[Federal Register Volume 71, Number 200 (Tuesday, October 17, 2006)]
[Rules and Regulations]
[Pages 61144-61233]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 06-8477]



[[Page 61143]]

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Part II





Environmental Protection Agency





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40 CFR Part 50



 National Ambient Air Quality Standards for Particulate Matter; Final 
Rule

Federal Register / Vol. 71, No. 200 / Tuesday, October 17, 2006 / 
Rules and Regulations

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 50

[EPA-HQ-OAR-2001-0017; FRL-8225-3]
RIN 2060-AI44


National Ambient Air Quality Standards for Particulate Matter

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

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SUMMARY: Based on its review of the air quality criteria and national 
ambient air quality standards (NAAQS) for particulate matter (PM), EPA 
is making revisions to the primary and secondary NAAQS for PM to 
provide increased protection of public health and welfare, 
respectively. With regard to primary standards for fine particles 
(generally referring to particles less than or equal to 2.5 micrometers 
([micro]m) in diameter, PM2.5), EPA is revising the level of 
the 24-hour PM2.5 standard to 35 micrograms per cubic meter 
([micro]g/m\3\) and retaining the level of the annual PM2.5 
standard at 15[micro]g/m\3\. With regard to primary standards for 
particles generally less than or equal to 10[mu]m in diameter 
(PM10), EPA is retaining the 24-hour PM10 and 
revoking the annual PM10 standard. With regard to secondary 
PM standards, EPA is making them identical in all respects to the 
primary PM standards, as revised.

DATES: This final rule is effective on December 18, 2006.

ADDRESSES: The EPA has established a docket for this action under 
Docket ID No. EPA-HQ-OAR-2001-0017. All documents in the docket are 
listed on the www.regulations.gov Web site. Although listed in the 
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 will be publicly available 
only in hard copy form. Publicly available docket materials are 
available either electronically through www.regulations.gov or in hard 
copy at the Air and Radiation Docket and Information Center, EPA/DC, 
EPA West, Room B102, 1301 Constitution Ave., NW., Washington, DC. This 
Docket Facility is open from 8:30 a.m. to 4:30 p.m., Monday through 
Friday, excluding legal holidays. The Docket telephone number is 202-
566-1741. The telephone number for the Public Reading Room is 202-566-
1744.
    The EPA Docket Center suffered damage due to flooding during the 
last week of June 2006. The Docket Center is continuing to operate. 
However, during the cleanup, there will be temporary changes to Docket 
Center telephone numbers, addresses, and hours of operation for people 
who wish to visit the Public Reading Room to view documents. Consult 
EPA's Federal Register notice at 71 FR 38147 (July 5, 2006) or the EPA 
Web site at www.epa.gov/epahome/dockets.htm for current information on 
docket status, locations and telephone numbers.

FOR FURTHER INFORMATION CONTACT: Ms. Beth M. Hassett-Sipple, Mail Code 
C504-06, Health and Environmental Impacts Division, Office of Air 
Quality Planning and Standards, U.S. Environmental Protection Agency, 
Research Triangle Park, North Carolina 27711, telephone: (919) 541-
4605, e-mail: hassett-sipple.beth@epa.gov.

SUPPLEMENTARY INFORMATION:

Table of Contents

    The following topics are discussed in today's preamble:

I. Background
    A. Summary of Revisions to the PM NAAQS
    B. Legislative Requirements
    C. Overview of Air Quality Criteria and Standards Review for PM
    D. Related Control Programs to Implement PM Standards
    E. Summary of Proposed Revisions to the PM NAAQS
    F. Organization and Approach to Final PM NAAQS Decisions
II. Rationale for Final Decisions on Primary PM2.5 
Standards
    A. Introduction
    1. Overview
    2. Overview of Health Effects Evidence
    3. Overview of Quantitative Risk Assessment
    B. Need for Revision of the Current Primary PM2.5 
Standards
    1. Introduction
    2. Comments on the Need for Revision
    3. Conclusions Regarding the Need for Revision
    C. Indicator for Fine Particles
    D. Averaging Time of Primary PM2.5 Standards
    E. Form of Primary PM2.5 Standards
    1. 24-Hour PM2.5 Standard
    2. Annual PM2.5 Standard
    F. Level of Primary PM2.5 Standards
    1. 24-Hour PM2.5 Standard
    2. Annual PM2.5 Standard
    G. Final Decisions on Primary PM2.5 Standards
III. Rationale for Final Decisions on Primary PM10 
Standards
    A. Introduction
    1. Overview
    2. Overview of Health Effects Evidence
    3. Overview of Quantitative Risk Assessment
    B. Need for Revision of the Current Primary PM10 
Standards
    1. Overview of the Proposal
    2. Comments on the Need for Revision
    C. Indicator for Thoracic Coarse Particles
    1. Introduction
    2. Comments on Indicator for Thoracic Coarse Particles
    3. Decision Not to Revise PM10 Indicator
    a. Unqualified PM10-2.5 Indicator
    b. PM10 Indicator
    c. Unqualified PM10 Indicator, with Adjustment to the 
PM2.5 Component
    4. Conclusions Regarding Indicator for Thoracic Coarse Particles
    D. Conclusions Regarding Averaging Time, Form, and Level of the 
Current PM10 Standards
    1. Averaging Time
    2. Level and Form of the 24-Hour PM10 Standard
    E. Final Decisions on Primary PM10 Standards
IV. Rationale for Final Decisions on Secondary PM Standards
    A. Visibility Impairment
    1. Visibility Impairment Related to Ambient PM
    2. Need for Revision of the Current Secondary PM2.5 
Standards to Protect Visibility
    3. Indicator of PM for Secondary Standard to Address Visibility 
Impairment
    4. Averaging Time of a Secondary PM2.5 Standard for 
Visibility Protection
    5. Final Decisions on Secondary PM2.5 Standards for 
Visibility Protection
    B. Other PM-Related Welfare Effects
    1. Evidence of Non-Visibility Welfare Effects Related to PM
    2. Need for Revision of the Current Secondary PM Standards to 
Address Other PM-Related Welfare Effects
    C. Final Decisions on Secondary PM Standards
V. Interpretation of the NAAQS for PM
    A. Amendments to Appendix N--Interpretation of the National 
Ambient Air Quality Standards for PM2.5
    1. General
    2. PM2.5 Monitoring and Data Reporting Considerations
    3. PM2.5 Computations and Data Handling Conventions
    4. Conforming Revisions
    B. Proposed Appendix P--Interpretation of the National Ambient 
Air Quality Standards for PM10-2.5
    C. Amendments to Appendix K--Interpretation of the 
National Ambient Air Quality Standards for PM10
VI. Reference Methods for the Determination of Particulate 
Matter as PM10-2.5 and PM2.5
    A. Appendix O to Part 50--Reference Method for the 
Determination of Coarse Particulate Matter as PM10-2.5 in 
the Atmosphere
    B. Amendments to Appendix L--Reference Method for the 
Determination of Fine Particulate Matter (as PM2.5) in 
the Atmosphere
VII. Issues Related to Implementation of PM10 Standards
    A. Summary of Comments Received on Transition

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    B. Impact of Decision on PM10 Designations
    C. Impact of Decision on State Implementation Plans (SIPs) and 
Control Obligations
    D. Consideration of Fugitive Emissions for New Source Review 
(NSR) Purposes
    E. Handling of PM10 Exceedances Due to Exceptional 
Events
VIII. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and 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 Advancement Act
    J. Executive Order 12898: Federal Actions to Address 
Environmental Justice in Minority Populations and Low-Income 
Populations
    K. Congressional Review Act
References

I. Background

A. Summary of Revisions to the PM NAAQS

    Based on its review of the air quality criteria and national 
ambient air quality standards (NAAQS) for particulate matter (PM), EPA 
is making revisions to the primary and secondary NAAQS for PM to 
provide increased protection of public health and welfare, 
respectively.
    With regard to primary standards for fine particles (generally 
referring to particles less than or equal to 2.5 micrometers ([micro]m) 
in diameter, PM2.5), EPA is revising the level of the 24-
hour PM2.5 standard to 35 micrograms per cubic meter 
[micro]g/m\3\), providing increased protection against health effects 
associated with short-term exposure (including premature mortality and 
increased hospital admissions and emergency room visits), and retaining 
the level of the annual PM2.5 standard at 15 [micro]g/m\3\, 
continuing protection against health effects associated with long-term 
exposure (including premature mortality and development of chronic 
respiratory disease). The EPA is revising the form of the annual 
PM2.5 standard with regard to the criteria for spatial 
averaging, such that averaging across monitoring sites is allowed if 
the annual mean concentration at each monitoring site is within 10 
percent of the spatially averaged annual mean, and the daily values for 
each monitoring site pair yield a correlation coefficient of at least 
0.9 for each calendar quarter.
    With regard to primary standards for particles generally less than 
or equal to 10[micro]m in diameter (PM10), EPA is retaining 
the 24-hour PM10 standard to protect against the health 
effects associated with short-term exposure to coarse particles 
(including hospital admissions for cardiopulmonary diseases, increased 
respiratory symptoms and possibly premature mortality). Given that the 
available evidence does not suggest an association between long-term 
exposure to coarse particles at current ambient levels and health 
effects, EPA is revoking the annual PM10 standard.
    With regard to secondary PM standards, EPA is revising the current 
24-hour PM2.5 secondary standard by making it identical to 
the revised 24-hour PM2.5 primary standard, retaining the 
annual PM2.5 and 24-hour PM10 secondary 
standards, and revoking the annual PM10 secondary standard. 
This suite of secondary PM standards is intended to provide protection 
against PM-related public welfare effects, including visibility 
impairment, effects on vegetation and ecosystems, and materials damage 
and soiling.

B. Legislative Requirements

    Two sections of the Clean Air Act (CAA) govern the establishment 
and revision of the NAAQS. Section 108 (42 U.S.C. 7408) directs the 
Administrator to identify and list ``air pollutants'' that ``in his 
judgment, may reasonably be anticipated to endanger public health and 
welfare'' and whose ``presence * * * in the ambient air results from 
numerous or diverse mobile or stationary sources'' and to issue air 
quality criteria for those that are listed. Air quality criteria are 
intended to ``accurately reflect the latest scientific knowledge useful 
in indicating the kind and extent of identifiable effects on public 
health or welfare which may be expected from the presence of [a] 
pollutant in ambient air * * * .''
    Section 109 (42 U.S.C. 7409) directs the Administrator to propose 
and promulgate ``primary'' and ``secondary'' NAAQS for pollutants 
listed under section 108. 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 include 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. Lead Industries Association v. EPA, 647 F.2d 1130, 1154 
(D.C. Cir 1980), cert. denied, 449 U.S. 1042 (1980); American Petroleum 
Institute v. Costle, 665 F.2d 1176, 1186 (D.C. Cir. 1981), cert. 
denied, 455 U.S. 1034 (1982). 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 include 
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 a background concentration 
level (see Lead Industries Association v. EPA, supra, 647 F.2d at 1156 
n. 51), 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, EPA 
considers such factors as the nature and severity of the health effects 
involved, the size of the sensitive population(s) at risk, and the kind 
and degree of the uncertainties that must be addressed. The selection 
of any particular approach to providing an adequate margin of safety is 
a policy choice left specifically to the Administrator's judgment. Lead

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Industries Association v. EPA, supra, 647 F.2d at 1161-62.
    In setting standards that are ``requisite'' to protect public 
health and welfare, as provided in section 109(b), EPA's task is to 
establish standards that are neither more nor less stringent than 
necessary for these purposes. In establishing primary and secondary 
standards, 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).
    Section 109(d)(1) of the CAA 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 in 
accordance with [the provisions in section 109(b) on primary and 
secondary standards].'' This includes the authority to modify or revoke 
a standard or standards, as appropriate under these provisions. 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 * * *.'' This 
independent review function is performed by the Clean Air Scientific 
Advisory Committee (CASAC) of EPA's Science Advisory Board.

C. Overview of Air Quality Criteria and Standards Review for PM

    Particulate matter is the generic term for a broad class of 
chemically and physically diverse substances that exist as discrete 
particles (liquid droplets or solids) over a wide range of sizes. 
Particles originate from a variety of anthropogenic stationary and 
mobile sources as well as from natural sources. Particles may be 
emitted directly or formed in the atmosphere by transformations of 
gaseous emissions such as sulfur oxides (SOX), nitrogen 
oxides (NOX), and volatile organic compounds (VOC). The 
chemical and physical properties of PM vary greatly with time, region, 
meteorology, and source category, thus complicating the assessment of 
health and welfare effects.
    More specifically, the PM that is the subject of the air quality 
criteria and standards reviews includes both fine particles and 
thoracic coarse particles, which are considered as separate subclasses 
of PM pollution based in part on long-established information on 
differences in sources, properties, and atmospheric behavior between 
fine and coarse particles (EPA, 2005, section 2.2). Fine particles are 
produced chiefly by combustion processes and by atmospheric reactions 
of various gaseous pollutants, whereas thoracic coarse particles are 
generally emitted directly as particles as a result of mechanical 
processes that crush or grind larger particles or the resuspension of 
dusts. Sources of fine particles include, for example, motor vehicles, 
power generation, combustion sources at industrial facilities, and 
residential fuel burning. Sources of thoracic coarse particles include, 
for example, traffic-related emissions such as tire and brake lining 
materials, direct emissions from industrial operations, construction 
and demolition activities, and agricultural and mining operations. Fine 
particles can remain suspended in the atmosphere for days to weeks and 
can be transported thousands of kilometers, whereas thoracic coarse 
particles generally deposit rapidly on the ground or other surfaces and 
are not readily transported across urban or broader areas.
    The last review of PM air quality criteria and standards was 
completed in July 1997 with notice of a final decision to revise the 
existing standards (62 FR 38652, July 18, 1997). In that decision, EPA 
revised the PM NAAQS in several respects. While EPA determined that the 
PM NAAQS should continue to focus on particles less than or equal to 10 
[mu]m in diameter (PM10), EPA also determined that the fine 
and coarse fractions of PM10 should be considered 
separately. The EPA added new standards, using PM2.5 as the 
indicator for fine particles (with PM2.5 referring to 
particles with a nominal aerodynamic diameter less than or equal to 2.5 
[mu]m), and using PM10 as the indicator for purposes of 
regulating the coarse fraction of PM10 (referred to as 
thoracic coarse particles or coarse-fraction particles; generally 
including particles with a nominal aerodynamic diameter greater than 
2.5 [mu]m and less than or equal to 10 [mu]m, or PM10-2.5). 
The EPA established two new PM2.5 standards: An annual 
standard of 15 [mu]g/m3, based on the 3-year average of 
annual arithmetic mean PM2.5 concentrations from single or 
multiple community-oriented monitors; and a 24-hour standard of 65 
[mu]g/m3, based on the 3-year average of the 98th percentile 
of 24-hour PM2.5 concentrations at each population-oriented 
monitor within an area. Also, EPA established a new reference method 
for the measurement of PM2.5 in the ambient air and adopted 
rules for determining attainment of the new standards. To continue to 
address thoracic coarse particles, EPA retained the annual 
PM10 standard, while revising the form, but not the level, 
of the 24-hour PM10 standard to be based on the 99th 
percentile of 24-hour PM10 concentrations at each monitor in 
an area. The EPA revised the secondary standards by making them 
identical in all respects to the primary standards.
    Following promulgation of the revised PM NAAQS, petitions for 
review were filed by a large number of parties, addressing a broad 
range of issues. In May 1999, a three-judge panel of the U.S. Court of 
Appeals for the District of Columbia Circuit issued an initial decision 
that upheld EPA's decision to establish fine particle standards, 
holding that ``the growing empirical evidence demonstrating a 
relationship between fine particle pollution and adverse health effects 
amply justifies establishment of new fine particle standards.'' 
American Trucking Associations v. EPA, 175 F.3d 1027, 1055-56 (D.C. 
Cir. 1999) (``ATA I'') rehearing granted in part and denied in part, 
195 F.3d 4 (D.C. Cir. 1999) (``ATA II''), affirmed in part and reversed 
in part, Whitman v. American Trucking Associations, 531 U.S. 457 
(2001). The Panel also found ``ample support'' for EPA's decision to 
regulate coarse particle pollution, but vacated the 1997 
PM10 standards, concluding that EPA's justification for the 
use of PM10 as an indicator for coarse particles was 
arbitrary. 175 F.3d at 1054-55. Pursuant to the court's decision, EPA 
removed the vacated 1997 PM10 standards from the regulations 
(CFR) (69 FR 45592, July 30, 2004) and deleted the regulatory provision 
(at 40 CFR 50.6(d)) that controlled the transition from the pre-
existing 1987 PM10 standards to the 1997 PM10 
standards (65 FR 80776, December 22, 2000). The pre-existing 1987 
PM10 standards remained in place. Id. at 80777.
    More generally, the panel held (over one judge's dissent) that 
EPA's approach to establishing the level of the standards in 1997, both 
for PM and for ozone NAAQS promulgated on the same day, effected ``an 
unconstitutional delegation of legislative authority.'' Id. at 1034-40. 
Although the panel stated that ``the factors EPA uses in determining 
the degree of public health concern associated with different levels of 
ozone and PM are reasonable,'' it remanded the rule to EPA, stating 
that when EPA considers these factors for potential non-threshold 
pollutants ``what EPA lacks is any determinate criterion for

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drawing lines'' to determine where the standards should be set. 
Consistent with EPA's long-standing interpretation and D.C. Circuit 
precedent, the panel also reaffirmed prior rulings holding that in 
setting NAAQS EPA is ``not permitted to consider the cost of 
implementing those standards.'' Id. at 1040-41.
    Both sides filed cross appeals on these issues to the United States 
Supreme Court, and the Court granted certiorari. In February 2001, the 
Supreme Court issued a unanimous decision upholding EPA's position on 
both the constitutional and cost issues. Whitman v. American Trucking 
Associations, 531 U.S. 457, 464, 475-76 (2001). On the constitutional 
issue, the Court held that the statutory requirement that NAAQS be 
``requisite'' to protect public health with an adequate margin of 
safety sufficiently guided EPA's discretion, affirming EPA's approach 
of setting standards that are neither more nor less stringent than 
necessary. The Supreme Court remanded the case to the Court of Appeals 
for resolution of any remaining issues that had not been addressed in 
that court's earlier rulings. Id. at 475-76. In March 2002, the Court 
of Appeals rejected all remaining challenges to the standards, holding 
under the traditional standard of judicial review that EPA's 
PM2.5 standards were reasonably supported by the 
administrative record and were not ``arbitrary and capricious.'' 
American Trucking Associations v. EPA, 283 F. 3d 355, 369-72 (D.C. Cir. 
2002) (``ATA III'').
    In October 1997, EPA published its plans for the current periodic 
review of the PM criteria and NAAQS (62 FR 55201, October 23, 1997), 
including the 1997 PM2.5 standards and the 1987 
PM10 standards. The approach in this review continues to 
address fine and thoracic coarse particles separately. This approach 
has been reinforced by new information that has advanced our 
understanding of differences in human exposure relationships and 
dosimetric patterns characteristic of these two subclasses of PM 
pollution, as well as the apparent independence of health effects that 
have been associated with them in epidemiologic studies (EPA, 2004a, 
section 3.2.3). See also ATA I, 175 F. 3d at 1053-54, 1055-56 (EPA 
justified in establishing separate standards for fine and thoracic 
coarse particles).
    As part of the process of preparing an updated Air Quality Criteria 
Document for Particulate Matter (henceforth, the ``Criteria 
Document''), EPA's National Center for Environmental Assessment (NCEA) 
hosted a peer review workshop in April 1999 on drafts of key Criteria 
Document chapters. The first external review draft Criteria Document 
was reviewed by CASAC and the public at a meeting held in December 
1999. Based on CASAC and public comment, NCEA revised the draft 
Criteria Document and released a second draft in March 2001 for review 
by CASAC and the public at a meeting held in July 2001. A preliminary 
draft of a staff paper, Review of the National Ambient Air Quality 
Standards for Particulate Matter: Assessment of Scientific and 
Technical Information (henceforth, the ``Staff Paper'') prepared by 
EPA's Office of Air Quality Planning and Standards (OAQPS) was released 
in June 2001 for public comment and for consultation with CASAC at the 
same public meeting. Taking into account CASAC and public comments, a 
third draft Criteria Document was released in May 2002 for review at a 
meeting held in July 2002.
    Shortly after the release of the third draft Criteria Document, the 
Health Effects Institute (HEI) \3\ announced that researchers at Johns 
Hopkins University had discovered problems with applications of 
statistical software used in a number of important epidemiological 
studies that had been discussed in that draft Criteria Document. In 
response to this significant issue, EPA took steps in consultation with 
CASAC and the broader scientific community to encourage researchers to 
reanalyze affected studies and to submit them expeditiously for peer 
review by a special expert panel convened at EPA's request by HEI. The 
results of this reanalysis and peer-review process were subsequently 
incorporated into a fourth draft Criteria Document, which was released 
in June 2003 and reviewed by CASAC and the public at a meeting held in 
August 2003.
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    \3\ The HEI is a non-profit, independent research institute 
jointly and equally funded by EPA and multiple industries that 
conducts research on the health effects of air pollution.
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    The first draft Staff Paper, based on the fourth draft Criteria 
Document, was released at the end of August 2003, and was reviewed by 
CASAC and the public at a meeting held in November 2003. During that 
meeting, EPA also consulted with CASAC on a new framework for the final 
chapter (integrative synthesis) of the Criteria Document and on ongoing 
revisions to other Criteria Document chapters to address previous CASAC 
comments. The EPA held additional consultations with CASAC at public 
meetings held in February, July, and September 2004, leading to 
publication of the final Criteria Document in October 2004 (EPA, 
2004a). The second draft Staff Paper, based on the final Criteria 
Document, was released at the end of January 2005, and was reviewed by 
CASAC and the public at a meeting held in April 2005. The CASAC's 
advice and recommendations to the Administrator, based on its review of 
the second draft Staff Paper, were further discussed during a public 
teleconference held in May 2005 and are provided in a June 6, 2005 
letter to the Administrator (Henderson, 2005a). The final Staff Paper 
takes into account the advice and recommendations of CASAC and public 
comments received on the earlier drafts of this document. The 
Administrator subsequently received additional advice and 
recommendations from the CASAC, specifically on potential standards for 
thoracic coarse particles, in a teleconference on August 11, 2005, and 
in a letter to the Administrator dated September 15, 2005 (Henderson, 
2005b). The final Staff Paper was reissued in December 2005 to add 
CASAC's final letter as an attachment (EPA, 2005).
    The schedule for completion of this review is governed by a consent 
decree resolving a lawsuit filed in March 2003 by a group of plaintiffs 
representing national environmental organizations. The lawsuit alleged 
that EPA had failed to perform its mandatory duty, under section 
109(d)(1), of completing the current review within the period provided 
by statute. American Lung Association v. Whitman (No. 1:03CV00778, 
D.D.C. 2003). An initial consent decree was entered by the court in 
July 2003 after an opportunity for public comment. The consent decree, 
as modified by the court, provides that EPA will sign for publication 
notices of proposed and final rulemaking concerning its review of the 
PM NAAQS no later than December 20, 2005 and September 27, 2006, 
respectively.
    On December 20, 2005, EPA issued its proposed decision to revise 
the NAAQS for PM (71 FR 2620, January 17, 2006) (henceforth 
``proposal''). In the proposal, EPA identified proposed revisions to 
the standards, based on the air quality criteria for PM, and to related 
data handling conventions and federal reference methods for monitoring 
PM. The proposal solicited public comments on alternative primary and 
secondary standards and related matters.
    The EPA held several public hearings across the country to provide 
direct opportunities for public comment on the proposed revisions to 
the PM NAAQS. On March 8, 2006, EPA held three concurrent 12-hour 
public hearings in Philadelphia, PA; Chicago, IL; and San Francisco, 
CA. At these public hearings, EPA heard testimony

[[Page 61148]]

from 280 individuals representing themselves or specific interested 
organizations.
    More than 120,000 comments were received from members of the public 
and various interested groups on the proposed revisions to the PM NAAQS 
by the close of the public comment period on April 17, 2006. CASAC 
provided additional advice to EPA in a letter to the Administrator 
requesting reconsideration of CASAC's recommendations for both the 
primary and secondary PM2.5 standards as well as standards 
for thoracic coarse particles (Henderson, 2006). Major issues raised in 
the public comments are discussed throughout the preamble of this final 
action. A comprehensive summary of all significant comments, along with 
EPA's responses (henceforth ``Response to Comments''), can be found in 
the docket for this rulemaking (Docket No. EPA-HQ-OAR-2001-0017).
    In the proposal, EPA recognized that there were a number of new 
scientific studies on the health effects of PM that had been published 
recently and therefore were not included in the Criteria Document.\4\ 
The EPA committed to conduct a review and assessment of any significant 
``new'' studies, including studies submitted during the public comment 
period. The purpose of this review was to ensure that the Administrator 
was fully aware of the ``new'' science before making a final decision 
on whether to revise the current PM NAAQS. The EPA screened and 
surveyed the recent literature, including studies submitted during the 
public comment period, and conducted a provisional assessment (EPA, 
2006a) that places the results of those studies of potentially greatest 
policy relevance in the context of the findings of the Criteria 
Document.
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    \4\ For ease of reference, these studies will be referred to as 
``new'' studies or ``new'' science, using quotation marks around the 
word new. Referring to studies that were published too recently to 
have been included in the 2004 Criteria Document as ``new'' studies 
is intended to clearly differentiate such studies from those that 
have been published since the last review and are included in the 
2004 Criteria Document (these studies are sometimes referred to as 
new (without quotation marks) or more recent studies, to indicate 
that they were not included in the 1996 Criteria Document and thus 
are newly available in this review).
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    The provisional assessment found that the ``new'' studies expand 
the scientific information and provide important insights on the 
relationship between PM exposure and health effects of PM. The 
provisional assessment also found that ``new'' studies generally 
strengthen the evidence that acute and chronic exposure to fine 
particles and acute exposure to thoracic coarse particles are 
associated with health effects; some of the ``new'' epidemiologic 
studies report effects in areas with lower concentrations of 
PM2.5 or PM10-2.5 than those in earlier reports; 
``new'' toxicology and epidemiologic studies link various health 
effects with a range of fine particle sources and components; and 
``new'' toxicology studies report effects of thoracic coarse particles 
but do not provide evidence to support distinguishing effects from 
exposure to urban and rural particles. Further, the provisional 
assessment found that the results reported in the studies do not 
dramatically diverge from previous findings, and, taken in context with 
the findings of the Criteria Document, the new information and findings 
do not materially change any of the broad scientific conclusions 
regarding the health effects of PM exposure made in the Criteria 
Document.
    The EPA believes it was important to conduct a provisional 
assessment in this case, so that the Administrator would be aware of 
the science that developed too recently for inclusion in the Criteria 
Document. However it is also important to note that EPA's review of 
that science to date has been limited to screening, surveying, and 
preparing a provisional assessment of these studies. Having performed 
this limited provisional assessment, EPA must decide whether to 
consider the newer studies in this review and take such steps as may be 
necessary to include them in the basis for the final decision, or to 
reserve such action for the next review of the PM NAAQS.
    As in prior NAAQS reviews, EPA is basing its decision in this 
review on studies and related information included in the Criteria 
Document and Staff Paper, which have undergone CASAC and public review. 
The studies assessed in the Criteria Document, and the integration of 
the scientific evidence presented in that document, have undergone 
extensive critical review by EPA, CASAC, and the public during the 
development of the Criteria Document. 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 EPA but also by the 
statutorily mandated independent advisory committee, as well as the 
public review that accompanies this process. As described above, the 
provisional assessment did not and could not provide that kind of in-
depth critical review.
    This decision is consistent with EPA's practice in prior NAAQS 
reviews. 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. See e.g., 36 FR 8186 (April 30, 1971) (EPA based 
original NAAQS for six pollutants on scientific studies discussed in 
air quality criteria documents and limited consideration of comments to 
those concerning validity of scientific basis); 38 FR 25678, 25679-
25680 (September 14, 1973) (EPA revised air quality criteria for sulfur 
oxides to provide basis for reevaluation of secondary NAAQS). 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. EPA has consistently 
followed this approach. 52 FR 24634, 24637 (July 1, 1987) (after review 
by CASAC, EPA issued a post-proposal addendum to the PM Criteria 
Document, to address certain new scientific studies not included in the 
1982 Criteria Document); 61 FR 25566, 25568 (May 22, 1996) (after 
review by CASAC, EPA issued a post-proposal supplement to the 1982 
Criteria Document to address certain new health studies not included in 
the 1982 Criteria Document or 1986 Addendum). The EPA recently 
reaffirmed this approach in its decision not to revise the ozone NAAQS 
in 1993, as well as in its final decision on the PM NAAQS in the 1997 
review. 58 FR 13008, 13013-13014 (March 9, 1993) (ozone review); 62 FR 
38652, 38662 (July 18, 1997) (The EPA conducted a provisional 
assessment but based the final PM decision on studies and related 
information included in the air quality criteria that had been reviewed 
by CASAC).
    As discussed in EPA's 1993 decision not to revise the NAAQS for 
ozone, new studies may sometimes be of such significance that it is 
appropriate to delay a decision on revision of NAAQS and to supplement 
the pertinent air quality criteria so the new studies can be taken into 
account (58 FR at 13013-13014, March 9, 1993). In the present case, the 
provisional assessment of recent studies concludes that, taken in 
context, the new information and findings do not materially change any 
of the broad scientific conclusions regarding the health effects of PM 
exposure made in the Criteria

[[Page 61149]]

Document. 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, EPA is 
basing the final decisions in this review on the studies and related 
information included in the PM air quality criteria that have undergone 
CASAC and public review. The EPA will consider the newly published 
studies for purposes of decision making in the next periodic review of 
the PM NAAQS, which will provide the opportunity to fully assess them 
through a more rigorous review process involving EPA, CASAC, and the 
public.
    In order to facilitate a comprehensive and timely review of the 
newly available science, the Administrator has directed EPA staff to 
begin the next review of the PM NAAQS immediately.\5\
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    \5\ The EPA has recently conducted a review of the process by 
which the Agency performs periodic NAAQS reviews to identify ways in 
which the process could be strengthened and streamlined (EPA, 
2006b). The EPA intends to incorporate recommendations from the 
NAAQS process review into the next PM NAAQS review.
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D. Related Control Programs To Implement PM Standards

    States are primarily responsible for ensuring attainment and 
maintenance of ambient air quality standards once EPA has established 
them. Under section 110 of the CAA (42 U.S.C. 7410) and related 
provisions, States are to submit, for EPA approval, State 
implementation plans (SIPs) that provide for the attainment and 
maintenance of such standards through control programs directed to 
sources of the pollutants involved. The States, in conjunction with 
EPA, also administer the prevention of significant deterioration (PSD) 
program under sections 160-169 of the CAA (42 U.S.C. 7470-7479) for 
these pollutants. In addition, the Act provides for nationwide 
reductions in emissions of these and other air pollutants through 
related programs, such as the Federal Mobile Source Control Program 
under Title II of the CAA (42 U.S.C. 7521-7574), which involves 
controls for automobile, truck, bus, motorcycle, nonroad and off-
highway engines and aircraft emissions; the new source performance 
standards under section 111 (42 U.S.C. 7411); and the national emission 
standards for hazardous air pollutants under section 112 (42 U.S.C. 
7412).
    As described in a recent EPA report, The Particle Pollution Report: 
Current Understanding of Air Quality and Emissions through 2003 (EPA, 
2004b), State and Federal programs have made substantial progress in 
reducing ambient concentrations of PM10 and 
PM2.5. For example, PM10 concentrations have 
decreased 31 percent nationally since 1988. Regionally, PM10 
concentrations decreased most in areas with historically higher 
concentrations--the Northwest (39 percent decline), the Southwest (33 
percent decline), and southern California (35 percent decline). Direct 
emissions of PM10 have decreased approximately 25 percent 
nationally since 1988.
    Programs aimed at reducing direct emissions of particles have 
played an important role in reducing PM10 concentrations, 
particularly in western areas. Some examples of PM10 
controls include paving unpaved roads and using best management 
practices for agricultural sources of resuspended soil. Of the 87 areas 
that were designated nonattainment for PM10 in the early 
1990s, 64 now meet those standards. In cities that have not attained 
the PM10 standards, the number of days above the standards 
is down significantly.
    Nationally, PM2.5 concentrations have declined by 10 
percent from 1999 to 2003. Generally, PM2.5 concentrations 
have also declined the most in regions with the highest 
concentrations--the Southeast (20 percent decline), southern California 
(16 percent decline), and the Industrial Midwest (9 percent decline). 
With the exception of the Northeast, the remaining regions posted 
modest declines in PM2.5 concentrations from 1999 to 2003. 
Direct emissions of PM2.5 have decreased by 5 percent 
nationally over the past 5 years.
    National programs that affect regional emissions have also 
contributed to lower sulfate concentrations and, consequently, to lower 
PM2.5 concentrations, particularly in the Industrial Midwest 
and Southeast. National ozone-reduction programs designed to reduce 
emissions of volatile organic compounds (VOCs) and nitrogen oxides 
(NOX) have also helped reduce carbon and nitrates, both of 
which are components of PM2.5. Additionally, EPA's Acid Rain 
Program has substantially reduced sulfur dioxide (SO2) 
emissions from power plants since 1995 in the eastern United States, 
contributing to lower PM concentrations. Nationally, SO2 
emissions have declined 9 percent, NOX emissions have 
declined 9 percent, and VOC emissions have declined by 12 percent from 
1999 to 2003. In eastern States affected by the Acid Rain Program, 
sulfates decreased 7 percent over the same period.
    Over the next 10 to 20 years, national and regional regulations 
will make major reductions in ambient PM2.5 levels. The 
Clean Air Interstate Rule (CAIR) and the NOX SIP Call will 
further reduce SO2 and NOX emissions from 
electric generating units and industrial boilers across the eastern 
half of the U.S.; regulations to implement the 1997 ambient air quality 
standards for PM2.5 will require direct PM2.5 and 
PM2.5 precursor controls in nonattainment areas; and new 
national mobile source regulations affecting heavy-duty diesel engines, 
highway vehicles, and other mobile sources will reduce emissions of 
NOX, direct PM2.5, SO2, and VOCs. The 
EPA estimates that these regulations for stationary and mobile sources 
will cut SO2 emissions by 6 million tons annually in 2015 
from 2001 levels. Emissions of NOX will be cut by 9 million 
tons annually in 2015 from 2001 levels. Emissions of VOCs will drop by 
3 million tons, and direct PM2.5 emissions will be cut by 
200,000 tons in 2015, compared to 2001 levels.
    In 2005, 39 nonattainment areas were designated as not attaining 
the PM2.5 standards established in 1997. SIPs for these 
areas are due in April 2008. Nonattainment areas are required to attain 
the standards as ``expeditiously as practicable'' based on 
implementation of federal measures already in place and the adoption of 
other reasonable control strategies for sources located in the 
nonattainment area and state. The presumptive timeframe for attainment 
is within five years of designation, although EPA may approve extended 
attainment dates of an additional one to five years for areas with more 
serious problems.
    Modeling done by EPA indicates that by 2010, 18 of the 39 currently 
designated nonattainment areas are projected to come into attainment 
with those standards just based on regulatory programs already in 
place, including CAIR, the Clean Diesel Rules, and other Federal 
measures. Between 2010 and 2015, further reductions in PM 
concentrations in the eastern U.S. are projected due to existing 
federal programs alone, on the order of 0.5 to 1.5 [mu]g/m\3\. All 
areas in the eastern U.S. will have lower PM2.5 
concentrations in 2015 relative to present-day conditions. In most 
cases, the predicted improvement in PM2.5 ranges from 10 
percent to 20 percent.

E. Summary of Proposed Revisions to the PM NAAQS

    For reasons discussed in the proposal, the Administrator proposed 
to revise the current primary and secondary PM2.5 and 
PM10 standards. With regard to the primary PM2.5 
standards, the Administrator proposed to revise the level of the 24-
hour PM2.5 standard to 35

[[Page 61150]]

[mu]g/m\3\, and to revise the form of the annual PM2.5 
standard by changing the constraints on the optional use of spatial 
averaging to include the criterion that the minimum correlation 
coefficient between monitor pairs to be averaged be 0.9 or greater, 
determined on a seasonal basis, and the criterion that differences 
between monitor values not exceed 10 percent. Related revisions for 
PM2.5 data handling conventions and for the reference method 
for monitoring PM as PM2.5 were also proposed.
    With regard to the primary PM10 standards, the 
Administrator proposed to revise the current standards to provide more 
targeted protection from thoracic coarse particles that are of concern 
to public health. In part, the Administrator proposed to establish a 
new indicator for thoracic coarse particles in terms of 
PM10-2.5, the definition of which included qualifications 
that identified both the mix of such particles that were provisionally 
determined to be of concern to public health, and were thus included in 
the indicator, and those for which currently available information was 
provisionally determined to be insufficient as a basis from which to 
infer a public health concern, and were thus excluded. More 
specifically, the proposed PM10-2.5 indicator was qualified 
so as to include any ambient mix of PM10-2.5 that is 
dominated by resuspended dust from high-density traffic on paved roads 
and PM generated by industrial sources and construction sources, and to 
exclude any ambient mix of PM10-2.5 that is dominated by 
rural windblown dust and soils and PM generated by agricultural and 
mining sources. The Administrator also proposed that agricultural 
sources, mining sources, and other similar sources of crustal material 
shall not be subject to control in meeting the proposed standard. The 
Administrator proposed to replace the current primary 24-hour 
PM10 standard with a 24-hour standard defined in terms of 
this new PM10-2.5 indicator. The proposed new standard would 
be met at an ambient air quality monitoring site when the 3-year 
average of the annual 98th percentile 24-hour average 
PM10-2.5 concentration is less than or equal to 70 [mu]g/
m\3\, which would generally maintain the degree of public health 
protection afforded by the current PM10 standards from 
short-term exposure to thoracic coarse particles of concern. 
Requirements for monitoring sites that would be appropriate for 
determining compliance with this proposed PM10-2.5 standard 
were included as part of proposed revisions to EPA's ambient air 
monitoring regulations (see 71 FR 2710, 2736-2728 and 71 FR 2706-2707 
(proposing to incorporate these requirements as part of the standard)). 
These proposed requirements included a five-part test for determining 
whether a potential monitoring site is suitable for comparison to the 
standard, all five parts of which had to be met. In summary, the 
suitability test included the following general provisions: a 
monitoring site must be within an urbanized area that has a population 
of at least 100,000 persons; the site must be within a block group with 
a population density greater than 500 people per square mile; the site 
must be a ``population-oriented'' site; the site may not be adjacent to 
a large emissions source or otherwise within the micro-scale 
environment affected by a large source; and, if the first four 
provisions are met, a site-specific assessment must show that the 
ambient mix of PM10-2.5 sampled at the site would be 
dominated by resuspended dust from high-density traffic on paved roads 
and PM generated by industrial sources and construction sources, and 
would not be dominated by rural windblown dust and soils and PM 
generated by agricultural and mining sources. Related new 
PM10-2.5 data handling conventions and a new reference 
method for monitoring PM as PM10-2.5 were also proposed. The 
Administrator also proposed to revoke and not replace the annual 
PM10 standard.
    With regard to the secondary PM2.5 and PM10 
standards, the Administrator proposed to revise the current standards 
by making them identical in all respects to the proposed primary 
PM2.5 and PM10-2.5 standards to address PM-
related welfare effects including visibility impairment, effects on 
vegetation and ecosystems, materials damage and soiling, and effects on 
climate change.

F. Organization and Approach to Final PM NAAQS Decisions

    This action presents the Administrator's final decisions on the 
review of the current primary and secondary PM2.5 and 
PM10 standards. Primary standards for fine particles and for 
thoracic coarse particles are addressed below in sections II and III, 
respectively. Consistent with the decisions made by EPA in the last 
review and with the conclusions in the Criteria Document and Staff 
Paper, fine and thoracic coarse particles continue to be considered as 
separate subclasses of PM pollution. Secondary standards for fine and 
thoracic coarse particles are addressed below in section IV. Related 
data handling conventions and federal reference methods for monitoring 
PM are addressed below in sections V and VI, respectively.
    Today's final decisions separately addressing fine and thoracic 
coarse particles are based on a thorough review in the Criteria 
Document of scientific information on known and potential human health 
and welfare effects associated with exposure to these subclasses of PM 
at levels typically found in the ambient air. These final decisions 
also take into account: (1) Staff assessments in the Staff Paper of the 
most policy-relevant information in the Criteria Document as well as a 
quantitative risk assessment based on that information; (2) CASAC 
advice and recommendations, as reflected in its letters to the 
Administrator, its discussions of drafts of the Criteria Document and 
Staff Paper at public meetings, and separate written comments prepared 
by individual members of the CASAC PM Review Panel \6\ (henceforth, 
``CASAC Panel''); (3) public comments received during the development 
of these documents, either in connection with CASAC meetings or 
separately; and (4) extensive public comments received on the proposed 
rulemaking.
---------------------------------------------------------------------------

    \6\ The CASAC PM Review Panel is comprised of the seven members 
of the chartered CASAC, supplemented by fifteen subject-matter 
experts appointed by the Administrator to provide additional 
scientific expertise relevant to this review of the PM NAAQS.
---------------------------------------------------------------------------

II. Rationale for Final Decisions on Primary PM2.5 Standards

A. Introduction

1. Overview
    This section presents the Administrator's final decisions regarding 
the need to revise the current primary PM2.5 NAAQS, and, 
more specifically, regarding revisions to the level of the 24-hour 
standard and to the form of the annual standard. As discussed more 
fully below, the rationale for the final decision on appropriate 
revisions to the primary PM2.5 NAAQS includes consideration 
of: (1) Evidence of health effects related to short- and long-term 
exposures to fine particles; (2) insights gained from a quantitative 
risk assessment; and (3) specific conclusions regarding the need for 
revisions to the current standards and the elements of PM2.5 
standards (i.e., indicator, averaging time, form, and level) that, 
taken together, are requisite to protect public health with an adequate 
margin of safety.
    In developing this rationale, EPA has drawn upon an integrative 
synthesis of the entire body of evidence on associations between 
exposure to

[[Page 61151]]

ambient fine particles and a broad range of health endpoints (EPA, 
2004a, Chapter 9), focusing on those health endpoints for which the 
Criteria Document concluded that the associations are likely to be 
causal. This body of evidence includes hundreds of studies conducted in 
many countries around the world, using various indicators of fine 
particles. In its assessment of the evidence judged to be most relevant 
to decisions on elements of the primary PM2.5 standards, EPA 
has placed greater weight on U.S. and Canadian studies using 
PM2.5 measurements, since studies conducted in other 
countries may well reflect different demographic and air pollution 
characteristics.
    As with virtually any policy-relevant scientific research, there is 
uncertainty in the characterization of health effects attributable to 
exposure to ambient fine particles, most generally with regard to 
whether observed associations are likely causal in nature and, if so, 
whether there are exposure levels below which such associations are no 
longer likely. As discussed below, an unprecedented amount of new 
research has been conducted since the last review, with important new 
information coming from epidemiologic, toxicologic, controlled human 
exposure, and dosimetric studies. Moreover, the newly available 
research studies evaluated in the Criteria Document have undergone 
intensive scrutiny through multiple layers of peer review, with 
extended opportunities for review and comment by CASAC and the public. 
While important uncertainties remain, the review of the health effects 
information has been extensive and deliberate. In the judgment of the 
Administrator, this intensive evaluation of the scientific evidence 
provides an adequate basis for regulatory decision making at this time. 
This review also provides important input to EPA's research plan for 
improving our future understanding of the relationships between 
exposures to ambient fine particles and health effects.
    The health effects information and quantitative risk assessment 
were summarized in sections II.A and II.B of the proposal (71 FR 2626-
2641) and are only briefly outlined below in sections II.A.2 and 
II.A.3. 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 it is appropriate to 
revise the current primary PM2.5 standards (section II.B), 
as well as a more complete discussion of the Administrator's rationale 
for retaining or revising the specific elements of the primary 
PM2.5 standards, namely the indicator (section II.C); 
averaging time (section II.D); form (section II.E); and level (section 
II.F). A summary of the final decisions on revisions to the primary 
PM2.5 standards is presented in section II.G.
2. Overview of Heath Effects Evidence
    This section briefly outlines the information presented in Section 
II.A of the proposal on the health effects associated with exposure to 
fine particles. As was true in the last review, evidence from 
epidemiologic studies plays a key role in the Criteria Document's 
evaluation of the scientific evidence. Some highlights of the new 
epidemiologic evidence available since the last review include:
    (1) New multi-city studies that use uniform methodologies to 
investigate the effects of various indicators of PM on health with data 
from multiple locations with varying climate and air pollution mixes, 
contributing to increased understanding of the role of various 
potential confounders, including gaseous co-pollutants, on observed 
associations with fine particles. These studies provide more precise 
estimates of the magnitude of an effect of exposure to PM, including 
fine particles, than most smaller-scale individual city studies.
    (2) More studies of various health endpoints evaluating 
associations between effects and exposures to fine particles and 
thoracic coarse particles (discussed below in section III), as well as 
ultrafine particles or specific components (e.g., sulfates, nitrates, 
metals, organic compounds, and elemental carbon) of fine particles.
    (3) Numerous studies of cardiovascular endpoints, with particular 
emphasis on assessment of cardiovascular risk factors or physiological 
changes.
    (4) Studies relating population exposure to fine particles and 
other pollutants measured at centrally located monitors to estimates of 
exposure to ambient pollutants at the individual level. Such studies 
have led to a better understanding of the relationship between ambient 
fine particle levels and personal exposures to fine particles of 
ambient origin.
    (5) New statistical approaches to addressing issues related to 
potential confounding by gaseous co-pollutants, possible thresholds for 
effects, and measurement error and exposure misclassification.\7\
---------------------------------------------------------------------------

    \7\ ``Confounding'' occurs when a health effect that is caused 
by one risk factor is attributed to another variable that is 
correlated with the causal risk factor; epidemiologic analyses 
attempt to adjust or control for potential confounders (EPA, 2004a, 
section 8.1.3.2; EPA, 2005, section 3.6.4). A ``threshold'' is a 
concentration below which it is expected that effects are not 
observed (EPA, 2004a, section 8.4.7; EPA, 2005, section 3.6.6). 
``Gaseous co-pollutants'' generally refer to other commonly-
occurring air pollutants, specifically O3, CO, 
SO2 and NO2. ``Measurement error'' refers to 
uncertainty in the air quality measurements, while ``exposure 
misclassification'' includes uncertainty in the use of ambient 
pollutant measurements in characterizing population exposures to PM 
(EPA, 2004a, section 8.4.5; EPA, 2005, section 3.6.2)
---------------------------------------------------------------------------

    (6) Efforts to evaluate the effects of fine particles from 
different sources (e.g., motor vehicles, coal combustion, vegetative 
burning, crustal \8\), using factor analysis or source apportionment 
methods with fine particle speciation data.
---------------------------------------------------------------------------

    \8\ ``Crustal'' is used here to describe particles of geologic 
origin, which can be found in both fine- and coarse-fraction PM.
---------------------------------------------------------------------------

    (7) New ``intervention studies'' providing evidence for 
improvements in respiratory or cardiovascular health with reductions in 
ambient concentrations of particles and gaseous co-pollutants.
    In addition, the body of evidence on PM-related effects has greatly 
expanded since the last review with findings from studies of potential 
mechanisms or pathways by which particles may result in the effects 
identified in the epidemiologic studies. These studies include 
important new dosimetry, toxicologic and controlled human exposure 
studies, as highlighted below.
    (8) Animal and controlled human exposure studies using concentrated 
ambient particles (CAPs), new indicators of response (e.g., C-reactive 
protein and cytokine levels, heart rate variability), and animal models 
simulating sensitive human subpopulations. The results of these studies 
are relevant to evaluation of plausibility of the epidemiologic 
evidence and provide insights into potential mechanisms for PM-related 
effects.
    (9) Dosimetry studies using new modeling methods that provide 
increased understanding of the dosimetry of different particle size 
classes and in members of potentially sensitive subpopulations, such as 
people with chronic respiratory disease.
    Section II.A of the proposal provides a detailed summary of key 
information contained in the Criteria Document (EPA, 2004a, Chapters 6-
9), and in the Staff Paper (EPA, 2005, Chapter 3), on the known and 
potential effects associated with exposure to fine particles including 
information on specific constituents and information on the effects of 
fine particles in combination with other pollutants that are routinely 
present in the ambient air

[[Page 61152]]

(71 FR 2626-2637). The information highlighted there summarizes:
    (1) Multiple biologic mechanisms that may be responsible for 
morbidity/mortality effects associated with exposure to ambient fine 
particles, including potential mechanisms or pathways related to direct 
effects on the respiratory system, systemic effects that are secondary 
to effects in the respiratory system including cardiovascular effects, 
or direct cardiovascular effects.
    (2) The nature of the effects that have been reported to be 
associated with fine particle exposures including premature mortality, 
aggravation of respiratory and cardiovascular disease (as indicated by 
increased hospital admissions and emergency department visits), changes 
in lung function and increased respiratory symptoms, as well as new 
evidence for more subtle indicators of cardiovascular health.
    (3) An integrated evaluation of the health effects evidence, with 
emphasis on key issues raised in interpreting epidemiological studies, 
along with supporting evidence from experimental (e.g., dosimetric and 
toxicologic) studies.
    (4) Sensitive or vulnerable subpopulations that appear to be at 
greater risk to such effects, including individuals with pre-existing 
heart and lung diseases, older adults, and children.
    (5) Conclusions, based on the magnitude of these subpopulations and 
risks identified in health studies, that exposure to ambient fine 
particles can have substantial public health impacts.
3. Overview of Quantitative Risk Assessment
    In addition to a comprehensive evaluation of the health effects 
evidence available in this review, EPA conducted a quantitative health 
risk assessment for selected health effects to provide additional 
information and insights that can help inform decision making on the 
NAAQS, while recognizing the limitations of such an assessment.\9\ As 
discussed in section II.B of the proposal, the approach used to develop 
quantitative risk estimates associated with exposures to 
PM2.5 was built upon the more limited risk assessment 
conducted during the last review (61 FR 65650). The expanded and 
updated assessment conducted in this review included estimates of risks 
of mortality (total non-accidental, cardiovascular, and respiratory), 
morbidity (hospital admissions for cardiovascular and respiratory 
causes), and respiratory symptoms (not requiring hospitalization) 
associated with recent short-term (daily) ambient PM2.5 
levels and risks of total, cardiopulmonary, and lung cancer mortality 
associated with long-term exposure to PM2.5 in a number of 
example urban areas.\10\
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    \9\ The EPA continues to support the development and application 
of risk assessment methods with the goal of improving the 
characterization of risks and the communication of uncertainties in 
such risk estimates.
    \10\ The risk assessment was discussed in the Staff Paper (EPA, 
2005, chapter 4) and presented more fully in a technical support 
document, Particulate Matter Health Risk Assessment for Selected 
Urban Areas (Abt Associates, 2005). The assessment scope and 
methodology were developed with considerable input from the CASAC 
Panel and the public, with CASAC concluding that the general 
assessment methodology and framework were appropriate (Hopke, 2002).
---------------------------------------------------------------------------

    The EPA recognized that there were many sources of uncertainty and 
variability inherent in the inputs to this assessment and that there 
was a high degree of uncertainty in the resulting PM2.5 risk 
estimates. Such uncertainties generally relate to a lack of clear 
understanding of a number of important factors, including, for example, 
the shape of concentration-response functions, particularly when, as 
here, effect thresholds can neither be discerned nor determined not to 
exist; issues related to selection of appropriate statistical models 
for the analysis of the epidemiologic data; the role of potentially 
confounding and modifying factors in the concentration-response 
relationships; issues related to simulating how PM2.5 air 
quality distributions will likely change in any given area upon 
attaining a particular standard, since strategies to reduce emissions 
are not yet defined; and whether there would be differential reductions 
in the many components within PM2.5 and, if so, whether this 
would result in differential reductions in risk. While some of these 
uncertainties were addressed quantitatively in the form of estimated 
confidence ranges around central risk estimates, other uncertainties 
and the variability in key inputs were not reflected in these 
confidence ranges, but rather were addressed through separate 
sensitivity analyses or characterized qualitatively.
    The concentration-response relationships used in the assessment 
were based on findings from human epidemiological studies that relied 
on fixed-site, population-oriented, ambient monitors as a surrogate for 
actual ambient PM2.5 exposures. The risk assessment included 
a series of base case estimates that, for example, included various 
cutpoints intended as surrogates for alternative assumed population 
thresholds. In its review of the Staff Paper and risk assessment, the 
CASAC Panel commented that for the purpose of estimating public health 
impacts, it ``favored the primary use of an assumed threshold of 10 
[mu]g/m\3\ '' and that ``a major research need is for more work to 
determine the existence and level of any thresholds that may exist or 
the shape of nonlinear concentration-response curves at low levels of 
exposure that may exist'' (Henderson, 2005a). Other uncertainties were 
addressed in various sensitivity analyses (e.g., the use of single-
versus multi-pollutant models, use of single-versus multi-city models, 
use of a distributed lag model) and had a more moderate and often 
variable impact on the risk estimates in some or all of the cities.
    Key observations and insights from the PM2.5 risk 
assessment, together with important caveats and limitations, were 
discussed in section II.B of the proposal. In general, estimated risk 
reductions associated with going from just meeting the current suite of 
PM2.5 standards to just meeting alternative suites of annual 
and 24-hour standards for all the various assumed cutpoints show 
patterns of increasing estimated risk reductions as either the annual 
or 24-hour standard, or both, were reduced over the range considered in 
this assessment, and the estimated percentage reductions in risk were 
strongly influenced by the assumed cutpoint level (see EPA, 2005, 
Figures 5-1, 5-2, 5A-1, and 5A-2). In comparing the risk estimates for 
the only two specific locations that were included in both the prior 
and current assessments, the magnitude of the estimates associated with 
just meeting the current annual standard, in terms of percentage of 
total incidence, were very similar for mortality associated with long-
term exposures. Current risk estimates for just meeting the current 
suite of PM2.5 standards were similar in one of the 
locations (Philadelphia) and somewhat lower in the other location (Los 
Angeles) for mortality associated with short-term exposures.

