as did the industry commenters.
These commenters variously noted their agreement with the rationale
provided by the EPA in the proposal or cited to CASAC comments,
including for a downward adjustment of its recommended values if a 3-
year average W126 was used rather than a single year index. Some other
commenters, including two groups of environmental organizations,
submitted comments recommending a focus on a W126 index level as low as
7 ppm-hrs based on reasons generally focused on consideration of
visible foliar injury.
Some aspects of these comments have been addressed in sections
IV.C.2.a and IV.C.2.b above. In the Response to Comments document, we
have additionally addressed other comments that recommend a focus on
W126 index values for specific reasons other than generally citing the
CASAC recommended range. Further, in her consideration of a target
level of protection for the revised secondary standard in section
IV.C.3 below, the Administrator has considered comments from the CASAC
regarding the basis for their recommended range.
An additional comment from an organization of western state air
quality managers indicated a concern with the use of W126 for
vegetation in arid and high altitude regions, such as those in the
western states, which the
[[Page 65397]]
commenter hypothesized may have reduced sensitivity. The commenters did
not provide evidence of this hypothesis, calling for further research
in order to characterize the sensitivity of vegetation in such areas.
The EPA agrees that additional research would be useful in more
completely characterizing the response of species in such areas, as
well as other less well studied areas, but does not find support in the
currently available evidence for the commenter's suggestion that
species in arid and high altitude regions may be less sensitive than
those in other areas.\202\
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\202\ For example, we note that among the 11 species for which
robust E-R functions have been established for O3 effects
on tree seedling growth, the sensitivity of ponderosa pine, a
species occurring in arid and high altitude regions of the western
U.S., is similar to the median (U.S. EPA, 2014c, Table 5C-1).
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Among the small number of commenters recommending against using the
W126 metric to assess O3 exposure, a few expressed the view
that some other, not-yet-identified cumulative exposure metric should
be used. These commenters cited a variety of concerns that they state
are not addressed by the W126 index: that plant exposure to and uptake
of O3 are not always equivalent because of variations in
stomatal conductance and plant defenses and their respective diel
patterns, which will also influence plant response; that the duration
between harmful O3 exposures affects the plant's ability to
repair damage; and, that night-time exposures may be important. These
commenters do not identify an alternative to the W126 index that they
conclude to better represent exposures relevant to considering
O3 effects on vegetation and particularly for growth
effects. The EPA has considered the items raised by these commenters,
recognizing some as areas of uncertainty (U.S. EPA, 2013, pp. 9-109 to
9-113), yet has concluded that based on the information available at
this time, exposure indices that cumulate and differentially weight the
higher hourly average concentrations while also including the ``mid-
level'' values offer the most appropriate approach for use in
developing response functions and comparing studies of O3
effects on vegetation (U.S. EPA, 2013, p. 9-117). When considering the
response of vegetation to O3 exposures represented by the
threshold (e.g., SUM06) and non-threshold (e.g., W126) indices, the ISA
notes that ``the W126 metric does not have a cut-off in the weighting
scheme as does SUM06 and thus it includes consideration of potentially
damaging exposures below 60 ppb'' and that ``[t]he W126 metric also
adds increasing weight to hourly concentrations from about 40 ppb to
about 100 ppb'' (U.S. EPA, 2013, p. 9-104). This aspect of W126 is one
way it differs from cut-off metrics such as the SUM06 where all
concentrations above 60 ppb are treated equally and is identified by
the ISA as ``an important feature of the W126 since as hourly
concentrations become higher, they become increasingly likely to
overwhelm plant defenses and are known to be more detrimental to
vegetation'' (U.S. EPA, 2013, p. 9-104). Further, we note the
concurrence by CASAC with the EPA's focus on the W126 exposure index,
as noted above.
Some commenters also raised concerns regarding the sensitivity of
vegetation in desert areas where plants take in ambient air during
nighttime rather than daylight hours, such that little exposure occurs
from 8 a.m. to 8 p.m., stating that the W126 index as defined by the
EPA to cumulate hourly O3 from 8 a.m. to 8 p.m. may result
in an overly stringent exposure level in areas with such vegetation.
The EPA recognizes that plants, such as cacti, that commonly occur in
desert systems exhibit a particular type of metabolism (referred to as
CAM photosynthesis) such that they only open their stomata at night
(U.S. EPA, 2013, p. 9-109). We note, however, that few if any
O3 exposure studies of these species are available \203\ to
further inform our characterization of these species' responses to
O3, and we have no basis on which to conclude that an
exposure level based on the studied species and a daylight exposure
metric would be overly or underly stringent in areas where only species
utilizing CAM photosynthesis occur. As summarized above, the CASAC
advice concurred with the use of an 8am to 8pm diurnal period for the
W126 exposure index. Thus, we conclude that for our purposes in this
review the focus on daylight hours is appropriate. Our use of the W126
index in this review has been for purposes of characterizing the
potential harm and conversely the potential protection that might be
afforded from the well-characterized effects of O3 on
vegetation, while recognizing associated uncertainties and limitations.
We note that different ecosystems across the U.S. will be expected to
be of varying sensitivities with regard to the effects of
O3. For example, large water bodies without vegetation
extending above the water's surface would be expected to be less
sensitive than forests of sensitive species. The EPA notes, however,
that the NAAQS are set with applicability to all ambient air in the
U.S., such that the secondary O3 standard provides
protection in areas across the U.S. regardless of site-specific aspects
of vegetation sensitivity to O3. In considering the evidence
on O3 and associated welfare effects, we recognize
variability in sensitivity that may relate to a number of factors, as
discussed in the ISA (U.S. EPA, 2013, section 9.4.8). This variability
is among the Administrator's considerations in setting the secondary
standard for O3 that is requisite to protect public welfare
against anticipated or known adverse effects.
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\203\ No O3 exposure studies on cacti or other
species that utilize CAM photosynthesis are reported in the ISA
(U.S. EPA, 2013).
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Further, some commenters who agreed with a focus on the W126
exposure index also stated that the EPA's definition of the index for
the daylight hours of 8 a.m. to 8 p.m. and a 3-month period was not
appropriate, stating that derivation of the W126 metric should involve
summing concentrations for all 24 hours in each day and all months in
each year to avoid underestimating O3 exposure that the
commenters viewed as pertinent. Support for the EPA's definition of the
W126 index, with which CASAC concurred (Frey, 2014c, p. iii), is based
on the assessment of the evidence in the ISA (U.S. 2013, section
9.5.3.2) and the context for use of the W126 index in relating
O3 exposure to magnitude and/or extent of O3
response. This context has a particular focus on growth effects for the
purposes of judging the potential for public welfare impacts, as well
as the level of protection, associated with different exposure
circumstances. We note that the ISA stated there is a lack of
information that would allow consideration of the extent to which
nocturnal exposures that may be of interest occur (U.S. EPA, 2013, p.
9-109). Additionally, in our use of the W126 index, we are relying on
E-R functions based on studies that were generally of 3-month duration
and involved controlled exposures during the daylight period.
Accordingly we have relied on the E-R function derived for 12-hour and
3-month W126 indices, as described in section IV.A.1 above. To apply
these E-R functions to the W126 estimates derived using 24 hours-per-
day index values would inaccurately represent the response observed in
the study (producing an overestimate). Similarly, with regard to the 3-
month duration, ``[d]espite the possibility that plants may be exposed
to ambient O3 longer than 3 months in some locations, there
is generally a lack of exposure experiments conducted for longer than
[[Page 65398]]
3 months'' (U.S. EPA, 2014c, p. 9-112). Thus, in consideration of the
lack of support in the current evidence for characterizing exposure for
purposes of estimating RBL based on cumulative exposures derived from a
combination of daytime and nighttime exposures and consideration of
year-round O3 concentrations across the U.S., we disagree
with the commenters' view of the appropriateness of using an exposure
index based on 24-hour, year-round O3 concentrations.
The commenters supporting the use of the W126 exposure index were
divided with regard to whether the EPA should focus on an annual index
or one averaged over three years. Some of the commenters indicating
support for the EPA's proposed focus on a 3-year average W126 index
stated that this was appropriate in light of the wide variations in
W126 index values that can occur on a year-to-year basis as a result of
the natural variation of climatic conditions that have a direct impact
on O3 formation; in their view, these factors are mitigated
by use of a 3-yr average, which thus provides ``stability'' in the
assessment dampening out the natural variation of climatic conditions
that have a direct impact on O3 formation. Others noted that
use of a 3-year average may be supported as matter of policy. We
generally concur with the relevance of these points, among others, to a
focus on the 3-year average W126. Other commenters expressed the view
that the EPA should focus on an annual W126 index, generally making
these comments in the context of expressing their support for a
secondary standard with a W126 form. These commenters variously cited
CASAC advice and its rationale for preferring a single year W126 form,
stated that vegetation damage occurs on an annual basis, and/or
questioned the EPA's statements of greater confidence in conclusions as
to O3 impacts based on a 3-year average exposure metric.
The EPA agrees with commenters that, as discussed in the PA and the
proposal, depending on the exposure conditions, O3 can
contribute to measurable effects on vegetation in a single year. We
additionally recognize that, as described in the PA and proposal, there
is generally a greater significance for effects associated with
multiple-year exposures. The proposal described a number of
considerations raised in the PA as influencing the Administrator's
decision to focus on a 3-year average W126 index (79 FR 75347, December
17, 2014). These included, among others, the observation of a greater
significance for effects associated with multiple-year exposures, and
the uncertainties associated with consideration of annual effects
relative to multiple-year effects.
Further, we note that among the judgments contributing to the
Administrator's decision on the level of protection appropriate for the
secondary standard are judgments regarding the weight to place on the
evidence of specific vegetation-related effects estimated to result
across a range of cumulative seasonal concentration-weighted
O3 exposures and judgments on the extent to which such
effects in such areas may be considered adverse to public welfare (79
FR 75312, December 17, 2014). Thus, conclusions regarding the extent to
which the size and/or prevalence of effects on vegetation in a single
year and any ramifications for future years represent an adverse effect
to the public welfare, conclusions that are also inherently linked to
overall magnitudes of exposures, are dependent on the Administrator's
judgment. Accordingly, the decision regarding the need to focus on a 1-
year or 3-year W126 index value is also a judgment of the
Administrator, informed by the evidence, staff evaluations and advice
from CASAC, as described in section IV.C.3 below.
d. Form and Averaging Time
In considering comments received on the proposed form for the
revised standard, the EPA first notes the advice and comments from the
CASAC, received in its review of the second draft PA. Similar to its
advice in the last review, the CASAC recommended ``establishing a
revised form of the secondary standard to be the biologically relevant
W126 index'' (Frey, 2014c, p. iii). With regard to its reasons for this
view, the CASAC cites the PA in stating that it ``concurs with the
justification in [section 5.7] that the form of the standard should be
changed from the current 8-hr form to the cumulative W126 index''
(Frey, 2014c, p. 12). In addressing specific aspects of this index, the
CASAC concurred with the EPA's focus on the 3-month period with the
highest index value and further states that ``[a]ccumulation over the
08:00 a.m.-08:00 p.m. daytime 12-hour period is a scientifically
acceptable and recommended means of generalizing across latitudes and
seasons'' (Frey, 2014c, p. 13). As section 5.7 of the PA discusses the
W126 index in the context of the support in the evidence for use of the
W126 exposure index for assessing impacts of O3 on
vegetation and the extent of protection from such impacts, we interpret
CASAC's statement on this point to indicate that the basis for CASAC's
view with regard to the form for the secondary standard relates to the
appropriateness of the W126 exposure index for those assessment
purposes.\204\ \205\
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\204\ Section 5.7 of the PA states that ``the evidence continues
to provide a strong basis for concluding that it is appropriate to
judge impacts of O3 on vegetation, related effects and
services, and the level of public welfare protection achieved, using
a cumulative, seasonal exposure metric, such as the W126-based
metric,'' references the support of CASAC for a W126-based secondary
standard, and then concludes that ``based on the consistent and
well-established evidence described above, . . . the most
appropriate and biologically relevant way to relate O3
exposure to plant growth, and to determine what would be adequate
protection for public welfare effects attributable to the presence
of O3 in the ambient air, is to characterize exposures in
terms of a cumulative seasonal form, and in particular the W126
metric'' (U.S. EPA, 2014c, p. 5-78).
\205\ The CASAC also mentioned its support for revising the
secondary standard to a W126 index-based form in its review of
Chapter 6 of the second draft PA (Frey, 2014c, p. 13). Similar to
section 5.7, in that chapter of the PA staff concluded that
``specific features associated with the W126 index still make it the
most appropriate and biologically relevant cumulative concentration-
weighted form for use in the context of the secondary O3 NAAQS
review'' (U.S. EPA, 2014c, p. 6-5) and also concluded that ``it is
appropriate to consider a revised secondary standard in terms of the
cumulative, seasonal, concentration-weighted form, the W126 index''
(U.S. EPA, 2014c, p. 6-57).
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The public comments on the form for a revised secondary standard
were divided. Most of the state and local environmental agencies or
governments, and all of the tribal agencies and organizations that
provided comments on the form for the secondary standard concurred with
the EPA's proposed decision, as did the industry commenters. These
commenters generally indicated agreement with the rationale provided in
the proposal that drew from the EPA analyses of recent air quality data
examining relationships at sites across the U.S. between values of the
fourth-high metric (the current design value) and values of a 3-year
average W126-based metric, stating that this analysis showed that a
standard in the form of the fourth-high metric, as proposed, can
provide air quality consistent with or below the range of 3-year W126
exposure index values identified in the proposal. Some commenters
additionally stated that the choice of form was a policy decision for
the EPA and that little or no additional protection of public welfare
would be gained by adopting a W126-based form. Some of these commenters
provided analyses of data for their state or region that further
supported this view. As
[[Page 65399]]
described in section IV.C.3 below, the EPA generally agrees with these
commenters.
Some commenters, including a regional organization of state
agencies and two groups of environmental organizations, submitted
comments recommending revision of the standard to a cumulative,
seasonal form based on the W126 index. In support of their position,
these commenters generally cited CASAC advice, variously additionally
indicating their view that the standard form should be a metric
described as biologically relevant, and that the existing form, with a
level in the proposed range, would not provide adequate ecosystem
protection. Some commenters additionally suggested that the EPA cannot
lawfully retain the form and averaging time that were initially
established for purposes of the primary standard when the EPA has
identified the W126 index as a metric appropriate for judging
vegetation-related effects on public welfare. With regard to the EPA
air quality analyses, summarized in the proposal, of the W126 index
values at sites where O3 concentrations met different levels
of fourth-high metric, some of these commenters stated that the
analyses showed widespread variation in W126 values for each fourth-
high metric examined. Further, some commenters disagreed with the EPA
that the analyses indicated that a revised standard level within the
proposed range would be expected to limit W126 exposures in the future
to the extent suggested by the analyses of data from the past.
We agree with public commenters and CASAC regarding the
appropriateness of the W126 index (the sum of hourly concentrations
over a specified period) as a biologically relevant metric for
assessing exposures of concern for vegetation-related public welfare
effects, as discussed in the proposal, PA and ISA. Accordingly, we
agree that this metric is appropriate for use in considering the
protection that might be expected to be afforded by potential
alternative secondary standards, as discussed in section IV.C.2.c
above. We disagree with commenters, however, that use of the W126
metric for this purpose dictates that we must establish a secondary
standard with a W126 index form.
In support of this position, we note the common use, in assessments
conducted for NAAQS reviews, of exposure metrics that differ in a
variety of ways from the ambient air concentration metrics of those
standards.\206\ Across reviews for the various NAAQS pollutants, we
have used a variety of exposure metrics to evaluate the protection
afforded by the standards. These exposure metrics are based on the
health or welfare effects evidence for the specific pollutant and
commonly, in assessments for primary standards, on established
exposure-response relationships or health-based benchmarks (doses or
exposures of concern) for effects associated with specific exposure
circumstances. Some examples of exposure metrics used to evaluate
health impacts in primary standard reviews include the concentration of
lead in blood of young children and a 5-minute exposure concentration
for sulfur dioxide. In contrast, the health-based standards for these
two pollutants are the 3-month concentration of lead in total suspended
particles and the average across three years of the 99th percentile of
1-hour daily maximum concentration of sulfur dioxide in ambient air,
respectively (73 FR 66964, November 12, 2008; 75 FR 35520, June 22,
2010). In somewhat similar manner, in the 2012 PM review, the EPA
assessed the extent to which the existing 24-hour secondary standard
for PM2.5, expressed as a 24-hour concentration (of
PM2.5 mass per cubic meter of air) not to be exceeded more
than once per year on average over three years, could provide the
desired protection from effects on visibility in terms of the 90th
percentile, 24-hour average PM2.5 light extinction, averaged
over three years, based on speciated PM2.5 mass
concentrations and relative humidity data (79 FR 3086, January 15,
2013). Additionally, in the case of the screening-level risk analyses
in the 2008 review of the secondary standard for lead, concentrations
of lead in soil, surface water and sediment were evaluated to assess
the potential for welfare effects related to lead deposition from air,
while the standard is expressed in terms of the concentration of lead
in particles suspended in air (73 FR 67009, November 12, 2008).
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\206\ The term design value is commonly used to refer to the
metric for the standard. Consistent with the summary in section I.D
above, a design value is the statistic that describes the air
quality of a given location in terms of the indicator, form and
averaging time of the standard such that it can then be compared to
the level of the standard.
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Further, depending on the evidence base, some NAAQS reviews may
consider multiple exposure metrics in assessing risks associated with a
particular pollutant in ambient air in order to judge the adequacy of
an existing standard in providing the required level of protection. And
a standard with an averaging time of one duration may provide
protection against effects elicited by exposures of appreciably shorter
or longer durations. For example, in the current review of the primary
O3 standard, as described in section II above, we have
considered the potential for effects associated with both short- and
long-term exposures and concluded, based on a combination of air
quality and risk analyses and the health effects evidence, that the
existing standard with its short (8-hour) averaging time provides
control of both the long and short term exposures (e.g., from one hour
to months or years) that may be of concern to public health. Similarly,
during the 1996 review of the NO2 primary standard, while
health effects were recognized to result from both long-term and short-
term exposures to NO2, the primary standard, which was a
long-term (annual) standard, was concluded to provide the requisite
protection against both long- and short-term exposures (61 FR 52852,
Oct 8 1996). In the subsequent review of the NO2 primary
standard in which the available air quality information indicated that
the annual standard was not providing the needed control of the shorter
term exposures, an additional short-term standard was established (75
FR 6474, February 9, 2010).
Thus, we note that different metrics may logically, reasonably, and
for technically sound reasons, be used in assessing exposures of
concern or characterizing risk as compared to the metric of the
standard which is used to control air quality to provide the desired
degree of protection. That is, exposure metrics are used to assess the
likely occurrence and/or frequency and extent of effects under
different air quality conditions, while the air quality standards are
intended to control air quality to the extent requisite to protect from
the occurrence of public health or welfare effects judged to be
adverse. In this review of the secondary standard for O3,
the EPA agrees that, for the reasons summarized in section IV.A.1 above
and described in the ISA, the W126 index--and not an 8-hour daily
maximum concentration that has relevance in human health risk
characterization, as described in section II above--is the appropriate
metric for assessing exposures of concern for vegetation,
characterizing risk to public welfare, and evaluating what air quality
conditions might provide the desired degree of public welfare
protection. We disagree, however, that the secondary standard must be
established using that same metric.
Moreover, we note that the CAA does not require that the secondary
O3 standard be established in a specific
[[Page 65400]]
form. Section 109(b)(2) provides only that any secondary NAAQS ``shall
specify a level of air quality the attainment and maintenance of which
in the judgment of the Administrator, based on [the air quality]
criteria, is requisite to protect the public welfare from any known or
anticipated adverse effects associated with the presence of such air
pollutant in the ambient air. . . . [S]econdary standards may be
revised in the same manner as promulgated.'' The EPA interprets this
provision to leave it considerable discretion to determine whether a
particular form is appropriate, in combination with the other aspects
of the standard (averaging time, level and indicator), for specifying
the air quality that provides the requisite protection, and to
determine whether, once a standard has been established in a particular
form, that form must be revised. Moreover, nothing in the Act or the
relevant case law precludes the EPA from establishing a secondary
standard equivalent to the primary standard in some or all respects, as
long as the Agency has engaged in reasoned decision-making.\207\
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\207\ In fact, the D.C. Circuit has upheld secondary NAAQS that
were identical to the corresponding primary standard for the
pollutant (e.g., ATA III, 283 F.3d at 375, 380 [D.C. Cir. 2002,
upholding secondary standards for PM2.5 and O3
that were identical to primary standards]).
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With regard to the commenter's emphasis on advice from CASAC on the
form of the secondary standard, the EPA agrees with the importance of
giving such advice careful consideration. The EPA further notes,
however, that the Administrator is not legally precluded from departing
from CASAC's recommendations, when she has provided an explanation of
the reasons for such differences.\208\ Accordingly, in reaching
conclusions on the revised secondary standard in this review, the
Administrator has given careful consideration to the CASAC advice in
this review and, when she has differed from CASAC recommendations, she
has fully explained the reasons and judgments that led her to a
different conclusion, as described in section IV.C.3 below.
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\208\ See CAA sections 307(d)(3) and 307(d)(6)(A); see also
Mississippi v. EPA, 744 F.3d 1334, 1354 (D.C. Cir. 2013) (``Although
EPA is not bound by CASAC's recommendations, it must fully explain
its reasons for any departure from them'').
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In disagreeing with the EPA's conclusions drawn from analyses of
recent air quality data on the extent to which cumulative seasonal
exposures might be limited to within or below the identified 3-year
average W126 index values by controlling air quality using different
values for the fourth-high metric, one group of environmental
organizations emphasized the range of W126 index values that occur at
monitors with concentrations at or below specific values for the
fourth-high metric. For monitor observations for which the fourth-high
metric was at or below 70 ppb, this commenter group stated that some
sites have 3-year average W126 index values above 17 ppm-hrs and noted
a maximum 3-year W126 index value of 19.1 ppm-hrs, while additionally
noting occurrences of other W126 values above the CASAC range of 7 to
15 ppm-hrs. This commenter additionally stated that the air quality
data ``do not support a claim of congruence'' between the fourth-high
and W126 metrics (e.g., ALA et al., p. 196), that there is no basis for
concluding that there is some fundamental underlying relationship that
assures meeting the fourth-high metric will mean meeting any of the
W126 options, and that the relationship between the metrics is non-
linear with significant spread in the data (citing visual inspection of
a graph).
The EPA does not agree with the commenter's statements regarding
the relationship between the two metrics.\209\ We have not, as stated
by the commenter, claimed there to be ``congruence'' between the two
metrics (e.g., ALA et al., p. 196), or that the two metrics coincide
exactly. Rather, at any location, values of both metrics are a
reflection of the temporal distribution of hourly O3
concentrations across the year and both vary in response to changes in
that distribution. While the EPA's air quality analysis shows that the
specific relationship differs among individual sites, it documents an
overall strong, positive, non-linear relationship between the two
metrics (Wells, 2014a, p. 6, Figures 5a and 5b; Wells, 2015b). Further,
this analysis finds the amount of year-to-year variability in the two
metrics tended to decrease over time with decreasing O3
concentrations, especially for the W126 metric, as described in section
IV.E.4 of the proposal (Wells, 2014a; Wells, 2015b).
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\209\ The EPA additionally notes that commenters contradict
their own assertion when, after stating their view that no
relationship exists between the 4th high and W126 metrics, the
commenter then states that there is a nonlinear relationship and yet
then relies on a predicted linear relationship to estimate W126
values occurring when air quality meets different values for the 4th
high metric at 11 national parks.
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With regard to the highest 3-year average W126 exposure index
values that might reasonably be expected in the future in areas where a
revised standard with a fourth-high form is met, we disagree with the
commenters as to the significance of the W126 index value of 19.1 ppm-
hrs in the 13-year dataset. This value, for a site during the period
2006-2008, is the only occurrence at or above 19 ppm-hrs in the nearly
4000 3-year W126 index values--across the 11 3-year periods extending
back in time from 2013--for which the fourth-high metric for the same
monitor location is at or below 70 ppb. This is clearly an isolated
occurrence.
In considering this comment, we have expanded the technical
memorandum that was available at the time of proposal (Wells, 2014a).
The expanded memorandum describes the same air quality analyses for 3-
year periods from 2001 through 2013 as the 2014 memorandum, and
includes additional summary tables for all 3-year periods from 2001
through 2013 as well as tables for the most recent period, 2011-2013
(Wells, 2015b). After the 3-year W126 index value of 19 ppm-hrs, the
next three highest 3-year average W126 index values, which are the only
other such values above 17 ppm-hrs in the 13-year dataset, and which
also occur during periods in the past, round to 18 ppm-hrs (Wells,
2015b). Additionally, we note that reductions in the fourth-high metric
over the 13-year period analyzed are strongly associated with
reductions in the cumulative W126 index (Wells, 2014a, Figure 11, Table
6; Wells, 2015b). Specifically, the regression analysis of changes in
W126 index between the 2001-2003 period and the 2011-2013 period with
changes in the fourth-high metric across the same periods indicates a
fairly linear and positive relationship between reductions of the two
types of metrics, with, on average, a change of approximately 0.7 ppm-
hr in the W126 index per ppb change in the fourth-high metric value.
From this information we conclude that W126 exposures above 17 ppm-hrs
at sites for which the fourth-high metric is at or below 70 ppb would
be expected to continue to be rare in the future, particularly as steps
are taken to meet a 70 ppb standard.
With regard to the comment that the relationship between the two
metrics varies across locations, the EPA agrees that there is variation
in cumulative seasonal O3 exposure (in terms of a 3-year
average W126 index) among locations that are at or below the same
fourth-high metric. As noted in the proposal, the analysis illustrates
this variation, with the locations in the West and Southwest NOAA
climatic regions tending to have the highest cumulative seasonal
exposures for the same fourth-high metric value. In considering
expectations for the future in light of this observation, however, we
note that
[[Page 65401]]
the regional regressions of reductions in W126 metric with reductions
in the fourth-high metric indicate that the Southwest and West regions,
which had the greatest potential for sites having 3-year W126 index
values greater than the various W126 values of interest when fourth-
high values are less than or equal to the various fourth-high metric
values of interest, also exhibited the greatest reduction in the W126
index values per unit reduction in the fourth-high values (Wells,
2015b). Thus, in considering the potential for occurrences of values
above 17 ppm-hrs in the future in areas that meet a fourth-high of 70
ppb, the EPA notes that the analysis indicates that those areas that
exhibited the greatest likelihood of occurrence of a 3-year W126 index
above a level of interest (e.g., the commenters' example in the
Southwest region of a value of 19.1 ppm-hrs [2006-2008] in comparison
to the W126 level of 17 ppm-hrs) also exhibit the greatest improvement
in W126 per unit decrease in fourth-high metric.\210\ It is expected
that future control programs designed to meet a standard with a fourth-
high form would provide similar improvements in terms of the W126
metric.
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\210\ Additionally, O3 levels at any location are
influenced by upwind precursor emissions, and many rural areas,
including the site referenced by the commenter, are impacted by
precursor emissions from upwind urban areas, such that as emissions
are reduced to meet a revised standard in the upwind locations,
reductions in those upwind emissions will contribute to reductions
at the downwind sites (Wells, 2014a; ISA, pp. 3-129 to 3-133).
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As part of their rationale in support of revising the current form
and averaging time, one commenter pointed to the regional variation in
the highest W126 index values expected at sites that just meet a
fourth-high metric of 70 ppb, based on the EPA's analysis of recent air
quality data available at the time of the proposal (Wells, 2014a). This
commenter observed that, while in some U.S. regions, locations that
meet a potential alternative standard with the current form and a level
of 70 ppb also have 3-year average W126 index values no higher than 17
ppm-hrs, the highest W126 index values in other parts of the country
are lower. As a result, the commenter concluded that such a standard
would result in regionally differing levels of welfare protection. The
commenter additionally states that, for extreme values, a W126 form for
the secondary standard would also offer different levels of protection,
although with the primary standard setting the upper boundary for such
values.
The EPA recognizes that a standard with the current form might be
expected to result in regionally differing distributions of W126
exposure index values (including different maximum values) depending on
precursor sources, local meteorology, and patterns of O3
formation. Variation in exposures is to be expected with any standard
(secondary or primary) of any form. In fact, variation in exposures and
any associated variation in welfare or health risk is generally an
inherent aspect of the Administrator's judgment on a specific standard,
and any associated variation in welfare or health protection may play a
role in the Administrator's judgment with regard to public welfare or
public health protection objectives for a national standard. In
considering the comment, however, we have focused only on the extent to
which the commenter's conclusion that a secondary standard of the
current form and averaging time would provide regionally varying
welfare protection might indicate that the specified air quality is
more (or less) than necessary to achieve the purposes of the standard.
In so doing, we additionally respond to a separate comment that the EPA
needs to address how the revised secondary standard is neither more or
less than necessary to protect the public welfare.
The CAA requirement in establishing a standard is that it be set at
a level of air quality that is requisite, meaning ``sufficient, but not
more than necessary'' (Whitman v. American Trucking Ass'ns, 531 U.S.
457, 473 [2001]). We note that the air quality that is specified by the
revised primary standard has been concluded to be ``necessary'' and it
may be reasonable and appropriate to consider the stringency of the
secondary standard in light of what is identified as ``necessary'' for
the primary standard. The EPA considered the stringency of the
O3 secondary standard in this way in the 1979 decision (44
FR 8211, February 8, 1979), which was upheld in subsequent litigation
(API v Costle, 665 F.2d 1176 [D.C. Cir. 1991]). We note that, in
similar manner, the commenter considered public welfare protection that
might be afforded by the primary standard in noting that the primary
standard would be expected to provide welfare protection from extreme
values.\211\
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\211\ As described earlier in this section, the EPA has also
considered the air quality specified by one secondary standard in a
decision on the need for a second secondary standard. In the
decision not to adopt a second PM2.5 secondary standard
specific to visibility-related welfare effects, the Administrator,
after describing the public welfare protection objective related to
visibility effects, considered analyses that related air quality
associated with the existing secondary standard to that expected for
the proposed visibility-focused secondary standard. From these
analyses, she concluded sufficient protection against visibility
effects would be provided by the existing standard, and to the
extent that the existing standard would provide more protection than
had been her objective for such effects, adoption of a second
secondary standard focused on visibility would not change that
result (78 FR 3227-3228, January 15, 2013). This decision responded
to a court remand of the prior EPA decision that visibility
protection would be afforded by a secondary standard set equal to
the primary standard based on the court's conclusion that the EPA
had not adequately described the Administrator's objectives for
visibility-related public welfare protection under the standard
(American Farm Bureau, 559 F.3d at 530-531).
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In addressing the remand of the 2008 secondary standard in this
rulemaking, as discussed in section IV.C.2.e below, the EPA recognizes
that it must explain the basis for concluding that the standard
selected by the Administrator specifies air quality that will provide
the degree of public welfare protection needed from the secondary
standard (Mississippi v. EPA, 744 F.3d 1334, 1360-61 [D.C. Cir. 2013]).
In this review, the Administrator describes the degree or level of
public welfare protection needed from the secondary standard and fully
explains the basis for concluding that the standard selected specifies
air quality that will provide that degree of protection. If the
Administrator concludes that the level of air quality specified by the
primary standard would provide sufficient protection against known or
anticipated adverse public welfare effects, the EPA believes that a
secondary standard with that indicator, level, form and averaging time
could be considered to be requisite. If the level of air quality that
areas will need to achieve or maintain for purposes of the primary
standard also provides a level of air quality that is adequate to
provide the level of protection identified for the secondary standard,
there would be little purpose in requiring the EPA to establish a less
stringent secondary standard. For these reasons, the expectation of
regionally differing cumulative exposures under a secondary standard of
the current form and averaging time does not lead us to conclude that
the air quality specified by such a standard would be more (or less)
than necessary (and thus not requisite) for the desired level of public
welfare protection.
e. Revisions to the Standard Level
Some comments specifically addressed the level for a revised
secondary standard of the current form and averaging time. Of the
comments that addressed this, some from states or industry groups
generally supported a level within the proposed range, frequently
specifying the upper end of the range (70 ppb), while comments
[[Page 65402]]
from tribes and tribal organizations, and a few others, recommended a
level no higher than 65 ppb. The Administrator has considered such
comments in reaching her decision on the appropriate revisions to the
standard, described in section IV.C.3. Detailed aspects of these
comments are discussed in the Response to Comments document.
f. 2013 Court Remand and Levels of Protection
Both industry groups and a group of environmental advocacy
organizations submitted comments on the extent to which the proposal
addressed the July 2013 remand of the secondary standard by the U.S.
Court of Appeals for the D.C. Circuit. The former generally concluded
that the proposal had adequately addressed the remand, while the latter
expressed the view that the EPA had failed to comply with the court's
remand because it had failed to identify the target levels of
vegetation protection for which the proposed range of standards would
provide the requisite protection, claiming that the identified W126
index range of 13-17 ppm-hrs was not based on a proposed level of
protection against biomass loss, carbon storage loss, or foliar injury
that the EPA had identified as requisite for public welfare.
We agree with the comments that state that we have addressed the
court's remand. More specifically, with this rulemaking, including
today's decision and the Administrator's conclusions described in
section IV.C.3 below, the EPA has fully addressed the remand of the
2008 secondary O3 standard. In Mississippi v. EPA, the D.C.
Circuit remanded the 2008 secondary O3 standard to the EPA
for reconsideration because it had not adequately explained why that
standard provided the requisite public welfare protection. 744 F.3d
1334, 1360-61 (D.C. Cir. 2013). In doing so, the court relied on the
language of CAA section 109(b)(2), and the court's prior decision,
American Farm Bureau Federation v. EPA, 559 F.3d 512, 528-32 (D.C. Cir.
2009), which came to the same conclusion for the 2006 secondary
PM2.5 standard. Both decisions recognize that the plain
language of section 109(b)(2) requires the EPA to ``specify a level of
air quality the maintenance of which . . . is requisite to protect the
public welfare from any known or anticipated adverse effects''
(Mississippi, 744 F.3d at 1360 [citing American Farm Bureau, 559 F.3d
at 530]). Further, explaining that it was insufficient for the EPA
``merely to compare the level of protection afforded by the primary
standard to possible secondary standards and to find the two roughly
equivalent'' (Mississippi, 744 F.3d at 1360), the court rejected the
EPA's justification for setting the secondary standard equivalent to
the primary standard because that justification was based on comparing
the protection from the primary standard to that expected from one
possible standard with a cumulative, seasonal form (21 ppm-hrs) without
stating that such a cumulative seasonal standard would be requisite to
protect welfare or explaining why that would be so. Because the EPA had
``failed to determine what level of protection was `requisite to
protect the public welfare'' (Mississippi, 744 F.3d at 1362), the court
found that the EPA's rationale failed to satisfy the requirements of
the Act.
Today's rulemaking both satisfies the requirements of section
109(b)(2) of the Act and addresses the issues raised in the court's
remand. In this rulemaking, the Administrator has established a revised
secondary standard that replaces the remanded 2008 secondary standard.
In so doing, based on her consideration of the currently available
evidence and quantitative exposure and air quality information, as well
as advice from CASAC and input from public comments, the Administrator
has described the requisite public welfare protection for the secondary
standard and explained how the standard selected specifies air quality
that will provide that protection. As explained in detail in IV.C.3
below, in this review the Administrator is describing the public
welfare protection she finds requisite in terms of seedling RBL in the
median species, which serves as a surrogate for a broader array of
O3 effects at the plant and ecosystem levels. This
description of the desired protection sufficiently articulates the
standard that the Administrator is using to evaluate welfare
protection. Further, the Administrator has considered air quality
analyses in determining how to achieve the air quality conditions
associated with the desired protection. Based on these analyses, the
Administrator is determining that revising the level of the secondary
standard to 70 ppb, while retaining the current form, averaging time,
and indicator, specifies a level of air quality that will provide the
requisite public welfare protection.
To the extent the comments suggest that the EPA is required in
establishing a standard to identify a precise and quantified level of
public welfare protection that is requisite with respect to every
potentially adverse public welfare impact (e.g., visible foliar injury,
crop yield loss) that is considered in establishing the standard, we
disagree. While the D.C. Circuit has required the EPA to
``qualitatively describe the standard governing its selection of
particular NAAQS,'' it has expressly ``rejected the notion that the
Agency must establish a measure of the risk to safety it considers
adequate to protect public health every time it establishes a NAAQS''
(ATA III, 283 F.3d at 369 [internal marks and citations omitted]). That
is, the EPA must ``engage in reasoned decision-making,'' but is not
required to ``definitively identify pollutant levels below which risks
to public health are negligible'' (ATA III, 283 F.3d at 370). This
principle recognizes that the Act requires the EPA to establish NAAQS
even when the risks or effects of a pollutant cannot be quantified or
precisely identified because of scientific uncertainty concerning such
effects at atmospheric concentrations (ATA III, 283 F.3d at 370).
Though these decisions specifically address setting a primary standard
under CAA section 109(b)(1), we believe the same principles apply to
the parallel provision in section 109(b)(2) governing secondary
standards. Accordingly, while the EPA recognizes that it must explain
the basis for concluding that the standard selected by the
Administrator specifies air quality that will provide the protection
against adverse effects on public welfare needed from the secondary
standard (Mississippi v. EPA, 744 F.3d 1334, 1360-61 [D.C. Cir. 2013]),
the CAA does not require the EPA to precisely quantify the measure of
protection that is necessary to protect the public welfare in
establishing a secondary standard. In light of the Administrator's
description of the desired public welfare protection in IV.C.3 below,
which has both qualitative and quantitative components, the EPA is not
required to further reduce this description to a precise, quantitative
target level of vegetation protection. Moreover, nothing in the CAA or
in case law requires the EPA to identify a target level of protection
for any particular public welfare effect, such as vegetation effects,
but rather leaves the Administrator discretion in judging how to
describe the public welfare protection that she concludes is requisite.
In IV.C.3 below, the Administrator explains her reasoning for giving
primary focus to growth-related effects in describing the requisite
welfare protection, rather than to other welfare effects such as foliar
injury, for which there are more uncertainties and less predictability
with respect to the severity of the effects that would be expected from
varying O3 exposures in the natural environment
[[Page 65403]]
and the significance of the associated impacts to public welfare.
3. Administrator's Conclusions on Revision
In reaching her decision on the appropriate revisions to the
secondary standard, the Administrator has drawn on (1) the ISA
conclusions regarding the weight of the evidence for a range of welfare
effects associated with O3 in ambient air, quantitative
findings regarding air quality and ecosystem exposures associated with
such effects, and associated limitations and uncertainties; (2) staff
evaluations in the PA of the evidence summarized in the ISA, the
exposure/risk information developed in the WREA and analyses of air
quality monitoring information; (3) additional air quality analyses of
relationships between air quality metrics based on form and averaging
time of the current standard and the W126 cumulative seasonal exposure
index; (4) CASAC advice; and (5) consideration of public comments.
After giving careful consideration to all of this information, the
Administrator believes that the conclusions and policy judgments
supporting her proposed decision remain valid.
The Administrator concludes it is appropriate to continue to use
O3 as the indicator for a secondary standard intended to
address adverse effects to public welfare associated with exposure to
O3 alone and in combination with related photochemical
oxidants. In this review, no alternatives to O3 have been
advanced as being a more appropriate surrogate for ambient
photochemical oxidants. Advice from CASAC concurs with the
appropriateness of retaining the current indicator. Thus, as is the
case for the primary standard (discussed above in section II.C.1), the
Administrator has decided to retain O3 as the indicator for
the secondary standard. In so doing, she recognizes that measures
leading to reductions in ecosystem exposures to O3 would
also be expected to reduce exposures to other photochemical oxidants.
In her decision on the other elements of the standard, the
Administrator has considered the body of evidence and information in a
systematic fashion, giving appropriate consideration to the important
findings of the ISA as to the effects of O3 in ambient air
that may present risks to the public welfare, measures of exposure best
formulated for assessment of these effects, associated evidence
regarding ecosystem exposures and air quality associated with such
effects; judgments regarding the weight to place on strengths,
limitations and uncertainties of this full body of information; and
public welfare policy judgments on the appropriate degree of protection
and the form and level of a revised standard that will provide such
protection. In reaching her decision, the Administrator recognizes that
the Act does not require that NAAQS be set at zero-risk or background
levels, but rather at levels that reduce risk sufficiently to protect
public welfare from known or anticipated adverse effects. In addition,
we note that the elements of the standard (indicator, level, form, and
averaging time) are considered together in assessing the protection
provided by a new or revised standard, and the EPA's approach for
considering the elements of a new or revised standard is part of the
exercise of the judgment of the Administrator.
As an initial matter, the Administrator recognizes the robustness
of the longstanding evidence, described in the ISA, of O3
effects on vegetation and associated terrestrial ecosystems. The newly
available studies and analyses have strengthened the evidence for the
current review that provides the foundation for the Administrator's
consideration of O3 effects, associated public welfare
protection objectives, and the revisions to the current standard needed
to achieve those objectives. In light of the extensive evidence base in
this regard, the Administrator focuses on protection against adverse
public welfare effects of O3 related effects on vegetation.
In so doing, she takes note of effects that compromise plant function
and productivity, with associated effects on ecosystems. She is
particularly concerned about such effects in natural ecosystems, such
as those in areas with protection designated by Congress for current
and future generations, as well as areas similarly set aside by states,
tribes and public interest groups with the intention of providing
similar benefits to the public welfare. She additionally recognizes
that providing protection for this purpose will also provide a level of
protection for other vegetation that is used by the public and
potentially affected by O3 including timber, produce grown
for consumption and horticultural plants used for landscaping.
A central issue in this review of the secondary standard, as in the
last review (completed in 2008), has been consideration of the role for
a cumulative seasonal exposure index. In the last review, the
Administrator proposed such an index as one of two options for the form
of a revised standard. The Administrator's decision in that review was
to retain the existing form and averaging time, while revising the
standard level to provide the desired level of protection. As described
in section IV.A above, this decision was remanded to the EPA in 2013 by
the DC Circuit. In the current review, the ISA evaluates the evidence
and concludes that, among the approaches investigated, quantifying
exposure with a cumulative seasonal index best captures the aspects of
exposure that relate to effects on vegetation, particularly those
related to growth and yield. The PA considered this finding both in the
context of assessing potential impacts, and, conversely, the protection
from such impacts that might be realized, as well as in the context of
using a cumulative seasonal exposure index as a form for the secondary
standard. In the proposal, the Administrator focused on the former
context, as an exposure index, while additionally soliciting comment on
use of the index as the form for the revised standard. Advice from
CASAC, all of which was received prior to the proposal, has largely
emphasized the latter context, and that was also the focus of some
comments.
In considering revisions to the secondary standard that will
specify a level of air quality to provide the necessary public welfare
protection, the Administrator focuses on use of a cumulative seasonal
exposure index, including specifically the W126 index as defined in the
proposal, for assessing exposure, both for making judgments with regard
to the potential harm to public welfare posed by conditions allowed by
various levels of air quality and for making the associated judgments
regarding the appropriate degree of protection against such potential
harm. In so doing, the Administrator takes note of the conclusions in
the ISA and PA, with which the CASAC concurred, that, based on the
currently available evidence, a cumulative seasonal concentration-
weighted index best captures the aspects of ecosystem exposure to
O3 in ambient air that impact vegetation. In considering the
public comments in this area, she notes the broad support for use of
such a metric as an exposure index, with many additionally supporting
its use as the form for a revised standard, in light of CASAC advice on
that point. Thus, based on the substantial support in the evidence and
CASAC advice, and in consideration of public comments, the
Administrator concludes that it is appropriate to use such a cumulative
seasonal concentration-weighted index for purposes of assessing the
potential
[[Page 65404]]
public welfare risks, and similarly, for assessing the potential
protection achieved against such risks on a national scale.