B. Need for Revision of the Current Primary PM2.5 Standards

1. Introduction
    The initial issue to be addressed in the current review of the 
primary PM2.5 standards is whether, in view of the advances 
in scientific knowledge reflected in the Criteria Document and Staff 
Paper, the existing standards should be revised. As discussed in 
section II.A of the proposal (71 FR 2625-2637), the Staff Paper 
concluded, based on the information and

[[Page 61153]]

conclusions presented in the Criteria Document, that while important 
uncertainties and research questions remain, much progress has been 
made since the last review in reducing some key uncertainties related 
to our understanding of the scientific evidence. The newly available 
information generally reinforces and provides increased confidence in 
the likely causal nature of the associations between short- and long-
term exposure to PM2.5 and mortality and morbidity effects 
observed in the last review, and provides additional information to 
inform judgments as to the extent to which such associations likely 
remain at lower exposure levels within the range of ambient air 
quality.
    The examination of short- and long-term exposures to specific 
components, properties, and sources of fine particles and mixtures of 
fine particles with gaseous co-pollutants that are linked with health 
effects, and the biological mechanisms underlying the observed 
linkages, remain important research needs. Other important research 
needs include better characterizing the shape of concentration-response 
functions, including identification of potential threshold levels, and 
methodological issues such as those associated with selecting 
appropriate statistical models in time-series studies to address time-
varying factors (such as weather) and other factors (such as other 
pollution variables), and better characterizing population exposures.
    Nonetheless, important progress has been made in advancing our 
understanding of potential mechanisms by which ambient 
PM2.5, alone and in combination with other pollutants, is 
causally linked with cardiovascular, respiratory, and lung cancer 
associations observed in epidemiologic studies. Due to reanalyses and 
extensions of key long-term exposure studies, there is now greater 
confidence in the causal nature of associations with long-term 
exposures to fine particles than in the last review. There is also an 
increased understanding of the populations that are the most 
susceptible to PM2.5-related effects. In addition, health 
effect associations reported in epidemiologic studies have been found 
to be generally robust to confounding by co-pollutants, especially for 
the more numerous short-term exposure studies. Further, while groups of 
commenters had differing views on the extent to which, if at all, newly 
available evidence increases confidence in associations between 
PM2.5 and mortality and morbidity effects, and on the extent 
of progress that has been made in reducing uncertainties since the last 
review, virtually no commenters argued for any relaxation of the 
current PM2.5 standards. Based on these considerations, EPA 
finds that overall the available evidence has increased the scientific 
basis supporting the health impacts of exposure to PM2.5, 
and not lessened it, providing clear support for fine particle 
standards that are at least as protective as the current 
PM2.5 standards.
    Having reached this initial conclusion, EPA addresses the question 
whether the available evidence supports consideration of standards that 
are more protective than the current PM2.5 standards. In 
considering this question, EPA first notes that the current standards 
were set as a suite that together would most effectively and 
efficiently protect the public against health effects related to both 
short- and long-term exposures to fine particles (62 FR at 38669). In 
so doing, the Agency set the annual standard to be the ``generally 
controlling'' standard for lowering both short- and long-term 
PM2.5 concentrations. In conjunction with such an annual 
standard, the current 24-hour standard was set to provide only 
supplemental protection against days with high peak PM2.5 
concentrations, localized ``hotspots,'' or risks arising from seasonal 
emissions that might not be well controlled by a national annual 
standard. As discussed below in section II.F, in considering what 
evidence to use as the basis for the 1997 annual standard, EPA placed 
greater emphasis on the short-term exposure studies, which were judged 
to be the strongest evidence at that time. The long-term exposure 
studies available at that time provided only supporting evidence for 
the annual standard, which was set primarily based on short-term 
exposure studies.
    In addressing the question whether the evidence now available in 
this review supports consideration of standards that are more 
protective than the current PM2.5 standards, the Staff Paper 
considered whether (1) statistically significant health effects 
associations with short-term exposures to fine particles occur in areas 
that would likely meet the current PM2.5 standards, or (2) 
associations with long-term exposures to fine particles extend down to 
lower air quality levels than had previously been observed.\11\
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    \11\ In addressing this question, the Criteria Document had 
recognized that although there are likely biologic threshold levels 
in individuals for specific health responses, the available 
epidemiologic evidence neither supports nor refutes the existence of 
thresholds at the population level for the effects of 
PM2.5 on mortality across the range of concentrations in 
the studies, for either long-term or short-term PM2.5 
exposures (EPA, 2004a, section 9.2.2.5).
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    In considering the available epidemiologic evidence in this review 
to address the question of whether more protective standards should be 
considered, the Staff Paper took a broader approach than was used in 
the last review. This approach reflects the more extensive and stronger 
body of evidence now available on health effects related to both short- 
and long-term exposure to PM2.5, and places relatively 
greater emphasis on evidence from long-term exposure studies than was 
done in the last review. As discussed below in section II.F, this 
broader approach was used at the time of proposal to consider the much 
expanded body of evidence from short-term exposure studies as the 
principal basis for setting the 24-hour standard to protect against 
health effects associated with short-term exposures to 
PM2.5, and to consider the stronger and more robust body of 
evidence from long-term exposure PM2.5 studies as the 
principal basis for setting the annual standard to protect against 
health effects associated with long-term exposures to PM2.5.
    In first considering whether areas in which short-term exposure 
studies have been conducted would likely meet the current 
PM2.5 standards, the focus is principally on comparing the 
long-term average PM2.5 concentration in a study area with 
the level of the current ``generally controlling'' annual 
PM2.5 standard. In considering the available epidemiologic 
evidence related to short-term exposures, the Staff Paper focused on 
specific epidemiologic studies that show statistically significant 
associations between PM2.5 and health effects for which the 
Criteria Document judged associations with PM2.5 to be 
likely causal (EPA, 2005, section 5.3.1.1). Many more U.S. and Canadian 
studies are now available that provide evidence of associations between 
short-term exposure to PM2.5 and serious health effects in 
areas with air quality at and above the level of the current annual 
PM2.5 standard (15 [mu]g/m3). Moreover, a few 
newly available short-term exposure mortality studies provide evidence 
of statistically significant associations with PM2.5 in 
areas with air quality levels below the levels of the current 
PM2.5 standards. In considering these studies, the Staff 
Paper focused on those that include adequate gravimetric 
PM2.5 mass measurements, and noted where the associations 
are generally robust to alternative model specification and to the 
inclusion of potentially confounding co-pollutants. Three

[[Page 61154]]

studies, conducted in Phoenix (Mar et al., 2003), Santa Clara County, 
CA (Fairley, 2003) and eight Canadian cities (Burnett and Goldberg, 
2003), report statistically significant associations between short-term 
PM2.5 exposure and total or cardiovascular mortality in 
areas in which long-term average PM2.5 concentrations ranged 
between 13 and 14 [mu]g/m\3\ and 98th percentile 24-hour concentrations 
ranged between 32 and 59 [mu]g/m\3\.\12\
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    \12\ As noted in the Staff Paper, these studies were reanalyzed 
to address questions about the application of the statistical 
software used in the original analyses, and the study results from 
Phoenix and Santa Clara County were little changed in alternative 
models (Mar et al., 2003; Fairley, 2003), although Burnett and 
Goldberg (2003) reported that their results were sensitive to using 
different temporal smoothing methods. Two of these studies also 
reported significant associations with gaseous pollutants (Mar et 
al., 2003; Fairley, 2003), and one of these studies included multi-
pollutant model results in reanalyses, reporting that associations 
with PM2.5 remained significant with gaseous pollutants 
(Fairley, 2003). The 98th percentile 24-hour concentrations were 
approximately 59 [mu]g/m\3\ in Fairley et al. (2003), 39 [mu]g/m\3\ 
in Burnett and Goldberg (2003), and 32 [mu]g/m\3\ in Mar et al. 
(2003).
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    In also considering the new epidemiologic evidence available from 
U.S. and Canadian studies of long-term exposure to fine particles, the 
Criteria Document noted that new studies have built upon studies 
available in the last review and concluded that these studies have 
confirmed and strengthened the evidence of associations for both 
mortality and respiratory morbidity (EPA, 2004a, section 9.2.3). For 
mortality, the Criteria Document placed greatest weight on the 
reanalyses and extensions of the Six Cities and ACS studies, finding 
that these studies provide strong evidence for associations with fine 
particles (EPA, 2004a, p. 9-34), notwithstanding the lack of consistent 
results in other long-term exposure studies. For morbidity, the 
Criteria Document found that new studies of a cohort of children in 
Southern California have built upon earlier limited evidence to provide 
fairly strong evidence that long-term exposure to fine particles is 
associated with development of chronic respiratory disease and reduced 
lung function growth (EPA, 2004a, pp. 9-33 to 9-34). In addition to 
strengthening the evidence of association, the new extended ACS 
mortality study (Pope et al., 2002) observed statistically significant 
associations with cardiorespiratory mortality (including lung cancer 
mortality) across a range of long-term mean PM2.5 
concentrations that was lower than was reported in the original ACS 
study available in the last review.
    Beyond the epidemiologic studies using PM2.5 as an 
indicator of fine particles, a large body of newly available evidence 
from studies that used PM10 in areas where fine particles 
would likely dominate this measurement, as well as other indicators or 
components of fine particles (e.g., sulfates, combustion-related 
components), provides additional support for the conclusions reached in 
the last review as to the likely causal role of ambient PM, and the 
likely importance of fine particles in contributing to observed health 
effects. Such studies notably include new multi-city studies, 
intervention studies (that relate reductions in ambient PM to observed 
improvements in respiratory or cardiovascular health), and source-
oriented studies (e.g., suggesting associations with combustion- and 
vehicle-related sources of fine particles). The Criteria Document also 
noted that new epidemiologic studies of asthma-related increased 
physician visits and symptoms, as well as new studies of cardiac-
related risk factors, suggest likely much larger public health impacts 
due to ambient fine particles than just those indexed by the mortality 
and morbidity effects considered in the last review (EPA, 2004a, p. 9-
94).
    In reviewing this information, the Staff Paper recognized that 
important limitations and uncertainties associated with this expanded 
body of evidence for PM2.5 and other indicators or 
components of fine particles need to be carefully considered in 
determining the weight to be placed on the body of studies available in 
this review. For example, the Criteria Document noted that although PM-
effects associations continue to be observed across most new studies, 
the newer findings do not fully resolve the extent to which the 
associations are properly attributed to PM acting alone or in 
combination with other gaseous co-pollutants or to the gaseous co-
pollutants themselves. The Criteria Document concluded, however, that 
overall the newly available epidemiologic evidence, especially for the 
more numerous short-term exposure studies, substantiates that 
associations for various PM indicators with mortality and morbidity are 
robust to confounding by co-pollutants (EPA, 2004a, p. 9-37).
    While the limitations and uncertainties in the available evidence 
suggest caution in interpreting the epidemiologic studies at the lower 
levels of air quality observed in the studies, the Staff Paper 
concluded that the evidence now available provides strong support for 
considering fine particle standards that would provide increased 
protection beyond that afforded by the current PM2.5 
standards. The Staff Paper noted that a more protective suite of 
PM2.5 standards would reflect the generally stronger and 
broader body of evidence of associations with mortality and morbidity 
now available in this review, both in short-term exposue studies at 
levels below the current standards and in long-term exposure studies 
that extend to lower levels of air quality than in earlier studies, as 
well as increased understanding of possible underlying mechanisms.
    In addition to this evidence-based evaluation, the Staff Paper also 
considered the extent to which health risks estimated to occur upon 
attainment of the current PM2.5 standards may be judged to 
be important from a public health perspective, taking into account key 
uncertainties associated with the quantitative health risk estimates, 
noted above in section II.A.3. In so doing, the Staff Paper first noted 
that the risk assessment addressed several key uncertainties through 
various base case analyses, as well as through sensitivity analyses, as 
noted above in section II.A.3 and discussed in section II.B of the 
proposal (71 FR 2637-2641). In considering the health risks estimated 
to occur upon attainment of the current PM2.5 standards, the 
Staff Paper focused in particular on a series of base case risk 
estimates, while recognizing that the confidence ranges in the selected 
base case estimates do not reflect all the identified uncertainties. 
These risks were estimated using not only the linear or log-linear 
concentration-response functions reported in the studies,\13\ but also 
using alternative modified linear functions as surrogates for assumed 
non-linear functions that would reflect the possibility that thresholds 
may exist in the reported associations within the range of air quality 
observed in the studies. Regardless of the relative weight placed on 
the risk estimates associated with the concentration-response functions 
reported in the studies or with the modified functions favored by CASAC 
(discussed above in section II.A.3), the risk assessment indicated the 
possibility that thousands of premature deaths per year would occur in 
urban areas across the U.S. upon attainment of the current 
PM2.5

[[Page 61155]]

standards.\14\ Beyond the estimated incidences of premature mortality, 
the Staff Paper also recognized that similarly substantial numbers of 
incidences of hospital admissions, emergency room visits, aggravation 
of asthma and other respiratory symptoms, and increased cardiac-related 
risk are also likely in many urban areas, based on risk assessment 
results (EPA, 2005, Chapter 4) and on the discussion related to this 
``pyramid of effects'' in the Criteria Document (EPA, 2004a, section 
9.2.5). Based on these considerations, the Staff Paper concluded that 
the estimates of risks likely to remain upon attainment of the current 
PM2.5 standards are indicative of risks that can reasonably 
be judged to be important from a public health perspective (EPA, 2005, 
section 5.3.1.).
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    \13\ As noted in section II.B of the proposal, the reported 
linear or log-linear concentration-response functions were applied 
down to 7.5 [mu]g/m\3\ in estimating risk associated with long-term 
exposure (i.e., the lowest measured level in the extended ACS 
study), and down to the estimated policy-relevant background level 
in estimating risk associated with short-term exposure (i.e., 3.5 
[mu]g/m\3\ for eastern urban areas and 2.5 [mu]g/m\3\ for western 
urban areas).
    \14\ The Staff Paper recognized how highly dependent any 
specific risk estimates are on the assumed shape of the underlying 
concentration-response functions, noting nonetheless that mortality 
risks are not completely eliminated when current PM2.5 
standards are met in a number of example urban areas even using the 
highest assumed cutpoint levels considered in the risk assessment 
(EPA, 2005, p. 5-15).
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    In considering available evidence, risk estimates, and related 
limitations and uncertainties, the Staff Paper concluded that the 
available information clearly calls into question the adequacy of the 
current suite of PM2.5 standards and provides strong support 
for revising the current suite of PM2.5 standards to provide 
increased public health protection. Also, taking into account these 
considerations, the CASAC advised the Administrator that a majority of 
CASAC Panel members were in agreement that the primary 24-hour and 
annual PM2.5 standards ``should be modified to provide 
increased public health protection'' (Henderson, 2005a). The CASAC 
further advised that changes to either the annual standard or the 24-
hour standard, or both, could be recommended, and expressed reasons 
that formed the basis for the consensus among the Panel members for 
placing more emphasis on lowering the 24-hour standard (Henderson, 
2005a).\15\
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    \15\ Of the individual Panel members who submitted written 
comments expressing views on appropriate levels of the 
PM2.5 standards, only one did not support changes to 
either the 24-hour or annual standard to provide additional public 
health protection (Henderson, 2005a).
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    At the time of proposal, in considering whether the suite of 
PM2.5 standards should be revised to provide requisite 
public health protection, the Administrator carefully considered the 
rationale and recommendations contained in the Staff Paper, the advice 
and recommendations from CASAC, and public comments to date on this 
issue. In so doing, the Administrator placed primary consideration on 
the evidence obtained from the studies, and provisionally found the 
evidence of serious health effects reported in short-term exposure 
studies conducted in areas that would attain the current standards to 
be compelling, especially in light of the extent to which such studies 
are part of an overall pattern of positive and frequently statistically 
significant associations across a broad range of studies that 
collectively represent a strong and robust body of evidence. As 
discussed in the Criteria Document and Staff Paper, the Administrator 
recognized that much progress has been made since the last review in 
addressing some of the key uncertainties that were important 
considerations in establishing the current suite of PM2.5 
standards. For example, progress made since the last review provides 
increased confidence in the long-term exposure studies as a basis for 
considering whether any revision of the annual standard is appropriate 
and increased confidence in the short-term exposure studies as a basis 
for considering whether any revision of the 24-hour standard is 
appropriate.\16\ In considering the risk assessment presented in the 
Staff Paper, the Administrator noted that the assessment contained a 
sensitivity analysis but not a formal uncertainty analysis, making it 
difficult to use the risk assessment to form a judgment of the 
probability of various risk estimates. Instead, the Administrator 
viewed the risk assessment in light of his evaluation of the underlying 
studies. Seen in this light, the risk assessment informs the 
determination of the public health significance of risks to the extent 
that the evidence is judged to support an effect at a particular level 
of air quality. Based on these considerations, the Administrator 
provisionally concluded that the current PM2.5 standards, 
taken together, are not requisite to protect public health with an 
adequate margin of safety and that revision is needed to provide 
increased public health protection.
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    \16\ The EPA notes that this increased confidence in the long- 
and short-term associations generally reflects less uncertainty as 
to the likely causal nature of such associations, but does not 
address directly the question of the extent to which such 
associations remain toward the lower end of the range of ambient 
PM2.5 concentrations. This question is central to the 
Agency's evaluation of the relevant evidence to determine 
appropriate standards levels, as discussed below in section II.F.
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2. Comments on the Need for Revision

    General comments based on relevant factors that either support or 
oppose any change to the current suite of PM2.5 primary 
standards are addressed in this section. Comments on specific short- 
and long-term exposure studies that relate to consideration of the 
appropriate levels of the 24-hour and annual PM2.5 standards 
are addressed below in sections II.F.1 and II.F.2, respectively. 
General comments based on implementation-related factors that are not a 
permissible basis for considering the need to revise the current 
standards are addressed in the Response to Comments document.
    Many public comments received on the proposal asserted that the 
current PM2.5 standards are insufficient to protect public 
health with an adequate margin of safety and revisions to the standards 
are appropriate. Among those calling for revisions to the current 
standards are medical groups, including the American Medical 
Association, the American Thoracic Society, the American Academy of 
Pediatrics, and the American College of Cardiology, as well as medical 
doctors and academic researchers. For example, the American Medical 
Association stated that PM air pollution is ``a national public health 
problem'' and supported more stringent standards based on studies that 
provide evidence of associations between PM2.5 and serious 
health effects in areas with PM2.5 concentrations that are 
below the 1997 standards. Other medical associations offered the 
following views in support of more protective standards:

As professional organizations that represent physicians treating 
patients with diseases either caused by or exacerbated by air 
pollution, we are keenly aware of the impact air quality has on the 
individual health of our patients. As such we are committed to 
supporting a standard for PM that is protective of the health of 
vulnerable populations including children, seniors and patients with 
respiratory and cardiac conditions * * *. In short, a significant 
body of research has described potential mechanisms for and the 
range of health effects caused by PM air pollution. The undersigned 
physician organizations find the body of scientific evidence to be 
rigorous, comprehensive and compelling enough to justify a 
significant tightening of the existing NAAQS PM standards. [American 
Thoracic Society et al.]

    In a letter signed from environmental health researchers and 
physicians, similar conclusions were drawn:

More than 2,000 peer-reviewed studies have been published since 1996 
* * *. These studies, as discussed and interpreted in the 2004 EPA 
Criteria Document, validate earlier epidemiologic studies linking 
both acute and chronic fine particle pollution with serious 
morbidity and mortality. The newer research has also expanded the 
list of health effects associated with PM, and has identified health 
effects at lower exposure levels than

[[Page 61156]]

previously reported. In fact, the science is now sufficiently strong 
that it is appropriate to conclude that PM2.5 is causally 
associated with numerous adverse health effects in humans, at 
exposure levels far below the current standards. [Schwartz et al., 
2005]

    Similar conclusions were also reached in comments by many national, 
state, and local public health organizations, including, for example, 
the American Lung Association, the American Heart Association, the 
American Cancer Society, the American Public Health Association, and 
the National Association of Local Boards of Health, as well as in 
letters to the Administrator from EPA's advisory panel on children's 
environmental health (Children's Health Protection Advisory Committee, 
2005, 2006). All of these medical and public health commenters stated 
that the current PM2.5 standards need to be revised, and 
that even more protective standards than those proposed by EPA are 
needed to protect the health of sensitive population groups. Many 
individual commenters also expressed such views.
    State and local air pollution control authorities who commented on 
the PM2.5 standards supported revision of the suite of 
current PM2.5 standards, as did the National Tribal Air 
Association. The State and Territorial Air Pollution Program 
Administrators and the Association of Local Air Pollution Control 
Officials (STAPPA/ALAPCO) urged that EPA revise the PM2.5 
standards in accordance with the recommendations of CASAC. Each of the 
individual State environmental/public health agencies that commented on 
the PM2.5 standards supported revisions to the current 
standards, with most supporting standards consistent with CASAC's 
recommendations. The Northeast States for Coordinated Air Use 
Management (NESCAUM) argued for even more stringent revisions to the 
standards.
    The commenters noted above primarily based their views on the body 
of evidence assessed in the Criteria Document, finding it to be 
stronger and more compelling than in the last review. These commenters 
generally placed much weight on CASAC's interpretation of the body of 
available evidence and the results of EPA's risk assessment, both of 
which formed the basis for CASAC's recommendation to revise the 
PM2.5 standards to provide increased public health 
protection was based.
    Some of these commenters specifically mentioned the independent 
reanalysis of the original ACS and Six Cities long-term exposure 
studies conducted by HEI (Krewski et al., 2000) that concluded that the 
original data were of high quality, the original results could be fully 
replicated, and the results were robust to alternative model 
specifications. Some also mentioned the ACS extended study (Pope et 
al., 2002) and the Southern California children's cohort study 
(Gauderman et al., 2002) as providing evidence of mortality and 
morbidity effects associated with long-term exposures to 
PM2.5 at lower levels than had previously been studied. A 
number of short-term exposure studies were also cited by some of these 
commenters as providing evidence of mortality and morbidity effects at 
levels well below the level of the current 24-hour PM2.5 
standard. In addition, many of these commenters generally concluded 
that progress had been made in reducing many of the uncertainties 
identified in the last review and in better understanding mechanisms by 
which PM2.5 may be causing the observed health effects.
    Some of these commenters also noted the results of EPA's risk 
assessment, concluding that it showed that the risks estimated to 
remain when the current standards are met are large and important from 
a public health perspective and warrant increased protection. Some of 
these commenters expressed the view that PM2.5-related risks 
are likely larger than those estimated in EPA's risk assessment, in 
part because EPA based its risk assessment on the ACS extended study 
which had greater exposure measurement error than other studies, 
leading to an underestimate of the relative risk, and because EPA 
incorporated an assumed ``cutpoint'' in its assessment that is not 
supported by studies that find no evidence of a threshold.
    In general, all of these commenters agreed on the importance of 
results from the large body of scientific studies reviewed in the 
Criteria Document and on the need to revise the suite of 
PM2.5 standards as articulated in EPA's proposal, while 
generally differing with EPA's proposed judgments about the extent to 
which the standards should be revised based on this evidence. The EPA 
generally agrees with these commenters' conclusion regarding the need 
to revise the current suite of PM2.5 standards. The 
scientific evidence noted by these commenters was generally the same as 
that assessed in the Criteria Document and the Staff Paper, and EPA 
agrees that this evidence provides a basis for concluding that the 
current PM2.5 standards, taken together, are not adequately 
protective of public health. For reasons discussed below in section 
II.F, however, EPA disagrees with aspects of these commenters' views on 
the level of protection that is appropriate and supported by the 
available scientific information.
    Some of these commenters also identified ``new'' studies that were 
not included in the Criteria Document as providing further support for 
the need to revise the PM2.5 standards. As discussed above 
in section I.C, EPA notes that, as in past NAAQS reviews, the Agency is 
basing the final decisions in this review on the studies and related 
information included in the PM air quality criteria that have undergone 
CASAC and public review, and will consider the newly published studies 
for purposes of decision making in the next PM NAAQS review. 
Nonetheless, in provisionally evaluating commenters' arguments (see 
Response to Comments document), EPA notes that its provisional 
assessment of ``new'' science found that such studies did not 
materially change the conclusions in the Criteria Document.
    Another group of commenters representing industry associations and 
businesses opposed revising the current PM2.5 standards. 
These views are most extensively presented in comments from the Utility 
Air Regulatory Group (UARG), representing a group of electric 
generating companies and organizations and several national trade 
associations, and from Pillsbury, Winthrop, Shaw and Pittman (Pillsbury 
et al.) on behalf of 19 industry and business associations (including, 
for example, the Alliance of Automobile Manufacturers, the American 
Iron and Steel Institute, the National Association of Manufacturers, 
the American Petroleum Institute, and the U.S. Chamber of Commerce).
    These and other commenters in this group generally mentioned many 
of the same studies that were cited by the commenters who supported 
revising the standards, as well as other studies, but highlighted 
different aspects of these studies in reaching 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 expressed the view that the current 
standards provide the requisite degree of public health protection. 
They then considered whether the evidence that has become available 
since the last review has established a more certain risk or a risk of 
effects that are significantly different in character to those that 
provided a basis for the current standards, or whether the evidence 
demonstrates that the risk to public health upon attainment of the 
current standards would be greater than

[[Page 61157]]

was understood when EPA established the current standards in 1997.
    In supporting their view that the present suite of primary 
PM2.5 standards continues to provide the requisite public 
health protection and should not be revised, UARG and others generally 
stated: (1) That the effects of concern have not changed significantly 
since 1997; (2) that the uncertainties in the underlying health science 
are as great or greater than in 1997; (3) that the estimated risk upon 
attainment of the current PM2.5 standards has decreased 
since 1997; and (4) that ``new'' studies not included in the Criteria 
Document continue to increase uncertainty about possible health risks 
associated with exposure to PM2.5. These comments are 
discussed in turn below.
    (1) In asserting that effects of concern have not changed 
significantly since 1997, some of these commenters stated that more 
subtle physiological changes in the cardiovascular system are the only 
type of new PM-related effect identified in this review. They stated 
that such subtle effects are far less serious than the cardiovascular 
effects such as aggravation of cardiovascular disease that had been 
considered in the last review. The EPA disagrees with the assertion 
that subtle changes in the cardiovascular system are the only type of 
new PM-related effect identified in this review. Further, EPA believes 
that evidence of physiological changes in the cardiovascular system is 
important in that it increases confidence in inferences about the 
causal nature of the associations between fine particles and 
cardiovascular-related mortality and hospital admissions.
    As discussed in the Criteria Document (EPA, 2004a, p. 9-75), 
epidemiologic studies published since the last review have expanded 
upon and extended the evidence examining possible links between long-
term exposures to fine particles and increased risk of lung cancer 
incidence and mortality, which was considered to be insufficient to 
support such a linkage in the last review. In this review, however, the 
epidemiologic evidence now available ``support(s) an association 
between long-term exposure to fine particles and lung cancer mortality; 
and the new toxicological studies provide credible evidence for the 
biological plausibility of these associations'' (EPA, 2004a, p. 9-76). 
More specifically, the Criteria Document highlighted ``the newer 
results of the extension of the ACS study analyses (that include more 
years of participant follow-up and address previous criticisms of the 
earlier ACS analyses), which indicate that long-term ambient PM 
exposures are associated with increased risk of lung cancer. That 
increased risk appears to be in about the same range as that seen for a 
nonsmoker residing with a smoker, with any consequent life-shortening 
due to lung cancer'' (EPA 2004a, p. 9-94).
    In addition, as noted earlier, the Criteria Document identified 
increased nonhospital medical visits (physician visits) and aggravation 
of asthma associated with short-term exposure to PM2.5 as 
being newly identified effects since the last review, and concluded 
that findings of such effects ``suggest likely much larger health 
impacts and costs to society due to ambient PM than just those indexed 
either by just hospital admissions/visits and/or mortality.'' Id. 
Further, the Criteria Document (EPA, 2004a, p. 9-79) noted that there 
may be PM-related health effects in infants and children, although only 
very limited evidence of such effects exists.
    (2) In asserting that the uncertainties in the underlying health 
science are as great or greater than in 1997, commenters in this group 
variously discussed a number of issues including: The lack of 
demonstrated mechanisms by which PM2.5 may be causing 
mortality and morbidity effects; uncertainty in the shape of the 
concentration-response functions; the potential for co-pollutant 
confounding; uncertainty in the role of individual constituents of fine 
particles; and the sensitivity of epidemiological results to 
statistical model specification. Each of these issues is addressed 
below. In summary, these commenters concluded that the substantial 
uncertainties present in the last review have not been resolved, that a 
previously unrecognized sensitivity to model specification has been 
newly identified, and/or that the uncertainty about the possible health 
risks associated with PM2.5 exposure has not diminished. As 
discussed below, although EPA agrees that important uncertainties 
remain, and that future research directed toward addressing these 
uncertainties is warranted, EPA believes that overall uncertainty about 
possible health risks associated with both short- and long-term 
PM2.5 exposure has diminished since the last review. As 
noted above, the greater confidence in short-term exposure studies 
supports the Administrator's increased reliance on those studies as the 
basis for the 24-hour standard, and greater confidence in long-term 
exposure studies supports the Administrator's increased reliance on 
those studies as the basis for the annual PM2.5.\17\
---------------------------------------------------------------------------

    \17\ As noted above, this increased confidence in the long- and 
short-term associations generally reflects less uncertainty as to 
the likely causal nature of such associations, but does not address 
directly the question of the extent to which such associations 
remain toward the lower end of the range of ambient PM2.5 
concentrations. This question is central to the Agency's evaluation 
of the relevant evidence to determine appropriate standards levels, 
as discussed below in section II.F.
---------------------------------------------------------------------------

    With regard to the issue of mechanisms, these commenters noted that 
although EPA recognizes that new evidence is now available on potential 
mechanisms and plausible biological pathways, the evidence still does 
not resolve all questions about how PM2.5 at ambient levels 
could produce the effects in question in this review. They further 
assert that even if more recent information has advanced our 
understanding of such mechanisms, it would not justify revision of the 
standard. The EPA notes that in the last review, the Agency considered 
the lack of demonstrated biologic mechanisms for the varying effects 
observed in epidemiologic studies to be an important caution in its 
integrated assessment of the health evidence, upon which the standards 
were based. Since the last review, there has been a great deal of 
research directed toward advancing our understanding of biologic 
mechanisms. While this research has not resolved all questions, and 
further research is warranted, it has provided important insights as 
discussed in section II.A.1 of the proposal (71 FR 2626-2627). As noted 
there, the findings from this new research indicate that different 
health responses are linked with different particle characteristics and 
that both individual components and complex particle mixtures appear to 
be responsible for many biologic responses relevant to fine particle 
exposures. The Criteria Document (EPA, 2004a, p. 7-206) concluded: 
``Thus, there appear to be multiple biologic mechanisms that may be 
responsible for observed morbidity/mortality due to exposure to ambient 
PM. It also appears that many biological responses are produced by PM 
whether it is composed of a single component or a complex mixture.'' 
Further, EPA believes that progress made in gaining insights into 
potential mechanisms lends support to the biologic plausibility of 
results observed in epidemiologic studies (71 FR 2636). The mechanistic 
evidence now available, taken together with newly available 
epidemiologic evidence, increases the Agency's confidence that observed 
associations are causal in nature, such that the risks of health 
effects attributed to short- and long-term exposure to 
PM2.5, acting alone and/or in combination with gaseous co-
pollutants, are now more

[[Page 61158]]

certain than was understood in the last review.
    With regard to uncertainty in concentration-response functions, 
these commenters concluded that ``because the actual shape of this 
function remains unknown, this uncertainty has not been reduced since 
1997'' (UARG, p. 17). The EPA notes that, in contrast to the last 
review when few studies had quantitatively assessed the form of the 
concentration-response function or the potential for a threshold, 
several new studies available in this review have used different 
methods to examine this question, and most have been unable to detect 
threshold levels in time-series mortality studies. The Criteria 
Document (EPA, 2004a, p. 9-44) recognized that in multi-city and most 
single-city time-series studies, statistical tests comparing linear and 
various nonlinear or threshold models have not shown statistically 
significant distinctions between them; where potential threshold levels 
have been suggested in single-city studies, they are at fairly low 
levels (Id. at p. 9-45). Further, the shape of concentration-response 
functions for long-term exposure to PM2.5 was evaluated 
using data from the ACS cohort, with the HEI reanalysis finding near-
linear increasing trends through the range of particle levels observed 
in this study, and the extended ACS study reporting that the various 
mortality associations were not significantly different from linear (71 
FR 2635).\18\ However, EPA agrees that uncertainties remain in our 
understanding of the shape of concentration-response functions, and, 
consistent with the conclusion in the Criteria Document, has concluded 
that the available evidence does not either support or refute the 
existence of population thresholds for effects associated with short- 
or long-term exposures to PM across the range of concentrations in the 
studies. Even while recognizing that uncertainties remain, EPA believes 
that our understanding of this issue for both short- and long-term 
exposure studies has been advanced since the last review.
---------------------------------------------------------------------------

    \18\ In assessing such uncertainties in this review relative to 
the last review, EPA notes that in the last review the level of 
uncertainty associated with long-term exposure studies was such that 
they were not relied on as the primary basis for the annual 
standard. In the last review, relative risk estimates from long-term 
exposure studies were deemed ``highly uncertain'' (62 FR 38668) and 
health effects from long-term exposure were characterized as 
``potentially independent'' (Id.) from those associated with short-
term exposure.
---------------------------------------------------------------------------

    With regard to co-pollutant confounding, these commenters asserted 
that EPA has been ``dismissive'' of this issue in assessing the 
epidemiologic evidence of associations between PM and mortality and 
morbidity endpoints (UARG, p. 18). These commenters asserted that EPA 
has inappropriately concluded that PM-related mortality and morbidity 
associations are generally robust to confounding, which is one of the 
criteria considered in drawing inferences about the extent to which 
observed statistical associations are likely causal in nature. The 
commenters focused on an examination of the extent to which 
statistically significant PM2.5 associations based on one-
pollutant models in a number of time-series studies, and in an analysis 
of associations with long-term exposures in the ACS cohort studies, 
often did not remain statistically significant in two-pollutant models.
    In general, EPA does not believe that the examination of this issue 
put forward by these commenters reflects the complexities inherent in 
assessing the issue of co-pollutant confounding. As discussed in the 
proposal (71 FR 2634) and more fully in the Criteria Document (EPA, 
2004a, section 8.4.3; chapter 9, section 9.2.2.2.2), although multi-
pollutant models may be useful tools for assessing whether gaseous co-
pollutants may be potential confounders, such models cannot determine 
whether in fact they are. Interpretation of the results of multi-
pollutant models is complicated by correlations that often exist among 
air pollutants, by the fact that some pollutants play a role in the 
atmospheric reactions that form other pollutants such as secondary fine 
particles, and by the inherent statistical power of the studies in 
question. While single-city multi-pollutant models have received a 
great deal of attention during this review, the Criteria Document also 
noted several other approaches to examining the question, including a 
more careful examination of personal exposures to PM and co-pollutants, 
the use of factor or principal component analyses, and the use of 
intervention studies (EPA, 2004a, pp. 8-245 to 8-246). The Criteria 
Document also recognized that it is important to consider the issue of 
potential co-pollutant confounding in the context of the more recent 
evidence available about the biological plausibility of associations 
between the various pollutants and health outcomes, model 
specification, and exposure error (EPA, 2004a, p. 8-254).
    An example of other approaches to examining potential co-pollutant 
confounding is the study of personal exposure to fine particles and co-
pollutant gases done in Baltimore (Sarnat et al., 2001). This study 
found that day-to-day variations in monitored ambient gases were not 
associated with day-to-day changes in personal exposures to those 
gases, but they were associated with day-to-day changes in personal 
exposure to PM2.5. One reasonable interpretation of this 
study is that for cities like Baltimore, changes in model results when 
ambient gases are included in multi-pollutant models may stem from such 
gases being surrogates for exposures to particles and not confounders 
at all.
    The broader examination of this issue in the Criteria Document 
included a focus on evaluating the stability of the size of the effect 
estimates in time-series studies using single- and multi-pollutant 
models, as illustrated in Figures 8-16 through 8-19 (EPA, 2004a, pp. 8-
248 to 8-251). This examination found that for most time-series 
studies, there was little change in effect estimates based on single- 
and multi-pollutant models, although recognizing that in some cases, 
the PM effect estimates were markedly reduced in size and lost 
statistical significance in models that included one or more gaseous 
pollutants. The Criteria Document also noted that PM and the gaseous 
co-pollutants were often highly correlated, and it is generally the 
case that high correlations existed between pollutants where PM effect 
estimates were reduced in size with the inclusion of gaseous co-
pollutants. With regard to the analysis of multiple pollutants from the 
ACS cohort, it is important to note that the effects estimates for fine 
particles actually increased in two pollutant models that incorporated 
CO, NO2, and ozone, and were reduced only for models that 
incorporated SO2. The Criteria Document recognized, however, 
that SO2 is a precursor for fine particle sulfates, which 
complicates the interpretation of multi-pollutant model results, and 
that mortality may be associated with not only PM2.5 but 
also with other components of the mix of ambient pollutants in this 
long-term exposure study.
    Far from being dismissive, EPA has examined this issue in detail 
based on the much more extensive body of relevant evidence available in 
this review. This Criteria Document concluded that ``the most 
consistent findings from amidst the diversity of multi-pollutant 
evaluation results for different sites is [sic] that the PM signal most 
often comes through most clearly.'' (EPA, 2004a, p. 8-254.) While 
acknowledging that these analyses have not fully disentangled the 
relative role of co-pollutants, EPA believes that this examination 
provides greater confidence than in the last review that

[[Page 61159]]

observed effects can be attributed to short- and long-term exposures to 
PM2.5, alone and in combination with other pollutants, while 
recognizing that potential confounding by co-pollutants remains a very 
challenging issue to address, even with well-designed studies.
    With regard to questions about the role of individual constituents 
within the mix of fine particles, these commenters pointed out that EPA 
recognized this issue as an important uncertainty in the last review 
and did so again in this review. These commenters then expressed the 
view that such continued uncertainty provides no grounds for 
reconsidering the Agency's 1997 conclusion that the current 
PM2.5 standards provide the requisite protection. As a 
general matter, EPA agrees that although new research directed toward 
this question has been conducted since the last review, important 
questions remain and the issue remains an important element in the 
Agency's ongoing research program. The EPA does not agree, however, 
that continued uncertainty with regard to the relative toxicity of 
components within the mix of fine particles, in and of itself, provides 
grounds for not revising the suite of PM2.5 standards. 
Rather, the full body of health effects evidence that has become 
available since the last review provides a basis for concluding that 
additional public health protection is warranted to protect against 
health effects that have been associated with exposure to fine 
particles measured as PM2.5 mass.
    At the time of the last review, the Agency determined that it was 
appropriate to control fine particles as a group, as opposed to 
singling out any particular component or class of fine particles. This 
distinction was based largely on epidemiologic evidence of health 
effects using various indicators of fine particles in a large number of 
areas that had significant contributions of differing components or 
sources of fine particles, together with some limited experimental 
studies that provided some evidence suggestive of health effects 
associated with high concentrations of numerous fine particle 
components. In this review, as discussed in section II.D of the 
proposal (71 FR 2643-2645) and below in section II.C, while most 
epidemiologic studies continue to be indexed by PM2.5, some 
epidemiologic studies also have continued to implicate various 
components within the mix of fine particles that have been more 
commonly studied (e.g., sulfates, nitrates, carbon, organic compounds, 
and metals) as being associated with adverse effects (EPA, 2004a, p. 9-
31, Table 9-3). In addition, several recent epidemiologic studies 
included in the Criteria Document have used PM2.5 speciation 
data to evaluate associations between mortality and fine particles from 
different sources, and some toxicologic studies have provided evidence 
for effects associated with various fine particle components or size-
differentiated subsets of fine particles.
    The available information continues to suggest that many different 
chemical components of fine particles and a variety of different types 
of source categories are all associated with, and probably contribute 
to, effects associated with PM2.5. Consequently, there 
continues to be no basis to conclude that any individual fine particle 
component cannot be associated with adverse health effects (EPA, 2005, 
p. 5-17). This information is relevant to the Agency's decision to 
retain PM2.5 as the indicator for fine particles (as discussed below in 
section II.C). The EPA also believes that it is relevant to the 
Agency's conclusion as to whether revision of the suite of 
PM2.5 standards is appropriate. Furthermore, while there 
remains uncertainty about the role and relative toxicity of various 
components of fine PM, the current evidence continues to support the 
view that fine particles should be addressed as a group for purposes of 
public health protection, and the remaining uncertainty does not call 
for delaying any increase in public health protection that other 
evidence indicates may be warranted.
    With regard to the sensitivity of epidemiologic associations to the 
use of different statistical models and different approaches to model 
specification used by researchers, these commenters identified this 
issue of model sensitivity as an area in which uncertainty in 
interpreting epidemiologic evidence has increased since the last 
review. Comments from UARG, Pillsbury et al., the Annapolis Center and 
others pointed to examples where individual study results are sensitive 
to the use of alternative models, and to reviews that recommend further 
exploration of this issue in future research, as a basis for asserting 
that current modeling approaches are too uncertain to use the available 
epidemiologic studies as a basis for revising the current 
PM2.5 standards. The EPA agrees that recent work on model 
sensitivity has raised new concerns and the Agency has given much 
attention to this issue. In so doing, EPA recognizes, as does the HEI 
and other researchers, that there is no clear consensus at this time as 
to what constitutes appropriate control of weather and temporal trends 
in time-series studies, and that no single statistical modeling 
approach is likely to be most appropriate in all cases (EPA 2004a, p. 
8-238).
    While recognizing the need for further research on this issue, EPA 
believes that the body of time-series epidemiologic studies considered 
in this review \19\ provides an appropriate basis for informing the 
Agency's decisions on whether to revise the 24-hour PM2.5 
standard, consistent with the conclusion of the HEI review panel (``* * 
* the revised findings will continue to help inform regulatory 
decisions regarding PM.'' HEI, 2003; EPA, 2004a, p. 8-237). More 
specifically, as discussed in the proposal (71 FR 2633-2634), the 
recent time-series epidemiologic studies evaluated in the Criteria 
Document have included some degree of control for variations in weather 
and seasonal variables. However, as summarized in the HEI review panel 
commentary, selecting a level of control to adjust for time-varying 
factors, such as temperature, in time-series epidemiologic studies 
involves a trade-off. For example, if the model does not sufficiently 
adjust for the relationship between the health outcome and temperature, 
some effects of temperature could be falsely ascribed to the pollution 
variable. Conversely, if an overly aggressive approach is used to 
control for temperature, the result would possibly underestimate the 
pollution-related effect and compromise the ability to detect a small 
but true pollution effect (EPA, 2004a, p. 8-236; HEI, 2003, p. 266). 
The selection of approaches to address such variables depends in part 
on prior knowledge and judgments made by the investigators, for 
example, about weather patterns in the study area and expected 
relationships between weather and other time-varying factors and health 
outcomes considered in the study.
---------------------------------------------------------------------------

    \19\ As discussed in section II.A.2.a of the proposal (71 FR 
2629-2630, 2633), this body of studies includes those that did not 
use generalized additive models or were reanalyzed to address 
problems with applications of statistical software used in a number 
of important studies, as noted above in section I.C.
---------------------------------------------------------------------------

    The HEI commentary also reached several other relevant conclusions 
about the reanalysis of time-series studies: upon reanalysis, the PM 
effect persisted in the majority of studies; in some of the large 
number of studies in which the PM effect persisted, the estimates of PM 
effects were substantially reduced; in the few studies in which further 
sensitivity analyses were performed, some showed marked sensitivity of 
the PM effect estimate to the degree of smoothing and/or the 
specification of

[[Page 61160]]

weather; and, in most studies, parametric smoothing approaches used to 
obtain correct standard errors of the PM effect estimates produced 
slightly larger standard errors than with the use of generalized 
additive models. However, the impact of these larger standard errors on 
the level of statistical significance of the PM effect was minor (EPA, 
2004a, pp. 8-237 to 8-238). While recognizing the need for further 
exploration of alternative modeling approaches for time-series 
analyses, the Criteria Document found that the studies included in this 
part of the reanalysis, in general, continued to demonstrate 
associations between PM and mortality and morbidity beyond those 
attributable to weather variables alone (EPA, 2004a, pp. 8-340, 8-341).
    For long-term exposure to fine particles, the reanalysis and 
extended analyses of data from prospective cohort studies have shown 
that reported associations between mortality and long-term exposure to 
fine particles are robust to alternative modeling strategies (Krewski 
et al., 2000). As stated in the reanalysis report, ``The risk estimates 
reported by the Original Investigators were remarkably robust to 
alternative specifications of the underlying risk models, thereby 
strengthening confidence in the original findings' (Krewski et al., 
2000, p. 232). In the extended analysis, Krewski et al. (2000) did 
identify model sensitivities related to education level and spatial 
patterns in the data (e.g., correlations in air pollutant 
concentrations between cities within a region of the country). However, 
these model sensitivities do not invalidate the findings of 
statistically significant associations between long-term exposure to 
PM2.5 and mortality. For example, while the association was 
stronger for the subset of the ACS cohort with the least education, 
there was an association with cardiorespiratory mortality in the entire 
population.\20\
---------------------------------------------------------------------------

    \20\ More specifically, in multivariate models, the association 
found between mortality and long-term PM2.5 exposure was 
little changed with addition of education level to the model 
(Krewski et al., 2000, p. 184). This indicates that education level 
was not a confounder in the relationship between fine particles and 
mortality, but the relationship between fine particles and mortality 
is larger in the population subsets with lower education in this 
study and not statistically significant in the population subset 
with the highest education (EPA, 2004, p. 8-100).
---------------------------------------------------------------------------

    In considering these issues related to uncertainties in the 
underlying health science, on balance, EPA believes that the available 
evidence interpreted in light of these remaining uncertainties does 
provide increased confidence relative to the last review in the 
reported associations between short- and long-term PM2.5 
exposures and mortality and morbidity effects, alone and in combination 
with other pollutants, and generally supports stronger inferences as to 
the causal nature of the associations. The EPA also believes that this 
increased confidence, when taken in context of the entire body of 
available health effects evidence and in light of the evidence from 
short-term exposure studies of associations observed in areas meeting 
the current suite of PM2.5 standards, adds support to its 
conclusion that the current suite of PM2.5 standards needs 
to be revised to provide increased public health protection. This 
increased confidence also adds support to the Administrator's decision 
to place greater reliance on the long-term exposure studies as the 
basis for the annual PM2.5 standard and to place greater 
reliance on the short-term exposure studies as the basis for the 24-
hour PM2.5 standard.
    (3) In asserting that the estimated risk upon attainment of the 
current PM2.5 standards has decreased since 1997 (UARG, p. 
23), these commenters compared results of EPA's risk assessment done in 
the last review with those from the Agency's risk assessment done as 
part of this review, and they concluded that risks upon attainment of 
the current PM2.5 standards ``are almost surely far below 
those that were predicted in 1997'' (UARG, p. 25). These commenters 
used this conclusion as the basis for a claim that there is no reason 
to revise the current PM2.5 standards. In particular, UARG 
and other commenters claimed that based on this purported reduction in 
risk estimates EPA cannot reconcile a decision to provide a greater 
level of health protection now than that afforded by the current 
standards with the ``not lower or higher than is necessary'' standard 
articulated by the Supreme Court in Whitman.
    The EPA believes that this claim is fundamentally flawed for three 
reasons, as discussed in turn below: (i) It mischaracterizes the use of 
the quantitative risk assessment in the 1997 rulemaking; (ii) it is 
factually incorrect in comparing the quantitative risks estimated in 
1997 with those estimated in the current rulemaking; and (iii) it fails 
to take into account that with similar risks, increased certainty in 
the risks presented by PM2.5 implies greater concern than in 
the last review.
    First, this claim mischaracterizes EPA's use of the risk assessment 
in 1997 in part by not recognizing that the illustrative risk 
assessment conducted for portions of two cities (Philadelphia and Los 
Angeles) in the last review was only used qualitatively to assess the 
need to revise the then-current PM10 standards. The EPA used 
the 1997 risk assessment estimates to confirm the conclusions drawn 
primarily from the epidemiological studies that ambient 
PM2.5 levels allowed under the then current PM10 
standards presented a serious public health problem. EPA did not use it 
as a basis for selecting the level of the 1997 PM standards. See 62 FR 
at 38656, 65; ATA III, 283 F. 3d at 373-74 (noting that EPA did not 
base the level of the standards on the numerical results of the risk 
assessment). In so doing, the Administrator concurred with CASAC's 
judgment that the quantitative risk estimates at the time were too 
uncertain for EPA to rely on in deciding the appropriate levels for the 
PM2.5 NAAQS. Therefore, the final decision on the level of 
the NAAQS was not based on the absolute or relative risk reductions 
estimated in the quantitative risk assessment. Instead, the decision 
was based on a direct assessment of the available epidemiological 
studies and the concentration levels observed in urban areas examined 
in the studies where statistically significant effects had been 
observed. Since EPA did not rely on the 1997 quantitative risk 
estimates in setting the level of the 1997 standards, the 1997 
estimates associated with those levels do not represent a decision on a 
requisite level of quantified risk from PM exposure, and therefore do 
not support the argument that a lower estimated risk is more than is 
necessary to provide the requisite level of protection. As a result, 
the suggested quantitative comparison between the 1997 estimates and 
the current estimates of risks at the levels of the current standards 
is not an appropriate basis for determining whether the current suite 
of PM2.5 standards needs to be revised.
    Second, EPA relies on the current risk estimates associated with 
meeting the current standards in a qualitative manner, as in 1997, to 
inform the conclusions drawn primarily from the epidemiological studies 
on whether ambient PM2.5 levels allowed under the current 
suite of PM2.5 standards present a serious public health 
problem warranting revision of the suite of PM2.5 standards. 
The 1997 estimate of these risks, or any comparison of the 1997 risk 
estimates to the current estimates, are irrelevant for that purpose, as 
the 1997 estimates reflect an outdated analysis that has been updated 
in this review to reflect the current science.
    Further, even if the 1997 and current risk assessments were 
legitimately comparable for decision-making purposes, it would still be 
factually

[[Page 61161]]

incorrect to conclude that EPA accepted significantly greater risk in 
1997 than is now estimated to be associated with the 1997 standards 
based on the most recent risk assessment. It is important to note that 
a very large proportion of the quantitative risks estimated in 1997 and 
today comes from long-term exposure mortality. The primary estimates 
from the current risk assessment (which assume a potential threshold of 
10 [mu]g/m\3\, as recommended by CASAC) result in residual risks in 
terms of percent of total incidence that are about the same in the 
current review as they were in the last review for both Philadelphia 
and Los Angeles.
    Third, it is important to take into account EPA's increased level 
of confidence in the associations between short- and long-term 
PM2.5 exposures and mortality and morbidity effects. In 
comparing the scientific understanding of the risk presented by 
exposure to PM2.5 between the last and current reviews, one 
must examine not only the quantitative estimate of risk from those 
exposures (e.g. the numbers of premature deaths or increased hospital 
admissions at various levels), but also the degree of confidence that 
the Agency has that the observed health effects are causally linked to 
PM2.5 exposure at those levels. As documented in the 
Criteria Document and the recommendations and conclusions of CASAC, EPA 
recognizes significant advances in our understanding of the health 
effects of PM2.5, based on reanalyses, extended analyses and 
new epidemiology studies, new human and animal studies documenting 
effects of concentrated ambient particles, new laboratory studies 
identifying and investigating biological mechanisms of PM toxicity, and 
new studies addressing the utility of using ambient monitors to assess 
population exposures to particles of outdoor origin. As a result of 
these advances, EPA is now more certain that fine particles, alone or 
in combination with other pollutants, present a significant risk to 
public health at levels at or above the range of levels that the Agency 
had considered for these standards in 1997. From this more 
comprehensive perspective, since the risks presented by 
PM2.5 are more certain and the overall current quantitative 
risk estimates are about the same as in 1997, PM2.5-related 
risks are now of greater concern than in the last review.
    In sum, quantitative risk estimates were not a basis for EPA's 
decision in setting a level for the PM2.5 standards in 1997, 
and they do not set any quantified ``benchmark'' for the Agency's 
decision to revise the PM2.5 standards at this time. In any 
case, there is not a significant difference in the risk estimates from 
1997 to now. Finally, EPA believes that confidence in the causal 
relationships between short- and long-term exposures to fine particles 
and various health effects has increased markedly since 1997. 
Therefore, similar or even somewhat lower quantitative risk estimates 
today would not be a basis to conclude that no revision to the suite of 
PM2.5 standards is ``requisite'' to protect public health 
with an adequate margin of safety.
    (4) Some of these commenters also identified ``new'' studies that 
were not included in the Criteria Document as showing ``continued 
erosion of the hypothesis that there is a causal connection between 
fine PM mass and health effects'' and further supporting ``the 
conclusion that more stringent PM2.5 standards are not 
justified'' (Pillsbury et al., p. 14). As discussed above in section 
I.C, EPA notes that, as in past NAAQS reviews, the Agency is basing the 
final decisions in this review on the studies and related information 
included in the PM air quality criteria that have undergone CASAC and 
public review, and will consider newly published studies for purposes 
of decision making in the next PM NAAQS review. Nonetheless, in 
provisionally evaluating commenters' arguments (see Response to 
Comments document), EPA notes that its provisional assessment of 
``new'' science found that such studies did not materially change the 
conclusions in the Criteria Document.
3. Conclusions Regarding the Need for Revision
    Having carefully considered the public comments, as discussed 
above, the Administrator believes the fundamental scientific 
conclusions on the effects of PM2.5 reached in the Criteria 
Document and Staff Paper, discussed above in section II.B.1, remain 
valid. In considering whether the suite of primary PM2.5 
standards should be revised, the Administrator places primary 
consideration on the evidence obtained from the epidemiologic studies, 
and finds the evidence of serious health effects reported in short-term 
exposure studies conducted in areas that would meet the current suite 
of PM2.5 standards to be compelling, especially in light of 
the extent to which such studies are part of an overall pattern of 
positive and frequently statistically significant associations across a 
broad range of studies. The Administrator believes that this literature 
collectively represents a strong and generally robust body of evidence 
of serious health effects associated with both short- and long-term 
exposures to PM2.5. Further, the Administrator believes that 
the increased confidence in the evidence of health effects associated 
with long-term exposure to PM2.5 supports relying on long-
term exposure studies as the basis for setting the annual standard in 
this review. This is in contrast to 1997 when EPA relied primarily on 
evidence from the then-available short-term exposure studies as the 
primary basis for setting the annual standard. As discussed in the 
Criteria Document and Staff Paper, the Administrator believes that much 
progress has been made since the last review in reducing some of the 
major uncertainties that were important considerations in establishing 
the current suite of PM2.5 standards.
    Extensive critical review of this body of evidence, the 
quantitative risk assessment, and related uncertainties during the 
criteria and standards review process, including review by CASAC and 
the public of the basis for EPA's proposed decision to revise the suite 
of primary PM2.5 standards, has identified a number of 
issues about which different reviewers disagree and for which 
additional research is warranted. Nonetheless, on balance, the 
Administrator believes that the remaining uncertainties in the 
available evidence do not diminish confidence in the associations 
between serious mortality and morbidity effects and exposure to fine 
particles, in particular as reported in peer-reviewed short-term 
exposure studies at levels allowed by the current standards. In this 
regard, the Administrator agrees with CASAC and the majority of public 
commenters that revision of the current suite of PM2.5 
standards to provide increased public health protection is both 
appropriate and necessary. Based on these considerations, the 
Administrator concludes that the current suite of primary 
PM2.5 standards, taken together, is not sufficient and thus 
not requisite to protect public health with an adequate margin of 
safety, and that revision is needed to provide increased public health 
protection.
    It is important to note that this conclusion, and the reasoning on 
which it is based, do not address the question of what specific 
revisions are appropriate. That requires looking specifically at the 
current indicator, averaging time, form, and level of the 24-hour and 
annual PM2.5 standards, and evaluating the evidence relevant 
to determining whether any of those elements should be revised. The 
analyses discussed above concerning the need to revise the current 
standards

[[Page 61162]]

go no further than determining whether the evidence, taken as a whole, 
indicates that greater public health protection is needed than that 
provided by the current suite of PM2.5 standards.

C. Indicator for Fine Particles

    In 1997, EPA established PM2.5 as the indicator for fine 
particles. In reaching this decision, the Agency first considered 
whether the indicator should be based on the mass of a size-
differentiated sample of fine particles or on one or more components 
within the mix of fine particles. Second, in establishing a size-based 
indicator, a size cut needed to be selected that would appropriately 
distinguish fine particles from particles in the coarse mode.
    In addressing the first question in the last review, EPA determined 
that it was appropriate to control fine particles as a group, as 
opposed to singling out any particular component or class of fine 
particles. Community health studies had found significant associations 
between various indicators of fine particles (including 
PM2.5 or PM10 in areas dominated by fine 
particles) and health effects in a large number of areas that had 
significant mass contributions of differing components or sources of 
fine particles, including sulfates, wood smoke, nitrates, secondary 
organic compounds and acid sulfate aerosols. In addition, a number of 
animal toxicologic and controlled human exposure studies had reported 
health effects associations with high concentrations of numerous fine 
particle components (e.g., sulfates, nitrates, transition metals, 
organic compounds), although such associations were not consistently 
observed. It also was not possible to rule out any component within the 
mix of fine particles as not contributing to the fine particle effects 
found in epidemiologic studies. For these reasons, EPA concluded that 
total mass of fine particles was the most appropriate indicator for 
fine particle standards rather than an indicator based on PM 
composition (62 FR 38667).
    Having selected a size-based indicator for fine particles, the 
Agency then based its selection of a specific size cut on a number of 
considerations. In focusing on a size cut within the size range of 1 to 
3 [mu]m (i.e., the intermodal range between fine and coarse mode 
particles), the Agency noted that the available epidemiologic studies 
of fine particles were based largely on PM2.5; only very 
limited use of PM1 monitors had been made. While it was 
recognized that using PM1 as an indicator of fine particles 
would exclude the tail of the coarse mode in some locations, in other 
locations it would miss a portion of the fine PM, especially under high 
humidity conditions, which would result in falsely low fine PM 
measurements on days with some of the highest fine PM concentrations. 
The selection of a 2.5 [mu]m size cut reflected the regulatory 
importance that was placed on defining an indicator for fine particle 
standards that would more completely capture fine particles under all 
conditions likely to be encountered across the U.S., especially when 
fine particle concentrations are likely to be high, while recognizing 
that some small coarse particles would also be captured by 
PM2.5 monitoring. Thus, EPA's selection of 2.5 [mu]m as the 
size cut for the fine particle indicator was based on considerations of 
consistency with the epidemiologic studies, the regulatory importance 
of more completely capturing fine particles under all conditions, and 
the potential for limited intrusion of coarse particles in some areas; 
it also took into account the general availability of monitoring 
technology (62 FR 38668).
    In this current review, the same considerations continue to apply 
for selection of an appropriate indicator for fine particles. As an 
initial matter, the available epidemiologic studies linking mortality 
and morbidity effects with short- and long-term exposures to fine 
particles continue to be largely indexed by PM2.5. Some 
epidemiologic studies also have continued to implicate various 
components within the mix of fine particles that have been more 
commonly studied (e.g., sulfates, nitrates, carbon, organic compounds, 
and metals) as being associated with adverse effects (EPA, 2004a p. 9-
31, Table 9-3). In addition, several recent studies have used 
PM2.5 speciation data to evaluate the association between 
mortality and particles from different sources (Schwartz, 2003; Mar et 
al., 2003; Tsai et al., 2000; EPA, 2004a, section 8.2.2.5). Schwartz 
(2003) reported statistically significant associations for mortality 
with factors representing fine particles from traffic and residual oil 
combustion that were little changed in reanalysis to address 
statistical modeling issues, and also an association between mortality 
and coal combustion-related particles that was reduced in size and lost 
statistical significance in reanalysis. In Phoenix, significant 
associations were reported between mortality and fine particles from 
traffic emissions, vegetative burning, and regional sulfate sources 
that remained unchanged in reanalysis models (Mar et al., 2003).\21\ 
Finally, a small study in three New Jersey cities reported significant 
associations between mortality and fine particles from industrial, oil 
burning, motor vehicle and sulfate aerosol sources, though the results 
were somewhat inconsistent between cities (Tsai et al., 2000).\22\ No 
significant increase in mortality was reported with a source factor 
representing crustal material in fine particles (EPA, 2004a, p. 8-85). 
Recognizing that these three studies represent a very preliminary 
effort to distinguish effects of fine particles from different sources, 
and that the results are not always consistent across the cities, the 
Criteria Document found that these studies indicate that exposure to 
fine particles from combustion sources, but not crustal material, is 
associated with mortality (EPA, 2004a, p. 8-77). Animal toxicologic and 
controlled human exposure studies have continued to link a variety of 
PM components or particle types (e.g., sulfates, notably primary metal 
sulfate emissions from residual oil burning, metals, organic 
constituents, bioaerosols, diesel particles) with health effects, 
though often at high concentrations (EPA, 2004a, section 7.10.2). In 
addition, some recent studies have suggested that the ultrafine subset 
of fine particles (generally including particles with a nominal 
aerodynamic diameter less than 0.1 [mu]m) may also be associated with 
adverse effects (EPA, 2004a, pp. 8-67 to 8-68).
---------------------------------------------------------------------------

    \21\ Mar et al. (2000) noted that sulfate alone in a single-
pollutant model was not associated with cardiovascular mortality, 
but that the sulfate ``factor,'' which was so associated, contained 
elevated levels of lead and bromine. The authors state that the 
health association with the sulfate (S) factor ``may be reflective 
of the contribution of Pb [lead] and Br [bromine] to the S factor.'' 
Mar et al. (2003) did not provide information about single-pollutant 
analysis of sulfate or about contribution of Pb and Br to the S 
factor.
    \22\ More specifically, statistically significant associations 
were reported with factors representing fine particles from oil 
burning, industrial and sulfate aerosol sources in Newark and with 
particles from oil burning and motor vehicle sources in Camden, and 
no statistically significant associations were reported in 
Elizabeth.
---------------------------------------------------------------------------

    The Criteria Document recognized that, for a given health response, 
some fine particle components are likely to be more closely linked with 
that response than others. The presumption that different PM 
constituents may have differing biological responses is toxicologically 
plausible and an important source of uncertainty in interpreting such 
epidemiologic evidence. For specific effects there may be stronger 
correlation with individual PM components than with aggregate particle 
mass. In addition, particles or particle-bound water can act as 
carriers to deliver other toxic agents into the respiratory tract, 
suggesting that

[[Page 61163]]

exposure to particles may elicit effects that are linked with a mixture 
of components more than with any individual PM component (EPA, 2004a, 
section 9.2.3.1.3).
    Thus, epidemiologic and toxicologic studies have provided evidence 
for effects associated with various fine particle components or size-
differentiated subsets of fine particles. The Criteria Document 
concluded: ``These studies suggest that many different chemical 
components of fine particles and a variety of different types of source 
categories are all associated with, and probably contribute to, 
mortality, either independently or in combinations'' (EPA, 2004a, p. 9-
31). Conversely, the Criteria Document provided no basis to conclude 
that any individual fine particle component cannot be associated with 
adverse health effects (EPA, 2005, p. 5-17). In short, there is not 
sufficient evidence that would lead toward the selection of one or more 
PM components as being primarily responsible for effects associated 
with fine particles, nor is there sufficient evidence to suggest that 
any component should be eliminated from the indicator for fine 
particles. The Staff Paper continued to recognize the importance of an 
indicator that not only captures all of the most harmful components of 
fine particles (i.e., an effective indicator), but also emphasizes 
control of those constituents or fractions, including sulfates, 
transition metals, and organics that have been associated with health 
effects in epidemiologic and/or toxicologic studies, and is thus most 
likely to result in the largest risk reduction (i.e., an efficient 
indicator). Taking into account the above considerations, the Staff 
Paper concluded that it remains appropriate to control fine particles 
as a group; i.e., that total mass of fine particles is the most 
appropriate indicator for fine particle standards (EPA, 2005, p. 5-17).
    With regard to an appropriate size cut for a size-based indicator 
of total fine particle mass, the Criteria Document concluded that 
advances in our understanding of the characteristics of fine particles 
continue to support the use of particle size as an appropriate basis 
for distinguishing between these subclasses, and that a nominal size 
cut of 2.5 [mu]m remains appropriate (EPA, 2004a, p. 9-22). This 
conclusion followed from a recognition that within the intermodal range 
of 1 to 3 [mu]m there is no unambiguous definition of an appropriate 
size cut for the separation of the overlapping fine and coarse particle 
modes. Within this range, the Staff Paper considered size cuts of both 
1 [mu]m and 2.5 [mu]m. Consideration of these two size cuts took into 
account that there is generally very little mass in this intermodal 
range, although in some circumstances (e.g., windy, dusty areas) the 
coarse mode can extend down to and below 1 [mu]m, whereas in other 
circumstances (e.g., high humidity conditions, usually associated with 
very high fine particle concentrations) the fine mode can extend up to 
and above 2.5 [mu]m. The same considerations that led to the selection 
of 2.5 [mu]m size cut in the last review--that the epidemiologic 
evidence was largely based on PM2.5 and that it was more 
important from a regulatory perspective to capture fine particles more 
completely under all conditions likely to be encountered across the 
U.S. (especially when fine particle concentrations are likely to be 
high) than to avoid some coarse-mode intrusion into the fine fraction 
in some areas--led to the same recommendation in the Staff Paper (EPA, 
2005, p. 5-18), which was endorsed by CASAC in its recommendations for 
PM2.5 standards (Henderson, 2005a, p. 6). In addition, the 
Staff Paper recognized that particles can act as carriers of water, 
oxidative compounds, and other components into the respiratory system, 
which adds to the importance of ensuring that larger accumulation-mode 
particles are included in the fine particle size cut (EPA, 2005, p. 5-
18).
    Consistent with the Staff Paper and CASAC recommendations, the 
Administrator proposed to retain PM2.5 as the indicator for 
fine particles. Further, the Administrator provisionally concluded that 
currently available studies do not provide a sufficient basis for 
supplementing mass-based fine particle standards with standards for any 
specific fine particle component or subset of fine particles, or for 
eliminating any individual component or subset of components from fine 
particle mass standards. Addressing the current uncertainties in the 
evidence of effects associated with various fine particle components 
and types of source categories is an important element in EPA's ongoing 
PM research program.
    In so doing, the Administrator also noted that some commenters had 
expressed views about the importance of evaluating health effect 
associations with various fine particle components and types of source 
categories as a basis for focusing ongoing and future research to 
reduce uncertainties in this area and for considering whether 
alternative indicator(s) are now or may be appropriate for standards 
intended to protect against the array of health effects that have been 
associated with fine particles as indexed by PM2.5. 
Information from such studies could also help inform the development of 
strategies that emphasize control of specific types of emission sources 
so as to address particles of greatest concern to public health. While 
recognizing that the studies evaluated in the Criteria Document 
provided some limited evidence of such associations that is helping to 
focus research activities, the Administrator solicited broad public 
comment on issues related to studies of fine particle components and 
types of source categories and their usefulness as a basis for 
consideration of alternative indicator(s) for fine particle standards. 
In general, comment was solicited on relevant new published research, 
recommendations for studies that would be appropriate for inclusion in 
future research activities, and approaches to assessing the available 
and future research results to determine whether alternative indicators 
for fine particles are warranted to provide effective protection of 
public health from effects associated with long- and short-term 
exposure to ambient fine particles (71 FR at 2645). More specifically, 
the proposal solicited comment on a number of related issues, including 
the extent to which reducing particular types of PM (differentiated by 
either size or chemistry) might alter the size and toxicity of 
remaining particles; the extent to which fine particles in urban and 
rural areas can be differentiated by size or chemistry; the extent to 
which the latest scientific information can be used to improve our 
understanding of the relationship of monitored pollution levels to 
human exposure; and on studies using concentrated ambient particles 
(CAPs) and their use in examining the toxicity of specific mixtures of 
pollutants or of particular source categories.
    The EPA received comparatively few public comments on issues 
related to the indicator for fine particles.\23\ Public comments from 
all major public and private sector groups received on the proposal 
were overwhelmingly in favor of EPA's proposal to retain 
PM2.5 as the indicator for fine particles. Commenters who 
supported retaining PM2.5 as an indicator argued that 
current scientific evidence does not identify specific components or 
sources of concern and therefore, that a mass-based indicator remains 
the appropriate indicator for fine particles (Engine Manufacturers 
Association; American Lung Association et al.). Some commenters 
emphasized the need to conduct additional research to more fully

[[Page 61164]]

understand the effect of specific PM components and/or sources on 
public health. For example, the Electric Power Research Institute 
highlighted specific new research studies that had been completed since 
the close of the Criteria Document addressing issues related to fine 
particle components and source apportionment, and noted its ongoing 
research on component-related health effects that includes coordinated 
epidemiology, toxicology, and exposure assessment studies. The 
Administrator recognizes the work of the Electric Power Research 
Institute and agrees that additional research is important to improve 
future understanding of the role of specific fine particle components 
and/or sources of fine particles. The Administrator also recognizes the 
ongoing efforts of HEI to conduct additional multidisciplinary research 
targeted at expanding the available data on the health effects 
associated with specific PM components (HEI, 2005).
---------------------------------------------------------------------------

    \23\ No public comments were submitted regarding the use of a 
different size for fine particles.
---------------------------------------------------------------------------

    Having considered the public comments on this issue, the 
Administrator concurs with the Staff Paper and CASAC recommendations 
and concludes that it is appropriate to retain PM2.5 as the 
indicator for fine particles.

D. Averaging Time of Primary PM2.5 Standards

    In the last review, EPA established two PM2.5 standards, 
based on annual and 24-hour averaging times, respectively (62 FR 38668-
70). This decision was based in part on evidence of health effects 
related to both short-term (from less than 1 day to up to several days) 
and long-term (from a year to several years) measures of PM. The EPA 
noted that the large majority of community epidemiologic studies 
reported associations based on 24-hour averaging times or on multiple-
day averages. Further, EPA noted that a 24-hour standard could also 
effectively protect against episodes lasting several days, as well as 
providing some degree of protection from potential effects associated 
with shorter duration exposures. The EPA also recognized that an annual 
standard would provide effective protection against both annual and 
multi-year, cumulative exposures that had been associated with an array 
of health effects, and that a much longer averaging time would 
complicate and unnecessarily delay control strategies and attainment 
decisions. The EPA considered the possibility of seasonal effects, 
although the very limited available evidence of such effects and the 
seasonal variability of sources of fine particle emissions across the 
country did not provide an adequate basis for establishing a seasonal 
averaging time.
    In considering whether the information available in this review 
supported consideration of different averaging times for 
PM2.5 standards, the Staff Paper concluded that the 
available information is generally consistent with and supportive of 
the conclusions reached in the last review to set PM2.5 
standards with both annual and 24-hour averaging times. In considering 
the new information, the Staff Paper made the following observations 
(EPA, 2005, section 5.3.3):
    (1) There is a growing body of studies that provide additional 
evidence of effects associated with exposure periods shorter than 24-
hours (e.g., one to several hours) (EPA, 2004a, section 3.5.5.1). While 
the Staff Paper concluded that this information remains too limited to 
serve as a basis for establishing a shorter-than-24-hour fine particle 
primary standard at this time, it also noted that this information 
gives added weight to the importance of a standard with a 24-hour 
averaging time.
    (2) Some recent PM10 studies have used a distributed lag 
over several days to weeks preceding the health event, although this 
modeling approach has not been extended to studies of fine particles 
(EPA, 2004a, section 3.5.5). While such studies continue to suggest 
consideration of a multiple day averaging time, the Staff Paper noted 
that limiting 24-hour concentrations of fine particles will also 
protect against effects found to be associated with PM averaged over 
many days in health studies. Consistent with the conclusion reached in 
the last review, the Staff Paper concluded that a multiple-day 
averaging time would add complexity without providing more effective 
protection than a 24-hour average.
    (3) While some newer studies have investigated seasonal effects 
(EPA, 2004a, section 3.5.5.3), the Staff Paper concluded that currently 
available evidence of such effects is still too limited to serve as a 
basis for considering seasonal standards.
    Based on the above considerations, the Staff Paper and CASAC 
(Henderson, 2005a, p. 6) recommended retaining the current annual and 
24-hour averaging times for PM2.5 primary standards. The 
Administrator concurred with the staff and CASAC recommendations and 
proposed that averaging times for PM2.5 standards should 
continue to include annual and 24-hour averages to protect against 
health effects associated with short-term (hours to days) and long-term 
(seasons to years) exposure periods.
    The EPA received very limited public comment on the issue of 
averaging time for the PM2.5 primary standards. A group of 
public health and environmental organizations agreed that ``the EPA has 
selected the appropriate averaging times for the fine particle 
standards'' (American Lung Association et al.).
    Having considered the public comments on this issue, the 
Administrator concurs with the recommendations presented in the Staff 
Paper and recommendations made by CASAC (Henderson, 2005a) and 
concludes, as proposed, that it is appropriate to retain the current 
annual and 24-hour averaging times for the primary PM2.5 
standards to protect against health effects associated with short-term 
and long-term exposure periods.

E. Form of Primary PM2.5 Standards

1. 24-Hour PM2.5 Standard
    In 1997 EPA established the form of the 24-hour PM2.5 
standard as the 98th percentile of the annual 24-hour concentrations at 
each population-oriented monitor within an area, averaged over three 
years (62 FR 38671-74). EPA found that, as compared to an exceedance-
based form used in earlier PM standards, a concentration-based form is 
more reflective of the health risk posed by elevated PM2.5 
concentrations because it gives proportionally greater weight to days 
when concentrations are well above the level of the standard than to 
days when the concentrations are just above the standard. Further, a 
concentration-based form better compensates for missing data and less-
than-every-day monitoring; and, when averaged over 3 years, it has 
greater stability and, thus, facilitates the development of more stable 
implementation programs. After considering a range of concentration 
percentiles from the 95th to the 99th, EPA selected the 98th percentile 
as an appropriate balance between adequately limiting the occurrence of 
peak concentrations and providing increased stability and robustness. 
Further, by basing the form of the standard on concentrations measured 
at population-oriented monitoring sites (as specified in 40 CFR part 
58), EPA intended to provide protection for people residing in or near 
localized areas of elevated concentrations.
    In this review, the Staff Paper concluded that it is appropriate to 
retain a concentration-based form that is defined in terms of a 
specific percentile of the distribution of 24-hour PM2.5 
concentrations at each population-oriented monitor within an area, 
averaged over 3 years. This staff

[[Page 61165]]

recommendation was based on the same reasons that were the basis for 
EPA's selection of this type of form in the last review. As to the 
specific percentile value to be considered, the Staff Paper took into 
consideration (1) the relative risk reduction afforded by alternative 
forms at the same standard level, (2) the relative year-to-year 
stability of the air quality statistic to be used as the basis for the 
form of a standard, and (3) the implications from a public health 
communication perspective of the extent to which either form allows 
different numbers of days in a year to be above the level of the 
standard in areas that attain the standard. Based on these 
considerations, the Staff Paper recommended either retaining the 98th 
percentile form or revising it to be based on the 99th percentile form, 
and noted that primary consideration should be given to the combination 
of form and level, as compared to looking at the form in isolation 
(EPA, 2005, p. 5-44).
    In considering the information provided in the Staff Paper, most 
CASAC Panel members favored continued use of the 98th percentile for a 
concentration-based form because it is more robust than the 99th 
percentile, such that it would provide more stability to prevent areas 
from moving in and out of attainment from year to year (Henderson 
2005a). In recommending retention of the 98th percentile form, the 
CASAC Panel recognized that it is the link between the form and level 
of a standard that determines the degree of public health protection 
the standard affords.
    In considering the available information and the Staff Paper and 
CASAC recommendations, the Administrator proposed to retain the form 
for the 24-hour standard. In so doing, the Administrator focused on the 
relative stability of the 98th and 99th percentile forms as a basis for 
selecting the 98th percentile form, while recognizing that the degree 
of public health protection likely to be afforded by a standard is a 
result of the combination of the form and the level of the standard.
    None of the public commenters raised objections to continuing the 
use of a concentration-based form for the 24-hour standard. Many of the 
individuals and groups who supported a more stringent 24-hour 
PM2.5 standard noted above in Section II.B, however, 
recommended a more restrictive concentration-based percentile form, 
specifically a 99th percentile form. The limited number of these 
commenters who provided a specific rationale for this recommendation 
generally expressed their concern that the 98th percentile form could 
allow too many days where concentrations exceeded the level of the 
standard, and thus fail to adequately protect public health. The EPA 
received comparatively few public comments from State and local air 
pollution control authorities and tribal organizations on the form of 
the 24-hour PM2.5 standard. Of the limited number of state 
air pollution control authorities that commented on the form of the 24-
hour PM2.5 standard, all supported retaining the 98th 
percentile form. Of the limited number of local air pollution control 
authorities and tribal organizations that commented on the form of the 
24-hour PM2.5 standard, some supported retaining the 98th 
percentile form while others supported the 99th percentile form. Beyond 
their support for retaining the current 24-hour PM2.5 
standard, which has a 98th percentile form, commenters representing 
industry associations and businesses provided no specific comments 
regarding the form of the 24-hour PM2.5 standard.
    The EPA notes that the viewpoints represented in this review are 
similar to comments submitted in the last review and through various 
NAAQS reviews. The EPA recognizes that the selection of the appropriate 
form includes maintaining adequate protection against peak 24-hour 
values while also providing a stable target for risk management 
programs, which serves to provide for the most effective public health 
protection in the long run.\24\ Nothing in the commenters' views has 
provided a reason to change the Administrator's previous conclusion 
regarding the appropriate balance represented in the proposed form of 
the 24-hour PM2.5 standard. Therefore, the Administrator 
concurs with CASAC recommendations and concludes that it is appropriate 
to retain the 98th percentile form for the 24-hour PM2.5 
standard.
---------------------------------------------------------------------------

    \24\ See ATA III, 283 F. 3d at 374-375 which concludes it is 
legitimate for EPA to consider promotion of overall effectiveness of 
NAAQS implementation programs, including their overall stability, in 
setting a standard that is requisite to protect the public health.
---------------------------------------------------------------------------

    In reaching this conclusion, EPA also recognizes that several 
states that otherwise supported EPA's proposal to retain the 98th 
percentile form of the 24-hour PM2.5 standard raised 
concerns regarding a technical problem associated with a potential bias 
in the method used to calculate the 98th percentile concentration for 
this form. NESCAUM, in particular, noted that ``the existing and 
proposed methodology yields a lower (i.e., less stringent) value on 
average for a 1 in 3 day frequency sample data-set compared to a daily 
sample data-set by approximately 1 [mu]g/m 3'' (NESCAUM, p. 
3), and recommended revisions to the methodology such that ``the 
calculation becomes insensitive to data capture rate or sampling 
frequency'' (NESCAUM, Attachment A, p.7). Another state commenter 
suggested the issue could be addressed by ``the addition of language 
that requires areas that are near the daily NAAQS to continue to use 
every day FRM/FEM sampling'' (Delaware Department of Natural Resources, 
p. 4). The EPA agrees with these commenters that the potential bias in 
calculating the design value of the 24-hour PM2.5 standard 
is a concern. To reduce this bias, EPA had proposed to increase the 
sampling frequency for monitoring sites that were within 10 percent of 
the standard to 1 in 3 day sampling (Part 58 section 12(d)(1)). The EPA 
is persuaded by these comments that it is appropriate to adjust the 
proposed sampling frequency requirements in order to further reduce 
this bias. Accordingly, EPA is modifying the final monitoring 
requirements such that areas that are within 5 percent of the standard 
will be required to increase the frequency of sampling to every day 
(Part 58 section 12(d)(1).\25\
---------------------------------------------------------------------------

    \25\ See final rulemaking notice regarding revisions to ambient 
air monitoring requirements, elsewhere in today's Federal Register.
---------------------------------------------------------------------------

2. Annual PM2.5 Standard
    In 1997 EPA established the form of the annual PM2.5 
standard as an annual arithmetic mean, averaged over 3 years, from 
single or multiple community-oriented monitors. This form of the annual 
standard was intended to represent a relatively stable measure of air 
quality and to characterize area-wide PM2.5 concentrations 
in conjunction with a 24-hour standard designed to provide adequate 
protection against localized peak or seasonal PM2.5 levels. 
The current annual PM2.5 standard level is to be compared to 
measurements made at the community-oriented monitoring site recording 
the highest level, or, if specific constraints are met, measurements 
from multiple community-oriented monitoring sites may be averaged (Part 
50 Appendix N section 1.0(c) and 2.1(a) and (b) and Part 58 Appendix D 
section 2.8.1.6.1; 62 FR 38672). Community-oriented monitoring sites 
were specified to be consistent with the intent that a spatially 
averaged annual standard protect persons living in smaller communities, 
as well as those in larger population centers. The constraints on 
allowing the use of spatially averaged measurements were

[[Page 61166]]

intended to limit averaging across poorly correlated or widely 
disparate air quality values.\26\ This approach was judged to be 
consistent with the short-term epidemiologic studies on which the 
annual PM2.5 standard was primarily based, in which air 
quality data were generally averaged across multiple monitors in an 
area or were taken from a single monitor that was selected to represent 
community-wide exposures, not localized ``hot spots'' (62 FR 38672). 
These criteria and constraints were intended to ensure that spatial 
averaging would not result in inequities in the level of protection 
afforded by the PM2.5 standards (Id.).
---------------------------------------------------------------------------

    \26\ The current constraints include the criteria that the 
correlation coefficient between monitor pairs to be averaged be at 
least 0.6, and that differences in mean air quality values between 
monitors to be averaged not exceed 20 percent and that areas in 
which monitoring results may be averaged should principally be 
affected by the same major emission source of PM2.5 (Part 
58 App. D section 2.8.1.6.1).
---------------------------------------------------------------------------

    In this review, there now exists a much larger set of 
PM2.5 air quality data than was available in the last 
review. Consideration in the Staff Paper of the spatial variability 
across urban areas that is revealed by this new data base has raised 
questions as to whether an annual standard that allows for spatial 
averaging, within currently specified or alternative constraints, would 
provide appropriate public health protection. Analyses in the Staff 
Paper to assess these questions, as discussed below, took into account 
both aggregate population risk across an entire urban area and the 
potential for disproportionate impacts on potentially vulnerable 
subpopulations within an area.
    The effect of allowing the use of spatial averaging on aggregate 
population risk was considered in sensitivity analyses included in the 
health risk assessment (EPA, 2005, section 4.4.3.2). In particular, 
this included analyses of several urban areas that compared estimated 
mortality risks based on calculating compliance with alternative 
standards (1) using air quality values from the highest community-
oriented monitor in an area and (2) using air quality values averaged 
across all such monitors within the constraints on spatial averaging 
allowed by the current standard.\27\ As expected, estimated risks 
associated with long-term exposures that remain upon just meeting the 
current annual standard are greater when spatial averaging is used than 
when the highest monitor is used (i.e., the estimated reductions in 
risk associated with just attaining the current or alternative annual 
standards are less when spatial averaging is used), as the use of the 
highest monitor leads to greater modeled reductions in ambient 
PM2.5 concentrations.\28\
---------------------------------------------------------------------------

    \27\ As discussed in the Staff Paper (EPA, 2005; section 4.2.2), 
the monitored air quality values were used to determine the design 
value for the annual standard in each area, as applied to a 
``composite'' monitor to reflect area-wide exposures. Changing the 
basis of the annual standard design value from the concentration at 
the highest monitor to the average concentration across all monitors 
changes the amount of reduction in PM2.5 levels that is 
needed to just meet the current or alternative annual standards. 
With averaging, less overall reduction in ambient PM2.5 
is needed to just meet the standards.
    \28\ For example, based on analyses conducted in three example 
urban areas, estimated mortality incidence associated with long-term 
exposure based on the use of spatial averaging is about 10 to more 
than 40 percent higher than estimated incidence based on the use of 
the highest monitor (EPA, 2005, p.5-41).
---------------------------------------------------------------------------

    In considering the potential for disproportionate impacts on 
potentially vulnerable subpopulations, EPA assessed whether any such 
groups are more likely than the general population to live in census 
tracts in which the monitors recording the highest air quality values 
in an area are located. Data used in this analysis included demographic 
parameters measured at the census tract level, including education 
level, income level, and percent minority population. Data from the 
census tract in each area in which the highest air quality value was 
monitored were compared to the area-wide average value (consistent with 
the constraints on spatial averaging provided by the current standard) 
in each area (Schmidt et al., 2005). Recognizing the limitations of 
such cross-sectional analyses, the Staff Paper observed that the 
results suggest that the highest concentrations in an area tend to be 
measured at monitors located in areas where the surrounding population 
is more likely to have lower education and income levels, and higher 
percentages of minority populations (EPA, 2005, p. 5-41).\29\ Noting 
the intended purposes of the form of the annual standard, as discussed 
above, the Staff Paper concluded that the existing constraints on 
spatial averaging may not be adequate to avoid substantially greater 
exposures in some areas, potentially resulting in disproportionate 
impacts on these potentially vulnerable subpopulations.
---------------------------------------------------------------------------

    \29\ As summarized in section II.A.4 of the proposal, the 
Criteria Document notes that some epidemiologic study results, most 
notably the associations between total mortality and long-term 
PM2.5 exposure in the ACS cohort, have shown larger 
effect estimates in the cohort subgroup with lower education levels 
(EPA, 2004a, p. 8-103). The Criteria Document also notes that lower 
education level can be a marker for lower socioeconomic status that 
may be related to increased vulnerability to the effects of fine 
particle exposures, for example, as a result of greater exposure 
from proximity to sources such as roadways and industry, as well as 
other factors such as poorer health status and access to health care 
(EPA, 2004a, section 9.2.4.5).
---------------------------------------------------------------------------

    In considering whether more stringent constraints on the use of 
spatial averaging may be appropriate, the Staff Paper presented results 
of an analysis of recent air quality data which assessed correlations 
and differences between monitor pairs in metropolitan areas across the 
country (Schmidt et al., 2005). For all pairs of PM2.5 
monitors, the median correlation coefficient based on annual air 
quality data is approximately 0.9, which is substantially higher than 
the current criterion (in Appendix D of Part 58, section 2.8.1.6.1) of 
a minimum correlation of at least 0.6, which was met by nearly all 
monitor pairs. The current criterion that differences in mean air 
quality values between individual monitors and the corresponding multi-
site spatial average not exceed 20 percent on an annual basis also was 
met for most monitor pairs, while the actual annual median and mean 
differences for all monitor pairs were 5 percent and 8 percent, 
respectively. This analysis also showed that in some areas with highly 
seasonal air quality patterns (e.g., due to seasonal wood smoke 
emissions), substantially lower seasonal correlations and larger 
seasonal differences can occur relative to those observed on an annual 
basis. This analysis provided some perspective on the constraints on 
spatial averaging that were adopted in the last review before data were 
widely available on spatial distributions of PM2.5 air 
quality levels.
    In considering the results of the analyses discussed above, the 
Staff Paper concluded that it is appropriate to consider either 
eliminating the provision that allows for spatial averaging from the 
form of an annual PM2.5 standard or narrowing the 
constraints on spatial averaging to be based on more restrictive 
criteria. More specifically, based on the analyses discussed above, the 
Staff Paper recommended consideration of revised criteria such that the 
correlation coefficient between monitor pairs to be averaged be at 
least 0.9, determined on a seasonal basis, and annual mean differences 
between individual monitors and corresponding spatial averages not 
exceed 10 percent (EPA, 2005, p. 5-42).\30\
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    \30\ In CASAC's review of the Second Draft Staff Paper, most of 
the members of the CASAC Review Panel found the fine particle 
sections to be ``generally well-written and scientifically well-
reasoned'' but, beyond their recommendation that the primary 
PM2.5 standards should be strengthened, CASAC provided no 
specific comments regarding the form of the annual standard 
(Henderson, 2005a, pp. 1-2).

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[[Page 61167]]

    In considering the Staff Paper recommendations based on the results 
of the analyses discussed above, and focusing on a desire to be 
consistent with the epidemiologic studies on which the PM2.5 
health effects are based and concern over the evidence of potential 
disproportionate impact on potentially vulnerable subpopulations, the 
Administrator proposed to revise the form of the annual 
PM2.5 standard consistent with the Staff Paper 
recommendation to change two of the criteria for use of spatial 
averaging such that the correlation coefficient between monitor pairs 
must be at least 0.9, determined on a seasonal basis, with differences 
between monitor values not to exceed 10 percent (71 FR 2647). The 
Administrator also solicited comment on the other Staff Paper-
recommended alternative of revising the form of the annual 
PM2.5 standard to one based on the highest community-
oriented monitor in an area, with no allowance for spatial averaging 
(Id. at 2647-48).
    Relatively few public comments were received on the form of the 
annual PM2.5 standard. Of the commenters noted above in 
Section II.B who supported a more stringent annual PM2.5 
standard, those who commented on the form of the annual 
PM2.5 standard argued that the EPA analyses described above 
demonstrated that the current form of the standard results in uneven 
public health protection leading to disproportionate impacts on 
potentially vulnerable subpopulations, and thus a change in the form of 
the standard is needed. However, these commenters argued that the 
proposed modifications to the spatial averaging criteria were not 
stringent enough and, in order to reduce the possibility of pollution 
hotspots and disproportionate impacts, especially in areas meeting the 
annual PM2.5 standard, spatial averaging should be 
eliminated (American Lung Association et al., 2006, pp. 44-47; 
Schwartz, 2005, p. 2). Of the commenters noted above in Section II.B 
who supported retaining the current annual PM2.5 standard, 
those who commented specifically on the form of the standard supported 
retaining the current spatial averaging criteria. These views are most 
extensively presented in comments from UARG who argued that changes to 
the spatial averaging criteria, effectively increasing the stringency 
of the standard, are not needed as the current standards provide the 
requisite degree of public health protection (UARG, 2006. pp. 33-36). 
In addition, one state air pollution control agency supported a more 
stringent level for the annual PM2.5 standard in the range 
recommended by CASAC but also supported retaining the option for 
spatial averaging for the form of the standard arguing that ``rarely is 
one monitor representative of an entire nonattainment area'' especially 
in the western U.S. (Utah Department of Environmental Quality, 2006, p. 
2).
    The Administrator emphasizes that the intent of the current spatial 
averaging criteria, as defined in 1997 based on a limited set of 
PM2.5 air quality data, was to ensure that spatial averaging 
would not result in inequities in the level of protection provided by 
the PM2.5 standards against health effects associated with 
short- and long-term exposures to PM2.5. Based on the 
analyses described above (Schmidt et al., 2005), which are based on the 
much larger set of air quality data that has become available since the 
last review, EPA now believes that tighter constraints on spatial 
averaging are necessary to address concerns over potential 
disproportionate impacts on the populations that EPA has identified as 
being potentially vulnerable to PM2.5-related health 
effects. The EPA believes that current information and analyses 
indicate that application of the current form has the clear potential 
to result in disproportionate impacts on potentially vulnerable 
subpopulations in some areas. The EPA recognizes that the proposed 
constraints have the potential to increase the stringency of the annual 
PM2.5 standard in some areas in which a State might choose 
to use spatial averaging. The EPA believes that in such cases this 
increased stringency is warranted so as to address possible 
disproportionate impacts on potentially vulnerable populations and more 
generally to avoid inequities across all population groups. The EPA 
disagrees with those commenters who support eliminating spatial 
averaging altogether. The EPA believes that the proposed narrowing of 
the spatial averaging criteria will adequately address the concerns 
about disproportionate impact raised by some commenters, as analyzed in 
the Staff Paper, by substantially reducing the amount of spatial 
variation in long-term ambient levels that will be allowed to be 
averaged together in determining compliance with the standard. 
Therefore, the Administrator concludes that the current form of the 
standard should be retained with the proposed modifications. The form 
of the annual PM2.5 standard is retained as an annual 
arithmetic mean, averaged over 3 years; however, the following two 
aspects of the spatial averaging criteria are narrowed: (1) The annual 
mean concentration at each site shall be within 10 percent of the 
spatially averaged annual mean, and (2) the daily values for each 
monitoring site pair shall yield a correlation coefficient of at least 
0.9 for each calendar quarter.

F. Level of Primary PM2.5 Standards

    In the last review, having concluded that it was appropriate to 
establish both 24-hour and annual PM2.5 standards, EPA 
selected a level for each standard that was appropriate for the 
function to be served by each (62 FR 38674, 38676-77). As noted above, 
EPA concluded at that time that the suite of PM2.5 standards 
could most effectively and efficiently protect public health by 
treating the annual standard as the generally controlling standard for 
lowering both short- and long-term PM2.5 concentrations.\31\ 
In conjunction with such an annual standard, the 24-hour standard was 
intended to provide protection against days with high peak 
PM2.5 concentrations, localized ``hotspots,'' and risks 
arising from seasonal emissions that would not be well controlled by an 
annual standard.\32\
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    \31\ In so doing, EPA noted that an annual standard would focus 
control programs on annual average PM2.5 concentrations, 
which would generally control the overall distribution of 24-hour 
exposure levels, as well as long-term exposure levels, and would 
also result in fewer and lower 24-hour peak concentrations. 
Alternatively, a 24-hour standard that focused controls on peak 
concentrations could also result in lower annual average 
concentrations. Thus, EPA recognized that either standard could 
provide some degree of protection from both short- and long-term 
exposures, with the other standard serving to address situations 
where the daily peaks and annual averages are not consistently 
correlated (62 FR 38669).
    \32\ See also ATA III, 283 F.3d at 373 (endorsing this 
reasoning).
---------------------------------------------------------------------------

    In selecting the level for the annual standard in the last review, 
EPA used an evidence-based approach that considered the evidence from 
both short- and long-term exposure studies. The risk assessment 
conducted in the last review, while providing qualitative insights 
about the distribution of risks, was considered by EPA to be too 
limited to serve as a quantitative basis for decisions on the standard 
levels. In accordance with Staff Paper and CASAC views on the relative 
strengths of the short- and long-term exposure studies, EPA placed 
greater emphasis on the short-term exposure studies. In so doing, EPA 
first determined a level for the annual standard based on the short-
term exposure studies, and then considered whether the long-term 
exposure studies suggested the need for a lower level. While 
recognizing that health effects could occur over the full range of 
concentrations observed in the studies, EPA concluded that the

[[Page 61168]]

strongest evidence for short-term PM2.5 effects occurs for 
air quality distributions with long-term concentrations near the long-
term (e.g., annual) average in those studies reporting statistically 
significant health effects. Thus, in the last review, EPA selected a 
level for the annual standard that was somewhat below the lowest long-
term average PM2.5 concentration in a short-term exposure 
study that reported statistically significant health effects. Further 
consideration of the average PM2.5 concentrations across the 
cities in the key long-term exposure studies available at that time did 
not provide a basis for establishing a lower annual standard level.
    In this review, the approach used in the Staff Paper as a basis for 
staff recommendations on standard levels built upon and broadened the 
general approach used by EPA in the last review. This broader approach 
reflected the more extensive and stronger body of evidence now 
available on health effects related to both short- and long-term 
exposure to PM2.5, together with the availability of much 
more extensive PM2.5 air quality data. This newly available 
information was used to conduct a more comprehensive risk assessment 
for PM2.5. As a consequence, the broader approach used in 
the Staff Paper discussed ways to take into account both evidence-based 
and quantitative risk-based considerations and placed relatively 
greater emphasis on evidence from long-term exposure studies than was 
done in the last review.
    Given the extensive body of new evidence based specifically on 
PM2.5 that is now available, and the resulting broader 
approach presented in the Staff Paper, the Administrator considered it 
appropriate to use a somewhat different evidence-based approach from 
that used in the last review to propose appropriate standard levels. In 
the Administrator's view, the very large numbers of PM2.5 
health effect studies that now make up the available body of evidence 
provide the most reliable basis for determining the level of the 
standards. More specifically, EPA's proposal relied on an evidence-
based approach that considered the much expanded body of evidence from 
short-term exposure PM2.5 studies as the principal basis for 
selecting the level of the 24-hour standard, with such standard aimed 
at protecting against health effects associated with short-term 
exposures to PM2.5. Likewise, the stronger and more robust 
body of evidence from the long-term exposure PM2.5 studies 
was considered as the principal basis for selecting the level of the 
annual standard, with such standard aimed at protecting against health 
effects associated with long-term exposures to PM2.5.
    With respect to the quantitative risk assessment, the Administrator 
recognized at proposal that it rests on a more extensive body of data 
and is more comprehensive in scope than the assessment conducted in the 
last review, but was mindful that significant uncertainties continue to 
underlie the resulting risk estimates. Such uncertainties generally 
relate to a lack of clear understanding of a number of important 
factors, including, for example, the shape of concentration-response 
functions, particularly when, as here, effect thresholds can neither be 
discerned nor determined not to exist; issues related to selection of 
appropriate statistical models for the analysis of the epidemiologic 
data; the role of potentially confounding and modifying factors in the 
concentration-response relationships; issues related to simulating how 
PM2.5 air quality distributions will likely change in any 
given area upon attaining a particular standard, since strategies to 
reduce emissions are not yet defined; and whether there would be 
differential reductions in the many components within PM2.5 
and, if so, whether this would result in differential reductions in 
risk. In the case of fine particles, the Administrator recognized that 
for purposes of developing quantitative risk estimates such 
uncertainties are likely to amplified by the complexity in the 
composition of the mix of fine particles generally present in the 
ambient air. Further, in the Administrator's view, this risk 
assessment, which is based on studies that do not resolve the issue of 
a threshold, has important limitations as a basis for standard setting, 
since if no threshold is assumed the assessment necessarily predicts 
that ever lower standards result in ever lower risks. This has the 
effect of masking the increasing uncertainty in the risk estimates that 
exists as lower levels are considered, even when a range of assumed 
thresholds is included. As a result, at the time of proposal the 
Administrator viewed the risk assessment as providing supporting 
evidence for the conclusion that there is a need to revise the current 
suite of PM2.5 standards, but he judged that it did not 
provide an appropriate basis to determine what specific quantitative 
revisions are appropriate.
1. 24-Hour PM2.5 Standard
    Based on the approach discussed above, the Administrator relied 
upon evidence from the short-term exposure PM2.5 studies as 
the principal basis for selecting the proposed level of the 24-hour 
standard. In considering these studies as a basis for the level of a 
24-hour standard, and having provisionally selected a 98th percentile 
form for the standard, the Administrator agreed with the focus in the 
Staff Paper of looking at the 98th percentile values in these studies. 
In so doing, the Administrator recognized that these studies provide no 
evidence of clear effect thresholds or lowest-observed-effects levels. 
Thus, in focusing on 98th percentile values in these studies, the 
Administrator was seeking to establish a standard level that will 
require improvements in air quality generally in areas in which the 
distribution of daily short-term exposure to PM2.5 can 
reasonably be expected to be associated with serious health effects. 
Although future air quality improvement strategies in any particular 
area are not yet defined, most such strategies are likely to move a 
broad distribution of PM2.5 air quality values in an area 
lower, resulting in reductions in risk associated with exposures to 
PM2.5 levels across a wide range of concentrations.
    Based on the information in the Staff Paper and in a supporting 
staff memorandum,\33\ the Administrator observed an overall pattern of 
statistically significant associations reported in studies of short-
term exposure to PM2.5 across a wide range of 24-hour 
average 98th percentile values. More specifically, the Administrator 
observed a strong predominance of studies with 98th percentile values 
down to about 39 [mu]g/m3 (in Burnett and Goldberg, 2003) 
reporting statistically significant associations with mortality, 
hospital admissions, and respiratory symptoms. For example, within this 
range of air quality, statistically significant associations were 
reported for mortality in the combined Six Cities study (and three of 
four individual cities within that study \34\) (Klemm and Mason, 2003), 
the Canadian 8-City Study (Burnett and Goldberg, 2003), and in studies 
in Santa Clara County, CA

[[Page 61169]]

(Fairley, 2003) and Philadelphia (Lipfert, 2000); for hospital 
admissions and emergency department visits in Seattle (Sheppard et al., 
2003), Toronto (Burnett et al., 1997; Thurston et al., 1994), Detroit 
(Ito, 2003, for heart failure \35\ and pneumonia, but not for other 
causes), and Montreal (Delfino et al., 1998,\36\ for some but not all 
age groups and years); and for respiratory symptoms in panel studies in 
a combined Six Cities study (Schwartz et al., 1994, as reanalyzed in 
Schwartz and Neas, 2000) and in two Pennsylvania cities (Uniontown in 
Neas et al., 1995; State College in Neas et al., 1996).\37\ Studies in 
this air quality range that reported positive but not statistically 
significant associations include mortality studies in Detroit (Ito, 
2003), Pittsburgh (Chock et al., 2000), Steubenville (Klemm and Mason, 
2003), and Montreal (Goldberg and Burnett, 2003), and a study of lung 
function in Philadelphia \38\ (Neas et al., 1999).
---------------------------------------------------------------------------

    \33\ As discussed in the Staff Paper (EPA, 2005, p. 5-30) and 
supporting staff memo (Ross and Langstaff, 2005), staff focused on 
U.S. and Canadian short-term exposure PM2.5 studies that 
had been reanalyzed as appropriate to address statistical modeling 
issues and considered the extent to which the reported associations 
are robust to co-pollutant confounding and alternative modeling 
approaches and are based on relatively reliable air quality data. 
Additional air quality data used in this analysis were documented in 
another staff memo (Ross and Langstaff, 2006) that was placed in the 
docket during the public comment period.
    \34\ Of the four cities in this study that were within this 
range of air quality, statistically significant results were 
reported for Boston, St. Louis, and Knoxville, but not for 
Steubenville.
    \35\ The proposal incorrectly listed this as an association with 
ischemic heart disease.
    \36\ The proposal incorrectly included Delfino et al., 1997 here 
as well as correctly including it in the next lower air quality 
range.
    \37\ Of the studies within this group that evaluated multi-
pollutant associations, as discussed above in section II.A.3, the 
results reported in Fairley (2003), Sheppard (2003), and Ito (2003) 
were generally robust to inclusion of gaseous co-pollutants.
    \38\ The proposal incorrectly identified this as a statistically 
significant association.
---------------------------------------------------------------------------

    Within the range of 24-hour average 98th percentile 
PM2.5 concentrations of about 35 to 30 [mu]g/m3, 
the Administrator no longer observed this strong predominance of 
statistically significant results. Rather, within this range, one study 
reports statistically significant results (Mar et al., 2003), other 
studies report mixed results in which some associations reported in the 
study are statistically significant and others are not (Delfino et al., 
1997; Peters et al., 2000),\39\ and other studies report associations 
that are not statistically significant (Ostro, 2003; \40\ two 
individual cities within Klemm and Mason, 2003). Further, the 
Administrator concluded that the very limited number of studies in 
which the 98th percentile values are below this range (Stieb et al., 
2000; Peters et al., 2001) do not provide a basis for reaching 
conclusions about associations at such levels. Thus, in the 
Administrator's view, this body of evidence provided confidence that 
statistically significant associations are occurring down close to this 
range, and it provided a clear basis for provisionally concluding that 
this range represents a range of reasonable values for a 24-hour 
standard level. The Administrator further noted that focusing on the 
range of 35 to 30 [mu]g/m3 is consistent with the 
interpretation of the evidence held by most CASAC Panel members as 
reflected in their recommendation to select a 24-hour PM2.5 
standard level within this range (Henderson, 2005a, p. 7). The 
Administrator recognized, however, the separate point that most CASAC 
Panel members favored the range of 35 to 30 [mu]g/m3 for the 
24-hour PM2.5 standard in concert with an annual standard 
set in the range of 14 to 13 [mu]g/m3 (Id.), as discussed in 
section II.F.2 below.
---------------------------------------------------------------------------

    \39\ For example, Delfino et al. (1997) report statistically 
significant associations between PM2.5 and respiratory 
emergency department visits for elderly people (>64 years old), but 
not children (<2 years old), in one part of the study period (summer 
1993) but not the other (summer 1992). Peters et al. (2000) report 
new findings of associations between fine particles and cardiac 
arrhythmia, but the Criteria Document observes that the strongest 
associations were reported for a small subset of the study 
population that had experienced 10 or more defibrillator discharges 
(EPA, 2004a, p. 8-164).
    \40\ The proposal incorrectly identified this as a statistically 
significant association.
---------------------------------------------------------------------------

    At proposal, in considering what level would be appropriate for a 
24-hour standard, the Administrator was mindful that this choice 
requires judgment based on an interpretation of the evidence that 
neither overstates nor understates the strength and limitations of the 
evidence, or the appropriate inferences to be drawn from the evidence. 
In the absence of evidence of any clear effects thresholds, EPA may 
select a specific standard level from within a range of reasonable 
values. In making this judgment, the Administrator noted that the 
general uncertainties related to the shape of the concentration-
response functions and to the selection of appropriate statistical 
models affect the likelihood that observed associations are causal down 
to the lowest concentrations in the studies. Further, and more 
specifically, the variation in results found in the short-term exposure 
studies in which the 98th percentile values were below 35 [mu]g/m\3\ 
indicated an increase in uncertainty as to whether likely causal 
associations extend down below this level (71 FR 2649).
    In considering the extent to which the quantitative risk assessment 
should inform EPA's selection of a 24-hour PM2.5 standard, 
the Administrator recognized that risk estimates based on simulating 
the attainment of standards set at lower levels within this range will 
inevitably suggest some additional reductions in risk at each lower 
standard level considered. However, these quantitative risk estimates 
largely depend upon assumptions made about the lowest level at which 
reported associations will likely persist and remain causal in nature. 
Thus, the Administrator was hesitant to use such risk estimates as a 
basis for proposing a specific standard level, particularly one below 
35 [mu]g/m\3\, and instead preferred to base the decision on level 
directly on the evidence in the studies themselves (71 FR 2649).
    Taking the above considerations into account, the Administrator 
proposed to set the level of the primary 24-hour PM2.5 
standard at 35 [mu]g/m\3\.\41\ In the Administrator's judgment at that 
time, based on the currently available evidence, a standard set at this 
level would protect public health with an adequate margin of safety 
from serious health effects, including premature mortality and hospital 
admissions for cardiorespiratory causes that are likely causally 
associated with short-term exposure to PM2.5. This judgment 
appropriately considered the requirement for a standard that is neither 
more nor less stringent than necessary for this purpose and recognized 
that the CAA does not require that primary standards be set at a zero-
risk level, but rather at a level that reduces risk sufficiently so as 
to protect public health with an adequate margin of safety.
---------------------------------------------------------------------------

    \41\ As noted above, the proposed form of the 24-hour standard 
was the same as the current standard.
---------------------------------------------------------------------------

    At the time of proposal, the Administrator recognized that sharply 
divergent views on the appropriate level of this standard had been 
presented to EPA as part of the NAAQS review process, and solicited 
comment on a wide range of standard levels and alternative approaches 
to characterizing and addressing scientific uncertainties. One such 
alternative view focused very strongly on the uncertainties inherent in 
the epidemiologic and toxicologic studies and the quantitative risk 
assessment as the basis for concluding that no change to the current 
24-hour PM2.5 standard of 65 [mu]g/m\3\ was warranted. In 
sharp contrast, others viewed the epidemiologic evidence and other 
health studies as strong and robust, and generally placed much weight 
on the results of the quantitative risk assessment as a basis for 
concluding that a much stronger policy response is warranted, generally 
consistent with a standard level at or below 25 [mu]g/m\3\. As 
discussed below, the same sharply divergent views were generally 
repeated in comments on the proposal by the two distinct groups of 
commenters identified in section II.B.2 above.
    In considering comments received on the proposal, the Administrator 
first notes that CASAC provided additional recommendations concerning 
the

[[Page 61170]]

proposed PM standards in a letter to the Administrator (Henderson, 
2006, p. 2), noting that members of the CASAC PM Panel were generally 
pleased that the proposed 24-hour PM2.5 primary standard was 
within the range that had previously been recommended by most members. 
Further, the Panel recognized that the proposed choice of the high end 
of the recommended range was a policy judgment. A number of commenters, 
including many States and Tribes, who supported the proposed level 
generally placed great weight on the recommendation of CASAC.
    Many more commenters expressed disagreement with the proposed 
level. As noted above, these commenters generally fell into two 
distinct groups that expressed sharply divergent views on their 
interpretations of the science (in some cases taking into consideration 
``new'' science not included in the Criteria Document), on the 
appropriate policy response based on the science, and on how the 
quantitative risk assessment should factor into a decision on the 
standard level.
    In interpreting the available scientific information, including 
consideration of ``new'' science, and advocating a policy response 
based on the science, one group of commenters focused strongly on the 
uncertainties they saw in the scientific evidence as a basis for 
concluding that no change to the current level of the 24-hour 
PM2.5 standard was warranted. This group included virtually 
all commenters representing industry associations and businesses. In 
commenting on the proposed level, these commenters most generally 
relied on the same arguments presented above in section II.B.2 as to 
why they believed it was inappropriate for EPA to make any revisions to 
the suite of primary PM2.5 standards. That is, they asserted 
that the health effects of concern associated with short-term exposure 
to PM2.5 have not changed significantly since 1997; that the 
uncertainties in the underlying time-series epidemiologic studies are 
as great or greater than in 1997; that the estimated risk upon 
attainment of the current PM2.5 standards is lower now than 
it was when the PM2.5 standards were set in 1997; and that 
``new'' science not included in the Criteria Document continues to 
increase uncertainty about possible health risks associated with 
exposure to PM2.5. These general comments are addressed 
above in section II.B.2.
    In more specific comments, UARG and other commenters in this group 
called into question EPA's rationale for the proposed level of 35 
[mu]g/m\3\. In so doing, these commenters primarily relied on an 
examination of this rationale included in an attachment to UARG's 
comments as the basis for concluding that the available studies do not 
support EPA's view of the overall pattern of statistically significant 
associations in studies of short-term exposure to PM2.5 
across a wide range of 98th percentile PM2.5 values. This 
examination of such studies concluded that there is no consistent 
pattern of associations at levels up to (and above) the 65 [mu]g/m\3\ 
98th percentile level of the current standard. This examination was 
based on an individual consultant's ranking of a set of short-term 
exposure studies by what is characterized as the ``overall 
significance'' of each study's results. A number of studies were 
included in this examination that EPA did not include in looking at the 
pattern of associations.
    In considering the approach used in this examination, EPA concludes 
that the categorical rankings were inappropriately defined in a very 
restrictive way that overly emphasized certain studies based on 
selection criteria that favored multi-pollutant models and alternative 
model specifications, which had the effect of dismissing statistically 
significant results in some studies. This conclusion reflects EPA's 
consideration of these issues as presented above in section II.B.2. As 
noted there, EPA believes in the importance of a comprehensive 
evaluation that considers and weighs a variety of evidence, including 
biological plausibility of associations between the various pollutants 
and health outcomes, and focuses on the stability of the size of the 
effect estimates in time-series studies using both single- and multi-
pollutant models, rather than just looking at statistical significance 
in a large number of alternative models and using it simplistically to 
delineate between real and suspect associations. In addition, the 
examination included several studies that, for a variety of reasons, 
EPA does not believe are appropriate for such an analysis. The 
inclusion of such studies, many of which had low statistical power, 
served to dilute the pattern of associations seen in studies considered 
by EPA as providing a more appropriate basis for this type of 
examination.
    Further, even if this examination were to be accepted at face 
value, it still would support a distinction between the patterns of 
associations above and below the proposed level, in that over half of 
the cited studies with 98th percentile values above 35 [mu]g/m\3\ were 
characterized as being of overall or mixed significance, and more than 
half of the cited studies with 98th percentile values below 35 [mu]g/
m\3\ were characterized as having no overall significant association. 
After fully considering this examination of patterns of study results, 
the Administrator believes that the observations of patterns of study 
results presented earlier in this section remain valid.\42\
---------------------------------------------------------------------------

    \42\ The EPA's consideration of this examination is discussed 
more fully in the Response to Comments document.
---------------------------------------------------------------------------

    The other group of commenters, including many medical groups, 
numerous physicians and academic researchers, many public health 
organizations, some States, and a large number of individual 
commenters, viewed the epidemiologic evidence and other health studies 
as strong and robust and expressed the belief that a much stronger 
policy response is warranted, generally consistent with a standard 
level at or below 25 [mu]g/m\3\. Some of these commenters generally 
expressed the view that the level of the standard should be set below 
the lowest level observed in any of the studies that report any 
statistically significant association. Some also expressed the view 
that important uncertainties inherently present in the evidence warrant 
a highly precautionary policy response, particularly in view of the 
serious nature of the health effects at issue, and should be addressed 
by selecting a standard level that incorporates a large margin of 
safety.
    More specifically, American Lung Association et al. and other 
commenters noted three studies included in the Criteria Document with 
98th percentile values below 35 [mu]g/m\3\, including a mortality study 
in Phoenix (Mar et al., 2000; reanalyzed in Mar et al., 2003) with a 
98th percentile value of 32 [mu]g/m\3\, a study of emergency department 
visits in Montreal (Delfino et al., 1997) with a 98th percentile value 
of 31 [mu]g/m\3\, and a study of increase in myocardial infarction in 
Boston (Peters et al., 2001) with a 98th percentile value of 28 [mu]g/
m\3\. Further, these commenters expressed the view that EPA's proposed 
approach to selecting a level of the 24-hour PM2.5 standard 
is fundamentally flawed because it ``relies unreasonably on point 
estimates of statistical significance at various concentrations, rather 
than on trends, and because it completely fails to consider issues of 
statistical power'' (American Lung Association et al., p. 57). In 
addition, these commenters found EPA's justification for the proposed 
level to be ``simply irrational'' in that it ``essentially fabricates 
uncertainty'' as a basis for avoiding setting a standard that