The Administrator has considered conclusions of the ISA and PA, as
well as advice from CASAC and public comments, regarding different
cumulative, concentration-weighted metrics, and different temporal
definitions of aspects of these metrics. The Administrator takes note
of the PA conclusions in support of the W126 exposure index, recognized
by the ISA for its strength in weighting potentially damaging
O3 concentrations that contributes to the advantages it
offers over other weighted cumulative indices. With regard to the
relevant definitions for the temporal aspects of this index,
conclusions in the ISA and PA, and such considerations in the last
review, have led to a focus on a maximum 3-month, 12-hour index,
defined by the 3-consecutive-month period within the O3
season with the maximum sum of W126-weighted hourly O3
concentrations during the period from 8:00 a.m. to 8:00 p.m. each day
(as explained in section IV.A.1.c above). The Administrator takes note
of the support in the ISA and PA, as well as CASAC recommendations for
consideration of the W126 index defined in this way. While recognizing
that no one definition of an exposure metric used for the assessment of
protection for multiple effects at a national scale will be exactly
tailored to every species or each vegetation type, ecosystem and region
of the country, as discussed in section IV.C.2 above, the Administrator
judges that on balance, a W126 index derived in this way, and averaged
over three years, as discussed below, will be appropriate for such
purposes.
In considering the appropriate exposure index to facilitate
assessment of the level of protection afforded to the public welfare by
alternative secondary standards in the proposal, the Administrator
concluded that a 3-year average W126 index was appropriate for these
purposes. A number of considerations raised in the PA influenced the
Administrator's conclusion at the time of proposal, in combination with
public welfare judgments regarding the weight to place on the evidence
of specific vegetation-related effects estimated to result across a
range of cumulative seasonal concentration-weighted O3
exposures and judgments on the extent to which such effects in such
areas may be considered adverse to public welfare (79 FR 76347, 75312,
December 17, 2014,). Some comments were received from the public on
this aspect of the proposed decision, as discussed in section IV.C.2
above, and have been considered in the conclusions reached here.
The Administrator continues to place weight on key aspects raised
in the PA and summarized in the proposal on the appropriateness of
considering a 3-year average index. The Administrator notes the PA
consideration of the potential for multiple consecutive years of
critical O3 exposures to result in larger impacts on
forested areas than intermittent occurrences of such exposures due to
the potential for compounding effects on tree growth. The Administrator
additionally notes the evidence, as considered in the PA and summarized
in the proposal, for some perennial species of some effects associated
with a single year's exposure of a critical magnitude that may have the
potential for some ``carry over'' of effects on plant growth or
reproduction in the subsequent season. Further, the Administrator notes
the occurrence of visible foliar injury and growth or yield loss in
annual plants or crops associated with exposures of a critical
magnitude. While the Administrator appreciates that the scientific
evidence documents the effects on vegetation resulting from individual
growing season exposures of specific magnitude, including those that
can affect the vegetation in subsequent years, she is also mindful,
both of the strengths and limitations of the evidence, and of the
information on which to base her judgments with regard to adversity of
effects on the public welfare. The Administrator also recognizes
uncertainties associated with interpretation of the public welfare
significance of effects resulting from a single-year exposure, and that
the public welfare significance of effects associated with multiple
years of critical exposures are potentially greater than those
associated with a single year of such exposure.
As she did for the proposal, the Administrator has considered
advice from CASAC in this area, including the CASAC comments that it
favors a W126-based secondary standard with a single year form, that
its recommended range of levels relates to such a form, and that a
lower range (e.g., with 13 ppm-hrs at the upper end) would pertain to a
3-year form. The Administrator also notes CASAC's recognition that her
decision on use of a 3-year average over a single-year W126 index may
be a matter of policy. While recognizing the potential for effects on
vegetation associated with a single-year exposure, the Administrator
concludes that use of a 3-year average metric can address the potential
for adverse effects to public welfare that may relate to shorter
exposure periods, including a single year.
While the Administrator recognizes the scientific information and
interpretations, as well as CASAC advice, with regard to a single-year
exposure index, she also takes note of uncertainties associated with
judging the degree of vegetation impacts for annual effects that would
be adverse to public welfare. Even in the case of annual crops, the
assessment of public welfare significance is unclear for the reasons
discussed below related to agricultural practices. The Administrator is
also mindful of the variability in ambient air O3
concentrations from year to year, as well as year-to-year variability
in environmental factors, including rainfall and other meteorological
factors, that influence the occurrence and magnitude of O3-
related effects in any year, and contribute uncertainties to
interpretation of the potential for harm to public welfare over the
longer term. As noted above, the Administrator also recognizes that the
public welfare significance of effects associated with multiple years
of critical exposures are potentially greater than those associated
with a single year of such exposure. Based on all of these
considerations, the Administrator recognizes greater confidence in
judgments related to public welfare impacts based on a 3-year average
metric. Accordingly, the considerations identified here lead the
Administrator to conclude it is appropriate to use an index averaged
across three years for judging public welfare protection afforded by a
revised secondary standard.
In reaching a conclusion on the amount of public welfare protection
from the presence of O3 in ambient air that is appropriate
to be afforded by a revised secondary standard, the Administrator has
given particular consideration to the following: (1) The nature and
degree of effects of O3 on vegetation, including her
judgments as to what constitutes an adverse effect to the public
welfare; (2) the strengths and limitations of the available and
relevant information; (3) comments from the public on the
Administrator's proposed decision, including comments related to
identification of a target level of protection; and (4) CASAC's views
regarding the strength of the evidence and its adequacy to inform
judgments on public welfare protection. The Administrator recognizes
that such judgments include judgments about the interpretation of the
evidence and other information, such as the quantitative analyses of
air quality monitoring,
[[Page 65405]]
exposure and risk. She also recognizes that such judgments should
neither overstate nor understate the strengths and limitations of the
evidence and information nor the appropriate inferences to be drawn as
to risks to public welfare. The CAA does not require that a secondary
standard be protective of all effects associated with a pollutant in
the ambient air but rather those known or anticipated effects judged
adverse to the public welfare (as described in section IV.A.3 above).
The Administrator additionally recognizes that the choice of the
appropriate level of protection is a public welfare policy judgment
entrusted to the Administrator under the CAA taking into account both
the available evidence and the uncertainties.
The Administrator finds the coherence and strength of the weight of
evidence concerning effects on vegetation from the large body of
available literature compelling. The currently available evidence
addresses a broad array of O3-induced effects on a variety
of tree species across a range of growth stages (i.e., seedlings,
saplings and mature trees) using diverse field-based (e.g., free air,
gradient and ambient) and OTC exposure methods. The Administrator gives
particular attention to the effects related to native tree growth and
productivity, recognizing their relationship to a range of ecosystem
services, including forest and forest community composition. She is
also mindful of the significance of community composition changes,
particularly in protected areas, such as Class I areas. At the same
time, she recognizes, while the evidence strongly supports conclusions
regarding O3 impacts on growth and the evidence showing
effects on tree seedlings, as well as on older trees, there are
limitations in our ability to predict impacts in the environment or to
estimate air quality or exposures that will avoid such impacts. Such
limitations relate to the variability of environmental factors or
characteristics that can influence the extent of O3 effects.
In recognition of the CASAC advice and the potential for adverse
public welfare effects, the Administrator has considered the nature and
degree of effects of O3 on the public welfare. In so doing,
the Administrator recognizes that the significance to the public
welfare of O3-induced effects on sensitive vegetation
growing within the U.S. can vary, depending on the nature of the
effect, the intended use of the sensitive plants or ecosystems, and the
types of environments in which the sensitive vegetation and ecosystems
are located. Any given O3-related effect on vegetation and
ecosystems (e.g., biomass loss, visible foliar injury), therefore, may
be judged to have a different degree of impact on the public depending,
for example, on whether that effect occurs in a Class I area, a
residential or commercial setting, or elsewhere. The Administrator
notes that such a distinction is supported by CASAC advice in this
review. In her judgment, like those of the Administrator in the last
review, it is appropriate that this variation in the significance of
O3-related vegetation effects should be taken into
consideration in making judgments with regard to the level of ambient
O3 concentrations that is requisite to protect the public
welfare from any known or anticipated adverse effects. As a result, the
Administrator concludes that of those known and anticipated
O3-related vegetation and ecosystem effects identified and
discussed in this notice, particular significance should be ascribed to
those that may occur on sensitive species that are known to or are
likely to occur in federally protected areas such as Class I areas or
on lands set aside by states, tribes and public interest groups to
provide similar benefits to the public welfare, for residents on those
lands, as well as visitors to those areas.
Likewise, the Administrator also notes that less protection related
to growth effects may be called for in the case of other types of
vegetation or vegetation associated with other uses or services. For
example, the maintenance of adequate agricultural crop yields is
extremely important to the public welfare and currently involves the
application of intensive management practices. With respect to
commercial production of commodities, the Administrator notes that
judgments about the extent to which O3-related effects on
commercially managed vegetation are adverse from a public welfare
perspective are particularly difficult to reach, given that the
extensive management of such vegetation (which, as CASAC noted, may
reduce yield variability) may also to some degree mitigate potential
O3-related effects. The management practices used on these
lands are highly variable and are designed to achieve optimal yields,
taking into consideration various environmental conditions. In
addition, changes in yield of commercial crops and commercial
commodities, such as timber, may affect producers and consumers
differently, further complicating the question of assessing overall
public welfare impacts. Thus, the Administrator concludes, while
research on agricultural crop species remains useful in illuminating
mechanisms of action and physiological processes, information from this
sector on O3-induced effects is considered less useful in
informing judgments on what specific standard would provide the
appropriate public welfare protection. In so doing, the Administrator
notes that a standard revised to increase protection for forested
ecosystems would also be expected to provide some increased protection
for agricultural crops and other commercial commodities, such as
timber.
The Administrator also recognizes that O3-related
effects on sensitive vegetation can occur in other areas that have not
been afforded special federal or other protections, including effects
on vegetation growing in managed city parks and residential or
commercial settings, such as ornamentals used in urban/suburban
landscaping or vegetation grown in land use categories involving
commercial production of commodities, such as timber. For vegetation
used for residential or commercial ornamental purposes, the
Administrator believes that there is not adequate information at this
time to establish a secondary standard based specifically on impairment
of these categories of vegetation, but notes that a secondary standard
revised to provide protection for sensitive natural vegetation and
ecosystems would likely also provide some degree of protection for such
vegetation.
Based on the above considerations, in identifying the appropriate
level of protection for the secondary standard, the Administrator finds
it appropriate to focus on sensitive trees and other native species
known or anticipated to occur in protected areas such as Class I areas
or on other lands set aside by the Congress, states, tribes and public
interest groups to provide similar benefits to the public welfare, for
residents on those lands, as well as visitors to those areas. In light
of their public welfare significance, the Administrator gives
particular weight to protecting such vegetation and ecosystems. Given
the reasons for the special protection afforded such areas (identified
in section I.A.3 above), she recognizes the importance of protecting
these natural forests from O3-induced impacts, including
those related to O3 effects on growth, and including those
extending in scale from individual plants to the ecosystem. The
Administrator also recognizes that the impacts identified for
O3 range from those for which the public welfare
significance may be more easily judged, but for which quantitative
relationships
[[Page 65406]]
with O3 in ambient air are less well established, such as
impacts on forest community composition in protected wilderness areas,
carbon storage and other important ecosystem services, to specific
plant-level effects, such as growth impacts (in terms of RBL) in tree
seedlings, for which our quantitative estimates are more robust.
For considering the appropriate public welfare protection objective
for a revised standard, the Administrator finds appropriate and useful
the estimates of tree seedling growth impacts (in terms of RBL)
associated with a range of W126-based index values developed from the
robust E-R functions for 11 tree species, that were described in the PA
and proposal and are summarized in Table 4 above. In making judgments
based on those observations, however, the Administrator has considered
the broader evidence base and public welfare implications, including
associated strengths, limitations and uncertainties. Thus, in drawing
on estimates from this table, she is not making judgments simply about
a specific magnitude of growth effect in seedlings that would be
acceptable or unacceptable in the natural environment. Rather, the
Administrator is using the estimates in the table, as suggested by
CASAC and emphasized by some commenters, as a surrogate or proxy for
consideration of the broader array of vegetation-related effects of
potential public welfare significance, that include effects on growth
of individual sensitive species and extend to ecosystem-level effects,
such as community composition in natural forests, particularly in
protected public lands, as well as forest productivity. In so doing,
she notes that CASAC similarly viewed biomass loss as ``a
scientifically valid surrogate of a variety of adverse effects to
public welfare'' (Frey, 2014c, p. 10). Thus, in considering the
appropriate level of public welfare protection for the revised
standard, the Administrator gives primary attention to the relationship
between W126 exposures and estimates of RBL in tree seedlings in Table
4, finding this to be a useful quantitative tool to inform her
judgments in this matter.
In considering the RBL estimates in Table 4 above (drawn from the
final PA), the Administrator takes note of comments from CASAC that
also give weight to these relationships in formulating its advice and
notes the CASAC comments on specific RBL values (Frey, 2014c). In so
doing, she considers and contrasts comments and their context on RBL
estimates of 2% and 6% for the median studied species.
With regard to the CASAC advice regarding 2% RBL for the median
studied tree species, the Administrator notes, as an initial matter,
the unclear basis for such a focus, as described in section IV.C.2
above and in the proposal. Further, she notes that the CASAC advice
related to this RBL value was that it would be appropriate for the
range of levels identified in the PA for the Administrator's
consideration to ``include[] levels that aim for not greater than 2%
RBL for the median tree species'' (Frey, 2014c, p. 14). As described in
the proposal, the range identified in the PA, which the Administrator
considered, extended down to W126 index levels for which the estimated
RBL in the median tree species is less than or equal to 2%, consistent
with the CASAC advice. In addition, the Administrator notes that only
the lowest portion of this range (7-8 ppm-hrs) corresponds to an
estimated RBL for the median tree species of less than or equal to 2%,
with the remainder of CASAC's range (up to 15 ppm-hrs) associated with
higher median RBL estimates. Thus, the Administrator understands CASAC
to have identified 2% RBL for the median tree species as a benchmark
falling within, and at one end of, the range of levels of protection
that the CASAC considers appropriate for the revised standard to
provide. However, the fact that the CASAC range included levels for
which the RBL estimates were appreciably greater than 2% indicates that
CASAC did not judge it necessary that the revised standard be based on
the 2% RBL benchmark. Accordingly, the Administrator proposed revisions
to the secondary standard based on options related to higher RBL
estimates and associated exposures. After also considering public
comments, the Administrator continues to consider the uncertainty
regarding the extent to which associated effects on vegetation at lower
O3 exposures would be adverse to public welfare to be too
great to provide a foundation for public welfare protection objectives
for a revised secondary standard.
With regard to the CASAC comments on a 6% RBL estimate, the
Administrator takes particular note of their characterization of this
level of effect in the median studied species as ``unacceptably high''
(Frey, 2014c, pp. iii, 13, 14). These comments were provided in the
context of CASAC's considering the significance of effects associated
with a range of alternatives for the secondary standard. Moreover, the
range recommended by CASAC excluded W126 index values for which the
median species was estimated to have a 6% RBL,\212\ based on the
information before CASAC at the time (Frey, 2014c, p. 12-13).
Accordingly, the EPA interprets these comments regarding 6% RBL to be
of a different nature than the CASAC advice regarding a 2% median RBL,
both because these two comments are framed to address different
questions and because CASAC treated them differently in its recommended
range.
---------------------------------------------------------------------------
\212\ As summarized in IV.C.2 above (and noted in section IV.E.3
of the proposal), revisions to this table in the final PA, made in
consideration of other CASAC comments, have resulted in changes to
the median species RBL estimates such that the median species RBL
estimate for a W126 index value of 17 ppm-hrs in this table in the
final PA (5.3%) is nearly identical to the median species estimate
for 15 ppm-hrs (the value corresponding to the upper end of the
CASAC-identified range) in the second draft PA (5.2%), the review of
which was the context for CASAC's advice on this point (Frey,
2014c). The median RBL estimate ranges from 5.3% to 3.8% across the
range of W126 exposures (17 ppm-hrs to 13 ppm-hrs) that the
Administrator proposed to conclude would provide the appropriate
public welfare protection for a revised secondary standard.
---------------------------------------------------------------------------
In the Administrator's consideration of the RBL estimates to inform
judgments on O3 exposures of concern to public welfare and
the appropriate protection that the secondary standard should provide
from such exposures, she has given particular consideration to the
current evidence for the relationship of reduced growth of sensitive
tree species with ecosystem effects (as described in the ISA), CASAC's
view of 6% RBL for the median studied species as unacceptably high, and
the role of the Administrator's judgments regarding public welfare
impacts of effects in specially protected natural systems, such as
Class I areas. With regard to a point of focus among the median RBL
estimates extending below 6% for purposes of judging the appropriate
public welfare protection objectives for a revised secondary standard,
the Administrator is mindful of the CASAC advice to consider lower
levels if using a 3-year average, rather than annual, W126 index value.
In considering the CASAC advice, the Administrator notes that her
judgments on a 3-year average index focus on the level of confidence in
conclusions that might be drawn with regard to single as compared to
multiple year impacts, as described above. For example, the
Administrator, while recognizing the strength of the evidence with
regard to quantitative characterization of O3 effects on
growth of tree seedlings and crops, and in addition to noting the
additional difficulties for assessing the welfare impacts of
O3 on crops, takes note of the uncertainty associated with
[[Page 65407]]
drawing conclusions with regard to the extent to which small percent
reductions in annual growth contribute to adverse effects on public
welfare and the role of annual variability in environmental factors
that affect plant responses to O3. Moreover, as explained
above, the Administrator concludes that concerns related to the
possibility of a single unusually damaging year, inclusive of those
described by the CASAC, can be addressed through use of a 3-year
average metric. Thus, similar to the CASAC's view that a lower level
would be appropriate with a 3-year form, the Administrator considers it
appropriate to focus on a standard that would generally limit
cumulative exposures to those for which the median RBL estimate would
be somewhat lower than 6%.
In focusing on cumulative exposures associated with a median RBL
estimate somewhat below 6%, the Administrator considers the
relationships in Table 4, noting that the median RBL estimate is 6% for
a cumulative seasonal W126 exposure index of 19 ppm-hrs. Considering
somewhat lower values, the median RBL estimate is 5.7% (which rounds to
6%) for a cumulative seasonal W126 exposure index of 18 ppm-hrs and the
median RBL estimate is 5.3% (which rounds to 5%) for 17 ppm-hrs. In
light of her decision that it is appropriate to use a 3-year cumulative
exposure index for assessing vegetation effects (described above), the
potential for single-season effects of concern, and CASAC comments on
the appropriateness of a lower value for a 3-year average W126 index,
the Administrator concludes it is appropriate to identify a standard
that would restrict cumulative seasonal exposures to 17 ppm-hrs or
lower, in terms of a 3-year W126 index, in nearly all instances. In
reaching this conclusion, based on the current information to inform
consideration of vegetation effects and their potential adversity to
public welfare, she additionally judges that the RBL estimates
associated with marginally higher exposures in isolated, rare instances
are not indicative of effects that would be adverse to the public
welfare, particularly in light of variability in the array of
environmental factors that can influence O3 effects in
different systems and uncertainties associated with estimates of
effects associated with this magnitude of cumulative exposure in the
natural environment.
While giving primary consideration to growth effects using the
surrogate of RBL estimates based on tree seedling effects, the
Administrator also recognizes the longstanding and robust evidence of
O3 effects on crop yield. She takes note of CASAC
concurrence with the PA description of such effects as of public
welfare significance and agrees. As recognized in the proposal, the
maintenance of adequate agricultural crop yields is extremely important
to the public welfare. Accordingly, research on agricultural crop
species remains important for further illumination of mechanisms of
action and physiological processes. Given that the extensive management
of such vegetation, which as CASAC noted may reduce yield variability,
may also to some degree mitigate potential O3-related
effects, however, judgments about the extent to which O3-
related effects on crop yields are adverse from a public welfare
perspective are particularly difficult to reach. Further, management
practices for agricultural crops are highly variable and generally
designed to achieve optimal yields, taking into consideration various
environmental conditions. As a result of this extensive role of
management in optimizing crop yield, the Administrator notes the
potential for greater uncertainty with regard to estimating the impacts
of O3 exposure on agricultural crop production than that
associated with O3 impacts on vegetation in natural forests.
For all of these reasons, the Administrator is not giving the same
weight to CASAC's statement regarding crop yield loss as a surrogate
for adverse effects on public welfare, or the magnitude that would
represent an adverse impact to public welfare, as to the CASAC's
comments on RBL as a surrogate for an array of growth-related effects.
Similarly, given the considerations summarized above and in the
proposal, the Administrator concludes that agricultural crops do not
have the same need for additional protection from the NAAQS as forested
ecosystems and finds protection of public welfare from crop yield
impacts to be a less important consideration in this review for the
reasons identified, including the extensive management of crop yields
and the dynamics of agricultural markets. Thus, the Administrator is
not giving a primary focus to crop yield loss in selecting a revised
secondary standard. She notes, however, that a standard revised to
increase protection for forested ecosystems would also be expected to
provide some increased protection for agricultural crops.
The Administrator has additionally considered the evidence and
analyses of visible foliar injury. In so doing, the Administrator notes
the ISA conclusion that ``[e]xperimental evidence has clearly
established a consistent association of visible injury with
O3 exposure, with greater exposure often resulting in
greater and more prevalent injury'' (U.S. EPA, 2013, section 9.4.2, p.
9-41). The Administrator also recognizes the potential for this effect
to affect the public welfare in the context of affecting values
pertaining to natural forests, particularly those afforded special
government protection, as discussed in section IV.A.3 above. However,
she recognizes significant challenges in judging the specific extent
and severity at which such effects should be considered adverse to
public welfare, in light of the variability in the occurrence of
visible foliar injury and the lack of clear quantitative relationships
with other effects on vegetation, as well as the lack of established
criteria or objectives that might inform consideration of potential
public welfare impacts related to this vegetation effect.
Further, the Administrator takes note of the range of evidence on
visible foliar injury and the various related analyses, including
additional observations drawn from the WREA biosite dataset in response
to comments, as summarized in section IV.C.2 above. In so doing, she
does not agree with CASAC's comment that a level of W126 exposure below
10 ppm-hrs is required to reduce foliar injury, noting some lack of
clarity in the WREA and PA presentations of the WREA cumulative
proportion analysis findings and their meaning (described in section
IV.C.2.b above). She notes that the additional observations summarized
in section IV.C.2 above indicate declines in proportions of sites with
any visible foliar injury and biosite index scores with reductions in
cumulative W126 exposure across a range of values extending at the high
end well above 20 ppm-hrs, down past and including 17 ppm-hrs. In
considering this information, however, the Administrator takes note of
the current lack of robust exposure-response functions that would allow
prediction of visible foliar injury severity and incidence under
varying air quality and environmental conditions, as recognized in
section IV.A.1.b above. Thus, while the Administrator notes that the
evidence is not conducive to use for identification of a specific
quantitative public welfare protection objective, due to uncertainties
and complexities described in sections IV.A.1.b and IV.A.3 above, she
concludes that her judgments above, reached with a focus on RBL
estimates, would also be expected to provide an additional
[[Page 65408]]
desirable degree of protection against visible foliar injury in
sensitive vegetation. Accordingly, she considers a conclusion on the
appropriateness of selecting a standard that will generally limit
cumulative exposures above 17 ppm-hrs to be additionally supported by
evidence for visible foliar injury, while not based on specific
consideration of this effect.
With the public welfare protection objectives identified above in
mind, the Administrator turns to her consideration of form and level
for the revised secondary standard. In considering whether the current
form should be retained or revised in order to provide the appropriate
degree of public welfare protection, the Administrator has considered
the analyses of air quality data from the last 13 years that describe
the cumulative exposures, in terms of a 3-year W126 index, occurring at
monitoring sites across the U.S. when the air quality metric at that
location, in terms of the current standard's form and averaging time,
is at or below different alternative levels. The Administrator notes
both the conclusions drawn from analyses of the strong, positive
relationship between these metrics and the findings that indicate the
amount of control provided by the fourth-high metric.
The Administrator has also considered advice from CASAC and public
commenters that support revision of the form to the W126 exposure
index. The Administrator concurs with the underlying premise that
O3 effects on vegetation are most directly assessed using a
cumulative seasonal exposure index, specifically the W126 exposure
index. The Administrator additionally recognizes, based on analyses of
the last 13 years of monitoring data, and consideration of modeling
analyses with associated limitations and uncertainties, that cumulative
seasonal exposures appear to have a strong relationship with design
values based on the current form and averaging time. She additionally
notes the correlation of reductions in W126 index values with
reductions in precursor emissions over the past decade that were
targeted at meeting the current O3 standards (with fourth-
high form), which indicate the control of cumulative seasonal exposures
that can be achieved with a standard of the current form and averaging
time.
With regard to recommendations from the CASAC that the form for the
revised secondary standard should be the biologically relevant exposure
metric, and related comments from the public indicating that the
secondary standard must have such a form, the Administrator disagrees.
In so doing, she notes that CAA section 109 does not impose such a
requirement on the form or averaging time for the NAAQS, as explained
in IV.C.2 above. She further notes that the averaging time and form of
primary standards are often not the same as the exposure metrics used
in reviews of primary standards, in which specific information on
quantitative relationships between different exposure metrics and
health risk is more often available than it is in reviews of secondary
NAAQS. As discussed in section IV.C.2 above, with examples, a primary
standard with a particular averaging time and form may provide the
requisite public health protection from health effects that are most
appropriately assessed using an exposure metric of a different
averaging time and form and indicator, and the same principle can apply
when establishing or revising secondary standards. The Administrator
recognizes that the exposure metric and the standard metric can be
quite similar, as in the case of consideration of short-term health
effects with the primary O3 standard. She also notes,
however, as illustrated by the examples described in section IV.C.2
above, that it is not uncommon for the EPA to retain or adopt elements
of an existing standard that the Administrator judges in combination
across all elements, including in some cases a revised level, to
provide the requisite protection under the Act, even if those elements
do not neatly correspond to the exposure metric. Accordingly, she
concludes that the Act does not require that the secondary
O3 standard be revised to match the exposure metric
identified as biologically relevant in this review, as long as the
revised standard provides the degree of protection required under CAA
section 109(b)(2).
Based on the considerations described here, including the use of an
exposure metric that CASAC has agreed to be biologically relevant and
appropriate, related considerations summarized in the proposal with
regard to air quality analyses and common uses of exposure metrics in
other NAAQS reviews, the Administrator finds that, in combination with
a revised level, the current form and averaging time for a revised
secondary standard can be expected to provide the desired level of
public welfare protection. Accordingly, she next turns to the important
consideration of a level that, in combination with the form and
averaging time, will yield a standard that specifies the requisite air
quality for protection of public welfare. In so doing, she has
recognized the recommendation by CASAC for revision of the form and
averaging time and provided the basis for her alternative view, as
described above. Further, in the context of the Administrator's
decision on objectives for public welfare protection of a revised
secondary standard, and with consideration of the advice from CASAC on
levels for a W126-based standard, the Administrator has also reached
the conclusion, as described above, that in order to provide the
appropriate degree of public welfare protection, the revised secondary
standard should restrict cumulative seasonal exposures to 17 ppm-hrs or
lower, in terms of a 3-year average W126 index, in nearly all
instances. Thus, the Administrator finds it appropriate to revise the
standard level to one that, in combination with the form and averaging
time, will exert this desired degree of control for cumulative seasonal
exposures.
In considering a revised standard level, the Administrator has, in
light of public comments, revisited the information she considered in
reaching her proposed decision on a level within the range of 65 to 70
ppb, and additional information or insights conveyed with public
comments. The primary focus of the Administrator's considerations in
reaching her proposed decision was the multi-faceted analysis of air
quality data from 2001 through 2013 documented in the technical memo in
the docket (Wells, 2014a), as well as the earlier analyses and related
information described in the PA (as summarized in section IV.E.4 of the
proposal). This analysis describes the occurrences of 3-year W126 index
values of a magnitude from 17 ppm-hrs through 7 ppm-hrs at monitor
locations where O3 concentrations met different alternative
standards with the current form and averaging time, and has been
expanded in consideration of public comments to present in summary form
the more extensive historical dataset accompanying this analysis
(Wells, 2015b). Focusing first on the air quality analyses for the most
recent period for which data are available (2011-2013) and with the
protection objectives identified above in mind, the Administrator
observes that across the sites meeting the current standard of 75 ppb,
the analysis finds 25 sites distributed across different NOAA climatic
regions with 3-year average W126 index values above 17 ppm-hrs, with
the values at nearly half of the sites extending above 19 ppm-hrs, with
some well above. In comparison, she observes that across sites meeting
an alternative
[[Page 65409]]
standard of 70 ppb, the analysis for the period from 2011-2013 finds no
occurrences of W126 metric values above 17 ppm-hrs and less than a
handful of occurrences that equal 17 ppm-hrs. The more than 500
monitors that would meet an alternative standard of 70 ppb during the
2011-2013 period are distributed across all nine NOAA climatic regions
and 46 of the 50 states (Wells, 2015b and associated dataset in the
docket).
The Administrator notes that some public commenters, who disagreed
with her proposed decision on form and averaging time, emphasized past
occurrences of cumulative W126 exposure values above the range
identified in the proposal (of 13 to 17 ppm-hrs). For example, these
commenters emphasize data from farther back across the full time period
of the dataset analyzed in the technical memorandum (2001-2013),
identifying a value of 19.1 ppm-hrs at a monitor for which the fourth-
high metric is 70 ppb for the 3-year period of 2006-2008. The
Administrator notes, as discussed in section IV.C.2 above, that this
was one of fewer than a handful of isolated occurrences of sites for
which the fourth-high was at or below 70 ppb and the W126 index value
was above 17 ppm-hrs, all but one of which were below 19 ppm-hrs. The
Administrator additionally recognizes her underlying objective of a
revised secondary standard that would limit cumulative exposures in
nearly all instances to those for which the median RBL estimate would
be somewhat lower than 6%. She observes that the single occurrence of
19 ppm-hrs identified by the commenter among the nearly 4000 3-year
W126 index values from across the most recently available 11 3-year
periods of data at monitors for which the fourth-high metric is at or
below 70 ppb is reasonably regarded as an extremely rare and isolated
occurrence (Wells, 2015b). As such, it is unclear whether it would
recur, particularly as areas take further steps to reduce O3
to meet revised primary and secondary standards. Further, based on the
currently available information, the Administrator does not judge RBL
estimates associated with marginally higher exposures in isolated, rare
instances to be indicative of adverse effects to the public welfare.
Thus, the Administrator concludes that a standard with a level of 70
ppb and the current form and averaging time may be expected to limit
cumulative exposures, in terms of a 3-year average W126 exposure index,
to values at or below 17 ppm-hrs, in nearly all instances, and
accordingly, to eliminate or virtually eliminate cumulative exposures
associated with a median RBL of 6% or greater.
The Administrator recognizes that any standard intended to exert a
very high degree of control on cumulative seasonal exposures, with the
objective of limiting exposures above 17 ppm-hrs across the U.S., in
nearly all instances, will, due to regional variation in meteorology
and sources of O3 precursors, result in cumulative seasonal
exposures well below 17 ppm-hrs in many areas. Even implementation of a
standard set in terms of the cumulative seasonal exposure metric, while
limiting the highest exposures, would, due to regional variation in
meteorology and sources of O3 precursors, result in many
areas with much lower exposures. Such variation in exposures occurring
under a specific standard is not unexpected and the overall
distribution of exposures estimated to occur with air quality
conditions associated with different alternative standards is a routine
part of the consideration of public health protection in reviews of
primary standards, and can also play a role in the review of secondary
standards. For these reasons, and in light of the discussion in section
IV.C.2.d above on consideration of ``necessary'' protection, the
Administrator notes that an expectation of differing exposures is not,
in itself, a basis for concluding that the air quality would be more
(or less) than necessary (and thus not requisite) for the desired level
of public welfare protection.
The Administrator has also considered the protection afforded by a
revised standard against other effects studied in this review, such as
visible foliar injury and reduced yield for agricultural crops, and
also including those associated with climate change. While noting the
evidence supporting a relationship of O3 in ambient air with
climate forcing effects, as concluded in the ISA, the Administrator
judges the quantitative uncertainties to be too great to support
identification of a standard specific to such effects such that she
concludes it is more important to focus, as she has done above, on
setting a standard based on providing protection against vegetation-
related effects which would be expected to also have positive
implications for climate change protection through the protection of
ecosystem carbon storage.
The Administrator additionally considers the extent of control for
cumulative seasonal exposures exerted by a revised standard level of 65
ppb, the lower end of the proposed range. In focusing on the air
quality analyses for the most recent 3-year period for which data are
available, the Administrator observes that across the sites meeting a
fourth-high metric of 65 ppb, the analysis finds no occurrences of W126
metric values above 11 ppm-hrs and 35 occurrences of a value between 7
ppm-hrs and 11 ppm-hrs, scattered across NOAA climatic regions. The
Administrator finds these magnitudes of cumulative seasonal exposures
to extend appreciably below the objectives she identified above for
affording public welfare protection. In considering this alternative
level, she additionally notes that data for only 276 monitors (less
than 25 percent of the total with valid fourth-high and W126 metric
values) were at or below a fourth-high value of 65 ppb during the
period from 2011-2013. In so noting, she recognizes the appreciably
smaller and less geographically extensive dataset available and the
associated uncertainty for conclusions based on such an analysis.
Thus, based on the support provided by currently available
information on air quality, the evidence base of O3 effects
on vegetation and her public welfare policy judgments, and after
carefully taking the above comments and considerations into account,
fully considering the scientific views of the CASAC, and also taking
note of CASAC's policy views, the Administrator has decided to retain
the current indicator, form and averaging time and to revise the
secondary standard level to 70 ppb. In the Administrator's judgment,
based on the currently available evidence and quantitative exposure and
air quality information, a standard set at this level, in combination
with the currently specified form, averaging time and indicator would
be requisite to protect the public welfare from known or anticipated
adverse effects. A standard set at this level provides an appreciable
increase in protection compared to the current standard. The
Administrator judges that such a standard would protect natural forests
in Class I and other similarly protected areas against an array of
adverse vegetation effects, most notably including those related to
effects on growth and productivity in sensitive tree species. The
Administrator believes that a standard set at 70 ppb would be
sufficient to protect public welfare from known or anticipated adverse
effects and believes that a lower standard would be more than what is
necessary to provide such protection. This judgment by the
Administrator appropriately recognizes
[[Page 65410]]
that the CAA does not require that standards be set at a zero-risk
level, but rather at a level that reduces risk sufficiently so as to
protect the public welfare from known or anticipated adverse effects.
Accordingly, the Administrator concludes that it is appropriate to
revise the level for the secondary standard to 70 ppb (0.070 ppm), in
combination with retaining the current form, indicator, and averaging
time, in order to specify the level of air quality that provides the
requisite protection to the public welfare from any known or
anticipated adverse effects associated with the presence of
O3 in the ambient air.
D. Decision on the Secondary Standard
For the reasons discussed above, and taking into account
information and assessments presented in the ISA and PA, the advice and
recommendations of CASAC, and the public comments, as well as public
welfare judgments, the Administrator is revising the level of the
current secondary standard. Specifically, the Administrator has decided
to revise the level of the secondary standard to a level of 0.070 ppm,
in conjunction with retaining the current indicator, averaging time and
form. Accordingly the revised secondary standard is 0.070 ppm
O3, as the annual fourth-highest daily maximum 8-hour
average concentration, averaged over three years.
V. Appendix U: Interpretation of the Primary and Secondary NAAQS for
O3
A. Background
The EPA is finalizing the proposed Appendix U to 40 CFR part 50:
Interpretation of the Primary and Secondary National Ambient Air
Quality Standards for Ozone. The proposed Appendix U addressed the
selection of ambient O3 monitoring data to be used in making
comparisons with the NAAQS, data reporting and data handling
conventions for comparing ambient O3 monitoring data with
the level of the NAAQS, and data completeness requirements. The EPA
solicited public comment on four elements where the proposed Appendix U
differed from Appendix P to 40 CFR part 50, which addressed data
handling conventions for the previous O3 NAAQS. These
included the following: (1) the addition of a procedure to combine data
collected from two or more O3 monitors operating
simultaneously at the same physical location, (2) the addition of a
provision allowing the Regional Administrator to approve ``site
combinations'', or the combination of data from two nearby monitoring
sites for the purpose of calculating a valid design value, (3) a change
from the use of one-half of the method detection limit (\1/2\ MDL) to
zero (0.000 ppm) as the substitution value in 8-hour average data
substitution tests, and 4) a new procedure for calculating daily
maximum 8-hour average O3 concentrations for the revised
NAAQS.
The EPA is also finalizing, as proposed, exceptional events
scheduling provisions in 40 CFR 50.14 that will apply to the submission
of information supporting claimed exceptional events affecting
pollutant data that are intended to be used in the initial area
designations for any new or revised NAAQS. The new scheduling
provisions will apply to initial area designations for the 2015
O3 NAAQS.
B. Data Selection Requirements
The EPA proposed this section in Appendix U to clarify which data
are to be used in comparisons with the revised O3 NAAQS. The
EPA is finalizing this section in Appendix U as proposed.
First, the EPA proposed to combine data at monitoring sites with
two or more O3 monitoring instruments operating
simultaneously into a single site-level data record for determining
compliance with the NAAQS, and proposed an analytical approach to
perform this combination (79 FR 75351-75352, December 17, 2014).
Several commenters supported the EPA's proposed approach, including the
State of Iowa, where 15 of the 20 monitoring sites currently operating
two O3 monitors simultaneously are located. Commenters
supporting the proposal noted that a similar approach is already being
used for lead and particulate monitoring, and that the proposed
approach will help states meet data completeness requirements.
A few commenters supported the EPA's proposed approach with the
additional restrictions that the monitoring instruments must use
identical methods and be operated by the same monitoring agency. The
EPA notes that at the time of this rulemaking, all monitors reporting
O3 concentration data to the EPA for regulatory use were
FEMs. All current O3 FEMs use an ultraviolet photometry
sampling methodology and have been found to meet the performance
criteria in 40 CFR part 53. Therefore, the EPA has no reason to believe
that O3 concentration data should not be combined across
monitoring methods at the site level. Regarding the commenters'
suggestion that data should not be combined when two or more monitors
at the same site are operated by different monitoring agencies, the EPA
is aware of only one instance where this presently occurs. In this
instance, the monitors have been assigned distinct site ID numbers in
the AQS database, so that data will not be combined across these
monitors. Should future instances arise where two or more monitoring
agencies decide to operate O3 monitors at the same site, the
EPA encourages these agencies to work together to establish a plan for
how the data collected from these monitors should be used in regulatory
decision making.
One state objected to combining data across monitors because the
secondary monitors at their sites were used only for quality assurance
purposes and data from these monitors should not be combined with data
reported from the primary monitors. The EPA notes that concentration
data collected to meet quality assurance requirements (i.e. precision
and bias data) are reported and stored in a separate location within
the AQS database and are not used for determining compliance with the
NAAQS. The required quality assurance data are derived from
O3 standards and not from a separate O3 monitor.
However, if a separate O3 monitor is used strictly for
quality assurance purposes and does not meet the applicable monitoring
requirements, it can be distinguished in AQS in such a manner that data
from the secondary monitor would not be combined with data from the
primary monitor.
Another commenter objected to the proposal because it would reduce
the total number of comparisons made with the NAAQS. While this is
true, the number of physical locations being compared with the NAAQS
will not decrease under the proposed approach, and in fact may increase
due to additional sites meeting the data completeness requirements.
Finally, two commenters submitted similar comments citing the EPA's
evaluation of collocated O3 monitoring data and precision
data in the ISA (U.S. EPA, 2013, section 3.5.2), and stated that
although the median differences in concentrations reported by the pairs
of monitoring instruments were near zero, the extreme values were close
to +/- 3.5%. The commenter argued that since the O3 NAAQS
are based on the fourth-highest annual value, data should not be
combined across monitors because of the imprecision in the extreme
values. The EPA disagrees, noting that the data presented in the ISA
are based on hourly concentrations, while design values for the
O3 NAAQS are based on a 3-year average of 8-hour average
concentrations. Thus, the random variability in the hourly
O3 concentration data due to monitoring
[[Page 65411]]
imprecision will be reduced when concentrations are averaged for
comparison with the NAAQS. Additionally, the precision data are
typically collected at concentrations at or above the level of the
NAAQS, thus the EPA expects that the level of precision documented in
the ISA analysis is consistent with the level of precision in the
fourth-highest daily maximum concentrations used for determining
compliance with the NAAQS.
The EPA is finalizing this addition in Appendix U as proposed. In
addition, the AQS database will be updated to require state agencies to
designate a primary monitor at O3 monitoring sites that
report data under more than one Pollutant Occurrence Code (POC), a
numeric indicator in AQS used to identify individual monitoring
instruments. O3 design value calculations in AQS will be
updated so that the data will automatically be combined across POCs at
a site, and a single design value will be reported for each site. The
EPA notes that the substitution approach described above will only be
applied to design value calculations for the revised O3
standards, and that design values for previous O3 standards
will continue to be calculated at the monitor level, in accordance with
the applicable appendices of 40 CFR part 50.
Second, the EPA proposed to add a provision in Appendix U that
would allow the Regional Administrator to approve ``site
combinations'', or to combine data across two nearby monitors for the
purpose of calculating a valid design value. Although data handling
appendices for previous O3 standards do not explicitly
mention site combinations, the EPA has approved over 100 site
combinations since the promulgation of the first 8-hour O3
NAAQS in 1997. Thus, the EPA's intention in proposing this addition was
merely to codify an existing convention, and to improve transparency by
implementing site combinations in AQS design value calculations.
Public commenters unanimously supported this proposed addition. Two
commenters suggested that the EPA should require monitoring agencies to
provide technical documentation supporting the similarities between
sites approved for combining data, including a requirement for
simultaneous monitoring whenever possible. One state requested that the
EPA provide more detailed acceptability criteria for approving site
combinations, while another state urged the EPA not to create a
regulatory burden by prescribing detailed requirements codified in
regulations.
The EPA is finalizing this addition as proposed in Appendix U. The
EPA believes that approval of site combinations should be handled on a
case-by-case basis, and that any requests for supporting documentation
should be left to the discretion of the Regional Administrator. The EPA
may issue future guidance providing general criteria for determining an
acceptable level of similarity in air quality concentrations between
monitored locations, but is not prescribing detailed criteria for
approval of site combinations in this rulemaking.
Additionally, the AQS database will be updated with new fields for
monitoring agencies to request site combinations, and an additional
field indicating Regional Administrator approval. All pre-existing site
combinations will be initially entered into the database as having
already been approved by the Regional Administrator. Since this
provision has already been used in practice under previous
O3 standards, site combinations will be applied to AQS
design value calculations for both the revised O3 standards
and previous O3 standards.
C. Data Reporting and Data Handling Requirements
First, the EPA proposed a change in Appendix U to the pre-existing
8-hour average data substitution test (40 CFR part 50, Appendix P,
section 2.1) which is used to determine if a site would have had a
valid 8-hour average greater than the NAAQS when fewer than 6 hourly
O3 concentration values are available for a given 8-hour
period. The EPA proposed to change the value substituted for the
missing hourly concentrations from one-half of the method detection
limit of the O3 monitoring instrument (\1/2\ MDL) to zero
(0.000 ppm).