[[Page 61171]]

the evidence ``clearly indicates is necessary'' (Id.).
    In considering these comments, the Administrator first notes that 
he generally agrees with CASAC's view that selecting a level within the 
range of 30 to 35 [mu]g/m\3\ is a public health policy judgment and 
that the science does not dictate the selection of any specific level 
within this range. The Administrator also believes that this policy 
judgment should take into consideration the important uncertainties 
that remain in issues that are central to interpreting these types of 
time-series epidemiologic studies. While the Administrator believes 
that progress has been made since the last review in addressing key 
uncertainties, as discussed above in section II.B.2, EPA and the 
scientific community, including CASAC and the National Research Council 
(NRC), recognize that important uncertainties remain that warrant 
further research (e.g., see NRC, 2004). Thus, the Administrator does 
not agree that the Agency is ``fabricating'' uncertainties that do not 
exist. More specifically, in considering the studies cited in these 
comments as a basis for a standard level below 35 [mu]g/m\3\, the 
Administrator continues to believe that it is necessary to consider not 
only the results of these studies and the inherent uncertainties in 
such studies, but also the pattern of results from other studies with 
similar air quality values. In so doing, EPA notes that the 
statistically significant results in Peters et al. (2001) were uniquely 
associated with 1 to 2 hour lag times, but not with 24-hour average 
PM2.5 concentrations, such that it would provide a very 
tenuous basis for the level of a 24-hour average national standard. 
While the studies in Phoenix and Montreal do provide some evidence of 
statistically significant associations within the range of 30 to 35 
[mu]g/m\3\, several other studies within this range of air quality that 
generally have somewhat greater statistical power and narrower 
confidence ranges do not provide such evidence. In making the public 
health policy judgment inherent in selecting a standard level, the 
Administrator believes that it is necessary to weigh the evidence and 
related uncertainties against the requirement that the standard is to 
be neither more nor less stringent than necessary to protect public 
health with an adequate margin of safety. See NRDC v. EPA, 902 F. 2d 
962, 971 (D.C. Cir. 1990) (in considering level of a NAAQS, EPA is 
required to take into account all of the relevant studies in the record 
and rationally determine what weight to give each study); API v. 
Costle, 665 F. 2d 1176, 1187 (DC Cir. 1981) (same). In so doing, the 
Administrator does not agree that this evidence presented by American 
Lung Association et al. warrants a level below 35 [mu]g/m\3\.
    These commenters also identified several ``new'' studies in support 
of their arguments for a lower level. As noted above, as in past NAAQS 
reviews, EPA is basing the final decisions in this review on the 
studies and related information included in the PM air quality criteria 
that have undergone CASAC and public review, and will consider the 
newly published studies for purposes of decision making in the next PM 
NAAQS review. Nonetheless, in provisionally evaluating commenters' 
arguments (see Response to Comments document), EPA notes that its 
provisional assessment of ``new'' science found that such studies did 
not materially change the conclusions in the Criteria Document.
    With regard to the other studies, EPA notes that neither the 
Vancouver nor the Atlanta studies found statistically significant 
associations with PM2.5, and that the Atlanta and California 
studies were conducted in areas with 98th percentile PM2.5 
values well above the proposed level. Thus, EPA concludes that, taken 
at face value, these studies would provide no basis for the commenters' 
claim that they would require a lower standard level than one based on 
the science included in the Criteria Document.
    With regard to considering how the quantitative risk assessment 
should factor into a decision on the standard level, EPA notes that 
both groups of commenters generally consider the risk assessment in 
their comments on the standard level, but they reach diametrically 
opposed conclusions as to what standard level is supported by the 
assessment. The general views of both groups on the implications of the 
risk assessment are presented above in section II.B.2, with one group 
arguing that it supports a decision not to revise either of the current 
PM2.5 standards, and the other group arguing that it 
supports a decision to revise both PM2.5 standards. More 
specifically, some of the medical/environmental health commenters 
consider the magnitude of risk estimated to remain upon meeting the 
proposed 24-hour standard as a strong reason to select a lower level. 
These commenters generally assert that the risks are likely even higher 
than EPA's primary estimates, in part because EPA incorporated a 
surrogate threshold of 10 [mu]g/m\3\ even though there is no clear 
evidence of a threshold in the relevant time-series studies. On the 
other hand, the industry/business commenters generally assert that the 
risks are likely lower than EPA's primary estimates, in part because 
EPA did not base its primary estimates on an assessment that included 
all statistical model results presented in the studies. Having 
considered comments based on the quantitative risk assessment from both 
groups of commenters, the Administrator finds no basis to change the 
position on the risk assessment that was taken at the time of proposal. 
That is, as discussed above, while the Administrator recognizes that 
the risk assessment rests on a more extensive body of data and is more 
comprehensive in scope than the assessment conducted in the last 
review, he is mindful that significant uncertainties continue to 
underlie the resulting quantitative risk estimates. Further, in the 
Administrator's view, as noted above in this section, this risk 
assessment, which is based on studies that do not resolve the issue of 
a threshold, has important limitations as a basis for standard setting 
in this review, since if no threshold is assumed the assessment 
necessarily predicts that ever lower standards result in ever lower 
risks. This has the effect of masking the increasing uncertainty that 
exists as lower levels are considered, even when a range of assumed 
thresholds are considered. As a result, the Administrator judges that 
the quantitative risk assessment does not provide an appropriate basis 
for selecting the level of the 24-hour PM2.5 standard.
    After carefully taking the above comments and considerations into 
account, the Administrator has decided to set the level of the primary 
24-hour PM2.5 standard at 35 [mu]g/m\3\. In the 
Administrator's judgment, based on the currently available evidence, a 
standard set at this level will protect public health with an adequate 
margin of safety from serious health effects including premature 
mortality and hospital admissions for cardiorespiratory causes that are 
likely causally associated with short-term exposure to 
PM2.5. A standard set at a higher level would not likely 
result in improvements in air quality in areas across the country in 
which short-term exposure to PM2.5 can reasonably be 
expected to be associated with serious health effects. A standard set 
at a lower level would only result in significant further public health 
protection if, in fact, there is a continuum of health risks down to 
the lower end of the ranges of air quality observed in the key 
epidemiologic studies and if the reported associations are, in fact, 
causally related to PM2.5 at

[[Page 61172]]

those lower levels. Based on the pattern of results observed in the 
available evidence, the Administrator is not prepared to make those 
assumptions. Taking into account the uncertainties that remain in 
interpreting the available epidemiologic studies, the likelihood of 
obtaining benefits to public health decreases at lower levels while the 
likelihood of requiring reductions in ambient concentrations that go 
beyond those that are needed to reduce risks to public health 
increases. On balance, the Administrator does not believe that a lower 
standard is necessary to provide the requisite degree of public health 
protection. This judgment by the Administrator appropriately considers 
the requirement for a standard that is neither more nor less stringent 
than necessary for this purpose and recognizes that the CAA does not 
require that primary standards be set at a zero-risk level, but rather 
at a level that reduces risk sufficiently so as to protect public 
health with an adequate margin of safety.
2. Annual PM2.5 Standard
    Based on the approach discussed above at the beginning of section 
II.F, at the time of proposal the Administrator relied upon evidence 
from the long-term exposure PM2.5 studies as the principal 
basis for selecting the proposed level of the annual standard. In 
considering these studies as a basis for the level of an annual 
standard, the Administrator agreed with the evidence-based focus in the 
Staff Paper of looking at the long-term mean PM2.5 
concentrations across the cities included in such long-term studies. In 
so doing, the Administrator recognized that these studies, like the 
short-term exposure studies, provide no evidence of clear effect 
thresholds or lowest-observed-effects levels. Thus, in focusing on the 
cross-city long-term mean concentrations in these studies, the 
Administrator was seeking to establish a standard level that will 
require improvements in air quality in areas in which long-term 
exposure to PM2.5 can reasonably be expected to be 
associated with serious health effects.
    Based on the characterization and assessment of the long-term 
PM2.5 exposure studies presented in the Criteria Document 
and Staff Paper, in the proposal the Administrator recognized the 
importance of the validation efforts and reanalyses that have been done 
since the last review of the original Six Cities and ACS mortality 
studies. These new assessments provide evidence of generally robust 
associations and provide a basis for greater confidence in the reported 
associations than in the last review, for example, in the extent to 
which they have made progress in understanding the importance of issues 
related to co-pollutant confounding and the specification of 
statistical models. Consistent with the information available in the 
last review, these two key long-term exposure mortality studies 
reported long-term mean PM2.5 concentrations across all the 
cities included in the studies of 18 and 21 [mu]g/m\3\, respectively. 
The Administrator also particularly recognized the importance of the 
extended ACS mortality study, published since the last review, which 
provides new evidence of mortality related to lung cancer and further 
substantiates the statistically significant associations with 
cardiorespiratory-related mortality observed in the original 
studies.\43\ The Administrator noted that the statistically significant 
associations reported in the extended ACS study, in a large number of 
cities across the U.S., provide evidence of effects at a lower long-
term mean PM2.5 concentration (17.7 [mu]g/m\3\) than had 
been observed in the original study, although the relative risk 
estimates are somewhat smaller in magnitude than those reported in the 
original study. The assessment in the Criteria Document of these 
mortality studies, taking into account study design, the strength of 
the study (in terms of statistical significance and precision of 
result), and the robustness of results, concluded that it would be 
appropriate to give the greatest weight to the reanalyses of the Six 
Cities and ACS studies, and in particular to the results of the 
extended ACS study (EPA, 2004a, p. 9-33) in weighing the evidence of 
mortality effects associated with long-term exposure to 
PM2.5. Consistent with that assessment, the Administrator 
placed greatest weight on these studies as a basis for selecting the 
proposed level of the annual PM2.5 standard.
---------------------------------------------------------------------------

    \43\ In the extended ACS study, significant lung cancer 
associations were found for those with high school education or 
less, but not for those with better than a high school education. 
When data are combined for all education levels, a significant 
association is found.
---------------------------------------------------------------------------

    In addition to these mortality studies, the Administrator also 
recognized the availability of relevant morbidity studies providing 
evidence of respiratory morbidity, including decreased lung function 
growth, in children with long-term exposure to PM2.5. 
Studies conducted in the U.S. and Canada include the 24-Cities study 
considered in the last review and more recent studies of cohorts of 
children in southern California, in which the long-term mean 
PM2.5 concentrations in all the cities included in the 
studies are approximately 14.5 and 15 [mu]g/m\3\, respectively. As 
discussed in section II.A. of the proposal (71 FR at 2632), in the 24 
Cities study, statistically significant associations were reported 
between long-term fine particle exposures and lung function measures at 
a single point in time, whereas positive but generally not 
statistically significant associations were reported with prevalence of 
several respiratory conditions. As interpreted in the last review, the 
results from the 24-Cities study are uncertain as to the extent to 
which the association extends below a long-term mean PM2.5 
concentration of approximately 15 [mu]g/m\3\. The more recent Southern 
California children's cohort study provides evidence of important 
respiratory morbidity effects in children, including evidence for a new 
measure of morbidity, decreased growth in lung function. Reports from 
this study suggest that long-term PM2.5 exposure is 
associated with decreases in lung function growth, as measured over a 
four-year follow-up period, although statistically significant 
associations are not consistently reported. The Administrator 
recognized that these are important new findings, indicating that long-
term PM2.5 exposure may be associated with respiratory 
morbidity in children. However, the Administrator also observed that 
this is the only study reporting decreased lung function growth, 
conducted in just one area of the country, such that further study of 
this health endpoint in other areas of the country would be needed to 
increase confidence in the reported associations. Thus, the 
Administrator provisionally concluded that this study provides an 
uncertain basis for establishing the level of a national standard (Id. 
at 2651).
    The Administrator generally agreed that, as discussed in the Staff 
Paper (EPA, 2005, p. 5-22), it was appropriate to consider a level for 
an annual PM2.5 standard that is below the averages of the 
long-term PM2.5 concentrations across the cities in the key 
long-term exposure mortality studies, recognizing that the evidence of 
an association in any such study is strongest at and around the long-
term average where the data in the study are most concentrated. The 
Administrator was mindful that considering what standard is requisite 
to protect public health with an adequate margin of safety requires 
public health policy judgments that neither overstate nor understate 
the strength and limitations of the evidence or the appropriate 
inferences to be drawn from the evidence. The Administrator 
provisionally concluded that these key mortality studies, together

[[Page 61173]]

with the morbidity studies, provide a basis for considering a standard 
level no higher than 15 [mu]g/m\3\. This level is somewhat below the 
long-term mean concentrations in the key mortality studies and 
consistent with the interpretation of the evidence from the morbidity 
studies discussed above. Further, in the Administrator's provisional 
view, these studies did not provide an appropriate basis for selecting 
a level lower than the current standard of 15 [mu]g/m\3\.
    In considering the extent to which the quantitative risk assessment 
can help to inform these judgments with regard to the annual 
PM2.5 standard, the Administrator again recognized that risk 
estimates based on simulating the attainment of standards set at lower 
levels, as expected, continue to suggest some additional reductions in 
risk at the lower standard levels considered in the assessment, and 
that these estimates largely depend upon assumptions made about the 
lowest level at which reported associations will likely persist and 
remain causal in nature. Thus, the Administrator was again hesitant to 
use such risk estimates as a basis for proposing a lower annual 
standard level than 15 [mu]g/m\3\, the level that is based directly on 
the evidence in the studies themselves, as discussed above.
    Taking the above considerations into account, the Administrator 
proposed to retain the level of the primary annual PM2.5 
standard at 15 [mu]g/m\3\. In the Administrator's judgment at that 
time, based on the currently available evidence, a standard set at this 
level would be requisite to protect public health with an adequate 
margin of safety from serious health effects, including premature 
mortality and respiratory morbidity that are likely causally associated 
with long-term exposure to PM2.5. This judgment by the 
Administrator appropriately considered the requirement for a standard 
that is neither more nor less stringent than necessary for this purpose 
and recognized that the CAA does not require that primary standards be 
set at a zero-risk level, but rather at a level that reduces risk 
sufficiently so as to protect public health with an adequate margin of 
safety.
    At the time of proposal, the Administrator recognized that the 
CASAC Panel did not endorse retaining the annual standard at the 
current level of 15 [mu]g/m\3\ (Henderson, 2005a, p. 7). In weighing 
the recommendation of the CASAC Panel, the Administrator carefully 
considered CASAC's stated rationale. In discussing its recommendation 
(Henderson, 2005a), the CASAC Panel first noted that changes to either 
the annual or 24-hour PM2.5 standard, or both, could be 
recommended. The Panel then gave three reasons for placing more 
emphasis on lowering the 24-hour standard than the annual standard: (1) 
The vast majority of studies indicating effects of short-term 
PM2.5 exposure were carried out in settings in which 
PM2.5 concentrations were largely below the current 24-hour 
standard level of 65 [mu]g/m\3\; (2) the amount of evidence on short-
term exposure effects, at least as reflected by the number of reported 
studies, is greater than for long-term exposure effects; and (3) 
toxicologic findings are largely related to the effects of short-term, 
rather than long-term, exposures. In not endorsing the option presented 
in the Staff Paper of retaining the level of the current annual 
standard in conjunction with lowering the 24-hour standard, the CASAC 
Panel observed that some cities have relatively high annual 
PM2.5 concentrations without much day-to-day variation and 
that such cities would only rarely exceed a 24-hour standard, even if 
it were set at a level below the current standard. In such a city, 
attaining a 24-hour standard would likely have minimal if any effect on 
the long-term mean PM2.5 concentration and consequently 
would be less likely to reduce health effects associated with long-term 
exposures. These observations indicate the desirability of lowering the 
level of the annual PM2.5 standard as well as that of the 
24-hour standard, so as to ensure that revisions to the standards 
achieve appropriate reductions in long-term exposures. Based on these 
considerations and taking into account the results of the risk 
assessment, most CASAC Panel members favored setting an annual standard 
in the range of 14 to 13 [mu]g/m\3\, along with lowering the 24-hour 
standard (Henderson, 2005a, p. 7).
    In considering these views, the Administrator noted that the 
appropriateness of setting an annual standard that would lower annual 
PM2.5 concentrations in cities across the country depends 
upon a policy judgment as to what annual level is required to protect 
public health with an adequate margin of safety from long-term 
exposures to PM2.5 in light of the available evidence. In 
considering the evidence of effects associated with long-term 
PM2.5 exposure as a basis for selecting an adequately health 
protective annual standard, as discussed above, the Administrator 
provisionally concluded that the evidence did not provide a basis for 
requiring annual levels below 15 [mu]g/m\3\. Thus, the Administrator 
agreed conceptually with the CASAC Panel that any particular 24-hour 
standard may not result in reductions in the level of long-term 
exposures to PM2.5 in all areas with relatively higher than 
typical annual PM2.5 concentrations and lower than typical 
ratios of peak-to-mean values (71 FR 2652). Further, the Administrator 
agreed that this general advice supported relying on the annual 
standard, and not the 24-hour standard, to achieve the appropriate 
level of protection from long-term exposures to PM2.5. 
However, the Administrator did not believe that this advice necessarily 
translated into a reason for setting the annual PM2.5 
standard at a level below the current level of 15 [mu]g/m\3\. As 
discussed above, the Administrator believed that the principal basis 
for selecting the appropriate level of an annual standard should be the 
evidence provided by the long-term studies, in conjunction with 
judgments concerning whether and over what range of concentrations the 
reported associations are likely causal, without reliance on the risk 
assessment, and that this evidence reasonably supported retaining the 
current level of the annual standard (Id.).
    Reflecting the great importance that EPA places on the advice of 
CASAC, the Administrator solicited broad public comment on the range of 
15 down to 13 [mu]g/m\3\ the low end of the range recommended by CASAC 
for the level of the annual PM2.5 standard, and on the 
reasoning that formed the basis for that recommendation. The 
Administrator recognized that a decision to select a standard in this 
range below 15 [mu]g/m\3\ would place greater weight on the strength of 
the associations reported in the key epidemiologic mortality and 
morbidity long-term exposure studies down to the lower part of the 
range of PM2.5 concentrations observed across all the cities 
included in these studies. Such a standard could also reflect greater 
reliance on the results of the quantitative risk assessment that 
suggested increased reductions in risk associated with meeting an 
annual standard at such lower levels (Id.).
    At the time of proposal, the Administrator also recognized that 
sharply divergent views on the appropriate level of this standard had 
been presented to EPA as part of the NAAQS review process, and 
solicited comments on a wider range of levels, down to 12 [mu]g/m\3\ on 
alternative views of the appropriate interpretation of the 
epidemiologic evidence and related uncertainties, and on relevant 
research that would improve our understanding of key issues and 
analytic approaches to

[[Page 61174]]

better inform policy judgments in the future. As was the case with the 
24-hour PM2.5 standard, the same sharply divergent views 
were again expressed by the two distinct groups of commenters 
identified above in section II.B.2, as discussed below.
    In considering comments received on the proposal, the Administrator 
first notes that CASAC requested that EPA reconsider its proposed 
decision on the level of the annual PM2.5 standard and set 
the level within the range that CASAC had previously recommended, 13 to 
14 [mu]g/m\3\ (Henderson, 2006, p. 1).\44\ In so doing, CASAC 
reiterated and elaborated on the scientific basis for its earlier 
recommendation (Henderson, 2006, pp. 3-4), which included consideration 
of the Agency's risk assessment (as ``the primary means of determining 
the effects on risk of changes in the 24-hour and annual 
PM2.5 standards in concert'') as well as the observations 
that ``a lower daily PM2.5 concentration limit alone cannot 
be relied on to provide protection against the adverse effects of 
higher annual average concentrations,'' that ``there is evidence that 
effects of long-term PM2.5 concentrations occur at or below 
the level of the current standard,'' and that ``short-term effects of 
PM2.5 persist in cities with annual PM2.5 
concentrations below the current standard'' down to approximately 13 
[mu]g/m\3\ (e.g., Burnett and Goldberg, 2003; Mar et al., 2003; and 
Lipsett et al., 1997). The CASAC concluded:

    \44\ Two PM Panel members did not agree with the views of the 
majority, expressing the view that there was an adequate scientific 
basis to choose an annual PM2.5 standard level within the 
range of 12 to 15 [mu]g/m\3\ and that the choice of a specific level 
within that range was a policy decision (Henderson, 2006, p. 6).
---------------------------------------------------------------------------

    In summary, the epidemiologic evidence, supported by emerging 
mechanistic understanding, indicates adverse effects of 
PM2.5 at current annual average levels below 15 [mu]g/
m3. The PM Panel realized the uncertainties involved in 
setting an appropriate, health-protective level for the annual 
standard, but noted that the uncertainties would increase rapidly 
below the level of 13 [mu]g/m3. That is the basis for the 
PM Panel recommendation of a level at 13-14 [mu]g/m3 
(Henderson, 2006, p. 4).

    In response to CASAC's request for reconsideration, the 
Administrator has carefully considered its stated views and the 
scientific basis for the range it recommended. As an initial matter, 
the Administrator notes that CASAC's recommendation to lower the level 
of the annual standard was based in large measure on the results of the 
Agency's risk assessment, which examined changes in both the 24-hour 
and annual standard levels in concert. In considering this information 
qualitatively, as discussed above in section II.B, the Administrator 
believes that the estimates of risks likely to remain upon attainment 
of the current suite of PM2.5 standards are indicative of 
risks that can reasonably be judged to be important from a public 
health perspective, and thus support revision of the current suite of 
standards. In addressing what revisions to the current suite of 
PM2.5 standards are appropriate, the Administrator has 
determined that the evidence of health effects associated with short-
term exposure to PM2.5 is such that it is appropriate to 
lower the level of the 24-hour PM2.5 standard (as discussed 
in section II.F.1 above). However, as discussed more fully above, the 
Administrator also believes that this risk assessment has important 
limitations as a basis for setting a standard level in this review, in 
part because the available studies do not resolve questions related to 
potential effect thresholds and because of other important 
uncertainties noted above in section II.A.3. As a result, the 
Administrator judges that the quantitative risk assessment does not 
provide an appropriate basis for selecting the level of either the 24-
hour or the annual PM2.5 standard. Thus, the Administrator 
more heavily weighs the implications of the uncertainties associated 
with the Agency's quantitative risk assessment than CASAC apparently 
does, and disagrees with CASAC that the risk assessment results 
appropriately serve as a primary basis for a decision on the level of 
the annual PM2.5 standard.
    The CASAC also considered the evidence from specific short-term 
exposure studies as part of the basis for its recommendation for a 
lower annual standard level, pointing to studies indicating that 
effects from short-term exposure of PM2.5 persist in cities 
with annual PM2.5 concentrations below the current standard. 
While the Administrator does not disagree with CASAC's factual 
statements regarding the findings of the studies of short-term exposure 
effects, he believes that, based on the evidence available in this 
review, it is more appropriate to consider the short-term exposure 
studies as a basis for the level of the 24-hour standard and to 
consider the long-term exposure studies as a basis for the level of the 
annual standard. The Administrator recognizes that the Agency used 
available short-term exposure studies as the primary basis for setting 
the level of a ``generally controlling'' annual standard in the last 
review, with the purpose that the annual standard would provide 
protection against both short-term exposures and long-term exposures, 
but notes that such a public health policy choice was made primarily 
because the short-term exposure studies were judged to be the strongest 
evidence available at that time and the evidence from long-term 
exposure studies was judged to be too limited to serve as other than a 
secondary consideration in setting the level of the annual standard. 
See 62 FR 38675 n. 41 and 38676. In this review, however, the bodies of 
evidence for both short- and long-term exposures have been 
substantially extended and strengthened, such that each 
PM2.5 standard can appropriately be evaluated based on the 
most directly relevant body of scientific studies, and can be focused 
on providing protection from the health risks evaluated in that body of 
scientific studies. The Administrator continues to believe, consistent 
with the evidence-based approach presented in the Staff Paper, that 
using evidence of effects associated with periods of exposure that are 
most closely matched to the averaging time of each standard is the most 
appropriate public health policy approach to evaluating the scientific 
evidence in selecting the level of each standard, with each standard 
designed to provide protection from the health risks associated with 
exposures reflecting that averaging time. Thus, the Administrator 
believes that the 24-hour standard should be set so as to provide an 
appropriate degree of protection from health effects associated with 
short-term exposures to PM2.5, and the annual standard 
should be set so as to provide an appropriate degree of protection from 
health effects associated with long-term exposures to PM2.5. 
In determining the level of each standard, the Administrator believes 
it is appropriate to rely on the short-term studies for purposes of 
determining the level of the 24-hour standard, and the long-term 
studies for purposes of determining the level of the annual 
standard.\45\ Therefore, the Administrator does not believe that 
evidence from short-term exposure studies is an appropriate basis for 
selecting any different level of the annual standard in this review 
than that selected based on the long-term exposure evidence. The EPA 
has instead

[[Page 61175]]

evaluated these short-term exposure studies in the context of 
determining the appropriate level for the 24-hour standard.
---------------------------------------------------------------------------

    \45\ This is consistent with the approach taken in the Staff 
Paper, sections 5.3.4.1 and 5.3.5.1, for evaluating the evidence-
based considerations related to setting the standards. The CASAC's 
letter of June 6, 2005 states that the Second Draft of the Staff 
Paper was ``Scientifically well-reasoned,'' with the exception of a 
section not relevant to the fine PM (Henderson, 2005a, pp. 1-2). The 
CASAC's general view thus includes this evidence-based approach 
presented in the Staff Paper.
---------------------------------------------------------------------------

    Finally, CASAC also expressed the view that there is evidence that 
effects of long-term PM2.5 concentrations occur at or below 
the level of the current standard. While the Administrator agrees that 
any such evidence would be directly relevant to his decision on the 
level of the annual PM2.5 standard, CASAC did not provide 
any specific information as to what studies it felt provided such 
evidence nor the considerations that played a role in its 
interpretation of the studies, including its assessment of the 
uncertainties inherent in any such studies.\46\ As discussed below, the 
Administrator has considered the available studies of long-term 
exposure to PM2.5, together with the uncertainties inherent 
in that body of evidence, to reach his final decision on the level of 
the annual standard. However, since CASAC did not provide any more 
specific statements as to its assessment of such mortality or morbidity 
studies, the Administrator cannot determine in what ways his judgments 
about that evidence may differ from CASAC's views.\47\ Lacking such 
specific statements to support CASAC's view that there is evidence that 
effects of long-term PM2.5 concentrations occur at or below 
the level of the current standard, the Administrator cannot discern a 
clear line of scientific reasoning that would preclude the current 
level of 15 [mu]g/m3 from being a reasonable policy choice 
based on the most relevant available evidence on the health effects of 
long-term exposures to PM2.5.
---------------------------------------------------------------------------

    \46\ The EPA does not believe that CASAC based this statement on 
the evidence it cites concerning effects associated with the long-
term means of the short-term studies. These studies address effects 
from short-term exposures, and do not address effects from long-term 
exposures.
    \47\ The CASAC did express the view that although the ``new'' 
scientific literature that was not included in the Criteria Document 
appears to support its findings, that literature was not needed to 
support its recommendation of a lower annual standard level 
(Henderson, 2006, p. 6).
---------------------------------------------------------------------------

    As noted above, EPA received other comments on the proposal from 
two distinct groups of commenters. One group that included virtually 
all commenters representing industry associations and businesses agreed 
with the Agency's proposed decision not to revise the level of the 
annual PM2.5 standard. The other group of commenters 
included many medical groups, numerous physicians and academic 
researchers, many public health organizations, many States, and a large 
number of individual commenters. They strongly disagreed with the 
Agency's proposed decision and argued that EPA should lower the level 
of the annual PM2.5 standard. While some of these commenters 
felt that the level should be set within the range recommended by 
CASAC, most such commenters advocated a level of 12 [mu]g/
m3. These commenters largely based their views on the same 
general considerations put forward by CASAC as a basis for its 
recommendation to lower the level of the annual PM2.5 
standard. To the extent that these commenters, like CASAC, relied upon 
the Agency's risk assessment or the evidence from short-term exposure 
studies as a basis for their views, their comments are addressed above. 
Comments that address how specific long-term PM2.5 exposure 
studies should be considered as a basis for the level of the annual 
PM2.5 standard are addressed below.
    A few commenters offered detailed comments on the key long-term 
exposure PM2.5 mortality studies discussed in the proposal, 
including the original analyses and reanalyses of the ACS and Six 
Cities cohorts and the extended ACS cohort study. In general, some 
medical/public health/researcher/State commenters expressed the view 
that EPA has downplayed the results of these studies to the extent that 
they provide evidence of effects below the level of the current 
standard. For example, American Lung Association et al. and Schwartz 
(2006) asserted that the ACS cohort study and the HEI reanalysis 
provide direct evidence of premature mortality associated with annual 
exposures below 15 [mu]g/m3 based on plots of the 
concentration-response function between long-term exposure to 
PM2.5 and risk of dying across 50 U.S. metropolitan areas 
that show no substantial deviation from linear, non-threshold 
relationships down through levels well below 15 [mu]g/m3. 
These commenters did not, however, discuss the uncertainties inherent 
in this type of epidemiologic study or the implications of these 
uncertainties on their interpretation of the results.
    In contrast, some industry/business commenters (e.g., Pillsbury et 
al.; Annapolis Center; UARG) emphasized that uncertainties remain in 
interpreting these studies with regard to issues such as potential 
confounding by co-pollutants, especially SO2, modeling to 
address spatial correlations in the data, and effect modification by 
education level or socioeconomic status. In addition, some industry/
business commenters raised additional questions about the appropriate 
interpretation of these key studies in light of other studies, which 
EPA did not rely on, that provided either mixed or no evidence of 
PM2.5-mortality associations, and in light of their view 
that the studies that EPA relied on report implausibly large effect 
estimates.
    In considering these commenters' sharply divergent assessments of 
the key mortality studies, the Administrator continues to believe that 
these studies provide strong evidence of an association between long-
term exposure to PM2.5 and mortality. However, the 
Administrator believes that the remaining uncertainties weigh against 
reaching the conclusion that the level of the annual PM2.5 
standard should be lowered on the basis of these studies. In reaching 
this conclusion, the Administrator notes that even though the long-term 
average PM2.5 concentration across the cities in the 
extended ACS study (17.7 [mu]g/m3) is lower than in the 
original study (21 [mu]g/m3), the level of the current 
standard is still appreciably below the long-term average of the 
extended ACS study and that of the Six Cities study (18 [mu]g/
m3). In commenting on alternative approaches to interpreting 
the study results as a basis for setting a standard level, American 
Lung Association et al. expressed the view that the level of the 
standard should more appropriately be based on the concentration that 
is one standard deviation below the cross-city long-term average in 
each relevant long-term exposure study. In considering such an 
approach, the Administrator notes that while that approach would by 
definition lead to a more precautionary standard, there is no basis for 
concluding that it is a more scientifically defensible approach or that 
it is more appropriate in this case where a number of key uncertainties 
in the evidence remain to be addressed in future research, and where 
the basic decision is a judgment by the Administrator as to what level 
is neither more nor less stringent than is necessary to protect public 
health with an adequate margin of safety. The Administrator continues 
to believe that it is reasonable to base the decision on the standard 
level on long-term average PM2.5 concentrations in the key 
long-term exposure studies, because the evidence of an association in 
any such study is strongest at and around the long-term average where 
the data in the study are most concentrated (71 FR 2651).
    Both groups of commenters also identified several ``new'' mortality 
studies not included in the Criteria Document in support of their 
various views. As noted above in Section I.C, as in past NAAQS reviews, 
EPA is basing

[[Page 61176]]

the final decisions in this review on the studies and related 
information included in the PM air quality criteria that have undergone 
CASAC and public review, and will consider the newly published studies 
for purposes of decision making in the next PM NAAQS review. 
Nonetheless, in provisionally evaluating commenters' arguments (see 
Response to Comments document), EPA notes that its provisional 
assessment of ``new'' science found that such studies did not 
materially change the conclusions in the Criteria Document.
    Some commenters who supported a lower annual standard level also 
asserted that EPA failed to adequately consider long-term exposure 
PM2.5 morbidity studies, especially studies of effects in 
children. For example, the Children's Health Protection Advisory 
Committee and other commenters noted that studies by Razienne et al. 
(1996) and Gauderman et al. (2002, 2004) showed effects on children's 
lung function at long-term cross-city average PM2.5 
concentrations of 14.5 [mu]g/m3 and 15 [mu]g/m3, 
respectively. The proposal notice included a careful discussion of the 
24-Cities study (Razienne et al., 1996) and the earlier Southern 
California children's health study (Gauderman et al., 2000, 2002), 
studies which were included in the Criteria Document,\48\ and explained 
the basis for the Administrator's provisional conclusion that these 
studies provide an uncertain basis for establishing the level of a 
national standard (71 FR 2651). These commenters offered no information 
that would change the Administrator's judgment with regard to these 
studies.\49\ In addition, the Children's Health Advisory Committee also 
cited several studies of ``traffic-related'' pollution (van Vliet et 
al., 1997; Brunekreef et al., 1997; Kim et al., 2004 \50\) as showing 
associations between fine particles and adverse respiratory outcomes, 
including asthma in children who live near major roadways, with mean 
annual average fine particle concentrations near and below 15 [mu]g/
m3.
---------------------------------------------------------------------------

    \48\ The Gaudermann et al. (2004) study cited by these 
commenters is a ``new'' study, and EPA's provisional consideration 
of this study is discussed in the Response to Comments document.
    \49\ The Administrator notes that CASAC's letter of March 21, 
2006 did not note any objection to his views on these morbidity 
studies as discussed in the proposal, or provide any reason to 
reconsider such views (Henderson, 2006).
    \50\ Kim et al. (2004) is a ``new'' study and EPA's provisional 
consideration of this study is discussed in the Response to Comments 
document.
---------------------------------------------------------------------------

    In considering these comments, EPA first notes that studies of 
traffic-related pollution generally do not disentangle potential 
effects of fine particles from those of other traffic-related 
pollutants, and thus provide an uncertain basis for establishing the 
level of a PM2.5 standard. Further, two of the studies cited 
by this commenter are ``new'' studies not included in the Criteria 
Document. As discussed above in section I.C, EPA is basing the final 
decisions in this review on the studies and related information 
included in the PM air quality criteria that have undergone CASAC and 
public review, and will consider the newly published studies for 
purposes of decision making in the next PM NAAQS review.
    The CARB and some other commenters who supported a lower annual 
standard level discussed the rationale used by the CARB in deciding to 
set the State's annual PM2.5 standard at a level of 12 
[mu]g/m3. Some of these commenters also pointed to the World 
Health Organization's annual PM2.5 guideline value of 10 
[mu]g/m3 in support of their view that the scientific 
evidence supports an annual PM2.5 standard in the U.S. at a 
level no higher than 12 [mu]g/m3. In considering these 
comments, the Administrator notes that his decision is constrained by 
the provision of the CAA that requires that the NAAQS be requisite to 
protect public health with an adequate margin of safety. This requires 
that his judgment is to be based on an interpretation of the evidence 
that neither overstates nor understates the strength and limitations of 
the evidence, or the appropriate inferences to be drawn from the 
evidence. This is not the same legal framework that governs the 
standards set by the State of California or the guidelines established 
by a working group of scientists within the World Health 
Organization.\51\ Thus, the Administrator does not agree that the 
California standard or the WHO guideline provide an appropriate basis 
for setting the level of the annual PM2.5 NAAQS in the U.S.
---------------------------------------------------------------------------

    \51\ For example, the California statute does not refer to 
setting a standard that is ``requisite'' to protect, as that term is 
used in the CAA, and California, unlike EPA, may take economic 
impacts into consideration in setting air quality standards. In 
addition, as with the WHO guidelines, the standards appear to be 
more in the nature of goals as compared to binding requirements that 
must be met.
---------------------------------------------------------------------------

    The Administrator further stresses, as explained at proposal, that 
he is placing the greatest weight in determining the level of the 
annual standard on the long-term means of the levels associated with 
mortality effects in the two key long-term studies in the record, the 
ACS and Six Cities studies (71 FR at 2651). The ACS and Six Cities 
studies are the two key long-term studies in this review, taking into 
account both ``study design, strength of the study (in terms of 
statistical significance and precision of result), and the consistency 
and robustness of results'' (71 FR 2651), and also the comprehensive 
reanalyses of these studies, which involved replication, validation, 
and sensitivity analyses. These reanalyses replicated the original 
results and confirmed the associations noted in the original studies 
(EPA 2005, p. 3-17). The Administrator has taken into account all the 
relevant studies but in evaluating the strengths and weaknesses of the 
various studies has determined that the greatest weight should be 
placed on these key studies, as compared to other studies, in 
determining the level of the annual standard. As discussed above, the 
level of the current annual standard is appropriate as it is 
appreciably below the long-term average of these key studies. This 
standard is also basically at the same level as the long-term average 
in the two morbidity studies, the 24 Cities study and the Southern 
California children's cohort study. These morbidity studies provide an 
uncertain basis for setting the level of the national standard, and, 
therefore, in the judgment of the Administrator do not warrant setting 
a lower level for the annual standard than the level warranted based on 
the key mortality studies.\52\
---------------------------------------------------------------------------

    \52\ The EPA is not required to base the level of the standard 
on either the highest or lowest level from any one study. Rather, 
the Administrator must ``make an informed judgment based on 
available evidence.'' American Petroleum Inst v. Costle, 665 F. 2d 
at 1187; NRDC v. EPA, 902 F. 2d at 971. Such an informed judgment 
can result in higher levels than shown in some of the studies in the 
record. See, e.g. NRDC v. EPA, 902 F. 2d at 971 (upholding 1987 
PM10 annual standard selected from ``near the middle of 
the `range of interest' ''); API v. Costle, 665 F. 2d at 1187 
(upholding 1979 hourly standard for ozone selected at level higher 
than a number of studies in the record).
---------------------------------------------------------------------------

    After carefully taking the above comments and considerations into 
account, the Administrator has decided to retain the level of the 
primary annual PM2.5 standard at 15 [mu]g/m3. In 
the Administrator's judgment, based on the currently available 
evidence, a standard set at this level would be requisite to protect 
public health with an adequate margin of safety from serious health 
effects including premature mortality and respiratory morbidity that 
are likely causally associated with long-term exposure to 
PM2.5. A standard set at a lower level would only result in 
significant further public health protection if, in fact, there is a 
continuum of health risks in areas with long-term average 
PM2.5 concentrations that are well below the cross-city 
long-term average concentrations observed in

[[Page 61177]]

the key epidemiologic studies and if the reported associations are, in 
fact, causally related to PM2.5 at those lower levels. Based 
on the available evidence, the Administrator is not prepared to make 
these assumptions. As was the case in considering the 24-hour 
PM2.5 standard, taking into account the uncertainties that 
remain in interpreting the available long-term exposure epidemiologic 
studies, the likelihood of obtaining benefits to public health 
decreases with a standard set below the current level, while the 
likelihood of requiring reductions in ambient concentrations that go 
beyond those that are needed to reduce risks to public health 
increases. On balance, the Administrator does not believe that a lower 
standard is needed to protect public health with an adequate margin of 
safety. This judgment by the Administrator appropriately considers the 
requirement for a standard that is neither more nor less stringent than 
necessary for this purpose and recognizes that the CAA does not require 
that primary standards be set at a zero-risk level, but rather at a 
level that reduces risk sufficiently so as to protect public health 
with an adequate margin of safety.

G. Final Decisions on Primary PM2.5 Standards

    For the reasons discussed above, and taking into account the 
information and assessments presented in the Criteria Document and 
Staff Paper, the advice and recommendations of CASAC, including its 
request to reconsider parts of the proposal, and public comments 
received on the proposal, the Administrator is revising the current 
primary PM2.5 standards. The suite of standards as revised 
will provide increased protection from the health risks associated with 
exposure to PM2.5, and in the judgment of the Administrator 
will be requisite to protect public health with an adequate margin of 
safety.
    Specifically, the Administrator is making the following revisions:
    (1) The level of the primary 24-hour PM2.5 standard is 
revised to 35 [mu]g/m3.
    (2) The form of the primary annual PM2.5 standard is 
revised with regard to the criteria for spatial averaging, such that 
averaging across monitoring sites is allowed if the annual mean 
concentration at each monitoring site is within 10 percent of the 
spatially averaged annual mean, and the daily values for each 
monitoring site pair yield a correlation coefficient of at least 0.9 
for each calendar quarter. Data handling conventions for the revised 
standards are specified in revisions to Appendix N, as discussed below 
in section V, and minor revisions to the reference method for 
monitoring PM as PM2.5 are specified in Appendix L, as 
discussed below in section VI.
    In a related rule on ambient air monitoring regulations (40 CFR 
Parts 53 and 58) published elsewhere in today's Federal Register, EPA 
is revising the requirements for reference and equivalent method 
determinations for fine particle monitors, monitoring network 
descriptions and periodic assessments, quality assurance, and data 
certification.
    Issues related to the implementation of revised PM2.5 
standards are discussed below in section VII. The EPA plans to propose 
related revisions to the Air Quality Index for PM2.5 at a 
later date.

III. Rationale for Final Decisions on Primary PM10 Standards

A. Introduction

1. Overview
    This section presents the Administrator's final decisions on the 
review of the primary NAAQS for PM10. The rationale for the 
final decisions on the primary PM10 NAAQS includes 
consideration of: (1) Evidence of health effects related to short- and 
long-term exposures to thoracic coarse particles; (2) insights gained 
from a quantitative risk assessment prepared by EPA; and (3) specific 
conclusions regarding the need for revisions to the current standards 
and the elements of standards for thoracic coarse particles (i.e., 
indicator, averaging time, form, and level) that, taken together, would 
be requisite to protect public health with an adequate margin of 
safety.
    In developing this rationale, EPA has taken into account the 
information available from a growing, but still limited, body of 
evidence on health effects associated with thoracic coarse particles 
from studies that use PM10-2.5 as a measure of thoracic 
coarse particles. The EPA has drawn upon an integrative synthesis of 
the body of evidence on associations between exposure to ambient 
thoracic coarse particles and a range of health endpoints (EPA, 2004a, 
Chapter 9), focusing on those health endpoints for which the Criteria 
Document concludes that the associations are suggestive of possible 
causal relationships. In its policy assessment of the evidence judged 
to be most relevant to making decisions on elements of the standards, 
EPA has placed greater weight on U.S. and Canadian epidemiologic 
studies using thoracic coarse particle measurements, since studies 
conducted in other countries may well reflect different demographic and 
air pollution characteristics.
    While there is little question that particles in the thoracic 
coarse particle size range can present a risk of adverse effects to the 
most sensitive regions of the respiratory tract at sufficient exposure 
levels, the characterization of health effects attributable to various 
levels of exposure to ambient thoracic coarse particles is subject to 
uncertainties that are markedly greater than is the case for fine 
particles. As summarized below, however, there is a growing body of 
evidence available since the last review of the PM NAAQS, with 
important new information coming from epidemiologic, toxicologic, and 
dosimetric studies. Moreover, the newly available research studies have 
undergone intensive scrutiny through multiple layers of peer review and 
extended opportunities for public review and comment. While important 
uncertainties remain, the review of the health effects information has 
been extensive and deliberate. In the judgment of the Administrator, 
this intensive evaluation of the scientific evidence provides an 
adequate basis for making final regulatory decisions at this time.
    In addition, this review has already provided important input to 
EPA's research and monitoring plans for improving our future 
understanding of the relationships between exposures to ambient 
thoracic coarse particles and health effects. As discussed in the 
proposal, the epidemiological evidence available in this review is 
almost entirely based on measurements of undifferentiated 
PM10-2.5 mass, without regard to the composition of thoracic 
coarse particles. Yet both fundamental toxicological considerations and 
the limited data available on this issue strongly suggest that the 
health effects could vary significantly depending upon the composition 
of the ambient coarse particle mix. The goal of the Agency's research 
and monitoring programs going forward is to provide scientific advances 
that will enable future PM NAAQS reviews to make more informed 
decisions that will provide more effective and efficient protection 
against the effects of those coarse particles and related source 
emissions that prove to be of concern to public health.
    The health effects information and human risk assessment were 
summarized in sections III.A and III.B of the proposal and are only 
briefly outlined in subsections III.A.2 and 3 below. Subsequent 
sections provide a more complete discussion of the Administrator's 
rationale, in light of key

[[Page 61178]]

issues raised in public comments, for his decision to retain the 
current 24-hour primary PM10 standard and to revoke the 
current annual PM10 standard. Specifically, these sections 
present a more complete discussion of the Administrator's rationale 
regarding the need to maintain protection against the health effects of 
coarse particles (section III.B) as well as the rationale for the 
decisions regarding specific elements of the primary PM10 
standards including indicator (section III.C); and averaging time, 
level and form (section III.D).
2. Overview of Health Effects Evidence
    The first PM NAAQS (36 FR 8186) used an indicator based solely on a 
preexisting monitor for total suspended particles (TSP) that was not 
designed to focus on particles of greatest risk to health. In preparing 
for the initial review of those standards, EPA placed a major emphasis 
on developing a new indicator that considered the significant amount of 
evidence on particle size, composition, and relative risk of effects 
from penetration and deposition to the major regions of the respiratory 
tract (Miller et al., 1979). The development and assessment of these 
lines of evidence in the PM Criteria Document and PM Staff Paper 
published between 1979 and 1986 culminated in revised standards for PM 
that used PM10 as the indicator (52 FR 24634). The major 
conclusion from that review, which remained unchanged in the 1997 
review, was that ambient particles smaller than or equal to 10 [mu]m in 
aerodynamic diameter are capable of penetrating to the deeper 
``thoracic'' \53\ regions of the respiratory tract and present the 
greatest concern to health (61 FR 65648). While considerable advances 
have been made, the available evidence in this review continues to 
support the basic conclusions reached in the 1987 and 1997 reviews 
regarding penetration and deposition of fine and thoracic coarse 
particles. As discussed in the Criteria Document, both fine and 
thoracic coarse particles penetrate to and deposit in the alveolar and 
tracheobronchial regions. For a range of typical ambient size 
distributions, the total deposition of thoracic coarse particles to the 
alveolar region can be comparable to or even larger than that for fine 
particles. For areas with appreciable coarse particle concentrations, 
thoracic coarse particles would tend to dominate particle deposition to 
the tracheobronchial region for mouth breathers (EPA, 2004a, p. 6-16). 
Deposition of particles to the tracheobronchial region is of particular 
concern with respect to aggravation of asthma.
---------------------------------------------------------------------------

    \53\ The ``thoracic'' regions of the respiratory tract are 
located in the chest (thorax) and are comprised of the tracheo-
bronchial region with connecting airways and the alveolar, or gas-
exchange region of the lung. For ease of communication, ``thoracic'' 
particles penetrating to these regions are often called 
``inhalable'' particles.
---------------------------------------------------------------------------

    In the last review, little new toxicologic evidence was available 
on potential effects of thoracic coarse particles and there were few 
epidemiologic studies that had included direct measurements of thoracic 
coarse particles. Evidence of associations between health outcomes and 
PM10 that were conducted in areas where PM10 was 
predominantly composed of thoracic coarse particles was an important 
part of EPA's basis for reaching conclusions about the requisite level 
of protection from coarse particles provided by the final standards. 
The new studies available in this review include epidemiologic studies 
that have reported associations with health effects using direct 
measurements of PM10-2.5, as well as new dosimetric and 
toxicologic studies.
    Section III.A of the proposal further outlines key information 
contained in the Criteria Document (Chapters 6-9) and the Staff Paper 
(Chapter 3) on known or potential effects associated with exposure to 
thoracic coarse particles and their major constituents. The information 
highlighted there includes:
    (1) New information available on potential mechanisms for health 
effects associated with exposure to thoracic coarse particles or their 
constituents.
    (2) The nature of the effects that have been associated with short-
term exposures to ambient thoracic coarse particles, particularly in 
urban and industrial settings, including aggravation of respiratory and 
cardiovascular disease (as indicated by increased hospital admissions), 
increased respiratory symptoms in children, and premature mortality.
    (3) An integrative assessment of the evidence on health effects 
related to thoracic coarse particles, with an emphasis on the key 
issues raised in assessing the available community-based epidemiologic 
studies, including alternative interpretations of the evidence, both 
for individual studies and the evidence as a whole.
    (4) Subpopulations that appear to be sensitive to effects from 
exposure to thoracic coarse particles, specifically including 
individuals with preexisting lung diseases such as asthma, and children 
and older adults.
    (5) Conclusions, based on the magnitude of these subpopulations and 
risks identified in health studies conducted in urban and industrial 
areas, that exposure to ambient thoracic coarse particles can have an 
important public health impact.
    The summary of the health effects evidence related to ambient 
coarse particles in the proposal will not be repeated here. The EPA 
emphasizes that the final decisions on these standards take into 
account the more comprehensive and detailed discussions of the 
scientific information on these issues contained in the Criteria 
Document and Staff Paper, which were reviewed by the CASAC and the 
public. For reasons summarized in section I.C above, EPA is not relying 
on studies published after completion of the Criteria Document as a 
basis for reaching final decisions on these standards.
3. Overview of Quantitative Risk Assessment
    The general overview and discussion of key components of the risk 
assessment used to develop risk estimates for PM2.5 
presented in section II.A above is also applicable to the assessment 
done for PM10-2.5 in this review. However, the scope of the 
risk assessment for PM10-2.5 is much more limited than that 
for PM2.5, reflecting the much more limited body of 
epidemiologic evidence and air quality information available for 
PM10-2.5. As discussed in chapter 4 of the Staff Paper, the 
PM10-2.5 risk assessment includes risk estimates for just 
three urban areas for two categories of health endpoints related to 
short-term exposure to PM10-2.5: hospital admissions for 
cardiovascular and respiratory causes, and respiratory symptoms.
    Estimates of hospital admissions attributable to short-term 
exposure to PM10-2.5 have been developed for Detroit 
(cardiovascular and respiratory admissions) and Seattle (respiratory 
admissions), and estimates of respiratory symptoms have been developed 
for St. Louis.\54\ While one of the goals of the PM10-2.5 
risk assessment was to provide estimates of the risk reductions 
associated with just meeting alternative PM10-2.5 standards, 
the nature and magnitude of the uncertainties and concerns associated 
with this portion of the risk assessment weigh against use of these 
risk estimates as a basis for recommending specific standard levels 
(EPA, 2005, p. 5-69).

[[Page 61179]]

These uncertainties and concerns are summarized in section III.B of the 
proposal and discussed more fully in the Staff Paper (Chapter 4) and 
the technical support document (Abt Associates, 2005).
---------------------------------------------------------------------------

    \54\ Quantitative risk estimates associated with recent air 
quality levels for these three cities are presented in Figures 4-11 
and 4-12 of the Staff Paper.
---------------------------------------------------------------------------

B. Need for Revision of the Current Primary PM10 Standards

    As presented in the proposal, taking into account both the nature 
of recent scientific evidence and legal considerations, this review of 
the primary PM10 standards has focused on whether to revise 
the indicator for thoracic coarse particles, and on the appropriate 
level, form and averaging time for any revised indicator. The basis for 
reaching a final decision on the indicator, as well as other facets of 
the standards, is presented below in sections III.C and III.D. This 
section provides an overview of the considerations that led to the 
Administrator's provisional conclusion, at the time of proposal, that 
it would be appropriate to revise the PM10 standards by 
adopting a new indicator (PM10-2.5).\55\ The section then 
presents a summary of public comments concerning whether the available 
evidence supports retention, revision, or revocation of standards to 
protect against exposure to thoracic coarse particles. For the reasons 
discussed below, the Administrator has concluded, consistent with CASAC 
and Staff Paper recommendations and conclusions drawn at the time of 
proposal, that continued protection against health effects associated 
with short-term exposure to thoracic coarse particles is requisite. 
However, EPA notes that, having considered the issues raised in 
extensive public comment on the proposal, the Administrator's final 
decision differs from that in the proposal regarding whether it is 
appropriate to revise the indicator in order to retain protection from 
coarse particles. This section, and the subsequent section on 
indicator, outline the rationale presented at the time of the proposal, 
and then describe how the Administrator has reached a different 
conclusion in his final decision.
---------------------------------------------------------------------------

    \55\ The Administrator also proposed qualifications to the 
indicator, and corresponding revisions to the level and form of the 
24-hour standard to provide protection that is generally equivalent 
to that afforded by the PM10 standard, and to revoke the 
annual PM10 standard.
---------------------------------------------------------------------------

1. Overview of the Proposal
    The initial issue addressed in the current review of the primary 
PM10 standards was whether, in view of the advances in 
scientific knowledge reflected in the Criteria Document and Staff 
Paper, the current standards should be revised. The Staff Paper 
addressed this question by first considering the conclusions reached in 
the last review, the subsequent litigation of that decision, and the 
nature of the new information available in this review.
    In 1997, in conjunction with establishing new PM2.5 
standards, EPA concluded that continued protection against potential 
effects associated with thoracic coarse particles in the size range of 
2.5 to 10 [mu]m was warranted based on particle dosimetry, toxicologic 
information, and limited epidemiologic evidence from studies that 
measured PM10 in areas where coarse particles were likely to 
dominate the distribution (62 FR 38677). This information indicated 
that thoracic coarse particles can deposit in those regions of the lung 
of most concern (i.e., the tracheobronchial and alveolar regions, which 
together make up the thoracic region),\56\ and that they can be 
expected to aggravate effects in individuals with asthma and contribute 
to increased upper respiratory illness (62 FR 38666-8).
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    \56\ EPA further concluded at that time that the risks of 
adverse health effects associated with deposition of particles in 
the thoracic region are ``markedly greater than for deposition in 
the extrathoracic (head) region,'' and that risks from extrathoracic 
deposition are ``sufficiently low that particles which deposit only 
in that region can safely be excluded from the standard indicator'' 
(62 FR 38666).
---------------------------------------------------------------------------

    Further, EPA decided that the new function of PM10 
standard(s) would be to provide such protection against effects 
associated with particles in the narrower size range between 2.5 to 10 
[mu]m. Although some consideration had been given to a more narrowly 
defined indicator that did not include fine particles (e.g., 
PM10-2.5), EPA decided that it was more appropriate to 
continue to use PM10 as the indicator for standards to 
control thoracic coarse particles. This decision was based in part on 
the recognition that the only studies of clear quantitative relevance 
to health effects most likely associated with thoracic coarse particles 
used PM10 in areas where the coarse fraction was the 
dominant fraction of PM10, namely two studies conducted in 
areas that substantially exceeded the 24-hour PM10 standard 
(62 FR 38679). The decision also reflected the fact that there were 
only very limited ambient air quality data then available specifically 
on thoracic coarse particles (i.e. PM10-2.5), in contrast to 
the extensive monitoring network already in place for PM10. 
In essence, EPA concluded at that time that it was appropriate to 
continue to control thoracic coarse particles, but that the only 
information available upon which to base such standards was indexed in 
terms of PM10.
    In subsequent litigation regarding the 1997 PM NAAQS revisions, 
however, the U.S. Court of Appeals (D.C. Circuit) held in part that EPA 
had not provided a reasonable explanation justifying use of 
PM10 as an indicator for thoracic coarse particles. ATA I, 
175 F.3d at 1054-55. Although the court found ``ample support'' (id. at 
1054) for EPA's decision to regulate thoracic coarse particles, it 
vacated the 1997 revised PM10 standards. The result of 
subsequent EPA actions, discussed above in section I.C, is that the 
1987 PM10 standards remain in place (65 FR 80776, 80777, 
Dec. 22, 2000) and the present review is consequently of those 1987 
standards.
    In this review, the Staff Paper focused on the recent information 
available in the Criteria Document from a growing, but still limited, 
body of evidence on health effects associated with thoracic coarse 
particles from studies that use PM10-2.5 as the measure of 
thoracic coarse particles. In addition, there is now much more 
information available to characterize air quality in terms of 
PM10-2.5 than was available in the last review. In 
considering this information, the Staff Paper found that the major 
considerations that formed the basis for EPA's 1997 decision to retain 
PM10 as the indicator for thoracic coarse particles, rather 
than a more narrowly defined indicator that does not include fine 
particles, no longer apply. More specifically, staff concluded that the 
continued use of PM10 as an indicator for standards intended 
to protect against health effects associated with thoracic coarse 
particles was no longer necessary since the information available in 
the Criteria Document could support the use of a more directly relevant 
indicator, PM10-2.5. Further, staff concluded that 
continuing to rely principally on health effects evidence indexed by 
PM10 to determine the appropriate averaging time, form, and 
level of a standard was no longer necessary or appropriate since a 
number of more directly relevant studies, indexed by 
PM10-2.5, were available. Thus, the Staff Paper concluded 
that it was appropriate to revise the current PM10 standards 
in part by revising the indicator for thoracic coarse particles, and by 
basing any such revised standard principally on the currently available 
evidence and air quality information indexed by PM10-2.5, 
but also considering evidence from studies using PM10 in 
locations where PM10-2.5 was the predominant fraction (EPA, 
2005, section 5.4.1). As noted in the introduction to this section,

[[Page 61180]]

having considered public comments on this issue, EPA has reached 
different conclusions regarding the appropriateness of revising the 
current indicator in this final decision; this is described in more 
detail below in section III.C.
    Recognizing that dosimetric evidence formed the basis for the 
initial establishment of the PM10 indicator in 1987 and 
supported the decision in 1997 to retain the PM10 indicator, 
the Staff Paper also considered whether currently available dosimetric 
evidence continues to support the basic conclusions reached in those 
reviews of the standards. In particular, consideration was given to 
available information about patterns of penetration and deposition of 
thoracic coarse particles in the sensitive thoracic region of the lung 
and to whether an aerodynamic size of 10 [mu]m remains a reasonable 
separation point for particles that penetrate and potentially deposit 
in the thoracic regions. The Staff Paper concluded that while 
considerable advances have been made in understanding particle 
dosimetry, the available evidence continues to support those basic 
conclusions from past reviews. More specifically, both fine particles, 
indexed by PM2.5, and thoracic coarse particles, indexed by 
PM10-2.5, penetrate to and deposit in the thoracic regions. 
Further, for a range of typical ambient size distributions, the total 
deposition of thoracic coarse particles to the alveolar region can be 
comparable to or even larger than that of fine particles (EPA, 2004a, 
p. 6-16).
    Beyond the dosimetric evidence, as noted in past reviews (EPA, 
1982, 1996b), toxicologic studies show that the deposition of a variety 
of particle types in the tracheobronchial region, including resuspended 
urban dust and coarse-fraction organic materials, has the potential to 
affect lung function and aggravate respiratory symptoms, especially in 
asthmatics. Of particular note are limited toxicologic studies that 
found urban road dust can produce cellular and immunological effects 
(e.g., Kleinman et al., 1995; Steerenberg et al., 2003).\57\ In 
addition, some very limited in vitro toxicologic studies show some 
evidence that coarse particles may elicit pro-inflammatory effects 
(EPA, 2004a, section 7.4.4). Further, the Staff Paper assessment of the 
physicochemical properties and occurrence of ambient coarse particles 
suggests that both the chemical makeup and the spatial distribution of 
coarse particles are likely to be more heterogeneous than for fine 
particles (EPA, 2005, chapter 2). In particular, as discussed below in 
section III.C, coarse particles in urban areas can contain all of the 
components found in more rural areas, but can also be contaminated by a 
number of additional materials, from motor-vehicle-related emissions to 
metals and transition elements associated with industrial operations. 
The Staff Paper concluded that the weight of the dosimetric, limited 
toxicologic, and atmospheric science evidence, taken together, lends 
support to the plausibility of the PM10-2.5-related effects 
reported in the urban epidemiologic studies discussed below, and 
provides support for retaining some standard for thoracic coarse 
particles so as to continue programs to protect public health from such 
effects (EPA, 2005, p. 5-49).\58\
---------------------------------------------------------------------------

    \57\ The Criteria Document notes that toxicologic studies, in 
general, use exposure concentrations that are generally much higher 
than ambient concentrations (EPA, 2004a, p. 9-51).
    \58\ Eventually, as a result of the data that will be gathered 
under EPA's new research and monitoring plan , the Agency may be 
able to further refine its regulation of coarse particles to better 
target those coarse particles of greatest concern to health.
---------------------------------------------------------------------------

    The available epidemiologic evidence, discussed in section III.A of 
the proposal, includes studies of associations between short-term 
exposure to thoracic coarse particles, indexed by PM10-2.5, 
and health endpoints. More specifically, several U.S. and Canadian 
studies now provide evidence of associations between short-term 
exposure to PM10-2.5 and various morbidity endpoints. Three 
such studies conducted in Toronto (Burnett et al., 1997), Seattle 
(Sheppard, 2003), and Detroit (Ito, 2003) report statistically 
significant associations between short-term PM10-2.5 
exposure and respiratory- and cardiac-related hospital admissions, and 
a fourth study (Schwartz and Neas, 2000), conducted in six U.S. cities 
(Boston, St. Louis, Knoxville, Topeka, Portage, and Steubenville), 
reports statistically significant associations across these six areas 
with respiratory symptoms in children. These studies were mostly done 
in areas in which PM2.5, rather than PM10-2.5, is 
the larger fraction of ambient PM10, and they are not 
representative of areas with relatively high levels of thoracic coarse 
particles (EPA, 2005, p. 5-49).
    In evaluating the epidemiologic evidence from health studies on 
associations between short-term exposure to PM10-2.5 and 
mortality, the Criteria Document concluded that such evidence was 
``limited and clearly not as strong'' as that for associations with 
PM2.5 or PM10 but nonetheless was suggestive of 
associations with mortality (EPA, 2004a, p. 9-28, 9-32). Statistically 
significant mortality associations were reported in short-term exposure 
studies conducted in areas with relatively high PM10-2.5 
concentrations, including Phoenix (Mar et al., 2003), Coachella Valley, 
CA (Ostro et al., 2003),\59\ and in the initial analysis of data from 
Steubenville (as part of the Six Cities study, Schwartz et al., 1996; 
reanalysis, Schwartz, 2003). In a separate reanalysis of the Six Cities 
study, the PM10-2.5 mortality association was not 
statistically significant for Steubenville (Klemm and Mason, 2003). In 
areas with lower PM10-2.5 concentrations, including the 
remaining five cities in the Six Cities study, no statistically 
significant associations were reported with mortality, though most were 
positive.
---------------------------------------------------------------------------

    \59\ The Coachella Valley study, like the Seattle study noted 
above, is subject to additional measurement uncertainties because it 
used regression techniques to impute PM10-2.5 
concentrations; this approach fills in missing PM10-2.5 
data based on relationships developed using data from days when data 
are available for both PM10 and PM2.5.
---------------------------------------------------------------------------

    The Staff Paper also considered relevant epidemiologic studies 
indexed by PM10 that were conducted in areas where the 
coarse fraction of PM10 is typically much greater than the 
fine fraction. Such studies include findings of associations between 
short-term exposure to PM10 and hospitalization for 
cardiovascular diseases in Tucson, AZ (Schwartz, 1997), hospitalization 
for COPD in Reno/Sparks, NV (Chen et al., 2000), and medical visits for 
asthma or respiratory diseases in Anchorage, AK (Gordian et al., 1996; 
Choudhury et al., 1997). In addition, a number of epidemiologic studies 
have reported significant associations with mortality, respiratory 
hospital admissions and respiratory symptoms in the Utah Valley area 
(e.g., Pope, 1989 and 1991; Pope et al., 1992). This group of studies 
provides additional supportive evidence for associations between short-
term exposure to thoracic coarse particles and health effects, 
particularly morbidity effects, generally in areas not meeting the 
PM10 standards (EPA, 2005, p. 5-50).\60\
---------------------------------------------------------------------------

    \60\ Based on recent air quality data, as well as the summary 
information provided for PM concentrations used in the studies, the 
existing PM10 standards are not met in any of these study 
cities except Tucson, AZ. Based on 2002-2004 air quality data, the 
98th percentile PM2.5 concentrations in three of these 
areas range from 15 to 25 [mu]g/m3, while in Utah Valley 
the concentrations range from 37 to 54 [mu]g/m3.
---------------------------------------------------------------------------

    In contrast to the findings from the short-term exposure studies 
discussed above, available epidemiologic studies do not provide 
evidence that long-term community-level exposure to thoracic coarse 
particles is associated with mortality or morbidity (EPA, 2005, p. 3-
25). More specifically, no association is

[[Page 61181]]

found between long-term exposure to thoracic coarse particles and 
mortality in the reanalyses and extended analysis of the ACS cohort 
(EPA, 2005, p. 8-306-07). Further, little evidence is available on 
potential respiratory and cardiovascular morbidity effects of long-term 
exposure to thoracic coarse particles (EPA, 2005, p. 3-23-24).
    The Staff Paper concluded that the available body of health 
evidence, including dosimetric, toxicologic and epidemiologic study 
findings, supports retaining a NAAQS that would continue to provide 
protection against the effects associated with short-term exposure to 
thoracic coarse particles. However, the substantial uncertainties 
associated with this limited body of epidemiologic evidence on health 
effects related to exposure to PM10-2.5 suggest a high 
degree of caution in interpreting this evidence, especially at the 
lower levels of ambient particle concentrations in the morbidity 
studies discussed above (EPA, 2005, p. 5-50).
    Beyond this evidence-based evaluation, the Staff Paper also 
considered the extent to which PM10-2.5-related health risks 
estimated to occur at current levels of ambient air quality may be 
judged to be important from a public health perspective, taking into 
account key uncertainties associated with the estimated risks. 
Consistent with the approach used to address this issue for 
PM2.5-related health risks, discussed above in section 
II.A.3, the Staff Paper considered the results of a series of base-case 
analyses that reflect in part the uncertainty associated with the form 
of the concentration-response functions drawn from the studies used in 
the assessment. In this assessment summarized above in section III.A.3, 
which is much more limited than the risk assessment conducted for 
PM2.5, health risks were estimated for three urban areas 
(Detroit, Seattle, and St. Louis) by using the reported linear or log-
linear concentration-response functions as well as modified functions 
that incorporate alternative assumed cutpoints as surrogates for 
potential population thresholds. In considering the risk estimates from 
this limited assessment, and recognizing the very substantial 
uncertainties inherent in basing an assessment on such limited 
information, the Staff Paper concluded that the results for the two 
areas in the assessment that did not meet the current PM10 
standards are indicative of risks that can reasonably be judged to be 
important from a public health perspective, in contrast to the 
appreciably lower risks estimated for the area that did meet the 
current standards (EPA, 2005, p. 5-52).
    The Staff Paper recognized the substantial uncertainties associated 
with the limited available epidemiologic evidence and the inherent 
difficulties in interpreting the evidence for purposes of setting 
appropriate standards for thoracic coarse particles. Nonetheless, in 
considering the available evidence, the public health implications of 
estimated risks associated with current levels of air quality, and the 
related limitations and uncertainties, the Staff Paper concluded that 
this information supports (1) revising the current PM10 
standards in part by revising the indicator for thoracic coarse 
particles, and (2) consideration of a standard that will continue to 
provide public health protection from short-term exposure to thoracic 
coarse particles of concern that have been associated with morbidity 
effects and possibly mortality at current levels in some urban areas 
(EPA, 2005, p. 5-52).
    In CASAC's review of these Staff Paper recommendations, there was 
unanimous agreement among CASAC Panel members that ``there was a need 
for a specific primary standard to address particles in the size range 
of 2.5 to 10 microns'' (Henderson, 2005b, p. 4). In making this 
recommendation, CASAC indicated its agreement with the summary of the 
scientific data regarding the potential adverse health effects from 
exposures to thoracic coarse particles in section 5.4 of the Staff 
Paper upon which the EPA staff recommendations were based.
    Unlike the case in the current PM2.5 review, neither EPA 
staff nor CASAC concluded that it was necessary to revise the 
PM10 standards to provide additional health protection 
against coarse particles beyond that afforded by the current standards. 
Rather, as noted above, staff and CASAC found that the most recent 
scientific information suggested it was possible to move to a more 
direct measurement of thoracic coarse particles via a 
PM10-2.5 indicator, and this was the major basis for 
recommending revisions to the current 24-hour PM10 standard. 
In considering what level of protection was appropriate, staff and 
CASAC recommended consideration of a range of levels for alternative 
24-hour coarse particle standards, from levels which would be more 
stringent than the current 24-hour PM10 standard to a level 
that would provide protection that was roughly equivalent to that 
provided by the current 24-hour PM10 standard.
    In considering whether the primary PM10 standards should 
be revised at the time of proposal, the Administrator considered the 
rationale and recommendations provided by the Staff Paper and CASAC, 
and the public comments received through the time of proposal. The 
Administrator provisionally concluded that the health evidence, 
including dosimetric, toxicologic and epidemiologic study findings, 
supported retaining a standard to provide continued protection against 
effects associated with short-term exposure to thoracic coarse 
particles. Further, the Administrator expressed the belief that the new 
evidence on health effects from studies that use PM10-2.5 as 
a measure of thoracic coarse particles, together with the much more 
extensive data now available to characterize air quality in terms of 
PM10-2.5, provided an appropriate basis for revising the 
current PM10 standards in part by revising the indicator to 
focus more narrowly on particles between 2.5 and 10 [mu]m. The 
Administrator also noted that the need for a standard for thoracic 
coarse particles had already been upheld based upon evidence of health 
effects considerably more limited than now available. ATA I, 175 F.3d 
at 1054. Based on these considerations, the Administrator provisionally 
concluded that the current suite of PM10 standards should be 
revised, and that the revised standard(s) should be set at a level that 
would ensure an equivalent level of protection to the current suite of 
standards (71 FR 2665).
2. Comments on the Need for Revision
    The vast majority of public comments on coarse particles raised 
issues related to the proposed revisions to the indicator for thoracic 
coarse standards, particularly the proposal to adopt a new 
PM10-2.5 indicator that was qualified to focus on particles 
associated with particular types of emissions sources and to impose 
stringent monitor site-suitability criteria for NAAQS-comparable 
monitors. These comments are addressed below in section III.C. Comments 
more specific to the 24-hour and annual standards (i.e., on averaging 
time, form, and level) are addressed below in section III.D. This 
section addresses those comments that, directly or indirectly, 
addressed the need to continue the kind of protection against coarse 
particles that is provided by the current PM10 standards.
    A substantial majority of commenters supported the Administrator's 
provisional conclusion that it is necessary to maintain a standard to 
continue protection against the health effects associated with short-
term exposure to thoracic coarse particles. Those advocating a coarse 
particle standard included public health organizations such as the 
American Lung Association, the American Heart

[[Page 61182]]

Association, and the American Cancer Society; environmental groups such 
as Environmental Defense, Earthjustice and Natural Resources Defense 
Council; the Children's Health Protection Advisory Committee, which 
provides the EPA Administrator with advice on children's health issues; 
all state and local air pollution control agencies commenting on the 
proposed coarse particle standard; and Tribal groups such as the 
National Tribal Caucus, the National Tribal Environmental Council, and 
numerous individual Tribes.
    These commenters agreed with EPA that the currently available 
scientific evidence clearly supports the need to provide continued 
protection from health effects associated with coarse particle 
exposure. Citing the Criteria Document and the Staff Paper, those 
commenters providing a more detailed rationale stressed the 
availability of epidemiologic, toxicologic and dosimetric studies 
showing associations between thoracic coarse particles and multiple 
morbidity and mortality endpoints. Many of these commenters also cited 
CASAC's recommendation in favor of continued protection. Moreover, some 
of these commenters pointed to particular studies, such as Ito (2003), 
Mar et al. (2003) and Ostro et al. (2003), which they concluded show 
that coarse particles are associated with hospital admissions or 
mortality and that coarse particles may even have stronger effects than 
fine particles in some instances. Several also cited two recent 
independent reviews (Brunekreef and Forsberg, 2005; WHO, 2005) which 
considered many of the same scientific studies on the health effects of 
coarse particles that were included in the Criteria Document as support 
for separate standards for coarse particles, in addition to standards 
for fine particles.
    In general, this body of commenters opposed revisions that they 
believed would reduce the level of protection provided by the current 
PM10 standards. For example, the comments of the American 
Lung Association and five environmental groups stated (American Lung 
Association et al., p. 81):

We strongly support the need for a coarse PM standard * * *. 
However, the coarse particle standard proposed by EPA is an 
egregious step backwards in protection of human health and welfare 
compared to the status quo * * *. If EPA feels it lacks adequate 
data to undertake the change in the coarse PM indicator to a 
PM10-2.5 standard, without reducing current protections * 
* * then the Agency must retain the existing PM10 NAAQS.

    Citing the more abundant evidence from studies focusing on short-
term exposures, these commenters advocated maintaining a 24-hour 
standard for thoracic coarse particles, at a minimum. Several of them 
also recommended an annual standard for thoracic coarse particles to 
protect against possible long-term effects, despite a significantly 
more limited body of evidence (for specific comments on averaging time, 
see section III.D.1 below).
    Many of these commenters, while recognizing that the epidemiologic 
evidence available to support specific coarse particle standards is 
weaker than that for fine particles, believed that the weight of 
evidence required revisions that provided a greater degree of 
protection, on a national basis, than that afforded by the current 
PM10 standards (for specific comments on level, see section 
III.D.2 below). Some commenters favoring a coarse particle standard 
supported their arguments by reference to emerging science from new 
toxicologic and epidemiologic studies that were not included in the 
Criteria Document. In general, however, these ``new'' studies were used 
in support of commenters' concerns about the proposal to qualify the 
indicator (discussed in section III.C.2 below), and not to support 
their comments on the need for coarse particle standards.
    The EPA generally agrees with these commenters regarding the need 
to provide continued protection from short-term exposure to coarse 
particles that may be harmful. The scientific evidence cited by these 
commenters was generally the same as that discussed in the Criteria 
Document and the Staff Paper and the commenters' recommendations for 
retaining a coarse particle standard are broadly consistent with staff 
and CASAC recommendations on this issue. To the limited extent that 
some commenters cited ``new'' scientific studies in support of their 
arguments in favor of retaining a coarse particle standard, EPA notes 
that it is basing the final decisions in this review on the studies and 
related information included in the PM air quality criteria that have 
undergone CASAC and public review. Although EPA is not basing its final 
decisions in this review on such information, the Agency will consider 
the newly published studies for purposes of decision making in the next 
PM NAAQS review, as discussed above in section I.C. Nonetheless, in 
provisionally evaluating commenters' arguments concerning the need for 
revision to or elimination of the current standards, the Agency notes 
that its preliminary analysis suggests such studies would not 
materially change the conclusions in the Criteria Document.
    In sharp contrast, a number of commenters, including virtually all 
of those representing industry associations and businesses, recommended 
revising the PM10 standards by revoking both the 24-hour and 
annual standards. These groups argued that the current body of 
scientific evidence is insufficient to justify either retaining the 
current PM10 standards or setting a revised standard for 
thoracic coarse particles at this time. These commenters included the 
National Cattlemen's Beef Association, the National Mining Association, 
the American Farm Bureau Federation, the Alliance of Automobile 
Manufacturers, the Engine Manufacturers Association, the National 
Association of Home Builders, and the Coarse Particle Coalition, which 
includes the National Stone, Sand and Gravel Association, the 
Industrial Minerals Association, the American Forest and Paper 
Association, the Portland Cement Association and the National Cotton 
Council. These commenters stressed the uncertainties, particularly 
those associated with interpreting the limited number of epidemiologic 
studies focusing on coarse particle health effects, and stated that EPA 
had failed to demonstrate that a coarse particle standard is necessary 
to protect public health. These commenters recommended deferring the 
decision on the appropriateness of setting a coarse particle standard 
pending additional monitoring and scientific research on health effects 
associated with exposure to coarse particles.
    These commenters criticized the key epidemiologic studies cited by 
EPA, referring especially to the alternative interpretations of the 
evidence presented in the proposal and citing a review and critique of 
key studies prepared by an academic consultant. They also argued that 
all coarse particle epidemiologic studies are flawed to the extent that 
they rely on air quality data from central monitors in exposure 
assessments. Based on these arguments, the commenters asserted that 
EPA's risk assessment cannot be used to demonstrate that ambient coarse 
particles present a significant risk to public health, and therefore 
EPA cannot maintain the existing PM10 NAAQS or establish a 
revised NAAQS to address coarse particles. Each of these issues is 
further summarized and discussed below.
    In discussing their disagreement with EPA's interpretation of four 
key epidemiologic studies (Ito, 2003; Burnett et al., 1997; Mar et al., 
2003; Ostro et al., 2003), these commenters placed significant weight 
on the alternative interpretations of these

[[Page 61183]]

studies that EPA provided in the proposal to encourage additional 
public comment (71 FR 2671-72). In particular, they criticized EPA's 
reliance on the single pollutant models in these and other studies as 
biased because the models omit PM2.5 and gaseous co-
pollutants. The commenters argued that when PM2.5 or gaseous 
co-pollutants were added to the underlying models, the effects 
associated with PM10-2.5 lost statistical significance. 
These commenters also stated that EPA failed to consider and give 
appropriate weight to a significant number of studies which relied on 
larger and more powerful data sets, were of longer duration, and 
assessed PM10-2.5 using multi-pollutant models, but did not 
find any statistically significant associations, including Schwartz et 
al. (1996), Thurston et al. (1994), Sheppard (2003), Fairley (2003), 
and Lipfert et al. (2000). They further summarized and attached a 
``detailed review of the cited studies'' prepared by an academic 
consultant, which they stated reveals numerous deficiencies that 
undermine the use of these studies to support the proposed coarse 
particle standard or any alternative standard. Based on all of the 
above, one commenter claimed that a ``fair and sound'' assessment of 
evidence would not conclude coarse particles have effects at ambient 
concentrations (National Mining Association, p. 14).
    The rationale for these commenters' conclusions, however, do not 
consider important aspects of the rationale for retaining coarse 
particle protection and are inconsistent with CASAC and other recent 
reviews of the scientific evidence. As summarized in section III.A of 
the proposal, the scientific evidence contained in the Criteria 
Document and Staff Paper, both of which have been reviewed and found 
acceptable for use in regulatory decision making by CASAC, supports the 
need for some standard to provide continued protection from coarse 
particles.\61\ The alternative interpretation of the evidence espoused 
by these commenters essentially argues that it is more reasonable to 
presume that the positive results from one-pollutant 
PM10-2.5 statistical models is the result of bias associated 
with omitting co-pollutants, especially PM2.5, for which the 
evidence is much stronger. EPA does not accept this argument for both 
technical and public health policy reasons. The Criteria Document and 
Staff Paper explain the rationale for reliance on single pollutant 
models in these studies, while recognizing the significant 
uncertainties in the limited number of studies available (EPA, 2004, 
section 8.4.3; EPA, 2005, p. 3-46). These documents illustrate the 
results of a number of studies that examined co-pollutants (Figures 8-
16 through 8-18 of the Criteria Document), where it can be seen that, 
in most cases, the inclusion of gaseous co-pollutants does little to 
change the effects estimate for PM2.5, although in some 
cases it does. Recognizing the additional uncertainties in measuring 
coarse particles (as discussed below), these documents further note the 
importance of the relative consistency in the size of effects estimates 
for coarse particles as well as the pattern of generally positive 
associations, and the need for considering the results of recent 
statistically significant associations found in PM10 studies 
where it is reasonable to expect that the coarse fraction dominated the 
distribution. It would be unwise to presume, in the face of this 
evidence, that the single pollutant result for coarse particles is 
generally the result of omitted gases in the model.
---------------------------------------------------------------------------

    \61\ The Response to Comments document contains more detailed 
responses to the specific issues these commenters raise regarding 
the interpretation of the epidemiologic evidence, which is important 
in terms of the use of these studies for supporting a coarse 
standard (this section of the preamble) as well as their use in 
deciding upon an appropriate level of protection (section III.D.2 of 
this preamble).
---------------------------------------------------------------------------

    EPA also believes that it is inappropriate to presume that coarse 
particle or PM10 associations in single or multi-pollutant models can 
be wholly explained by fine particles. In studies where 
PM2.5 and PM10-2.5 have similar effect estimates, 
it is difficult to determine whether one or both contribute to the 
result (e.g. EPA 2004a, p 8-61). The comparison of PM2.5 and 
PM10-2.5 is further complicated by the differential 
measurement error between the two pollutants, which is generally 
greater for coarse particles (as discussed below). When both pollutants 
have similar effect estimates, it is difficult to determine whether one 
or both contribute to the result (e.g. EPA, 2004a, p. 8-61). Some 
studies conducted in urban areas, however, have found significant 
associations for coarse particles, but not fine particles. The Criteria 
Document summarizes a case cross-over study (Lin et al., 2002) 
conducted in Toronto, that found a significant association of 
PM10-2.5 with asthma hospital admissions in children ages 6-
12 that was robust to the inclusion of gaseous co-pollutants, but did 
not report significant associations for PM2.5.\62\ Three 
different studies used essentially the same air quality data set to 
examine coarse and fine particles in Phoenix (Mar et al., 2000, 2003; 
Clyde, 2000; Smith et al., 2000). All three studies found significant 
associations between mortality and PM10-2.5, but only one 
found a significant association for PM2.5 (EPA, 2004a, p. 8-
57 to 66). Ito (2003) found a significant association in Detroit 
between hospital admissions for ischemic heart disease and exposure to 
coarse particles, but not fine particles. While all of these studies 
have limitations, it is difficult to ignore the fact that, despite the 
differential measurement error associated with coarse particles, a 
number of these studies find statistically significant associations for 
coarse particles, but not for fine particles. For these reasons, EPA 
believes that it would be inappropriate, based on the limited data 
currently available, to presume that all of the effects associated with 
coarse particles in single pollutant models are actually the result of 
confounding by fine particles.
---------------------------------------------------------------------------

    \62\ Unlike more commonly used time series studies, the design 
used in this study has the advantage of controlling for confounding 
by having each case serve as its own control. The Criteria Document 
notes limitations in available measurement information and 
adjustment for season that may have influenced the relative results 
for fine and coarse particles (EPA, 2004a, pp. 185-186).
---------------------------------------------------------------------------

    It is also important to note that in the NAAQS reviews that 
concluded in 1987 and 1997, EPA found that the scientific evidence then 
available supported the need to continue regulation of thoracic coarse 
particles through appropriate NAAQS. This evidence included mechanistic 
considerations developed from particle dosimetry and toxicology, as 
well as an integrated assessment of particle composition and both 
community and occupational epidemiologic studies. By 1997, EPA judged 
the evidence to be strong enough to propose separate standards for fine 
and coarse particles. While the D.C. Circuit found problems with the 
indicator for thoracic coarse particles promulgated in 1997, the court 
upheld EPA's determination that a standard was needed (ATA I, 175 F.3d 
at 1054). In EPA's judgment, the more recent studies included in the 
2004 Criteria Document, even with their recognized limitations, serve 
to add to, not reduce, the concern present in previous reviews over 
ambient exposures to coarse particles, particularly in urban areas.
    The business and industry commenters also suggested that the 
epidemiologic studies were flawed by the reliance on data from central 
monitors to estimate community-level exposures to coarse fraction 
particles. According to these commenters, this would result in an 
overestimation of

[[Page 61184]]

exposure due to the significant spatial variability associated with 
coarse particle distributions. Such overestimation, in the commenters' 
view, would invalidate any statistical associations found between 
ambient data, as measured by the central monitors, and adverse health 
effects. The National Mining Association (p. 16-17), for example, 
noted:

The spatial variability of coarse PM renders even the few, limited, 
uncertain epidemiological studies that have been cited by EPA 
invalid, as well as imprecise * * *. Given that the purported 
associations between PM coarse and health effects is small to begin 
with, 71 FR at 2659, the logical conclusion should be that the lack 
of a demonstrable connection between the monitored ambient data and 
the level of exposure of the subject population is a fatal flaw that 
precludes reliance on the studies for any connection between PM 
coarse and health effects.

    These commenters also provided supporting information regarding 
correlations among monitors and an air quality modeling analysis 
purporting to show that significant quantities of coarse particles 
cannot travel more than 1 kilometer from sources.\63\
---------------------------------------------------------------------------

    \63\ This issue is discussed in more detail in the Response to 
Comments document.
---------------------------------------------------------------------------

    The Criteria Document and Staff Paper contain detailed analyses of 
the spatial variability of coarse particle concentrations, as well as 
other issues that generally result in greater exposure measurement 
error for coarse particles as compared to fine particles (EPA, 2004a, 
p. 3-52-53, Appendix 3A; EPA, 2005, pp. 2-36-40, 2-70-73). While EPA 
agrees that coarse particle measurements from central monitors is 
subject to potentially large measurement error when used to reflect 
population exposures in epidemiologic studies, the Agency disagrees 
with the commenters' assessment of the direction of the resulting bias 
and with their conclusion that any statistically significant 
associations between centrally monitored air quality concentrations and 
adverse health effects measured in these studies are invalid as a 
result. This issue received substantial attention in the Criteria 
Document (EPA, 2004a, section 8.4.5). The Criteria Document concluded 
that such measurement errors are more likely to underestimate the 
strength and the significance of any association between coarse 
particles and any adverse health effects observed in the study (EPA, 
2004a, pp. 5-126, 8-341). While the spatial variation of coarse 
particle data is larger than for fine particles, the Staff Paper notes 
that, on a day-to-day basis, coarse particle data from monitor sites 
within an urban area can be fairly well correlated, even when 
substantial differences exist in the absolute concentrations between 
the sites (EPA, 2005, p. 3-41). The signal that drives statistical 
associations between ambient concentrations and health effects in time-
series studies is the day-to-day changes in concentration, not the 
absolute daily values. To the extent possible, EPA examined both the 
day-to-day correlations and annual averages in PM10-2.5 
taken from multiple monitors in key study locations, such as Detroit, 
Phoenix and Coachella Valley (Ross and Langstaff, 2005).\64\
---------------------------------------------------------------------------

    \64\ In Phoenix, for example, two key sites were highly 
correlated with similar means. In Detroit/Windsor, correlations were 
moderate to good, but absolute values were significantly higher in 
Detroit (Ross and Langstaff, 2005).
---------------------------------------------------------------------------

    In reacting to this issue in opposing comments, the California Air 
Resources Board similarly stated:

The current scientific consensus suggests that measurement of coarse 
particles will typically involve greater errors than that of fine 
particles. However we reject the * * * implication that therefore 
these studies are not reliable. In fact, the larger measurement 
error, which is likely to be random, would make it more difficult to 
find an association with mortality. It is well accepted in the 
epidemiological literature that such measurement error will tend to 
obscure a relationship between an exposure and a given health 
outcome, assuming that such a relationship exists. Therefore, the 
measurement error argument cannot be used to nullify an effect that 
has been observed. If anything, it is likely that the real effects 
are likely to be larger than those that were estimated. (CARB, p. 
11)

    The EPA agrees with CARB's analysis of the issue. Therefore, for 
the purposes of determining whether public health protection is 
warranted in light of the available evidence, EPA believes that it has 
interpreted the evidence from these epidemiologic studies correctly, 
and that despite the uncertainties, the evidence of statistically 
significant relationships between exposure to coarse particles and 
adverse health effects is sufficiently strong to support continued 
regulation of coarse particles.
    Some commenters opposed to maintaining a coarse particle standard 
criticized EPA's risk assessment. These commenters stated that current 
short-term epidemiologic data are insufficient to serve as the basis 
for a scientifically sound quantitative risk assessment, without which, 
they claim, EPA lacks sufficient evidence to establish a standard based 
on those data. According to these commenters, while EPA may exercise 
its judgment about future risks and set standards that are preventive 
in nature, as long as an adequate scientific rationale is presented, 
the Agency does not have the authority to engage in ``crystal ball 
speculation'' in the absence of support in the record considered as a 
whole. (See e.g., Coarse Particle Coalition, p. 8-9, citing Lead 
Industries Assoc v. EPA, 647 F. 2d 1130, 1146-7 (DC Cir. 1980), NRDC v. 
EPA, 902 F.2d 962, 968, 971 (D.C. Cir. 1990) and Ethyl Corp. v. EPA, 
541 F.2d 1, 13 (D.C. Cir. 1976).) These commenters stated that the 
NAAQS must address only ``significant risk'', not any risk, and that 
EPA has failed to demonstrate that coarse particles pose a significant 
enough risk to human health to warrant a coarse particle standard.
    The EPA disagrees on technical, policy, and legal grounds. For 
reasons specified in the proposal and summarized above, EPA believes 
that the available scientific evidence is more than adequate to support 
a decision to continue regulation of coarse particles under the NAAQS. 
Although the data are weaker than for fine particles and subject to 
greater measurement error, in several of the studies where comparisons 
are possible, the normalized relative risk estimates for coarse 
particles from the new urban/industrial-area studies that were included 
in the Criteria Document often fall into a similar range as those for 
fine particles (EPA, 2004a, p. 8-64; EPA, 2005, pp. 3-13 and 3-20). 
Furthermore, as summarized above, EPA did produce a risk assessment for 
thoracic coarse particles, which was reviewed by CASAC and included in 
the Staff Paper (EPA, 2005, Chapter 4). While the limited number of 
cities and the significant uncertainties noted in the risk assessment 
and the proposal limit their quantitative usefulness, EPA staff 
concluded that the risk assessment results for the two urban areas in 
the assessment that did not meet the current PM10 standards 
are indicative of risks that can reasonably be judged to be important 
from a public health perspective.
    Furthermore, there is no requirement that EPA develop a 
``scientifically sound quantitative risk assessment'' before adopting 
or revising a NAAQS (ATA III, 283 F.3d at 374), or that the Agency must 
demonstrate significant risk before promulgating a NAAQS.\65\ EPA's 
reliance on evidence from peer-

[[Page 61185]]

reviewed scientific studies in this review, as well as its reliance on 
CASAC's unanimous recommendation that there is a need for a standard 
for thoracic coarse particles, cannot be considered ``crystal ball 
speculation.''
---------------------------------------------------------------------------

    \65\ See e.g., American Petroleum Inst. v. Costle, 665 F. 2d at 
1186-87: ``In setting margins of safety the Administrator need not 
regulate only the known dangers to health, but may ``err'' on the 
side of overprotection by setting a fully adequate margin of safety. 
Of course the Administrator's conclusions must be supported by the 
record, and he may not engage in sheer guesswork. Where the 
Administrator bases his conclusion as to an adequate margin of 
safety on a reasoned analysis and evidence of risk, the court will 
not reverse.''
---------------------------------------------------------------------------

    After careful consideration of all of these comments, EPA continues 
to believe that the health evidence, including dosimetric, toxicologic 
and epidemiologic study findings, supports retaining a standard to 
protect against effects associated with short-term exposure to thoracic 
coarse particles. As noted above and summarized in section III.A of the 
proposal, there is a growing body of evidence suggesting causal 
associations between short-term exposure to thoracic coarse particles 
and morbidity effects, such as respiratory symptoms and hospital 
admissions for respiratory diseases, and possibly mortality. As 
summarized in the proposal (71 FR 2659), the available body of evidence 
also suggests there is a lack of such effects associated with long-term 
exposure to thoracic coarse particles. Considering the magnitude of the 
risks identified in health studies, and the size of potentially 
susceptible subpopulations such as people with preexisting respiratory 
diseases, including asthma, and children and older adults, EPA 
concludes that short-term exposure to thoracic coarse particles can 
have an important public health impact. The health evidence regarding 
effects of thoracic coarse particles is limited in some respects and 
still subject to significant uncertainty. The Administrator has 
concluded that it is a priority to establish a robust research program 
that will enable future PM NAAQS reviews to make more informed 
decisions that will provide more targeted protection against the 
effects only of those coarse particles and related source emissions 
that prove to be of concern to public health. The Administrator also 
notes that the need for a standard for thoracic coarse particles has 
already been upheld based upon evidence of health effects considerably 
more limited than now available (ATA I, 175 F.3d at 1054).
    In the judgment of the Administrator, it is appropriate at this 
time to retain a standard to address the known and potential public 
health risks associated with exposure to coarse particles. The 
Administrator's specific decisions regarding the indicator, averaging 
time, level and form of a standard for thoracic coarse particles are 
described below.

C. Indicator for Thoracic Coarse Particles

1. Introduction

    As outlined above, at the time of proposal the Administrator judged 
it appropriate, based on an evaluation of the available scientific 
evidence, to propose a new indicator of thoracic coarse particles 
defined to include those particles between 2.5 and 10 [mu]m in 
diameter, or PM10-2.5, and qualified to focus on the mix of 
thoracic coarse particles generally present in urban environments. In 
making this determination, the Administrator relied heavily on key 
findings and observations from the Criteria Document and Staff Paper, 
and on recommendations from CASAC. The Staff Paper made the following 
general observations about the PM10-2.5 indicator:
    (1) The most obvious choice for a thoracic coarse particle standard 
is the size-differentiated, mass-based indicator used in the 
epidemiologic studies that provide the most direct evidence of such 
health effects, PM10-2.5.
    (2) The upper size cut of a PM10-2.5 indicator is 
consistent with dosimetric evidence that continues to reinforce the 
finding from past reviews that an aerodynamic size of 10 [mu]m is a 
reasonable separation point for particles that penetrate to and 
potentially deposit in the thoracic regions of the respiratory tract.
    (3) The lower size cut of such an indicator is consistent with the 
choice of 2.5 [mu]m as a reasonable separation point between fine and 
coarse fraction particles.
    (4) Further, the limited available information is not sufficient to 
define an indicator for thoracic coarse particles solely in terms of 
metrics other than size-differentiated mass, such as specific chemical 
components.
    (5) The available epidemiologic evidence for effects of 
PM10-2.5 exposure is quite limited and is inherently 
characterized by large uncertainties, reflective in part of the more 
heterogeneous nature of the spatial distribution and chemical 
composition of thoracic coarse particles and the more limited and 
generally uncertain measurement methods that have historically been 
used to characterize their ambient concentrations.
    In evaluating relevant information from atmospheric sciences, 
toxicology, and epidemiology related to thoracic coarse particles, the 
Staff Paper also noted that there appear to be clear distinctions 
between (1) the character of the ambient mix of particles generally 
found in urban areas as compared to that found in non-urban and, more 
specifically, rural areas, and (2) the nature of the evidence 
concerning health effects associated with thoracic coarse particles 
generally found in urban versus rural areas.\66\ Based on such 
information, and on specific initial advice from CASAC (Henderson, 
2005a), the Staff Paper considered a more narrowly defined indicator 
for thoracic coarse particles that would focus on the mix of such 
particles that is characteristic of the mix generally found in urban 
areas where thoracic coarse particles are strongly influenced by 
traffic-related or industrial sources. In so doing, the Staff Paper 
focused on comparing the potential health effects associated with 
thoracic coarse particles in urban and rural settings, as discussed 
below.
---------------------------------------------------------------------------

    \66\ In general, EPA believes it is appropriate to draw a 
distinction between two general types of ambient mixes of coarse 
particles: ``urban'' and ``non-urban''. The first term characterizes 
the mix in more heavily populated urban areas, where sources such as 
motor vehicles and industry contribute heavily to ambient coarse 
particle concentrations and composition. The term ``non-urban,'' on 
the other hand, encompasses mixes in a variety of other locations 
outside of urbanized areas, including mixes in rural areas which are 
likely to be dominated by natural crustal materials (and where urban 
types of sources are largely absent or, in the case of motor 
vehicles, are not present to the same degree). It should be noted 
that some types of sources are present in both urban and non-urban 
areas. Industrial sources, for example, are found in non-urban 
areas, though they are more commonly located in urban areas. 
Similarly, agricultural and mining sources are primarily non-urban 
sources, but may be found in or near urban areas as well.
---------------------------------------------------------------------------

    The Staff Paper also noted that atmospheric science and monitoring 
information indicates that exposures to thoracic coarse particles tend 
to be higher in urban areas than in nearby rural locations. Further, 
the mix of thoracic coarse particles typically found in urban areas 
contains a number of contaminants that are not commonly present to the 
same degree in the mix of natural crustal particles that is typical of 
rural areas. The elevation of PM10-2.5 levels in urban 
locations as compared to those at nearby rural sites suggests that 
sources located within urban areas are generally the cause of elevated 
urban concentrations; conversely, PM10-2.5 concentrations in 
such urban areas are not largely composed of particles blown in from 
more distant regions (EPA, 2005, sections 2.4.5 and 5.4.2.1). Important 
sources of thoracic coarse particles in urban areas include dense 
traffic that suspends significant quantities of dust from paved roads, 
as well as industrial and combustion sources and construction 
activities that contribute to ambient coarse particles both directly 
and through deposition to soils and roads (EPA, 2005, Table 2-2).

[[Page 61186]]

The Staff Paper concluded that the mix of thoracic coarse particles in 
urban areas would likely differ in composition from that in rural 
areas, being influenced to a relatively greater degree by components 
from urban mobile and stationary source emissions.
    While detailed composition data are more limited for 
PM10-2.5 than for PM2.5, available measurements 
from some areas as well as studies of road dust components do show a 
significant influence of urban sources on both the composition and mass 
of thoracic coarse particles generally found in urban areas. Although 
crustal elements and natural biological materials represent a 
significant fraction of thoracic coarse particles in urban areas, both 
their relative quantity and character may be altered by urban sources 
(EPA, 2005, p. 5-54). Traffic-related activities can also grind and 
resuspend vegetative materials into forms not as common in more natural 
areas (Rogge et al., 1993). Studies of urban road dusts find that 
levels of a variety of components are increased from traffic as well as 
from other anthropogenic urban sources, including products of 
incomplete combustion (e.g. polycyclic aromatic hydrocarbons) from 
motor vehicle emissions and other sources, brake and tire wear, rust, 
salt and biological materials (EPA, 2004a, p. 3D-3). Limited ambient 
coarse fraction composition data from various comparisons show that 
metals and sometimes elemental carbon contribute a greater proportion 
of thoracic coarse particle mass in urban areas than in nearby rural 
areas. In addition, while large uncertainties exist in emissions 
inventory data, the Staff Paper observed that major sources of 
PM10-2.5 emissions in the urban counties in which 
epidemiologic studies have been conducted are paved roads and ``other'' 
sources (largely construction), and that such areas also have larger 
contributions from industrial emissions, whereas unpaved roads and 
agriculture are the main sources of PM10-2.5 emissions 
outside of urban areas.
    In the proposal, EPA also stated that toxicologic studies, although 
quite limited, support the view that thoracic coarse particles from 
sources common in urban areas are of greater concern than 
uncontaminated materials of geologic origin. One major source of 
thoracic coarse particles in urban areas is paved road dust; the 
Criteria Document discussed results from a recent toxicologic study in 
which road tunnel dust particles had greater allergy-related activity 
than several other particle samples (Steerenberg et al., 2003; EPA, 
2004a, pp. 7-136-137). This study supports evidence available in the 
last review regarding potential effects of road dust particles (EPA, 
1996b, p. V-70). In contrast, a number of studies have reported that 
Mt. St. Helens volcanic ash, an example of uncontaminated natural 
crustal material of geologic origin, has very little toxicity in animal 
or in vitro toxicologic studies (EPA, 2004a, p. 7-216).
    A few toxicologic studies have used ambient thoracic coarse 
particles from urban/suburban locations (PM10-2.5), and the 
results suggest that effects can be linked with several components of 
PM10-2.5. These in vitro toxicologic studies linked thoracic 
coarse particles with effects including cytotoxicity, oxidant 
formation, and inflammatory effects (EPA, 2005, sections 3.2 and 
5.4.1). While these studies cannot be used for quantitative assessment 
of morbidity or mortality effects, they suggest that several components 
(e.g., metals, endotoxin, other materials) may have roles in various 
health responses but do not suggest a focus on any individual 
component.
    Although largely focused on undifferentiated PM10, the 
series of epidemiologic observations and toxicologic experiments 
related to the Utah Valley suggest that directly emitted (fine and 
coarse) and resuspended (coarse) urban industrial emissions are of 
concern. Of particular interest are area studies spanning a 13-month 
period when a major source of PM10 in the area, a steel 
mill, was not operating. Observational studies found that respiratory 
hospital admissions for children were lower when the plant was shut 
down (Pope, 1989). More recently, a set of toxicologic and controlled 
human exposure studies have used particles extracted from filters from 
ambient PM10 monitors from periods when the plant did and 
did not operate. In both human volunteers and animals, greater lung 
inflammatory responses were reported with particles collected when the 
source was operating, as compared to the period when the plant was 
closed (EPA, 2004a, p. 9-73). In addition, in some studies it was 
suggested that the metal content of the particles was most closely 
related to the effects reported (EPA, 2004a, p. 9-74). While peak days 
in the Utah Valley occur in conditions that enhance fine particle 
concentrations, over the long run, over half of the PM10 was 
in the coarse fraction. The aggregation of particles collected on the 
filters during the study period reflects this long-term composition and 
represent the kinds of industrial components that would be incorporated 
in road dusts in the area.
    The Staff Paper also noted that epidemiologic studies that have 
examined exposures to thoracic coarse particles generally found in 
urban environments, together with studies that have taken into account 
exposures to natural crustal materials typical of rural areas, 
generally support the view that the mix of thoracic coarse particles 
generally found in urban areas is of concern to public health, in 
contrast to natural crustal dusts of geologic origin. With respect to 
the urban results, several recent studies have shown associations 
between PM10-2.5 and health outcomes in a few sites across 
the U.S. and Canada. Associations have been reported with morbidity in 
a few urban areas, some of which had relatively low PM10-2.5 
concentrations. For mortality, statistically significant associations 
have been reported only for two urban areas that have notably higher 
ambient PM10-2.5 concentrations. These associations are with 
short-term exposures to aggregated PM10-2.5 mass, and no 
epidemiologic evidence is available on associations with different 
components or sources of PM10-2.5. However, these studies 
have all been conducted in urban areas of the U.S., and thus reflect 
effects associated with the ambient mix of thoracic coarse particles 
generally present in urban environments, which includes PM from traffic 
and industrial sources.
    The Staff Paper also pointed to other evidence from epidemiologic 
studies suggesting that mortality and possibly other health effects are 
not associated with thoracic coarse particles from dust storms or other 
such wind-related events that result in suspension of natural crustal 
materials of geologic origin. The clearest example is a study in 
Spokane, WA, which specifically assessed whether mortality was 
increased on dust-storm days using case-control analysis methods. The 
average PM10 level was more than 200 [mu]g/m3 
higher on dust storm days than on control days, and the authors report 
no evidence of increased mortality on these specific days (Schwartz et 
al., 1999). One caveat of note is the possibility that people may 
reduce their exposure to ambient particles on the dustiest days (e.g., 
Gordian et al., 1996; Ostro et al., 2000). Nevertheless, these studies 
provide no suggestion of significant health effects from uncontaminated 
natural crustal materials that would typically form a major fraction of 
coarse particles in rural areas.
    Beyond the urban and rural distinctions discussed above, the Staff 
Paper also considered the extent to which there is evidence of effects 
from

[[Page 61187]]

exposure to the ambient thoracic coarse particles in communities 
predominantly influenced by agricultural or mining sources.\67\ For 
example, in the last review, EPA considered health evidence related to 
long-term silica exposures from mining activities, but found that there 
was a lack of evidence that such emissions contribute to effects linked 
with ambient PM exposures (EPA, 1996b, p. V-28). Similarly in this 
review, there is an absence of evidence related to such community 
exposures. While crustal and organic dusts generated from agricultural 
activity can include a variety of biological materials, and some 
occupational studies discussed in the Criteria Document report effects 
at occupational exposure levels (EPA, 2004a, Table7B-3, p. 7B-11), such 
studies do not provide relevant evidence for effects at the much lower 
levels of community exposure. Further, it is unlikely that such 
predominantly non-urban sources contribute to the effects reported in 
the recent urban epidemiologic studies.
---------------------------------------------------------------------------

    \67\ As used in the Staff Paper, the term ``mining sources'' is 
intended to include all activities that encompass extraction and/or 
mechanical handling of natural geologic crustal materials. In the 
context of this rulemaking, neither mining nor agricultural sources 
are included in the more general category of ``industrial sources.''
---------------------------------------------------------------------------

    The Criteria Document concluded its integrated assessment of the 
effects of natural crustal materials as follows:

Certain classes of ambient particles appear to be distinctly less 
toxic than others and are unlikely to exert human health effects at 
typical ambient exposure concentrations (or perhaps only under 
special circumstances). For example, particles of crustal origin, 
which are predominately in the coarse fraction, are relatively non-
toxic under most circumstances, compared to combustion-related 
particles (such as from coal and oil combustion, wood burning, etc.) 
However, under some conditions, crustal particles may become 
sufficiently toxic to cause human health effects. (EPA, 2004a, p. 8-
344)

    The Staff Paper assessment of the available evidence relevant to 
the appropriate scope of an indicator for coarse particles can be 
summarized as follows. Ambient concentrations of thoracic coarse 
particles generally reflect contributions from local sources, and the 
limited information available from speciation of thoracic coarse 
particles and emissions inventory data indicate that the sources of 
thoracic coarse particles in urban areas generally differ from those 
found in non-urban areas. As a result, the mix of thoracic coarse 
particles people are typically exposed to in urban areas can be 
expected to differ appreciably from the mix typically found in non-
urban or rural areas. Ambient PM10-2.5 exposure is 
associated with health effects in studies conducted in urban areas, and 
the limited available health evidence more strongly implicates the 
ambient mix of thoracic coarse particles that is dominated by traffic-
related and industrial sources than that dominated by uncontaminated 
soil or geologic sources. The limited evidence does not support either 
the existence or the lack of causative associations for community 
exposures to thoracic coarse particles from agricultural or mining 
industries. Given the apparent differences in composition and in the 
epidemiologic evidence, the Staff Paper concluded that it is not 
appropriate to generalize the available evidence of associations with 
health effects that have been related to thoracic coarse particles 
generally found in urban areas and apply it to the mix of particles 
typically found in non-urban or rural areas (EPA, 2005, p. 5-57). The 
Staff Paper concluded that the available evidence collectively suggests 
that a more narrowly defined indicator for thoracic coarse particles 
should be considered that would protect public health against effects 
that have been linked with the mix of thoracic coarse particles 
generally present in urban areas. Such an indicator would be 
principally based on particle size, but also reflect a focus on the mix 
of thoracic coarse particles that is generally present in urban 
environments and the sources that principally generate that mix. The 
Staff Paper recommended consideration of thoracic coarse urban 
particulate matter (UPM10-2.5) as an indicator for a 
thoracic coarse particle standard, referring to the mix of airborne 
particles between 2.5 and 10 [mu]m in diameter that are generally 
present in urban environments, which, as discussed above, are 
principally comprised of resuspended road dust typical of high traffic-
density areas and emissions from industrial sources and construction 
activities (EPA, 2005, p. 5-54, 5-57-58). The Staff Paper concluded 
that such an indicator would more likely be an effective indicator for 
standards to protect against health effects that have been associated 
with thoracic coarse particles than a more broadly focused 
PM10-2.5 indicator. This indicator would also be consistent 
with a cautious interpretation of the epidemiologic evidence that does 
not potentially over-generalize the results of the limited available 
studies.
    In conjunction with this recommendation of an indicator defined in 
terms of the mix of thoracic coarse particles that are generally 
present in urban areas, the Staff Paper also discussed the importance 
of a monitoring network designed to be consistent with the intent of 
such an indicator and to facilitate implementation of such a standard. 
It should be noted that EPA has historically used other implementation-
related policies, specifically its guidelines regarding the handling of 
data affected by exceptional or natural events, to address elevations 
in thoracic coarse particle levels that may occur in urban areas as a 
result of dust storms or other such events for which the staff-
recommended indicator was not intended to apply. The Staff Paper 
recommended that both new criteria for monitor network design and 
revised natural/exceptional events policies should work in concert with 
a revised thoracic coarse particle indicator to ensure the most 
effective application of a thoracic coarse particle standard.
    In its review of the Staff Paper recommendation for a thoracic 
coarse particle indicator (Henderson, 2005b, p. 4), the CASAC generally 
agreed that ``thoracic coarse particles in urban areas can be expected 
to differ in composition from those in rural areas;'' that ``coarse 
particles in urban or industrial areas are likely to be enriched by 
anthropogenic pollutants that tend to be inherently more toxic than the 
windblown crustal material which typically dominates coarse particle 
mass in arid rural areas;'' and that ``evidence of associations with 
health effects related to urban coarse-mode particles would not 
necessarily apply to non-urban or rural coarse particles.'' Further, 
most CASAC Panel members concurred that ``the current scarcity of 
information on the toxicity of rural dusts makes it necessary'' for EPA 
to base its standard for thoracic coarse particles ``on the known 
toxicity of urban-derived coarse particles.'' While most Panel members 
concurred with the thoracic coarse particle indicator recommended in 
the Staff Paper, a few members recommended specifying an unqualified 
PM10-2.5 indicator in conjunction with monitoring network 
design criteria and natural/exceptional events policies that would 
emphasize urban influences. In either case, CASAC indicated that the 
intent of any such indicator should be to ``provide protection against 
those components of PM10-2.5 that arise from anthropogenic 
activities occurring in or near urban and industrial areas.''
    Based on these considerations, the Administrator proposed to 
establish a new indicator for thoracic coarse particles in terms of 
PM10-2.5, qualified so as to include any ambient mix of 
PM10-2.5 that is dominated by resuspended dust from high-
density traffic on paved roads and PM generated

[[Page 61188]]

by industrial sources and construction sources, and to exclude any 
ambient mix of PM10-2.5 that is dominated by rural windblown 
dust and soils and PM generated by agricultural and mining sources (71 
FR 2667-68). Furthermore, EPA proposed that ``[a]gricultural sources, 
mining sources, and other similar sources of crustal material shall not 
be subject to control in meeting this standard'' (71 FR 2699). As 
summarized above in section I.E, the proposed standard also included 
specific monitor site-suitability requirements which any monitor would 
have to meet in order to be used for comparison to the NAAQS, including 
a requirement that such monitors be sited in urbanized areas with a 
minimum population of 100,000. These requirements were designed to 
ensure that the monitors were capturing the ambient mix of 
PM10-2.5 dominated by the sources of concern.
    Subsequent to the proposal, CASAC provided additional comments to 
the Administrator on the proposed indicator for thoracic coarse 
particles. In a letter dated March 21, 2006, the Committee stated that 
``the PM Panel was pleased to see that the indicator for coarse 
thoracic particles of concern to public health took into account some 
of the various approaches that the PM Panel identified for 
consideration'' (Henderson 2006, p. 4). The CASAC reiterated its 
earlier statement that ``the current scarcity of information on the 
toxicity of rural dusts makes it necessary for the Agency to base its 
regulations on the known toxicity of urban-derived coarse particles.'' 
However, the Committee went on to say that ``the CASAC neither foresaw 
nor endorsed a standard that specifically exempts all agricultural and 
mining sources, and offers no protection against episodes of urban-
industrial PM10-2.5 in areas of populations less than 
100,000.'' The Committee recommended the ``expansion of our knowledge 
of the toxicity of rural dusts rather than exempting specific 
industries (e.g. mining, agriculture)'' from control under the standard 
(id at 5).
2. Comments on Indicator for Thoracic Coarse Particles
    The EPA received a large number of comments on its proposed 
decision with regard to the indicator of thoracic coarse particles 
which overwhelmingly opposed the proposed indicator. Few commenters 
unconditionally supported EPA's proposal to replace the PM10 
indicator with a qualified PM10-2.5 indicator that would 
provide targeted protection by including certain ambient mixes of 
thoracic coarse particles and excluding others. Support for the 
proposed approach came almost entirely from those industrial sectors 
whose sources were excluded from the proposed qualified 
PM10-2.5 indicator (i.e., agriculture and mining interests). 
While these commenters argued that EPA should not maintain any standard 
for thoracic coarse particles, they conditionally supported the 
qualified indicator if any standard were to be set. In contrast, all 
other commenters, including environmental and public health groups, 
State and local agencies, and industries not excluded from the proposed 
indicator (e.g., transportation and construction), opposed the proposed 
qualified indicator. Representatives from a variety of groups who 
otherwise disagreed on various aspects of the proposed indicator 
commented on the need for additional research to address the 
uncertainties in the current body of evidence regarding coarse 
particles and health effects. In addition, a variety of commenters 
urged EPA to deploy additional PM10-2.5 monitors in both 
urban and rural areas, consistent with the advice of CASAC, to provide 
a more robust and complete body of evidence regarding coarse particle 
effects.
    Commenters conditionally supporting the proposal expressed the view 
that EPA should exclude non-urban wind-blown dust and soil from the 
PM10-2.5 indicator. According to these commenters, ``such 
particles have been shown to be nontoxic, and the scientific studies 
show that they are not associated with adverse health effects' 
(American Farm Bureau Federation, p. 1). Furthermore, these commenters 
agreed with the proposed exclusion for agricultural and mining sources, 
stating that ``the preponderance of scientific evidence continues to 
demonstrate that fugitive dust from agricultural and mining operations 
presents no substantial health or welfare concerns' (National Mining 
Association, p. 1; see also National Cattlemen's Beef Association, p. 
1). These commenters quoted extensively from the Criteria Document and 
Staff Paper, and made points that were in many cases conceptually 
similar to the arguments in these documents and in the proposal. These 
commenters also tended to argue that there is substantial scientific 
evidence showing an absence of health effects from rural particles.
    These commenters cited differences in the composition of the mix of 
particles in urban areas versus the mix of particles in non-urban 
areas, which they stated is dominated by wind-blown soil fractions 
including silicates, primary organic materials including ground plant 
matter, residential wood smoke, and dust from unpaved roads. Though the 
coarse particle mix in urban areas also contains significant crustal 
materials, the commenters stated that it is contaminated by a wide 
variety of industrial and combustion-related byproducts, such as metals 
and organic materials (tire and brake wear, vehicle exhaust, industrial 
emissions, residential fuel combustion). These commenters noted that 
studies conducted in urban areas have linked health effects 
specifically to these urban-industrial contaminants. For example, the 
American Farm Bureau Federation cited the distinction between studies 
that found health effects related to traffic emissions in urban areas 
(Pearson et al., 2000; Kramer et al., 2000; and Lin et al., 2002) and a 
study they suggested found a strong association between cardiovascular 
mortality and motor vehicle exhaust components, but a negative 
association between soil and total mortality (Mar et al., 2000).\68\ 
Some of these commenters argued that coarse mode particles, especially 
crustal coarse mode particles, are unlikely to serve as carriers of 
urban-area contaminants because they have less surface area, do not 
adsorb contaminants easily, and have short atmospheric residence times. 
These commenters conditionally agreed with EPA's proposed goal of 
focusing regulatory efforts on the sources known to be associated with 
toxic coarse particles, especially traffic (Coarse Particle Coalition). 
Some of these commenters cited new studies completed after the close of 
the Criteria Document as providing additional evidence of associations 
between traffic-related emissions and adverse health effects (e.g. Kim 
et al., 2004; Ryan et al., 2005; Garshick et al., 2003; McDonald et 
al., 2004; and Ostro et al., 2006).
---------------------------------------------------------------------------