Several commenters supported the proposed change, stating that the
use of a constant substitution value instead of \1/2\ MDL, which can
vary across O3 monitoring methods, would simplify design
value calculations. One commenter noted that with a substitution value
of zero, the data substitution test for an 8-hour average value greater
than the NAAQS is equivalent to a sum of hourly O3
concentrations greater than 0.567 ppm (i.e., if the sum is 0.568 ppm or
higher, the resulting 8-hour average must be at least 0.071 ppm, which
is greater than the revised O3 NAAQS of 0.070 ppm). Finally,
one commenter opposed the proposed change in favor of some type of
mathematical or statistical interpolation approach, but did not provide
a specific recommendation.
The EPA is finalizing the proposed change in Appendix U, with the
addition of a short clause making note of the equivalent summation
approach described above. The purpose of the data substitution test is
to identify 8-hour periods that do not meet the requirements for a
valid 8-hour average, yet the reported hourly concentration values are
so high that the NAAQS would have been exceeded regardless of the
magnitude of the missing concentration values. The EPA believes that
zero, being the lowest measured O3 concentration physically
possible, is the most appropriate value to substitute in this
situation. Additionally, the EPA does not support the use of
interpolation or other means of filling in missing monitoring data for
O3 NAAQS comparisons. Such an approach would be contrary to
the EPA's long-standing policy of using only quality-assured and
certified ambient air quality measurement data to determine compliance
with the O3 NAAQS.
Second, the EPA proposed a new procedure in Appendix U for
determining daily maximum 8-hour O3 concentrations for the
revised NAAQS.\213\ The EPA proposed to determine the daily maximum 8-
hour O3 concentration based on 17 consecutive moving 8-hour
periods in each day, beginning with the 8-hour period from 7:00 a.m. to
3:00 p.m., and ending with the 8-hour period from 11:00 p.m. to 7:00
a.m. In addition, the EPA proposed that a daily maximum value would be
considered valid if 8-hour averages were available for at least 13 of
the 17 consecutive moving 8-hour periods, or if the daily maximum value
was greater than the level of the NAAQS. This procedure is designed to
eliminate ``double counting'' exceedances of the NAAQS based on
overlapping 8-hour periods from two consecutive days with up to 7 hours
in common, which was allowed under previous 8-hour O3 NAAQS.
A dozen public commenters expressed support for the proposed procedure,
including several states.
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\213\ This procedure will be adopted only for the revised
O3 NAAQS. Design values for the 1997 8-hour O3
NAAQS and the 2008 8-hour O3 NAAQS will continue to be
calculated according to Appendix I and Appendix P of 40 CFR part 50,
respectively.
---------------------------------------------------------------------------
One regional air quality management organization and three of its
member states submitted similar comments stating that they agreed with
the principle of eliminating ``double counting'' exceedances of the
NAAQS
[[Page 65412]]
based on overlapping 8-hour periods, but suggested an alternative
calculation procedure that would accomplish the same objective. The
alternative procedure iteratively finds the highest 8-hour period in a
given year, then removes this 8-hour period and all other 8-hour
periods associated with that day, including any overlapping 8-hour
periods on adjacent days, from the data until a daily maximum value is
determined for each day of the year with sufficient monitoring data.
The EPA examined a similar iterative procedure in a previous data
analysis supporting the proposal (Wells, 2014b, Method 1). The EPA
compared this procedure to the procedure proposed by the commenters
using the data from the original analysis and found the resulting daily
maximum 8-hour values to be nearly identical (Wells, 2015a).
Additionally, the commenters' procedure suffers from the same
limitations the EPA identified previously in the original analysis:
added complexity in design value calculations, longer computational
time, and challenges to real-time O3 data reporting systems,
which would have to re-calculate daily maximum 8-hour values for the
entire year each time the system was updated with new data.
Three states submitted comments stating that they agreed with the
proposed calculation procedure, but disagreed with the proposed
requirements for determining a valid daily maximum 8-hour O3
concentration. These states were primarily concerned that the proposed
requirements would only allow a monitoring site to have four missing 8-
hour averages during a day before the entire day would be invalidated,
compared with six missing 8-hour averages allowed previously. Two of
these states also stated concerns that the proposed requirements would
be more difficult to meet while maintaining compliance with existing
monitoring requirements such as biweekly quality assurance checks. The
EPA compared annual data completeness rates calculated using the
Appendix U requirements to annual data completeness rates calculated
using the requirements under the previous O3 standards
across all U.S. monitoring sites based on data from 2004-2013 (Wells,
2015a). The national mean annual data completeness rate was 0.1% higher
under the proposed Appendix U requirements than under the previous
O3 standards, and the national median annual data
completeness rates were identical. In addition, the EPA notes that the
Appendix U requirements allow for biweekly quality assurance checks and
other routine maintenance to be performed between 5:00 a.m. and 9:00
a.m. local time without affecting data completeness. Thus, the EPA does
not believe that the proposed daily data completeness requirements in
Appendix U will be more difficult for monitoring agencies to meet.
Finally, two public commenters opposed the proposed procedures for
determining daily maximum 8-hour concentrations. These commenters
expressed similar concerns, primarily that not considering 8-hour
periods starting midnight to 6:00 a.m. is less protective of public
health than the procedure used to determine daily maximum 8-hour
concentrations for the previous O3 standards. The EPA
believes that this approach provides the appropriate degree of
protection for public health, noting that the hourly concentrations
from midnight to 7:00 a.m. are covered under the 8-hour period from
11:00 p.m. to 7:00 a.m., which is included in the design value
calculations proposed in Appendix U. At the same time, the proposed
approach ensures that individual hourly concentrations may not
contribute to multiple exceedances of the NAAQS, which the EPA believes
is inappropriate given that people are only exposed once.
The EPA is finalizing as proposed in Appendix U the procedure for
determining daily maximum 8-hour concentrations. The EPA does not
believe that daily maximum 8-hour concentrations for two consecutive
days should be based on overlapping 8-hour periods, since the exposures
experienced by individuals only occur once. The EPA believes that the
new procedure will avoid this outcome while continuing to make use of
all hourly concentrations in determining attainment of the standards,
without introducing unnecessary complexity into design value
calculations, and without creating additional difficulties for
monitoring agencies to meet the data completeness requirements.
D. Exceptional Events Information Submission Schedule
The ``Treatment of Data Influenced by Exceptional Events; Final
Rule'' (72 FR 13560, March 22, 2007), known as the Exceptional Events
Rule and codified at 40 CFR 50.14, contains generic deadlines for an
air agency to submit to the EPA specified information about exceptional
events and associated air pollutant concentration data. As discussed in
this section and in more detail in the O3 NAAQS proposal,
without revisions to 40 CFR 50.14, an air agency may not be able to
flag and submit documentation for some relevant data either because the
generic deadlines may have already passed by the time a new or revised
NAAQS is promulgated or because the generic deadlines require
submission of documentation at least 12 months prior to the date by
which the EPA must make a regulatory decision, which may be before air
agencies have collected some of the potentially affected data. Specific
to the revised O3 NAAQS, revisions to 40 CFR 50.14 are
needed because it is not possible for air agencies to flag and submit
documentation for any exceptional events that occur in October through
December of 2016 by 1 year before the designations are made in October
2017, as is required by the existing generic schedule.
The EPA is finalizing exceptional events scheduling provisions in
40 CFR 50.14, as proposed and as supported by multiple commenters, that
will apply to the submission of information supporting claimed
exceptional events affecting pollutant data that are intended to be
used in the initial area designations for any new or revised NAAQS. The
new scheduling provisions will apply to initial area designations for
the revised O3 NAAQS. The provisions that we are
promulgating use a ``delta schedule'' that calculates the timelines
associated with flagging data potentially influenced by exceptional
events, submitting initial event descriptions and submitting
exceptional events demonstrations based on the promulgation date of a
new or revised NAAQS. The general data flagging deadlines in the
Exceptional Events Rule at 40 CFR 50.14(c)(2)(iii) and the general
schedule for submission of demonstrations at 40 CFR 50.14(c)(3)(i)
continue to apply to data used in regulatory decisions other than those
related to the initial area designations process under a new or revised
NAAQS.\214\
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\214\ The EPA intends to consider changes to these retained
scheduling requirements as part of the planned notice and comment
rulemaking revisions to the 2007 Exceptional Events Rule.
---------------------------------------------------------------------------
The EPA acknowledges the concern raised by several commenters that
a strengthened O3 NAAQS may result in numerous
demonstrations for exceptional events occurring between 2014 and 2016,
the data years that the EPA will presumably use for initial area
designation decisions made in October 2017.\215\ Commenters noted that
the proposed schedule is particularly burdensome for agencies needing
to submit exceptional events packages for
[[Page 65413]]
the third year to be used in a 3-year design value (i.e., 2016 data).
Several commenters recommended that the EPA either establish no defined
schedule for data flagging and exceptional events demonstration
submittal or allow a minimum of 2 years from the setting of any new or
revised NAAQS for air agencies to provide a complete exceptional events
demonstration. Given the CAA requirement that the EPA follow a 2-year
designations schedule, the EPA cannot remove submittal schedules
entirely for data influenced by exceptional events or provide a minimum
2-year period from the setting of a new or revised NAAQS for
documentation submittal. Neither of these options would ensure that the
EPA has time to consider event-influenced data in initial area
designation decisions. Rather, the EPA is promulgating in this action
an exceptional events schedule that provides air agencies with the
maximum amount of time available to prepare exceptional events
demonstrations and will still allow the EPA sufficient time to consider
such exceptional events demonstrations in the designations process in
advance of the date by which the EPA must send 120-day notification
letters to states.\216\ The EPA recognizes that the schedule
promulgated in this action is compressed, particularly for the third
year of data to be used in a 3-year design value, and we will work
cooperatively with air agencies to accommodate this scenario.
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\215\ Governors may also use 2013 data to formulate their
recommendations regarding designations.
\216\ See Section VIII.B for additional detail on the initial
area designations process for the revised O3 NAAQS.
---------------------------------------------------------------------------
Under the schedule promulgated in this action and assuming initial
area designation decisions in October 2017 for the revised
O3 NAAQS, affected air agencies would need to flag data,
submit initial event descriptions and submit demonstrations for
exceptional events occurring in 2016 by May 31, 2017. This schedule
provides approximately 5 months between the EPA's receipt of the
demonstration package and the expected date of designation decisions
and approximately 1 month between the EPA's receipt of a package and
the date by which the EPA must notify states and tribes of intended
modifications to the Governors' recommendations for designations (i.e.,
120-day letters).
While, for the third year of data anticipated to be used in a 3-
year design value for the revised O3 NAAQS, the promulgated
schedule provides for demonstration submission 5 months after the end
of the calendar year, the EPA expects that most submitting agencies
will have additional time to prepare documentation as we expect the
majority of potential O3-related exceptional events to occur
during the warmer months (e.g., March through October). Additionally,
the EPA will soon propose rule revisions to the 2007 Exceptional Events
Rule and will release through a Federal Register Notice of Availability
a draft guidance document to address Exceptional Events Rule criteria
for wildfires that could affect O3 concentrations. We expect
to promulgate Exceptional Events Rule revisions and finalize the new
guidance document before the October 2016 date by which states, and any
tribes that wish to do so, are required to submit their initial
designation recommendations for the revised O3 NAAQS.
Considered together, the EPA believes the exceptional events scheduling
dates promulgated in this action, the upcoming Exceptional Events Rule
revisions, the forthcoming guidance, and the existing guidance and
examples of submitted demonstrations currently on the EPA's exceptional
events Web site at http://www2.epa.gov/air-quality-analysis/treatment-data-influenced-exceptional-events, will help air agencies submit
information in a timely manner.
Applying the ``delta schedule'' promulgated in this action for air
quality data collected in 2013 through 2014 that could be influenced by
exceptional events and be considered during the initial area
designations process for the revised O3 NAAQS, results in
extending to July 1, 2016, the otherwise applicable generic deadlines
of July 1, 2014, and July 1, 2015, respectively, for flagging data and
providing an initial description of an event (40 CFR 50.14(c)(2)(iii)).
The schedule promulgated in this action also results in a July 1, 2016,
date for flagging data and providing an initial description of an event
for air quality data collected in 2015. The July 1, 2016, date for data
collected in 2015 is the same as that which would apply under the
existing generic deadline in the 2007 Exceptional Events Rule. Under
the schedule promulgated in this action, October 1, 2016 is the
deadline for submitting exceptional events demonstrations for data
years 2013 through 2015. As noted previously, under the schedule
promulgated in this action, affected air agencies would need to flag,
submit initial event descriptions and submit demonstrations for
exceptional events occurring in 2016 by May 31, 2017. The EPA believes
these revisions will provide adequate time for air agencies to review
potential O3 exceptional events influencing compliance with
the revised O3 NAAQS, to notify the EPA by flagging the
relevant data and providing an initial event description in AQS, and to
submit documentation to support exceptional events demonstrations. The
schedule revisions promulgated in this action will also allow the EPA
to consider and act on the submitted information during the initial
area designation process.
While the EPA will make every effort to designate areas for any new
or revised NAAQS on a 2-year schedule, the EPA recognizes that under
some circumstances we may need up to an additional year for the
designations process to ensure that air agencies and the EPA base
designations decisions on complete and sufficient information. The
promulgated schedule accounts for the possibility that the EPA might
announce after promulgating a new or revised NAAQS that we are
extending the designations schedule beyond 2 years using authority
provided in CAA section 107(d)(B)(i). If the EPA determines that we
will follow a 3-year designation schedule, the deadline is 2 years and
7 months after promulgation of a new or revised NAAQS for states to
flag data influenced by exceptional events, submit initial event
descriptions and submit exceptional events demonstrations for the last
year of data that will be used in the designations (e.g., if the EPA
were to designate areas in October 2018, the exceptional events
submittal deadline for 2017 data would be May 31, 2018). If the EPA
notifies states and tribes of a designations schedule between 2 and 3
years, the deadline for states to flag data affected by exceptional
events, submit initial event descriptions, and submit exceptional
events demonstrations associated with data from the last year to be
considered would be 5 months prior to the date specified for
designation decisions.
Therefore, using the authority provided in CAA section 319(b)(2)
and in the 2007 Exceptional Events Rule at 40 CFR 50.14(c)(2)(vi), the
EPA is modifying the schedule for flagging data and submitting
exceptional events demonstrations considered for initial area
designations by replacing the deadlines and information in Table 1 in
40 CFR 50.14 with the deadlines and information presented in Table 5.
As we did in the O3 NAAQS proposal, we are also providing
Table 6 to illustrate how the promulgated schedule might apply to the
designations process for the revised O3 NAAQS and to
designations
[[Page 65414]]
processes for other future new or revised NAAQS.\217\
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\217\ The range of dates identified in Table 6 is illustrative
of the dates for the revised O3 NAAQS. Users could
increment these dates by any constant number (for example by 6 years
for a hypothetical NAAQS promulgated in 2021) to develop a table
with dates relevant to NAAQS promulgated in the future.
---------------------------------------------------------------------------
Additionally, in conjunction with promulgating exceptional events
schedules for initial area designations for new or revised NAAQS, the
EPA, as proposed, is removing obsolete regulatory language in 40 CFR
50.14(c)(2)(iv) and (v) and 40 CFR 50.14(c)(3)(ii) and (iii) associated
with exceptional events schedules for all historical standards.
Table 5--Schedule for Flagging and Documentation Submission for Data
Influenced by Exceptional Events for Use in Initial Area Designations
------------------------------------------------------------------------
Exceptional events deadline
Exceptional events/Regulatory action schedule \d\
------------------------------------------------------------------------
Flagging and initial event description If state and tribal initial
deadline for data years 1, 2 and 3 \a\. designation recommendations
for a new/revised NAAQS are
due August through January,
then the flagging and initial
event description deadline
will be the July 1 prior to
the recommendation deadline.
If state and tribal
recommendations for a new/
revised NAAQS are due February
through July, then the
flagging and initial event
description deadline will be
the January 1 prior to the
recommendation deadline.
Exceptional events demonstration No later than the date that
submittal deadline for data years 1, 2 state and tribal
and 3 \a\. recommendations are due to the
EPA.
Flagging, initial event description and By the last day of the month
exceptional events demonstration that is 1 year and 7 months
submittal deadline for data year 4 \b\ after promulgation of a new/
and, where applicable, data year 5 \c\. revised NAAQS, unless either
option a or b applies.
a. If the EPA follows a 3-year
designation schedule, the
deadline is 2 years and 7
months after promulgation of a
new/revised NAAQS.
b. If the EPA notifies the
state/tribe that it intends to
complete the initial area
designations process according
to a schedule between 2 and 3
years, the deadline is 5
months prior to the date
specified for final
designations decisions in such
EPA notification.
------------------------------------------------------------------------
\a\ Where data years 1, 2, and 3 are those years expected to be
considered in state and tribal recommendations.
\b\ Where data year 4 is the additional year of data that the EPA may
consider when it makes final area designations for a new/revised NAAQS
under the standard designations schedule.
\c\ Where data year 5 is the additional year of data that the EPA may
consider when it makes final area designations for a new/revised NAAQS
under an extended designations schedule.
\d\ The date by which air agencies must certify their ambient air
quality monitoring data in AQS is annually on May 1 of the year
following the year of data collection as specified in 40 CFR
58.15(a)(2). In some cases, however, air agencies may choose to
certify a prior year's data in advance of May 1 of the following year,
particularly if the EPA has indicated its intent to promulgate final
designations in the first 8 months of the calendar year. Data
flagging, initial event description and exceptional events
demonstration deadlines for ``early certified'' data will follow the
deadlines for ``year 4'' and ``year 5'' data.
[[Page 65415]]
[GRAPHIC] [TIFF OMITTED] TR26OC15.001
[[Page 65416]]
VI. Ambient Monitoring Related to O3 Standards
A. Background
The EPA proposed to revise the state-by-state O3
monitoring seasons; the PAMS monitoring requirements; the FRM for
measuring O3; and the FEM performance requirement
specifications for automated O3 analyzers. The EPA also
proposed to make additional minor changes to the FEM analyzer
performance testing requirements for NO2 and particulate
matter in part 53.
The EPA is finalizing changes to the length of the required
O3 monitoring season for 32 states and the District of
Columbia. Section VI.B of this preamble provides an overview of the
proposed changes to the length of the required O3 monitoring
seasons, a summary of significant public comments and our responses,
and a summary of the final decisions made to the O3
monitoring seasons for each state.
The EPA is finalizing changes to the PAMS monitoring requirements
in 40 CFR part 58, Appendix D Section 5. Section VI.C of this preamble
provides background on the PAMS program and current monitoring
requirements, a summary of the proposed changes to the PAMS
requirements, a summary of significant public comments and our
responses, and a summary of the changes to the PAMS requirements in
this final rule.
The EPA is finalizing changes to the FRM for O3 in
Section VI.D of this preamble and to the associated FEM performance
requirement specifications for automated O3 analyzers in
Section VI.E. A summary of significant public comments and our
responses are provided and a summary of the final changes to the FRM
and FEM requirements in this final rule. The EPA is also finalizing
minor additional changes to Part 53 including conforming changes to the
FEM performance testing requirements in Table B-1 and Figure B-5 for
NO2; extending the period of time for the Administrator to
take action on a request for modification of a FRM or FEM from 30 days
to 90 days in part 53.14; and removing an obsolete provision for
manufacturers to submit Product Manufacturing Checklists for fine and
coarse particulate matter monitors in part 53.9.
B. Revisions to the Length of the Required O3 Monitoring
Seasons
Unlike the ambient monitoring requirements in 40 CFR part 58 for
other criteria pollutants that mandate year-round monitoring at State
and Local Air Monitoring Stations (SLAMS), O3 monitoring is
only required during the seasons of the year that are conducive to
O3 formation. These seasons vary in length from place-to-
place as the conditions conducive to the formation of O3
(i.e., seasonally-dependent factors such as ambient temperature,
strength of solar insolation, and length of day) differ by location. In
some locations, conditions conducive to O3 formation are
limited to the summer months of the year. In other states with warmer
climates (e.g., California, Nevada, and Arizona), the currently
required O3 season is year-round. Elevated levels of winter-
time O3 have also been measured in some western states where
precursor emissions can interact with sunlight off the snow cover under
very shallow, stable boundary layer conditions (U.S. EPA 2013).
The EPA has determined that the proposed lengthening of the
O3 monitoring seasons in 32 states and the District of
Columbia is appropriate. Ambient O3 concentrations in these
areas could approach or exceed the level of the NAAQS, more frequently
and during more months of the year compared with the current season
lengths. It is important to monitor for O3 during the
periods when ambient concentrations could approach the level of the
NAAQS to ensure that the public is informed when exposure to
O3 could reach or has reached a level of concern.
The EPA completed an analysis to address whether extensions of
currently required monitoring seasons are appropriate (Rice, 2014). In
this analysis, we used all available data in AQS, including data from
monitors that collected O3 data year-round during 2010-2013.
More than half of O3 monitors are voluntarily operated on a
year-round basis by monitoring agencies. We determined the number of
days where one or more monitors had a daily maximum 8-hour
O3 average equal to or above 0.060 ppm in the months outside
each state's current O3 monitoring season and the pattern of
those days in the out-of-season months. We believe that a threshold of
0.060 ppm, taking into consideration reasonable uncertainty, serves as
an appropriate indicator of ambient conditions that may be conducive to
the formation of O3 concentrations that approach or exceed
the NAAQS. We also considered regional consistency, particularly for
those states with little available data. We note that seasonal
O3 patterns vary year-to-year due primarily to highly
variable meteorological conditions conducive to the formation of
elevated O3 concentrations early or late in the season in
some years and not others. The EPA believes it is important that
O3 monitors operate during all periods when there is a
reasonable possibility of ambient levels approaching the level of the
NAAQS.
Basing O3 monitoring season requirements on the goal of
ensuring monitoring when ambient O3 levels approach or
exceed the level of the NAAQS supports established monitoring network
objectives described in Appendix D of Part 58, including the
requirement to provide air pollution data to the general public in a
timely manner \218\ and to support comparisons of an area's air
pollution levels to the NAAQS. The operation of O3 monitors
during periods of time when ambient levels approach or exceed the level
of the NAAQS ensures that unusually sensitive people and sensitive
groups are alerted to O3 levels of potential health concern
allowing them to take precautionary measures. The majority of
O3 monitors in the U.S. report to AIRNOW,\219\ as well as to
state-operated Web sites and automated phone reporting systems. These
programs support many objectives including real-time air quality
reporting to the public, O3 forecasting, and the
verification of real-time air quality forecast models.
---------------------------------------------------------------------------
\218\ Public reporting requirements are detailed in 40 CFR part
58 Appendix G, Uniform Air Quality Index (AQI) and Daily Reporting.
\219\ See http://airnow.gov/.
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1. Proposed Changes to the Length of the Required O3
Monitoring Seasons
The EPA proposed to extend the length of the required O3
monitoring season in 32 states and the District of Columbia. The
proposed changes were an increase of one month for 22 states
(Connecticut, Delaware, Idaho, Illinois, Iowa, Kansas, Maryland,
Massachusetts, Minnesota, Missouri, Nebraska, New Hampshire, New
Jersey, New York, North Carolina, Ohio, Pennsylvania, Rhode Island,
South Carolina, Texas (northern portion only), Virginia, and West
Virginia) and the District of Columbia, an increase of one and one half
months for Wisconsin, an increase of two months for four states
(Indiana, Michigan, Montana, and North Dakota), an increase of four
months for Florida and South Dakota, an increase of five months for
Colorado, and an increase of seven months for Utah. For Wyoming, we
proposed to add three months at the beginning of the season and remove
one month at the end of the season, resulting in a net increase of two
months. Ozone season requirements are currently split by Air Quality
Control Region (AQCR) in Louisiana and Texas. We proposed lengthening
the required season in the northern part of Texas (AQCR 022, 210,
[[Page 65417]]
211, 212, 215, 217, and 218) by one month and leaving the year-round
O3 season in the southern part of Texas (AQCRs 106, 153,
213, 214, and 216) unchanged. No changes were proposed for the AQCRs in
Louisiana. As noted earlier, in a few states with limited available
data and few exceedance days outside the currently-required season
(Iowa, Missouri, and West Virginia), the proposed changes were made by
considering supporting information from the surrounding states. These
changes involved the proposed addition of one month (March) to the
currently-required O3 seasons for these states.
The EPA also proposed that O3 monitors at all National
Core Multipollutant Monitoring Stations (NCore) be operated year-round,
January through December, regardless of the length of the required
O3 season for the remainder of the SLAMS within each state.
We noted that the EPA Regional Administrators have previously
approved deviations from the required O3 monitoring seasons
as allowed by paragraph 4.1(i) of 40 CFR part 58, Appendix D. We
proposed to retain the rule language permitting such deviations from
the required O3 monitoring seasons, but note that finalized
changes to O3 monitoring season requirements would revoke
all existing Regional Administrator-granted waiver approvals. As
appropriate, monitoring agencies could seek new approvals for seasonal
deviations. Any seasonal deviations based on the Regional
Administrator's waiver of requirements must be described in the state's
annual monitoring network plan and updated in the AQS.
Given the timing of the final rulemaking and any associated burden
on state/local monitoring agencies to implement the extended
O3 seasons, we proposed that implementation of the revised
O3 seasons would become effective at SLAMS (including NCore
sites) on January 1, 2017. We solicited comment on whether the revised
seasons could be implemented beginning January 1, 2016, for all
monitors or for a subset of monitors, such as those currently operating
year-round or on a schedule that corresponds to the proposed
O3 season.
2. Comments on the Length of the Required O3 Monitoring
Seasons
We received several comments on the proposed revisions to
O3 monitoring seasons. Several commenters supported the
proposed O3 season length changes and agreed that
O3 monitoring seasons should reflect the times of year when
O3 may approach or exceed the level of the NAAQS. A few
commenters noted the complexities that would arise in the
implementation of multi-state planning agreements if states that shared
an MSA had different required O3 monitoring seasons. Two
state agencies that supported season length changes also recommended
changes to neighboring states' O3 seasons. New York
recommended that Connecticut's proposed O3 season be further
extended (adding the month of October) to match the proposed season in
New York (March-October) because they share a major MSA and
nonattainment area, and the highest design value monitor in the
nonattainment area is often in Connecticut. The results from the EPA's
analysis did not support the addition of October for Connecticut. The
EPA recognizes that there may be value in having a consistent
O3 season across multi-state planning areas. We recommend
that monitoring agency representatives from New York and Connecticut
contact their respective EPA Regional Office to jointly develop a
monitoring plan to provide coverage of the MSA for a longer period of
time. Consistent with the results from the EPA's analysis and
consistent with our proposal, the EPA is finalizing the March-October
season in New York and the March-September season in Connecticut.
Although no changes were proposed for Arkansas, the Arkansas
Department of Environmental Quality recommended that the O3
season in the nonattainment area that includes Crittenden County,
Arkansas (March-November) be consistent with the O3 seasons
in Tennessee (March-October) and Mississippi (March-October) by either
shortening the O3 season in Arkansas or lengthening the
O3 season by one month in Tennessee and Mississippi. Based
on the results from the EPA's analysis and consistent with our
proposal, the EPA is not finalizing any changes to the current
O3 seasons in Arkansas, Tennessee, or Mississippi. There is
currently one monitor operating in Crittenden County. We recommend that
Arkansas work with their EPA Regional Administrator to consider a
waiver for the monitor(s) in Crittenden County to allow a deviation
(shortened season) from the required O3 season if the agency
demonstrates that such a deviation is appropriate for consistency in
the nonattainment area.
Two commenters noted the need to extend seasons to capture
wintertime O3 events. One commenter urged the EPA to extend
monitoring to year-round in the intermountain west (specifically
Wyoming) to adequately capture summer and winter O3 problem
days and noted especially two monitors in the Pinedale area of Wyoming
that should be operated year-round. The EPA's analysis showed that
there were no days that were >= 0.060 ppm in Wyoming for the months of
October-December and that the Wyoming Department of Environmental
Quality is currently operating about 70% of their O3
monitors year-round including all O3 monitors in Sublette
County, which includes the Pinedale area. Another commenter supported
lengthening the seasons for states in the western U.S. where wintertime
O3 could be an issue in light of the unique and growing
O3 pollution problems caused by oil and gas development
activities. They also recommended that the EPA expand the O3
monitoring season to year-round for North Dakota, South Dakota, and
Montana beyond what was proposed. The number of observed days that were
>= 0.060 ppm in the months outside the season proposed for these states
(one day for North Dakota and no days observed for South Dakota and
Montana) do not support a further extension to the length of the
O3 monitoring season beyond what was proposed. These states
are already operating a large percentage of their monitors year-round
(89% in North Dakota, 100% in South Dakota, and 78% in Montana). The
EPA is finalizing the seasons as proposed in Wyoming (January-
September), North Dakota (March-September), South Dakota (March-
October), and Montana (April-September). The EPA encourages these
states to continue year-round operation of their monitors to determine
what areas are affected by elevated levels of winter-time
O3.
The commenters who opposed lengthening the O3 monitoring
seasons noted concerns with the threshold (0.060 ppm) used as the basis
for the changes and the length of time (2010-2013) for which ambient
data were retrieved and analyzed. Many of those with concerns
recommended that levels in the proposed range (e.g., 0.065 ppm or 0.070
ppm) or the current NAAQS level of 0.075 ppm be used as the appropriate
threshold for determining the O3 season. With regard to the
0.060 ppm threshold used, this value is consistent with the 85 percent
threshold used to require additional O3 monitoring based on
Appendix D requirements, which include the MSA population and design
value.\220\ As noted previously, year-to-year variability occurs in
seasonal O3 patterns based on highly variable and
unpredictable meteorological
[[Page 65418]]
conditions, which can support the formation of early or late season
elevated O3 concentrations in some years and not in other
years. This threshold serves as an appropriate indicator of ambient
conditions that may be conducive to the formation of O3
concentrations that approach or exceed the level of the NAAQS.
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\220\ See 40 CFR part 58, appendix D, Table D-2.
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Certain logistical complexities were noted if longer seasons were
required, including site access during winter and the challenge of
getting the monitoring equipment ready in time. Four states noted
concerns with operator safety and anticipated their inability to access
sites due to early spring snowfall. The EPA agrees that site access
could be an issue depending on weather conditions and notes that
specific site monitoring season deviations may be appropriate. We
suggest that this be addressed through the monitoring season waiver
process with the EPA Regional Administrator. Any deviations based on
the Regional Administrator's waiver of requirements must be described
in the state's annual monitoring network plan and updated in AQS.
Several commenters had concerns about the additional cost and
resources needed to expand the O3 monitoring seasons. There
was some disagreement with the EPA's total annual average cost estimate
of $230,000 which took into account the number of O3
monitors already operating year-round across the country. Commenters
noted specifically that the proposed extension of required monitoring
seasons would increase operational costs and potentially impact the
resources available for other monitoring efforts. The added cost of
operating O3 monitors over a longer period was noted by some
commenters, referencing both the cost of staff to operate the monitors,
as well as the additional wear and tear those O3 monitors
would experience over a longer operational period. They noted that
extending their required monitoring season by adding the month of March
would increase staffing requirements for monitor operation and quality
assurance. They also noted that the life expectancy of equipment would
be reduced due to increased wear and tear. The EPA acknowledges that
operational costs for O3 monitoring networks will
incrementally increase in states where required seasons have been
lengthened. We encourage monitoring agencies to review available
technology and operational procedures to institute practices that could
potentially reduce such costs, such as the automation of quality
control and calibration checks and remote access to evaluate monitor
operations. As noted earlier, all states operated at least a portion of
their O3 monitoring network outside of the required
O3 season during the 2010-2013 data period and reported the
data to AQS. In addition, many states are operating more than the
minimum number of monitors required to support the basic monitoring
objectives described in 40 CFR part 58, Appendix D. Some states have a
large percentage of their total O3 monitors operating
outside the currently-required O3 season and some states
have a small percentage. In situations where states are already
operating a large number of their O3 monitors outside their
current O3 season, the actual cost increase will be less. In
cases where states have a small number of monitors operating outside
their current O3 season, in addition to automation and
remote access, those states could investigate with their Regional
Administrator the process in 40 CFR part 58.14 for reducing the total
number of operating monitors that are above the number required by 40
CFR, part 58, appendix D to offset the cost of extending the
O3 monitoring season in their state.
Two commenters had concerns about the 4-year period of time
evaluated in the EPA's analysis and noted that the 4-year period of
time evaluated does not take into account meteorological anomalies and
other weather induced situations and is not consistent with the 3 years
used to calculate design values. One state agency's comments referenced
their own analysis showing concentrations going back 20 years. They
noted that 2010 was an unusual year and inclusion of such an unusual
year in the 4-year period (2010-2013) of the EPA's analysis provides
too much weight on those data. As noted earlier, year-to-year
variability occurs in seasonal O3 patterns based on variable
meteorological conditions and given the impracticality of forecasting
such conditions that affect O3 photochemistry, the EPA
believes it is important that O3 monitors operate when there
is a reasonable possibility of ambient levels approaching the level of
the NAAQS. Another state agency commented that 4 years appeared to be
an unusual number of years given that design values are based on 3
years. To support the proposed rule in 2014, the EPA's analysis of
O3 seasons began in 2013. At that time the EPA's analysis
considered the most recent 3 years of certified data (2010-2012) and
updated the analysis to add a fourth year (2013) when the data were
quality-assured, certified, and available in AQS. We used 4 years of
data, including the most recent year (2013) to include an additional
year of potentially-variable meteorological conditions to propose
changes to the seasons. The EPA treated all years equally and did not
put any more weight on the 2010 data than any of the other years used
in the analysis. The EPA believes that using recently-available data
across multiple years to capture varying meteorological conditions was
appropriate to support the decisions on extending the O3
seasons. One commenter disagreed with the EPA's definition of year-
round (at least 20 daily observations in all 12 months of at least 1
year of the 4-year period). The definition of year-round was used to
estimate the number of monitors being operated outside a state's
required O3 season and also used for the EPA's Information
Collection Request (ICR). All available data in AQS were used for the
O3 season analysis, including data from year-round monitors.
Two commenters noted that ``regional consistency'' is not a
scientific reason and is not needed for making changes to the
O3 seasons. One commenter noted that significant
geographical, meteorological and demographic differences exist between
neighboring states that may not warrant identical monitoring seasons.
The EPA notes that regional consistency was considered, but only
important for a few states where little data were available and the
neighboring states had more available data and a sufficient number of
days that were >= 0.060 ppm to support the proposed O3
season changes. Regional consistency was not important for other
states.
Some commenters expressed support for the proposed requirement that
NCore O3 sites operate year-round. They questioned whether
data from NCore stations outside the O3 season will be used
for designations and requested that the EPA exclude those data from the
designations process. Consistent with the designations process for all
criteria pollutants, the states, tribes, and the EPA use all data
available in AQS that meet the quality assurance requirements in 40 CFR
part 58, Appendix A for the designations process. Given that
O3 data from NCore stations will meet these requirements,
there is no rational basis for excluding these data from comparison to
the NAAQS. Accordingly, such data from NCore stations cannot be
excluded and will be treated in a manner equivalent to all other
O3 data in AQS. The EPA expects that the highest
O3 values will occur during the required O3
season; therefore, we don't anticipate that NCore data from the out-of-
season months will contribute to the design value used in
[[Page 65419]]
the designations process. The EPA is finalizing the requirement for
year-round O3 monitoring at NCore stations.
The EPA Regional Administrators have previously approved deviations
from the required O3 monitoring seasons through rulemakings
(64 FR 3028, January 20, 1999; 67 FR 57332, September 10, 2002; and 69
FR 52836, August 30, 2004). The current ambient monitoring rule, in
paragraph 4.1(i) of 40 CFR part 58, Appendix D (71 FR 61319, October
17, 2006), allows the EPA Regional Administrators to approve changes to
the O3 monitoring season without rulemaking. The EPA is
retaining the rule language allowing such deviations from the required
O3 monitoring seasons without rulemaking. In the finalized
revision to paragraph 4.1(i) of 40 CFR part 58, Appendix D, the EPA is
clarifying the minimum considerations that should be taken into account
when reviewing requests, and clarifying that changes to the
O3 seasons finalized in this rule revoke all previously
approved seasonal deviations. The EPA clarifies that all O3
season waivers will be revoked when this final rule becomes effective.
We encourage monitoring agencies with existing waivers to engage their
EPA Regions as soon as possible to evaluate whether new or continued
waivers are appropriate given the level of the revised O3
NAAQS.
We received three comments for and three comments against early
implementation of the revised O3 seasons by the start of the
applicable O3 season in each state by January 1, 2016. Those
commenters in favor of early implementation of the revised
O3 seasons are already operating a large percentage of
O3 monitors year-round or outside the current O3
monitoring season in their state. Those commenters against early
implementation cited concerns with the need for additional time to
implement the revised O3 seasons, especially in areas where
access in order to service and support the monitoring equipment may be
problematic during winter weather conditions, and the undue burden on
already constrained state resources. One commenter noted that given the
date for the final rule (October 1, 2015) that there is insufficient
time for public review of their annual monitoring network plan due July
1, 2015, for early implementation in 2016. The EPA encourages those
agencies who are able to implement the O3 season changes
early to do so by the start of the applicable O3 season in
their state in 2016. However, taking into consideration the timing and
potential burden on monitoring agencies, the EPA is finalizing the
requirement for implementing the revised O3 seasons no later
than the start of the applicable O3 monitoring season in
2017, as proposed.
3. Final Decisions on the Length of the Required O3
Monitoring Seasons
Final changes to the required O3 monitoring seasons are
summarized in this section as well as in revised Table D-3 in 40 CFR
part 58, Appendix D.
Detailed state-by-state technical information has been placed in
the docket to document the basis for the EPA's decision on each state.
This information includes state-by-state maps and number of days that
were >= 0.060 ppm; distribution charts of the number of days that were
>= 0.060 ppm by month and state; and detailed information regarding AQS
site IDs, dates and concentrations of all occurrences of the 8-hour
daily maximum of at least 0.060 ppm between 2010 and 2013. Summaries
have also been prepared for each state including the former and
proposed O3 monitoring seasons.
No changes to the required O3 monitoring season were
proposed or finalized for these states: Alabama, Alaska, Arizona,
Arkansas, California, Georgia, Hawaii, Kentucky, Northern Louisiana
(AQCR \221\ 019, 022), Southern Louisiana (AQCR 106), Maine,
Mississippi, Nevada, New Mexico, Oklahoma, Oregon, Tennessee, Southern
Texas (AQCR 106, 153, 213, 214, 216), Vermont, Washington, Puerto Rico,
Virgin Islands, Guam, and American Samoa. All existing O3
season deviations or waivers are revoked.
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\221\ Air Quality Control Region.
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Changes to the required O3 monitoring seasons are
finalized as follows for these states and the District of Columbia and
all existing O3 season deviations or waivers are revoked.
Colorado: Proposed addition of January, February, October,
November, and December is finalized. The required season is revised to
January-December.
Connecticut: Proposed addition of March is finalized, revising
season to March-September.
Delaware: Proposed addition of March is finalized, revising season
to March-October.
District of Columbia: Proposed addition of March is finalized,
revising season to March-October.
Florida: Proposed addition of January, February, November, and
December is finalized. The required season is revised to January-
December.
Idaho: Proposed addition of April is finalized, revising season to
April-September.
Illinois: Proposed addition of March is finalized, revising season
to March-October.
Indiana: Proposed addition of March and October, revising season to
March-October.
Iowa: Proposed addition of March is finalized, revising season to
March-October.
Kansas: Proposed addition of March is finalized, revising season to
March-October.
Maryland: Proposed addition of March is finalized, revising season
to March-October.
Massachusetts: Proposed addition of March is finalized, revising
season to March-September.
Michigan: Proposed addition of March and October is finalized,
revising season to March-October.
Minnesota: Proposed addition of March is finalized, revising season
to March-October.
Missouri: Proposed addition of March is finalized, revising season
to March-October.
Montana: Proposed addition of April and May is finalized, revising
season to April-September.
Nebraska: Proposed addition of March is finalized, revising season
to March-October.
New Hampshire: Proposed addition of March is finalized, revising
season to March-September.
New Jersey: Proposed addition of March is finalized, revising
season to March-October.
New York: Proposed addition of March is finalized, revising season
to March-October.
North Carolina: Proposed addition of March is finalized, revising
season to March-October.
North Dakota: Proposed addition of March and April is finalized,
revising season to March-September.
Ohio: Proposed addition of March is finalized, revising season to
March-October.
Pennsylvania: Proposed addition of March is finalized, revising
season to March-October.
Rhode Island: Proposed addition of March is finalized, revising
season to March-September.
South Carolina: Proposed addition of March is finalized, revising
season to March-October.
South Dakota: Proposed addition of March, April, May, and October
is finalized, revising season to March-October.
Texas (Northern AQCR 022, 210, 211, 212, 215, 217, 218): Proposed
addition of November is finalized, revising season to March-November.
Utah: Proposed addition of January, February, March, April,
October,
[[Page 65420]]
November, and December is finalized. The required season is revised to
January-December.
Virginia: Proposed addition of March is finalized, revising season
to March-October.
West Virginia: Proposed addition of March is finalized, revising
season to March--October.
Wisconsin: Proposed addition of March and April 1--15 is finalized,
revising season to March--October 15.
Wyoming: Proposed addition of January, February, March, and removal
of October is finalized, revising season to January--September.
Finally, we are finalizing the required O3 monitoring
season for all NCore stations to be year-round (January--December)
regardless of the required monitoring season for the individual state
in which the NCore station is located.
C. Revisions to the PAMS Network Requirements
Section 182 (c)(1) of the CAA required the EPA to promulgate rules
for enhanced monitoring of O3, NOX, and VOCs for
nonattainment areas classified as serious (or above) to obtain more
comprehensive and representative data on O3 air pollution.
In addition, Section 185B of the CAA required the EPA to work with the
National Academy of Sciences (NAS) to conduct a study on the role of
O3 precursors in tropospheric O3 formation and
control. As a result of this study, the NAS issued the report entitled,
``Rethinking the Ozone Problem in Urban and Regional Air Pollution'',
(NAS, 1991).
In response to the CAA requirements and the recommendations of the
NAS report, on February 12, 1993 (58 FR 8452), the EPA revised the
ambient air quality surveillance regulations to require PAMS in each
O3 nonattainment area classified as serious, severe, or
extreme (``PAMS areas''). As noted in the EPA's Technical Assistance
Document (TAD) for Sampling and Analysis of Ozone Precursors (U.S. EPA,
1998), the current objectives of the PAMS program are to: (1) Provide a
speciated ambient air database that is both representative and useful
in evaluating control strategies and understanding the mechanisms of
pollutant transport by ascertaining ambient profiles and distinguishing
among various individual volatile organic compounds (VOCs); (2) provide
local, current meteorological and ambient data to serve as initial and
boundary condition information for photochemical grid models; (3)
provide a representative, speciated ambient air database that is
characteristic of source emission impacts to be used in analyzing
emissions inventory issues and corroborating progress toward
attainment; (4) provide ambient data measurements that would allow
later preparation of unadjusted and adjusted pollutant trends reports;
(5) provide additional measurements of selected criteria pollutants for
attainment/nonattainment decisions and to construct NAAQS maintenance
plans; and (6) provide additional measurements of selected criteria and
non-criteria pollutants to be used for evaluating population exposure
to air toxics as well as criteria pollutants.