    \68\ Commenters cite the original publication. In the subsequent 
reanalysis, the investigators report ``our original findings 
remained unchanged'' (Mar et al. 2003).
---------------------------------------------------------------------------

    These commenters also stated that while urban contaminants may 
increase the toxicity of coarse particles, studies have demonstrated a 
lack of adverse effects associated with exposure to coarse particles in 
non-urban areas (e.g., Buist et al. (1983) study of exposure to Mount 
St. Helens' ash among diabetic children). Furthermore, these commenters 
argued that studies have found a lack of effects associated with 
exposure to crustal materials in general. They cited the lack of an 
association between mortality and dust storms found in Schwartz et al. 
(1999) and also noted that studies such as the 6-city study by Laden et 
al. (2000) have found

[[Page 61189]]

that crustal material, in both the fine and coarse fractions, is not 
associated with increased mortality. Thus, these commenters argued that 
there is sufficient evidence to show that crustal particulate matter is 
essentially benign and therefore should be excluded from the coarse 
particle indicator.
    The EPA agrees with these commenters that the strongest available 
evidence relates to the toxicity of the ambient mix of coarse particles 
found in urban environments. The limited evidence available from 
epidemiologic and toxicologic studies indicates exposure to ambient 
thoracic coarse particulate in urban areas is associated with health 
effects, and the health evidence more strongly implicates coarse 
particles from urban types of sources such as resuspended dust from 
high-density traffic on paved roads and PM generated by industrial 
sources and construction sources than coarse particles from 
uncontaminated soil or geologic sources. The EPA also agrees that there 
is far more evidence concerning health effects associated with thoracic 
coarse particles in urban areas than in non-urban areas. However, EPA 
disagrees with these commenters that there is sufficient evidence to 
demonstrate that there are no adverse health effects from community-
level exposure to coarse particles in non-urban areas. Rather, the 
existing evidence is inconclusive with regard to whether or not 
community-level exposures to thoracic coarse particles are associated 
with adverse health effects in non-urban areas. However, EPA does agree 
with these commenters that additional research is needed to clarify 
this issue and to reduce some of the other uncertainties regarding the 
effects associated with coarse particles. As discussed above, the EPA 
is, in fact, expanding both its research and monitoring programs to 
collect additional evidence on the differences between coarse particles 
typically found in urban areas and those typically found in rural 
areas. Specifically, EPA notes that the Agency's National Center for 
Environmental Research recently issued a Request for Proposals on 
``Sources, Composition, and Health Effects of Coarse Particulate 
Matter'' which is designed to (1) improve understanding of the type and 
severity of health outcomes associated with exposure to 
PM10-2.5; (2) improve understanding of subpopulations that 
may be especially sensitive to PM10-2.5 exposures including 
minority populations, highly exposed groups, and other susceptible 
groups; (3) characterize and compare the influence of mass, 
composition, source characteristics and exposure estimates in different 
locations and differences in health outcomes, including comparisons in 
rural and urban areas; and (4) characterize the composition and 
variability of PM10-2.5 in towns, cities or metropolitan 
areas, including comparisons of rural and urban areas. In addition, as 
described in the final monitoring rule published elsewhere in today's 
Federal Register, EPA and the states will require measurement of 
PM10-2.5 at 75 new multipollutant monitoring sites around 
the country. These sites will provide continuous measurements of mass 
as well as chemical speciation. EPA will locate 55 of these sites in 
urban areas and 20 in rural areas in order to gather information on the 
composition and transport of coarse particles in urban and rural areas. 
In addition, these monitors will employ the latest in speciation 
technology to advance the science so that future regulation will 
provide more targeted protection against the effects only of those 
coarse particles and related source emissions that prove to be of 
concern to public health.
    In addition, EPA disagrees with these commenters that there is 
sufficient evidence to exclude crustal materials from the coarse 
particle indicator regardless of the degree of contamination. Although 
there is some evidence that coarse particles of natural geologic origin 
are relatively non-toxic in their uncontaminated form, the Criteria 
Document notes that such particles may become sufficiently 
``contaminated by toxic trace elements or other components from 
previously deposited fine PM,'' to cause health effects (EPA, 2004a, 8-
344). Indeed, the urban coarse PM associated with adverse health 
effects in the studies discussed above was, by mass, predominantly 
crustal in origin.\69\ As noted in the proposal and in the response to 
these commenters on the need to maintain a coarse particle standard, 
EPA is aware of the studies that found no effects on mortality at lower 
coarse particle concentrations, but believes, consistent with the Staff 
Paper and Criteria Document conclusions, that the evidence is 
suggestive of a coarse particle effect in urban or industrial 
areas.\70\ The EPA continues to believe that urban sources may 
significantly alter both the relative quantity and character of crustal 
and natural biological materials in ambient mixes in urban areas. As 
noted above in section III.C.1, metals and other contaminants such as 
elemental carbon tend to appear in higher concentrations in the urban 
PM10-2.5 mix, and vegetative materials are ground and 
resuspended by traffic-related activities into forms not common outside 
urban areas.
---------------------------------------------------------------------------

    \69\ The American Farm Bureau Federation's summary of the 
results of Mar et al. (2000), offered in support of their arguments 
about the lack of effect of soil or crustal materials, misses some 
important elements of the study results. A major finding of the 
original study as well as the reanalysis (Mar et al., 2003) was an 
association between PM10-2.5 particles and mortality. The 
analysis in this work that examined sources and components examined 
contributions to the effects of PM2.5, not to 
PM10-2.5. In the opinion of the authors, the factor 
commenters call motor vehicle exhaust ``probably represents the 
influence of motor vehicle exhaust and resuspended road dust'' (Mar 
et al., 2000, p. 351). The negative association for ``soil'' in the 
fine fraction cited by the commenter was apparently related to 
problems in the PM2.5 measurement. When the data were 
reassessed for the period with an improved sampler, the authors 
report that the association between soil and mortality was 
``positive and significant at 0 days lag'' (ibid., p. 352).
    \70\ The Laden et al. (2000) study cited by commenters was 
reanalyzed in Schwartz (2003), with qualitatively similar findings. 
As in Mar et al. (2000, 2003), this study examined the associations 
of crustal materials in the fine particle fraction, in which they 
make up such a small fraction of fine mass that one of the six 
cities had to be excluded from the analysis (Laden et al., 2000, p. 
945). While this result does not provide any support for 
associations between coarse crustal materials and mortality, given 
the lower concentrations of coarse particles in five of the six 
cities and the lack of examination of coarse particle composition, 
the results are inconclusive with respect to the potential effects 
of higher concentrations of coarse particles.
---------------------------------------------------------------------------

    In contrast to those few commenters who conditionally supported 
EPA's proposed indicator, the vast majority of commenters opposed one 
or more aspects of EPA's proposed indicator, including: (1) The basic 
decision to qualify the indicator to focus on particles associated with 
certain types of sources and to exclude other ambient mixes; and (2) 
the particular qualifications applied to the indicator, including the 
proposed siting requirements for coarse particle monitors suitable for 
comparison with the NAAQS and the proposed exclusion of agricultural, 
mining, and other similar sources from control under the standard. This 
large group of commenters advanced scientific as well as legal and 
policy arguments against drawing a distinction between particles 
typical of urban versus non-urban or rural areas. These commenters 
included public health groups such as the American Lung Association, 
the American Heart Association, the American Cancer Society, the 
American Diabetes Association, and the American Public Health 
Association, and environmental groups such as Earthjustice, 
Environmental Defense, and the Natural Resources Defense Council. It 
also included the State and Territorial Air Pollution Program

[[Page 61190]]

Administrators and the Association of Local Air Pollution Control 
Officials (STAPPA/ALAPCO) and numerous individual State and local air 
pollution control agencies, as well as dozens of Tribes and Tribal 
organizations such as the National Tribal Caucus, the National Tribal 
Air Association and its parent organization, the National Tribal 
Environmental Council. In addition, a number of industry groups 
expressed opposition to the proposal to qualify the coarse particle 
indicator; in general, these comments came from groups representing 
industry categories that were not excluded from the proposed indicator, 
such as the Engine Manufacturers Association, the Alliance of 
Automobile Manufacturers, and the National Association of Home 
Builders. Though these industry commenters primarily argued against 
setting any coarse particle standard at this time, they stated that if 
a standard were to be adopted, scientific evidence did not support the 
proposal to qualify the indicator based on the mix of sources present.
    Commenters opposed to a qualified coarse particle indicator 
advanced numerous scientific arguments to support their position. They 
criticized EPA's interpretation of key epidemiologic studies, such as 
Gordian et al. (1996), Choudhury et al. (1997), Ostro et al. (2003), 
Smith et al. (2000) and Mar et al. (2003), arguing that these studies 
linked thoracic coarse particles to adverse health effects in 
environments where crustal components formed a significant part of the 
ambient mix of PM10-2.5. For example, commenters argued that 
the study conducted by Ostro et al. (2003) in Coachella Valley, which 
found statistically significant associations between exposure to coarse 
particles and mortality, provides direct evidence of harm from exposure 
to rural particles. These commenters also challenged the results of 
Schwartz et al. (1999), attributing the lack of statistically 
significant mortality results in that study to avoidance behavior 
(i.e., people may stay inside during dust storms) and noting that the 
study might have drawn different conclusions if morbidity endpoints had 
been considered. In support of this argument, they pointed to Hefflin 
et al. (1994), which looked at hospitalizations for bronchitis and 
sinusitis during dust storms and did find a small increase in these 
effects in the same area.
    In addition, a number of commenters, including States, researchers, 
environmental and public health groups, and industry commenters, cited 
studies of particle composition as showing that the coarse PM found in 
rural areas is commonly contaminated with the same toxic components as 
particles found in urban areas (e.g. Alaska Department of Environmental 
Conservation; American Lung Association; Engine Manufacturers 
Association; Veranth). Moreover, these commenters noted that rural 
dusts may contain additional toxic contaminants such as molds, fungi, 
endotoxins, pesticides, and carbonaceous compounds including polycyclic 
aromatic hydrocarbons (PAHs), all of which are associated with rural 
sources and have been shown to produce toxic effects (citing studies 
including: Monn and Becker 1999; Soukup and Becker 2001; Horvath et 
al., 1996; Offenberg and Baker, 2000; Eleftheriadis and Colbeck, 2001). 
(See American Lung Association et al., pp. 92-100.) In addition, some 
commenters pointed to studies of the composition of coarse particles in 
particular locations, such as Owens and Mono Lakes in California, as 
evidence of the dangerous nature of rural particles. Commenters noted 
that coarse particles from these areas are contaminated by heavy 
metals, arsenic, and other toxic contaminants, but would be excluded 
from the proposed indicator.
    Commenters critical of the proposed decision to qualify the coarse 
particle indicator also stated that EPA had inappropriately relied on 
the relatively few studies involving exposure to crustal materials, 
especially the Mt. St. Helens' studies. These commenters expressed the 
view that EPA should not equate exposure to volcanic ash to exposure to 
coarse particles emitted from agricultural and mining industries. 
Commenters noted that volcanic ash lacks many of the organic components 
typical of rural coarse PM, including pesticides and PAHs. Commenters 
pointed to specific components of coarse particles emitted by 
agricultural or mining activities, including endotoxins, pesticides, 
and metals, that they claim are associated with adverse health effects. 
These commenters argued that coarse particles in rural and other non-
urban areas are not generally ``uncontaminated materials of geologic 
origin'' or ``uncontaminated natural crustal dusts.'' They argued that 
some of the effects noted in epidemiologic studies of thoracic coarse 
particles, such as Mar et al. (2003), occurred in areas dominated by 
agricultural or mining dusts (Maricopa County Air Quality Department, 
p. 3-4). Some commenters also stated that EPA had not demonstrated or 
even claimed that coarse particles associated with agricultural and 
mining activities are harmless. Citing a long history of occupational 
studies documenting effects and EPA's statement in the proposal that 
``in the 1987 review, EPA found that occupational and toxicological 
studies provided ample cause for concern related to higher levels of 
thoracic coarse particles' (71 FR 2654), these commenters urged EPA to 
give greater weight to the results of such studies.
    A number of commenters opposing a qualified PM10-2.5 
indicator referenced ``new'' epidemiologic and toxicologic studies 
which were not included in the Criteria Document in support of their 
arguments in favor of an unqualified PM10-2.5 indicator. 
Specifically, the commenters pointed to recent epidemiologic studies 
showing statistically significant adverse health effects from exposure 
to coarse particles of varying composition, such as one study that 
found an association between exposure to volcanic ash and wheeze and 
exercise-induced bronchoconstriction (Forbes et al., 2003). In 
addition, commenters cited several ``new'' studies of health effects 
associated with exposure to coarse particles during Asian dust storms 
(Chen Y-S et al., 2004; Chen and Yang, 2005; Yang C-Y et al., 2005; 
Chang et al., 2006). Commenters also pointed to ``new'' toxicologic 
studies such as Schins et al. (2004), Veranth (2004, 2006), Becker 
(2005), Labban et al. (2004, 2006), and Steerenberg et al. (2006), 
arguing that toxicological studies do not show consistent differences 
between urban and rural dusts.
    In response to these commenters' first point regarding the 
epidemiologic studies that were included in the Criteria Document, EPA 
does not agree with the commenters that these epidemiologic studies 
provide direct evidence of harm from non-urban or rural crustal 
material. While EPA acknowledges that crustal particles may have 
dominated the ambient mix in some of the locations in which these 
studies were done, it is also the case that these areas are all urban, 
so the crustal materials in the ambient mix typically would be 
contaminated by metals, road dust, and other combustion byproducts. At 
the same time, EPA notes that CASAC cited the studies by Ostro et al. 
(2000, 2003) as suggestive of health effects associated with exposure 
to rural crustal materials: ``Little is known about the potential 
toxicity of rural dusts, although the 2000 and 2003 Coachella Valley, 
CA studies from Ostro et al. showed significant adverse health effects, 
primarily involving exposures to coarse-mode particles arising from

[[Page 61191]]

crustal sources' (Henderson, 2005a, p. 4). Thus while EPA does not 
agree with these commenters that the epidemiologic studies demonstrate 
that non-urban or rural crustal particles are harmful, at the same time 
EPA believes the studies do raise credible concerns and suggest the 
need to be cautious in interpreting the epidemiologic and other 
evidence.
    The EPA agrees with these commenters that the observations of 
Hefflin et al. (1994) suggest it is possible that the lack of mortality 
effects on dust storm days observed in Schwartz et al. (1999) may be 
due to avoidance behavior. As noted in the proposal (71 FR 2666), there 
is a possibility that people may reduce their exposure to ambient 
particles on the most dusty days. This argues for caution in 
interpreting the results of Schwartz et al. (1999) with regard to the 
potential health effects associated with exposure to natural crustal 
material.
    The EPA acknowledges the limitations on the scientific evidence 
identified by these commenters regarding the differences in composition 
and toxicologic effects of urban and rural thoracic coarse particles. 
As noted in the Criteria Document and Staff Paper, there is clear 
evidence of toxicity of certain components of thoracic coarse 
particles, such as metals and endotoxins, as well as evidence that 
natural crustal materials of geologic origin, such as Mt. St. Helens 
volcanic ash, may have very little toxicity. There is largely an 
absence of evidence regarding the presence or absence of toxicologic 
effects associated with other types of coarse particles in non-urban 
areas. However, EPA agrees that thoracic coarse particles in non-urban 
areas may become contaminated with a wide variety of toxic materials 
(EPA, 2004a, p. 8-344). Clearly, however, crustal material associated 
with particular locations, such as the dry lakebeds of Owens and Mono 
Lakes, can be highly contaminated with metals, salts, and other toxic 
constituents. The EPA agrees with commenters that the potential 
toxicity of these components is well recognized; however, such 
locations tend to be isolated and not representative of other 
locations.
    In response to other comments raised by this group of commenters, 
EPA continues to find it inappropriate to assume that effects observed 
in occupational studies should be considered representative of effects 
that would occur at community exposure levels. However, EPA agrees with 
commenters that the presence of occupational exposure studies 
demonstrating adverse effects lends further support to a cautious 
approach in considering revisions to the standards affording protection 
from thoracic coarse particles. Finally, to the extent that commenters 
cited new scientific studies that were not considered in the Criteria 
Document in support of their arguments against a qualified coarse 
particle indicator, EPA notes that as discussed above in section I.C, 
EPA it is basing the final decisions in this review on the studies and 
related information included in the PM air quality criteria that have 
undergone CASAC and public review, and will consider the newly 
published studies for purposes of decision making in the next PM NAAQS 
review.
    Overall, the scientific evidence supports a conclusion that the 
risks of adverse health effects associated with thoracic coarse 
particles typically found in urban or industrial areas warrant targeted 
protection. Although the limited and inconclusive evidence does not 
support such a conclusion concerning thoracic coarse particles 
typically found in non-urban or rural areas, it supports a cautious 
approach concerning thoracic coarse particles. The EPA agrees with all 
the commenters who pointed to the need for additional research to 
strengthen the current body of evidence to reduce some of the 
uncertainties regarding the health effects associated with coarse 
particles.
    In addition to their criticisms of the scientific basis for EPA's 
proposed indicator, commenters opposed to a qualified indicator also 
advanced legal and policy arguments against EPA's proposed approach. In 
particular, commenters criticized the proposal's provision that 
``agricultural sources, mining sources, and other similar sources of 
crustal materials shall not be subject to control in meeting this 
standard'' (71 FR 2699); a large number of commenters expressed the 
view that the exclusion is flatly illegal, citing CAA section 101 (a) 
(3) and case law in support. These commenters also pointed to CASAC's 
March 21, 2006 letter to the Administrator which stated that EPA had 
misconstrued the finding of the Committee and that the proposed rule--
particularly the source-category exclusions--was not consistent with 
the Committee's recommendations.
    These commenters also stated that EPA had failed to demonstrate 
that its proposed qualified indicator would protect public health with 
an adequate margin of safety. Pointing again to the relative paucity of 
data regarding health effects associated with coarse particles of 
differing compositions, and the almost complete lack of evidence 
regarding health effects in rural areas, these commenters expressed the 
view that EPA must demonstrate affirmatively that the coarse particle 
standards will ensure an absence of adverse effects on sensitive 
individuals (American Lung Association, p. 82, citing Lead Industries 
Ass'n v. EPA, 647 F.2d 1130, 1153 (D.C. Cir. 1980) and American Lung 
Ass'n v. EPA, 134 F.3d 388, 389 (D.C. Cir. 1998)), and that in the 
absence of evidence, or in the face of significant uncertainty, the CAA 
requirement to provide an adequate margin of safety obligates EPA to 
regulate all coarse particles equally (Lead Industries Ass'n v. EPA, 
647 F.2d 1154-55). Some of these commenters pointed to the DC Circuit 
Court's instruction in ATA III that ``[t]he Act requires EPA to 
promulgate protective primary NAAQS even where * * * the pollutant's 
risks cannot be quantified or `precisely identified as to nature or 
degree' '' (ATA III, 283 F.3d 355, 369 (quoting PM NAAQS, 62 FR 
28653)).
    Commenters also argued that, under the CAA, EPA is charged with 
setting ambient standards that are national in scope and application, 
and that the proposed qualified indicator fails this test. Citing 
Whitman, 531 U.S. at 473, some of these commenters stated that the 
proposed qualified indicator is a thinly veiled attempt to establish a 
coarse particle standard that only applies to urban areas, and that it 
denies citizens in non-urban areas adequate health protection. Several 
commenters, including numerous Tribes, argued that the qualified 
indicator, by virtue of depriving non-urban populations of protection 
from coarse particles, violated principles of environmental justice and 
the government's Trust Responsibility to Tribes.
    Commenters pointed to other concerns as well, many of them focused 
on specific aspects of the proposed PM10-2.5 indicator. 
First, some commenters stated that the proposed qualified indicator 
inadequately describes the substance(s) being regulated. These 
commenters argued that EPA is attempting to establish a composition-
based indicator without being able to define adequately which 
particular chemical or physical components are associated with adverse 
health effects. Furthermore, commenters pointed out that the indicator 
was defined in large part through an implementation strategy--i.e. via 
the placement of monitors--rather than in scientific terms. The 
Alliance of Automobile Manufacturers expressed concern that the result 
would be that two sources of coarse particulate matter with similar 
composition that presumably produce similar health

[[Page 61192]]

impacts would be ``given different regulatory treatment based merely on 
the non-scientific qualifiers established in EPA's indicator'' 
(Alliance of Automobile Manufacturers, p. 9).
    In addition, some commenters pointed to a logical paradox inherent 
in the proposed PM10-2.5 indicator, which is defined to 
include any ambient mix ``dominated by'' particles from particular 
types of sources. Commenters noted the potential for the same 
concentration of ``harmful'' coarse particles--i.e. particles from 
high-density traffic, industrial sources and construction sources--to 
be regulated differently in different locations depending on what 
percentage of the ambient mix it constitutes relative to ``crustal'' 
particles. These commenters stated that the coarse particle standard 
must provide a consistent level of protection from particles of 
concern, and that use of a 50 percent domination threshold would result 
in a variable level of protection from particles of concern.
    The EPA also received an extremely large number of comments from 
diverse stakeholder groups--some of whom conditionally supported a 
qualified indicator--regarding perceived problems with implementing the 
proposed PM10-2.5 indicator. Many commenters pointed out 
that EPA failed to specify which source types were included in the 
broad source category descriptions listed in the indicator. They 
requested further definition of what could be considered an 
``agricultural source,'' a ``mining source,'' or ``other similar 
sources of crustal material'' (i.e. those sources that would be 
excluded from control under the proposed standard), and which 
``industrial'' and ``construction'' sources were included in the 
indicator. Furthermore, some commenters inquired about the treatment of 
sources that were neither explicitly included in nor excluded from the 
proposed indicator, such as residential and commercial sources. In 
addition, commenters wondered how EPA or the States would make the 
determination that one set of sources was ``dominant,'' given the 
scarcity of knowledge about coarse particle emissions and air quality 
concentrations, and the lack of suitable source attribution techniques.
    Commenters also objected to the proposed five-part test for siting 
NAAQS-comparable monitors, noting that as written, the monitor siting 
criteria arbitrarily would prohibit monitoring and regulation of coarse 
particles outside urbanized areas of 100,000 population, regardless of 
the presence of large or numerous sources of the types of coarse 
particles of concern or the nature of the ambient mix. Commenters 
pointed out that the monitor siting criteria, by virtue of their highly 
prescriptive role in defining where the pollutant can and cannot be 
measured, in essence define the indicator itself, and artificially 
narrow its scope such that in many instances, coarse particles of 
concern would not be covered by the indicator. These commenters argued 
that by failing to provide protection from coarse particles of concern 
in non-urban areas even though the composition of those particles may 
be identical to that of coarse particles found in large urban areas, 
the qualified indicator, as EPA proposed to implement it, would be 
under inclusive. Many Tribes and some other commenters raised concerns 
about the environmental justice implications of the proposal and stated 
that EPA had violated its Trust Responsibility toward Tribes, because 
Tribal lands would be virtually excluded from coverage under the 
proposed monitor siting criteria, regardless of the mix of particles 
present. Furthermore, numerous commenters stated that the siting 
criteria would be impossible to implement, so the criteria undermined 
the proposed standard on a practical level. Commenters particularly 
objected to the fifth part of the monitor-site suitability test, which 
as proposed would require an affirmative demonstration that the ambient 
mix at the site was dominated by sources of concern, even if all of the 
other four monitor site-suitability criteria were met. Commenters 
stated that this demonstration would be impossible to execute due to 
the lack of suitable data and techniques, undermining the siting of any 
NAAQS-comparable PM10-2.5 monitors.
    In response to these perceived problems with the proposed qualified 
indicator, commenters suggested a number of remedies. A few commenters, 
mostly industry representatives who preferred that no coarse particle 
standard be set at the current time, stated that if EPA does set a 
standard, it should be based on a qualified PM10-2.5 
indicator, but EPA should fix specific problematic aspects of the 
proposal (e.g. clarify the definition of included vs. excluded 
industries). Most commenters, including States, Tribes, and 
environmental and public health groups, urged EPA to adopt an 
unqualified PM10-2.5 indicator to ensure adequate public 
health protection and to avoid some of their perceived legal and/or 
policy issues associated with the qualified indicator. A few of these 
commenters recommended that EPA utilize the Exceptional Events Rule, 
proposed on March 10, 2006 (71 FR 12592-12610), to exclude violations 
caused by rural windblown dust. According to these commenters, this 
would be consistent with historical practice, because in the past the 
Natural Events Policy has been applied in many instances to exclude 
data associated with dust storms and other events from consideration 
under the PM10 standard (see New Mexico Air Quality Bureau, 
p. 10).
    Some commenters advocating an unqualified PM10-2.5 
indicator stated that, given the limitations on the scientific 
evidence, and in light of some of the other problems identified with 
the proposed qualified indicator, EPA should consider retaining the 
current PM10 standards to continue protection from coarse 
particles. They expressed particular concern about the absence of 
control in the interim period between the issuance of the final PM 
NAAQS rule (which as proposed would include the revocation of existing 
PM10 standards in almost all locations) and the completion 
of designations under a new PM10-2.5 standard (which would 
require deployment of a new monitoring network followed by 3 years of 
data collection). A few of the commenters advocating the retention of 
the PM10 standards suggested that measurements of 
PM10 could be adjusted by subtracting out PM2.5 
to avoid double regulating the fine fraction, to satisfy a concern 
voiced by the D.C. Circuit in ATA I (e.g., Alliance of Automobile 
Manufacturers; also some Tribes and States). Some Tribal, State and 
local commenters suggested that the 24-hour PM10 standard be 
retained permanently in all areas where the PM10-2.5 
standard did not apply by virtue of the monitoring requirements, which 
limited NAAQS-comparable monitors to sites that met the five-point site 
suitability test outlined in the monitoring rule.
    While EPA proposed a qualified indicator that attempted to include 
certain ambient mixes of thoracic coarse particles and exclude others, 
EPA's evaluation of the large number of adverse comments received on 
the proposed qualified indicator has led it to the conclusion that 
significant caution is warranted in considering such revisions to the 
scope of the indicator affording public health protection from coarse 
particles. As discussed below, there are two main issues that arise 
from consideration of a qualified indicator for thoracic coarse 
particles: (1) The inability to effectively and precisely identify 
which coarse particles are included in the indicator

[[Page 61193]]

and which are not; \71\ and (2) the importance of providing some level 
of protection from exposure to all thoracic coarse particles while 
targeting protection at those kinds of thoracic coarse particles for 
which there is more evidence regarding adverse health effects.
---------------------------------------------------------------------------

    \71\ These concerns apply both to defining the qualified 
indicator and implementing the standard.
---------------------------------------------------------------------------

    As explained earlier in this section, EPA continues to believe 
that, from a scientific standpoint, it is appropriate to draw a 
distinction between the character of the ambient mix of thoracic coarse 
particles generally found in urban areas and that found in non-urban 
and, more specifically, rural areas, recognizing that the mix of coarse 
particles in urban areas is influenced to a relatively greater degree 
by components from urban mobile and stationary source emissions and 
that the evidence of health effects associated with exposure to these 
urban types of coarse particles should not be generalized to other 
types of coarse particles. In the presence of significant, though 
limited, evidence of effects in urban areas, it remains EPA's view that 
a targeted indicator that focuses control on areas with ambient mixes 
of coarse particles known to be associated with adverse health effects 
will provide the most certain and substantial public health benefits.
    However, EPA also recognizes a number of flaws in the proposed 
qualified indicator, as noted by numerous commenters, most specifically 
the difficulties inherent in attempting to effectively and precisely 
identify the ambient mixes of concern. These include: (1) The 
artificial constraints on the reach of the indicator resulting from the 
application of quantitative monitor site-suitability criteria such as 
the requirement that NAAQS-comparable monitors can only be sited in 
urbanized areas with minimum 100,000 population even if there is an 
ambient mix of concern around such an area; and (2) the difficulties 
associated with attempting to determine with any precision which 
sources ``dominate'' the ambient mix of coarse particles in different 
locations.
    The quantitative constraints in the monitor site-suitability 
criteria result in an under-inclusive indicator that fails to include 
all ambient mixes of concern. Smaller urban and/or industrial areas, 
for example, would not meet the proposed monitor siting criteria, but 
might have an ambient mix of concern. Consequently, EPA agrees with 
commenters that unless the constraints were changed, the proposed 
indicator would be under-inclusive. The EPA has considered several 
options to modify the quantitative criteria, including those discussed 
in the proposal (see Weinstock, 2006). For example, EPA evaluated 
different possible minimum population thresholds (e.g., 25,000 or 
50,000 instead of 100,000) for areas eligible to site NAAQS-comparable 
monitors, and/or the possibility of adding additional criteria to 
include areas that do not meet a quantitative population threshold but 
are dominated by industrial or traffic-oriented sources. Each of these 
options, however, was found too inflexible to capture all relevant 
areas or too difficult to implement in practice. Thus, EPA believes 
that even a more complex set of quantitative criteria would fail to 
resolve the basic problem inherent in precisely identifying those 
ambient mixes to include and those to exclude. Based on the data 
available to us in this review, there still remains a clear risk of 
failing to capture all ambient mixes of concern, or of capturing 
ambient mixes that are intended to be excluded from the qualified 
indicator.
    Moreover, as a general matter, the use of a qualified indicator 
without such objective monitor site-suitability criteria would still 
present serious problems because it is currently impossible to 
determine with any precision which sources ``dominate'' the ambient mix 
in many different locations. Although it may be easy in certain 
instances to identify an ambient mix dominated by urban and/or 
industrial sources, in many cases it would be difficult to determine 
whether that precise ambient mix presents the types of health risks 
identified in the epidemiologic and other studies. The EPA is currently 
unable to identify any set of objective criteria or techniques such as 
chemical air quality speciation or modeling that could be practically 
employed to ensure adequate inclusion of all areas with particles of 
concern, and exclusion of areas without such particles.
    The EPA is also aware that the legal concerns raised by commenters 
with regard to the exemption of agricultural and mining sources from 
control under the standard, and the specific sections of the Clean Air 
Act that speak to this issue, would require careful consideration if 
the proposed qualified indicator were to be adopted. The logical 
paradox noted by commenters is also a flaw in the qualified indicator 
that would need to be resolved. It is another example of the lack of 
precision in the use of such a qualified indicator.
    After careful consideration of the concerns raised by commenters 
and the options available, EPA now agrees with commenters that the 
proposed qualified indicator is fundamentally flawed, because it cannot 
effectively and precisely identify the ambient mixes of concern and 
because modifications to the indicator that could rectify this and 
other problems highlighted by the commenters have not been identified. 
At the present time, therefore, EPA believes that there is an inherent 
risk that a qualified indicator would not include all of the ambient 
mixes of concern which the indicator is intended to capture.
    Furthermore, in light of the significant scientific uncertainty 
surrounding the health effects associated with different ambient mixes 
of coarse particles, EPA agrees with commenters that the proposed 
qualified indicator would be insufficiently protective and further 
concludes that, given the limitations on the evidence regarding the 
health risks associated with different ambient mixes, some protection 
from exposure to thoracic coarse particles is warranted in all areas. 
The EPA recognizes that additional data will be collected and analyzed 
that will be useful to inform the next review.
    The EPA has already set out the reasons for providing protection 
from exposure to ambient mixes dominated by the types of thoracic 
coarse particles found in urban or industrial areas. With respect to 
other ambient mixes, some commenters have argued that the scientific 
evidence, including epidemiologic, dosimetric, toxicologic, and 
occupational studies, demonstrates that non-urban mixes of thoracic 
coarse particles are harmful, and therefore that EPA should maintain an 
unqualified indicator. Other commenters argue that the evidence 
demonstrates that non-urban mixes of thoracic coarse particles are 
benign and therefore EPA should retain a qualified indicator. The EPA 
disagrees with both of these views regarding the strength of the 
evidence. The existing evidence is inconclusive with regard to whether 
or not community-level exposures to thoracic coarse particles are 
associated with adverse health effects in non-urban areas. In light of 
this uncertainty and the need for caution in considering the evidence, 
and recognizing the large population groups potentially exposed to non-
urban thoracic coarse particles and the nature and degree of the health 
effects at issue, it is the judgment of the Administrator that the 
proper response to this body of evidence is to provide some protection 
from thoracic coarse particles in all areas. Congress ``specifically 
directed the Administrator to allow an adequate margin of safety to 
protect against effects which have not

[[Page 61194]]

yet been uncovered by research and effects whose medical significance 
is a matter of disagreement * * * Congress' directive to the 
Administrator to allow an ``adequate margin of safety'' alone plainly 
refutes any suggestion that the Administrator is only authorized to set 
primary air quality standards which are designed to protect against 
health effects that are known to be clearly harmful.'' Lead Industries 
v. EPA, 647 F.2d at 1154-55; see also American Petroleum Inst. v. 
Costle, 665 F.2d at 1186 (``in setting margins of safety the 
Administrator need not regulate only the known dangers to health'').
    The Administrator has carefully reviewed the scientific evidence 
and recommendations contained in the Staff Paper, the advice and 
recommendations from CASAC, and the public comments received regarding 
the appropriate indicator for coarse particles. After doing so, the 
Administrator has decided that it would not be appropriate at this time 
to revise the indicator for coarse particles by adopting a qualified 
PM10-2.5 indicator, either as proposed or with 
modifications. At the same time, the Administrator believes it is 
appropriate to target protection from thoracic coarse particles 
principally towards those types of coarse particles that have been 
demonstrated to be associated with significant adverse health effects, 
specifically urban and industrial ambient mixes of coarse particles.
    In general, EPA believes these conclusions regarding the potential 
health effects associated with thoracic coarse particles, and the 
conclusion that an unqualified indicator that provides targeted 
protection is the most appropriate approach for regulating coarse 
particles, are consistent with views expressed by CASAC. In its June 6, 
2005 letter, CASAC expressed the view that it was ``important to 
qualify the PM10-2.5 standard by somehow allowing exceptions 
for regions where the coarse fraction was composed largely of material 
that was not contaminated by industrial- or motor vehicle traffic-
associated sources. Options discussed by members of the Panel for 
attempting to achieve this approach included limiting the standard to 
cover ``all'' urban areas, the judicious siting of monitors with a 
focus on urban areas, or regulatory exceptions for regions where road 
dust is not an issue or where rural components dominate the source. No 
single option was favored'' (Henderson, 2005a, p. 8, emphasis added). 
CASAC thus recognized that there were numerous ways to approach the 
need for targeted protection. In its September 2005 letter responding 
to the recommendations regarding a qualified PM10-2.5 
indicator in the final Staff Paper, the PM Panel noted that some 
members did not favor adoption of a qualified indicator. Moreover, 
CASAC clearly anticipated the difficulties associated with adopting a 
qualified PM10-2.5 indicator:

CASAC generally agrees with EPA staff conclusions that thoracic 
coarse particles in urban areas can be expected to differ in 
composition from those in rural areas and that evidence of 
associations with health effects related to urban coarse-mode 
particles would not necessarily apply to non-urban or rural coarse 
particles (although it is likely that there will be some overlap of 
the same contaminants in both areas). Most Panel members concurred 
that the current scarcity of information on the toxicity of rural 
dusts makes it necessary for the Agency to base its regulations on 
the known toxicity of urban-derived coarse particles, and that an 
urban coarse particle indicator should be specified as 
UPM10-2.5. Other Panel members recommended specifying a 
national PM10-2.5 standard accompanied by monitoring and 
exceptional-events guidance that emphasized urban influences. Some 
members also expressed concerns whether EPA would be able to specify 
a clear definition of ``urban'' to effectively determine in advance 
the specific conditions in which the standard would (and would not) 
apply. It is recognized that, as more information on the toxicity of 
rural dusts is acquired, the name and/or geographical focus of a 
coarse-particle indicator may need to be reconsidered* * *. There is 
a paucity of data currently available on health outcomes related to 
thoracic coarse particles in rural areas and limited information on 
the composition and toxicity of rural area coarse particles. 
(Henderson 2005b, p. 4)

CASAC also commented negatively on the proposed qualified indicator, 
raising concerns about the quantitative criteria for monitor siting and 
the source exclusions, as well as flagging the need for more 
information about health effects in non-urban areas (Henderson, 2006, 
p.4).
    The comments and concerns expressed by CASAC are consistent with 
the difficulties EPA has encountered in attempting to craft a qualified 
indicator, and the Committee correctly anticipated these difficulties. 
Furthermore, CASAC's advice is generally consistent with the ultimate 
decision by the Administrator not to move to a qualified 
PM10-2.5 indicator at present. The practical difficulties 
and imprecision associated with a qualified indicator, as well as the 
substantial scientific uncertainty regarding the health effects 
associated with different components and mixes of coarse particles, the 
large population groups potentially exposed to non-urban thoracic 
coarse particles and the nature and degree of the health effects at 
issue, have convinced the Administrator that it is inappropriate to 
adopt a qualified PM10-2.5 indicator at this time. In the 
following section, EPA considers what indicator would most 
appropriately provide the type of targeted but comprehensive protection 
judged appropriate based on its review of the scientific evidence.
3. Decision Not To Revise PM10 Indicator
    For reasons discussed in the previous section, in the view of the 
Administrator it is not appropriate to revise the PM10 
indicator by replacing it with a qualified indicator for thoracic 
coarse particles at this time. Based on the scientific evidence already 
summarized, the Administrator believes it is necessary to maintain some 
protection from all ambient mixes of thoracic coarse particles, and 
also to have that level of protection reflect the varying degree of 
public health concern presented by the different ambient mixes of 
thoracic coarse particulate matter. This would mean allowing lower 
ambient concentrations of thoracic coarse particles in urban areas, 
where the evidence indicates the public health risks to be significant, 
and higher levels in non-urban areas where the public health concerns 
are less certain. The difficulty of the task is compounded because 
there presently is no means of achieving this objective by linking 
allowable concentrations to specific coarse particle chemical 
components. As CASAC noted, ``[s]ufficient data are lacking at the 
present time to set standards [for thoracic coarse particulate matter] 
based specifically on composition'' (Henderson 2005b, p. 5).
    Given these objectives and constraints, EPA carefully considered 
various possibilities regarding the indicator for coarse particles, 
including adopting an unqualified PM10-2.5 indicator, 
retaining the existing PM10 indicator, and/or retaining the 
PM10 indicator with adjustment to avoid double-counting the 
PM2.5 fraction. These options are discussed below.
    a. Unqualified PM10-2.5 Indicator. The EPA evaluated 
whether an unqualified PM10-2.5 indicator would satisfy the 
goals for public health protection described above. However, if such an 
indicator were utilized as part of a standard with a single unvarying 
level, it would not reflect the critical difference in evidence 
regarding the relative public health risks associated with urban and 
non-urban thoracic coarse particles. If the level were selected to 
provide appropriate protection against effects associated

[[Page 61195]]

with exposure to the ambient mixes typical of urban or industrial 
areas, the standard would likely be more stringent than necessary to 
protect against effects associated with exposure to the ambient mixes 
in non-urban areas. In the judgment of the Administrator, the evidence 
warrants a lower ambient concentration of ambient coarse particles in 
urban areas than in non-urban areas, where the coarse particles are 
typically from different sources and there is less evidence of public 
health risk. Conversely, if a less stringent level were adopted on the 
grounds that there is less certainty that the ambient mix in non-urban 
areas poses a health risk, then the standard would not provide 
sufficient protection from the ambient mix found in urban or industrial 
areas. In both instances the standard would not be requisite overall, 
i.e., ``not lower or higher than is necessary,'' to protect the public 
health with an adequate margin of safety. Whitman, 531 U.S. at 476.
    Arguably this dilemma could be resolved by adopting a standard 
based on a PM10-2.5 indicator with a varying level depending 
on whether the area is urban or non-urban. However, determining 
appropriate levels for different kinds of ambient mixes is not feasible 
at this time. The EPA notes that given the variety of sources 
contributing to PM10-2.5 concentrations in different 
locations, a wide variety of ``ambient mixes'' are likely to exist, 
greatly complicating the determination of the appropriate standard 
level for each location. There is a lack of evidence to support 
establishing specific quantitative distinctions in level based on 
variations in coarse particle composition and differential toxicity. In 
addition, there is insufficient evidence regarding coarse particle 
composition in different areas to allow for the proper assignment of 
different standard levels in different locations, and the technical 
capabilities necessary to make such determinations are currently 
lacking. Even if EPA tried to assign only two levels, urban and non-
urban, the same problems identified earlier with respect to a qualified 
indicator would apply here, given the inability to effectively and 
precisely identify different ambient mixes. Therefore, EPA finds that 
the current state of the science does not provide an adequate basis 
upon which to establish a PM10-2.5 standard with an 
appropriately varying level. As EPA's new research program produces 
speciated monitoring data, thereby improving scientific knowledge, 
revealing more specific and precise information about coarse particle 
composition and relative toxicity, and about the distribution of 
ambient coarse particle mixes of varying composition, it will be 
appropriate in a future review to revisit the option of a 
PM10-2.5 standard with a variable level or a qualified 
indicator.
    b. PM10 Indicator. An alternative approach would be to 
retain PM10 as an indicator. The EPA recognizes, as did many 
commenters, that the D.C. Circuit concluded that EPA's 1997 choice of 
PM10 as the indicator for coarse particles was arbitrary and 
capricious. ATA I, 175 F.3d at 1027, 1054-55. In that case, the court 
noted the tension between EPA's conclusion that coarse and fine 
particles are different kinds of particles and pose independent and 
distinct threats to public health, and its choice to address the public 
health risks associated with coarse particles indirectly, using an 
indicator for coarse particles that nonetheless includes both fine and 
coarse particles. Although EPA adopted PM10 as a ``surrogate 
for coarse fraction particles,'' the court also noted EPA's recognition 
``that PM10-2.5 would have served as a satisfactory coarse 
particle indicator.'' With this backdrop, the court evaluated EPA's 
three bases for selecting PM10 as the indicator: (a) That 
the two epidemiologic studies underlying the standards for coarse 
particles used PM10 rather than PM10-2.5 as the 
indicator; (b) that the PM10 standards would work in 
conjunction with the PM2.5 standards ``by regulating the 
portion of particulate pollution not regulated by the PM2.5 
standards''; and (c) that a nationwide monitoring network for 
PM10 already existed. Id. at 1054.
    The court rejected the first two arguments for two interrelated 
reasons. First, use of PM10 as the indicator regulates both 
fine and coarse particles, contrary to EPA's argument that the 
PM10 indicator would work in conjunction with the 
PM2.5 standard to regulate only the coarse particle fraction 
of PM10. The court concluded: ``we cannot discern exactly 
how a PM10 standard, instead of a PM10-2.5 
standard, will work alongside a PM2.5 standard to regulate 
only the coarse fraction of PM10. EPA provides no 
explanation to aid us in understanding its decision.'' Id. at 1054. 
Second, because the PM10 indicator regulates both fine and 
coarse particles, the amount of coarse particles allowed ``will depend 
(quite arbitrarily) on the amount of PM2.5 pollution in the 
air.'' Id. EPA failed to explain why this result was consistent with 
its argument that a PM10 indicator would increase the 
likelihood that the standard would achieve the desired level of 
protection from exposure to coarse particles. The resulting combination 
of PM2.5 and PM10 standards would lead to double 
regulation of fine particles and the potential under-regulation of 
coarse particles, since the amount of allowable coarse particles would 
always depend on the amount of fine particles in the air. Id. The court 
rejected the third of EPA's arguments, the pragmatic, administrative 
convenience of using the existing monitoring network, on the grounds 
that only factors related to public health can be considered in 
establishing a NAAQS. Id. at 1054-55. In sum, the court rejected EPA's 
adoption of a PM10 indicator as arbitrary because of the 
inadequacy of the reasons provided by the Agency as support for the 
decision.
    Based on the current review of the scientific evidence, EPA feels 
it is now appropriate to reconsider utilizing PM10 as an 
indicator for coarse particles. Unlike its view in 1997, EPA views 
PM10-2.5 as an unsatisfactory indicator in this review, for 
the reasons described in the previous subsection. In addition, EPA is 
not maintaining, as it did in 1997, that a PM10 indicator 
will work in conjunction with the PM2.5 standard to regulate 
coarse particles exclusively, nor is the Agency justifying its choice 
of the PM10 indicator on grounds of administrative 
convenience. Instead, after careful consideration, it is the view of 
the Administrator that the PM10 indicator will in fact 
provide the type of targeted protection from thoracic coarse particles 
which is justified by the emerging body of scientific evidence, that it 
will do so more effectively and more appropriately than all other 
indicators evaluated by EPA during the course of this review, and that 
the inclusion of PM2.5 in the PM10 indicator does 
not over-regulate fine particles or under-regulate coarse particles.
    To the contrary, the inclusion of PM2.5 in the 
PM10 indicator plays two important roles in effectively 
providing the kind of targeted health protection called for under the 
current state of the science. Because the PM10 indicator 
includes both coarse PM (PM10-2.5) and fine PM (PM2.5), the 
concentration of PM10-2.5 allowed by a PM10 standard set at 
a single level declines as the concentration of PM2.5 
increases. Thus, the level of coarse particles allowed varies depending 
on the level of fine particles present. At the same time, 
PM2.5 levels tend to be lower in rural areas and higher in 
urban areas. EPA, 2005, p. 2-54, and Figures 2-23 and 2-24 at pp. 2-52 
and 2-53. Thus, to the extent that higher PM2.5 levels lead 
to a lower allowable level of coarse particles in some areas compared 
to others, this will occur in precisely those locations--

[[Page 61196]]

i.e. urban or industrial areas--where the science has shown the 
strongest evidence of adverse health effects associated with exposure 
to coarse particles. The EPA's recent Particle Pollution Report (EPA, 
2004b, Figure 5, p. 8) provides evidence that annual average 
concentrations of PM2.5 in selected eastern and western 
urban areas consistently exceed the annual average levels of 
PM2.5 in nearby rural areas. This means that a 
PM10 standard set at a single, unvarying level will permit, 
on average, lower levels of coarse particles in urban areas, where 
PM2.5 concentrations tend to be higher. The varying levels 
of coarse particles allowed by a PM10 indicator will 
therefore target protection in urban and industrial areas where the 
evidence of adverse health effects associated with exposure to coarse 
particles is strongest. For the same reason, lower levels of 
PM2.5 lead to a higher allowable level of coarse particles 
in non-urban areas, again an appropriate result given the inconclusive 
evidence of health risks associated with coarse particles in these 
areas. The varying amounts of coarse particles that are allowed in 
urban vs. non-urban areas under the 24-hour PM10 standard, 
based on the varying levels of PM2.5 present, appropriately 
reflect the differences in the strength of evidence regarding coarse 
particle effects in urban and non-urban areas.\72\
---------------------------------------------------------------------------

    \72\ The EPA recognizes that this relationship is qualitative. 
That is, the varying coarse particle concentrations allowed under 
the PM10 standard do not precisely correspond to the 
variable toxicity of thoracic coarse particles in different areas. 
While currently available information does not allow any more 
precise adjustment for relative toxicity, EPA believes the standard 
will generally ensure that the coarse particle levels allowed will 
be lower in urban areas and higher in non-urban areas. While the 
allowable levels will vary with location due to differing levels of 
fine particles, that variability will ultimately be limited by 
implementation of the PM2.5 standards. Areas that do not 
meet these standards are taking steps to reduce PM2.5, 
Currently, the annual fine particle standard places limits on both 
the long- and short-term levels of fine particles in a number of 
cities, particularly in the east and in some California cities. In 
the long run, this will serve to make the ``headroom'' allowed for 
thoracic coarse particles (i.e. the allowable PM10 level 
minus the corresponding PM2.5 concentration) more uniform 
among cities. The new 24-hour PM2.5 standard of 35 [mu]g/
m\3\ will promote this same result. It should cause areas that now 
meet the annual PM2.5 standard, but have high 24-hour 
PM2.5 concentrations, to adopt additional controls, 
further reducing the variability in the ``headroom'' for allowable 
thoracic coarse particle concentrations. In combination with the 
annual standard, the revised 24-hour PM2.5 standard thus 
will provide for more consistent allowable levels of thoracic coarse 
particles in cities under the PM10 standard.
---------------------------------------------------------------------------

    This result is consistent with our current understanding of the 
strength of the evidence regarding the toxicity of different ambient 
mixes of thoracic coarse particles in urban and non-urban or rural 
areas, and also is in accord with our current understanding of the 
observed toxicity in urban and industrial areas. As noted in both the 
proposal and the Criteria Document, the observed toxicity of coarse 
particles in urban and industrial areas comes from the kind of coarse 
particles found in these environments, for example direct emissions 
from industrial sources or materials released to road dust from motor 
vehicles such as brake and tire wear, as well as from the contamination 
of coarse particles that can occur. This contamination can come from 
both mobile and stationary sources. In particular, specific components, 
such as byproducts of incomplete combustion (e.g. polycyclic aromatic 
hydrocarbons) most commonly emitted from motor vehicles and other 
sources in the form of PM2.5, as well as metals and other 
contaminants emitted from other anthropogenic sources, appear in higher 
levels in urban areas (EPA, 2004a, p. 8-344; 71 FR 2665). Many of these 
contaminants in PM10-2.5 come originally from fine 
particles, which may become attached in the atmosphere or be deposited 
and mixed into coarse materials on the ground. Thus the greater the 
concentration of PM2.5, with higher levels typically found 
in urban areas, the greater the level of contamination of coarse 
particles by fine particles. This contamination increases the potential 
health risk posed by those coarse particles. For that reason, it is 
logical to allow lower levels of coarse particles when fine particle 
concentrations are high. In other words, inclusion of PM2.5 
in the PM10 indicator for purposes of coarse particle 
protection would appropriately reflect the contribution that 
contaminants emitted in fine particle form can make to the overall 
health risk posed by coarse particles.
    Moreover, due to the contamination of PM10-2.5 by 
PM2.5, use of a PM10 indicator will not result in 
inappropriate double regulation of the PM2.5 component. To 
the extent that use of a PM10 indicator would result in any 
reduction in PM2.5 concentrations in an area, this would 
reduce the potential health risk from coarse particles in the area as 
well. There is no certainty that the contribution of PM2.5 
to the health risk associated with exposure to contaminated coarse 
particles would be appropriately addressed through the fine particle 
standards alone. Thus, to the extent that the inclusion of the 
PM2.5 fraction in the PM10 indicator amounts to 
double regulation of PM2.5, its inclusion is non-duplicative 
and reasonable: it ensures that this risk of contamination of coarse 
particles by PM2.5 is addressed in the suite of fine and 
coarse PM standards.
    Some commenters nonetheless maintained that the court's opinion in 
ATA I bars use of PM10 as an indicator for coarse particles, 
stressing the court's statement that ``[i]t is the very presence of a 
separate PM2.5 standard that makes retention of the 
PM10 indicator arbitrary and capricious.'' 175 F. 3d at 
1054. The EPA disagrees that the ATA I decision precludes use of a 
PM10 indicator. The court did not hold that it was unlawful 
per se to use PM10 as an indicator for thoracic coarse 
particles. Instead, the court noted two particular problems--the 
variable level of allowable concentrations of PM10-2.5 and 
double regulation of PM2.5--and found that EPA either failed 
to address these issues, or provided explanations that were 
inconsistent and unsupported. Id. In large part, the court's decision 
was an important factor in EPA's close evaluation and subsequent 
proposal of a qualified PM10-2.5 indicator as part of this 
NAAQS review. See EPA, 2005, p. 1-5. However, EPA now believes that a 
qualified PM10-2.5 indicator is inappropriate, and that an 
unqualified PM10-2.5 indicator is more problematic and less 
effective than a PM10 indicator at providing the requisite 
level of protection from the varying risks associated with thoracic 
coarse particles. Indeed, for the reasons described above, 
PM10 is an effective indicator for targeting coarse 
particles because it provides the desired variability in allowable 
coarse particle concentrations.
    Far from being arbitrary and capricious, inclusion of 
PM2.5 serves two important functions: first, it is the 
mechanism that provides for the variation in allowable 
PM10-2.5 concentrations, targeting lower allowable levels 
where there is greater public health concern; and second, to the extent 
that there is ``double regulation'' of PM2.5 by virtue of 
its inclusion in the PM10 indicator (175 F.3d at 1054), 
regulation of PM2.5 via this indicator serves valid, non-
duplicative purposes in providing requisite protection from thoracic 
coarse particles. The EPA also notes that ``double regulation'' of a 
pollutant, in the context of multiple NAAQS standards, is neither 
impermissible nor even unusual. For example, there are both annual and 
24-hour standards for PM2.5, as well as both primary and 
secondary standards for PM2.5. The key is that the different 
standards reasonably serve different purposes `` they are directed at 
different effects, or

[[Page 61197]]

are not inconsistent when directed at the same effect--as is the case 
here.
    The EPA also recognizes that selection of PM10 as the 
indicator for thoracic coarse particles differs in some degree from the 
specific advice provided by CASAC to use a qualified 
PM10-2.5 indicator directed at urban or industrial thoracic 
coarse particles (71 FR 2665). However, EPA believes that the 
PM10 indicator is consistent with the central thrust of 
CASAC's advice--to utilize an indicator directed at urban types of 
coarse particulate matter, given the known toxicity of these 
particles--because it would generally allow lower levels of 
PM10-2.5 in urban areas. The EPA has also explained why it 
has rejected a qualified PM10-2.5 indicator at this time, 
and notes that CASAC itself considered multiple ways to achieve some 
degree of targeted protection and voiced strong objections to the 
qualified PM10-2.5 indicator which the Agency proposed 
(Henderson, 2006, p. 4). The EPA has carefully considered CASAC's views 
in making its decision, and believes the final decision is consistent 
with the critical part of CASAC's advice, i.e., to focus the indicator 
(and standard) on the type of thoracic coarse particles known to be 
harmful, which are found in urban and/or industrial environments.
    c. Unqualified PM10 Indicator, with Adjustment to the 
PM2.5 Component. EPA also solicited comment on an approach 
that would use PM10 as an indicator but subtract out the 
amount of PM2.5 in excess of the 24-hour daily standard for 
PM2.5 to avoid the double regulation of PM2.5 in 
the situations where this would have the most regulatory consequence 
(71 FR 2673). Specifically, this option would retain the indicator, 
form and level of the 1987 PM10 standard, but on days when 
the measured concentration of PM10 exceeds the level of the 
standard and the measured concentration of PM2.5 exceeds the 
level of the daily PM2.5 standard, the amount of 
PM2.5 in excess of the daily PM2.5 standard would 
be subtracted from the total PM10. A few commenters, 
including certain industry commenters and several local agencies and 
Tribes, expressed conditional support for pursuing this approach: 
though they preferred either no coarse particle standard (in the case 
of industry commenters) or an unqualified PM10-2.5 standard 
applied nationally (in the case of Tribes or local agencies), they 
suggested that an adjusted PM10 indicator would be an 
acceptable alternative. This alternative, like an unadjusted 
PM10 indicator, would allow variable ambient concentrations 
of coarse particles. The net result, however, would be that 
PM10-2.5 levels would be allowed to increase relative to the 
current PM10 standard when PM2.5 levels are 
highest. As explained above, this is the opposite result from that 
desired from a public health perspective. There should be less 
allowable coarse particulate matter as PM2.5 levels increase 
because these are the conditions under which PM10-2.5 tends 
to become more contaminated and therefore more harmful. Furthermore, it 
would essentially relax the level of protection afforded by the current 
24-hour PM10 standard because it would allow higher total 
PM10 levels on days with high PM2.5 levels. As 
explained below in section III.D.2, EPA believes it is important to 
maintain the current level of protection from health effects associated 
with exposure to thoracic coarse particles. For both of these reasons, 
therefore, EPA rejected this approach.
4. Conclusions Regarding Indicator for Thoracic Coarse Particles
    After extensive evaluation of the evidence, the alternatives 
available to the Agency, the advice and recommendations of CASAC, and 
all of the public comments, EPA concludes that retaining the 
PM10 indicator will be more effective in providing targeted 
public health protection than all other options available and, based on 
the current state of the science, is the most appropriate indicator to 
protect against the health effects associated with exposure to thoracic 
coarse particles. Thus, in the judgment of the Administrator, it is 
appropriate to retain PM10 as the indicator for coarse 
particles at this time. The conclusions that led to this decision can 
be summarized as follows:
    (1) All thoracic coarse particulate matter can deposit in the 
sensitive regions of the lung of most concern, the tracheobronchial and 
alveolar regions.
    (2) It remains appropriate to provide, to the extent possible, 
targeted protection from thoracic coarse particles that have been 
demonstrated to be associated with significant adverse health effects. 
Urban or industrial ambient mixes of coarse particulate matter 
dominated by high density vehicular, industrial, and construction 
emissions are of greatest concern, and should be the focus of 
protection.
    (3) The proposed qualified PM10-2.5 indicator was beset 
by numerous problems. Possible modifications to the qualifications 
considered by EPA failed to resolve these problems, which stem from the 
basic inability at this time to effectively and precisely identify 
which ambient mixes are included in the indicator and which are not.
    (4) The evidence of health effects associated with non-urban 
ambient mixes of coarse particles is limited and inconclusive: in 
general, the evidence does not demonstrate that community-level 
exposures in non-urban areas are associated with either the existence 
or absence of adverse health effects.
    (5) In light of the entire body of evidence concerning thoracic 
coarse particles, and given the potentially serious nature of the 
health risks posed by at least some thoracic coarse particles and the 
potential size of the population exposed, it is appropriate to provide 
some protection for all types of thoracic coarse particles, consistent 
with the requirement of the Act to allow an adequate margin of safety.
    With all of the foregoing considerations in mind, the Administrator 
judges it appropriate not to revise the current PM10 
indicator at this time. In the view of the Administrator, the 
PM10 indicator provides the type of targeted variation in 
allowable coarse particle concentrations that is justified by the 
emerging body of scientific evidence, while providing some protection 
in all areas. A decision not to revise the PM10 indicator 
reflects an appropriately cautious approach in two respects. First, it 
ensures inclusion of all ambient mixes of coarse particles of known 
concern in the indicator; and second, it addresses the potential that 
additional scientific research may reveal that non-urban or rural 
ambient mixes of thoracic coarse particles present public health risks 
that the evidence does not clearly identify at this time. It is EPA's 
goal that its new research and speciated monitoring program will 
produce data to determine what effect differences in particle 
composition may have on health outcomes. Such results have the 
potential to provide the kind of certainty and specificity required for 
making future decisions on indicators for thoracic coarse particles 
that might incorporate qualifications, such as the proposed qualified 
indicator related to coarse particles from agriculture and mining.