The original requirements called for two to five fixed sites per
PAMS area depending on the area's population. Four types of PAMS sites
were identified including upwind (Type 1), maximum precursor emission
rate (Type 2), maximum O3 concentration (Type 3), and
extreme downwind (Type 4) sites. Each PAMS site was required to measure
O3, nitrogen oxide (NO), NO2, speciated VOCs,
selected carbonyl compounds, and selected meteorological parameters. In
addition, upper air meteorological monitoring was required at one site
in each PAMS area.
In the October 17, 2006 monitoring rule (71 FR 61236), the EPA
revised the PAMS requirements to only require two sites per PAMS area.
The intent of the revision was to ``allow PAMS monitoring to be more
customized to local data needs rather than meeting so many specific
requirements common to all subject O3 nonattainment areas;
the changes also gave states the flexibility to reduce the overall size
of their PAMS programs--within limits--and to use the associated
resources for other types of monitoring they consider more useful.'' In
addition to reducing the number of required sites per PAMS area, the
2006 revisions also limited the requirement for carbonyl measurements
(specifically formaldehyde, acetaldehyde, and acetone) to areas
classified as serious or above for the 8-hour O3 standards.
This change was made in recognition of carbonyl sampling issues which
were believed to cause significant uncertainty in the measured
concentrations.
Twenty-two areas were classified as serious or above O3
nonattainment at the time the PAMS requirements were promulgated in
1993. On July 18, 1997 (62 FR 38856), the EPA revised the averaging
time of the O3 NAAQS from a 1-hour averaging period to an 8-
hour averaging period. On June 15, 2005 (70 FR 44470), the EPA revoked
the 1-hour; however, PAMS requirements were identified as requirements
that had to be retained in the anti-backsliding provisions included in
that action. Therefore, PAMS requirements continue to be applicable to
areas that were classified as serious or above nonattainment for the 1-
hour O3 standards as of June 15, 2004. Currently, 25 areas
are subject to the PAMS requirements with a total of 75 sites. As will
be discussed in detail later, the current PAMS sites are concentrated
in the Northeast U.S. and California with relatively limited coverage
in the rest of the country (Cavender, 2014).
The first PAMS sites began operation in 1994, and have been in
operation for over 20 years. Since the start of the program, there have
been many changes to the nature and scope of the O3 problem
in the U.S. as well as to our understanding of it. The O3
standards has been revised multiple times since the PAMS program was
first implemented. On July 18, 1997, the EPA revised the O3
NAAQS to a level of 0.08 parts per million (ppm), with a form based on
the 3-year average of the annual fourth-highest daily maximum 8-hour
average O3 concentration. On March 28, 2008 (73 FR 16436),
the EPA revised the O3 standards to a level of 0.075 ppm,
with a form based on the 3-year average of the annual fourth-highest
daily maximum 8-hour average O3 concentration. These changes
in the level and form of the O3 NAAQS, along with notable
decreases in O3 levels in most parts of the U.S., have
changed the landscape of O3 NAAQS violations in the U.S. At
the time of the first round of designations for the 8-hour standards
(June 15, 2005), only 5 areas were classified as serious or above for
the 8-hour standards as compared to 22 areas that were classified as
serious or above for the 1-hour standards. While the number of serious
and above areas decreased, the number of nonattainment areas remained
nearly the same. In addition to the change in the landscape of
O3 nonattainment issues, much of the equipment used at PAMS
sites is outdated and in need of replacement. New technologies have
been developed since the inception of the PAMS program that should be
considered for use in the network to simplify procedures and improve
data quality. For these reasons, the EPA determined that it would be
appropriate to re-evaluate the PAMS program as explained below.
In 2011, the EPA initiated an effort to re-evaluate the PAMS
requirements in light of changes in the needs of PAMS data users and
the improvements in monitoring technology. The EPA consulted with the
Clean Air Science Advisory Committee (CASAC), Air
[[Page 65421]]
Monitoring and Methods Subcommittee (AMMS) to seek advice on potential
revisions to the technical and regulatory aspects of the PAMS program;
including changes to required measurements and associated network
design requirements. The EPA also requested advice on appropriate
technology, sampling frequency, and overall program objectives in the
context of the most recently revised O3 NAAQS and changes to
atmospheric chemistry that have occurred over the past 10-15 years in
the significantly impacted areas. The CASAC AMMS met on May 16 and May
17, 2011, and provided a report with their advice on the PAMS program
on September 28, 2011 (U.S. EPA, 2011f). In addition, the EPA met
multiple times with the National Association of Clean Air Agencies
(NACAA) Monitoring Steering Committee (MSC) to seek advice on the PAMS
program. The MSC includes monitoring experts from various State and
local agencies actively engaged in ambient air monitoring and many
members of the MSC have direct experience with running PAMS sites.
Specific advice obtained from the CASAC AMMS and the MSC that was
considered in making the proposed changes to the PAMS requirements is
discussed in the appropriate sections below.
Based on the findings of the PAMS evaluation and the consultations
with the CASAC AMMS and NACAA MSC, the EPA proposed to revise several
aspects of the PAMS monitoring requirements including changes in (1)
network design, (2) VOC sampling, (3) carbonyl sampling, (4) nitrogen
oxides sampling, and (5) meteorology measurements. The following
paragraphs summarize the proposed changes, the comments received, and
the final changes and supporting rationale.
1. Network Design
As discussed above, the current PAMS network design calls for two
sites (a Type 2, and a Type 1 or Type 3) per PAMS area. In their report
(U.S EPA, 2011f), the CASAC AMMS found ``that the existing uniform
national network design model for PAMS is outdated and too resource
intensive,'' and recommended ``that greater flexibility for network
design and implementation of the PAMS program be transferred to state
and local monitoring agencies to allow monitoring, research, and data
analysis to be better tailored to the specific needs of each
O3 problem area.'' While stating that the current PAMS
objectives were appropriate, the AMMS report also stated that
``objectives may need to be revised to include both a national and
regional focus because national objectives may be different from
regional objectives.'' The NACAA MSC also advised the EPA that the
existing PAMS requirements were too prescriptive and may hinder state
efforts to collect other types of data that were more useful in
understanding their local O3 problems.
The EPA agrees with CASAC that the PAMS objectives include both
local and national objectives, and believes that the current PAMS
network design is no longer suited for meeting either sets of
objectives. As part of the PAMS evaluation, it was determined that at
the national level the primary use of the PAMS data has been to
evaluate photochemical model performance. Due to the locations of the
current PAMS areas and the current network design, existing PAMS sites
are clustered along the northeast and west coasts leading to
significant redundancy in these areas and very limited coverage
throughout the remainder of the country (Cavender, 2014). The resulting
uneven spatial coverage greatly limits the value of the PAMS data for
evaluation of model performance. CASAC (U.S. EPA, 2011f) noted the
spatial coverage issue and advised that the EPA should consider
requiring PAMS measurements in areas in addition to ``areas classified
as serious and above for the O3 NAAQS to improve spatial
coverage.'' The EPA also agrees with CASAC and NACAA that the PAMS
requirements should be revised to provide monitoring agencies greater
flexibility in meeting local objectives.
The EPA proposed changes to the network design requirements to
better serve both national and local objectives. The EPA proposed a two
part network design. The first part of the design included a network of
fixed sites (``required PAMS sites'') intended to support O3
model development and the tracking of trends of important O3
precursor concentrations. The second part of the network design
required states with O3 non-attainment areas to develop and
implement Enhanced Monitoring Plans (EMPs) which were intended to allow
monitoring agencies the needed flexibility to implement additional
monitoring capabilities to suit the needs of their area.
To implement the fixed site portion of the network design, the EPA
proposed to require PAMS measurements at any existing NCore site in an
O3 nonattainment area in lieu of the current PAMS network
design requirements.\222\ The NCore network is a multi-pollutant
monitoring network consisting of 80 sites (63 urban, 17 rural) sited in
typical neighborhood scale locations and supports multiple air quality
objectives including some of the objectives of the PAMS program
including the development and evaluation of photochemical models
(including both PM2.5 and O3 models), development
and evaluation of control strategies, and the tracking of regional
precursor trends.
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\222\ The EPA noted that the proposed change would expand the
PAMS applicability beyond that required in 182(c)(1) of the CAA.
Thus, in this final rule, the EPA is relying on the authority
provided in Sections 103(c), 110(a)(2)(B), 114(a) and 301(a)(1) of
the CAA to expand the PAMS applicability to areas other than those
that are serious or above O3 nonattainment.
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The EPA recognized that in limited situations existing NCore sites
may not be the most appropriate locations for making PAMS measurements.
For example, an existing PAMS site in an O3 nonattainment
area may be sited at a different location than the existing NCore site.
In this case, it may be appropriate to continue monitoring at the
existing PAMS site to support ongoing research and to maintain trends
information. To account for these situations, the EPA also proposed to
provide the EPA Regional Administrator the authority to approve an
alternative location for a required PAMS site where appropriate. The
EPA also solicited comments on alternative frameworks using other
benchmarks such as attainment status or population to ensure an
appropriately sized fixed PAMS monitoring network. The EPA received
several comments on the proposed changes to the network design,
primarily from state and local monitoring agencies. The following
paragraphs summarize the major comments made on the proposed network
design, our response, and final network design requirements.
Most commenters agreed with the need to revise the existing network
design. One commenter agreed that ``requiring PAMS monitoring at
already existing NCore locations will benefit national and local
objectives to understand ozone formation and would also provide
significant cost efficiencies.'' Another commenter stated that they
supported the proposed changes, ``especially the flexibility provided
by EMPs designed to meet local objectives and achieve a better
understanding of photochemical precursors.'' Another commenter
supporting the changes stated that the ``proposed network revision will
provide states the flexibility to use their resources effectively.''
One commenter stated that the proposed changes ``reflect a more
efficient use of state and local monitoring resources by availing
[[Page 65422]]
monitoring agencies of existing NCore infrastructure to fulfill PAMS
requirements.''
A number of concerns were also raised with the proposed network
design. Several commenters stated that the proposal ``would drastically
reduce the PAMS network in the Northeast.'' One commenter stated that
``this is not acceptable for the Northeast and Mid-atlantic Corridor,
which requires monitoring of the complex transport from multiple large
metropolitan areas in the region.'' One commenter recognized that the
EPA had intended to allow states to use EMPs to address upwind and
downwind data needs, but raised concerns that states with historically
important upwind and downwind sites in the Ozone Transport Region \223\
(OTR) may not be required to develop an EMP since those sites would be
in states that are attaining the O3 NAAQS. One commenter
suggested that ``the EPA consider the entire OTR when designing a PAMS
network rather than pockets of nonattainment areas in the region.'' The
EPA agrees that the reduction of sites in the OTR is a potential issue
and that many important existing PAMS sites would not be part of the
required PAMS sites based on the proposed network design. As noted by
several commenters, the EPA intended the state directed EMPs to give
states flexibility in determining data needed to understand local
O3 formation, including transport in the Northeast. However,
the EPA also agrees that as proposed many states in the OTR would not
be required to develop EMPs and, therefore, may not be provided PAMS
resources. To address these concerns and ensure adequate network
coverage in the OTR, the EPA is adding a requirement that all states in
the OTR develop and implement an EMP regardless of O3
attainment status. This change will help ensure that an EMP appropriate
for the entire OTR can be implemented.
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\223\ Section 184(c) of the CAA establishes the OTR as comprised
of the states of Connecticut, Delaware, Maine, Maryland,
Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania,
Rhode Island, Vermont, and Consolidated Metropolitan Statistical
Area that includes the District of Columbia.
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Concerns were raised by some states that existing NCore sites may
not be the most appropriate location for making PAMS measurements. One
commenter noted that their NCore site was inland but that their ``most
significant ozone problems occur along the shoreline due to transport
along the lake'', and that ``the NCore site cannot provide insight into
these important lakeshore ozone processes.'' Another commenter stated
that ``while it was laudable to leverage sites where data is already
being collected, it is unclear whether NCore sites adequately meet the
objectives of the PAMS program'', and that ``the current NCore network
may not be adequate to depict boundary conditions or areas of maximum
emissions.'' One commenter stated that ``in some nonattainment areas an
NCore site may be an appropriate location for a PAMS monitor, but in
other areas it would be preferable to install the PAMS monitoring in a
location downwind of a source region where higher ozone exposures
occur'' and that ``State and local boundaries should not be part of the
network design criteria.'' One commenter noted that while the EPA had
proposed to allow waivers, it was unclear if waivers would be allowed
where the alternative site was in a different CBSA or state than the
required PAMS site. As stated in our proposal, the EPA recognizes that
in some cases existing PAMS sites (or other sites) may be better suited
to meet local and national data needs. For this reason, we had proposed
to allow waivers in these situations. We do agree that it is
appropriate in some cases to allow these waivers to cross CBSA and
state boundaries. Therefore, we have added specific language to the
final waiver provisions to clarify that waivers can be allowed to cross
CBSA and state boundaries. Where a monitoring agency receives a waiver
from siting a monitor in reliance on a monitor operated by a different
monitoring agency (e.g., across state lines), the waiver will be
conditioned on the monitor being properly included in the other
agency's network plan, and operated in accordance with the requirements
of Part 58, including the relevant appendices.
In addition to the concerns raised about closing important existing
PAMS sites discussed above, some commenters raised concerns that many
of the newly required PAMS sites would be in locations that were
expected to attain the revised O3 NAAQS soon after the new
sites would be installed. One commenter noted that ``requiring marginal
nonattainment areas to install PAMS sites would result in a large
undertaking at an area that would most likely be back in attainment at
or around the time the PAMS site started collecting data.'' One
commenter stated that by tying the network requirement to NAAQS
attainment ``threatens to underserve areas that are very close to
exceeding the revised ozone NAAQS and results in significant gaps in
the spatial coverage of the PAMS network'' and ``has the potential to
introduce undesirable uncertainty on the size and spatial extent of the
PAMS network over the long term.'' Another commenter was concerned that
the proposed network would be unstable, and would experience frequent
changes as areas came into attainment or went out of attainment thus
reducing the value of the data collected, and resulting in inefficient
use of resources. One commenter noted that ``a more stable monitoring
network design will allow for the examination of trends from spatially
robust, long running sites and will allow states to firmly establish
the infrastructure costs.''
The EPA noted in the proposal that the size and locations of the
proposed required PAMS network is sensitive to the level of the revised
O3 NAAQS and future O3 concentrations. We
recognize and agree that if current downward trends in O3
concentrations continue, many initially required sites may no longer be
required to make PAMS measurements soon after the sites were installed.
Non-required sites could be closed, soon after being installed, at the
state's discretion. We agree this would result in an inefficient use of
resources. We also note that if these sites were closed following a
potential reclassification to attainment, the loss of those sites could
lead to a network with poor spatial coverage. Therefore, the EPA is
making changes to the proposed revisions to the network design to
improve the stability of the fixed site network. As explained below,
the final requirements are based on options for which we requested
comments in the proposal and the comments we have received.
We requested comments on additional options to define the fixed
PAMS network component of the new network design. These options were
further discussed in a memorandum to the docket (Cavender, 2014). One
option discussed was to require PAMS measurements at all NCore sites
irrespective of the O3 attainment status of the area. One
commenter noted that ``requiring PAMS monitoring at all NCore sites,
regardless of ozone attainment status, provides the most spatially
robust and stable monitoring network.'' We noted that this requirement
would result in a network of approximately 80 sites, which would be
larger than the current network. In the supporting memorandum, we noted
that a fixed network of 80 sites would strain existing resources and
would not allow adequate resources to implement the state directed
EMPs.
Another option discussed in the proposal included requiring PAMS
measurements at NCore sites in O3
[[Page 65423]]
nonattainment areas with a population greater than 1,000,000. We noted
that this option would result in a network of between 31 and 37 sites
depending on the level of the revised O3 NAAQS. We also
noted that focusing the applicability of PAMS to those NCore sites in
larger CBSAs would still provide the desired improvement in geographic
distribution while reducing the number of required sites down to a
level that would provide sufficient resources to implement the state-
directed EMP portion of the network. One commenter stated that they
``supported a 1,000,000 population threshold because it would help
prioritize resources to areas based on the greatest human health
impacts.'' In addition, a number of commenters, while not commenting on
the need for a population limit, did raise concerns about their ability
to acquire and retain staff with the necessary expertise to collect
PAMS measurements in less urbanized areas. As with the proposed network
design, we recognize that the total number of sites and the ultimate
spatial coverage under this option is also sensitive to changes in
O3 concentrations. If current downward trends in
O3 concentrations continue, many initially required sites
would not be required soon after they were installed. As with the
proposed option, this option could result in an unstable network
resulting in an inefficient use of resources and inadequate spatial
coverage to meet the network goals discussed above.
Upon further consideration and in response to the comments
received, we are finalizing a network design that includes a
requirement for states to make PAMS measurements at all NCore sites in
CBSAs with a population of 1,000,000 people or more, irrespective of
O3 attainment status. We believe this requirement will
result in an appropriately sized network (roughly 40 sites) that will
provide adequate spatial coverage to meet national model evaluation
needs (Cavender, 2015). Redundancy is greatly reduced while important
network coverage is added in the midwest, southeast, and mountain west.
The improved spatial coverage will also strengthen the EPA's ability to
track trends in precursor concentrations regionally.
Because the network requirement is not tied to attainment status,
this final requirement will ensure network stability and allows for
more efficient use of available resources. This final requirement also
removes uncertainty as to applicability and aids planning and logistics
involved with implementing the new requirements. Monitoring agencies
can determine the applicability of the fixed site requirements to their
areas today, and begin to make plans for investments in equipment,
shelter improvements, and staffing and training needs necessary to
implement the fixed site requirements without having to wait for the
designations process to be completed. In addition, this final
requirement should alleviate concerns raised by monitoring agencies in
more rural locations over the ability to attract and retain staff with
the skills necessary to make PAMS measurements.
By adding the PAMS measurements to existing NCore sites,
significant efficiencies can be obtained which should further reduce
the costs of the fixed site network as NCore sites currently make many
of the PAMS measurements. Furthermore, adding the additional PAMS
measurements (e.g., speciated VOCs, carbonyls, and mixing height) to
existing NCore sites will improve our ability to assess other
pollutants (e.g., air toxics and PM2.5).
Although, as discussed in comment and summarized above, we believe
there are good reasons for not tying the requirement for fixed PAMS
sites to O3 attainment status, we continue to believe that
requiring PAMS measurements in areas that historically have had low
O3 concentrations is unlikely to provide data of significant
value to warrant the expense and effort of making such measurements.
Therefore, we have included a provision that would allow a monitoring
agency to obtain a waiver, based on Regional Administrator approval, in
instances where CBSA-wide O3 design values are equal to or
less than 85% of the 8-hour O3 NAAQS and where the site is
not considered an important upwind or downwind site for other
nonattainment areas. The EPA selected 85% as the threshold for this
waiver provision as it has been used historically to identify locations
needing additional monitoring for both the O3 and
PM2.5 NAAQS. The EPA will work with the monitoring agencies
and the Regions to help ensure consistent implementation of this waiver
provision.
The second part of the proposed PAMS network design included
monitoring agency directed enhanced O3 monitoring activities
intended to provide data needed to understand an area's specific
O3 issues. To implement this part of the PAMS network
design, the EPA proposed to add a requirement for states with
O3 nonattainment areas to develop an EMP. The purpose of the
EMP was to improve monitoring for ambient concentrations of
O3, NOX, total reactive nitrogen (NOy)
\224\, VOC, and meteorology. The EPA suggested that types of activities
that might be included in the state's EMP could include additional PAMS
sites (e.g., upwind or downwind sites), additional O3 and
NOX monitoring, ozonesondes or other aloft measurements,
rural measurements, mobile PAMS sites, additional meteorological
measurements, and episodic or intensive studies. The intent of the EMPs
is to allow monitoring agencies flexibility in determining and
collecting the information they need to understand their specific
O3 problems.
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\224\ NOy includes NO, NO2, and other
oxidized nitrogen compounds (NOz).
---------------------------------------------------------------------------
We received comments on the proposed requirement for an EMP in
states with O3 nonattainment areas. Most comments supported
the requirement, but other comments raised a number of concerns. A
number of commenters questioned the need for EMPs in Marginal and
Moderate O3 nonattainment areas. They noted that in most
cases, Marginal O3 nonattainment areas were expected to come
into compliance without state-specific controls. One commenter stated
that ``nonattainment areas projected to attain the standard without
additional state-level actions may not need the PAMS resources and
additional monitoring to develop a better understanding of their ozone
issues.'' One commenter noted that ``marginal ozone nonattainment areas
are given only a few requirements because it is assumed that the areas
will reach attainment within three years.'' Another commenter stated
``requiring enhanced monitoring for any marginal or moderate area
should only be implemented where such analyses show the need for this
data.'' The EPA agrees that based on current trends in O3
concentrations and the EPA's own projections, states in Marginal
nonattainment areas likely will comply with the revised NAAQS without
additional state-directed controls, and as such, an EMP is not
necessary in Marginal O3 attainment areas. Accordingly, the
EPA is finalizing a requirement for EMPs in areas classified as
Moderate or above O3 nonattainment and, thereby, removing
the applicability of the requirement for Marginal areas. We believe
this final requirement will provide the desired flexibility to allow
states to identify enhanced monitoring needs while focusing resources
for EMPs in areas of greater need of enhanced monitoring data.
Commenters expressed concerns over the lack of detail on what an
approvable EMP would entail. As proposed, the
[[Page 65424]]
EMPs would be reviewed and approved by the EPA Regional Administrator
as part of the annual monitoring plan review process. One commenter
recommended that the ``EPA detail the requirements of the EMPs for
ozone nonattainment areas in future implementation guidance.'' One
commenter stated that the ``EPA should provide some coordination
between regional offices and technical guidance to state agencies that
would be of assistance in developing and executing the EMPs.'' The
requirements for the EMPs were intentionally left quite general in
order to maximize the flexibility for states in identifying their
specific data needs. Regional approval of the plans is required to
ensure the enhanced monitoring planned will be commensurate with grant
funds provided for EMPs. Nonetheless, the EPA understands the need for
guidance on developing EMPs and commits to working with monitoring
agencies and the regions to develop appropriate guidance on developing
and reviewing EMPs.
2. Speciated VOC Measurements
Measurement of speciated VOCs important to O3 formation
is a key aspect of the PAMS program. The existing PAMS requirements
allow for a number of options in measuring speciated VOCs at PAMS sites
which include (1) hourly measurements using an automatic gas
chromatograph (``autoGC''), (2) eight 3-hour samples daily using
canisters, or (3) one morning and one afternoon sample with a 3-hour or
less averaging time daily using canisters plus continuous Total Non-
methane Hydrocarbon (TNMHC) measurements.
The EPA believes that the current options provided for VOC
measurement limit the comparative value of the data being collected,
and proposed that required PAMS sites must measure and report hourly
speciated VOCs, which effectively would require them to use an autoGC
to measure VOCs in lieu of canisters. More complete and consistent
speciated VOC data nationally would better help meet certain objectives
of the PAMS program described above (e.g., a speciated ambient air
database useful in evaluating control strategies, analyzing emissions
inventory issues, corroborating progress toward attainment, and
evaluating population exposure to air toxics). Furthermore, as noted by
the CASAC AMMS, hourly VOC data are ``particularly useful in evaluating
air quality models and performing diagnostic emission attribution
studies. These data can be provided on a near real-time basis and
presented along with other precursor species (e.g., oxides of nitrogen
and carbon monoxide) collected over similar averaging times.'' Longer
time-averaged data are of significantly lower value for model
evaluation. In addition, creating consistent monitoring requirements
across the network would provide better data for analyzing regional
trends and spatial patterns.
At the time the original PAMS requirements were promulgated, the
canister options were included because the EPA recognized that the
technologies necessary to measure hourly average speciated VOCs
concentrations were relatively new and may not have been suitable for
broad network use. At that time, GCs designed for laboratory use were
equipped with auto-samplers designed to ``trap'' the VOC compounds from
a gas sample, and then ``purge'' the compounds onto the GC column. The
EPA did not believe that autoGCs were universally appropriate due to
the technical skill and effort necessary at that time to properly
operate an autoGC.
While the basic principles of autoGC technology have not changed,
the hardware and software of modern autoGCs are greatly improved over
that available at the time of the original PAMS requirements. Based on
advice from the CASAC AMMS, the EPA initiated an evaluation of current
autoGCs potentially suitable for use in the PAMS network. Based on the
preliminary results, the EPA believes that typical site operators, with
appropriate training, will have the skill necessary to operate a modern
autoGC successfully. Considering the advances in autoGC technology, the
added value obtained from hourly data, and the proposed move of PAMS
measurements to NCore sites in O3 nonattainment areas, the
EPA proposed to require hourly speciated VOC sampling at all PAMS
sites. The EPA noted that this proposed requirement would effectively
prevent the use of canisters to collect speciated VOCs at the required
PAMS sites but that canister sampling may continue to be an appropriate
method for collecting speciated VOCs at other locations as part of
discretionary monitoring designed within the EMPs.
While the EPA believes that the proposed transition to hourly
speciated VOC sampling is the appropriate strategy to take advantage of
improved technology and to broaden the utility of collected data, we
are also mindful of the additional rigidity that the proposed mandatory
use of autoGCs may have for monitoring agencies, especially those that
have experience with and have established effective and reliable
canister sampling programs. Therefore, the EPA requested comment on the
proposed requirement for hourly VOC sampling as well as the range of
alternatives that might be appropriate in lieu of a strict requirement.
The EPA received a number of comments on the requirement to measure
hourly VOCs at required PAMS sites. Many commenters agreed with
requiring hourly VOC data. One commenter agreed that ``hourly VOC data
collection is the most appropriate and useful for PAMS monitors'' and
that ``it is only appropriate to approve an alternative data collection
interval if it is believed that the high ozone in an area is due to
other pollutants, such as NOX or methane.'' One commenter
stated they ``supported the movement towards hourly PAMS VOC speciated
measurements with flexibility to use canisters if programmatic or
logistical needs indicate.''
However, some commenters raised concerns with the hourly VOC
requirement. Some commenters questioned if autoGCs would be capable of
measuring important VOC species in their environment. One commenter
noted that in their location (high desert) ``the largest VOC present in
our inventory is creosote, a compound not commonly measured with this
instrumentation.'' One commenter stated that the ``Southeastern United
States is dominated by biogenic VOC emissions'' and questioned ``the
benefits of an autoGC in understanding ozone formation in any potential
nonattainment area in our State.'' \225\ Some questioned the detection
capabilities of autoGCs as compared to canister sampling. One commenter
found that the method detection limit (MDL) for their canister sampling
was ``consistently equal to or less than the autoGC instrumentation''
based on the EPA's autoGC evaluation laboratory report (RTI, 2014).
Another commenter noted that the MDLs for many of the compounds and
systems reported in the laboratory report were too high to be useful at
PAMS sites. Another commenter stated that they found that ``retention-
time shifts made it difficult for instant identification of chemical
peaks'' and that ``states should be allowed the flexibility to continue
using canisters instead of autoGC.''
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\225\ The EPA notes that isoprene (the dominant biogenic
compound in the Southeast) is well measured using autoGCs. The EPA
is also evaluating the potential of modern autoGC's to measure alpha
and beta pinene; however that work is not complete.
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As noted in the preamble, and the comments received, the EPA is
currently completing an evaluation of
[[Page 65425]]
commercially available autoGCs. A copy of the report for the laboratory
phase of the study is available in the docket (RTI, 2014). As noted in
the laboratory report, the MDL estimates made for the laboratory study
were not conducted according to normal MDL testing procedures and as
such the results should only be used to compare the various instruments
being tested against each other.\226\ As part of the evaluation, the
EPA identified the manufacturer's specifications for MDL. Most of the
systems that are being evaluated have a manufacturer's estimated MDL in
the range of 0.1 ppb to 0.5 ppb. Based on the evaluation of MDL
capabilities and typical ambient concentrations of O3
precursors, the EPA believes that autoGCs are an appropriate method for
gathering VOC data at most urban locations. However, canister sampling
may be more appropriate in locations with low VOC concentrations.
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\226\ Several factors combined to result in the high relative
MDL estimates reported in laboratory report. The MDL testing in the
laboratory was conducted during concurrent tests for interferences
from humidity and temperature. In addition, the MDL testing was
conducted at relatively high concentrations compared to the
concentrations testing would be conducted at for conventional MDL
testing. Finally, as noted in the laboratory report, a number of
instruments were having technical difficulties during the testing
which greatly impacted their MDL results. The EPA is continuing the
autoGC evaluation and has conducted a field study during the summer
of 2015. A final report is expected in early 2016.
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For the reasons discussed above and in the proposed rule, the EPA
is finalizing a requirement for hourly speciated VOC measurements at
required PAMS sites. The EPA believes that hourly VOC measurements will
provide a more complete and consistent speciated VOC database to help
meet the PAMS program objectives described above. Hourly VOC data are
particularly useful in evaluating air quality models and performing
diagnostic emission attribution studies. Longer time-averaged data are
of lower value for model evaluation. Consistent monitoring requirements
across the network will provide better data for analyzing regional
trends and spatial patterns.
However, the EPA agrees that there may be locations where an autoGC
may not be the most appropriate method for VOC measurement and that it
is appropriate to allow for canister sampling in limited situations.
Accordingly, the EPA is adding a waiver option (to be approved by the
EPA Regional Administrator) to allow three 8-hour average samples every
3rd day as an alternative in cases where VOCs are not well measured by
autoGC due to low concentrations of target compounds or where the
predominant VOC compounds cannot be measured using autoGC technology
(e.g., creosote in high desert environments). This alternative sampling
frequency was selected to be consistent with the sampling frequency
selected for carbonyls, which is discussed later in this preamble.
3. Carbonyl Measurements
Carbonyls include a number of compounds important to O3
formation that cannot currently be measured using the autoGCs or
canisters used at PAMS sites to measure speciated VOCs. The current
method for measuring carbonyls in the PAMS program is Compendium Method
TO-11A (U.S. EPA, 1999). In this method, carbonyl compounds are
adsorbed and converted into stable hydrazones using
dinitrophenylhydrazine (DNPH) cartridges. These cartridges are then
analyzed for the individual carbonyl compounds using liquid
chromatography (LC) techniques. Three carbonyls are currently required
to be measured in the PAMS program--formaldehyde, acetaldehyde, and
acetone.
In 2006, the EPA revised the PAMS requirements such that carbonyl
sampling was only required in areas classified as serious or above
nonattainment for O3 under the 8-hour O3 standard
which effectively reduced the applicability of carbonyl sampling to a
few areas in California. This change was made in recognition that there
were a number of issues with Method TO-11A that raised concerns with
the uncertainty in the carbonyl data being collected. These issues
include interferences (humidity and O3) and breakthrough
(i.e., overloading of the DNPH cartridge) at high concentrations. While
solutions for these issues have been investigated, these improvements
have not been incorporated into Method TO-11A.
A recent evaluation of the importance of VOCs and carbonyls to
O3 formation determined that carbonyls, especially
formaldehyde, are very important to O3 formation (Cavender,
2013). CASAC AMMS (U.S. EPA, 2011f) also noted the importance of
carbonyls stating that ``There are many compelling scientific reasons
to measure carbonyls. They are a very important part of O3
chemistry almost everywhere.'' Although the EPA recognizes the issues
that have been raised about the current method of measuring carbonyls,
due to the importance of carbonyls to understanding O3
chemistry, the EPA proposed to require all required PAMS sites to
measure carbonyls.
Several commenters agreed with the need for carbonyl data at PAMS
sites. However, a number of commenters questioned the proposed
frequency of eight 3-hour samples every day during the PAMS sampling
season (June through August). Several commenters indicated that the
frequency was too high. One commenter noted that the requirement would
require 800 samples per season at each PAMS site and pointed out that
this requirement, which was required at the inception of the PAMS
program in the 1990s was ``found to be prohibitively expensive,
technically unsustainable, and qualitatively compromised.'' Another
commenter stated that ``this level of sampling would require a
substantial amount of agency resources and seems unduly burdensome.'' A
number of commenters also questioned the commercial availability of an
8-channel carbonyl sampler that would be needed to take eight 3-hour
samples daily. In light of the comments and upon further review, the
EPA agrees that the proposed frequency is unduly burdensome and is
finalizing a requirement with a lower frequency.
A number of alternative frequencies were suggested in the comments.
Several commenters suggested a frequency of three 8-hour samples on
either a 1-in-6 day or 1-in-3 day basis. Another commenter suggested a
frequency of eight 3-hour samples on a 1 in 6 day basis. The EPA notes
that sampling on a 1-in-6 day frequency would lead to as little as 15
sampling days per PAMS sampling season. The EPA believes that 15
sampling days is too few to provide a meaningful representation of
carbonyl concentrations over the PAMS sampling period. A sampling
frequency of 1-in-3 days would lead to 30 sampling days per season with
each day of the week being represented at least 4 times per sampling
season. With regards to samples per day, a 3-hour sampling duration
provides a better diurnal representation of carbonyl sampling compared
with an 8-hour sampling duration; however 8-hour sampling can provide
information useful for evaluating diurnal differences in carbonyl
concentrations. Upon further consideration and in light of the comments
received, the EPA is finalizing a carbonyl sampling requirement with a
frequency of three 8-hour samples on a 1-in-3 day basis. This final
requirement will result in approximately 90 samples per PAMS sampling
season which the EPA believes is not unduly burdensome and
[[Page 65426]]
will provide a reasonable representation of carbonyl concentrations.
A number of commenters noted the ongoing development of continuous
formaldehyde instruments, and recommended that EPA allow for continuous
formaldehyde measurements as an alternative to the manual cartridge
based TO-11A method. The EPA agrees that continuous formaldehyde, with
the ability to obtain hourly averaged measurements, would be a
significantly more valuable that the longer averaged measurements. As a
result, the EPA has added an option to allow for continuous
formaldehyde as an alternative to the carbonyl measurements using TO-
11A.
4. Nitrogen Oxides Measurements
It is well known that NO and NO2 play important roles in
O3 formation (U.S. EPA, 2013, Section 3.2.2). Under the
current network design, Type 2 PAMS sites are required to measure
NOX (which by definition is the sum of NO and
NO2), and Types 1, 3, and 4 sites are required to measure
NOy. NCore sites are currently required to measure
NOy but are not required to measure NO2
separately.
In conventional NOX analyzers, NO2 is
determined as the difference between the measured NO and NOX
concentrations. However, due to the non-selective reduction of oxidized
nitrogen compounds by the molybedenum converter used in conventional
NOX monitors, the NO2 measurement made by
conventional NOX monitors can be biased high due to the
varying presence of NOz compounds that may be reported as
NO2. The unknown bias from the NOz compounds is undesirable
when attempting to understand O3 chemistry.
Improvements in reactive nitrogen measurements have been made since
the original PAMS requirements were promulgated that allow for improved
NO2 measurements. Selective photolytic converters have been
developed that are not significantly biased by NOz compounds (Ryerson
et al., 2000). Monitors using photolytic converters are commercially
available and have been approved as FEMs for the measurement of
NO2. In addition, methods that directly read NO2
have been developed that allow for very accurate readings of
NO2 without some of the issues inherent to the ``difference
method'' used in converter-based NOX analyzers. However,
these direct reading NO2 analyzers generally do not provide
an NO estimate, and would need to be paired with a converter-based
NOX monitor or NOy monitor in order to also
measure NO.
As discussed above, the EPA is finalizing a PAMS network design
such that PAMS measurements will be required at existing NCore sites in
CBSAs with a population of 1,000,000 people or more. NCore sites
currently are required to measure NO and NOy. NCore sites
are not currently required to measure NO2. Due to the
importance of accurate NO2 data to the understanding of
O3 formation, the EPA proposed to require NO2
measurements at required PAMS sites. Since existing NCore sites
currently measure NOy, either a direct reading
NO2 analyzer or a photolytic-converter NOX
analyzer could be used to meet the proposed requirement. The EPA
believes conventional NOX analyzers would not be appropriate
for making PAMS measurements due to the uncertainty caused by
interferences from NOz compounds.
A number of commenters questioned the need for both NOy
and NO2 measurements at PAMS sites. One commenter stated
that ``in dense urban areas an NO/NO2/NOX
instrument may be adequate but in a more rural area an NO/
NOy instrument may be preferable.'' Another commenter stated
that due to the size of the grid cells used in grid models that ``the
impact of NOz interferences would be very small compared to other
modeling uncertainties such as emission inventories and mixing
heights.'' Another commenter suggested that ``EPA should provide clear
and specific guidance on how agencies can request that the
NOy monitoring be eliminated from the NCore suite based on
comparative data between the NO2 and NOy
monitors.''
The comments suggest that the model's ability to simulate the
partitioning of reactive nitrogen is unimportant because there may be
other errors in the model. The EPA believes that measurements should be
routinely collected so that it can be demonstrated that the chemistry,
meteorology, and emissions in the model are all of sufficient
reliability for use in informing air quality management decisions.
Monitoring sites rarely fall into simple categories of urban or rural,
and the speciation of NOy varies considerably as a function
of meteorology and time of day at a given site. The state-of-the-
science in regulatory air quality modeling is such that accurate
measurements of key O3 precursors must be available to
demonstrate the credibility of the model predictions. The increased
availability of special field study observations is leading to
increased scrutiny of the chemical mechanisms used in regulatory
modeling. Comprehensive and accurate measurement sites are needed to
demonstrate the adequacy of the models and to respond to these
challenges.
Measurements of NO, NO2, and NOy
concentrations are critical to understanding atmospheric aging and
photochemistry. These measurements will provide essential information
about whether NOy compounds are fresh or aged which is
important for understanding both local photochemistry (i.e. through
indicator ratios to distinguish NOX vs VOC limited
conditions) as well as for characterizing transport from upwind
regions. These evaluations may be conducted using observations, box
modeling or through complex photochemical grid based modeling. Accurate
speciated and total NOy measurements are necessary for all
three types of analysis. For these reasons, the EPA is finalizing the
requirement for required PAMS sites to measure true NO2 in
addition to NO and NOy.
5. Meteorology Measurements
The current PAMS requirements require monitoring agencies to
collect surface meteorology at all required PAMS sites. As noted in the
EPA's Technical Assistance Document (U.S. EPA, 1998) for the PAMS
program, the PAMS requirements do not provide specific surface
meteorological parameters to be monitored. As part of the
implementation efforts for the original PAMS program, a list of
recommended parameters was developed and incorporated into the TAD
which includes wind direction, wind speed, temperature, humidity,
atmospheric pressure, precipitation, solar radiation, and ultraviolet
(UV) radiation. Currently, NCore sites are required to measure the
above parameters with the exceptions of atmospheric pressure,
precipitation, solar radiation, and UV radiation. In recognition of the
importance of these additional measurements for understanding
O3 formation, the EPA proposed to specify that
required PAMS sites are required to collect wind direction, wind speed,
temperature, humidity, atmospheric pressure, precipitation, solar
radiation, and UV radiation. Since NCore sites are currently required
to measure several of these surface meteorological parameters, the net
impact of the proposal was to add the requirement for the monitoring of
atmospheric pressure, precipitation, solar radiation, and UV radiation
at affected NCore sites. The EPA received no significant comments on
this portion of the proposal, and therefore is finalizing the
requirement as proposed.
[[Page 65427]]
The existing PAMS requirements also require the collection of upper
air meteorological measurements at one site in each PAMS area. The term
upper air meteorological is not well defined in the existing PAMS
requirements. As part of the implementation efforts for the original
PAMS program, mixing height was added to the PAMS TAD as a recommended
meteorological parameter to be monitored. Most monitoring agencies
installed radar profilers to meet the requirement to collect upper air
meteorology. Radar profilers provide data on wind direction and speed
at multiple heights in the atmosphere. Radio acoustic sounding system
(RASS) profilers are often included with radar profilers to obtain
atmospheric temperature at multiple heights in the atmosphere and to
estimate mixing height. The EPA recognizes that the upper air data on
wind speed and wind direction from radar profilers can be very useful
in O3 modeling. However, many of the current PAMS radar
profilers are old and in need of replacement or expensive maintenance.
In addition, the cost to install and operate radar profilers at all
required PAMS sites would be prohibitive. Therefore, the EPA did not
propose to add upper air wind speed and direction as required
meteorological parameters to be monitored at required PAMS sites. Where
monitoring agencies find the radar profiler data valuable, continued
operation of existing radar profilers or the installation of new radar
profilers would be appropriate to consider as part of the state's EMP.
As discussed above, mixing height is one upper air meteorological
measurement that has historically been measured at PAMS sites. A number
of methods can be used to measure mixing height in addition to radar
profiler technology discussed above. Recent developments in ceilometer
technology allow for the measurement of mixing height by changes in
particulate concentrations at the top of the boundary layer (Eresmaa et
al., 2006). Ceilometers provide the potential for continuous mixing
height data at a fraction of the cost of radar profilers. Due to the
importance of mixing height measurements for O3 modeling,
the EPA proposed to add the requirement for monitoring agencies to
measure mixing height at required PAMS sites.
A number of commenters questioned the need for mixing height
measurements at PAMS sites. One commenter stated, ``the photochemical
modeling community has a long history of relying upon National Weather
Service measurements for mixing height.'' Another commenter stated that
``in some areas of the country the models used to predict mixing height
are adequate, but in other mountainous or marine areas model-predicted
mixing height data is inadequate.'' Accurate estimates of mixing height
are important for appropriately characterizing concentrations of
O3 and O3 precursors. Mixing height is also
important for characterizing how modeled O3 may change as a
result of changing NOX and VOC concentrations. For instance,
if the modeled mixing height is too low causing unrealistically high
concentration of NOX, then O3 destruction could
be predicted when O3 production may be happening in the
atmosphere. When this or the opposite situation exists in modeling it
may lead O3 response to emissions changes that are less
reliable for air quality planning purposes. While models are believed
to do a reasonable job of predicting mixing height during the day,
there is considerably more uncertainty in predicting this parameter
during morning and evening transition periods and at night. Model
O3 predictions are particularly sensitive to mixing height
during the time periods for which uncertainty in this parameter is
greatest.
Several commenters noted that nearby National Oceanic and
Atmospheric Administration (NOAA) Automated Surface Observing System
(ASOS) sites may be a better alternative for collection of mixing
height data. As indicated in the proposal, the EPA is aware of the
network of ceilometers operated by NOAA as part of ASOS. The EPA has
been in discussions with NOAA regarding the potential for these systems
to provide the needed mixing height data. However, the ASOS ceilometers
are not currently equipped to provide mixing height data and NOAA has
no current plans to measure continuous mixing height in the future.
Nonetheless, the EPA will continue to work with NOAA to determine if
the ASOS ceilometers can be upgraded to meet the need for mixing height
data, and included proposed regulatory language that will allow states
a waiver to use nearby mixing height data from ASOS (or other sources)
to meet the requirement to collect mixing height data at required PAMS
sites when such data are suitable and available.
The EPA is finalizing the requirement for the measurement of mixing
height at required PAMS sites due to the importance of mixing height in
O3 modeling. A waiver option, to be approved by the Regional
Administrator, is also being included to allow mixing height
measurements to be obtained from other nearby sites (e.g., NOAA ASOS
sites).
6. PAMS Season
Currently, PAMS measurements are required to be taken during the
months of June, July, and August. This 3-month period is referred to as
the ``PAMS Season.'' As part of the PAMS re-evaluation, the EPA
considered changes to the PAMS season. The 3-month PAMS season was
originally selected to represent the most active period for
O3 formation. However, the EPA notes that in many areas the
highest O3 concentrations are observed outside of the PAMS
season. As an example, the highest O3 concentrations in the
mountain-west often occur during the winter months. Data collected
during the current PAMS season would have limited value in
understanding winter O3 episodes.
The CASAC AMMS (U.S. EPA, 2011f) noted in their report to the EPA
that ``it would be desirable to extend the PAMS monitoring season
beyond the current June, July, August sampling period.'' But that ``the
monitoring season should not be mandated and rigid; it should be
flexible and adopted and coordinated on a regional airshed basis.'' The
EPA agrees with CASAC on the need for flexibility in determining when
PAMS measurements should be taken to meet local monitoring needs but
also agrees with CASAC that the flexibility ``should not conflict with
national goals for the PAMS program.'' A significant benefit of the
standard PAMS season is that it ensures data availability from all PAMS
sites for national- or regional-scale modeling efforts.
While the EPA agrees with the potential benefit of extending the
availability of PAMS measurements outside of the current season, we
also considered the burden of requiring monitoring agencies to operate
additional PAMS measurements (e.g., hourly speciated VOC) for periods
that in some cases, might be much longer than the current 3-month
season, for example, if the PAMS season was extended to match each
state's required O3 monitoring season. Being mindful of the
potential burden associated with a lengthening of the PAMS season as
well as the potential benefits of the additional data, the EPA proposed
to maintain the current 3-month PAMS monitoring season for required
PAMS sites rather than extending the PAMS season to other periods where
elevated O3 may be expected. No significant comments were
received on the proposed PAMS season, and as such, for the reasons
stated here and in the proposal, the EPA is not changing the 3-month
PAMS season of June, July, and August.
[[Page 65428]]
The EPA believes that the 3-month PAMS season will provide a
consistent data set of O3 and O3 precursor
measurements for addressing the national PAMS objectives. Monitoring
agencies are strongly encouraged to consider collecting PAMS
measurements in additional periods beyond the required PAMS season as
part of their EMP. The monitoring agencies should consider factors such
as the periods of expected peak O3 concentrations and
regional consistency when determining potential expansion of their
specific monitoring periods beyond the required PAMS season.
7. Timing and Other Implementation Issues
The EPA recognizes that the changes to the PAMS requirements will
require resources and a reasonable timeline in order to be successfully
implemented. The PAMS program is funded, in part, as part of the EPA's
section 105 grants. The EPA believes that the current national funding
level of the PAMS program is sufficient to support these final changes,
but changes in the distribution of PAMS funds will need to be made. The
network design changes will require some monitoring agencies to start
collection of new PAMS measurements, while other monitoring agencies
will see reductions in PAMS measurement requirements. The EPA will work
with the NAACA, AAPCA, and other monitoring agencies to develop an
appropriate PAMS grant distribution strategy.
In addition to resources, the affected monitoring agencies will
need time to implement the revised PAMS requirements. For the required
PAMS sites, monitoring agencies can determine now which NCore sites
will be required to make PAMS measurements based on readily available
census data. However, monitoring agencies will still need time to
evaluate and seek approval for alternative sites or alternative VOC
methods. In addition, monitoring agencies will need time to make
capital investments (primarily for the installation of autoGCs,
NO2 monitors, and ceilometers), prepare appropriate QA
documents, and develop the expertise needed to successfully collect
PAMS measurements via training or otherwise. In order to ensure
monitoring agencies have adequate time to plan and successfully
implement the revised PAMS requirements, the EPA is requiring that
monitoring agencies identify their plans to implement the PAMS
measurements at NCore sites in their Annual Network Plan due July 1,
2018, and to begin making PAMS measurements at NCore sites by June 1,
2019. The EPA believes some monitoring agencies may be able to begin
making PAMS measurements sooner than June 2019 and encourages early
deployment where possible.
Monitoring agencies will need to wait until O3
designations are made to officially determine the applicability of the
EMP requirement. The EPA proposed to allow two years after designations
to develop EMPs, and that the EMPs would be submitted as part of their
Annual Network Plan. Several commenters stated that due to the level of
planning and coordination required for the EMPs, that the plans should
instead be included as part of the 5-year network assessment. While the
EPA agrees that the EMPs will require a substantial amount of planning
and coordination, the next 5 year network assessment will not be due
until July 1, 2020--nearly 5 years from the date of this final
rulemaking. The EPA believes that it would be inappropriate to wait 5-
years from the date of this rulemaking to develop plans for enhanced
O3 monitoring. In addition, the EPA believes that the first
round of EMP development should receive additional focus and review
that may not be afforded as part of the larger network assessment.
Finally, most monitoring agencies will be aware of their likely
O3 attainment status well in advance of the official
designations. In order to ensure timely development of the initial
EMPs, the EPA is requiring affected monitoring agencies to submit their
initial EMPs no later than two years following designations. States in
the OTR do not need to wait until designations to determine EMP
applicability and may not be classified as Moderate or above. As such,
the final rule includes a requirement for states in the OTR to submit
their initial EMPs by October 1, 2019 (which is consistent with the
expected timeline for the remaining EMPs). However, subsequent review
and revisions to the EMPs are to be made as part of the 5-year network
assessments beginning with the assessments due in 2025.
D. Addition of a New FRM for O3
The use of FRM analyzers for the collection of air monitoring data
provides uniform, reproducible measurements of concentrations of
criteria pollutants in ambient air. FRMs for various pollutants are
described in several appendixes to 40 CFR part 50. For most gaseous
criteria pollutants (including O3 in Appendix D of part 50),
the FRM is described as a particular measurement principle and
calibration procedure to be implemented, with further reference to
specific analyzer performance requirements specified in 40 CFR part 53.
The EPA allows new or alternative monitoring technologies--
identified as FEMs--to be used in lieu of FRMs, provided that such
alternative methods produce measurements closely comparable to
corresponding FRM measurements. Part 53 sets forth the specific
performance requirements as well as the performance test procedures
required by the EPA for determining and designating both FRM and FEM
analyzers by brand and model.
To be used in a determination of compliance with the O3
NAAQS, ambient O3 monitoring data must be obtained using
either a FRM or a FEM, as defined in parts 50 and 53. For
O3, nearly all the monitoring methods currently used by
state and local monitoring agencies are FEM (not FRM) continuous
analyzers that utilize an alternative measurement principle based on
quantitative measurement of the absorption of UV light by
O3. This type of O3 analyzer was introduced into
monitoring networks in the 1980s and has since become the predominant
type of method used because of its all-optoelectronic design and its
ease of installation and operation.
The existing O3 FRM specifies a measurement principle
based on quantitative measurement of chemiluminescence from the
reaction of ambient O3 with ethylene (ET-CL). Ozone
analyzers based on this FRM principle were once widely deployed in
monitoring networks, but now they are no longer used for routine
O3 field monitoring because readily available UV-type FEMs
are substantially less difficult to install and operate. In fact, the
extent of the utilization of UV-type FEMs over FRMs for O3
monitoring is such that FRM analyzers have now become commercially
unavailable. The last new commercial FRM analyzer was designated by the
EPA in 1979. The current list of all approved FRMs and FEMs capable of
providing ambient O3 data for use in NAAQS attainment
decisions may be found on the EPA's Web site and in the docket for this
action (U.S. EPA, 2014e). However, that list does not indicate whether
or not each listed method is still commercially available.
1. Proposed Changes to the FRM for O3
Although the existing O3 FRM is still a technically
sound methodology, the lack of commercially available FRM O3
analyzers severely impedes the use of FRM analyzers, which are needed
for quality control purposes and as the standard to which candidate
FEMs are
[[Page 65429]]
required to be compared. Therefore, the EPA proposed to establish a new
FRM measurement technique for O3 based on NO-
chemiluminescence (NO-CL) methodology. This new chemiluminescence
technique is very similar to the existing ET-CL methodology with
respect to operating principle, so the EPA proposed to incorporate it
into the existing O3 FRM as a variation of the existing ET-
CL methodology, coupled with the same existing FRM calibration
procedure.
A revised Appendix D to 40 CFR part 50 was proposed to include both
the original ET-CL methodology as well as the new NO-CL methodology,
such that use of either measurement technique would be acceptable for
implementation in commercial FRM analyzers. Currently, two
O3 analyzer models (from the same manufacturer) employing
the NO-CL methodology have been designated by the EPA as FEMs and would
qualify for re-designation as FRMs under the revised O3 FRM.
The rationale for selecting the new NO-CL FRM methodology, including
what other methodologies were also considered, and additional
information to support its selection are discussed in the preamble to
the proposal for this action (79 FR 75366-75368). No substantive change
was proposed to the existing O3 FRM calibration procedure,
which would be applicable to both chemiluminescence FRM methodologies.
The proposed FRM in part 50, Appendix D also included numerous
editorial changes to provide clarification of some provisions, some
revised wording, additional details, and a more refined numbering
system and format consistent with that of two other recently revised
FRMs (for SO2 and CO).
As noted in the proposal, there is substantial similarity between
the new and previously existing FRM measurement techniques, and
comparative field data show excellent agreement between ambient
O3 measurements made with the two techniques (U.S. EPA
2014f). Therefore, the EPA believes that there will be no significant
impact on the comparability between existing ambient O3
monitoring data based on the original ET-CL methodology and new
monitoring data that may be based on the NO-CL methodology.
The proposed FRM retains the original ET-CL methodology, so all
existing FEMs, which were designated under part 53 based on
demonstrated comparability to that ET-CL methodology, will retain their
FEM designations. Thus, there will be no negative consequences or
disruption to monitoring agencies, which will not be required to make
any changes to their O3 monitors due to the revised
O3 FRM. New FEMs would be designated under part 53, based on
demonstrated acceptable comparability to either FRM methodology.
2. Comments on the FRM for O3
Comments that were received from the public on the proposed new
O3 FRM technique are addressed in this section. Most
commenters expressed general support for the proposed changes, although
a few commenters expressed some concerns. The most significant issue
discussed in comments was the relatively small but nevertheless
potentially significant interference of water vapor observed in the ET-
CL technique. As some comments pointed out, this interference is
positive and could possibly affect NAAQS attainment decisions. The
available NO-CL FEM analyzers include a sample dryer, which minimizes
this interference. As noted previously, very few, if any, ET-CL FRM
analyzers are still in operation. The ET-CL (with and without a sample
dryer), the proposed NO-CL FRM, and all designated FEM analyzers have
demonstrated compliance with the substantially reduced water vapor
interference equivalent limit specified in 40 CFR part 53.
The proposed FRM mentioned the need for a sample air dryer for both
ET-CL and NO-CL FRM analyzers. In response to these comments, the
wording of the ET-CL FRM has been augmented to clarify the requirement
for a dryer in all newly designated FRMs (the only change being made by
the EPA to the existing ET-CL FRM as proposed). Also, the interference
equivalent limit for water vapor in part 53 was proposed to be
substantially reduced from the current 0.02 ppm to 0.002 ppm. The
interference equivalent test for water vapor applicable to the new NO-
CL candidate FRM analyzers (specified in Table B-3 of part 53) was
proposed to be more stringent than the corresponding existing test for
ET-CL FRM analyzers by requiring that water vapor be mixed with
O3. This mixing requirement was not part of the existing
test for ET-CL candidate analyzers (denoted by footnote 3 in Table B-
3). However, in further response to these commenters' concerns, the EPA
has modified Table B-3 to extend this water vapor mixing requirement to
newly designated ET-CL analyzers, as well. These measures should insure
that potential water vapor interference is minimized in all newly
designated FRM analyzers.
Several comments indicated concern that currently-designated FEM
analyzers retain their designation without retesting if the new FRM
were promulgated. The current ET-CL FRM is being retained; therefore,
it is not necessary to make these new requirements retroactive to
existing designated FEM analyzers. The existing FEM analyzers will not
be required to be retested, and their FEM designation will be retained
so that there will be no disruption to current monitoring networks.
Although beyond the scope of this rulemaking, other comments
concerned potential hazards of the NO compressed gas supply required
for NO-CL analyzer operation, and the current non-availability of a
photolytic converter to provide an alternative source of NO from a less
hazardous nitrous oxide gas supply. With regard to the photolytic
converter, the EPA would approve such a converter as a source of NO if
requested by an FRM analyzer manufacturer, upon demonstration of
adequate functionality.
A few commenters liked the ``scrubberless UV absorption'' (SL-UV)
measurement technique. The EPA has identified the SL-UV method as a
potentially advantageous candidate for the O3 FRM, but could
not propose adopting it until additional test and performance
information becomes available. A related comment requested
clarification that promulgation of the proposed revised FRM would not
preclude future consideration of other O3 measurement
techniques such as SL-UV. In response, the EPA can always consider new
technologies for FRMs under 40 CFR 53.16 (Supersession of reference
methods). However, a revised or amended FRM that included the SL-UV
technique, as set forth in Appendix D of 40 CFR part 50, would have to
be promulgated as part of a future rulemaking, before a SL-UV analyzer
could be approved as an FRM under 40 CFR part 53.
One comment suggested that the value for the absorption cross
section of O3 at 254 nm used by the FRM's calibration
procedure should be changed. The comment indicated that the nearly 2%
difference effectively lowers the O3 NAAQS by that amount.
Using the corrected value would resolve much of the difference observed
between O3 measurements calibrated against the UV standard
reference photometer versus those calibrated using NO gas phase
titration and it would allow the EPA to adopt the less complex and more
economical Gas Phase Titration (GPT) technique as the primary
calibration standard for the
[[Page 65430]]
FRM. The EPA will await the results of further studies determining the
value of the O3 cross section at 254 nm before making a
change to the calibration procedures and will not finalize changes to
the calibration procedures in this final rule.
E. Revisions to the Analyzer Performance Requirements
1. Proposed Changes to the Analyzer Performance Requirements
In close association with the proposed O3 FRM, the EPA
also proposed changes to the associated analyzer performance
requirements for designation of FRMs and FEMs for O3, as set
forth in 40 CFR part 53. These changes were largely confined to Table
B-1, which specifies performance requirements for FRM and FEM analyzers
for SO2, CO, O3, and NO2, and to Table
B-3, which specifies test concentrations for the various interfering
agent (interferent) tests. Minor changes were also proposed for Figure
B-5 and the general provisions in subpart A of part 53. All of these
proposed changes are described and discussed more fully in the preamble
to the proposal for this action (79 FR 75368-75369).
Modest changes proposed for Table B-3 would add new interferent
test concentrations specifically for NO-CL O3 analyzers,
which include a test for NO2 interference.
Several changes to Table B-1 were proposed. Updated performance
requirements for ``standard range'' analyzers were proposed to be more
consistent with current O3 analyzer performance
capabilities, including reduced limits for noise allowance, lower
detectable limit (LDL), interference equivalent, zero drift, span
drift, and lag, rise, and fall times. The previous limit on the total
of all interferents was proposed to be withdrawn as unnecessary and to
be consistent with that same change made previously for SO2
and CO analyzers. Also, the span drift limit at 20% of the upper range
limit (URL) was proposed to be withdrawn because it has similarly been
shown to be unnecessary and to maintain consistency with that same
change made previously for SO2 and CO analyzers.
The form of the precision limits at both 20% and 80% of the URL was
proposed to be changed from ppm to percent. The proposed new limits (in
percent) were set to be equivalent to the previously existing limits
(in ppm) and thus remain effectively unchanged. This change in form of
the precision limits in Table B-1 has been previously made for
SO2 and CO analyzers, and was proposed to extend also to
analyzers for NO2, (again with equivalent limits) for
consistency and to simplify Table B-1 across all types of analyzers to
which the table applies. A new footnote proposed for Table B-1
clarifies the new form for precision limits as ``standard deviation
expressed as percent of the URL.'' Also proposed was a revision to
Figure B-5 (Calculation of Zero Drift, Span Drift, and Precision) to
reflect the changes proposed in the form of the precision limits and
the withdrawal of the limits for total interference equivalent.
Concurrent with the proposed changes to the performance
requirements for candidate O3 analyzers, the EPA conducted a
review of all designated FRM and FEM O3 analyzers currently
in production or being used, and verified that all meet the proposed
new performance requirements. Therefore, none would require withdrawal
or cancellation of their current FRM or FEM respective designations.
Finally, the EPA proposed new, optional, ``lower range''
performance limits for O3 analyzers operating on measurement
ranges lower (i.e., more sensitive) than the standard range specified
in Table B-1. The new performance requirements are listed in a new
``lower range'' column in Table B-1 and will provide for more stringent
performance in applications where more sensitive O3
measurements are needed.
Two minor changes were proposed to the general, administrative
provisions in Subpart A of part 53. These include an increase in the
time allowed for the EPA to process requests for approval of
modifications to previously designated FRMs and FEMs in 53.14 and the
withdrawal of a requirement for annual submission of Product
Manufacturing Checklists associated with FRMs and FEMs for
PM2.5 and PM10-2.5 in 53.9. No comments were
received on these proposed changes and the EPA will be finalizing these
revisions in this rulemaking.
2. Comments on the Analyzer Performance Requirements
Several comments were received related to the proposed changes to
the analyzer performance requirements of part 53, and most were
supportive. Comments from a few monitoring agencies suggested that the
more stringent performance requirements proposed might be difficult to
achieve or would increase monitor maintenance and cost. The EPA is also
clarifying that these requirements apply only to the performance
qualification requirements for designations of new FRM and FEM
analyzers and will have no impact on a monitoring agency's operation of
existing O3 analyzers.
More specific comments from an analyzer manufacturer pointed out
that the proposed lower limits for noise and LDL may be too stringent,
the former because low-cost portable analyzers may have shorter
absorption cells, and the latter because of limitations of current
calibration technology. After further consideration of available
analyzer performance data in light of these comments, the EPA agrees
and is changing the noise limits from the proposed values of 1 ppb and
0.5 ppb (for the standard and lower ranges, respectively) to 2.5 ppb
and 1 ppb (respectively). The EPA is also changing the LDL limit from
the proposed values of 3 ppb and 1 ppb (respectively) to 5 ppb and 2
ppb (respectively). These new limits are still considerably more
stringent than the previous limits (for the standard range) and are
also consistent with those recommended by the commenter and the current
performance capabilities of existing analyzer/calibration technology.
This commenter also pointed out that the proposed lower limit for
12-hour zero drift, together with the way the prescribed test is
carried out, resulted in the test being dominated by analyzer noise
rather than drift. The EPA agrees with this comment in general but
believes that further study is needed before any specific changes can
be proposed for the 12-hour zero drift test, particularly since any
such changes would affect analyzers for other gaseous pollutants, as
well.
Other comments suggested that there was no need for the proposed
new, low-range performance requirements, because of cost and that
available calibrators would be inadequate for calibration of such low
ranges. The EPA disagrees with these comments and believes, as noted in
the proposal preamble, that there is a definite need for low-level
O3 measurements in some applications and that suitable
calibration for such low-level measurement ranges can be adequately
carried out. As stated previously, the new ``low range'' specifications
for O3 analyzers are optional.
Several comments pointed out some typographical errors related to
footnotes in Table B-3, as proposed; these errors have been corrected
in the version of Table B-3 being finalized today.
EPA is finalizing the proposed amendments to both the O3
FRM in Appendix D of part 50 and provisions in part 53, modified as
described above, in response to the comments received.
[[Page 65431]]
VII. Grandfathering Provision for Certain PSD Permits
This section addresses the grandfathering provision for certain
Prevention of Significant Deterioration (PSD) permit applications that
is being finalized in this rule. Section VIII.C of this preamble
contains a description of the PSD and Nonattainment New Source Review
(NNSR) permitting programs and additional discussion of the
implementation of those programs for the O3 NAAQS.
A. Summary of the Proposed Grandfathering Provision
The EPA proposed to amend the PSD regulations to add a transition
plan that would address the extent to which the revised O3
NAAQS will apply to pending PSD permit applications. This transition
plan is reflected in a grandfathering provision that applies to permit
applications that meet certain milestones in the review process prior
to either the signature date or effective date of the revised
O3 NAAQS. Absent such a grandfathering provision in the
EPA's regulations, the EPA interprets section 165(a)(3)(B) of the CAA
and the implementing PSD regulations at 40 CFR 52.21(k)(1) and
51.166(k)(1) to require that PSD permit applications include a
demonstration that emissions from the proposed facility will not cause
or contribute to a violation of any NAAQS that is in effect as of the
date the PSD permit is issued. The proposal included a grandfathering
provision that would enable eligible PSD applications to make the
demonstration that the proposed project would not cause or contribute
to a violation of any NAAQS with respect to the O3 NAAQS in
effect at the time the relevant permitting benchmark for grandfathering
was reached, rather than the revised O3 NAAQS. We proposed
that the grandfathering provision would apply specifically to either of
two categories of pending PSD permit applications: (1) Applications for
which the reviewing authority has formally determined that the
application is complete on or before the signature date of the final
rule revising the O3 NAAQS; and (2) applications for which
the reviewing authority has first published a public notice of the
draft permit or preliminary determination before the effective date of
the revised NAAQS.
In the proposal, we also noted that for sources subject to the
federal PSD program under 40 CFR 52.21, the EPA and air agencies that
have been delegated authority to implement the federal PSD program for
the EPA would apply the grandfathering provision to any PSD application
that satisfies either of the two criteria that make an application
eligible for grandfathering. Accordingly, if a particular application
does not qualify under the first criterion based on a complete
application determination, it may qualify under the second criterion
based on a public notice announcing the draft permit or preliminary
determination. Conversely, a source may qualify for grandfathering
under the first criterion, even if it does not satisfy the second.
The EPA also proposed revisions to the PSD regulations at 40 CFR
51.166 that would afford air agencies that issue PSD permits under a
SIP-approved PSD permit program the discretion to adopt provisions into
the SIP that allow for grandfathering of pending PSD permits under the
same circumstances as set forth in the federal PSD regulations. With
regard to implementing the grandfathering provision, we also explained
that air agencies with EPA-approved PSD programs in their SIPs would
have additional flexibility for implementing the proposed
grandfathering provision to the extent that any alternative approach is
at least as stringent as the federal provision. In addition, the
proposal recognized that some air agencies do not make formal
completeness determinations; thus, only the latter criterion based on
the issuance of a public notice would be relevant in such cases and the
state could elect to adopt only that criterion into its SIP.
Accordingly, the EPA proposed to add a grandfathering provision to 40
CFR 51.166 containing the same two criteria as proposed for 40 CFR
52.21.
B. Comments and Responses
Many of the comments supported the concept of grandfathering. Some
of these comments, mostly by state and local air agencies, supported
the grandfathering provision as proposed. Many others recommended
alternative approaches to grandfathering based on several different
dates. Several comments recommended that air agencies be allowed to
grandfather certain PSD permit applications and issue a PSD permit
based on the 2008 O3 NAAQS after the area is designated
nonattainment for the revised O3 NAAQS. An opposing set of
comments, representing a coalition of eight environmental groups and
one health advocacy group, strongly objected to the proposal for
grandfathering, claiming that the EPA did not have any authority under
the CAA to exempt or grandfather permit applicants from the statutory
PSD permitting requirements. We are addressing some of these comments
below and others in the Response to Comment Document that is included
in the docket for this rule.
Comments that recommended broadening the scope of the proposed
grandfathering provision suggested a variety of approaches. Some air
agency and industry comments recommended that the EPA adopt a
grandfathering provision applicable only to those PSD applications for
which the reviewing authority has determined the application to be
complete on or before the signature date of the revised NAAQS. Other
air agency and industry comments recommended that grandfathered status
be determined only on the basis of whether the relevant permitting
milestone has been achieved by the effective date of the revised NAAQS.
The EPA disagrees with these comments; the final rule uses separate
dates for the two grandfathering milestones, as proposed. If the
effective date of the revised NAAQS were used as the date for the
complete application milestone, this could lead to pressure on state
permitting authorities to prematurely issue completeness determinations
in order to qualify for the grandfathering provision in the time period
between signature of this final rule and the effective date. Using the
signature date of the revised O3 NAAQS as the date for the
grandfathering milestone based on the completeness determination is
thus intended to help preserve the integrity of the completeness
determination process. Permit applications that have not yet been
determined complete can be supplemented or revised to address the
revised O3 standards before the completeness determination
is issued. Conversely, the amount and type of work required for a
preliminary determination or a draft permit reduces the risk that such
a document would be released prematurely merely to qualify for
grandfathering. Similarly, because these documents are released for the
purpose of providing an adequate opportunity for public participation
in the permitting process, it would not behoove a reviewing authority
to precipitately release such documents merely to satisfy the
grandfathering milestone. Accordingly, the EPA does not have the same
concerns about using the effective date of this final rule for the
preliminary determination or draft permit milestone and further finds
it reasonable to provide additional time for satisfying this milestone.
Moreover, using the proposed milestones and corresponding dates is
consistent with the milestones and corresponding dates that were used
in the grandfathering provisions for the 2012 PM2.5 NAAQS.
[[Page 65432]]
Several other comments recommended that the grandfathering
provision apply to all PSD applications for which a final PSD permit
will be issued prior to the effective date of the area designations for
the revised NAAQS. Some of these comments explained that without some
transition provisions in the final rule, it may be impossible for a
source to demonstrate attainment if the current ambient air monitoring
data indicates a revised, lowered standard is not being met. The
comments also suggested that the extended period for grandfathering a
source from the revised NAAQS would provide states with additional time
to establish offset banks or similar systems for new nonattainment
areas.
Other comments recommended that air agencies be allowed to
grandfather either all or certain PSD permit applications received
before the effective date of the final nonattainment designations for
the revised O3 NAAQS. These comments supported allowing air
agencies to issue PSD permits to grandfathered sources even after the
area in which the source proposes to locate is designated nonattainment
for the revised O3 NAAQS. One comment saw this as being
necessary because the development of the regulatory framework that will
support the revised NAAQS, such as development of a credit market or
even a transition into NNSR permitting, does not instantaneously
accompany the revised standard. Hence, the comment added that
``[d]uring the Interim Period (the time between the revision of the
NAAQS rule and development of the regulatory framework) the project may
be unable to secure offsets and no offsets would be available for
purchase.'' Another comment explained that the extended period for
grandfathering sources from the revised O3 NAAQS was needed
to ``minimize disruption to complex projects that may have been under
development since before the EPA published the proposed NAAQS
revision.'' This comment noted the ``PSD projects commonly undergo
years of engineering and other development resources before an air
permit application can be prepared.''
The EPA does not agree with the comments recommending that the EPA
use a date after the effective date of the revised O3 NAAQS
as the date by which the permit application must reach the relevant
milestone to qualify for grandfathering. The EPA does not believe it is
appropriate to unreasonably or unnecessarily delay implementation of
these revised standards under the PSD program. As explained in more
detail below, the purpose of the grandfathering provision is to provide
a reasonable transition mechanism for certain PSD applications and the
EPA believes that the milestones proposed and finalized here strike the
appropriate balance in providing for such a reasonable transition.
Moreover, in some cases, some of these recommended approaches could
enable a situation where a PSD permit would be issued to a source
during a future period when the area is designated nonattainment for
the revised O3 NAAQS. As explained below, the EPA does not
believe that this specific outcome is permissible under the CAA.
The EPA does not agree with the comments suggesting that the
grandfathering provision should be expanded to apply to any PSD
application received before the effective date of the final
nonattainment designations for the revised O3 NAAQS. Because
the process for reviewing PSD permit applications and issuing a final
PSD permit is time consuming, such an approach could allow issuance of
PSD permits to grandfathered sources even after the area in which the
source proposes to locate is designated nonattainment for the revised
O3 NAAQS. The EPA does not agree that grandfathering should
be extended in a way that would allow a source located in an area
designated as nonattainment for a pollutant at the time of permit
issuance to obtain a PSD permit for that pollutant rather than a NNSR
permit. The EPA does not interpret the CAA or its implementing
regulations to allow such an outcome. The PSD requirements under CAA
section 165 only apply in areas designated attainment or unclassifiable
for the pollutant. Alabama Power v. Costle, 636 F.2d 323, 365-66, 368
(D.C. Cir. 1980). Accordingly, the PSD implementing regulations at 40
CFR 52.21(i)(2) contain an exemption that provides that the substantive
PSD requirements shall not apply to a pollutant if the owner or
operator demonstrates that the facility is located in an area
designated nonattainment for that pollutant under CAA section 107 of
the Act. See also 40 CFR 51.166(i)(2) (allowing for the same exemption
in SIP-approved PSD permitting programs). In addition, under CAA
section 172(c)(5) implementation plans must require that permits issued
to new or modified stationary sources ``anywhere in the nonattainment
area'' meet the requirements of CAA section 173, which contains the
NNSR permit requirements. See 40 CFR part 51, Appendix S, IV.A
(providing that, if a major new source or major modification that would
locate in an area designated as nonattainment for a pollutant for which
the source or modification would be major, approval to construct may be
granted only if the specific conditions for NNSR are met, including
obtaining emission offsets and an emission limitation that specifies
the lowest achievable emissions rate). Moreover, given the adverse air
quality conditions that already exist in a nonattainment area and the
congressional directive to reach attainment as expeditiously as
practicable, construction of a major stationary source that
significantly increases emissions in such an area should be expected to
address all of the NNSR requirements, which are designed to ensure that
a new or modified major stationary source will not interfere with
reasonable progress toward attainment, even if this could cause delay
to the permit applicant.
With respect to the comments that suggested the effective date of
the NAAQS should be used as the date for both milestones, the EPA does
not agree that such a change is necessary. The purpose of the
grandfathering provision is to provide a reasonable transition
mechanism in the following circumstances: first, the PSD application is
one for which both the applicant and the reviewing authority have
committed substantial resources; and, second, this situation is one
where the need to satisfy the demonstration requirement under CAA
section 165(a)(3) could impact the reviewing authority's ability to
meet the statutory deadline for issuing a permit within one year of the
completeness determination. In situations where the reviewing authority
has not yet issued a completeness determination as of the signature
date of the revised O3 NAAQS, both the permit applicant and
the reviewing authority have sufficient notice of the revised standard
so that it can be addressed before the completeness determination is
issued and the one-year clock begins to run. The grandfathering
provision issued in this rulemaking is crafted to draw a reasonable
balance that accommodates the requirements under both CAA sections
165(a)(3) and 165(c). Any modification of the dates further than is
necessary to accommodate these concerns could upset this balance.
With respect to the comments that suggested adopting a
grandfathering provision applicable only to those PSD applications for
which the reviewing authority has determined the application to be
complete on or before the signature date of the revised NAAQS, the EPA
is not making this change because we understand that not all reviewing
authorities issue formal completeness determinations. Including
[[Page 65433]]
a grandfathering provision based on the publication of a public notice
of the draft permit or preliminary determination provides a reasonable
transition mechanism for PSD applications in situations where the
reviewing authority does not issue formal completeness determinations,
but the applicant and the reviewing authority have both committed
substantial resources to the pending permit application at the time the
revisions to the O3 NAAQS are finalized.
An opposing set of comments--submitted by a consortium of eight
environmental groups and one health advocacy group--challenged the
proposed grandfathering provision on the basis that the EPA did not
have the legal authority to grandfather sources from PSD requirements.
These commenters argued that the plain language of CAA section 165
forecloses the EPA's proposed approach and raised several other legal
considerations. The EPA disagrees with these comments, including the
interpretations of the CAA that they offer. As summarized in the
rationale for the final action below in section VII.C of this preamble,
the EPA believes that the CAA provides it authority and discretion to
establish a PSD grandfathering provision such as the one being adopted
today through a rulemaking process. The EPA is providing a further,
detailed analysis fully responding to this set of comments, as well as
other comments related to the grandfathering provision, in the Response
to Comment Document in the docket for this rule.
C. Final Action and Rationale
After consideration and evaluation of all the public comments
received on the grandfathering provision, the EPA is finalizing this
provision as proposed, with minor revisions that enhance the clarity of
the grandfathering provision, without changing its substantive effect.
While these revisions lead to slight differences in wording for the
grandfathering provision for the 2012 PM2.5 NAAQS and the
grandfathering provision finalized in this rulemaking, those
differences are not intended to create a different meaning; rather, the
grandfathering provision finalized in this rulemaking is intended to
have the same substantive effect and meaning for the revised
O3 standards as the grandfathering provision for the 2012
PM2.5 NAAQS had for the revised PM standards. Other than
those clarifying revisions, this final rule includes the same rule
language for the grandfathering provision as previously proposed for
the PSD regulations at 40 CFR 52.21(i)(12) and 51.166(i)(11),
respectively. The provision in the final rule reflects the same two
milestones and corresponding dates as the proposed grandfathering
provision. Thus, under the grandfathering provision as finalized,
either of the following two categories of pending PSD permit
applications would be eligible for grandfathering: (1) Applications for
which the reviewing authority has formally determined that the
application is complete on or before the signature date of the revised
O3 NAAQS, or (2) applications for which the reviewing
authority has first published a notice of a draft permit or preliminary
determination before the effective date of the revised O3
NAAQS. The EPA believes that it continues to be appropriate to include
the two proposed milestones for pending permit applications to be
eligible for grandfathering. While a completeness determination is
often the first event, some air agencies do not determine applications
complete as part of their permit process.
Under 40 CFR 52.21, a permit application may qualify for
grandfathering under either of the two sets of milestones and dates
contained in the provision. Where the EPA is the reviewing authority,
the EPA intends to apply the grandfathering provision to PSD applicants
pursuant to PSD regulations at 40 CFR 52.21 primarily through the use
of the completeness determination milestone because the EPA Regional
Offices make a formal completeness determination for any PSD
application that they receive and review. The EPA is including the
second criterion in 40 CFR 52.21 so that pending applications can still
qualify for grandfathering under the second criterion if any air agency
that incorporates 40 CFR 52.21 into a SIP-approved program does not
make formal completeness determinations as part of its permit review
process.
The EPA is also amending the PSD regulations at 40 CFR 51.166 to
enable states and other air agencies that issue PSD permits under SIP-
approved PSD programs to adopt a comparable grandfathering provision.
Nevertheless, such air agencies have discretion to not grandfather PSD
applications or to apply grandfathering under their approved PSD
programs in another manner as long as that program is at least as
stringent as the provision being added to 40 CFR 51.166. Accordingly,
an air agency may elect to rely on both sets of milestones and dates or
it may grandfather on the sole basis of only one set. However, the EPA
anticipates that once a decision is made concerning the use of either
set of milestones and dates, the air agency will apply grandfathering
consistently to all pending PSD permit applications.
As explained in more detail in the proposal, absent a regulatory
grandfathering provision, the EPA interprets section 165(a)(3)(B) of
the CAA and the implementing PSD regulations at 40 CFR 52.21(k)(1) and
51.166(k)(1) to require that PSD permit applications include a
demonstration that emissions from the proposed facility will not cause
or contribute to a violation of any NAAQS that is in effect as of the
date the PSD permit is issued. However, reading CAA section
165(a)(3)(B) in context with other provisions of the Act and the
legislative history, the EPA interprets the Act to provide the EPA with
authority to establish grandfathering provisions through regulation.
The EPA has explained its interpretation of its authority to promulgate
grandfathering provisions in previous rulemaking actions, most recently
in the rule establishing the grandfathering provision for the 2012
PM2.5 NAAQS (78 FR 3086, 3254-56, January 15, 2013), as well
as in the proposal for this final action. The EPA is providing
additional discussion of this authority in the Response to Comment
Document contained in the docket for this final action.
To summarize briefly, the addition of this grandfathering provision
is permissible under the discretion provided by the CAA for the EPA to
craft a reasonable implementation regulation that balances competing
objectives of the statutory PSD program found in CAA section 165.
Specifically, section 165(a)(3) requires a permit applicant to
demonstrate that its proposed project will not cause or contribute to a
violation of any NAAQS, while section 165(c) requires that a PSD permit
be granted or denied within one year after the permitting authority
determines the application for such permit to be complete. Section
109(d)(1) of the CAA requires the EPA to review existing NAAQS and make
appropriate revisions every five years. When these provisions are
considered together, a statutory ambiguity arises concerning how the
requirements under CAA section 165(a)(3)(B) should be applied to a
limited set of pending PSD permit applications when the O3
NAAQS is revised. The Act does not clearly address how the requirements
of CAA section 165(a)(3)(B) should be met for PSD permit applications
that are pending when the NAAQS are revised, particularly when the EPA
also determines that complying with the
[[Page 65434]]
demonstration requirement for the revised NAAQS could hinder compliance
with the requirement under section 165(c) to issue a permit within one
year of the completeness determination for a certain subset of pending
permits. The CAA also does not address how the requirements of CAA
sections 165(a)(3) and 165(c) should be balanced in light of the
statutory requirement to review the NAAQS every five years. As Congress
has not spoken precisely to this issue, the EPA has the discretion to
apply a permissible interpretation of the Act that balances the
statutory requirements to make a decision on a permit application
within one year and to ensure the new and modified sources will only be
authorized to construct after showing they can meet the substantive
permitting criteria. See Chevron, U.S.A., Inc. v. Natural Res. Def.
Council, Inc., 467 U.S. 837, 843-44 (1984).
In addressing these gaps in the CAA and the tension that may arise
in section 165 in these circumstances, the EPA also applies CAA section
301, where the Administrator is authorized ``to prescribe such
regulations as are necessary to carry out his functions under this
chapter.'' Sections 165(a)(3) and 165(c) of the CAA make clear that the
interests behind CAA section 165 include both protection of air quality
and timely decision-making on pending permit applications. The
legislative history illustrates congressional intent to avoid delays in
permit processing. S. Rep. No. 94-717, at 26 (1976) (``nothing could be
more detrimental to the intent of this section and the integrity of
this Act than to have the process encumbered by bureaucratic delay'').
Thus, when read in combination, these provisions of the CAA provide the
EPA with the discretion to issue regulations to grandfather pending
permit applications from having to address a revised NAAQS where
necessary to achieve both CAA objectives--to protect the NAAQS and to
avoid delays in processing PSD permit applications. Accordingly, the
EPA is seeking in this action to balance the requirements in the CAA to
make a decision on a permit application within one year and to ensure
that new and modified sources will only be authorized to construct
after showing they can meet the substantive permitting criteria that
apply to them. The EPA is achieving this balance by determining through
rulemaking which O3 NAAQS apply to certain permit
applications that are pending when the EPA finalizes the revisions to
the O3 NAAQS in this final rule. We are clarifying, for the
limited purpose of satisfying the requirements under section
165(a)(3)(B) for those permits, which O3 NAAQS are
applicable to those permit applications and must be addressed in the
source's demonstration that its emissions do not cause or contribute to
a violation of the NAAQS.
This approach is consistent with a recent opinion by the U.S. Court
of Appeals for the Ninth Circuit, which recognized the EPA's
traditional exercise of grandfathering authority through rulemaking.
The court observed that this approach was consistent with the statutory
requirement to ``enforce whatever regulations are in effect at the time
the agency makes a final decision'' because it involved identifying
``an operative date, incident to setting the new substantive standard,
and the grandfathering of pending permit applications was explicitly
built into the new regulations.'' Sierra Club v. EPA, 762 F.3d 971, 983
(9th Cir. 2014). As discussed in more detail in the EPA's Response to
Comment Document contained in the docket for this rule, this case
supports the EPA's action in this rulemaking. The court favorably
discussed prior adoption of regulatory grandfathering provisions that
are similar to the action in this rulemaking, such as the
grandfathering provision that the EPA promulgated when revising the
PM2.5 NAAQS that became effective in 2013. See id. at 982-
83.\227\
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\227\ This case specifically involved an action by the EPA to
issue an individual PSD permit, which grandfathered a specific
permit applicant from certain requirements without any revision to
the regulations that were in effect. The court's reasoning in this
case distinguishes that type of permit-specific grandfathering from
establishing grandfathering provisions through a rulemaking process.
While the court was not persuaded that there was a conflict between
the requirements of sections 165(a)(3) and 165(c) of the CAA that
supported the permit-specific grandfathering at issue in that case,
it did not extend that uncertainty to its discussion of the EPA's
rulemaking authority. In fact, in its favorable discussion of the
EPA's authority to grandfather pending permit applications through
regulation, the court noted that the power of an administrative
agency ``to administer a congressionally created and funded program
necessarily requires the formulation of policy and the making of
rules to fill any gap left, implicitly or explicitly, by Congress''
though ``such decision cannot be made on an ad hoc basis.'' Sierra
Club v. EPA, 762 F.3d 971, 983 (9th Cir. 2014) (internal quotations
and marks omitted). This indicates that the court believed there is
a gap in the CAA that supports including grandfathering provisions
in regulations.
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This adoption of a grandfathering provision in this action is also
consistent with previous actions in which the EPA has recognized that
the CAA provides discretion for the EPA to establish grandfathering
provisions for PSD permit applications through regulations. Some
examples of previous references to the EPA's authority to grandfather
certain applications through rulemaking include 45 FR 52683, August 7,
1980; 52 FR 24672, July 1, 1987; and most recently 78 FR 3086, January
15, 2013.
This grandfathering provision does not apply to any applicable PSD
requirements related to O3 other than the requirement to
demonstrate that the proposed source does not cause or contribute to a
violation of the revised O3 NAAQS. Sources with projects
qualifying under the grandfathering provision will be required to meet
all the other applicable PSD requirements, including applying BACT to
all applicable pollutants, demonstrating that emissions from the
proposed facility will not cause or contribute to a violation of the
O3 NAAQS in effect at the time of the relevant
grandfathering milestone, and addressing any Class I area and
additional O3-related impacts in accordance with the
applicable PSD requirements. In addition, this grandfathering provision
would not apply to any permit application for a new or modified major
stationary source of O3 located in an area designated
nonattainment for O3 on the date the permit is issued.
VIII. Implementation of the Revised O3 Standards
This section provides background information for understanding the
implications of the revised O3 NAAQS and describes the EPA's
plans for providing revised rules or additional guidance on some
subjects in a timely manner to assist states with their implementation
efforts under the requirements of the CAA. This section also describes
existing EPA rules, interpretations of CAA requirements, and other EPA
guidance relevant to implementation of the revised O3 NAAQS.
Relevant CAA provisions that provide potential flexibility with regard
to meeting implementation timelines are highlighted and discussed. This
section also contains a discussion of how existing requirements to
reduce the impact on O3 concentrations from the stationary
source construction in permit programs under the CAA are affected by
the revisions to the O3 NAAQS. These are the PSD and
Nonattainment New Source Review (NNSR) programs. As discussed in
section VII of this preamble, to facilitate a smooth transition to the
PSD requirements for the revised O3 NAAQS, the EPA is
finalizing as part of this rulemaking a grandfathering provision that
applies to certain PSD permit applications that are pending and have
met certain milestones in the permitting process
[[Page 65435]]
when the revised O3 NAAQS is signed or before the effective
date of the revised O3 NAAQS, depending on the milestone.
In the preamble for the O3 NAAQS proposal, the EPA
solicited comments on several issues related to implementing the
revised O3 NAAQS that the agency anticipated addressing in
future guidance or regulatory actions, but for which the EPA was not at
that time proposing any action. The EPA received numerous comments on
those and other implementation issues. Consistent with what the EPA
indicated in the O3 NAAQS proposal (79 FR 75370), the agency
is not responding to the implementation comments that are not related
to a specific proposal. However, the EPA intends to take these comments
under advisement as the agency develops rules and guidance to assist
with implementation of the revised NAAQS. Because the EPA did
specifically propose and is finalizing provisions in the regulations
addressing grandfathering for certain PSD permit applications and
requirements, as discussed in section VII of this preamble, the EPA is
responding to comments on the proposed PSD grandfathering provisions.