D. Conclusions Regarding Averaging Time, Form, and Level of the Current 
PM10 Standards

1. Averaging Time
    In the last review, EPA retained both 24-hour and annual 
PM10 standards to provide protection against the known and 
potential effects of short- and long-term exposures to thoracic coarse 
particles (62 FR 38677-79). That

[[Page 61198]]

decision was based in part on qualitative considerations related to the 
expectation that deposition of thoracic coarse particles in the 
respiratory system could aggravate effects in individuals with asthma. 
In addition, quantitative support for retaining a 24-hour standard came 
from limited epidemiologic evidence suggesting that aggravation of 
asthma and respiratory infection and symptoms may be associated with 
daily or episodic increases in PM10, where dominated by 
thoracic coarse particles including fugitive dust. The decision to 
retain an annual standard as well was generally based on considerations 
of the plausibility of the potential build-up of insoluble thoracic 
coarse particles in the lung after long-term exposures to high levels 
of such particles.
    New information available in this review, discussed above, includes 
several epidemiologic studies that report statistically significant 
associations between short-term (24-hour) exposure to 
PM10-2.5 and various morbidity effects and mortality. With 
regard to long-term exposure studies, while one study conducted in 
southern California reported a link between reduced lung function 
growth and long-term exposure to PM10-2.5 and 
PM2.5, other such studies reported no associations (EPA, 
2005, p. 3-19, 3-23-24). Thus, the Criteria Document concluded that the 
available evidence does not suggest an association with long-term 
exposure to PM10-2.5 (EPA, 2004a, p. 9-79).
    Based on these considerations, the Staff Paper concluded that the 
newly available evidence continues to support a 24-hour averaging time 
for a standard intended to control thoracic coarse particles, based 
primarily on evidence suggestive of associations between short-term 
(24-hour) exposure and morbidity effects and, to a lesser degree, 
mortality. Noting the absence of evidence judged to be suggestive of an 
association with long-term exposures, the Staff Paper concluded that 
there is no quantitative evidence that directly supports an annual 
standard, while recognizing that it could be appropriate to consider an 
annual standard to provide a margin of safety against possible effects 
related to long-term exposure to thoracic coarse particles that future 
research may reveal. The Staff Paper observed, however, that a 24-hour 
standard that would reduce 24-hour exposures would also likely reduce 
long-term average exposures, thus providing some margin of safety 
against the possibility of health effects associated with long-term 
exposures (EPA, 2005, p. 5-61). Based on its review of the Staff Paper, 
CASAC recommended retention of a 24-hour averaging time and agreed that 
an annual averaging time is not currently warranted for the coarse 
particle standard (Henderson, 2005b, p.5).
    The EPA received relatively few comments regarding the appropriate 
averaging time of the coarse particle standard. Most of those who did 
comment generally supported the retention of a 24-hour, but not annual, 
averaging time, as proposed. A few of the commenters who concurred with 
EPA's proposal to revoke the annual standard urged reconsideration of 
the appropriateness of an annual averaging time in the next PM NAAQS 
review. Several commenters, however, including a few States and several 
environmental and public health groups, urged EPA to retain an annual 
standard as well as a 24-hour standard. The American Lung Association, 
in particular, stated that EPA had inappropriately ignored evidence of 
long-term morbidity effects in several studies, including Gauderman et 
al. (2000, 2002) and Avol et al. (2001), and had also ignored 
substantial evidence from European studies as well as the 
recommendations for an annual PM10 standard made by a WHO 
working group. These commenters argued that an annual standard was 
requisite to protect public health with an adequate margin of safety.
    EPA disagrees that it ignored the evidence that is relevant to 
evaluating the health effects associated with long-term exposure to 
thoracic coarse particles. The EPA's assessment, both in this review 
and the previous review, placed greatest weight on studies that 
measured PM10-2.5 or on studies conducted in areas where it 
is reasonable to expect the PM10 measurements to be 
dominated by coarse particles (EPA, 2005). By contrast, these 
commenters have placed inappropriate reliance on studies that measured 
PM10, and were conducted in Southern California cities 
(Gauderman et al., 2000, 2002) or in European cities where it is not 
reasonable to assume that PM10 associations are dominated by 
coarse particles.\73\ In such cases, it is difficult to draw meaningful 
conclusions about the relative role of coarse as opposed to fine 
particles. The WHO panel recommendations for PM10 limits 
cited by commenters also do not provide any independent scientific 
justification regarding the need for a separate long-term standard for 
coarse particles.\74\
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    \73\ The only one of these studies (Gauderman et al., 2000) to 
include measurements of coarse particles found an association 
between lung function growth for PM10, PM2.5, 
PM10-2.5, NO2, and acids. The authors were 
unable to cite any single pollutant as responsible for these 
results, but they chose not to include measures for coarse particles 
in their follow-up study (Gauderman et al., 2002). As noted in the 
1996 PM Staff Paper, the other major study of lung function and 
long-term air pollution in children found no associations with 
coarse particles (EPA, 1996, p. 5-67a).
    \74\ The WHO panel essentially developed their recommendations 
for PM10 standards by deriving a ratio of fine particles 
to PM10 and adjusting their recommended levels for 
PM2.5 to derive an equivalent PM10 metric, for 
areas that do not yet have access to PM2.5 monitors (WHO, 
2005, p. 8).
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    The long-term exposure studies of mortality and morbidity that 
permit comparisons of fine and coarse particles continue to suggest 
that, at current ambient levels in the US, fine particles are 
associated with health effects and coarse particles are not.\75\ The 
EPA believes that the PM2.5 standards it is establishing in 
today's notice address the major risk suggested in the PM10 
studies cited by commenters. To the extent that additional concerns may 
exist with regard to long-term exposures to coarse particles that have 
not been fully identified by scientific research, the Staff Paper notes 
that the short-term standard for coarse particles, which is generally 
controlling, has and will continue, as a practical matter, to limit 
such long-term exposures.\76\
---------------------------------------------------------------------------

    \75\ See EPA 2004a, pp. 8-306 to 307 (``no statistically 
significant associations have been reported between long-term 
exposure to coarse fraction particles and cause-specific 
mortality''); pp. 8-313 to 314 (``[t]he recent studies suggest that 
long-term exposure to fine particles is associated with development 
of chronic respiratory disease and reduced lung function growth; 
little evidence is available on potential effects of exposure to 
coarse fraction particles'').
    \76\ The Staff Paper analysis of PM10 air quality 
data indicates that the current 24-hour PM10 standard is 
``controlling'' in virtually every area in the US; that is, 
virtually all areas that violate the PM10 standards 
violate the 24-hour PM10 standard. Some of them may 
violate the annual PM10 standard as well, but (depending 
on the year) few, if any, areas violate the annual PM 
without violating the 24-hour PM10 standard (EPA, 2005, 
p. 2-31 to 32). A supplemental analysis in the Response to Comments 
document shows that for 2003-2005, all of the areas that would 
violate the annual PM10 standard also violate the 24-hour 
standard.
---------------------------------------------------------------------------

    After reviewing the available evidence, the Administrator concurs 
with staff and CASAC recommendations and concludes that the evidence 
continues to support a 24-hour averaging time for a coarse particle 
standard, based primarily on evidence suggestive of associations 
between short-term (24-hour) exposure and morbidity effects and, to a 
lesser degree, mortality. As noted above, a 24-hour standard would in 
effect also provide protection against any as yet unidentified 
potential effects of long-term exposure at ambient levels. Further, the 
Administrator concludes

[[Page 61199]]

that an annual coarse particle standard is not warranted at this time. 
Thus, the Administrator is retaining the 24-hour PM10 
standard and revoking the annual PM10 standard.
2. Level and Form of the 24-Hour PM10 Standard
    This section summarizes the major considerations that led to the 
proposed decision regarding the appropriate level and form for the 24-
hour standard for thoracic coarse particles, summarizes and addresses 
public comments on the appropriate level of protection to be provided 
by the standard, and presents the Administrator's final conclusions 
regarding the level and form of the 24-hour standard. The proposed 
level and form for the 24-hour standard for thoracic coarse particles 
were based primarily on an assessment of studies that measured 
PM10-2.5, as well as studies that measured PM10 
in areas that were dominated by PM10-2.5. Now that the 
Administrator has concluded that it is appropriate to retain 
PM10 as the indicator for thoracic coarse particles, rather 
than adopting a PM10-2.5 indicator as proposed, the 
Administrator relied on this same body of studies as the principal 
basis for determining an appropriate level and form for a standard 
based on the PM10 indicator. Therefore, in this section EPA 
reviews the basis for its conclusions in the proposal, and then 
discusses how this evidence informs the choice of level and form for 
the 24-hour PM10 standard.
    In considering the available evidence as a basis for setting a 24-
hour standard for thoracic coarse particles, the Staff Paper focused on 
relevant U.S. and Canadian epidemiologic studies showing associations 
between short-term PM10-2.5 concentrations and morbidity and 
mortality effects, as discussed above in section III.A. As an initial 
matter, the Staff Paper recognized that these individual short-term 
exposure studies provide no evidence of clear population thresholds, or 
lowest-observed-effects levels, in terms of 24-hour average 
concentrations. As a consequence, this body of evidence is difficult to 
translate directly into a specific 24-hour standard that would protect 
against the range of effects that have been associated with short-term 
exposures to coarse particles.
    In considering the evidence, the Staff Paper noted the significant 
uncertainties and the limited nature of the available evidence. In 
examining the available evidence to identify a basis for a range of 
standard levels that would be appropriate for consideration, the Staff 
Paper focused on the upper end of the distributions of daily 
PM10-2.5 concentrations in the relevant studies in terms of 
the 98th and 99th percentile values.\77\
---------------------------------------------------------------------------

    \77\ This examination of the evidence is based on air quality 
information and analyses presented in two staff memos which were 
part of the materials reviewed by CASAC (Ross and Langstaff, 2005; 
Ross, 2005).
---------------------------------------------------------------------------

    In looking first at the morbidity studies that report statistically 
significant associations with respiratory- and cardiac-related hospital 
admissions in Toronto (Burnett et al., 1997), Seattle (Sheppard, 2003), 
and Detroit (Ito, 2003), the 98th percentile PM10-2.5 values 
reported in these studies range from approximately 30 to 36 [mu]g/
m3. To provide some perspective on these PM10-2.5 
levels, the Staff Paper noted that the level of the 24-hour 
PM10 standard was exceeded on only a few occasions during 
the time periods of the studies in Detroit and Seattle.\78\ In the 
mortality studies that report statistically significant and generally 
robust associations with short-term exposures to PM10-2.5 in 
Phoenix (Mar et al., 2003) and Coachella Valley, CA (Ostro et al., 
2003), the reported 98th percentile values were approximately 70 and 
107 [mu]g/m3, respectively. These studies were conducted in 
areas with air quality levels that did not meet the current 
PM10 standards. In addition, as part of the Six Cities 
study, Schwartz et al. (1996 and reanalysis 2003a) reported a 
statistically significant association between PM10-2.5 and 
mortality in Steubenville, where the PM10-2.5 concentrations 
were fairly high, with a reported 98th percentile value of 53 [mu]g/
m3, although in a second reanalysis, the association did not 
remain statistically significant (Klemm and Mason, 2003). On the other 
hand, the Staff Paper noted that no statistically significant mortality 
associations were reported in a number of other studies, including 
those in the five other cities that were part of the Six Cities study 
(Boston, St. Louis, Knoxville, Topeka, and Portage), and in Santa Clara 
County, CA, Detroit, Philadelphia, and Pittsburgh. With the exception 
of Pittsburgh, these cities had much lower 98th percentile 
PM10-2.5 values, ranging from 18 to 49 [mu]g/m3. 
Thus, in mortality studies that reported statistically significant 
associations, the reported 98th percentile PM10-2.5 values 
were all above 50 [mu]g/m3, and all in areas that exceeded 
the level of the daily PM10 standard, whereas in the 
mortality studies that reported no statistically significant 
associations, the reported 98th percentile PM10-2.5 values 
were generally below 50 [mu]g/m3.
---------------------------------------------------------------------------

    \78\ As shown in air quality data trends reports: for Seattle, 
1997 Air Quality Annual Report for Washington State, p. 17, at 
http://www.ecy.wa.gov/pubs/97208.pdf; for Detroit, Michigan's 2003 
Annual Air Quality Report, p. 46, at http://www.deq.state.mi.us/
documents/deq-aqd-air-reports-03AQReport.pdf.
---------------------------------------------------------------------------

    In examining the air quality data used in the key morbidity and 
mortality studies considered in the Staff Paper, EPA recognized that 
the uncertainty related to exposure measurement error associated with 
using ambient concentrations to represent area-wide population exposure 
levels can be potentially quite large. For example, in looking 
specifically at the Detroit study, the Staff Paper noted that the 
PM10-2.5 air quality values were based on air quality 
monitors located in Windsor, Canada. While the study authors concluded 
that these monitors were appropriate for use in exploring the 
association between air quality and hospital admissions in Detroit, a 
close examination of air quality levels at Detroit and Windsor sites in 
recent years led to the conclusion that the statistically significant, 
generally robust association with hospital admissions in Detroit likely 
reflects population exposures that may be appreciably higher in the 
central city area, but not necessarily across the broader study area, 
than would be estimated using data from the Windsor monitors (EPA, 
2005, p. 5-64).
    The Staff Paper also looked more specifically at the Coachella 
Valley mortality study (Ostro et al., 2003), in which data were used 
from a single monitoring site in one city, Indio, within the study area 
where daily measurements were available. A close examination of air 
quality levels across the Coachella Valley suggested that while the 
association of mortality with PM10-2.5 measurements made at 
the Indio site was statistically significant, a portion of the study 
population would have been expected to experience appreciably lower 
ambient exposure levels. In contrast to the Detroit study, air quality 
data used in the mortality study conducted in Coachella Valley appeared 
to represent concentrations on the high end of PM10-2.5 
levels for Coachella Valley communities. On the other hand, a close 
examination of the air quality data used in the other studies discussed 
above generally showed less disparity between air quality levels at the 
monitoring sites used in the studies and the broader pattern of air 
quality levels across the study areas than that described above in the 
Detroit and Coachella Valley studies.
    The Staff Paper noted that this close examination of air quality 
information generally reinforced the view that exposure measurement 
error is potentially quite large in studies focusing on thoracic coarse 
particles. As

[[Page 61200]]

a consequence, the air quality levels reported in these studies as 
measured by ambient concentrations at monitoring sites within the study 
areas are not necessarily good surrogates for population exposures that 
are likely associated with the observed effects in the study areas or 
that would likely be associated with effects in other urban areas 
across the country. The Detroit example suggests that population 
exposures were probably appreciably underestimated in the Detroit 
morbidity study, such that the observed effects are likely associated 
with higher PM10-2.5 levels than reported. In contrast, the 
Coachella Valley mortality study provides an example in which 
PM10-2.5 levels to which the study populations were exposed 
were probably appreciably overestimated, such that the observed effects 
may well be associated with lower PM10-2.5 levels than 
reported. At relatively low levels of air quality, population exposures 
implied by these studies as being associated with the observed effects 
become more uncertain, suggesting a high degree of caution in 
interpreting the air quality levels from the group of morbidity studies 
as a basis for identifying a standard level that would protect against 
the observed effects. See generally EPA, 2005, pp. 5-65-66.
    Taking into account this close examination of the air quality data 
associated with health effects in these studies, the Staff Paper 
concluded that this evidence suggests that EPA could consider a 
standard for urban thoracic coarse particles at a PM10-2.5 
level at least down to 50 [mu]g/m3, in conjunction with a 
98th percentile form. This view takes into account the conclusion that 
this evidence is particularly uncertain as to population exposures, 
especially from the morbidity studies reporting effects at relatively 
low concentrations, as well as the general lack of evidence of 
associations from the group of mortality studies with reported 
concentrations below these levels. Id. at p. 5-66.
    The Staff Paper also outlined another view that reflected a more 
cautious or restrained approach to interpreting the limited body of 
PM10-2.5 epidemiologic evidence. This approach would judge 
that the uncertainties as to population exposures associated with the 
observed effects in this whole group of studies were too large to 
permit direct use of the reported effects levels as a basis for setting 
a specific standard level. Such a judgment would be consistent with 
concluding that these studies, together with other dosimetric and 
toxicologic evidence, provide support for retaining standards for 
thoracic coarse particles at some level to protect against the 
morbidity and mortality effects observed in the studies, regardless of 
whether an associated population exposure level can be clearly 
discerned from the studies.
    Based on this more cautious approach, the Staff Paper concluded 
that it would be reasonable to interpret the available epidemiologic 
evidence more qualitatively. Considering the available evidence in this 
way led to the following observations:
    (1) The statistically significant mortality associations with 
short-term exposure to PM10-2.5 reported in the Phoenix and 
Coachella Valley studies were observed in areas that did not meet the 
current PM10 standards.
    (2) The statistically significant morbidity associations with 
short-term exposure to PM10-2.5 reported in the Detroit and 
Seattle studies were observed in areas that exceeded the level of the 
current 24-hour PM10 standard on just a few occasions during 
the time periods of the studies.
    (3) All but one of the statistically significant morbidity and 
mortality associations with short-term exposure to PM10 that 
were reported in areas in which PM10 was dominated by the 
coarse particle fraction (including Reno/Sparks, NV, Tucson, AZ, 
Anchorage, AK, and the Utah Valley area) were observed in areas that 
did not meet the current PM10 standards. Id. at p. 5-67.
    Based on these considerations, the Staff Paper found little basis 
for concluding that the degree of protection afforded by the current 
PM10 standards in urban areas is greater than warranted, 
since potential mortality effects have been associated with air quality 
levels not allowed by the current 24-hour standard, but have not been 
associated with air quality levels that would generally meet that 
standard, and morbidity effects have been associated with air quality 
levels that exceeded the current 24-hour standard only a few times. 
Further, the Staff Paper found little basis for concluding that a 
greater degree of protection is warranted in light of the very high 
degree of uncertainty in the relevant population exposures implied by 
the morbidity studies. The Staff Paper concluded, therefore, that it is 
reasonable to interpret the available evidence as supporting 
consideration of a short-term standard for urban thoracic coarse 
particles, so as to provide generally ``equivalent'' protection to that 
afforded by the current 24-hour PM10 standard, recognizing 
that no one PM10-2.5 level will be strictly equivalent to a 
specific PM10 level in all areas (EPA, 2005, p. 5-67). Such 
a standard would likely provide protection against morbidity effects 
especially in those urban areas where, unlike several of the study 
areas, PM10 is generally dominated by coarse-fraction rather 
than fine-fraction particles. Such a standard would also likely provide 
protection against the more serious, but less certain, coarse-particle-
related mortality effects observed in some studies, generally at 
somewhat higher concentrations.
    The Staff Paper went on to consider what level for a 24-hour 
PM10-2.5 standard for urban coarse particles would provide 
an equivalent level of protection to that afforded by the current 24-
hour PM10 standard. This consideration of a 
PM10-2.5 standard providing generally ``equivalent'' 
protection reflected a judgment that while the epidemiologic evidence 
supported establishing a short-term standard for urban thoracic coarse 
particles at such a generally ``equivalent'' level, the evidence 
concerning air quality levels of thoracic coarse particles in the 
studies was not strong enough to provide a basis for changing the level 
of protection generally afforded by the current PM10 
standards (EPA, 2005, pp. 5-68-69). The Staff Paper examined various 
approaches to providing this equivalent level of protection, including 
establishing a level of 70 [mu]g/m\3\ (98th percentile form) for the 
qualified PM10-2.5 standard (Id. at 5-67-68), which is what 
EPA proposed (71 FR 2671).
    CASAC generally supported the Agency's proposed range of 50-70 
[mu]g/m\3\ (98th percentile) for the 24-hour PM10-2.5 
standard. As noted, the upper end of this range was based on EPA's 
assessment of a level for an urban coarse particle standard that would 
provide a generally equivalent level of protection to that afforded by 
the current PM10 standards. The lower end of the range was 
developed in consideration of an approach that would place greater 
weight on the effects levels reported in several studies with lower 
ambient coarse particle concentrations. The CASAC Panel noted that 
``there was general agreement among Panel members that Agency staff had 
presented a reasonable justification for the ranges of levels 
proposed'' (Henderson 2005b, p. 6).
    Relatively few public commenters addressed the issue of whether 
``general equivalence'' was an appropriate goal for the level and form 
of the proposed coarse particle standard. Some commenters, particularly 
those industry commenters advocating that no coarse

[[Page 61201]]

particle standard be adopted,\79\ stated that seeking ``equivalence'' 
to the PM10 standard was fundamentally flawed because, in 
their view: (1) The level of the current PM10 standard was 
not based on coarse particle studies; (2) the proposed standard is not 
equivalent to the PM10 standard; and (3) the court had 
already declared any standard based directly or indirectly on 
PM10 to be invalid. The EPA agrees that the 1987 
PM10 standards were designed to protect against the health 
effects of both fine and coarse particles, and based in part on 
epidemiological studies that variously measured particles both smaller 
and larger than PM10. However, the arguments regarding the 
origin of the 1987 standards as well as commenters' claims about the 
basis for the PM10 standards promulgated in 1997 \80\ are 
not relevant to the current review. In determining whether to revise 
the standards in this review, EPA has examined the degree of protection 
provided by the current 24-hour PM10 standard in light of 
the quantitative evidence from the expanded epidemiological data base 
that includes studies using direct PM10-2.5 measurements as 
well as studies using PM10 measurements in areas where 
coarse particles dominate the distribution.
---------------------------------------------------------------------------

    \79\ As discussed in section III.B.2, these commenters call 
EPA's interpretation of the key studies discussed in this section 
into question. EPA's response to the criticisms of use of these 
studies for standard setting is summarized in section III.B.2 and 
presented in more detail in the Response to Comments document.
    \80\ Commenters also suggested that, in promulgating revised 
PM10 standards in 1997, EPA did not consider whether the 
level of the PM10 standards it promulgated was lower than 
necessary and did not base the levels on coarse particle health 
effects data. While EPA disagrees with both of these claims--for 
example, EPA relied on two PM10 studies done in areas 
dominated by coarse particles in selecting the level (62 FR 38679)--
this argument is not relevant to this review.
---------------------------------------------------------------------------

    Because as discussed in section III.C.3 above, the Administrator 
has decided that it is appropriate to retain PM10 as the 
indicator for thoracic coarse particles, there can be no uncertainty as 
to whether the final standard is equivalent to the current standard, 
making the commenters' second point above moot. With regard to their 
third point, for reasons outlined in section III.C.3, EPA believes that 
it has addressed the concerns raised by the court regarding 
PM10 as an indicator, and in any case, the D.C. Circuit did 
not address the issue of the level of protection from thoracic coarse 
particles afforded by the 1997 or 1987 24-hour PM10 
standard.
    Other commenters, particularly environmental and public health 
groups, disagreed with EPA's proposal to seek an ``equivalent level of 
protection'' because they believe the scientific evidence mandates a 
lower level to protect against adverse health effects. These commenters 
cited studies reviewed in the Staff Paper and noted above, which they 
claimed showed significant associations between health effects and 
PM10-2.5 concentrations at levels between 30-40 [mu]g/m\3\, 
and recent decisions by the European Union and the State of California 
to adopt 24-hour PM10 standards of 50 [mu]g/m\3\.
    These commenters argued that, even considering EPA's analyses of 
the uncertainties in the relevant ambient concentration measurements, 
these studies, particularly those in Atlanta, Seattle, and Toronto and 
the six-cities study of respiratory symptoms in children (Schwartz and 
Neas, 2000), demonstrate the need for a more stringent level of 
protection than that provided by the current standards. These 
commenters also argued that EPA's approach to determining an equivalent 
level resulted in less protection than the current standard, even in 
urban areas. In addition, these commenters pointed to the study review 
conducted by Brunekreef and Forsberg (2005) and numerous ``new'' 
studies published too recently for inclusion in the Criteria Document 
such as Mar et al. (2004), Chen Y et al. (2005), and Lin et al. (2005), 
as supportive of lower levels.
    As noted above, EPA has conducted a careful assessment of the 
studies cited by commenters \81\ from the Staff Paper assessment but 
reaches substantially different conclusions about their implications 
for the level of a 24-hour standard for thoracic coarse particles. 
Based on that assessment, EPA staff recommended consideration of a 
range of levels for a 24-hour PM10-2.5 standard extending 
from a level equivalent to the current PM10 standard down to 
a level of 50 [mu]g/m\3\, which is clearly above that suggested by 
these commenters. CASAC found general agreement that the ``staff had 
presented a reasonable justification'' for this range oflevels. While 
EPA strongly agrees that the available scientific evidence supports and 
requires maintaining the level of protection provided by the current 
24-hour PM10 standard, the limited extent of epidemiological 
evidence as well as the unusually large uncertainties in measuring 
exposures to thoracic coarse particles, particularly at lower levels, 
argue for the more cautious interpretation advocated by EPA staff and 
CASAC. Because the Administrator has decided to continue the use of 
PM10 as the indicator for coarse particles, commenters' 
remaining concerns about whether the proposed levels for 
PM10-2.5 are as protective as current standards are no 
longer relevant.
---------------------------------------------------------------------------

    \81\ As detailed in the Response to Comment document, EPA had 
various reasons for not placing primary reliance on the reported air 
quality results in these studies for selecting a standard level. The 
Atlanta study (Tolbert et al, 2000), found a significant effect for 
PM10, but not for coarse particles. Both the Six Cities 
children's diary study (Schwartz and Neas, 2000) and the Toronto 
hospital admissions study (Burnett et al.,, 1997) were conducted for 
a periods of less than one year, making it difficult to determine 
what peak value across all seasons in a year might represent 
exposures of concern.
---------------------------------------------------------------------------

    For reasons summarized in section II.F above, EPA does not believe 
that standards adopted by the State of California or, by extension, the 
European Union, which operates under a different legal and policy 
structure, provide a relevant guide for establishing U.S. National 
Ambient Air Quality Standards. While EPA agrees that the assessment of 
Brunekreef and Forsberg (2005) supports separate regulation of fine and 
coarse particles, these authors make no recommendations with respect to 
appropriate levels of protection. To the extent that commenters cited 
``new'' studies in support of their argument for a more stringent 
standard to protect against health effects associated with exposure to 
coarse particles, EPA notes that as in past NAAQS reviews, it is basing 
the final decisions in this review on the studies and related 
information included in the PM air quality criteria that have undergone 
CASAC and public review, and will consider the newly published studies 
for purposes of decision making in the next PM NAAQS review, as 
discussed above in section I.C. As evidenced by the uncertainties found 
in the detailed assessment of key coarse particle studies in the Staff 
Paper, the kind of assessment and analysis provided by the formal 
criteria and standards review process is particularly crucial for 
coarse particle studies that may be relevant to selecting the level of 
the standard.
    After considering the public comments on this issue, EPA continues 
to believe that the available evidence leads to the conclusion that the 
degree of protection afforded by the current 24-hour PM10 
standard is requisite to protect public health with an adequate margin 
of safety. Having chosen to retain the current indicator for the 
standard (PM10), and to retain the same degree of 
protection, it is still necessary to determine the appropriate form and 
level for the standard. In the context of proposing a standard based on 
a qualified PM10-2.5 indicator, EPA proposed to change the 
form of the 24-hour standard from a one-expected

[[Page 61202]]

exceedance form to a 98th percentile form. The 98th percentile form was 
intended to be consistent with the goal of providing protection 
equivalent to that afforded by the current 24-hour PM10 
standard (71 FR at 2671; EPA, 2005, p. 5-68). The few commenters 
addressing the proposed form supported it, largely because the 98th 
percentile would provide a more stable statistical basis for making 
nonattainment determinations. However, some commenters objected to the 
98th percentile form because they felt it was inappropriate to allow as 
many as 21 days over the level of the standard over the course of a 
three-year period. These commenters argued for a more restrictive form 
(generally 99th percentile) to ensure the protection of public health 
with an adequate margin of safety. The EPA notes that the current one-
expected-exceedance form of the 24-hour PM10 standard allows 
only three days above the standard over a three-year period.
    While EPA generally favors the concentration-based form for short-
term standards for reasons noted above, EPA also notes that adopting 
such a form in this review without changing the level would result in a 
standard that would not provide the same protection as the current 
standard, and the level of the standard would have to be adjusted 
downward to achieve the desired protection. Given the overall decision 
to provide the same protection as the current standards, the 
Administrator concludes it is best to retain both the form and the 
level of the current primary 24-hour PM10 standard.
    In conclusion, it is EPA's view, as expressed in the Staff Paper 
and proposal and supported by CASAC and by the available health effects 
evidence, that the level of protection afforded by the current 24-hour 
PM10 standard of 150 [mu]g/m\3\, one-expected-exceedance 
form, continues to be appropriate for the types of thoracic coarse 
particles typically found in urban or industrial areas. As explained 
above, mortality effects observed in epidemiologic studies for coarse 
particles are generally associated with exposure levels that exceed the 
current standards, and morbidity effects are generally associated with 
exposure levels that exceeded the current standards on only a few 
occasions. This suggests the level of protection afforded by the 
current PM10 standards is not greater than warranted. 
Furthermore, the very high degree of uncertainty in the relevant 
population exposures implied by the morbidity studies suggests there is 
little basis for concluding at this time that a greater degree of 
protection is warranted.
    Moreover, as explained above in section III.C.3.b, the 
PM10 indicator provides appropriate variation in allowable 
coarse particle concentrations in different areas based on the relative 
proportions of PM2.5 and PM10-2.5 in the ambient 
mix. In urban areas where PM2.5 concentrations tend to be 
higher, the current 24-hour PM10 standard level of 150 
[mu]g/m\3\ will result in lower allowable levels of 
PM10-2.5. In non-urban areas, the higher allowable levels of 
coarse particles provided by the current 24-hour PM10 
standard will also provide appropriate protection of public health, 
given the body of evidence discussed above. The EPA therefore believes 
that the level of protection from coarse particles provided by the 
current 24-hour PM10 standard remains requisite to protect 
public health with an adequate margin of safety. Revising either the 
level or the form of this standard would alter the current level of 
protection and therefore would not be appropriate based on the 
scientific evidence available at this time.
    Therefore, after considering the available scientific evidence, the 
rationale and recommendations contained in the Staff Paper, the advice 
and recommendations of CASAC, and the public comments received 
regarding the appropriate level and form for a 24-hour standard 
intended to afford requisite protection of public health from effects 
associated with exposure to coarse particles, the Administrator has 
determined to retain the current level of 150 [mu]g/m\3\ for the 24-
hour PM10 standard, and the current one-expected-exceedance 
form. In the Administrator's judgment, based on the currently available 
evidence, a standard set at this level remains requisite to protect 
public health with an adequate margin of safety from the morbidity and 
possibly mortality effects that have been associated with short-term 
exposures to thoracic coarse particles in urban or industrial areas, as 
well as to protect against the potential for risks from exposure to 
thoracic coarse particles in other areas. The EPA intends to address 
the considerable uncertainties in the currently available information 
on thoracic coarse particles as part of the Agency's ongoing PM 
research program.

E. Final Decisions on Primary PM10 Standards

    For the reasons discussed above in this section, and taking into 
account the information and assessments presented in the Criteria 
Document and Staff Paper, the advice and recommendations of CASAC, and 
public comments received on the proposal, the Administrator is 
retaining the current primary 24-hour PM10 standard at the 
level of 150 [mu]g/m\3\, which is met when this level is not exceeded 
more than once per year on average over a three-year period measured at 
each monitor within an area. The Administrator also is revoking and not 
replacing the annual PM10 standard.
    As discussed in more detail in section VI, EPA is promulgating a 
new reference method (FRM) for measurement of mass concentrations of 
PM10-2.5 in the atmosphere. Although NAAQS for 
PM10-2.5 have not been established by EPA, this new FRM will 
nevertheless be defined as the standard of reference for measurements 
of PM10-2.5 concentrations in ambient air. This should 
provide a basis for approving Federal Equivalent Methods (FEMs) and 
promote the gathering of scientific data to support future reviews of 
the PM NAAQS. One of the reasons for not finalizing a 
PM10-2.5 standard was the limited body of evidence on health 
effects associated with thoracic coarse particles from studies that use 
PM10-2.5 measurements of ambient thoracic coarse particle 
concentrations. If an FRM is available, researchers will likely include 
PM10-2.5 measurements of thoracic coarse particles in health 
studies either by directly using the FRM or by utilizing approved 
equivalent methods based on the FRM.
    In addition, EPA published elsewhere in today's Federal Register a 
requirement for a new multi-pollutant monitoring network that takes an 
integrated approach to air quality measurements. One of the required 
measurements at these multi-pollutant monitoring stations is 
PM10-2.5. The availability of an FRM, and subsequently 
approved equivalent methods for PM10-2.5, will support State 
and local agencies' efforts to deploy robust methods at these 
monitoring stations for the measurement of thoracic coarse particles 
that do not include fine particles. These multi-pollutant monitoring 
stations will provide a readily available dataset at approximately 75 
urban and rural locations for atmospheric and health researchers to 
compare particle and gaseous air pollutants.
    Finally, the PM10-2.5 FRM, by definition, provides a 
reference measurement. Because it is a filter based system, this method 
can itself be used to provide speciated data and EPA will be issuing 
guidance to ensure the use of a consistent national approach for 
speciated coarse particle monitors as soon as possible. The reference 
measurement from this instrument is

[[Page 61203]]

also important in the development of alternative PM10-2.5 
speciation samplers. We will be developing dichotomous samplers to meet 
the requirements of SAFETEA-LU. Appropriate guidance to ensure that the 
use of a consistent national approach for speciated coarse particle 
monitors will be issued with this method. As discussed in more detail 
in the final monitoring rule published elsewhere in today's Federal 
Register, EPA is requiring the deployment of PM10-2.5 
speciation samplers at all 75 multi-pollutant monitoring stations. Such 
speciation monitoring will help States in developing SIPs and will 
address a key research need for thoracic coarse particles by providing 
a better understanding of the chemistry of the collected samples.

IV. Rationale for Final Decisions on Secondary PM Standards

    This section presents the Administrator's final decisions regarding 
the review of the current secondary NAAQS for PM. The existing suite of 
secondary PM standards, which is identical to the suite of primary PM 
standards, includes annual and 24-hour PM2.5 standards and 
annual and 24-hour PM10 standards. The existing suite of 
secondary standards is intended to address visibility impairment 
associated with fine particles,\82\ and materials damage and soiling 
related to both fine and coarse particles. The following discussion of 
the rationale for the final decisions on revising the secondary PM 
standards focuses on those considerations most influential in the 
Administrator's decisions, first addressing visibility impairment as it 
relates to the PM2.5 secondary standards and then addressing 
the other welfare effects as they relate to both the PM2.5 
and PM10 secondary standards. The other welfare effects 
considered in this review include effects on vegetation and ecosystems, 
materials damage and soiling, and climate change.\83\
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    \82\ The Administrator recognized in establishing the levels of 
the secondary standards for PM2.5 that these standards 
would work ``in conjunction with implementation of a regional haze 
program'' under Section 169A to provide appropriate national 
protection against visibility impairment in both urban and non-urban 
areas (62 FR 38683).
    \83\ As noted in section I.A above, in establishing secondary 
standards that are requisite to protect the public welfare from any 
known or anticipated adverse effects, EPA may not consider the costs 
of implementing the standards.
---------------------------------------------------------------------------

    Sections IV.A and IV.B of the proposal (71 FR 2675-2685) provide a 
detailed summary of key information contained in the Criteria Document 
(EPA, 2004a, Chapters 4 and 9) and in the Staff Paper (EPA, 2005, 
Chapters 6 and 7) on the known and potential welfare effects associated 
with PM, including PM-related visibility impairment and PM-related 
effects on vegetation and ecosystems, materials damage and soiling, and 
climate change, respectively. This information is only briefly outlined 
in subsections IV.A.1 and IV.B.1 below. Subsequent sections provide a 
more complete discussion of the Administrator's rationale, having 
considered the evidence in light of public comments and his final 
decisions on the primary standards for PM, for his decision to revise 
the current PM secondary standards by making them identical in all 
respects to the revised suite of primary PM standards.

A. Visibility Impairment

    This section presents the rationale for the Administrator's 
decision to revise the current secondary PM2.5 standards to 
address PM-related visibility impairment by setting secondary standards 
identical in all respects to the revised PM2.5 primary 
standards. As discussed below, the rationale includes consideration of: 
(1) The latest scientific information on visibility effects associated 
with PM; (2) insights gained from assessments of correlations between 
ambient PM2.5 and visibility impairment prepared by EPA 
staff; and (3) specific conclusions regarding the need for revisions to 
the current standards (i.e., indicator, averaging time, form, and 
level) that, taken together, would be requisite to protect the public 
welfare from adverse effects of PM2.5 on visual air quality.
1. Visibility Impairment Related to Ambient PM
    Section IV.A.1 of the proposal (71 FR 2675-2678) outlined key 
information contained in the Criteria Document and Staff Paper relevant 
to considering visibility impairment that is related to ambient PM. The 
information highlighted there summarizes:
    (1) The nature of visibility impairment, including trends in visual 
air quality and the characterization of current visibility conditions, 
with a particular focus on visibility impairment in urban areas.
    (2) Direct, quantitative relationships that exist between ambient 
PM constituents and light extinction, and thus visibility impairment, 
based in part on analyses of the extensive new data now available on 
PM2.5 concentrations, primarily in urban areas, that 
explored factors that have historically complicated efforts to address 
visibility impairment nationally, including regional differences 
related to levels of primarily fine particles and to relative humidity.
    (3) The impacts of urban visibility impairment on public welfare, 
based in part on valuation studies of benefits associated with 
improvements in visibility and in part on recognition of a number of 
programs, standards, and planning efforts to address visibility 
impairment, in the U.S. and abroad, that illustrate the value that the 
public places on improved visibility.
    (4) Approaches to evaluating public perceptions and attitudes about 
visibility impairment, including new methods and tools that have been 
developed to communicate and evaluate public perceptions of varying 
visual effects associated with alternative levels of visibility 
impairment relative to varying pollution levels and environmental 
conditions.
    The summary of the evidence on visibility impairment related to 
ambient fine particles in the proposal will not be repeated here. The 
EPA emphasizes that the final decisions on the secondary standards take 
into account the more comprehensive and detailed discussions of the 
scientific information on visibility impairment contained in the 
Criteria Document and Staff Paper.
2. Need for Revision of the Current Secondary PM2.5 
Standards To Protect Visibility
    In 1997, EPA decided to address the effects of PM on visibility by 
setting secondary standards identical to the suite of PM2.5 
primary standards, in conjunction with the future establishment of a 
regional haze program under sections 169A and 169B of the Act (62 FR 
38679-83). In reaching this decision, EPA first concluded that PM, 
especially fine particles, impairs visibility in various locations 
across the country, including multi-state regions, urban areas, and 
remote Class I Federal areas (e.g., national parks and wilderness 
areas). The EPA also concluded that addressing visibility impairment 
solely through setting more stringent national secondary standards 
would not be an appropriate means to protect the public welfare from 
adverse impacts of PM on visibility in all parts of the country. As a 
consequence, EPA determined that an approach that combined national 
secondary standards with a regional haze program was the most 
appropriate and effective way to address visibility impairment (EPA 
2005, p. 7-2).
    As anticipated in the last review, EPA promulgated a regional haze 
program in 1999 (65 FR 35713). That program requires States to 
establish goals for improving visibility in Class I areas and

[[Page 61204]]

to adopt control strategies to achieve these goals. Since strategies to 
meet these goals are to reflect a coordinated approach among States, 
multi-state regional planning organizations have been formed and are 
now developing strategies, to be adopted over the next few years, that 
will make reasonable progress in meeting these goals.
    The initial issue to be addressed in the current review of the 
secondary PM standards is whether, in view of the information now 
available, the existing secondary standards should be revised to 
provide requisite protection from PM-related adverse effects on visual 
air quality. As discussed in the Criteria Document and Staff Paper, 
while new research has led to improved understanding of the optical 
properties of particles and the effects of relative humidity on those 
properties, it has not changed the fundamental characterization from 
the last review of the role of PM, and especially fine particles, in 
visibility impairment. However, extensive new information from 
visibility and fine particle monitoring networks since the last review 
has allowed for updated characterizations of visibility trends and 
current levels in urban areas, as well as Class I areas. As discussed 
in section IV.A.1.b. of the proposal (71 FR 2676-2677), these new data 
were a critical component of analyses that better characterized 
visibility impairment in urban areas and the relationship between 
visibility and PM2.5 concentrations, and led to the finding 
that PM2.5 concentrations can be used as a general surrogate 
for visibility impairment in urban areas.
    Taking into account the most recent monitoring information and 
analyses, and recognizing that efforts are now underway to address all 
human-caused visibility impairment in Class I areas through the 
regional haze program implemented under sections 169A and 169B of the 
CAA, as discussed above, this review focused on visibility impairment 
primarily in urban areas. In so doing, given the stronger link between 
visibility impairment and short-term PM2.5 concentrations, 
EPA gave significant consideration to the question of whether 
visibility impairment in urban areas allowed by the current 24-hour 
secondary PM2.5 standard can be considered adverse to public 
welfare.
    As discussed in section IV.A.1.c. of the proposal (71 FR 2677-
2678), studies in the U.S. and abroad have provided the basis for the 
establishment of standards and programs to address specific visibility 
concerns in a number of local areas. These studies (e.g., in Denver, 
Phoenix, British Columbia) have produced reasonably consistent results 
in terms of the visual ranges found to be generally acceptable by the 
participants in the various studies, which spanned from approximately 
40 to 60 km in visual range. Standards targeting protection within this 
range have also been set by the State of Vermont and by California for 
the Lake Tahoe area, in contrast to the statewide California standard 
that targets a visual range of approximately 16 km.
    In addition to the information available from such programs, 
photographic representations (simulated images and actual photographs) 
of visibility impairment are available, as discussed in section 
IV.A.1.d of the proposal (71 FR 2678), to help inform judgments about 
the acceptability of varying levels of visual air quality in urban 
areas across the U.S. In considering these images for Phoenix, 
Washington, DC, and Chicago (for which PM2.5 concentrations 
are reported), the Staff Paper observed that:
    (1) At concentrations at or near the level of the current 24-hour 
PM2.5 standard (65 [mu]g/m\3\), which equates to visual 
ranges roughly around 10 km (6 miles), scenic views (e.g., mountains, 
historic monuments), as depicted in these images around and within the 
urban areas, are significantly obscured from view.
    (2) Appreciable improvement in the visual clarity of the scenic 
views depicted in these images occurs at PM2.5 
concentrations below 35 to 40 [mu]g/m\3\, which equate to visual ranges 
generally above 20 km for the urban areas considered (EPA, 2005, p. 7-
6).
    (3) Visual air quality appears to be good in these images at 
PM2.5 concentrations generally below 20 [mu]g/m\3\, 
corresponding to visual ranges of approximately 25 to 35 km (EPA, 2005, 
p. 7-8).
    While being mindful of the limitations inherent in using visual 
representations from a small number of areas as a basis for considering 
national visibility-based secondary standards, the Staff Paper 
nonetheless concluded that these observations, together with 
information from the analyses and other programs discussed above, 
support revising the current secondary PM2.5 standards to 
improve visual air quality, particularly in urban areas. As discussed 
below, the Staff Paper recommended the establishment of a new short-
term secondary PM2.5 standard to provide increased and more 
targeted protection, primarily in urban areas, from visibility 
impairment related to fine particles (EPA, 2005, p. 7-12). Based on its 
review of the Staff Paper, the CASAC advised the Administrator that 
most CASAC PM Panel members strongly supported the Staff Paper 
recommendation to establish a new distinct secondary PM2.5 
standard to protect urban visibility (Henderson, 2005a).\84\ Most Panel 
members considered such a standard to be a reasonable complement to the 
Regional Haze Rules that protect Class I areas.
---------------------------------------------------------------------------

    \84\ A dissenting view was expressed in one Panel member's 
individual review comments to the effect that any urban visibility 
standard should be voluntary and locally adopted (Henderson, 2005a).
---------------------------------------------------------------------------

    In the proposal, the Administrator carefully considered the 
rationale and recommendations in the Staff Paper, the advice and 
recommendations from CASAC, and initial public comments on the issue of 
whether the secondary PM standards should be revised to provide 
increased PM-related visibility impairment primarily in urban areas. In 
so doing, the Administrator first recognized that PM-related visibility 
impairment is principally related to fine particle levels, such that it 
is appropriate to focus the review on whether the current secondary 
PM2.5 standards should be revised. The Administrator also 
recognized that perception of visibility impairment is most directly 
related to instantaneous levels of visual air quality, such that in 
considering whether the current suite of secondary standards would 
provide the appropriate degree of protection, he first considered 
whether the current 24-hour secondary PM2.5 standard 
provides an appropriate level of protection from visibility impairment, 
principally in urban areas.
    In the proposal, the Administrator called attention to the Staff 
Paper finding that, at concentrations at or near the level of the 
current 24-hour PM2.5 secondary standard (65 [mu]g/m\3\) 
visual ranges are degraded to a distance of about 10 km (6 miles) and 
images of scenic views (e.g., mountains, historic monuments, urban 
skylines) around and within a number of urban areas are significantly 
obscured from view. Further, the Administrator took note of the various 
State and local standards and programs that have been established to 
protect visual air quality beyond the degree of protection that would 
be afforded by the current 24-hour secondary PM2.5 standard. 
Based on all of the above considerations, the Administrator 
provisionally concluded that it was appropriate to revise the current 
24-hour secondary PM2.5 standard to provide an appropriate 
level of protection from visibility impairment principally in urban 
areas, in conjunction with the regional haze

[[Page 61205]]

program for protection of rural air quality in Class I areas.
    The majority of commenters who expressed an opinion on the 
secondary standards, including NESCAUM, STAPPA/ALAPCO, a number of 
individual States, Tribal associations, and local organizations, and 
combined comments from various environmental groups supported the 
position that the secondary PM2.5 standards should be 
revised to increase protection against visibility impairment. A number 
of these commenters cited the studies and evidence in the PM Staff 
Paper, as well as the recommendations of CASAC, in support of their 
views that a more protective standard is warranted. NESCAUM noted that, 
though monitors in the northeast region do not exceed the current 
secondary PM2.5 standards, their regional haze camera 
network (CAMNET) routinely documents extremely hazy days obscuring city 
skylines and views. NESCAUM stated that ``this shows that virtually all 
of PM2.5 effects on visibility in the Northeast are 
occurring below the present secondary standard, justifying EPA's 
proposal to revise the existing standard to a more stringent level 
adequately protective of public welfare'' (NESCAUM, attachment C, p. C-
1) In general, EPA agrees with these commenters that the more recent 
information on visibility values, photographic evidence, and air 
quality/visibility relationships supports the need to revise the 
current secondary PM2.5 standards.
    Other commenters, including UARG, American Public Power 
Association, and American Electric Power, opposed a revision to 
strengthen the secondary PM2.5 standards at this time. UARG 
stated that:

Because the record does not establish that the risks to public 
welfare from ambient PM2.5 are greater, different in 
character, or more certain than was understood when the present 
standards were established, the Agency lacks a basis for revising 
its conclusion that those standards provide the requisite protection 
of public welfare. (UARG, p. 36).