A. NAAQS Implementation Plans
1. Cooperative Federalism
As directed by the CAA, reducing pollution to meet national air
quality standards always has been a shared task, one involving the
federal government, states, tribes and local air quality management
agencies. The EPA develops regulations and strategies to reduce
pollution on a broad scale, while states and tribes are responsible for
implementation planning and any additional emission reduction measures
necessary to bring specific areas into attainment. The agency supports
implementation planning with technical resources, guidance, and program
rules where necessary, while air quality management agencies use their
knowledge of local needs and opportunities in designing emission
reduction strategies that will work best for their industries and
communities.
This partnership has proved effective since the EPA first issued
O3 standards more than three decades ago. For example, 101
areas were designated as nonattainment for the 1-hour O3
standards issued in 1979. As of the end of 2014, air quality in all but
one of those areas meets the 1-hour standards. The EPA strengthened the
O3 standards in 1997, shifting to an 8-hour standard to
improve public health protection, particularly for children, the
elderly, and other sensitive individuals. The 1997 standards drew
significant public attention when they were proposed, with numerous
parties voicing concerns about states' ability to comply. However,
after close collaboration between the EPA, states, tribes and local
governments to reduce O3-forming pollutants, significant
progress has been made. Air quality in 108 of the original 115 areas
designated as nonattainment for the 1997 O3 NAAQS now meets
those standards. Air quality in 18 of the original 46 areas designated
as nonattainment for the 2008 O3 NAAQS now meets those
standards.
The revisions to the primary and secondary O3 NAAQS
discussed in sections II.D and IV.D of this preamble trigger a process
under which states \228\ make recommendations to the Administrator
regarding area designations. Then, the EPA promulgates the final area
designations. States also are required to review capacity and
authorities in their existing SIPs to ensure the CAA requirements
associated with the new standards can be carried out, and modify or
supplement their existing SIPs as needed. The O3 NAAQS
revisions also apply to the transportation conformity and general
conformity determinations, and affect which preconstruction permitting
requirements apply to sources of O3 precursor emissions, and
the nature of those requirements.
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\228\ This and all subsequent references to ``state'' are meant
to include state, local, and tribal agencies responsible for the
implementation of an O3 control program.
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The EPA has regulations in place addressing the general
requirements for SIPs, and there are also provisions in these existing
rules that cover O3 SIPs (40 CFR part 51). States likewise
have provisions in their existing SIPs to address air quality for
O3 and to implement the existing O3 NAAQS. In the
course of the past 45 years of regulating criteria pollutants,
including O3, the EPA has also provided general guidance on
the development of SIPs and administration of construction permitting
programs, as well as specific guidance on implementing the
O3 NAAQS in some contexts under the CAA and the EPA
regulations.
The EPA has considered the extent to which existing EPA regulations
and guidance are sufficient to implement the revised standards. The CAA
does not require that the EPA promulgate new implementing regulations
or issue new guidance for states every time that a NAAQS is revised.
Likewise, the CAA does not require the issuance of additional
implementing regulations or guidance by the EPA before a revised NAAQS
becomes effective. It is important to note that the existing EPA
regulations in 40 CFR part 51 applicable to SIPs generally and to
particular pollutants, including O3 and O3
precursors, continue to apply unless and until they are updated.
Accordingly, the discussion below provides the EPA's current thoughts
about the extent to which revisions to existing regulations and
additional guidance are appropriate to aid in the implementation of the
revised O3 NAAQS.
2. Additional New Rules and Guidance
The EPA has received comments from a variety of states and
organizations asking for rules and guidance associated with a revised
NAAQS to be issued in a timely manner. As explained above, and
consistent with the proposal, the EPA is not responding to these
comments at this time because they are not related to any changes to
existing regulations that EPA proposed in this rule. Moreover, although
issuance of such rules and guidance is not a part of the NAAQS review
process, National Ass'n of Manufacturers v. EPA, 750 F. 3d 921, 926-27
(D.C. Cir. 2014), toward that end, the EPA intends to develop
appropriate revisions to necessary implementation rules and provide
additional guidance in time frames that are useful to states when
developing implementation plans that meet CAA requirements.
Certain requirements under the PSD preconstruction permit review
program apply immediately to a revised NAAQS upon the effective date of
that NAAQS, unless the EPA has established a grandfathering provision
through rulemaking. To ensure a smooth transition to a revised
O3 NAAQS, the EPA is finalizing a grandfathering provision
similar to the provision finalized in the 2012 PM2.5 NAAQS
Rule. See section VII.C of this preamble for more details on the PSD
program and the final grandfathering provision.
Promulgation or revision of the NAAQS starts a clock for the EPA to
designate areas as either attainment or nonattainment. State
recommendations for area designations are due to the EPA within 12
months of promulgating or revising the NAAQS. In an effort to allow
states to make more informed recommendations for these particular
standards, the EPA intends to issue additional guidance concerning the
designations process for these standards within four months of
promulgation of the NAAQS, or approximately eight months before state
recommendations are due. The EPA generally completes
[[Page 65436]]
area designations two years after promulgation of a NAAQS. See section
VIII.B of this preamble for additional information on the initial area
designation process.
Under CAA section 110, a NAAQS revision triggers the review and, as
necessary, revision of SIPs to be submitted within three years of
promulgation of a revised NAAQS. These SIPs are referred to as
``infrastructure SIPs.'' The EPA issued general guidance on submitting
infrastructure SIPs on September 13, 2013.\229\ It should be noted that
this guidance did not address certain state planning and emissions
control requirements related to interstate pollution transport. This
guidance remains relevant for the revised O3 NAAQS. See
section VIII.A.4 of this preamble for additional information on
infrastructure SIPs.
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\229\ See memorandum from Stephen D. Page to Regional Air
Directors, ``Guidance on Infrastructure State Implementation Plan
(SIP) Elements under Clean Air Act Sections 110(a)(1) and
110(a)(2)'' September 13, 2013, which is available at http://www3.epa.gov/airquality/urbanair/sipstatus/docs/Guidance_on_Infrastructure_SIP_Elements_Multipollutant_FINAL_Sept_2013.pdf.
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While much of the existing rules and guidance for prior ozone
standards remains applicable to the new standards, the EPA intends to
propose to adopt revised rules on some subjects to facilitate air
agencies' efforts to implement the revised O3 NAAQS within
one year after the revised NAAQS is established. The rules would
address nonattainment area classification methodologies and attainment
dates, attainment plan and NNSR SIP submission due dates, and any other
necessary revisions to existing regulations for other required
implementation programs. The EPA anticipates finalizing these rules by
the time areas are designated nonattainment. Finalizing rules and
guidance on these subjects by this time would assist air quality
management agencies with development of any CAA-required SIPs
associated with nonattainment areas. See section VIII.A.5 of this
preamble for additional information on nonattainment SIPs and section
VIII.C.3 for additional information on nonattainment New Source Review
requirements applicable to new major sources and major modifications of
existing sources.
3. Background O3
The EPA and state, local and tribal air agencies, strive to
determine how to most effectively and efficiently use the CAA's various
provisions to provide required public health and welfare protection
from the harmful effects of O3. In most cases, reducing man-
made emissions of NOX and VOCs within the U.S. will reduce
O3 formation and provide additional health and welfare
protection. The EPA recognizes, however, that there can be infrequent
events where daily maximum 8-hour O3 concentrations approach
or exceed 70 ppb largely due to the influence of wildfires or
stratospheric intrusions, which contribute to U.S. background (USB)
levels but may also qualify for consideration under the Exceptional
Events Rule. See section I.D; but see section II.A.2.a above
(percentage of anthropogenic O3 tends to increase on high
O3 days relative to percentage of background, including in
intermountain west).
The term ``background'' O3 is often used to refer to
O3 that originates from natural sources of O3
(e.g., wildfires and stratospheric O3 intrusions) and
O3 precursors, as well as from man-made international
emissions of O3 precursors. Using the term generically,
however, can lead to confusion as to what sources of O3 are
being considered. Relevant to the O3 implementation
provisions of the CAA, we define background O3 the same way
the EPA defines USB: O3 that would exist in the absence of
any man-made emissions inside the U.S.
While the great majority of modeled O3 exceedances have
local and regional emissions as their primary cause, there can be
events where O3 levels approach or exceed the concentration
level of the revised O3 standards in large part due to
background sources. These cases of high USB levels on high
O3 days typically result from stratospheric intrusions of
O3 or wildfire O3 plumes. These events are
infrequent and the CAA contains provisions that can be used to help
deal, in particular, with stratospheric intrusion and wildfire events
with O3 contributions of this magnitude, including providing
varying degrees of regulatory relief for air agencies and potential
regulated entities. The EPA intends to work closely with states to
identify affected locations and ensure that the appropriate regulatory
mechanisms are employed.
Statutory and regulatory relief associated with U.S. background
O3 may include: \230\
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\230\ Note that the relief mechanisms discussed here do not
include the CAA's interstate transport provisions found in sections
110(a)(2)(D) and 126. The interstate transport provisions are
intended to address the cross-state transport of O3 and
O3 precursor emissions from man-made sources within the
continental U.S. rather than background O3 as it is
defined in this section. As noted in section II.A.2.a above, many of
the instances where commenters pointed to remote monitored locations
having O3 exceedances due to background O3 in
fact reflected sizeable contributions from domestic sources,
including interstate contributions (including from the Los Angeles
Basin and other California locations).
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Relief from designation as a nonattainment area through
exclusion of data affected by exceptional events;
Relief from the more stringent requirements of higher
nonattainment area classifications through treatment as a rural
transport area, through exclusion of data affected by exceptional
events, or through international transport provisions;
Relief from having to demonstrate attainment and having to
adopt more than reasonable controls on local sources through
international transport provisions.
Further discussion of these mechanisms is provided in sections
VIII.B.2 (exceptional events), VIII.B.1 (rural transport areas), and
VIII.E.2 (international transport).
Although these relief mechanisms require some level of assessment
or demonstration by a state and/or the EPA to invoke, they have been
used successfully in the past under appropriate circumstances. For
example, the EPA has historically acted on every exceptional events
demonstration that has affected a regulatory decision regarding initial
area designations. See e.g., Idaho: West Silver Valley Nonattainment
Area--Area Designations for the 2012 primary annual PM2.5
NAAQS Technical Support Document, pp. 10-14, December 2014. For the
revised O3 standards, the areas that would most likely need
to use the mechanisms discussed in this section as part of attaining
the revised O3 standards are locations in the western U.S.
where we have estimated the largest seasonal average values of
background O3 occur. We expect some of these areas to use
the provisions in the Exceptional Events Rule during the designations
process for the revised O3 standards. The EPA will then give
priority to exceptional events demonstrations submitted by air agencies
with areas whose designation decision could be influenced by the
exclusion of data under the Exceptional Events Rule. In addition, as
discussed in more detail in sections V.D and VIII.B.2 of this action,
to streamline the exceptional events process, the EPA will soon propose
revisions to the 2007 Exceptional Events Rule and will release through
a Federal Register Notice of Availability a draft guidance document to
address Exceptional Events Rule criteria for wildfires that could
affect O3 concentrations. We expect to
[[Page 65437]]
promulgate Exceptional Events Rule revisions and finalize the new
guidance document before the October 2016 date by which states, and any
tribes that wish to do so, are required to submit their initial
designation recommendations for the revised O3 NAAQS.
4. Section 110 State Implementation Plans
The CAA section 110 specifies the general requirements for SIPs.
Within three years after the promulgation of revised NAAQS (or such
shorter period as the Administrator may prescribe \231\) each state
must adopt and submit ``infrastructure'' SIPs to the EPA to address the
requirements of section 110(a)(1) and (2), as applicable. These
``infrastructure SIP'' submissions establish the basic state programs
to implement, maintain, and enforce revised NAAQS and provide
assurances of state resources and authorities. States are required to
develop and maintain an air quality management infrastructure that
includes enforceable emission limitations, a permitting program, an
ambient monitoring program, an enforcement program, air quality
modeling capabilities, and adequate personnel, resources, and legal
authority. Because the revised primary NAAQS and secondary NAAQS are
identical, the EPA does not at present discern any need for there to be
any significant substantive difference in the infrastructure SIP
elements for the two standards and thus believes it would be more
efficient for states and the EPA if each affected state submits a
single section 110 infrastructure SIP that addresses both standards at
the same time (i.e., within three years of promulgation of the
O3 NAAQS). Accordingly the EPA is not extending the SIP
deadline for purposes of a revised secondary standard.
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\231\ While the CAA allows the EPA to set a shorter time for
submission of these SIPs, the EPA does not currently intend to do so
for this revision to the O3 NAAQS.
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It is the responsibility of each state to review its air quality
management program's compliance with the infrastructure SIP provisions
in light of each new or revised NAAQS. Most states have revised and
updated their infrastructure SIPs in recent years to address
requirements associated with the 2008 O3 NAAQS. We expect
that the result of these prior updates is that, in most cases, states
will already have adequate state regulations previously adopted and
approved into the SIP to address a particular requirement with respect
to the revised O3 NAAQS. For such portions of the state's
infrastructure SIP submission, the state may provide a
``certification'' specifying that certain existing provisions in the
SIP are adequate to meet applicable requirements. Although the term
``certification'' does not appear in the CAA as a type of
infrastructure SIP submittal, the EPA sometimes uses the term in the
context of infrastructure SIPs, by policy and convention, to refer to a
state's SIP submission. If a state determines that its existing EPA-
approved SIP provisions are adequate in light of the revised
O3 NAAQS with respect to a given infrastructure SIP element
(or sub-element), then the state may make a ''certification'' that the
existing SIP contains provisions that address those requirements of the
specific CAA section 110(a)(2) infrastructure elements. In the case of
a certification, the submittal does not have to include another copy of
the relevant provision (e.g., rule or statute) itself. Rather, the
submission may provide citations to the already SIP-approved state
statutes, regulations, or non-regulatory measures, as appropriate,
which meet the relevant CAA requirement. Like any other SIP submission,
such certification can be made only after the state has provided
reasonable notice and opportunity for public hearing. This ``reasonable
notice and opportunity for public hearing'' requirement for
infrastructure SIP submittals appears at section 110(a), and it
comports with the more general SIP requirement at section 110(l) of the
CAA. Under the EPA's regulations at 40 CFR part 51, if a public hearing
is held, an infrastructure SIP submission must include documentation by
the state that the public hearing was held in accordance with the EPA's
procedural requirements for public hearings. See 40 CFR part 51,
Appendix V, paragraph 2.1(g), and 40 CFR 51.102. In the event that a
state's existing SIP does not already meet applicable requirements,
then the infrastructure SIP submission must include the modifications
or additions to the state's SIP in order to update it to meet the
relevant elements of section 110(a)(2).
5. Nonattainment Area Requirements
Part D of the CAA describes the various program requirements that
apply to states with nonattainment areas for different NAAQS. Clean Air
Act Section 182 (found in subpart 2 of part D) includes the specific
SIP requirements that govern the O3 program, and supplements
the more general nonattainment area requirements in CAA sections 172
and 173. Under CAA section 182, states generally are required to submit
attainment demonstration SIPs within three or four years after the
effective date of area designations promulgated by the EPA, depending
on the classification of the area.\232\ These SIP submissions need to
show how the nonattainment area will attain the primary O3
standard ``as expeditiously as practicable,'' but no later than within
the relevant time frame from the effective date of designations
associated with the classification of the area.
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\232\ Section 181(a)(1) of the CAA establishes classification
categories for areas designated nonattainment for the primary
O3 NAAQS. These categories range from ``Marginal,'' the
lowest O3 classification with the fewest requirements
associated with it, to ``Extreme,'' the highest classification with
the most required programs. Areas with worse O3 problems
are given more time to attain the NAAQS and more associated emission
control requirements.
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The EPA believes that the overall framework and policy approach of
the implementation rules associated with the 2008 O3 NAAQS
provide an effective and appropriate template for the general approach
states would follow in planning for attainment of the revised
O3 standard.\233\ However, to assist with the implementation
of the revised O3 standards, the EPA intends to develop and
propose an additional O3 NAAQS Implementation Rule that will
address certain subjects specific to the new O3 NAAQS
finalized here. This will include establishing air quality thresholds
associated with each nonattainment area classification (i.e., Marginal,
Moderate, etc.), associated attainment deadlines, and deadlines for
submitting attainment planning SIP elements (e.g., RACT for major
sources, RACT VOC control techniques guidelines, etc.). The rulemaking
will also address whether to revoke the 2008 O3 NAAQS, and
to impose appropriate anti-backsliding requirements to ensure that the
protections afforded by that standard are preserved. The EPA intends to
propose this implementation rule within one year after the revised
O3 NAAQS is promulgated, and finalize this implementation
rule by no later than the time the area designations process is
finalized (approximately two years after promulgation of the revised
O3 NAAQS).
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\233\ Implementation of the 2008 National Ambient Air Quality
Standards for Ozone: State Implementation Plan Requirements; Final
Rule (80 FR 12264; March 6, 2015) and Implementation of the 2008
National Ambient Air Quality Standards for Ozone: Nonattainment Area
Classifications Approach, Attainment Deadlines and Revocation of the
1997 Ozone Standards for Transportation Conformity Purposes (77 FR
30160; May 21, 2012).
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We know that developing the implementation plans that outline the
steps a nonattainment area will take to
[[Page 65438]]
meet an air quality standard requires a significant amount of work on
the part of state, tribal or local air agencies. The EPA routinely
looks for ways to reduce this workload, including assisting with air
quality modeling by providing inputs such as emissions, meteorological
and boundary conditions; and sharing national-scale model results that
states can leverage in their development of attainment demonstrations.
B. O3 Air Quality Designations
1. Area Designation Process
After the EPA establishes or revises a NAAQS, the CAA directs the
EPA and the states to take steps to ensure that the new or revised
NAAQS is met. One of the first steps, known as the initial area
designations, involves identifying areas of the country that either
meet or do not meet the new or revised NAAQS, along with any nearby
areas that contribute to areas that do not meet the new or revised
NAAQS.
Section 107(d)(1) of the CAA provides that, ``By such date as the
Administrator may reasonably require, but not later than 1 year after
promulgation of a new or revised national ambient air quality standard
for any pollutant under section 109, the Governor of each state shall .
. . submit to the Administrator a list of all areas (or portions
thereof) in the state'' that designates those areas as nonattainment,
attainment, or unclassifiable. The EPA must then promulgate the area
designations according to a specified process, including procedures to
be followed if the EPA intends to modify a state's initial
recommendation.
Clean Air Act Section 107(d)(1)(B)(i) further provides, ``Upon
promulgation or revision of a national ambient air quality standard,
the Administrator shall promulgate the designations of all areas (or
portions thereof) . . . as expeditiously as practicable, but in no case
later than 2 years from the date of promulgation of the new or revised
national ambient air quality standard. Such period may be extended for
up to one year in the event the Administrator has insufficient
information to promulgate the designations.'' By no later than 120 days
prior to promulgating area designations, the EPA is required to notify
states of any intended modifications to their recommendations that the
EPA may deem necessary. States then have an opportunity to demonstrate
why any proposed modification is inappropriate. Whether or not a state
provides a recommendation, the EPA must timely promulgate the
designation that the agency deems appropriate.
While section 107 of the CAA specifically addresses states, the EPA
intends to follow the same process for tribes to the extent
practicable, pursuant to CAA section 301(d) regarding tribal authority
and the Tribal Authority Rule (63 FR 7254, February 12, 1998). To
provide clarity and consistency in doing so, the EPA issued a 2011
guidance memorandum on working with tribes during the designation
process.\234\
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\234\ Page, S. (2011). Guidance to Regions for Working with
Tribes during the National Ambient Air Quality Standards (NAAQS)
Designations Process, Memorandum from Stephen D. Page, Director, EPA
Office of Air Quality Planning and Standards to Regional Air
Directors, Regions I-X, December 20, 2011. Available: http://www.epa.gov/ttn/oarpg/t1/memoranda/20120117naaqsguidance.pdf.
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As discussed in sections II and IV of this preamble, the EPA is
revising both the primary and secondary O3 NAAQS.
Accordingly, the EPA intends to complete designations for both NAAQS
following the standard 2-year process discussed above. In accordance
with section 107(d)(1) of the CAA, state Governors (and tribes, if they
choose) should submit their initial designation recommendations for a
revised primary and secondary NAAQS by 1 year after October 1, 2015. If
the EPA intends to modify any state recommendation, the EPA would
notify the appropriate state Governor (or tribal leader) no later than
120 days prior to making final designation decisions. A state or tribe
that believes the modification is inappropriate would then have the
opportunity to demonstrate to the EPA why it believes its original
recommendation (or a revised recommendation) is more appropriate. The
EPA would take any additional input into account in making the final
designation decisions.
The CAA defines an area as nonattainment if it is violating the
NAAQS or if it is contributing to a violation in a nearby area.
Consistent with previous area designations processes, the EPA intends
to use area-specific analysis of multiple factors to support area
boundary decisions. The EPA intends to evaluate information related to
the following factors for designations: air quality data, emissions and
emissions-related data, meteorology, geography/topography, and
jurisdictional boundaries. Additional guidance on the designation
process and how these factors may be evaluated and inform the process
will be issued by the EPA early in 2016 to assist states in developing
their recommendations.
Areas that are designated as nonattainment are also classified at
the time of designation by operation of law according to the severity
of their O3 problem. The classification categories are
Marginal, Moderate, Serious, Severe, and Extreme. Ozone nonattainment
areas are subject to specific mandatory measures depending on their
classification. As indicated previously, the thresholds for the
classification categories will be established in a future O3
implementation rule.
Clean Air Act section 182(h) authorizes the EPA Administrator to
determine that an area designated nonattainment can be treated as a
rural transport area. Regardless of its classification, a rural
transport area is deemed to have fulfilled all O3-related
planning and control requirements if it meets the CAA's requirements
for areas classified Marginal, which is the lowest classification
specified in the CAA. In accordance with the statute, a nonattainment
area may qualify for this determination if it meets the following
criteria:
The area does not contain emissions sources that make a
significant contribution to monitored O3 concentrations in
the area, or in other areas; and
The area does not include and is not adjacent to a
Metropolitan Statistical Area.
Historically, the EPA has listed four nonattainment areas as rural
transport areas under this statutory provision.\235\ The EPA has not
issued separate written guidance to further elaborate on the
interpretation of these CAA qualification criteria. However, the EPA
developed draft guidance in 2005 that explains the kinds of technical
analyses that states could use to establish that transport of
O3 and/or O3 precursors into the area is so
overwhelming that the contribution of local emissions to an observed 8-
hour O3 concentration above the level of the NAAQS is
relatively minor and determine that emissions within the area do not
make a significant contribution to the O3 concentrations
measured in the area or in other areas.\236\ While this guidance
[[Page 65439]]
was not prepared specifically for rural transport areas, it could be
useful to states for developing technical information to support a
request that the EPA treat a specific O3 nonattainment area
as a rural transport area. The EPA will work with states to ensure
nonattainment areas eligible for treatment as rural transport areas are
identified.
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\235\ For the 1979 1-hour O3 standard, Door County
Area, Wisconsin; Edmonson County Area, Kentucky; Essex County Area
(Whiteface Mountain), New York; and Smyth County Area (White Top
Mountain), Virginia were recognized by the EPA as rural transport
areas. No rural transport areas were recognized for the 1997 or 2008
8-hour O3 standards.
\236\ U.S. Environmental Protection Agency (2005). Criteria For
Assessing Whether an Ozone Nonattainment Area is Affected by
Overwhelming Transport [Draft EPA Guidance]. U.S. Environmental
Protection Agency, Research Triangle Park, NC. June 2005. Available
at http://www.epa.gov/scram001/guidance/guide/owt_guidance_07-13-05.pdf.
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2. Exceptional Events
During the initial area designations process, the EPA intends to
evaluate multiple factors, including air quality data, when identifying
and determining boundaries for areas of the country that meet or do not
meet the revised O3 NAAQS. In some cases, these data may be
influenced by exceptional events. Under the Exceptional Events Rule, an
air agency can request and the EPA can agree to exclude data associated
with event-influenced exceedances or violations of a NAAQS, including
the revised O3 NAAQS, provided the event meets the statutory
requirements in section 319(b) of the CAA, which requires that:
the event ``affects air quality;''
the event ``is not reasonably controllable or
preventable;''
the event is ``caused by human activity that is unlikely
to recur at a particular location or [is] a natural event,'' \237\ and
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\237\ A natural event is further described in 40 CFR 50.1(k) as
``an event in which human activity plays little or no direct causal
role.''
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that ``a clear causal relationship must exist between the
measured exceedances of a [NAAQS] and the exceptional event. . . .''
The EPA's implementing regulations, the Exceptional Events Rule,
further specify certain requirements for air agencies making
exceptional events demonstrations.\238\
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\238\ 72 FR 13,560 (March 22, 2007), ``Treatment of Data
Influenced by Exceptional Events,'' Final Rule; see also 40 CFR
parts 50 and 51.
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The ISA contains discussions of natural events that may contribute
to O3 or O3 precursors. These include
stratospheric O3 intrusion and wildfire events.\239\ As
indicated above, to satisfy the exceptional events requirements and to
qualify for data exclusion under the Exceptional Events Rule, an air
agency must develop and submit a demonstration, including evidence,
addressing each of the identified criteria. The extent to which a
stratospheric O3 intrusion event or a wildfire event
contributes to O3 levels can be uncertain, and in most cases
requires detailed analyses to determine.
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\239\ The preamble to the Exceptional Events Rule (72 FR 13560)
identifies both stratospheric O3 intrusions and wildfires
as natural events that could also qualify as exceptional events
under the CAA and Exceptional Event Rule criteria. Note that
O3 resulting from routine natural emissions from
vegetation, microbes, animals and lightning are not exceptional
events authorized for exclusion under the section 319 of the CAA.
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Strong stratospheric O3 intrusion events, most prevalent
at high elevation sites during winter or spring, can be identified
based on measurements of low relative humidity, evidence of deep
atmospheric mixing, and a low ratio of CO to O3 based on
ambient measurements. Accurately determining the extent of weaker
intrusion events remains challenging (U.S. EPA 2013, p. 3-34). Although
states have submitted only a few exceptional events demonstrations for
stratospheric O3 intrusion, the EPA recently approved a
demonstration from Wyoming for a June 2012 stratospheric O3
event.\240\
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\240\ U.S. EPA (2014) Treatment of Data Influenced by
Exceptional Events: Examples of Reviewed Exceptional Event
Submissions. U.S. Environmental Protection Agency, Research Triangle
Park, NC, available at http://www.epa.gov/ttn/analysis/exevents.htm.
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While stratospheric O3 intrusions can increase monitored
ground-level ambient O3 concentrations, wildfire plumes can
either suppress or enhance O3 depending upon a variety of
factors including fuel type, combustion stage, plume chemistry, aerosol
effects, meteorological conditions and distance from the fire (Jaffe
and Wigder, 2012). As a result, determining the impact of wildfire
emissions on specific O3 observations is challenging. The
EPA recently approved an exceptional events demonstration for wildfires
affecting 1-hour O3 levels in Sacramento, California in 2008
that successfully used a variety of analytical tools (e.g., regression
modeling, back trajectories, satellite imagery, etc.) to support the
exclusion of O3 data affected by large fires.\241\
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\241\ U.S. EPA (2014) Treatment of Data Influenced by
Exceptional Events: Examples of Reviewed Exceptional Event
Submissions. U.S. Environmental Protection Agency, Research Triangle
Park, NC. Examples of O3-related exceptional event
submissions, available at http://www.epa.gov/ttn/analysis/exevents.htm.
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In response to previously expressed stakeholder feedback regarding
implementation of the Exceptional Events Rule and specific stakeholder
concerns regarding the burden of exceptional events demonstrations, the
EPA is currently engaged in a rulemaking process to amend the
Exceptional Events Rule. As part of an upcoming notice and comment
rulemaking effort (and related activities, including the issuance of
relevant guidance documents), the EPA sees opportunities to standardize
best practices for collaboration between the EPA and air agencies,
clarify and simplify demonstrations, and improve tools and consistency.
Additionally, the EPA intends to develop guidance to address
implementing the Exceptional Events Rule criteria for wildfires that
could affect ambient O3 concentrations. Wildfire emissions
are a component of background O3 (Jaffe and Wigder, 2012)
and in some locations can significantly contribute to periodic high
O3 levels (Emery, 2012). The threat from wildfires can be
mitigated through management of wildland vegetation. Planned and
managed fires are one tool that land managers can use to reduce fuel
load, unnatural understory and tree density, thus helping to reduce the
risk of catastrophic wildfires. Allowing some wildfires to continue and
the thoughtful use of prescribed fire can influence the occurrence of
catastrophic wildfires, which may reduce the probability of fire-
induced smoke impacts and subsequent health effects. Thus, appropriate
use of prescribed fire may help manage the contribution of wildfires to
both background and periodic peak O3 air pollution. Several
commenters expressed concern that the revised O3 NAAQS could
limit the future use of prescribed fire. Under the current Exceptional
Events Rule, prescribed fires meeting the rule criteria may also
qualify as exceptional events. The EPA intends to further clarify the
Exceptional Events Rule criteria for prescribed fire on wildland in its
upcoming rulemaking.
The EPA is committed to working with federal land managers, other
federal agencies, tribes and states to effectively manage prescribed
fire use to reduce the impact of wildfire-related emissions on
O3 through policies and regulations implementing these
standards.
C. How do the New Source Review (NSR) requirements apply to the revised
O3 NAAQS?
1. NSR Requirements for Major Stationary Sources for the Revised
O3 NAAQS
The CAA, at parts C and D of title I, contains preconstruction
review and permitting programs applicable to new major stationary
sources and major modifications of existing major sources. The
preconstruction review of each new major stationary source and major
modification applies on a pollutant-specific basis, and the
requirements that apply for each pollutant depend on whether the area
in which the source is situated is designated as attainment (or
[[Page 65440]]
unclassifiable) or nonattainment for that pollutant. In areas
designated attainment or unclassifiable for a pollutant, the PSD
requirements under part C apply to construction at major sources. In
areas designated nonattainment for a pollutant, the NNSR requirements
under part D apply to major source construction. Collectively, those
two sets of permit requirements are commonly referred to as the ``major
New Source Review'' or ``major NSR'' programs.
Until an area is formally designated with respect to the revised
O3 NAAQS, the NSR provisions applicable under that area's
current designation for the 2008 O3 NAAQS (including any
applicable anti-backsliding requirements) will continue to apply. That
is, for areas designated as attainment/unclassifiable for the 2008
O3 NAAQS, PSD will apply for new major stationary sources
and major modifications that trigger major source permitting
requirements for O3; areas designated nonattainment for the
2008 O3 NAAQS must comply with the NNSR requirements for new
major stationary sources and major modifications that trigger major
source permitting requirements for O3. When the new
designations for the revised O3 NAAQS become effective,
under the current rules, those designations will generally serve to
determine whether PSD or NNSR applies to O3 and its
precursors. The PSD regulations at 40 CFR 51.166(i)(2) and 52.21(i)(2)
provide that the substantive PSD requirements do not apply for a
particular pollutant if the owner or operator of the new major
stationary source or major modification demonstrates that the area in
which the source is located is designated nonattainment for that
pollutant under CAA section 107. Thus, new major sources and
modifications will generally be subject to the PSD program requirements
for O3 if they are locating in an area that does not have a
current nonattainment designation under CAA section 107 for
O3. These rules further provide that nonattainment
designations for a revoked NAAQS, as contained in 40 CFR part 81, are
not viewed as current designations under CAA section 107 for purposes
of determining the applicability of such PSD requirements.\242\
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\242\ This description of paragraph (i)(2) of the PSD
regulations at 40 CFR 51.166 and 52.21 reflects revisions made in
the final 2008 O3 NAAQS SIP Requirements Rule. See 80 FR
12264 at 12287 (March 6, 2015).
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The EPA's major NSR regulations define the term ``regulated NSR
pollutant'' to include any pollutant for which a NAAQS has been
promulgated and any pollutant identified in EPA regulations as a
constituent or precursor to such pollutant.\243\ Both the PSD and NNSR
regulations identify VOC and NOX as precursors to
O3. Accordingly, the major NSR programs for O3
are applied to emissions of VOC and NOX as precursors of
O3.\244\
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\243\ The definition of ``regulated NSR pollutant'' is found in
the PSD regulations at 40 CFR 51.166(b)(49) and 52.21(b)(50), and in
the NNSR regulations at 40 CFR 51.165(a)(1)(xxxvii).
\244\ VOC and NOX are defined as precursors of ozone
in the PSD regulations at 40 CFR 51.166(b)(49)(i)(b)(1) and
52.21(b)(50)(i)(b)(1), and in the NNSR regulations at 40 CFR
51.165(a)(1)(xxxvii)(B) and (C)(1) and part 51, Appendix S,
II.A.31(ii)(b)(1).
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2. Prevention of Significant Deterioration (PSD) Program
The statutory requirements for a PSD permit program set forth under
part C of title I of the CAA (sections 160 through 169) are addressed
by the EPA's PSD regulations found at 40 CFR 51.166 (minimum
requirements for an approvable PSD SIP) and 40 CFR 52.21 (PSD
permitting program for permits issued under the EPA's federal
permitting authority). Both sets of regulations already apply for
O3 when the area is designated attainment or unclassifiable
for O3 and when the new source or modification triggers PSD
requirements for O3.
For PSD, a ``major stationary source'' is one that emits or has the
potential to emit 250 tons per year (tpy) or more of any regulated NSR
pollutant, unless the new or modified source is classified under a list
of 28 source categories contained in the statutory definition of
``major emitting facility'' in section 169(1) of the CAA. For those 28
source categories, a ``major stationary source'' is one that emits or
has the potential to emit 100 tpy or more of any regulated NSR
pollutant. A ``major modification'' is a physical change or a change in
the method of operation of an existing major stationary source that
results first, in a significant emissions increase of a regulated NSR
pollutant for the project, and second, in a significant net emissions
increase of that pollutant at the source. See 40 CFR 51.166(b)(2)(i),
40 CFR 52.21(b)(2)(i).
Among other things, for each regulated NSR pollutant emitted or
increased in significant amounts, the PSD program requires a new major
stationary source or a major modification to apply Best Available
Control Technology and to conduct an air quality impact analysis to
demonstrate that the proposed source or project will not cause or
contribute to a violation of any NAAQS or PSD increment (see CAA
section 165(a)(3)-(4), 40 CFR 51.166(j)-(k), 40 CFR 52.21(j)-(k)). The
PSD requirements may also include, in appropriate cases, an analysis of
potential adverse impacts on Class I areas (see CAA sections 162 and
165).\245\ The EPA has generally interpreted the requirement for an air
quality impact analysis under CAA section 165(a)(3) and the
implementing regulations to include a requirement to demonstrate that
emissions from the proposed facility will not cause or contribute to a
violation of any NAAQS that is in effect as of the date a PSD permit is
issued.\246\ See, e.g., 73 FR 28321, 28324, 28340 (May 16, 2008); 78 FR
3253 (Jan. 15, 2013); Memorandum from Stephen D. Page, Director, Office
of Air Quality Planning & Standards, ``Applicability of the Federal
Prevention of Significant Deterioration Permit Requirements to New and
Revised National Ambient Air Quality Standards'' (April 1, 2010).
Consistent with this interpretation, the demonstration required under
CAA section 165(a)(3) and 40 CFR 51.166(k) and 52.21(k) will apply to
any revised O3 NAAQS when such NAAQS become effective,
except to the extent that a pending permit application is subject to a
grandfathering provision that the EPA establishes through rulemaking.
In addition, the other existing requirements of the PSD program will
remain applicable to O3 after the revised O3
NAAQS takes effect.
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\245\ Congress established certain Class I areas in section
162(a) of the CAA, including international parks, national
wilderness areas, and national parks that meet certain criteria.
Such Class I areas, known as mandatory federal Class I areas, are
afforded special protection under the CAA. In addition, states and
tribal governments may establish Class I areas within their own
political jurisdictions to provide similar special air quality
protection.
\246\ An exception occurs in cases where the EPA has included a
grandfathering provision in its PSD regulations for a particular
pollutant. The EPA historically has exercised its discretion to
transition the implementation of certain new requirements through
grandfathering, under appropriate circumstances, either by
rulemaking or through a case-by-case determination for a specific
permit application. In 2014, the United States Court of Appeals for
the Ninth Circuit vacated a decision by the EPA to issue an
individual PSD permit grandfathering a permit applicant from certain
requirements. See Sierra Club v. EPA, 762 F.3d 971 (9th Cir. 2014).
In light of that decision, the EPA is no longer asserting authority
to grandfather permit applications on a case-by-case basis. This
decision is addressed in more detail in the discussion of the
grandfathering provisions that the EPA is issuing through this
rulemaking in section VII of this preamble.
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Because the complex chemistry of O3 formation in the
atmosphere poses significant challenges for the assessing the impacts
of individual stationary sources on O3 formation, the EPA's
judgment historically has been that it is not technically sound to
designate a
[[Page 65441]]
specific air quality model that must be used in the PSD permitting
process to make this demonstration for O3. To address
ambient impacts of emissions from proposed individual stationary
sources on O3, the EPA proposed amendments to Appendix W to
40 CFR part 51 in July 2015 that would, among other things, revise the
Appendix W provisions relating to the analytical techniques for
demonstrating that an individual PSD source or modification does not
cause or contribute to a violation of the O3 NAAQS (80 FR
45340, July 29, 2015). Until any revisions are finalized and in effect,
PSD permit applicants should continue to follow the current provisions
in the applicable regulations and Appendix W in order to demonstrate
that a proposed source or modification does not cause or contribute to
a violation of the O3 NAAQS.
a. What transition plan is the EPA providing for implementing the PSD
requirements for the revised O3 NAAQS?
In this rulemaking, the EPA is amending the PSD regulations at 40
CFR 51.166 and 40 CFR 52.21 to include a grandfathering provision that
will allow reviewing authorities to continue to review certain pending
PSD permit applications in accordance with the O3 NAAQS that
was in effect when a specific permitting milestone was reached, rather
than the revised O3 NAAQS. The EPA is finalizing the
grandfathering provision as proposed with two trigger dates--the
signature date of the revised O3 NAAQS rule for complete
applications and the effective date of the revised O3 NAAQS
for a draft permit or preliminary determination. A more detailed
discussion of the final provision, comments received and our responses
to those comments is provided in section VII of this preamble, which
addresses this change to the PSD regulations, as well as the Response
to Comment Document contained in the docket for this rulemaking.
b. What screening and compliance demonstration tools are used to
implement the PSD program?
The EPA has historically allowed the use of screening and
compliance demonstration tools to help facilitate the implementation of
the NSR program by reducing the source's burden and streamlining the
permitting process for circumstances where the emissions or ambient
impacts of a particular pollutant could be considered de minimis. For
example, the EPA has established significant emission rates, or SERs,
that are used as screening tools to determine when a pollutant would be
considered to be emitted in a significant amount and, accordingly, when
the NSR requirements should be applied to that pollutant. See 40 CFR
51.166(b)(23) and 52.21(b)(23). For O3, the EPA established
a SER of 40 tpy for emissions of each O3 precursor--VOC and
NOX. For PSD, the O3 SER applies independently to
emissions of VOC and NOX (emissions of precursors are not
added together) to determine when the proposed major stationary source
or major modification must undergo PSD review for that precursor and
whether individual PSD requirements, such as BACT, apply to that
precursor.\247\
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\247\ See In re Footprint Power Salem Harbor Development, LP, 16
E.A.D ___, PSD Appeal No. 14-02, at 20-25 (EAB, Sept. 2, 2014)
(including description of EPA's position on application of BACT to
ozone precursors) available at http://yosemite.epa.gov/oa/EAB_Web_Docket.nsf/PSD+Permit+Appeals+(CAA)?OpenView.
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In the context of the PSD air quality impact analysis, the EPA has
also used a value called a significant impact level (SIL) as a
compliance demonstration tool. The SIL, expressed as an ambient
concentration of a pollutant, may be used first to determine the
geographical scope of the ambient impact analysis that must be
completed for the applicable pollutant to satisfy the air quality
demonstration requirement under CAA section 165(a)(3). A second use is
to guide the determination of whether the impact of the source is
considered to cause or contribute to a violation of any NAAQS. The EPA
has not established a SIL for O3. The EPA is currently
considering development of a SIL for O3 through either
guidance or a rulemaking process. Such a SIL would complement proposed
revisions to Appendix W mentioned above (80 FR 45340, July 29, 2015)
and would assist in the implementation of the PSD air quality analysis
requirement for protection of the O3 NAAQS. However, the EPA
is not making revisions in this rulemaking to address the PSD air
quality analysis for O3. Until any rulemaking to amend
existing PSD regulations for O3 is completed, permitting
decisions should continue to be based on the existing provisions in the
applicable regulations.
Several commenters addressed statements that the EPA made
concerning screening tools for O3 in the preamble to the
O3 NAAQS proposal. These statements were not linked to any
proposed amendments to EPA regulations. Aside from adopting the
grandfathering provision addressed in section VII of this preamble, the
EPA is not revising the PSD requirements for O3 in this
final rule. Therefore, the EPA is not responding to those comments at
this time, consistent with the EPA's general approach to comments on
implementation topics described above.
c. Other PSD Transition Issues
The EPA anticipates that the existing O3 air quality in
some areas currently designated attainment of unclassifiable for
O3 will not meet the revised O3 NAAQS upon its
effective date and that some of these areas will ultimately be
designated ``nonattainment'' for the revised O3 NAAQS
through the formal area designation process set forth under the CAA
(see section VIII.B above). However, until the EPA issues such
nonattainment designations, proposed new major sources and major
modifications situated in any area designated attainment or
unclassifiable for the 2008 O3 NAAQS will continue to be
required to address O3 in a PSD permit.\248\ As mentioned
above, the PSD permitting program requires that proposed new major
stationary sources and major modifications must demonstrate that the
emissions from the proposed source or modification will not cause or
contribute to a violation of any NAAQS. In the notice of proposed
rulemaking, the EPA provided information concerning its views on the
possibility that some PSD permit applications could satisfy the air
quality analysis requirements for O3 by obtaining air
quality offsets (called PSD offsets).\249\ Several commenters expressed
concern that without some transition provisions in the final rule
exempting PSD permit applications for sources located in such areas
from meeting the air quality analysis requirements for the revised
O3 NAAQS, such applications might not be able to satisfy the
demonstration requirement, as the current ambient air monitoring data
indicate the revised lower standards are not being met. The
O3 NAAQS proposal included no proposed revisions to PSD
regulations on this
[[Page 65442]]
topic and the EPA is not making any revisions to the PSD requirements
for O3 in this action to address this issue. Therefore, the
EPA is not responding to those comments at this time, consistent with
its general approach to comments on implementation topics described
above. However, to help address this concern raised by commenters, the
EPA is considering issuing additional guidance on how PSD offsets can
be implemented.
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\248\ Any proposed major stationary source or major modification
subject to PSD for O3 that does not receive its PSD
permit by the effective date of a new O3 nonattainment
designation for the area where the source would locate would then be
required to satisfy all of the applicable NNSR preconstruction
permit requirements for O3, even if such source had been
grandfathered under the PSD regulations from the demonstration
requirement under CAA section 165(a)(3) for O3.
\249\ The EPA has historically recognized in regulations and
through other actions that sources applying for PSD permits may have
the option of utilizing offsets as part of the required PSD
demonstration under CAA section 165(a)(3)(B). See, e.g., In re
Interpower of New York, Inc., 5 E.A.D. 130, 141 (EAB 1994)
(describing an EPA Region 2 PSD permit that relied in part on
offsets to demonstrate the source would not cause or contribute to a
violation of the NAAQS). 52 FR 24698 (July 1, 1987); 78 FR 3261-62
(Jan. 15, 2013).
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3. Nonattainment NSR
Part D of title I of the CAA includes preconstruction review and
permitting requirements for new major stationary sources and major
modifications when they locate in areas designated nonattainment for a
particular pollutant. The relevant part D requirements are typically
referred to as the nonattainment NSR (NNSR) program. The EPA
regulations for the NNSR program are contained at 40 CFR 51.165, 52.24
and part 51 Appendix S. The EPA's minimum requirements for a NNSR
program to be approvable into a SIP are contained in 40 CFR 51.165.
Appendix S to 40 CFR part 51 contains an interim NNSR program. This
interim program enables implementation of NNSR permitting in
nonattainment areas that lack a SIP-approved NNSR permitting program
for the particular nonattainment pollutant, and the interim program can
be applied during the time between the date of the relevant
nonattainment designation and the date on which the EPA approves into
the SIP a NNSR program or additional components of an NNSR program for
a particular pollutant.\250\ This interim program is commonly known as
the Emissions Offset Interpretative Rule, and is applicable to all
criteria pollutants, including O3.\251\
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\250\ See Appendix S, Part I; 40 CFR 52.24(k).
\251\ As appropriate, certain NNSR requirements under 40 CFR
51.165 or Appendix S can also apply to sources and modifications
located in areas that are designated attainment or unclassifiable in
the Ozone Transport Region. See, e.g., CAA 184(b)(2), 40 CFR
52.24(k).
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The EPA is not modifying any existing NNSR requirements in this
rulemaking. Under the CAA, area designations for new or revised NAAQS
are addressed subsequent to the effective date of the new or revised
NAAQS. If the EPA determines that any revisions to the existing NNSR
requirements, including those in Appendix S, are appropriate, the EPA
expects, at a later date contemporaneous with the designation process
for the revised O3 NAAQS, to propose those revisions. If any
changes are proposed to Appendix S requirements, the EPA anticipates
that it would intend for those changes to become effective no later
than the effective date of the area designations. This timing would
allow air agencies that lack an approved NNSR program for O3
to use the relevant Appendix S provisions to issue NNSR permits
addressing O3 on and after the effective date of
designations of new nonattainment areas for O3 until such
time as a NNSR program for O3 is approved into the SIP.\252\
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\252\ States with SIP-approved NNSR programs for O3
should evaluate that program to determine whether they can continue
to issue permits under their approved program or whether revisions
to their program are necessary to address the revised O3
NAAQS.
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For NNSR, new major stationary sources and major modifications for
O3 must comply with the Lowest Achievable Emission Rate
(LAER) requirements as defined in the CAA and NNSR rules, and must
perform other analyses and satisfy other requirements under section 173
of the CAA. For example, under CAA section 173(c) emissions reductions,
known as emissions offsets, must be secured to offset the increased
emissions of the air pollutant (including the relevant precursors) from
the new or modified source by an equal or greater reduction, as
applicable, of such pollutant. The appropriate emissions offset needed
for a particular source will depend upon the classification for the
O3 nonattainment area in which the source or modification
will locate, such that areas with more severe nonattainment
classifications have more stringent offset requirements. This ranges
from 1.1:1 for areas classified as Marginal to 1.5:1 for areas
classified as Extreme. See, e.g., CAA section 182, 40 CFR 51.165(a)(9)
and 40 CFR part 51 Appendix S section IV.G.2.
To facilitate continued economic development in nonattainment
areas, many states have established offset banks or registries.\253\
Such banks or registries can help new or modified major stationary
source owners meet offset requirements by streamlining identification
and access to available emissions reductions. Some states have
established offset banks to help ensure a consistent method for
generating, validating and transferring NOX and VOC offsets.
Offsets in these areas are generated by emissions reductions that meet
specific creditability criteria set forth by the SIP consistent with
the EPA regulations. See 40 CFR 51.165(a)(3)(ii)(A)-(J) and part 51
Appendix S section IV.C. The EPA received comments expressing concern
about the limited availability of offsets in nonattainment areas. Since
the EPA did not propose, and is not finalizing, any amendments related
to the NNSR offset provisions, the EPA is not responding to those
comments at this time, consistent with the EPA's general approach to
comment on implementation topics as described above.
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\253\ See, for example, emission reduction credit banking
programs in Ohio (OAC Chapter 3745-1111) and California (H&SC
Section 40709).
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D. Transportation and General Conformity
1. What are transportation and general conformity?
Conformity is required under CAA section 176(c) to ensure that
federal actions are consistent with (``conform to'') the purpose of the
SIP. Conformity to the purpose of the SIP means that federal activities
will not cause new air quality violations, worsen existing violations,
or delay timely attainment of the relevant NAAQS or interim reductions
and milestones. Conformity applies to areas that are designated
nonattainment, and those nonattainment areas redesignated to attainment
with a CAA section 175A maintenance plan after 1990 (``maintenance
areas'').
The EPA's Transportation Conformity Rule (40 CFR 51.390 and part
93, subpart A) establishes the criteria and procedures for determining
whether transportation activities conform to the SIP. These activities
include adopting, funding or approving transportation plans,
transportation improvement programs (TIPs) and federally supported
highway and transit projects. For further information on conformity
rulemakings, policy guidance and outreach materials, see the EPA's Web
site at http://www.epa.gov/otaq/stateresources/transconf/index.htm. The
EPA may issue future transportation conformity guidance as needed to
implement a revised O3 NAAQS.
With regard to general conformity, the EPA first promulgated
general conformity regulations in November 1993. (40 CFR part 51,
subpart W, 40 CFR part 93, subpart B) Subsequently the EPA finalized
revisions to the general conformity regulations on April 5, 2010. (75
FR 17254-17279). Besides ensuring that federal actions not covered by
the transportation conformity rule will not interfere with the SIP, the
general conformity program also fosters communications between federal
agencies and state/local air quality agencies, provides for public
notification of and access to federal agency conformity determinations,
and allows for air quality review of
[[Page 65443]]
individual federal actions. More information on the general conformity
program is available at http://www.epa.gov/air/genconform/.
2. When would transportation and general conformity apply to areas
designated nonattainment for the revised O3 NAAQS?
Transportation and general conformity apply one year after the
effective date of nonattainment designations for the revised
O3 NAAQS. This is because CAA section 176(c)(6) provides a
1-year grace period from the effective date of initial designations for
any revised NAAQS before transportation and general conformity apply in
areas newly designated nonattainment for a specific pollutant and
NAAQS.
3. Impact of a Revised O3 NAAQS on a State's Existing
Transportation and/or General Conformity SIP
In this final rule, the EPA is revising the O3 NAAQS,
but is not making specific changes to its transportation or general
conformity regulations. Therefore, states should not need to revise
their transportation and/or general conformity SIPs. While we are not
making any revisions to the general conformity regulations at this
time, we recommend, when areas develop SIPs for a revised O3
NAAQS, that state and local air quality agencies work with federal
agencies with large emitting activities that are subject to the general
conformity regulations to establish an emissions budget for those
facilities and activities in order to facilitate future conformity
determinations under the conformity regulations. Finally, states with
existing conformity SIPs and new nonattainment areas may also need to
revise their conformity SIPs in order to ensure the state regulations
apply in any newly designated areas.
Because significant tracts of land under federal management may be
included in nonattainment area boundaries, the EPA encourages state and
local air quality agencies to work with federal agencies to assess and
develop emissions budgets that consider emissions from projects subject
to general conformity, including emissions from fire on wildland, in
any baseline, modeling and SIP attainment inventory. Where appropriate,
states, land managers, and landowners may also consider developing
plans to ensure that fuel accumulations are addressed Information is
available from DOI and USDA Forest Service on the ecological role of
fire and on smoke management programs and basic smoke management
practices.\254\
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\254\ USDA Forest Service and Natural Resources Conservation
Service, Basic Smoke Management Practices Tech Note, October 2011,
http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1046311.pdf.
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If this is the first time that transportation conformity will apply
in a state, such a state is required by the statute and EPA regulations
to submit a SIP revision that addresses three specific transportation
conformity requirements that address consultation procedures and
written commitments to control or mitigation measures associated with
conformity determinations for transportation plans, TIPs or projects.
(40 CFR 51.390) Additional information and guidance can be found in the
EPA's ``Guidance for Developing Transportation Conformity State
Implementation Plans'' (http://www.epa.gov/otaq/stateresources/transconf/policy/420b09001.pdf).
E. Regional and International Pollution Transport
1. Interstate Transport
The CAA contains provisions that specifically address and require
regulation of the interstate transport of air pollution that does not
otherwise qualify for data exclusion under the Act's exceptional events
provisions. As previously noted, emissions from events, such as
wildfires, may qualify as exceptional events and may be transported
across jurisdictional boundaries. The EPA intends to address the
transport of event-related emissions in our upcoming proposed revisions
to the Exceptional Events Rule and draft guidance document addressing
the Exceptional Events Rule criteria for wildfires that could affect
O3 concentrations. The EPA encourages affected air agencies
to coordinate with their EPA regional office to identify approaches to
evaluate the potential impacts of transported event-related emissions
and determine the most appropriate information and analytical methods
for each area's unique situation.
CAA section 110(a)(2)(D)(i)(I), Interstate Transport--CAA section
110(a)(2)(D)(i)(I) requires states to develop and implement a SIP to
address the interstate transport of emissions. Specifically, this
provision requires the SIP to prohibit ``any source or other type of
emissions activity within the state'' that would ``significantly
contribute to nonattainment'' of any NAAQS in another state, or that
would ``interfere with maintenance'' of any NAAQS in another state.
When EPA promulgates or revises a NAAQS, each state is required to
submit a SIP addressing this interstate transport provision within 3
years.
CAA section 126, Interstate Transport--CAA section 126(b) provides
states and political subdivisions with a mechanism to petition the
Administrator for a finding that ``any major source or group of
stationary sources emits or would emit any air pollution in violation
of the prohibition of [CAA section 110(a)(2)(D)(i)(I)].'' \255\ Where
the EPA makes such finding, the source is allowed to operate beyond a
3-month period after such finding only if the EPA establishes emissions
limitations and a compliance schedule designated to bring the source
into compliance as expeditiously as practicable, but no later than
three years after such finding. This mechanism is available to downwind
states and political subdivisions, regardless of designation status,
that would be affected by emissions from upwind states.
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\255\ The text of section 126 codified in the United States Code
cross references section 110(a)(2)(D)(ii) instead of section
110(a)(2)(D)(i). The courts have confirmed that this is a
scrivener's error and the correct cross reference is to section
110(a)(2)(D)(i), See Appalachian Power Co. v. EPA, 249 F.3d 1032,
1040-44 (D.C. Cir. 2001).
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2. International Transport
The agency is active in work to reduce the international transport
of O3 and other pollutants that can contribute to
``background'' O3 levels in the U.S. Under the Convention on
Long-Range Transboundary Air Pollution (LRTAP) of the United Nations
Economic Commission for Europe, the U.S. has been a party to the
Protocol to Abate Acidification, Eutrophication, and Ground-level Ozone
(known as the Gothenburg Protocol) since 2005. The U.S. is also active
in the LRTAP Task Force for Hemispheric Transport of Air Pollution. The
U.S. has worked bilaterally with Canada under the US-Canada Air Quality
Agreement to adopt an Ozone Annex to address transboundary
O3 impacts and continues to work with China on air quality
management activities. This work includes supporting China's efforts to
rapidly deploy power plant pollution controls that can achieve
NOX reductions of at least 80 to 90%. The U.S. also
continues to work bilaterally with Mexico on the Border 2020 program to
support efforts to improve environmental conditions in the border
region. One of the main goals of the program is to reduce air
pollution, including emissions that can cause transboundary
O3 impacts.
[[Page 65444]]
Clean Air Act section 179B recognizes the possibility that certain
nonattainment areas may be impacted by O3 or O3
precursor emissions from international sources beyond the regulatory
jurisdiction of the state. The EPA's science review suggests that the
influence of international sources on U.S. O3 levels will be
largest in locations that are in the immediate vicinity of an
international border with Canada or Mexico. The science review also
cites two recent studies which indicate that intercontinental transport
of pollution, along with other natural sources and local pollutant
sources, can affect O3 air quality in the western U.S. under
specific conditions. (U.S. EPA 2013, p. 3-140). Section 179B allows
states to consider in their attainment plans and demonstrations whether
an area might meet the O3 NAAQS by the attainment date ``but
for'' emissions contributing to the area originating outside the U.S.
If a state is unable to demonstrate attainment of the NAAQS in such an
area impacted by international transport after adopting all reasonably
available control measures (e.g., RACM, including RACT, as required by
CAA section 182(b)), the EPA can nonetheless approve the CAA-required
state attainment plan and demonstration using the authority in section
179B.
When the EPA approves this type of attainment plan and
demonstration, and there would be no adverse consequence for a finding
that the area failed to attain the NAAQS by the relevant attainment
date. States can also avoid potential sanctions and FIPs that would
otherwise apply for failure to submit a required SIP submission or
failure to submit an approvable SIP submission. For example, section
179B explicitly provides that the area shall not be reclassified to the
next highest classification or required to implement a section 185
penalty fee program if a state meets the applicable criteria.
Section 179B authority does not allow an area to avoid a
nonattainment designation or for the area to be classified with a lower
classification than is indicated by actual ambient air quality. Section
179B also does not provide for any relaxation of mandatory emissions
control measures (including contingency measures) or the prescribed
emissions reductions necessary to achieve periodic emissions reduction
progress requirements. In this way, section 179B insures that states
will take actions to mitigate the public health impacts of exposure to
ambient levels of pollution that violate the NAAQS by imposing
reasonable control measures on the sources that are within the
jurisdiction of the state while also authorizing EPA to approve such
attainment plans and demonstrations even though they do not fully
address the public health impacts of international transport. Also,
generally, monitoring data influenced by international transport may
not be excluded from regulatory determinations. However, depending on
the nature and scope of international emissions events affecting air
quality in the U.S., the event-influenced data may qualify for
exclusion under the Exceptional Events Rule. The EPA encourages
affected air agencies to coordinate with their EPA regional office to
identify approaches to evaluate the potential impacts of international
transport and to determine the most appropriate information and
analytical methods for each area's unique situation. The EPA will also
work with states that are developing attainment plans for which section
179B is relevant, and ensure the states have the benefit of the EPA's
understanding of international transport of ozone and ozone precursors.
The EPA has used section 179B authority previously to approve
attainment plans for Mexican border areas in El Paso, TX
(O3, PM10, and CO plans); and Nogales, AZ
(PM10 plan). The 24-hour PM10 attainment plan for
Nogales, AZ, was approved by EPA as sufficient to demonstrate
attainment of the NAAQS by the Moderate classification deadline, but
for international emissions sources in the Nogales Municipality, Mexico
area (77 FR 38400, June 27, 2012).
States are encouraged to consult with their EPA Regional Office to
establish appropriate technical requirements for these analyses.
IX. Statutory and Executive Order Reviews
Additional information about these statutes and Executive Orders
can be found at http://www2.epa.gov/laws-regulations/laws-and-executive-orders.
A. Executive Order 12866: Regulatory Planning and Review and Executive
Order 13563: Improving Regulation and Regulatory Review
This action is an economically significant regulatory action that
was submitted to the Office of Management and Budget (OMB) for review.
Any changes made in response to OMB recommendations have been
documented in the docket. The EPA prepared an analysis of the potential
costs and benefits associated with this action. This analysis is
contained in the document, Regulatory Impact Analysis of the Final
National Ambient Air Quality Standards for Ground-Level Ozone, October
2015. A copy of the analysis is available in the RIA docket (EPA-HQ-
OAR-2013-0169) and the analysis is briefly summarized here. The RIA
estimates the costs and monetized human health and welfare benefits of
attaining three alternative O3 NAAQS nationwide.
Specifically, the RIA examines the alternatives of 65 ppb and 70 ppb.
The RIA contains illustrative analyses that consider a limited number
of emissions control scenarios that states and Regional Planning
Organizations might implement to achieve these alternative
O3 NAAQS. However, the CAA and judicial decisions make clear
that the economic and technical feasibility of attaining ambient
standards are not to be considered in setting or revising NAAQS,
although such factors may be considered in the development of state
plans to implement the standards. Accordingly, although an RIA has been
prepared, the results of the RIA have not been considered in issuing
this final rule.
B. Paperwork Reduction Act
The information collection requirements in this final rule have
been submitted for approval to the Office of Management and Budget
(OMB) under the Paperwork Reduction Act (PRA). The information
collection requirements are not enforceable until OMB approves them.
The Information Collection Request (ICR) document prepared by the EPA
for these revisions has been assigned EPA ICR #2313.04.
The information collected and reported under 40 CFR part 58 is
needed to determine compliance with the NAAQS, to characterize air
quality and associated health and ecosystems impacts, to develop
emission control strategies, and to measure progress for the air
pollution program. We are extending the length of the required
O3 monitoring season in 32 states and the District of
Columbia and the revised O3 monitoring seasons will become
effective on January 1, 2017. We are also revising the PAMS monitoring
requirements to reduce the number of required PAMS sites while
improving spatial coverage, and requiring states in moderate or above
O3 non-attainment areas and the O3 transport
region to develop an enhanced monitoring plan as part of the PAMS
requirements. Monitoring agencies will need to comply with the PAMS
requirements by June 1, 2019. In addition, we are revising the
O3 FRM to establish a new, additional technique for
measuring O3 in the ambient air. It will be
[[Page 65445]]
incorporated into the existing O3 FRM, using the same
calibration procedure in Appendix D of 40 CFR part 50. We are also
making changes to the procedures for testing performance
characteristics and determining comparability between candidate FEMs
and reference methods.
For the purposes of ICR number 2313.04, the burden figures
represent the burden estimate based on the requirements contained in
this rule. The burden estimates are for the 3-year period from 2016
through 2018. The implementation of the PAMS changes will occur beyond
the time frame of this ICR with implementation occurring in 2019. The
cost estimates for the PAMS network (including revisions) will be
captured in future routine updates to the Ambient Air Quality
Surveillance ICR that are required every 3 years by OMB. The addition
of a new FRM in 40 CFR part 50 and revisions to the O3 FEM
procedures for testing performance characteristics in 40 CFR part 53
does not add any additional information collection requirements.
The ICR burden estimates are associated with the changes to the
O3 seasons in this final rule. This information collection
is estimated to involve 158 respondents for a total cost of
approximately $24,597,485 (total capital, labor, and operation and
maintenance) plus a total burden of 339,930 hours for the support of
all operational aspects of the entire O3 monitoring network.
The labor costs associated with these hours are $20,209,966. Also
included in the total are other costs of operations and maintenance of
$2,254,334 and equipment and contract costs of $2,133,185. The actual
labor cost increase to expand the O3 monitoring seasons is
$2,064,707. In addition to the costs at the state, local, and tribal
air quality management agencies, there is a burden to EPA of 41,418
hours and $2,670,360. Burden is defined at 5 CFR 1320.3(b). State,
local, and tribal entities are eligible for state assistance grants
provided by the federal government under the CAA which can be used for
related activities. An agency may not conduct or sponsor, and a person
is not required to respond to, a collection of information unless it
displays a currently valid OMB control number. The OMB control numbers
for EPA's regulations in 40 CFR are listed in 40 CFR part 9.
C. Regulatory Flexibility Act (RFA)
I certify that this action will not have a significant economic
impact on a substantial number of small entities under the RFA. This
action will not impose any requirements on small entities. Rather, this
rule establishes national standards for allowable concentrations of
O3 in ambient air as required by section 109 of the CAA. See
also American Trucking Associations v. EPA, 175 F. 3d at 1044-45 (NAAQS
do not have significant impacts upon small entities because NAAQS
themselves impose no regulations upon small entities). Similarly, the
revisions to 40 CFR part 58 address the requirements for states to
collect information and report compliance with the NAAQS and will not
impose any requirements on small entities. Similarly, the addition of a
new FRM in 40 CFR part 50 and revisions to the FEM procedures for
testing in 40 CFR part 53 will not impose any requirements on small
entities.
D. Unfunded Mandates Reform Act (UMRA)
This action does not contain an unfunded federal mandate of $100
million or more as described in UMRA, 2 U.S.C. 1531-1538, and does not
significantly or uniquely affect small governments. The revisions to
the O3 NAAQS impose no enforceable duty on any state, local,
or tribal governments or the private sector beyond those duties already
established in the CAA. The expected costs associated with the
monitoring requirements are described in the EPA's ICR document, and
these costs are not expected to exceed $100 million in the aggregate
for any year.
Furthermore, as indicated previously, in setting NAAQS the EPA
cannot consider the economic or technological feasibility of attaining
ambient air quality standards, although such factors may be considered
to a degree in the development of state plans to implement the
standards (see American Trucking Associations v. EPA, 175 F. 3d at 1043
[noting that because the EPA is precluded from considering costs of
implementation in establishing NAAQS, preparation of a RIA pursuant to
the UMRA would not furnish any information which the court could
consider in reviewing the NAAQS]). With regard to the sections of the
rule preamble discussing implementation of the revisions to the
O3 NAAQS, the CAA imposes the obligation for states to
submit SIPs to implement the NAAQS for O3. To the extent the
EPA's discussion of implementation topics in this final rule may
reflect some interpretations of those requirements, those
interpretations do not impose obligations beyond the duties already
established in the CAA and thus do not constitute a federal mandate for
purposes of UMRA. The EPA is also adopting a grandfathering provision
for certain PSD permits in this action, as described above. However,
that provision does not impose any mandate on any state, local, or
tribal government or the private sector, but rather provides relief
from requirements that would otherwise result from the new standards.
In addition, the EPA is not requiring states to revise their SIPs to
include such a provision.
E. Executive Order 13132: Federalism
This action does not have federalism implications. It will not have
substantial direct effects on the states, on the relationship between
the national government and the states, or on the distribution of power
and responsibilities among the various levels of government.
F. Executive Order 13175: Consultation and Coordination With Indian
Tribal Governments
This action does not have tribal implications as specified in
Executive Order 13175. It does not have a substantial direct effect on
one or more Indian tribes. This rule provides increased protection from
adverse effects of ozone for the entire country, including for
sensitive populations, and tribes are not obligated to adopt or
implement any NAAQS. In addition, tribes are not obligated to conduct
ambient monitoring for O3 or to adopt the ambient monitoring
requirements of 40 CFR part 58. Even if this action were determined to
have tribal implications within the meaning of Executive Order 13175,
it will neither impose substantial direct compliance costs on tribal
governments, nor preempt tribal law. Thus, consultation under Executive
Order 13175 was not required.
Nonetheless, consistent with the ``EPA Policy on Consultation and
Coordination with Indian Tribes'', the EPA offered government-to-
government consultation on the proposed rule. No tribe requested
government-to-government consultation with the EPA on this rule. In
addition, the EPA conducted outreach to tribal environmental
professionals, which included participation in the Tribal Air call
sponsored by the National Tribal Air Association, and two other calls
available to tribal environmental professionals. During the public
comment period we received comments on the proposed rule from seven
tribes and three tribal organizations.
G. Executive Order 13045: Protection of Children From Environmental
Health & Safety Risks
This action is subject to Executive Order 13045 because it is an
[[Page 65446]]
economically significant regulatory action as defined by Executive
Order 12866, and the EPA believes that the environmental health risk
addressed by this action may have a disproportionate effect on
children. The rule will establish uniform NAAQS for O3;
these standards are designed to protect public health with an adequate
margin of safety, as required by CAA section 109. However, the
protection offered by these standards may be especially important for
children because children, especially children with asthma, along with
other at-risk populations \256\ such as all people with lung disease
and people active outdoors, are at increased risk for health effects
associated with exposure to O3 in ambient air. Because
children are considered an at-risk lifestage, we have carefully
evaluated the environmental health effects of exposure to O3
pollution among children. Discussions of the results of the evaluation
of the scientific evidence, policy considerations, and the exposure and
risk assessments pertaining to children are contained in sections II.B
and II.C of this preamble.
---------------------------------------------------------------------------
\256\ As used here and similarly throughout this document, the
term population refers to people having a quality or characteristic
in common, including a specific pre-existing illness or a specific
age or lifestage.
---------------------------------------------------------------------------
H. Executive Order 13211: Actions That Significantly Affect Energy
Supply, Distribution, or Use
This action is not a ``significant energy action'' because it is
not likely to have a significant adverse effect on the supply,
distribution, or use of energy. The purpose of this rule is to
establish revised NAAQS for O3, establish an additional FRM,
revise FEM procedures for testing, and revises air quality surveillance
requirements. The rule does not prescribe specific pollution control
strategies by which these ambient standards and monitoring revisions
will be met. Such strategies will be developed by states on a case-by-
case basis, and the EPA cannot predict whether the control options
selected by states will include regulations on energy suppliers,
distributors, or users. Thus, the EPA concludes that this rule is not
likely to have any adverse energy effects and does not constitute a
significant energy action as defined in Executive Order 13211.
I. National Technology Transfer and Advancement Act
This rulemaking involves environmental monitoring and measurement.
Consistent with the Agency's Performance Based Measurement System
(PBMS), the EPA is not requiring the use of specific, prescribed
analytical methods. Rather, the Agency is allowing the use of any
method that meets the prescribed performance criteria. Ambient air
concentrations of O3 are currently measured by the FRM in 40
CFR part 50, Appendix D (Measurement Principle and Calibration
Procedure for the Measurement of Ozone in the Atmosphere) or by FEM
that meet the requirements of 40 CFR part 53. Procedures are available
in part 53 that allow for the approval of an FEM for O3 that
is similar to the FRM. Any method that meets the performance criteria
for a candidate equivalent method may be approved for use as an FEM.
This approach is consistent with EPA's PBMS. The PBMS approach is
intended to be more flexible and cost-effective for the regulated
community; it is also intended to encourage innovation in analytical
technology and improved data quality. The EPA is not precluding the use
of any method, whether it constitutes a voluntary consensus standard or
not, as long as it meets the specified performance criteria.
J. Executive Order 12898: Federal Actions To Address Environmental
Justice in Minority Populations and Low-Income Populations
The EPA believes that this action will not have disproportionately
high and adverse human health or environmental effects on minority
populations, low-income populations or indigenous peoples. The action
described in this notice is to strengthen the NAAQS for O3.
The primary NAAQS are established at a level that is requisite to
protect public health, including the health of sensitive or at-risk
groups, with an adequate margin of safety. The NAAQS decisions are
based on an explicit and comprehensive assessment of the current
scientific evidence and associated exposure/risk analyses. More
specifically, EPA expressly considers the available information
regarding health effects among at-risk populations, including that
available for low-income populations and minority populations, in
decisions on NAAQS. Where low-income populations or minority
populations are among the at-risk populations, the decision on the
standard is based on providing protection for these and other at-risk
populations and lifestages. Where such populations are not identified
as at-risk populations, a NAAQS that is established to provide
protection to the at-risk populations would also be expected to provide
protection to all other populations, including low-income populations
and minority populations.
The ISA, HREA, and PA for this review, which include identification
of populations at risk from O3 health effects, are available
in the docket, EPA-HQ-OAR-2008-0699. The information on at-risk
populations for this NAAQS review is summarized and considered earlier
in this preamble (see section II.A). This final rule increases the
level of environmental protection for all affected populations without
having any disproportionately high and adverse human health or
environmental effects on any population, including any minority
populations, low-income populations or indigenous peoples. This rule
establishes uniform national standards for O3 in ambient air
that, in the Administrator's judgment, protect public health, including
the health of sensitive groups, with an adequate margin of safety.
Although it is part of a separate docket (EPA-HQ-OAR-2013-0169) and
is not part of the rulemaking record for this action, EPA has prepared
a RIA of this decision. As part of the RIA, a demographic analysis was
conducted. While, as noted in the RIA, the demographic analysis is not
a full quantitative, site-specific exposure and risk assessment, that
analysis examined demographic characteristics of persons living in
areas with poor air quality relative to the proposed standard.
Specifically, Chapter 9, section 9.10 (page 9-7) and Appendix 9A of the
RIA describe this proximity and socio-demographic analysis. This
analysis found that in areas with poor air quality relative to the
revised standard,\257\ the representation of minority populations was
slightly greater than in the U.S. as a whole. Because the air quality
in these areas does not currently meet the revised standard,
populations in these areas would be expected to benefit from
implementation of the strengthened standard, and, thus, would be more
affected by strategies to attain the revised standard. This analysis,
which evaluates the potential implications for minority populations and
low-income populations of future air pollution control actions that
state and local agencies may consider in implementing the revised
O3 NAAQS described in this decision notice are discussed in
Appendix 9A of the RIA. The RIA is available on the Web, through the
EPA's Technology Transfer Network Web site at http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_index.html and
[[Page 65447]]
in the RIA docket (EPA-HQ-OAR-2013-0169). As noted above, although an
RIA has been prepared, the results of the RIA have not been considered
in issuing this final rule.
---------------------------------------------------------------------------
\257\ This refers to monitored areas with O3 design
values above the revised and alternative standards.
---------------------------------------------------------------------------
K. Congressional Review Act (CRA)
This action is subject to the CRA, and the EPA will submit a rule
report to each House of the Congress and to the Comptroller General of
the United States. This action is a ``major rule'' as defined by 5
U.S.C. 804(2).
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cohorts with potentially predisposing diseases. Am J Respir Crit
Care Med 184:836-841. http://dx.doi.org/10.1164/rccm.201102-0227OC.
Zanobetti, A; Schwartz, J. (2008). Mortality displacement in the
association of ozone with mortality: An analysis of 48 cities in the
United States. Am J Respir Crit Care Med 177:184-189. http://dx.doi.org/10.1164/rccm.200706-823OC.
Zanobetti, A; Schwartz, J. (2006). Air pollution and emergency
admissions in Boston, MA. J Epidemiol Community Health 60:890-895.
http://dx.doi.org/10.1136/jech.2005.039834.
List of Subjects
40 CFR Part 50
Environmental protection, Air pollution control, Carbon monoxide,
Lead, Nitrogen dioxide, Ozone, Particulate matter, Sulfur oxides.
40 CFR Part 51
Environmental protection, Administrative practices and
[[Page 65452]]
procedures, Air pollution control, Intergovernmental relations.
40 CFR Part 52
Environmental Protection, Administrative practices and procedures,
Air pollution control, Incorporation by reference, Intergovernmental
relations.
40 CFR Part 53
Environmental protection, Administrative practice and procedure,
Air pollution control, Reporting and recordkeeping requirements.
40 CFR Part 58
Environmental protection, Administrative practice and procedure,
Air pollution control, Intergovernmental relations, Reporting and
recordkeeping requirements.
Dated: October 1, 2015.
Gina McCarthy,
Administrator.
For the reasons set forth in the preamble, chapter I of title 40 of
the Code of Federal Regulations is amended as follows:
PART 50--NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY
STANDARDS
0
1. The authority citation for part 50 continues to read as follows:
Authority: 42 U.S.C. 7401 et seq.
0
2. Amend Sec. 50.14 by:
0
a. Revising paragraphs (c)(2)(iii) and (vi) and (c)(3)(i); and
0
b. Removing and reserving paragraphs (c)(2)(iv) and (v) and (c)(3)(ii)
and (iii).
The revisions read as follows:
Sec. 50.14 Treatment of air quality monitoring data influenced by
exceptional events.
* * * * *
(c) * * *
(2) * * *
(iii) Flags placed on data as being due to an exceptional event
together with an initial description of the event shall be submitted to
EPA not later than July 1st of the calendar year following the year in
which the flagged measurement occurred, except as allowed under
paragraph (c)(2)(vi) of this section.
* * * * *
(vi) Table 1 identifies the data submission process for a new or
revised NAAQS. This process shall apply to those data that will or may
influence the initial designation of areas for any new or revised
NAAQS.
Table 1--Schedule for Flagging and Documentation Submission for Data
Influenced by Exceptional Events for Use in Initial Area Designations
------------------------------------------------------------------------
Exceptional events/regulatory Exceptional events deadline schedule
action \d\
------------------------------------------------------------------------
Flagging and initial event If state and tribal initial
description deadline for data designation recommendations for a
years 1, 2 and 3.\a\. new/revised NAAQS are due August
through January, then the flagging
and initial event description
deadline will be the July 1 prior
to the recommendation deadline. If
state and tribal recommendations
for a new/revised NAAQS are due
February through July, then the
flagging and initial event
description deadline will be the
January 1 prior to the
recommendation deadline.
Exceptional events demonstration No later than the date that state
submittal deadline for data years and tribal recommendations are due
1, 2 and 3.\a\. to EPA.
Flagging, initial event By the last day of the month that is
description and exceptional 1 year and 7 months after
events demonstration submittal promulgation of a new/revised
deadline for data year 4 \b\ and, NAAQS, unless either option a or b
where applicable, data year 5.\c\. applies.
a. If the EPA follows a 3-year
designation schedule, the deadline
is 2 years and 7 months after
promulgation of a new/revised
NAAQS.
b. If the EPA notifies the state/
tribe that it intends to complete
the initial area designations
process according to a schedule
between 2 and 3 years, the deadline
is 5 months prior to the date
specified for final designations
decisions in such EPA notification.
------------------------------------------------------------------------
\a\ Where data years 1, 2, and 3 are those years expected to be
considered in state and tribal recommendations.
\b\ Where data year 4 is the additional year of data that the EPA may
consider when it makes final area designations for a new/revised NAAQS
under the standard designations schedule.
\c\ Where data year 5 is the additional year of data that the EPA may
consider when it makes final area designations for a new/revised NAAQS
under an extended designations schedule.
\d\ The date by which air agencies must certify their ambient air
quality monitoring data in AQS is annually on May 1 of the year
following the year of data collection as specified in 40 CFR
58.15(a)(2). In some cases, however, air agencies may choose to
certify a prior year's data in advance of May 1 of the following year,
particularly if the EPA has indicated its intent to promulgate final
designations in the first 8 months of the calendar year. Data
flagging, initial event description and exceptional events
demonstration deadlines for ``early certified'' data will follow the
deadlines for ``year 4'' and ``year 5'' data.
(3) Submission of demonstrations. (i) Except as allowed under
paragraph (c)(2)(vi) of this section, a State that has flagged data as
being due to an exceptional event and is requesting exclusion of the
affected measurement data shall, after notice and opportunity for
public comment, submit a demonstration to justify data exclusion to EPA
not later than the lesser of 3 years following the end of the calendar
quarter in which the flagged concentration was recorded or 12 months
prior to the date that a regulatory decision must be made by EPA. A
State must submit the public comments it received along with its
demonstration to EPA.
* * * * *
0
3. Section 50.19 is added to read as follows:
Sec. 50.19 National primary and secondary ambient air quality
standards for ozone.
(a) The level of the national 8-hour primary ambient air quality
standard for ozone (O3) is 0.070 parts per million (ppm),
daily maximum 8-hour average, measured by a reference method based on
appendix D to this part and designated in accordance with part 53 of
this chapter or an equivalent method designated in accordance with part
53 of this chapter.
(b) The 8-hour primary O3 ambient air quality standard
is met at an ambient air quality monitoring site when the 3-year
average of the annual fourth-highest daily maximum 8-hour average
O3 concentration is less than or equal to 0.070 ppm, as
determined in accordance with appendix U to this part.
(c) The level of the national secondary ambient air quality
standard for O3 is 0.070 ppm, daily maximum 8-hour
[[Page 65453]]
average, measured by a reference method based on appendix D to this
part and designated in accordance with part 53 of this chapter or an
equivalent method designated in accordance with part 53 of this
chapter.
(d) The 8-hour secondary O3 ambient air quality standard
is met at an ambient air quality monitoring site when the 3-year
average of the annual fourth-highest daily maximum 8-hour average
O3 concentration is less than or equal to 0.070 ppm, as
determined in accordance with appendix U to this part.
0
4. Revise appendix D to part 50 to read as follows:
Appendix D to Part 50--Reference Measurement Principle and Calibration
Procedure for the Measurement of Ozone in the Atmosphere
(Chemiluminescence Method)
1.0 Applicability.
1.1 This chemiluminescence method provides reference
measurements of the concentration of ozone (O3) in
ambient air for determining compliance with the national primary and
secondary ambient air quality standards for O3 as
specified in 40 CFR part 50. This automated method is applicable to
the measurement of ambient O3 concentrations using
continuous (real-time) sampling and analysis. Additional quality
assurance procedures and guidance are provided in 40 CFR part 58,
appendix A, and in Reference 14.
2.0 Measurement Principle.
2.1 This reference method is based on continuous automated
measurement of the intensity of the characteristic chemiluminescence
released by the gas phase reaction of O3 in sampled air
with either ethylene (C2H4) or nitric oxide
(NO) gas. An ambient air sample stream and a specific flowing
concentration of either C2H4 (ET-CL method) or
NO (NO-CL method) are mixed in a measurement cell, where the
resulting chemiluminescence is quantitatively measured by a
sensitive photo-detector. References 8-11 describe the
chemiluminescence measurement principle.
2.2 The measurement system is calibrated by referencing the
instrumental chemiluminescence measurements to certified
O3 standard concentrations generated in a dynamic flow
system and assayed by photometry to be traceable to a National
Institute of Standards and Technology (NIST) standard reference
photometer for O3 (see Section 4, Calibration Procedure,
below).
2.3 An analyzer implementing this measurement principle is shown
schematically in Figure 1. Designs implementing this measurement
principle must include: an appropriately designed mixing and
measurement cell; a suitable quantitative photometric measurement
system with adequate sensitivity and wavelength specificity for
O3; a pump, flow control, and sample conditioning system
for sampling the ambient air and moving it into and through the
measurement cell; a sample air dryer as necessary to meet the water
vapor interference limit requirement specified in subpart B of part
53 of this chapter; a means to supply, meter, and mix a constant,
flowing stream of either C2H4 or NO gas of
fixed concentration with the sample air flow in the measurement
cell; suitable electronic control and measurement processing
capability; and other associated apparatus as may be necessary. The
analyzer must be designed and constructed to provide accurate,
repeatable, and continuous measurements of O3
concentrations in ambient air, with measurement performance that
meets the requirements specified in subpart B of part 53 of this
chapter.
2.4 An analyzer implementing this measurement principle and
calibration procedure will be considered a federal reference method
(FRM) only if it has been designated as a reference method in
accordance with part 53 of this chapter.
2.5 Sampling considerations. The use of a particle filter on the
sample inlet line of a chemiluminescence O3 FRM analyzer
is required to prevent buildup of particulate matter in the
measurement cell and inlet components. This filter must be changed
weekly (or at least often as specified in the manufacturer's
operation/instruction manual), and the sample inlet system used with
the analyzer must be kept clean, to avoid loss of O3 in
the O3 sample air prior to the concentration measurement.
3.0 Interferences.
3.1 Except as described in 3.2 below, the chemiluminescence
measurement system is inherently free of significant interferences
from other pollutant substances that may be present in ambient air.
3.2 A small sensitivity to variations in the humidity of the
sample air is minimized by a sample air dryer. Potential loss of
O3 in the inlet air filter and in the air sample handling
components of the analyzer and associated exterior air sampling
components due to buildup of airborne particulate matter is
minimized by filter replacement and cleaning of the other inlet
components.
4.0 Calibration Procedure.
4.1 Principle. The calibration procedure is based on the
photometric assay of O3 concentrations in a dynamic flow
system. The concentration of O3 in an absorption cell is
determined from a measurement of the amount of 254 nm light absorbed
by the sample. This determination requires knowledge of (1) the
absorption coefficient ([alpha]) of O3 at 254 nm, (2) the
optical path length (l) through the sample, (3) the transmittance of
the sample at a nominal wavelength of 254 nm, and (4) the
temperature (T) and pressure (P) of the sample. The transmittance is
defined as the ratio I/I0, where I is the intensity of
light which passes through the cell and is sensed by the detector
when the cell contains an O3 sample, and I0 is
the intensity of light which passes through the cell and is sensed
by the detector when the cell contains zero air. It is assumed that
all conditions of the system, except for the contents of the
absorption cell, are identical during measurement of I and
I0. The quantities defined above are related by the Beer-
Lambert absorption law,
[GRAPHIC] [TIFF OMITTED] TR26OC15.002
Where:
[alpha] = absorption coefficient of O3 at 254 nm = 308
4 atm-1 cm-1 at 0 [deg]C and 760 torr,\1, 2, 3, 4, 5, 6,
7\
c = O3 concentration in atmospheres, and
l = optical path length in cm.
A stable O3 generator is used to produce
O3 concentrations over the required calibration
concentration range. Each O3 concentration is determined
from the measurement of the transmittance (I/I0) of the
sample at 254 nm with a photometer of path length l and calculated
from the equation,
[GRAPHIC] [TIFF OMITTED] TR26OC15.003
[[Page 65454]]
The calculated O3 concentrations must be corrected
for O3 losses, which may occur in the photometer, and for
the temperature and pressure of the sample.
4.2 Applicability. This procedure is applicable to the
calibration of ambient air O3 analyzers, either directly
or by means of a transfer standard certified by this procedure.
Transfer standards must meet the requirements and specifications set
forth in Reference 12.
4.3 Apparatus. A complete UV calibration system consists of an
O3 generator, an output port or manifold, a photometer,
an appropriate source of zero air, and other components as
necessary. The configuration must provide a stable O3
concentration at the system output and allow the photometer to
accurately assay the output concentration to the precision specified
for the photometer (4.3.1). Figure 2 shows a commonly used
configuration and serves to illustrate the calibration procedure,
which follows. Other configurations may require appropriate
variations in the procedural steps. All connections between
components in the calibration system downstream of the O3
generator must be of glass, Teflon, or other relatively inert
materials. Additional information regarding the assembly of a UV
photometric calibration apparatus is given in Reference 13. For
certification of transfer standards which provide their own source
of O3, the transfer standard may replace the
O3 generator and possibly other components shown in
Figure 2; see Reference 12 for guidance.
4.3.1 UV photometer. The photometer consists of a low-pressure
mercury discharge lamp, (optional) collimation optics, an absorption
cell, a detector, and signal-processing electronics, as illustrated
in Figure 2. It must be capable of measuring the transmittance, I/
I0, at a wavelength of 254 nm with sufficient precision
such that the standard deviation of the concentration measurements
does not exceed the greater of 0.005 ppm or 3% of the concentration.
Because the low-pressure mercury lamp radiates at several
wavelengths, the photometer must incorporate suitable means to
assure that no O3 is generated in the cell by the lamp,
and that at least 99.5% of the radiation sensed by the detector is
254 nm radiation. (This can be readily achieved by prudent selection
of optical filter and detector response characteristics.) The length
of the light path through the absorption cell must be known with an
accuracy of at least 99.5%. In addition, the cell and associated
plumbing must be designed to minimize loss of O3 from
contact with cell walls and gas handling components. See Reference
13 for additional information.
4.3.2 Air flow controllers. Air flow controllers are devices
capable of regulating air flows as necessary to meet the output
stability and photometer precision requirements.
4.3.3 Ozone generator. The ozone generator used must be capable
of generating stable levels of O3 over the required
concentration range.
4.3.4 Output manifold. The output manifold must be constructed
of glass, Teflon, or other relatively inert material, and should be
of sufficient diameter to insure a negligible pressure drop at the
photometer connection and other output ports. The system must have a
vent designed to insure atmospheric pressure in the manifold and to
prevent ambient air from entering the manifold.
4.3.5 Two-way valve. A manual or automatic two-way valve, or
other means is used to switch the photometer flow between zero air
and the O3 concentration.
4.3.6 Temperature indicator. A device to indicate temperature
must be used that is accurate to 1 [deg]C.
4.3.7 Barometer or pressure indicator. A device to indicate
barometric pressure must be used that is accurate to 2
torr.
4.4 Reagents.
4.4.1 Zero air. The zero air must be free of contaminants which
would cause a detectable response from the O3 analyzer,
and it must be free of NO, C2H4, and other
species which react with O3. A procedure for generating
suitable zero air is given in Reference 13. As shown in Figure 2,
the zero air supplied to the photometer cell for the I0
reference measurement must be derived from the same source as the
zero air used for generation of the O3 concentration to
be assayed (I measurement). When using the photometer to certify a
transfer standard having its own source of O3, see
Reference 12 for guidance on meeting this requirement.
4.5 Procedure.
4.5.1 General operation. The calibration photometer must be
dedicated exclusively to use as a calibration standard. It must
always be used with clean, filtered calibration gases, and never
used for ambient air sampling. A number of advantages are realized
by locating the calibration photometer in a clean laboratory where
it can be stationary, protected from the physical shock of
transportation, operated by a responsible analyst, and used as a
common standard for all field calibrations via transfer standards.
4.5.2 Preparation. Proper operation of the photometer is of
critical importance to the accuracy of this procedure. Upon initial
operation of the photometer, the following steps must be carried out
with all quantitative results or indications recorded in a
chronological record, either in tabular form or plotted on a
graphical chart. As the performance and stability record of the
photometer is established, the frequency of these steps may be
reduced to be consistent with the documented stability of the
photometer and the guidance provided in Reference 12.
4.5.2.1 Instruction manual. Carry out all set up and adjustment
procedures or checks as described in the operation or instruction
manual associated with the photometer.
4.5.2.2 System check. Check the photometer system for integrity,
leaks, cleanliness, proper flow rates, etc. Service or replace
filters and zero air scrubbers or other consumable materials, as
necessary.
4.5.2.3 Linearity. Verify that the photometer manufacturer has
adequately established that the linearity error of the photometer is
less than 3%, or test the linearity by dilution as follows: Generate
and assay an O3 concentration near the upper range limit
of the system or appropriate calibration scale for the instrument,
then accurately dilute that concentration with zero air and re-assay
it. Repeat at several different dilution ratios. Compare the assay
of the original concentration with the assay of the diluted
concentration divided by the dilution ratio, as follows
[GRAPHIC] [TIFF OMITTED] TR26OC15.004
Where:
E = linearity error, percent
A1 = assay of the original concentration
A2 = assay of the diluted concentration
R = dilution ratio = flow of original concentration divided by the
total flow
The linearity error must be less than 5%. Since the accuracy of
the measured flow-rates will affect the linearity error as measured
this way, the test is not necessarily conclusive. Additional
information on verifying linearity is contained in Reference 13.
4.5.2.4 Inter-comparison. The photometer must be inter-compared
annually, either directly or via transfer standards, with a NIST
standard reference photometer (SRP) or calibration photometers used
by other agencies or laboratories.
4.5.2.5 Ozone losses. Some portion of the O3 may be
lost upon contact with the photometer cell walls and gas handling
components. The magnitude of this loss must be determined and used
to correct the calculated O3 concentration. This loss
must not exceed 5%. Some guidelines for quantitatively determining
this loss are discussed in Reference 13.
4.5.3 Assay of O3 concentrations. The operator must
carry out the following steps to properly assay O3
concentrations.
4.5.3.1 Allow the photometer system to warm up and stabilize.
4.5.3.2 Verify that the flow rate through the photometer
absorption cell, F, allows the cell to be flushed in a reasonably
short period of time (2 liter/min is a typical flow). The precision
of the measurements is inversely related to the time required for
flushing, since the photometer drift error increases with time.
4.5.3.3 Ensure that the flow rate into the output manifold is at
least 1 liter/min greater than the total flow rate required by the
photometer and any other flow demand connected to the manifold.
[[Page 65455]]
4.5.3.4 Ensure that the flow rate of zero air, Fz, is at least 1
liter/min greater than the flow rate required by the photometer.
4.5.3.5 With zero air flowing in the output manifold, actuate
the two-way valve to allow the photometer to sample first the
manifold zero air, then Fz. The two photometer readings must be
equal (I = I0).
Note: In some commercially available photometers, the operation
of the two-way valve and various other operations in section 4.5.3
may be carried out automatically by the photometer.
4.5.3.6 Adjust the O3 generator to produce an
O3 concentration as needed.
4.5.3.7 Actuate the two-way valve to allow the photometer to
sample zero air until the absorption cell is thoroughly flushed and
record the stable measured value of Io.
4.5.3.8 Actuate the two-way valve to allow the photometer to
sample the O3 concentration until the absorption cell is
thoroughly flushed and record the stable measured value of I.
4.5.3.9 Record the temperature and pressure of the sample in the
photometer absorption cell. (See Reference 13 for guidance.)
4.5.3.10 Calculate the O3 concentration from equation
4. An average of several determinations will provide better
precision.
[GRAPHIC] [TIFF OMITTED] TR26OC15.005
Where:
[O3]OUT = O3 concentration, ppm
[alpha] = absorption coefficient of O3 at 254 nm = 308
atm-1 cm-1 at 0[deg] C and 760 torr
l = optical path length, cm
T = sample temperature, K
P = sample pressure, torr
L = correction factor for O3 losses from 4.5.2.5 = (1-
fraction of O3 lost).
Note: Some commercial photometers may automatically evaluate
all or part of equation 4. It is the operator's responsibility to
verify that all of the information required for equation 4 is
obtained, either automatically by the photometer or manually. For
``automatic'' photometers which evaluate the first term of equation
4 based on a linear approximation, a manual correction may be
required, particularly at higher O3 levels. See the
photometer instruction manual and Reference 13 for guidance.
4.5.3.11 Obtain additional O3 concentration standards
as necessary by repeating steps 4.5.3.6 to 4.5.3.10 or by Option 1.
4.5.4 Certification of transfer standards. A transfer standard
is certified by relating the output of the transfer standard to one
or more O3 calibration standards as determined according
to section 4.5.3. The exact procedure varies depending on the nature
and design of the transfer standard. Consult Reference 12 for
guidance.
4.5.5 Calibration of ozone analyzers. Ozone analyzers must be
calibrated as follows, using O3 standards obtained
directly according to section 4.5.3 or by means of a certified
transfer standard.
4.5.5.1 Allow sufficient time for the O3 analyzer and
the photometer or transfer standard to warm-up and stabilize.
4.5.5.2 Allow the O3 analyzer to sample zero air
until a stable response is obtained and then adjust the
O3 analyzer's zero control. Offsetting the analyzer's
zero adjustment to +5% of scale is recommended to facilitate
observing negative zero drift (if any). Record the stable zero air
response as ``Z''.
4.5.5.3 Generate an O3 concentration standard of
approximately 80% of the desired upper range limit (URL) of the
O3 analyzer. Allow the O3 analyzer to sample
this O3 concentration standard until a stable response is
obtained.
4.5.5.4 Adjust the O3 analyzer's span control to
obtain the desired response equivalent to the calculated standard
concentration. Record the O3 concentration and the
corresponding analyzer response. If substantial adjustment of the
span control is necessary, recheck the zero and span adjustments by
repeating steps 4.5.5.2 to 4.5.5.4.
4.5.5.5 Generate additional O3 concentration
standards (a minimum of 5 are recommended) over the calibration
scale of the O3 analyzer by adjusting the O3
source or by Option 1. For each O3 concentration
standard, record the O3 concentration and the
corresponding analyzer response.
4.5.5.6 Plot the O3 analyzer responses (vertical or
Y-axis) versus the corresponding O3 standard
concentrations (horizontal or X-axis). Compute the linear regression
slope and intercept and plot the regression line to verify that no
point deviates from this line by more than 2 percent of the maximum
concentration tested.
4.5.5.7 Option 1: The various O3 concentrations
required in steps 4.5.3.11 and 4.5.5.5 may be obtained by dilution
of the O3 concentration generated in steps 4.5.3.6 and
4.5.5.3. With this option, accurate flow measurements are required.
The dynamic calibration system may be modified as shown in Figure 3
to allow for dilution air to be metered in downstream of the
O3 generator. A mixing chamber between the O3
generator and the output manifold is also required. The flow rate
through the O3 generator (Fo) and the dilution air flow
rate (FD) are measured with a flow or volume standard that is
traceable to a NIST flow or volume calibration standard. Each
O3 concentration generated by dilution is calculated
from:
[GRAPHIC] [TIFF OMITTED] TR26OC15.006
Where:
[O3]'OUT = diluted O3
concentration, ppm
FO = flow rate through the O3 generator, liter/min
FD = diluent air flow rate, liter/min
Note: Additional information on calibration and pollutant
standards is provided in Section 12 of Reference 14.
5.0 Frequency of Calibration.
5.1 The frequency of calibration, as well as the number of
points necessary to establish the calibration curve, and the
frequency of other performance checking will vary by analyzer;
however, the minimum frequency, acceptance criteria, and subsequent
actions are specified in Appendix D of Reference 14: Measurement
Quality Objectives and Validation Templates. The user's quality
control program shall provide guidelines for initial establishment
of these variables and for subsequent alteration as operational
experience is accumulated. Manufacturers of analyzers should include
in their instruction/operation manuals information and guidance as
to these variables and on other matters of operation, calibration,
routine maintenance, and quality control.
6.0 References.
1. E.C.Y. Inn and Y. Tanaka, ``Absorption coefficient of Ozone in
the Ultraviolet and Visible Regions'', J. Opt. Soc. Am., 43, 870
(1953).
2. A. G. Hearn, ``Absorption of Ozone in the Ultraviolet and Visible
Regions of the Spectrum'', Proc. Phys. Soc. (London), 78, 932
(1961).
3. W. B. DeMore and O. Raper, ``Hartley Band Extinction Coefficients
of Ozone in the Gas Phase and in Liquid Nitrogen, Carbon Monoxide,
and Argon'', J. Phys. Chem., 68, 412 (1964).
4. M. Griggs, ``Absorption Coefficients of Ozone in the Ultraviolet
and Visible Regions'', J. Chem. Phys., 49, 857 (1968).
5. K. H. Becker, U. Schurath, and H. Seitz, ``Ozone Olefin Reactions
in the Gas Phase. 1. Rate Constants and Activation Energies'', Int'l
Jour. of Chem. Kinetics, VI, 725 (1974).
6. M. A. A. Clyne and J. A. Coxom, ``Kinetic Studies of Oxy-halogen
Radical Systems'', Proc. Roy. Soc., A303, 207 (1968).
7. J. W. Simons, R. J. Paur, H. A. Webster, and E. J. Bair, ``Ozone
Ultraviolet Photolysis. VI. The Ultraviolet Spectrum'', J. Chem.
Phys., 59, 1203 (1973).
8. Ollison, W.M.; Crow, W.; Spicer, C.W. ``Field testing of new-
technology
[[Page 65456]]
ambient air ozone monitors.'' J. Air Waste Manage. Assoc., 63 (7),
855-863 (2013).
9. Parrish, D.D.; Fehsenfeld, F.C. ``Methods for gas-phase
measurements of ozone, ozone precursors and aerosol precursors.''
Atmos. Environ., 34 (12-14), 1921-1957(2000).
10. Ridley, B.A.; Grahek, F.E.; Walega, J.G. ``A small, high-
sensitivity, medium-response ozone detector suitable for
measurements from light aircraft.'' J. Atmos. Oceanic Technol., 9
(2), 142-148(1992).
11. Boylan, P., Helmig, D., and Park, J.H. ``Characterization and
mitigation of water vapor effects in the measurement of ozone by
chemiluminescence with nitric oxide.'' Atmos. Meas. Tech. 7, 1231-
1244 (2014).
12. Transfer Standards for Calibration of Ambient Air Monitoring
Analyzers for Ozone, EPA publication number EPA-454/B-13-004,
October 2013. EPA, Office of Air Quality Planning and Standards,
Research Triangle Park, NC 27711. [Available at www.epa.gov/ttnamti1/files/ambient/qaqc/OzoneTransferStandardGuidance.pdf.]
13. Technical Assistance Document for the Calibration of Ambient
Ozone Monitors, EPA publication number EPA-600/4-79-057, September,
1979. [Available at www.epa.gov/ttnamti1/files/ambient/criteria/4-79-057.pdf.]
14. QA Handbook for Air Pollution Measurement Systems--Volume II.
Ambient Air Quality Monitoring Program. EPA-454/B-13-003, May 2013.
[Available at http://www.epa.gov/ttnamti1/files/ambient/pm25/qa/QA-Handbook-Vol-II.pdf.]
[GRAPHIC] [TIFF OMITTED] TR26OC15.007
[[Page 65457]]
[GRAPHIC] [TIFF OMITTED] TR26OC15.008
[[Page 65458]]
[GRAPHIC] [TIFF OMITTED] TR26OC15.009
0
5. Add appendix U to Part 50 to read as follows:
Appendix U to Part 50--Interpretation of the Primary and Secondary
National Ambient Air Quality Standards for Ozone
1. General
(a) This appendix explains the data handling conventions and
computations necessary for determining whether the primary and
secondary national ambient air quality standards (NAAQS) for ozone
(O3) specified in Sec. 50.19 are met at an ambient
O3 air quality monitoring site. Data reporting, data
handling, and computation procedures to be used in making
comparisons between reported O3 concentrations and the
levels of the O3 NAAQS are specified in the following
sections.
(b) Whether to exclude or retain the data affected by
exceptional events is determined by the requirements under
Sec. Sec. 50.1, 50.14 and 51.930.
(c) The terms used in this appendix are defined as follows:
8-hour average refers to the moving average of eight consecutive
hourly O3 concentrations measured at a site, as explained
in section 3 of this appendix.
Annual fourth-highest daily maximum refers to the fourth highest
value measured at a site during a year.
Collocated monitors refers to the instance of two or more
O3 monitors operating at the same physical location.
Daily maximum 8-hour average O3 concentration refers to the
maximum calculated 8-hour average value measured at a site on a
particular day, as explained in section 3 of this appendix.
Design value refers to the metric (i.e., statistic) that is used
to compare ambient O3 concentration data measured at a
site to the NAAQS in order to determine compliance, as explained in
section 4 of this appendix.
Minimum data completeness requirements refer to the amount of
data that a site is required to collect in order to make a valid
determination that the site is meeting the NAAQS.
Monitor refers to a physical instrument used to measure ambient
O3 concentrations.
O3 monitoring season refers to the span of time
within a year when individual states are required to measure ambient
O3 concentrations, as listed in Appendix D to part 58 of
this chapter.
Site refers to an ambient O3 air quality monitoring
site.
Site data record refers to the set of hourly O3
concentration data collected at a site for use in comparisons with
the NAAQS.
Year refers to calendar year.
2. Selection of Data for use in Comparisons With the Primary and
Secondary Ozone NAAQS
(a) All valid hourly O3 concentration data collected
using a federal reference method specified in Appendix D to this
part, or an equivalent method designated in accordance with part 53
of this chapter, meeting all applicable requirements in part 58 of
this chapter, and submitted to EPA's Air Quality System (AQS)
database or otherwise available to EPA, shall be used in design
value calculations.
(b) All design value calculations shall be implemented on a
site-level basis. If data are reported to EPA from collocated
monitors, those data shall be combined into a single site data
record as follows:
(i) The monitoring agency shall designate one monitor as the
primary monitor for the site.
(ii) Hourly O3 concentration data from a secondary
monitor shall be substituted into
[[Page 65459]]
the site data record whenever a valid hourly O3
concentration is not obtained from the primary monitor. In the event
that hourly O3 concentration data are available for more
than one secondary monitor, the hourly concentration values from the
secondary monitors shall be averaged and substituted into the site
data record.
(c) In certain circumstances, including but not limited to site
closures or relocations, data from two nearby sites may be combined
into a single site data record for the purpose of calculating a
valid design value. The appropriate Regional Administrator may
approve such combinations after taking into consideration factors
such as distance between sites, spatial and temporal patterns in air
quality, local emissions and meteorology, jurisdictional boundaries,
and terrain features.
3. Data Reporting and Data Handling Conventions
(a) Hourly average O3 concentrations shall be
reported in parts per million (ppm) to the third decimal place, with
additional digits to the right of the third decimal place truncated.
Each hour shall be identified using local standard time (LST).
(b) Moving 8-hour averages shall be computed from the hourly
O3 concentration data for each hour of the year and shall
be stored in the first, or start, hour of the 8-hour period. An 8-
hour average shall be considered valid if at least 6 of the hourly
concentrations for the 8-hour period are available. In the event
that only 6 or 7 hourly concentrations are available, the 8-hour
average shall be computed on the basis of the hours available, using
6 or 7, respectively, as the divisor. In addition, in the event that
5 or fewer hourly concentrations are available, the 8-hour average
shall be considered valid if, after substituting zero for the
missing hourly concentrations, the resulting 8-hour average is
greater than the level of the NAAQS, or equivalently, if the sum of
the available hourly concentrations is greater than 0.567 ppm. The
8-hour averages shall be reported to three decimal places, with
additional digits to the right of the third decimal place truncated.
Hourly O3 concentrations that have been approved under
Sec. 50.14 as having been affected by exceptional events shall be
counted as missing or unavailable in the calculation of 8-hour
averages.
(c) The daily maximum 8-hour average O3 concentration
for a given day is the highest of the 17 consecutive 8-hour averages
beginning with the 8-hour period from 7:00 a.m. to 3:00 p.m. and
ending with the 8-hour period from 11:00 p.m. to 7:00 a.m. the
following day (i.e., the 8-hour averages for 7:00 a.m. to 11:00
p.m.). Daily maximum 8-hour average O3 concentrations
shall be determined for each day with ambient O3
monitoring data, including days outside the O3 monitoring
season if those data are available.
(d) A daily maximum 8-hour average O3 concentration
shall be considered valid if valid 8-hour averages are available for
at least 13 of the 17 consecutive 8-hour periods starting from 7:00
a.m. to 11:00 p.m. In addition, in the event that fewer than 13
valid 8-hour averages are available, a daily maximum 8-hour average
O3 concentration shall also be considered valid if it is
greater than the level of the NAAQS. Hourly O3
concentrations that have been approved under Sec. 50.14 as having
been affected by exceptional events shall be included when
determining whether these criteria have been met.
(e) The primary and secondary O3 design value
statistic is the annual fourth-highest daily maximum 8-hour
O3 concentration, averaged over three years, expressed in
ppm. The fourth-highest daily maximum 8-hour O3
concentration for each year shall be determined based only on days
meeting the validity criteria in 3(d). The 3-year average shall be
computed using the three most recent, consecutive years of ambient
O3 monitoring data. Design values shall be reported in
ppm to three decimal places, with additional digits to the right of
the third decimal place truncated.
4. Comparisons With the Primary and Secondary Ozone NAAQS
(a) The primary and secondary national ambient air quality
standards for O3 are met at an ambient air quality
monitoring site when the 3-year average of the annual fourth-highest
daily maximum 8-hour average O3 concentration (i.e., the
design value) is less than or equal to 0.070 ppm.
(b) A design value greater than the level of the NAAQS is always
considered to be valid. A design value less than or equal to the
level of the NAAQS must meet minimum data completeness requirements
in order to be considered valid. These requirements are met for a 3-
year period at a site if valid daily maximum 8-hour average
O3 concentrations are available for at least 90% of the
days within the O3 monitoring season, on average, for the
3-year period, with a minimum of at least 75% of the days within the
O3 monitoring season in any one year.
(c) When computing whether the minimum data completeness
requirements have been met, meteorological or ambient data may be
sufficient to demonstrate that meteorological conditions on missing
days were not conducive to concentrations above the level of the
NAAQS. Missing days assumed less than the level of the NAAQS are
counted for the purpose of meeting the minimum data completeness
requirements, subject to the approval of the appropriate Regional
Administrator.
(d) Comparisons with the primary and secondary O3
NAAQS are demonstrated by examples 1 and 2 as follows:
Example 1--Site Meeting the Primary and Secondary O3 NAAQS
--------------------------------------------------------------------------------------------------------------------------------------------------------
Percent valid
days within O3 1st highest 2nd highest 3rd highest 4th highest 5th highest
Year monitoring daily max 8- daily max 8- daily max 8- daily max 8- daily max 8-
season (Data hour O3 (ppm) hour O3 (ppm) hour O3 (ppm) hour O3 (ppm) hour O3 (ppm)
completeness)
--------------------------------------------------------------------------------------------------------------------------------------------------------
2014.................................................... 100 0.082 0.080 0.075 0.069 0.068
2015.................................................... 96 0.074 0.073 0.065 0.062 0.060
2016.................................................... 98 0.070 0.069 0.067 0.066 0.060
Average................................................. 98 .............. .............. .............. 0.065
--------------------------------------------------------------------------------------------------------------------------------------------------------
As shown in Example 1, this site meets the primary and secondary
O3 NAAQS because the 3-year average of the annual fourth-
highest daily maximum 8-hour average O3 concentrations
(i.e., 0.065666 ppm, truncated to 0.065 ppm) is less than or equal
to 0.070 ppm. The minimum data completeness requirements are also
met (i.e., design value is considered valid) because the average
percent of days within the O3 monitoring season with
valid ambient monitoring data is greater than 90%, and no single
year has less than 75% data completeness.
Example 2--Site Failing to Meet the Primary and Secondary O3 O3 NAAQS
--------------------------------------------------------------------------------------------------------------------------------------------------------
Percent valid
days within O3 1st highest 2nd highest 3rd highest 4th highest 5th highest
Year monitoring daily max 8- daily max 8- daily max 8- daily max 8- daily max 8-
season (Data hour O3 (ppm) hour O3 (ppm) hour O3 (ppm) hour O3 (ppm) hour O3 (ppm)
completeness)
--------------------------------------------------------------------------------------------------------------------------------------------------------
2014.................................................... 96 0.085 0.080 0.079 0.074 0.072
[[Page 65460]]
2015.................................................... 74 0.084 0.083 0.072 0.071 0.068
2016.................................................... 98 0.083 0.081 0.081 0.075 0.074
Average................................................. 89 .............. .............. .............. 0.073
--------------------------------------------------------------------------------------------------------------------------------------------------------
As shown in Example 2, this site fails to meet the primary and
secondary O3 NAAQS because the 3-year average of the
annual fourth-highest daily maximum 8-hour average O3
concentrations (i.e., 0.073333 ppm, truncated to 0.073 ppm) is
greater than 0.070 ppm, even though the annual data completeness is
less than 75% in one year and the 3-year average data completeness
is less than 90% (i.e., design value would not otherwise be
considered valid).
PART 51--REQUIREMENTS FOR PREPARATION, ADOPTION, AND SUBMITTAL OF
IMPLEMENTATION PLANS
0
6. The authority citation for part 51 continues to read as follows:
Authority: 23 U.S.C. 101; 42 U.S.C. 7401-7671q.
Subpart I---Review of New Sources and Modifications
0
8. Amend Sec. 51.166 by adding paragraph (i)(11) to read as follows:
Sec. 51.166 Prevention of significant deterioration of air quality.
* * * * *
(i) * * *
(11) The plan may provide that the requirements of paragraph (k)(1)
of this section shall not apply to a permit application for a
stationary source or modification with respect to the revised national
ambient air quality standards for ozone published on October 26, 2015
if:
(i) The reviewing authority has determined the permit application
subject to this section to be complete on or before October 1, 2015.
Instead, the requirements in paragraph (k)(1) of this section shall
apply with respect to the national ambient air quality standards for
ozone in effect at the time the reviewing authority determined the
permit application to be complete; or
(ii) The reviewing authority has first published before December
28, 2015 a public notice of a preliminary determination or draft permit
for the permit application subject to this section. Instead, the
requirements in paragraph (k)(1) of this section shall apply with
respect to the national ambient air quality standards for ozone in
effect at the time of first publication of a public notice of the
preliminary determination or draft permit.
* * * * *
PART 52--APPROVAL AND PROMULGATION OF IMPLEMENTATION PLANS
0
8. The authority citation for part 52 continues to read as follows:
Authority: 42 U.S.C. 7401 et seq.
0
9. Amend Sec. 52.21 by adding paragraph (i)(12) to read as follows:
Sec. 52.21 Prevention of significant deterioration of air quality.
* * * * *
(i) * * *
(12) The requirements of paragraph (k)(1) of this section shall not
apply to a permit application for a stationary source or modification
with respect to the revised national ambient air quality standards for
ozone published on October 26, 2015 if:
(i) The Administrator has determined the permit application subject
to this section to be complete on or before October 1, 2015. Instead,
the requirements in paragraph (k)(1) of this section shall apply with
respect to the national ambient air quality standards for ozone in
effect at the time the Administrator determined the permit application
to be complete; or
(ii) The Administrator has first published before December 28, 2015
a public notice of a preliminary determination or draft permit for the
permit application subject to this section. Instead, the requirements
in paragraph (k)(1) of this section shall apply with respect to the
national ambient air quality standards for ozone in effect on the date
the Administrator first published a public notice of a preliminary
determination or draft permit.
* * * * *
PART 53--AMBIENT AIR MONITORING REFERENCE AND EQUIVALENT METHODS
0
10. The authority citation for part 53 continues to read as follows:
Authority: Sec. 301(a) of the Clean Air Act (42 U.S.C.
1857g(a)), as amended by sec. 15(c)(2) of Pub. L. 91-604, 84 Stat.
1713, unless otherwise noted.
Subpart A--General Provisions
Sec. 53.9 [Amended]
0
11. Amend Sec. 53.9 by removing paragraph (i).
0
12. Amend Sec. 53.14 by revising paragraph (c) introductory text to
read as follows:
Sec. 53.14 Modification of a reference or equivalent method.
* * * * *
(c) Within 90 calendar days after receiving a report under
paragraph (a) of this section, the Administrator will take one or more
of the following actions:
* * * * *
Subpart B--Procedures for Testing Performance Characteristics of
Automated Methods for SO2, CO, O3, and
NO2
0
13. Amend Sec. 53.23 by revising paragraph (e)(1)(vi) to read as
follows:
Sec. 53.23 Test procedures.
* * * * *
(e) * * *
(1) * * *
(vi) Precision: Variation about the mean of repeated measurements
of the same pollutant concentration, denoted as the standard deviation
expressed as a percentage of the upper range limits.\258\
---------------------------------------------------------------------------
\258\ NO2 precision in Table B-1 is also changed to
percent to agree with the calculation specified in 53.23(e)(10)(vi).
---------------------------------------------------------------------------
* * * * *
0
14. Revise Table B-1 to Subpart B of Part 53 to read as follows:
[[Page 65461]]
Table B-1 to Subpart B of Part 53--Performance Limit Specifications for Automated Methods
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SO2 O3 CO
------------------------------------------------------------------------------------------------ NO2 (Std. Definitions and test
Performance parameter Units \1\ Lower range 2 Lower range 2 Lower range 2 range) procedures
Std. range \3\ 3 Std. range \3\ 3 Std. range \3\ 3
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1. Range.......................... ppm.................. 0-0.5 <0.5 0-0.5 <0.5 0-50 <50 0-0.5 Sec. 53.23(a)
2. Noise.......................... ppm.................. 0.001 0.0005 0.0025 0.001 0.2 0.1 0.005 Sec. 53.23(b)
3. Lower detectable limit......... ppm.................. 0.002 0.001 0.005 0.002 0.4 0.2 0.010 Sec. 53.23(c)
4. Interference equivalent
Each interferent.............. ppm.................. 0.005 minus>0.005 minus>0.005 minus>0.005 minus>1.0 minus>0.5 minus>0.02
Total, all interferents....... ppm.................. - - - - - - 0.04 Sec. 53.23(d)
5. Zero drift, 12 and 24 hour..... ppm.................. 0.004 minus>0.002 minus>0.004 minus>0.002 minus>0.5 minus>0.3 minus>0.02
6. Span drift, 24 hour
20% of upper range limit...... Percent.............. - - - - - - 20.0
80% of upper range limit...... Percent.............. 3.0 minus>3.0 minus>3.0 minus>3.0 minus>2.0 minus>2.0 minus>5.0
7. Lag time....................... Minutes.............. 2 2 2 2 2.0 2.0 20 Sec. 53.23(e)
8. Rise time...................... Minutes.............. 2 2 2 2 2.0 2.0 15 Sec. 53.23(e)
9. Fall time...................... Minutes.............. 2 2 2 2 2.0 2.0 15 Sec. 53.23(e)
10. Precision
20% of upper range limit...... - - - - - - Sec. 53.23(e)
Percent \5\.......... 2 2 2 2 1.0 1.0 4 Sec. 53.23(e)
80% of upper range limit...... - - - - - - Sec. 53.23(e)
Percent \5\.......... 2 2 2 2 1.0 1.0 6 Sec. 53.23(e)
Sec. 53.23(e)
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ To convert from parts per million (ppm) to [mu]g/m\3\ at 25 [deg]C and 760 mm Hg, multiply by M/0.02447, where M is the molecular weight of the gas. Percent means percent of the upper
measurement range limit.
\2\ Tests for interference equivalent and lag time do not need to be repeated for any lower range provided the test for the standard range shows that the lower range specification (if
applicable) is met for each of these test parameters.
\3\ For candidate analyzers having automatic or adaptive time constants or smoothing filters, describe their functional nature, and describe and conduct suitable tests to demonstrate their
function aspects and verify that performances for calibration, noise, lag, rise, fall times, and precision are within specifications under all applicable conditions. For candidate analyzers
with operator-selectable time constants or smoothing filters, conduct calibration, noise, lag, rise, fall times, and precision tests at the highest and lowest settings that are to be
included in the FRM or FEM designation.
\4\ For nitric oxide interference for the SO2 UVF method, interference equivalent is 0.0003 ppm for the lower range.
\5\ Standard deviation expressed as percent of the URL.
[[Page 65462]]
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[[Page 65463]]
[GRAPHIC] [TIFF OMITTED] TR26OC15.011
[[Page 65464]]
[GRAPHIC] [TIFF OMITTED] TR26OC15.012
[[Page 65465]]
[GRAPHIC] [TIFF OMITTED] TR26OC15.013
[[Page 65466]]
* * * * *
Subpart C--Procedures for Determining Comparability between
Candidate Methods and Reference Methods
0
17. Amend Sec. 53.32 by revising paragraph (g)(1)(iii) to read as
follows:
Sec. 53.32 Test procedures for methods for SO2, CO,
O3, and NO2.
* * * * *
(g) * * *
(1) * * *
(iii) The measurements shall be made in the sequence specified in
table C-2 of this subpart.
* * * * *
Figure E-2 to Subpart E of Part 53 [Removed]
0
18. Amend subpart E by removing figure E-2 to subpart E of part 53.
PART 58--AMBIENT AIR QUALITY SURVEILLANCE
0
19. The authority citation for part 58 continues to read as follows:
Authority: 42 U.S.C. 7403, 7405, 7410, 7414, 7601, 7611, 7614,
and 7619.
Subpart B--Monitoring Network
0
20. Amend Sec. 58.10 by adding paragraphs (a)(9) through (11) to read
as follows:
Sec. 58.10 Annual monitoring network plan and periodic network
assessment.
(a) * * *
(9) The annual monitoring network plan shall provide for the
required O3 sites to be operating on the first day of the
applicable required O3 monitoring season in effect on
January 1, 2017 as listed in Table D-3 of appendix D of this part.
(10) A plan for making Photochemical Assessment Monitoring Stations
(PAMS) measurements, if applicable, in accordance with the requirements
of appendix D paragraph 5(a) of this part shall be submitted to the EPA
Regional Administrator no later than July 1, 2018. The plan shall
provide for the required PAMS measurements to begin by June 1, 2019.
(11) An Enhanced Monitoring Plan for O3, if applicable,
in accordance with the requirements of appendix D paragraph 5(h) of
this part shall be submitted to the EPA Regional Administrator no later
than October 1, 2019 or two years following the effective date of a
designation to a classification of Moderate or above O3
nonattainment, whichever is later.
* * * * *
0
21. Section Sec. 58.11 is amended by revising paragraph (c) to read as
follows:
Sec. 58.11 Network technical requirements.
* * * * *
(c) State and local governments must follow the network design
criteria contained in appendix D to this part in designing and
maintaining the SLAMS stations. The final network design and all
changes in design are subject to approval of the Regional
Administrator. NCore and STN network design and changes are also
subject to approval of the Administrator. Changes in SPM stations do
not require approvals, but a change in the designation of a monitoring
site from SLAMS to SPM requires approval of the Regional Administrator.
* * * * *
0
22. Amend Sec. 58.13 by adding paragraphs (g) and (h) to read as
follows:
Sec. 58.13 Monitoring network completion.
* * * * *
(g) The O3 monitors required under appendix D, section
4.1 of this part must operate on the first day of the applicable
required O3 monitoring season in effect January 1, 2017.
(h) The Photochemical Assessment Monitoring sites required under 40
CFR part 58 Appendix D, section 5(a) must be physically established and
operating under all of the requirements of this part, including the
requirements of appendix A, C, D, and E of this part, no later than
June 1, 2019.
Subpart F--Air Quality Index Reporting
0
23. Amend Sec. 58.50 by revising paragraph (c) to read as follows:
Sec. 58.50 Index reporting.
* * * * *
(c) The population of a metropolitan statistical area for purposes
of index reporting is the latest available U.S. census population.
Subpart G--Federal Monitoring
0
24. Amend appendix D to part 58, under section 4, by revising section
4.1(i) and table D-3 to appendix D of part 58, and by revising section
5 to read as follows:
Appendix D to part 58--Network Design Criteria for Ambient Air Quality
Monitoring
* * * * *
4. Pollutant-Specific Design Criteria for SLAMS Sites
* * * * *
4.1 * * *
(i) Ozone monitoring is required at SLAMS monitoring sites only
during the seasons of the year that are conducive to O3
formation (i.e., ``ozone season'') as described below in Table D-3
of this appendix. These O3 seasons are also identified in
the AQS files on a state-by-state basis. Deviations from the
O3 monitoring season must be approved by the EPA Regional
Administrator. These requests will be reviewed by Regional
Administrators taking into consideration, at a minimum, the
frequency of out-of-season O3 NAAQS exceedances, as well
as occurrences of the Moderate air quality index level, regional
consistency, and logistical issues such as site access. Any
deviations based on the Regional Administrator's waiver of
requirements must be described in the annual monitoring network plan
and updated in AQS. Changes to the O3 monitoring season
requirements in Table D-3 revoke all previously approved Regional
Administrator waivers. Requests for monitoring season deviations
must be accompanied by relevant supporting information. Information
on how to analyze O3 data to support a change to the
O3 season in support of the 8-hour standard for the
entire network in a specific state can be found in reference 8 to
this appendix. Ozone monitors at NCore stations are required to be
operated year-round (January to December).
Table D-3 \1\ to Appendix D of part 58. Ozone Monitoring Season by state
------------------------------------------------------------------------
State Begin Month End Month
------------------------------------------------------------------------
Alabama......................... March............. October.
Alaska.......................... April............. October.
Arizona......................... January........... December.
Arkansas........................ March............. November.
California...................... January........... December.
Colorado........................ January........... December.
Connecticut..................... March............. September.
Delaware........................ March............. October.
District of Columbia............ March............. October.
[[Page 65467]]
Florida......................... January........... December.
Georgia......................... March............. October.
Hawaii.......................... January........... December.
Idaho........................... April............. September.
Illinois........................ March............. October.
Indiana......................... March............. October.
Iowa............................ March............. October.
Kansas.......................... March............. October.
Kentucky........................ March............. October.
Louisiana (Northern) AQCR 019, March............. October.
022.
Louisiana (Southern) AQCR 106... January........... December.
Maine........................... April............. September.
Maryland........................ March............. October.
Massachusetts................... March............. September.
Michigan........................ March............. October.
Minnesota....................... March............. October.
Mississippi..................... March............. October.
Missouri........................ March............. October.
Montana......................... April............. September.
Nebraska........................ March............. October.
Nevada.......................... January........... December.
New Hampshire................... March............. September.
New Jersey...................... March............. October.
New Mexico...................... January........... December.
New York........................ March............. October.
North Carolina.................. March............. October.
North Dakota.................... March............. September.
Ohio............................ March............. October.
Oklahoma........................ March............. November.
Oregon.......................... May............... September.
Pennsylvania.................... March............. October.
Puerto Rico..................... January........... December.
Rhode Island.................... March............. September.
South Carolina.................. March............. October.
South Dakota.................... March............. October.
Tennessee....................... March............. October.
Texas (Northern) AQCR 022, 210, March............. November.
211, 212, 215, 217, 218.
Texas (Southern) AQCR 106, 153, January........... December.
213, 214, 216.
Utah............................ January........... December.
Vermont......................... April............. September.
Virginia........................ March............. October.
Washington...................... May............... September.
West Virginia................... March............. October.
Wisconsin....................... March............. October 15.
Wyoming......................... January........... September.
American Samoa.................. January........... December.
Guam............................ January........... December.
Virgin Islands.................. January........... December.
------------------------------------------------------------------------
\1\ The required O3 monitoring season for NCore stations is January
through December.
* * * * *
5. Network Design for Photochemical Assessment Monitoring Stations
(PAMS) and Enhanced Ozone Monitoring
(a) State and local monitoring agencies are required to collect
and report PAMS measurements at each NCore site required under
paragraph 3(a) of this appendix located in a CBSA with a population
of 1,000,000 or more, based on the latest available census figures.
(b) PAMS measurements include:
(1) Hourly averaged speciated volatile organic compounds (VOCs);
(2) Three 8-hour averaged carbonyl samples per day on a 1 in 3
day schedule, or hourly averaged formaldehyde;
(3) Hourly averaged O3;
(4) Hourly averaged nitrogen oxide (NO), true nitrogen dioxide
(NO2), and total reactive nitrogen (NOy);
(5) Hourly averaged ambient temperature;
(6) Hourly vector-averaged wind direction;
(7) Hourly vector-averaged wind speed;
(8) Hourly average atmospheric pressure;
(9) Hourly averaged relative humidity;
(10) Hourly precipitation;
(11) Hourly averaged mixing-height;
(12) Hourly averaged solar radiation; and
(13) Hourly averaged ultraviolet radiation.
(c) The EPA Regional Administrator may grant a waiver to allow
the collection of required PAMS measurements at an alternative
location where the monitoring agency can demonstrate that the
alternative location will provide representative data useful for
regional or national scale modeling and the tracking of trends in
O3 precursors. The alternative location can be outside of
the CBSA or outside of the monitoring agencies jurisdiction. In
cases where the alternative location crosses jurisdictions the
waiver will be contingent on the monitoring agency responsible for
the alternative location including the required PAMS measurements in
their annual monitoring plan required under Sec. 58.10 and
continued successful collection of PAMS measurements at the
alternative location. This waiver can be revoked in cases where the
Regional Administrator determines the PAMS measurements are not
being collected at the alternate location in compliance with
paragraph (b) of this section.
(d) The EPA Regional Administrator may grant a waiver to allow
speciated VOC measurements to be made as three 8-hour averages on
every third day during the PAMS
[[Page 65468]]
season as an alternative to 1-hour average speciated VOC
measurements in cases where the primary VOC compounds are not well
measured using continuous technology due to low detectability of the
primary VOC compounds or for logistical and other programmatic
constraints.
(e) The EPA Regional Administrator may grant a waiver to allow
representative meteorological data from nearby monitoring stations
to be used to meet the meteorological requirements in paragraph 5(b)
where the monitoring agency can demonstrate the data is collected in
a manner consistent with EPA quality assurance requirements for
these measurements.
(f) The EPA Regional Administrator may grant a waiver from the
requirement to collect PAMS measurements in locations where CBSA-
wide O3 design values are equal to or less than 85% of
the 8-hour O3 NAAQS and where the location is not
considered by the Regional Administrator to be an important upwind
or downwind location for other O3 nonattainment areas.
(g) At a minimum, the monitoring agency shall collect the
required PAMS measurements during the months of June, July, and
August.
(h) States with Moderate and above 8-hour O3
nonattainment areas and states in the Ozone Transport Region as
defined in 40 CFR 51.900 shall develop and implement an Enhanced
Monitoring Plan (EMP) detailing enhanced O3 and
O3 precursor monitoring activities to be performed. The
EMP shall be submitted to the EPA Regional Administrator no later
than October 1, 2019 or two years following the effective date of a
designation to a classification of Moderate or above O3
nonattainment, whichever is later. At a minimum, the EMP shall be
reassessed and approved as part of the 5-year network assessments
required under 40 CFR 58.10(d). The EMP will include monitoring
activities deemed important to understanding the O3
problems in the state. Such activities may include, but are not
limited to, the following:
(1) Additional O3 monitors beyond the minimally
required under paragraph 4.1 of this appendix,
(2) Additional NOX or NOy monitors beyond
those required under 4.3 of this appendix,
(3) Additional speciated VOC measurements including data
gathered during different periods other than required under
paragraph 5(g) of this appendix, or locations other than those
required under paragraph 5(a) of this appendix, and
(4) Enhanced upper air measurements of meteorology or pollution
concentrations.
* * * * *
0
25. Appendix G of Part 58 is amended by revising table 2 to read as
follows:
Appendix G to Part 58--Uniform Air Quality Index (AQI) and Daily
Reporting
* * * * *
TABLE 2--BREAKPOINTS FOR THE AQI
----------------------------------------------------------------------------------------------------------------
These breakpoints Equal these AQI's
----------------------------------------------------------------------------------------------------------------
PM2.5 PM10
O3 (ppm) 8- O3 (ppm) 1- ([micro]g/ ([micro]g/ CO (ppm) 8- SO2 (ppb) NO2 (ppb)
hour hour\1\ m\3\) 24- m\3\) 24- hour 1-hour 1-hour AQI Category
hour hour
----------------------------------------------------------------------------------------------------------------
0.000-0.05 -- 0.0--12.0 0-54 0.0-4.4 0-35 0-53 0-50 Good.
4
0.055-0.07 -- 12.1--35.4 55-154 4.5-9.4 36-75 54-100 51-100 Moderate.
0
0.071-0.08 0.125-0.16 35.5--55.4 155-254 9.5-12.4 76-185 101-360 101-150 Unhealthy for
5 4 Sensitive
Groups.
0.086-0.10 0.165-0.20 \3\ 55.5-- 255-354 12.5-15.4 \4\ 186- 361-649 151-200 Unhealthy.
5 4 150.4 304
0.106-0.20 0.205-0.40 \3\ 150.5-- 355-424 15.5-30.4 \4\ 305- 650-1249 201-300 Very Unhealthy.
0 4 250.4 604
0.201- 0.405-0.50 \3\ 250.5-- 425-504 30.5-40.4 \4\ 605- 1250-1649 301-400 Hazardous.
(\2\) 4 350.4 804
(\2\) 0.505-0.60 \3\ 350.5-- 505-604 40.5-50.4 \4\ 805- 1650-2049 401-500
4 500.4 1004
----------------------------------------------------------------------------------------------------------------
\1\ Areas are generally required to report the AQI based on 8-hour ozone values. However, there are a small
number of areas where an AQI based on 1-hour ozone values would be more precautionary. In these cases, in
addition to calculating the 8-hour ozone index value, the 1-hour ozone index value may be calculated, and the
maximum of the two values reported.
\2\ 8-hour O3 values do not define higher AQI values (>301). AQI values > 301 are calculated with 1-hour O3
concentrations.
\3\ If a different SHL for PM2.5 is promulgated, these numbers will change accordingly.
\4\ 1-hr SO2 values do not define higher AQI values (>=200). AQI values of 200 or greater are calculated with 24-
hour SO2 concentration.
[FR Doc. 2015-26594 Filed 10-23-15; 8:45 am]
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