    UARG questioned the usefulness of the photographic images and urban 
studies of acceptable visibility highlighted in the proposal for 
determining appropriate levels of urban visibility. They further noted 
that, for most areas, the annual PM2.5 standard would 
prevent any exceedances of 65 [mu]g/m\3\.
    While, as summarized above, the key optical aspects of the 
relationship between fine particles and visibility have been 
established for a long time, EPA strongly disagrees that the more 
recent visibility-related evidence and analyses presented in the 
Criteria Document and Staff Paper provide no basis for considering more 
protective PM2.5 standards. As discussed in the Staff Paper, 
one of the key issues in the last review was whether the differences in 
humidity between East and West complicated the establishment of a 
nationally uniform PM2.5 secondary standard, even for urban 
areas (EPA, 2005, p. 7-3). With the substantial addition to the air 
quality and visibility data made possible by the national urban 
PM2.5 monitoring networks, an analysis conducted for this 
review found that, in urban areas, visibility levels show far less 
difference between eastern and western regions on a 24-hour or shorter 
time basis than implied by the largely non-urban data available in the 
1997 review (EPA, 2005, p. 7-5). Of equal importance, more recent 
studies of visibility values conducted for several urbanized areas have 
found results generally consistent with an earlier study done for the 
city of Denver. While such studies are still limited in number and 
subject to uncertainty, they suggest a remarkable consistency in public 
reaction to urban visibility impairment caused by fine particles (EPA 
2005, p. 6-18 to 23).
    Furthermore, staff and CASAC agreed on the utility of photographic 
evidence in characterizing the nature of particle-induced haze. At the 
level of the current 24-hour PM2.5 standard, the potential 
subtleties associated with alternative photographic views alluded to by 
UARG would be obscured by the density of the accompanying haze, which 
would restrict the distance of the farthest discernable dark objects to 
only 6 miles and greatly reduce the contrast for objects at 
significantly shorter distances. Although, as suggested by these 
commenters, the annual standard serves to limit excursions above the 
level of the current 24-hour standard, particularly in eastern urban 
areas, continuation of the current 24-hr PM2.5 standard 
would permit a large number of exceedances of this level especially in 
some western urban areas, even when the standard is just attained. In 
summary, contrary to the views of this set of commenters, EPA believes 
that the combination of new insights from air quality analyses, the 
standards and studies developed to address urban visibility in several 
areas, as well as an evaluation of the photographic evidence, supports 
the need to revise the current secondary PM2.5 standards.
    Having considered the evidence and analysis of visibility and fine 
particles in the Criteria Document and Staff Paper, the advice and 
recommendations of the CASAC, as well as the public comments on this 
issue, the Administrator concludes that it is appropriate to revise the 
current secondary PM2.5 standards to provide increased 
protection from visibility impairment in urban areas. Consistent with 
the considerations and rationale summarized above and in the proposal, 
the Administrator believes that emphasis should be placed on revisions 
to the current 24-hour PM2.5 standard that would provide an 
appropriate level of protection against visibility impairment 
principally in urban areas, in conjunction with the regional haze 
program for protection of visual air quality in Class I areas.
3. Indicator of PM for Secondary Standard To Address Visibility 
Impairment
    As discussed in the Staff Paper, fine particles contribute to 
visibility impairment directly in proportion to their concentration in 
the ambient air. Hygroscopic components of fine particles, in 
particular sulfates and nitrates, contribute disproportionately to 
visibility impairment under high humidity conditions. Particles in the 
coarse mode generally contribute only marginally to visibility 
impairment in urban areas. In analyzing how well PM2.5 
concentrations correlate with visibility in urban locations across the 
U.S. (see EPA, 2005, section 6.2.3), the Staff Paper concluded that the 
observed correlations are strong enough to support the use of 
PM2.5 as the indicator for such standards. More 
specifically, clear correlations exist between 24-hour average 
PM2.5 concentrations and reconstructed light extinction, 
which is directly related to visual range. These correlations are 
similar in the eastern and western regions of the U.S. Further, these 
correlations are less influenced by relative humidity and more 
consistent across regions when PM2.5 concentrations are 
averaged over shorter, daylight time periods (e.g., 4 to 8 hours). 
Thus, the Staff Paper concluded that it is appropriate to use 
PM2.5 as an indicator for standards to address visibility 
impairment in urban areas, especially when the indicator is defined for 
a relatively short period of daylight hours. Based on its review of the 
Staff Paper, most CASAC Panel members endorsed a PM2.5 
indicator for a secondary standard to address visibility impairment 
(Henderson, 2005a, p. 9).
    The Administrator provisionally concurred with the EPA staff and 
CASAC recommendations, and proposed that PM2.5 should be 
retained as the indicator for fine particles as part

[[Page 61206]]

of a secondary standard to address visibility protection. No commenters 
disputed the appropriateness of continuing to use PM2.5 as 
the indicator for fine particle secondary standards to address 
visibility impairment.
    Having considered the scientific information discussed in the 
proposal and summarized above, as well as the recommendations of the 
staff and CASAC and the public comments on this issue, the 
Administrator concludes that PM2.5 should be retained as the 
indicator for fine particles as part of a secondary standard to address 
visibility protection.
4. Averaging Time of a Secondary PM2.5 Standard for 
Visibility Protection
    As discussed in the Staff Paper, averaging times from 24 to 4 hours 
were considered for a revised standard to address visibility 
impairment. Within this range, clear and similarly strong correlations 
were found between visibility and 24-hour average PM2.5 
concentrations in eastern and western areas, while somewhat stronger 
correlations were found with PM2.5 concentrations averaged 
over a 4-hour time period. In general, correlations between 
PM2.5 concentrations and light extinction were found to be 
generally less influenced by relative humidity and more consistent 
across regions as shorter, sub-daily averaging times, within daylight 
hours from approximately 10 a.m. to 6 p.m., were considered. The Staff 
Paper concluded that an averaging time from 4 to 8 hours, generally 
within this daylight time period, should be considered for a standard 
to address visibility impairment.
    In reaching this conclusion, the Staff Paper recognized that the 
PM2.5 Federal Reference Method (FRM) monitoring network 
provides 24-hour average concentrations, and, in some cases, on a 
third- or sixth-day sample schedule, such that implementing a standard 
with a less-than-24-hour averaging time would necessitate the use of 
continuous monitors that can provide hourly time resolution. Given that 
the data used in the Staff Paper analysis discussed above were from 
commercially available PM2.5 continuous monitors, such 
monitors clearly could provide the hourly data that would be needed for 
comparison with a potential visibility standard with a less-than-24-
hour averaging time.
    Most CASAC Panel members supported the Staff Paper recommendation 
of a sub-daily (4 to 8 daylight hours) averaging time, finding it to be 
an innovative approach that strengthens the quality of the 
PM2.5 indicator for visibility effects by targeting the 
driest part of the day (Henderson, 2005a, p. 9). In its advice to the 
Administrator, CASAC noted an indirect but important benefit to 
advancing EPA's monitoring program goals that would come from the 
direct use of hourly data from a network of continuous PM2.5 
mass monitors.
    In considering the Staff Paper recommendation and CASAC's advice, 
the Administrator provisionally concluded that averaging times from 24 
hours to 4 daylight hours would represent a reasonable range of choices 
for a standard to address urban visibility impairment. A 24-hour 
averaging time could be selected and applied based on the extensive 
data base currently available from the existing PM2.5 FRM 
monitoring network, whereas a sub-daily averaging time would 
necessarily depend upon an expanded network of continuous 
PM2.5 mass monitors. While the Administrator agreed that 
broader deployment of continuous PM2.5 mass monitors is a 
desirable goal, working toward that goal does not depend upon nor 
provide an appropriate basis for setting a sub-daily standard. The 
Administrator believed that it was appropriate to evaluate averaging 
time in conjunction with reaching decisions on the form and level of a 
standard. Public comments on these issues, as well as the rationale for 
the final decisions on averaging time, form, and level of the secondary 
standards, are presented in the following section.
5. Final Decisions on Secondary PM2.5 Standards for 
Visibility Protection
    In considering PM2.5 standards that would provide an 
appropriate level of protection against PM-related impairment of 
visibility primarily in urban areas, the Administrator took into 
account the results of the public perception and attitude surveys in 
the U.S. and Canada, State and local visibility standards within the 
U.S., and visual inspection of photographic representations of several 
urban areas across the U.S. summarized in section IV.A.1 of the 
proposal. In the Administrator's judgment, these sources provide useful 
but still quite limited information on the range of levels appropriate 
for consideration in setting a national visibility standard primarily 
for urban areas, given the generally subjective nature of the public 
welfare effect involved. In considering alternative forms for such 
standards, the Administrator took into account the same general factors 
that were considered in selecting an appropriate form for the 24-hour 
primary PM2.5 standard (as discussed above in section 
II.E.1), as well as additional information on the percent of areas not 
likely to meet various alternative PM2.5 standards, 
consistent with CASAC advice to consider such information (Henderson, 
2005a, p. 10).
    In considering the remaining elements of a secondary 
PM2.5 standard (averaging time, form, and level) for 
purposes of the proposal, the Administrator looked to the rationale 
presented in the Staff Paper and to CASAC's advice and recommendations 
for such a standard. Based on photographic representations of varying 
levels of visual air quality, public perception studies, and local and 
State visibility standards, as discussed above, the Staff Paper 
concluded that 30 to 20 [mu]g/m\3\ PM2.5 represents a 
reasonable range for a national visibility standard primarily for urban 
areas, based on a sub-daily averaging time. The upper end of this range 
is below the levels at which the illustrative scenic views are 
significantly obscured, and the lower end is around the level at which 
visual air quality generally appears to be good based on observation of 
the illustrative views. Analyses of 4-hour average PM2.5 
concentrations indicate that this concentration range can be expected 
generally to correspond to median visual ranges in urban areas within 
regions across the U.S. of approximately 25 to 35 km (see EPA, 2005, 
Figure 7-1).\85\ This range of visual range values is bounded above by 
the visual range targets selected in specific areas where State or 
local agencies placed particular emphasis on protecting visual air 
quality.
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    \85\ The Staff Paper notes that a standard set at any specific 
PM2.5 concentration will necessarily result in visual 
ranges that vary somewhat in urban areas across the country, 
reflecting the variability in the correlations between 
PM2.5 concentrations and light extinction (EPA, 2005, p. 
7-8).
---------------------------------------------------------------------------

    In considering a reasonable range of forms for a PM2.5 
standard within this range of levels, the Staff Paper concluded that a 
concentration-based percentile form is appropriate for the same reasons 
as those discussed in section II.F.1 above (on the form of the 24-hour 
primary PM2.5 standard). The Staff Paper also concluded that 
the upper end of the range of concentration percentiles should be 
consistent with the percentile used for the primary standard, which was 
proposed to be the 98th percentile, and that the lower end of the range 
should be the 92nd percentile, which represents the mean of the 
distribution of the 20 percent most impaired days, as targeted in the 
regional haze program (EPA, 2005, p. 7-11 to 12).
    In its advice to the Administrator, the CASAC Panel recognized that 
it is difficult to select any specific level and

[[Page 61207]]

form based on currently available information (Henderson, 2005a, p. 9). 
Some Panel members felt that the range of levels recommended in the 
Staff Paper was on the high side, but recognized that developing a more 
specific (and more protective) level in future reviews would require 
updated and refined public visibility valuation studies, which CASAC 
strongly encouraged the Agency to support prior to the next review. 
With regard to the form of the standard, the recommendations in the 
final Staff Paper reflected CASAC's advice to consider percentiles in 
the range of the 92nd to the 98th percentile. Some Panel members 
recommended considering a percentile within this range in conjunction 
with a level toward the upper end of the range recommended in the Staff 
Paper.\86\
---------------------------------------------------------------------------

    \86\ Some CASAC Panel members also recommended that such a 
standard be implemented in conjunction with an ``exceptional 
events'' policy so as to avoid having non-compliance with the 
standard be driven by natural source influences such as dust storms 
and wild fires (Henderson, 2005a).
---------------------------------------------------------------------------

    Based on the above considerations, for purposes of the proposal the 
Administrator believed that it was appropriate to first consider the 
level of protection that would be afforded by the proposed suite of 
primary PM2.5 standards (71 FR 2681). The limited and 
uncertain evidence currently available for use in evaluating the 
appropriate level of protection suggested that a cautious approach was 
warranted in establishing a distinct secondary PM2.5 
standard to address visibility impairment. While significantly more 
information is available since the last review concerning the 
relationship between fine PM levels and visibility across the country, 
there is still little available information for use in making the 
relatively subjective value judgment needed in selecting the 
appropriate degree of protection to be afforded by such a standard. 
Given this, the Administrator first evaluated the level of protection 
that the proposed primary PM2.5 standards would likely 
provide, and then determined whether the available evidence warranted 
adopting a standard with a different level, form, or averaging time.
    In comparing the extent to which the proposed suite of primary 
standards would require areas across the country to improve visual air 
quality with the extent of increased protection likely to be afforded 
by a standard based on a sub-daily averaging time, the Administrator 
looked to an analysis of the predicted percent of areas not likely to 
meet various alternative secondary and primary PM2.5 
standards (EPA, 2005, Tables 7A-1 and 5B-1(a) \87\). In so doing, the 
Administrator observed that the predicted percent of counties with 
monitors not likely to meet the proposed suite of primary 
PM2.5 standards (i.e., a 24-hour standard set at 35 [mu]g/
m\3\, with a 98th percentile form, and an annual standard of 15 [mu]g/
m\3\) was actually somewhat greater (27 percent) than the predicted 
percent of counties with monitors not likely to meet a sub-daily 
secondary standard with an averaging time of 4 daylight hours, a level 
toward the upper end of the range recommended in the Staff Paper (e.g., 
up to 30 [mu]g/m\3\), and a form within the recommended range (e.g., 
around the 95th percentile) (24 percent). A similar comparison was seen 
in considering the predicted percentages of the population living in 
such areas.
---------------------------------------------------------------------------

    \87\ The information in these Tables is based on analysis of 
2001-2003 air quality data, including 562 counties with FRM monitors 
that met specific data completeness criteria for developing 
predicted percentages of counties not likely to meet the suite of 
primary PM2.5 standards and 168 counties with continuous 
PM2.5 monitors that met less restrictive data 
completeness criteria for developing predicted percentages for a 4-
hour secondary PM2.5 standard.
---------------------------------------------------------------------------

    Considering the evidence in light of these comparisons, the 
Administrator provisionally concluded that revising the current 
secondary 24-hour standard for PM2.5 to be identical to the 
proposed revised primary PM2.5 standard and retaining the 
current annual secondary PM2.5 standard was a reasonable 
policy approach to addressing visibility protection primarily in urban 
areas. Consistent with CASAC's recommendation, the Administrator also 
solicited comment on a sub-daily (4- to 8-hour averaging time) 
secondary PM2.5 standard.
    In additional comments responding to EPA's proposed revision of the 
secondary PM2.5 standards for visibility protection (71 FR 
2675-2781), the CASAC requested that a sub-daily standard to protect 
visibility be favorably reconsidered (Henderson, 2006, p. 2). As noted 
above, most of the CASAC Panel recommended a sub-daily standard for 
PM2.5 with a level in the 20 to 30 [mu]g/m\3\ range for a 
four- to eight-hour (4-8 hr) mid-day time period with a 92nd to 98th 
percentile form. The CASAC members noted three cautions regarding the 
Agency's proposed reliance on a secondary PM2.5 standard 
identical to the proposed 24-hour primary PM2.5 standard 
(Id. at pp. 5-6):
    (1) They noted that the PM2.5 mass measurement is a 
better indicator of visibility impairment during daylight hours, when 
humidities are low; the sub-daily standard more clearly matches the 
nature of visibility impairment, whose adverse effects are most evident 
during the daylight hours; using a 24-hour standard as a proxy 
introduces error and uncertainty in protecting visibility; and sub-
daily standards are used for other NAAQS and should be the focus for 
visibility.
    (2) They noted that CASAC and its monitoring subcommittees have 
repeatedly commended EPA's initiatives promoting the introduction of 
continuous and near-continuous PM monitoring, and that expanded 
deployment of continuous PM2.5 monitors is consistent with 
setting a sub-daily standard to protect visibility.
    (3) They cautioned that the analysis showing a similarity between 
percentages of counties not likely to meet what they considered to be a 
lenient 4- to 8-hour secondary standard and a secondary standard 
identical to the proposed 24-hour primary standard is a numerical 
coincidence that is not indicative of any fundamental relationship 
between visibility and health.
    The CASAC Panel further stated that ``visual air quality is 
substantially impaired at PM2.5 concentrations of 35 [mu]g/
m\3\'' and that ``it is not reasonable to have the visibility standard 
tied to the health standard, which may change in ways that make it even 
less appropriate for visibility concerns.'' (Id. at p. 6.)
    Many of the public commenters who supported a more stringent 
visibility standard also supported the more specific EPA staff and 
CASAC recommendations and urged EPA to adopt a sub-daily (4- to 8-hour 
averaging time) PM2.5 standard to address visibility 
impairment, within the range of 20 to 30 [mu]g/m\3\ and with a form 
within the range of the 92nd to 98th percentile. In general, these 
commenters based their recommendations on the same studies, analyses, 
and considerations presented in the Staff Paper and in section IV.A of 
the proposal.\88\
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    \88\ The American Lung Association et al. disagreed with the 
Administrator's view that the secondary standards should be focused 
primarily on providing protection in urban areas, with protection of 
Class I areas provided by the Regional Haze Rule. These commenters 
suggested that EPA should not rely on the regional haze program and 
must set national standards to protect all areas. As discussed in 
the Response to Comments document, EPA believes that this issue was 
settled in ATA I. (See 175 F.3d at 1056-1057.)
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    EPA agrees with several of the key technical points made in CASAC's 
original recommendations and their request for reconsideration. The 
Administrator recognizes that there is a significant body of data and 
information indicating that a sub-daily standard has

[[Page 61208]]

strong technical merit. The fine particle/visibility relationship is 
most consistent across regions for shorter averaging times during the 
daylight hours, when humidity tends to be lowest. The EPA also agrees 
that visibility impairment has the greatest impact on public welfare 
during the daylight hours, but notes that daylight is not limited to a 
four to eight hour period.
    The Administrator believes, however, that it is appropriate to 
consider the protection the revised suite of primary PM2.5 
standards would provide against adverse effects on public welfare. The 
analysis summarized above found that the relative protection provided 
by the proposed primary standards was equivalent or more protective 
than several of the 4-hour secondary standard alternatives in the range 
recommended by the Staff Paper and CASAC. Given the limitations in the 
underlying studies and the subjective nature of the judgment required, 
the Administrator continues to believe that caution is warranted in 
establishing a distinct secondary standard for visibility impairment. 
Contrary to commenters who recommended a distinct standard providing 
greater protection, in this case, the Administrator does not believe 
that these studies warrant adopting a secondary standard that would 
provide either more or less protection against visibility impairment in 
urban areas than would be provided by secondary standards set equal to 
the proposed primary PM2.5 standards. While EPA agrees that 
the use of 24-hour and annual averages will result in more variability 
in visibility across urban areas, as the Staff Paper notes, any 
PM2.5 secondary standard would result in some variability in 
protection in different locations (EPA, 2005, p. 7-8).
    While, as noted above and in the proposal, the Administrator agrees 
with CASAC's point that broader deployment of continuous 
PM2.5 mass monitors is a desirable goal, working toward that 
goal does not depend upon nor provide an appropriate basis for setting 
a sub-daily standard. Moreover, pursuant to CASAC recommendations, EPA 
is today issuing modifications to the PM2.5 reference and 
equivalent methods that will encourage the certification and deployment 
of more continuous monitors (in a separate document published in 
today's Federal Register). With respect to the third CASAC comment 
summarized above, EPA agrees that the result of the analysis showing a 
similarity in the percentages of counties not likely to meet the 
revised 24-hour primary PM2.5 standard or a sub-daily 
standard set toward the upper end of the range of protectiveness 
recommended by CASAC is not indicative of any fundamental relationship 
between visibility and public health. However, EPA does not believe 
that this coincidental similarity weighs against considering making the 
secondary standard identical to the revised primary standard.
    Having considered the evidence, the advice of CASAC, and public 
comments, the Administrator believes that revising the current 
secondary PM2.5 standards to be identical to the revised 
suite of primary PM2.5 standards adopted in today's notice 
is a reasonable policy approach to addressing visibility impairment 
primarily in urban areas. The current annual and revised 24-hour 
secondary PM2.5 standards will result in improvements in 
visual air quality in as many or more urban areas across the country as 
would the alternative approach of setting a sub-daily standard 
consistent with the upper portion of the ranges recommended by CASAC. 
This approach recognizes the substantial limitations in the available 
hourly air quality data and in available studies of public perception 
and attitudes with regard to the acceptability of various degrees of 
visibility impairment in urban areas across the country. Given these 
limitations, the Administrator believes that a distinct secondary 
standard with a different averaging time, level, or form is not 
warranted at this time, because the available evidence does not support 
a decision to achieve a level of protection different from that 
provided by the revised suite of primary standards, and because no 
further change in averaging time, level, or form appears needed to 
achieve a comparable level of protection. A decision in this review to 
make secondary standards equivalent in all respects to the primary 
standards, as revised, does not limit the ability of the Agency to 
establish a distinct secondary standard in the future if and when the 
underlying evidence indicates that it is appropriate. Further, the 
Administrator notes that continuing to advance the use of continuous 
PM2.5 monitors is not dependant on establishing a sub-daily 
secondary PM2.5 standard.
    The Administrator believes that any secondary NAAQS for visibility 
protection should be considered in conjunction with the regional haze 
program as a means of achieving appropriate levels of protection 
against PM-related visibility impairment in urban, non-urban, and Class 
I areas across the country. Programs implemented to meet the national 
primary standards can be expected to improve visual air quality not 
just in urban areas but in surrounding non-urban areas as well; 
similarly, programs now being developed to address the requirements of 
the regional haze rule established for protection of visual air quality 
in Class I areas can be expected to improve visual air quality in 
surrounding areas as well. The Administrator further believes that the 
development of local programs continues to be an effective and 
appropriate approach to provide additional protection for unique scenic 
resources in and around certain urban areas that are highly valued by 
people living in those areas.
    Based on all of the considerations discussed above, the 
Administrator concludes that it is appropriate to revise the current 
secondary PM2.5 standards to be identical in all respects to 
the revised suite of primary PM2.5 standards adopted in 
today's notice to provide an appropriate level of visibility protection 
primarily in urban areas.

B. Other PM-Related Welfare Effects

    In considering the currently available evidence on non-visibility 
PM-related welfare effects, the Staff Paper noted that there was much 
information linking ambient PM to potentially adverse effects on 
vegetation and ecosystems and on materials damage and soiling, and on 
characterizing the role of atmospheric particles in climatic and 
radiative processes. However, given the evaluation of this information 
in the Criteria Document and Staff Paper, which highlighted the 
substantial limitations in the evidence, especially the lack of 
evidence linking various effects to specific levels of ambient PM, the 
Administrator provisionally concluded in the proposal that the 
available evidence did not provide a sufficient basis for establishing 
distinct secondary standards for PM based on any of these effects 
alone.
    In the proposal, the Administrator also addressed the question 
whether reductions in PM likely to result from the current secondary PM 
standards, or from the range of revised primary PM standards, would 
provide appropriate protection against any of these PM-related welfare 
effects. As discussed below, these considerations included the latest 
scientific information characterizing the nature of these non-
visibility PM-related effects and judgments as to whether revision of 
the current secondary standards is appropriate based on that 
information.
1. Evidence of Non-Visibility Welfare Effects Related to PM
    Particulate matter contributes to adverse effects on a number of 
welfare effects categories other than visibility impairment, including 
vegetation and

[[Page 61209]]

ecosystems, soiling and materials damage, and climate. These welfare 
effects result predominantly from exposure to excess amounts of 
specific chemical species, regardless of their source or predominant 
form (particle, gas, or liquid). Reflecting this fact, the Criteria 
Document concluded that regardless of size fraction, particles 
containing nitrates and sulfates have the greatest potential for 
widespread environmental significance. The nature of these welfare 
effects is discussed in the Criteria Document (Chapters 4 and 9) and 
Staff Paper (Chapter 6) and summarized in section IV.B.1 of the 
proposal. The information highlighted there includes:
    (1) PM-related effects on vegetation, specifically those associated 
with excess levels of particulate nitrate and sulfate in acidifying 
deposition to foliage, leading to accelerated weathering of leaf 
cuticular surfaces; increased permeability of leaf surfaces to toxic 
materials, water, and disease agents; increased leaching of nutrients 
from foliage; and altered reproductive processes--all which serve to 
weaken trees so that they are more susceptible to other stresses (e.g., 
extreme weather, pests, pathogens).
    (2) PM-related effects on ecosystems, specifically those resulting 
from the nutrient or acidifying characteristics of deposited PM on both 
terrestrial and aquatic ecosystems, which contribute to adverse impacts 
on essential ecological attributes such as species shifts, loss of 
diversity, impacts to threatened and endangered species and alteration 
of native fire cycles.
    (3) Characterization of ecosystem exposure to PM deposition, 
specifically the currently available deposition monitoring network and 
the lack of sufficient long-term monitoring of ecosystem response 
needed for PM-related ecological risk assessment.
    (4) The critical loads concept and its applicability as an 
assessment tool in the context of the PM secondary NAAQS review.
    (5) PM-related effects on materials, specifically the physical 
damage caused mainly by deposited particulate nitrates and sulfates and 
the impaired aesthetic qualities due to soiling caused mainly by 
particles consisting primarily of carbonaceous compounds.
    (6) PM-related effects on climate, specifically through scattering 
and absorption of radiation by ambient particles, as well as effects on 
the radiative properties of clouds through changes in the number and 
size distribution of cloud droplets, and by altering the amount of 
ultraviolet solar radiation (especially UV-B) penetrating through the 
atmosphere to ground level.
2. Need for Revision of the Current Secondary PM Standards To Address 
Other PM-Related Welfare Effects
    At the time of proposal, in considering the currently available 
evidence on each type of PM-related welfare effects discussed above, 
the Administrator noted that there was much information linking the 
sulfur- and nitrogen-containing components of ambient PM to potentially 
adverse effects on ecosystems and vegetation, as well as links between 
PM and its constituents and materials damage and soiling, as well as 
climatic and radiative processes. However, after reviewing the extent 
of relevant studies and other information available since the 1997 
review of the PM standards, which highlighted the substantial 
limitations in the evidence, especially with regard to the lack of 
evidence linking various effects to specific levels of ambient PM, the 
Administrator concurred with conclusions reached in the Staff Paper and 
by CASAC (Henderson, 2005a) that the available data do not provide a 
sufficient basis for establishing distinct secondary PM standards based 
on any of these non-visibility PM-related welfare effects.
    While recognizing that PM-related impacts on vegetation and 
ecosystems and PM-related soiling and materials damage are associated 
with chemical components in both fine and coarse-fraction PM, the 
Administrator provisionally concluded that sufficient information was 
not available at this time to consider either an ecologically based 
indicator or an indicator based distinctly on soiling and materials 
damage, in terms of specific chemical components of PM. Further, 
consistent with the rationale and recommendations in the Staff Paper, 
the Administrator agreed that it was appropriate to continue control of 
ambient fine and coarse-fraction particles, especially long-term 
deposition of particles such as particulate nitrates and sulfates that 
contribute to adverse impacts on vegetation and ecosystems and/or to 
materials damage and soiling. The Administrator also agreed with the 
Staff Paper that the available information did not provide a sufficient 
basis for the development of distinct secondary standards to protect 
against such effects beyond the protection likely to be afforded by the 
proposed suite of primary PM standards. In considering those proposed 
standards in combination, including the proposed more protective 24-
hour standard for PM2.5 and the proposed 24-hour standard 
for PM10-2.5, which was intended to provide an equivalent 
degree of protection to the current PM10 standards in areas 
where the proposed PM10-2.5 indicator would apply (which 
tend to be more densely populated areas where materials damage would be 
of greater concern), the Administrator believed that this proposed 
suite of standards would afford at least the degree of protection as 
that afforded by the current secondary PM standards.
    Finally, the Administrator believed that such standards should be 
considered in conjunction with the protection afforded by other 
programs intended to address various aspects of air pollution effects 
on ecosystems and vegetation, such as the acid deposition program and 
other regional approaches to reducing pollutants linked to nitrate or 
acidic deposition. Based on these considerations, and taking into 
account the information and recommendations discussed above, the 
Administrator proposed to revise the current secondary PM2.5 
and PM10 standards to address these other welfare effects by 
making them identical in all respects to the proposed suite of primary 
PM2.5 and PM10-2.5 standards.
    In response to the proposal, in addition to their recommendation 
for a PM2.5 secondary standard, CASAC recommended 
(Henderson, 2006, p. 4) ``that a secondary PM10-2.5 standard 
be set at the same level as the primary PM coarse standard to protect 
against the various irritant, soiling and nuisance welfare or 
environmental effects of coarse particles. Since these effects are not 
uniquely related to urban sources or receptors, the standard should not 
be limited to urban areas.'' Only limited public comments were received 
on this aspect of the proposal.
    In general, public comments relating to secondary standards and 
other welfare effects focused on issues related to the current 
secondary PM10 standards. Most of these commenters, 
including the groups who objected to the use of a qualified indicator 
for the primary thoracic coarse particle standard, argued that current 
levels of PM dust contribute or potentially contribute to nuisance, 
soiling, and irritant impacts on personal comfort and well being, 
especially in non-urban areas. The same commenters agreed with CASAC 
that, in the absence of a demonstration to the contrary, EPA is not 
justified in eliminating or reducing the level of protection to rural 
areas that is provided by the current suite of secondary standards. 
Most of these commenters recommended that EPA either retain the current 
PM10 secondary standard or replace it with a 
PM10-2.5

[[Page 61210]]

standard set identical to the proposed primary standard without the 
proposed qualifications that limited application of the standard to 
urban areas.
    A few commenters argued against retaining any secondary standard 
for coarse particles. Many of these same commenters argued that if EPA 
did set a secondary PM10-2.5 standard, it should be set 
equal to the primary PM10-2.5 standard because there was 
insufficient evidence to support adoption of a distinct secondary 
standard for PM10-2.5 at this time. Furthermore, these 
commenters noted that in the proposal, EPA had correctly excluded from 
both primary and secondary standards ``any ambient mix of 
PM10-2.5 that is dominated by rural windblown dust and soils 
and PM generated by agricultural and mining sources'' because these 
particles are nontoxic and generally settle quickly.
    In reaching a final decision on the need to revise the PM secondary 
standards regarding these non-visibility related welfare effects, the 
Administrator has taken into account several key factors, including: 
(1) The latest scientific information on non-visibility welfare effects 
associated with PM, as previously described; (2) the post-proposal 
recommendations of CASAC, (3) comments received during the public 
comment period, and (4) the final decisions reached in today's notice 
on the primary standards for fine and coarse particles, as well as the 
decision presented above on secondary PM2.5 standards to 
protect against visibility impairment. The Administrator notes that 
extending today's decision not to revise the current 24-hour primary 
PM10 standard to the secondary standard would be consistent 
with the recommendations of CASAC and would address the issues raised 
by the first group of commenters summarized above. Consistent with the 
assessment of the evidence in the Staff Paper and the CASAC 
recommendations, the Administrator disagrees with those who assert that 
no secondary standard is needed to protect against the welfare effects 
associated with coarse particles.
    On the other hand, the Administrator does not believe that distinct 
secondary standards for fine or coarse particles are warranted for any 
of the effects considered in this section. The available evidence is 
not sufficient to support the selection of an ecologically based 
indicator or an indicator based distinctly on materials damage, 
soiling, irritant or nuisance effects, or other effects of PM. However, 
the Administrator recognizes that it is appropriate to continue control 
of ambient fine and coarse particles, especially long-term deposition 
of particles such as particulate nitrates and sulfates that contribute 
to the total input of nitrogen and sulfur to ecosystems that has been 
shown to adversely affect sensitive aquatic and terrestrial ecosystems, 
and/or particles that contribute to materials damage and soiling. The 
Administrator notes that setting the secondary PM standards identical 
to the revised suite of primary standards directionally improves the 
level of protection afforded vegetation, ecosystems, and materials. In 
addition, the Administrator continues to believe that the secondary 
NAAQS should be considered in conjunction with the protection afforded 
by other programs intended to address various aspects of air pollution 
effects on ecosystems and vegetation, such as the acid deposition 
program and other regional approaches to reducing pollutants linked to 
nitrate or acidic deposition.
    Based on the above considerations, the Administrator concludes that 
it is appropriate to address the other welfare effects summarized in 
this section by revising the current suite of PM2.5 
secondary standards, making them identical in all respects to the suite 
of primary PM2.5 standards, while retaining the current 24-
hour PM10 secondary standard and revoking the current annual 
PM10 secondary standard. For the reasons noted in section 
III.D.1 above, the 24-hour PM10 standard will provide 
adequate protection against the known and potential effects related to 
long-term PM10 concentrations.

C. Final Decisions on Secondary PM Standards

    For the reasons discussed above, and taking into account the 
information and assessments presented in the Criteria Document and 
Staff Paper, the advice and recommendations of CASAC, and public 
comments received on the proposal, the Administrator is revising the 
current secondary PM standards by making them identical in all respects 
to the suite of primary PM standards, as revised by today's action. In 
the Administrator's judgment, these standards, in conjunction with the 
regional haze program, will provide appropriate protection to address 
PM-related welfare effects, including visibility impairment, effects on 
vegetation and ecosystems, materials damage and soiling, and effects on 
climate change.

V. Interpretation of the NAAQS for PM

    This section presents EPA's final decisions regarding the revision, 
addition, and/or revocation of appendices to 40 CFR Part 50 on 
interpreting the primary and secondary NAAQS for PM.

A. Amendments to Appendix N--Interpretation of the National Ambient Air 
Quality Standards for PM2.5

    The EPA proposed to revise the data handling procedures in appendix 
N to 40 CFR Part 50 for the annual and 24-hour PM2.5 
standards (71 FR 2685-2686). The proposed amendments to appendix N 
detailed the computations necessary for determining when the proposed 
primary and secondary PM2.5 NAAQS were met. The proposed 
amendments also addressed data reporting, monitoring considerations, 
and rounding conventions. Key elements of the proposed revisions to 
appendix N were presented in section V of the preamble to the proposed 
rule and are summarized below, together with EPA's final decisions on 
revisions to appendix N.
1. General
    As proposed, EPA is adding several new definitions to section 1.0 
and using these definitions throughout the appendix, most notably ones 
for ``design values.'' Also, the 24-hour sampling timeframe has been 
clarified as representing ``local standard (word inserted) time.'' This 
revision reflects EPA's previous intent as well as majority practice, 
and also avoids ambiguity since local clock time varies according to 
daylight savings periods. No opposing comments were received on these 
changes.
2. PM2.5 Monitoring and Data Reporting Considerations
    As proposed, two new sections are being added to appendix N to more 
specifically stipulate and highlight monitoring and data considerations 
(71 FR 2685). New section 2.0 includes statistical requirements for 
spatial averaging (which is part of the form of the annual standard for 
PM2.5). As discussed in section II.F.2 above, EPA is 
tightening two of the constraints on the use of spatial averaging to 
provide an adequate margin of safety to susceptible subpopulations by 
reflecting enhanced knowledge of typical monitor relationships in 
metropolitan areas.
    New section 3.0 to appendix N codifies aspects of raw data 
reporting and raw data time interval aggregation including 
specifications of number of decimal places. Previously, these reporting 
instructions resided only in associated guidance documents. Section 3.0 
also notes the process for assimilating monitored concentration data 
from collocated instruments into a

[[Page 61211]]

single ``site'' record; data for the site record would originate mainly 
from the designated ``primary'' monitor at the site location, but would 
be augmented with collocated Federal reference method (FRM) or Federal 
equivalent method (FEM) monitor data whenever valid data are not 
generated by the primary monitor. This procedure will enhance the 
opportunity for sites to meet data completeness requirements. This 
language likewise codifies existing practice, since the technique was 
previously documented in guidance documentation and implemented as EPA 
standard operating procedure. Commenters agreed that this was a valid 
approach and should be implemented.
3. PM2.5 Computations and Data Handling Conventions
    As proposed, EPA is maintaining a spatially-averaged annual mean, 
with revisions to the criteria for when spatial averaging can be used 
(see section 1 above, as well as section II.E.2), as the form of the 
annual PM2.5 standard and is retaining a 98th percentile 
concentration as the form of the 24-hour PM2.5 standard. 
Although no actual computational change was proposed for a spatially-
averaged annual mean, the proposed Appendix N differentiated, in 
language and formulae, between a spatial average of more than one site 
and a spatial average of only one site. We are adopting these changes 
throughout Appendix N as appropriate to alleviate confusion caused by 
the current ``catch-all'' generic reference (i.e., ``spatial average'' 
or ``spatially averaged'') found throughout the existing Appendix N.
    As proposed, appendix N identifies the NAAQS metrics and explains 
data capture requirements and comparisons to the standards for the 
annual PM2.5 standard and the 24-hour standard (in sections 
4.1, and 4.2, respectively); data rounding conventions (in section 
4.3); and formulas for calculating the annual and 24-hour metrics (in 
sections 4.4 and 4.5, respectively). A significant comment related to 
the 98th percentile formula and an associated bias for periodic 
sampling is discussed above in section II.E.1.
    With regard to the annual PM2.5 standard, EPA proposed 
to retain current data capture requirements with two exceptions. The 
current appendix N had reduced data capture requirements for years that 
exceeded the level of the annual NAAQS; specifically, a minimum of 11 
valid samples per quarter as opposed to a more stringent 75 percent (of 
scheduled samples) was considered sufficient in those instances where 
the annual mean exceeded the NAAQS level. See existing Part 50 App. N 
2.1 (b). The EPA proposed to also allow 11 or more samples per quarter 
as an acceptable minimum if the calculated annual standard design value 
exceeds the level of the standard. The intent of this change was to 
prevent a site with a violating design value that is made up of one (or 
more) annual means under the level of the NAAQS from not being used for 
regulatory purposes just because one (or more) of the quarters of the 
year(s) under the NAAQS level has less than 75% data capture. One 
commenter voiced a general concern over the lack of uniformity in 
completeness criteria but the other commenters supported the change. 
Taking these comments into consideration, EPA is revising appendix N as 
proposed with regard to this issue.
    A second proposed change in the data completeness requirements 
would incorporate data substitution logic for situations where the 
proposed 11 samples per quarter minimum is not met. Consistent with 
existing guidance and practice (implementing current App. N 2.1 (c)), 
EPA proposed to incorporate the following requirement into appendix N: 
a quarter with less than 11 samples would be complete and valid if, by 
substituting an historically low 24-hr value for the missing samples 
(up to the 11 minimum), the results yield an annual mean, spatially 
averaged annual mean, and/or annual standard design value that exceeds 
the level of the standard. The EPA proposed to implement this procedure 
for making comparisons to the NAAQS and not to permanently alter the 
reported data. The EPA considered this a very conservative means of 
imputing data (and increasing the opportunities for using monitoring 
data that otherwise are valid), but solicited comment on the proposed 
approach. Several comments were received on this approach and the 
majority favored it. However, two commenters (NESCAUM and a constituent 
State) suggested a limit of one quarter (out of the 12 in a 3-year 
period) where the substitutions could be made. They suggested the 
limitation because they were concerned that the absence of a 
significant amount of data is an indication that site operator and/or 
equipment problems exist. The EPA shares this concern but observes that 
the method protocol itself guards against excessive utilization. The 
more missing values that are potentially substituted with the method 
effectively reduce the chance of a valid result (i.e., a usable design 
value). Taking these comments into consideration, EPA is revising 
appendix N as proposed with regard to this issue.
    With regard to the 24-hour PM2.5 standard, EPA proposed 
to revise appendix N to include a special formula (Equation 6 in the 
proposed rule, 71 FR 2702) for computing annual 98th percentile values 
when a site operates on an approved seasonal sampling schedule. This 
formula was previously stated only in guidance documentation (EPA, 
1999) but was utilized, where appropriate, in official OAQPS design 
value calculations. No adverse comments were received on this addition.
    The proposed revisions to appendix N also incorporated language 
explicitly stating that 98th percentiles (for both regular and seasonal 
sampling schedules) were to be based on the applicable number of 
samples rather than the actual number of samples. The EPA proposed that 
both annual 98th percentile equations (proposed Equations 5 and 6) 
would reflect this approach. The EPA acknowledges that it made an error 
in the placement of the ``applicable number of samples'' references 
into the denominator of the special seasonal 98th percentile formula 
(Equation 6) and has restored the equation to its original form. The 
EPA notes that the special season formula already takes into 
consideration oversampling in low periods. Furthermore, because the 
``applicable number of samples'' was removed from the seasonal formula, 
there was no need to stipulate that ``seasons'' could not divide 
months; that proposed requirement was only necessary to accommodate the 
calculation of ``applicable number.''
    The EPA solicited comment on the ``applicable number of samples'' 
concept and calculation and received several comments on the concept. 
One commenter endorsed it without discussion, one commenter did not 
object to it but noted that it was difficult to program, and another 
commenter thought that the concept unnecessarily complicates matters 
and favored the use of ``scheduled number of samples'' instead. Two 
commenters said that it would be an acceptable approach if it still 
permitted ``extra'' sampling at the end of a month to make up for 
missed samples. The EPA notes that it has never endorsed this ``extra'' 
sampling practice for the 24-hour PM2.5 standard, so that 
the commenter's premise is incorrect. The EPA agrees with comments that 
expressed concerns about this calculation being too complicated and, 
therefore, has simplified the procedure in a manner that corresponds to 
the calculation of

[[Page 61212]]

data capture. The applicable number of samples for a given year is now 
defined as simply the sum of the number of completed scheduled 
(``creditable'') samples for the year. The new appendix N defines the 
new term, ``creditable'' and describes its use in calculating data 
capture rates and ``applicable number.'' For sites that sample 
correctly (i.e. don't oversample at the end of the month), the simpler 
``applicable number'' procedure will produce the same result as the 
proposed calculation.
    To simplify the regulatory language, as proposed, EPA is revising 
appendix N to eliminate the equation computational examples. The EPA 
will provide extensive computational examples in forthcoming guidance 
documents.
4. Conforming Revisions
    As proposed, EPA is revising terminology and data handling 
procedures associated with exceptional events to conform to rules which 
EPA proposed to implement the recent amendment to CAA section 319 (42 
U.S.C. 7619) by section 6013 of the Safe, Accountable, Flexible 
Efficient Transportation Equity Act: A Legacy for Users (SAFETEA-LU) 
(Pub. L. 109-59). The EPA proposed rules to address exceptional events 
on March 10, 2006 (71 FR 12592). The EPA is replacing the term 
currently used in appendix N.1(b)--uncontrollable or natural events--
with ``exceptional events,'' corresponding with the term used in the 
recent amendment. (Because this revision makes only a semantic change 
to existing appendix N, EPA believes the change is consistent with 
section 6013(b)(4) of SAFETEA-LU, which provided that EPA continue to 
apply existing appendix N of part 50 (among others) until the effective 
date of rules implementing the exceptional event provisions in amended 
section 319 of the CAA.)\89\
---------------------------------------------------------------------------

    \89\ EPA will answer all comments raising substantive issues 
relating to the natural events policy when it finalizes the pending 
exceptional events proposal.
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B. Proposed Appendix P--Interpretation of the National Ambient Air 
Quality Standards for PM10	2.5

    The EPA proposed to add appendix P to 40 CFR Part 50 in order to 
add data handling procedures for the proposed 24-hour 
PM10-2.5 standard. Since the current 24-hour PM10 
standard is being retained and a PM10-2.5 standard is not 
being implemented, the proposed new appendix P (on interpreting the 
proposed 24-hour PM10-2.5 standard) is not being added.

C. Amendments to Appendix K--Interpretation of the National Ambient Air 
Quality Standards for PM10

    Because the Administrator has decided to retain the current 24-hour 
PM10 standard but to revoke and not replace the annual 
PM10 standard, some changes are required to appendix K to 40 
CFR Part 50 on interpreting the primary and secondary NAAQS for 
PM10. The modifications principally entailed simply removing 
the obsolete annual standard related sections. However some 
typographical corrections were also made to some of the remaining 
sections related to the 24-hour standard; a spelling error was 
corrected and certain equal signs (=) were changed to plus signs (+) in 
the illustrative examples found in section 3 of the appendix in order 
to correct obvious mistakes in arithmetic. For readers' convenience, 
EPA is reprinting the entire Appendix K in the rule section of this 
notice, but is not reopening or reconsidering any parts of the Appendix 
except those discussed above.

VI. Reference Methods for the Determination of Particulate Matter as 
PM10-2.5 and PM2.5

A. Appendix O to Part 50--Reference Method for Determination of Coarse 
Particulate Matter as PM10	2.5 in the Atmosphere

    The EPA proposed a new reference method (FRM) for measuring mass 
concentrations of coarse particles (PM10-2.5) in ambient air 
as a new Appendix O to 40 CFR part 50.71 FR 2703. Although this method 
can fulfill a variety of PM monitoring objectives, its primary purpose 
is to serve as the standard of comparison for determining the adequacy 
of alternative ``equivalent'' methods for use in lieu of the FRM. Id. 
at 2687-88. In conjunction with additional analysis, this method may be 
used to develop speciated data. The EPA expects to designate such 
alternative methods as equivalent methods (FEMs) under revised 
provisions of 40 CFR part 53, published elsewhere in today's Federal 
Register. The EPA is finalizing the FRM for PM10-2.5, even 
though a NAAQS for PM10-2.5 is not being adopted. An 
official FRM will be an important element in facilitating consistent 
research on PM10-2.5 air quality and health effects and in 
promoting the commercial development of FEMs. In a separate final rule 
amending 40 CFR part 58 elsewhere in today's Federal Register, the EPA 
is promulgating a requirement that States deploy about 60 FRM or FEM 
PM10-2.5 monitors as part of a new National Core (NCore) 
multi-pollutant monitoring stations. The EPA also plans to negotiate 
with some States for additional NCore stations which would include 
PM10-2.5 monitors.
    The PM10-2.5 reference method is a difference method 
based on separate, concurrent measurements of PM10 and 
PM2.5, with the PM10-2.5 measurement being the 
result of subtraction of the PM2.5 measurement from the 
corresponding PM10 measurement. The 24-hour integrated 
measurements are based on conventional, low-volume filter samples of 
particulate matter analyzed gravimetrically after a period of moisture 
and temperature equilibration. Although the component PM10 
and PM2.5 filter samples can be subsequently analyzed 
chemically, no actual, physically separated PM10-2.5 sample 
is produced by the method for chemical species analysis. The EPA 
anticipates that one or more alternative methods that do provide 
PM10-2.5 samples that are completely or nearly completely 
separated physically for species analysis (such as the dichotomous 
sampler method) will become available as an FEM.
    The substantial advantages of the method and the rationale for its 
selection as the FRM for PM10-2.5 are discussed in the 
proposal (71 FR 2687). In that discussion, EPA acknowledges that the 
method does not provide a direct measurement of PM10-2.5, 
has some significant shortcomings, and likely will not ideally meet all 
needs for monitoring PM10-2.5 in the ambient air. The EPA 
indicated that although the method is readily usable in routine 
monitoring networks, it is clearly less than optimally suited for such 
use. Instead, EPA expects that alternative FEMs that typically offer 
some substantial advantage or advantages over the FRM will become the 
principle methods deployed for routine monitoring. Further, EPA 
anticipates that self-contained, automated FEMs will become available 
to provide near real-time, hourly monitoring data availability and ease 
the monitoring burdens of monitoring agencies. Although the FRM will 
likely be used initially in monitoring applications because of its 
conventional nature and similarity to the widely used PM2.5 
FRM, ultimately its principle purpose will be as the standard of 
reference for determining the adequacy of alternative, candidate FEMs 
and for assessing the quality of PM10-2.5 monitoring data 
obtained in monitoring networks, particularly networks using 
alternative FEMs. The FRM may thus be used on a voluntary basis by 
states wishing to deploy PM10-2.5 monitors prior to the

[[Page 61213]]

January 1, 2011 deadline for operation of PM10-2.5 monitors 
at NCore multi-pollutant sites (a requirement of the final rule 
amending 40 CFR part 58, elsewhere in today's Federal Register), 
although many of the required monitors operating at NCore sites in 2011 
and beyond may be FEMs.
    After considering alternative methodologies and weighing the 
various pros and cons of other methods, as also discussed in the 
proposal preamble, the EPA concluded that the proposed method is the 
best method currently available to serve these purposes, while also 
being readily usable for many initial monitoring applications. The 
Ambient Air Monitoring and Methods Subcommittee of the Clean Air 
Scientific Advisory Committee (CASAC) concurs with this assessment and 
approach, recommending that EPA adopt the difference method as the FRM, 
but that it ultimately be used primarily as a benchmark for evaluating 
the performance of continuous as well as other direct-measuring filter-
based integrated methods (Henderson, 2005c).
    Of the relatively few comments received on the proposed FRM, most 
raised concern about some of the same shortcomings of the method that 
had already been considered by EPA in selecting the method (and by the 
CASAC in concurring with EPA's approach). No comments presented any 
issues that resulted in any changes to the method. Thus, the FRM is 
being promulgated today (in Appendix O), with the only change being 
deletion of the reference to national ambient air quality standards in 
section 1.1 of the method, since the EPA is not using 
PM10-2.5 as the indicator in the NAAQS addressing thoracic 
coarse particles.
    One comment raised concern about the relationship of the new 
PM10-2.5 FRM to the requirements of Section 6012 of the 
SAFETEA-LU, under which the EPA is to ``develop a Federal reference 
method to measure directly particles that are larger than 2.5 
micrometers in diameter without reliance on subtracting from coarse 
particle measurements those particles that are equal to or smaller than 
2.5 micrometers in diameter.'' As discussed in the proposal preamble at 
71 FR 2690, EPA believes that this FRM does not conflict with either 
the specific language or intent of the SAFETEA-LU Act. The new FRM, 
together with the additions to part 53 (published elsewhere in this 
Federal Register) that will allow designation of FEMs for monitoring 
PM10-2.5, will provide a strong incentive to stimulate the 
further commercial development and refinement of new or existing 
methods for PM10-2.5, most of which will not rely on 
subtraction of fine mode particle measurements from coarse mode 
particle measurements. Further, EPA is actively investigating the 
possibility that a dichotomous-based method might ultimately provide a 
more direct means of measuring the coarse fraction of PM10. 
Within the time frame prescribed by the SAFETEA-LU, it appears very 
likely that at least one such method will be shown to achieve an 
adequate level of performance and may therefore be identified and 
utilized as a ``reference method''. The terms of the SAFETEA-LU Act do 
not require that the Agency promulgate a non-difference method as 
either the sole FRM or as an alternative FRM as specifically defined in 
part 53. Until such a new, more direct method is demonstrated to be 
suitable and adequate and becomes commercially available, the 
difference-based FRM of Appendix O provides a reliable, proven 
measurement method which can be successfully implemented immediately. 
The CASAC agreed that none of the direct sampling methods is presently 
sufficiently reliable for use as an FRM, Henderson, 2005c, but that 
suitable direct measurement methods could be developed quickly enough 
to become approved as equivalent methods in a planned monitoring 
network.
    The salient technical aspects of the FRM are provided in the 
proposal preamble (71 FR 2690). The dual samplers specified in the FRM 
are essentially identical to the sampler specified in the 
PM2.5 FRM (40 CFR part 50, appendix L) except for removal of 
the PM2.5 WINS impactor particle separator from the sampler 
used for PM10. Operational procedures and most other aspects 
are also similar or identical to those for the PM2.5 FRM. 
One notable condition is that the PM10 sampler of the 
PM10-2.5 FRM must meet the higher standards of performance 
and manufacture of appendix L rather than the somewhat lesser 
requirements for conventional PM10 samplers in 40 CFR part 
50, appendix J. Thus, conventional PM10 FRM samplers will 
not be acceptable for use as part of a PM10-2.5 FRM sampler 
pair. But both the PM10 and PM2.5 component 
measurements obtained incidental to PM10-2.5 measurements 
would be valid as PM10 or PM2.5 measurements 
under the monitoring requirements of 40 CFR part 58, provided they are 
sited at the appropriate spatial scale. However, since such 
PM10 samplers meet higher standards of performance than 
conventional PM10 samplers, the measurements need to be 
differentiated from conventional PM10 measurements (e.g. by 
a descriptor such as PM10c). Also, conventional 
PM10 measurements are reported based on standard temperature 
and pressure, whereas PM10c measurements are reported based 
on actual local conditions of temperature and pressure.
    The EPA designation of specific, commercial candidate 
PM10-2.5 FRM samplers will be based on an application and on 
consideration in accordance with new or revised provisions of 40 CFR 
part 53, published elsewhere in this Federal Register. Since 
PM2.5 FRM samplers have been in use for several years and 
are readily available, EPA designation of PM10-2.5 FRM 
sampler models based on one or more currently available 
PM2.5 sampler models is expected to occur soon after 
promulgation. The two samplers of the PM10-2.5 FRM sampler 
pair would be required to be of the same make and model and matched 
design and fabrication so that they are essentially identical (except 
that one would not have a PM2.5 particle separator). The 
samplers may be of either single-filter or multiple-filter (sequential-
sample) design, as long as both are of the same type, design, and 
configuration. For a commercial sampler that has already been 
designated as a PM2.5 FRM, no further testing under part 53 
would be required for designation as a PM10-2.5 FRM, 
although the sampler manufacturer would have to submit a formal, brief 
application under part 53. Users may assemble their own 
PM10-2.5 sampler pair using existing PM2.5 
samplers of matched model or design by converting one of the samplers 
to a PM10c sampler, provided that the specific sampler pair 
has been previously designated by the EPA as a PM10-2.5 FRM 
under part 53.
    A PM2.5 sampler pair consisting of samplers that are 
slightly dissimilar or have some minor design or model variations (and 
one sampler is configured as a PM10c sampler) may be 
considered for designation by EPA as a Class I FEM under revised part 
53. An application for an FEM determination would need to be submitted 
under part 53, and some supplemental or special tests may be required. 
Also, a pairing of slightly dissimilar samplers that has not been 
designated by EPA as an FRM or Class I FEM may be considered for 
approved use in PM10-2.5 monitoring networks as a user-
modification of an FRM under section 2.8 of appendix C to 40 CFR part 
58.

[[Page 61214]]

B. Amendments to Appendix L--Reference Method for the Determination of 
Fine Particulate Matter (as PM2.5) in the Atmosphere

    In connection with the proposal of a new FRM for 
PM10-2.5, the EPA also proposed (71 FR 2691) minor technical 
changes to the FRM for PM2.5 (40 CFR Part 50, appendix L). 
EPA is adopting these changes as proposed. These changes are to provide 
improvements in the efficiency of the method in monitoring network 
operations without altering the method's performance.
    The most significant change is the addition of an alternative 
PM2.5 particle size separator, specifically, a very sharp 
cut cyclone (VSCCTM) manufactured by BGI Incorporated, 
Waltham, MA. FRM samplers now may be configured with either the 
original WINS impactor or the alternative cyclone separator, and 
existing FRM samplers may be retrofitted by users with the cyclone, if 
desired. Sampler users wishing to retrofit their samplers should 
contact the sampler manufacturer to obtain the correct BGI 
VSCCTM model along with the associated installation, 
operation, and maintenance instructions specific to the sampler model, 
and a new designated method label to be attached to the sampler. The 
seven sampler models configured with the BGI VSCCTM that 
have been designated as FEMs will be re-designated as reference 
methods, and owners of such sampler should contact the sampler 
manufacturer to receive a new reference method label for the sampler.
    Another change is substitution of an improved type of impactor oil 
for the original PM2.5 WINS particle size separator to 
correct an occasional cold-weather performance issue with the 
originally specified oil. Finally, minor increases in the time limits 
for sample retrieval and sample weighing were proposed, as were minor 
reductions in the sampler data output reporting requirements. 
Justifications for these changes are discussed in the proposal 
preamble. Of the very few comments received in connection with these 
proposed changes, all were supportive. Accordingly, the changes are 
adopted as proposed.

VII. Issues Related to Implementation of PM10 Standards

    Issues related to implementation of the NAAQS are not relevant to 
the Administrator's decisions regarding whether it is appropriate to 
set or revise a standard. For this reason, EPA has not addressed 
implementation-related issues in preceding sections, nor has it 
addressed public comments regarding implementation. The EPA identified 
issues regarding transition to or implementation of the standards 
promulgated in this rule in an advance notice of proposed rulemaking 
(ANPR) on Transition to New or Revised Particulate Matter National 
Ambient Air Quality Standards (71 FR 6718-6729, February 9, 2006). In 
the ANPR, EPA solicited comment on a wide range of issues related to 
both the fine and coarse particle NAAQS, including the schedules for 
implementation of these standards and the requirements that would be 
applicable if any PM NAAQS were revoked. The public comment period for 
the ANPR ended on July 10, 2006. The EPA is currently reviewing the 
public comments received. In the near future, EPA intends to address, 
as necessary, issues such as designations, conformity, and new source 
review, related to implementation of today's final rule. In this 
section, EPA highlights a few issues that may arise as an immediate 
consequence of today's final decision to retain the 24-hour 
PM10 standards but revoke the annual PM10 
standards, and restates existing policies and practices to address 
several concerns raised by commenters.

A. Summary of Comments Received on Transition

    Many commenters, particularly State and local air pollution control 
agencies and Tribes, but also environmental and public health groups, 
voiced strong concerns about EPA's proposal to revoke current annual 
PM10 standards everywhere upon promulgation of this final 
rule, and to revoke, upon finalization of a primary 24-hour standard 
for PM10-2.5, the current 24-hour PM10 standard 
everywhere except in 15 large urbanized areas (with population greater 
than 100,000) that have at least one monitor violating the 24-hour 
PM10 standard based on the most recent three years of air 
quality data. For these few areas, EPA proposed to retain the 24-hour 
PM10 standard until designations were completed under a 
final 24-hour PM10-2.5 standard. While a few local 
government commenters recommended that one or another of the 15 areas 
be dropped from this list--i.e., recommended that the 24-hour 
PM10 standard should be retained in fewer locations--most 
commenters expressing views on transition suggested that EPA was being 
too hasty in dismantling existing PM10 protections. Pointing 
to long delays in the implementation timeline for the 1997 
PM2.5 standards due to litigation, such that designations 
were not completed for eight years after promulgation of the final 
rule, these commenters suggested that the 24-hour PM10 
standard should remain in place everywhere until designations were 
complete under the 24-hour PM10-2.5 standard, or even until 
PM10-2.5 SIPs had been submitted by States. Some Tribal, 
State and local commenters suggested that the PM10 standard 
should be retained permanently in all areas where the 
PM10-2.5 standard did not apply by virtue of the monitoring 
requirements, which limited NAAQS comparable monitors to sites that met 
the five-point site suitability test outlined in the monitoring rule. 
Other commenters maintained that EPA has no authority to revoke the 
PM10 standards or the specific pollution controls mandated 
in Title I Subpart 4 for PM10 nonattainment areas.\90\
---------------------------------------------------------------------------

    \90\ These comments and EPA's responses to the issues raised by 
commenters are discussed in greater detail in the Response to 
Comments document.
---------------------------------------------------------------------------

    The EPA notes that the Administrator's decision to retain the 
current 24-hour PM10 standard alleviates these concerns. 
Because the 24-hour PM10 standard is generally controlling, 
as described above in section III.D.2, retention of this standard 
ensures the continuation of existing public health protections. The EPA 
further believes that it has the legal authority to revoke the annual 
PM10 standard, and addresses this issue in detail in the 
Response to Comments document.

B. Impact of Decision on PM10 Designations

    The EPA notes that because it is retaining the current 24-hour 
PM10 standards, new nonattainment designations for 
PM10 will not be required under the provisions of the Clean 
Air Act. As established in Section 107(d)(1) of the Act, the only time 
EPA is obligated to designate areas as attainment or nonattainment is 
after it promulgates or revises a NAAQS. Under an existing standard, 
all redesignations are at the Administrator's discretion: EPA has no 
legal obligation to redesignate an area even if a monitor should 
register a violation of that standard (see CAA Section 107(d)(3)). 
Thus, this final decision does not affect existing PM10 
nonattainment designations. This is consistent with past practice. For 
example, when EPA decided not to revise the ozone standards in 1993 or 
the SO2 standards in 1996, it did not revisit prior 
designations or designate any new areas as nonattainment. The EPA does 
regard air quality violations seriously, and does expect States to take 
actions to reduce

[[Page 61215]]

air quality to healthy levels in any areas that are experiencing 
violations. However, EPA recognizes that there are other ways to 
address such violations besides redesignating an area as nonattainment. 
For example, EPA can work directly with a State and nearby industries 
to take appropriate actions to reduce emissions that are contributing 
to the violation. The EPA has worked in this way with States in the 
past. Of course, States may request redesignation of an area, either 
from nonattainment to attainment, or from attainment to nonattainment, 
based on the most recent air quality data available, if they choose to 
do so. In addition, both transportation and general conformity will 
continue to apply to all PM10 nonattainment and maintenance 
areas since no designations are changing. However, because EPA is 
revoking the annual PM10 standard in this final rule, after 
the effective date of this rule conformity determinations in 
PM10 areas will only be required for the 24-hour 
PM10 standard; conformity to the annual PM10 
standard will no longer be required. The EPA will address specific 
conformity issues related to the revocation of the annual 
PM10 standard either in future guidance or in another public 
document. The EPA also notes that PSD increments and baseline years 
will not be affected by this decision.
    The EPA is retaining the current 24-hour PM10 standards 
and revoking the annual PM10 standards. Today's rule does 
not change any existing guidance related to the PM10 NAAQS 
as it applies to the 24-hour PM10 standards, and to the 
extent that modifications to the existing guidance are needed in 
response to today's action, EPA will make such modifications in the 
near future.
    As described in the revisions to Part 53/58 appearing elsewhere in 
today's Federal Register, EPA believes a reduction in the size of the 
existing monitoring networks for certain pollutants, including 
PM10, for which the large majority of monitors record no 
NAAQS violations, is appropriate as a way to free up resources for 
higher priority monitoring objectives. The current minimum 
PM10 network requirements are based on the population of a 
metropolitan statistical area (MSA) and its historical PM10 
air quality. This focus on larger urban areas is consistent with EPA's 
belief that it is appropriate to target an indicator for thoracic 
coarse particles toward urban and industrial areas, where the ambient 
mix of thoracic coarse particles is dominated by emissions from 
particular types of sources. See sections III.C.2 and III.C.3 above. To 
the extent that States and Tribes are considering reducing the total 
number of PM10 monitors deployed, EPA believes, consistent 
with the basis for retaining the 24-hour PM10 standard, that 
priority should be given to maintaining monitors sited in urban and 
industrial areas.
    In addition, if States and Tribes are considering deploying new 
PM10 monitors, EPA recommends, again consistent with the 
basis for retaining the 24-hour PM10 standard, that those 
monitors be placed in areas where there are urban and/or industrial 
sources of thoracic coarse particles. Furthermore, consistent with the 
monitors used in studies that informed the Administrator's decision on 
the level of the standard (see section III.D above), EPA recommends 
that any new PM10 monitors be placed in locations that are 
reflective of community exposures at middle and neighborhood scales of 
representation, and not in source-oriented hotspots.
    As summarized briefly above in section III.E and described in 
detail in section V.E.1 of the monitoring rule published elsewhere in 
today's Federal Register, EPA is also establishing requirements for a 
new multi-pollutant monitoring network that will include approximately 
75 PM10-2.5 monitors that will speciate according to the 
composition as well as size of the particles. These speciated 
PM10-2.5 monitors are a critical part of EPA's research 
program on coarse particles, and will be sited in both urban and rural 
locations. It is EPA's expectation that these monitors will help 
alleviate the current deficit of information regarding the public 
health impacts of PM10-2.5 mixes in different locations.\91\
---------------------------------------------------------------------------

    \91\ In addition, EPA notes that the Agency's National Center 
for Environmental Research recently issued a Request for Proposals 
on ``Sources, Composition, and Health Effects of Coarse Particulate 
Matter'' which is designed to (1) improve understanding of the type 
and severity of health outcomes associated with exposure to 
PM10-2.5; (2) improve understanding of subpopulations 
that may be especially sensitive to PM10-2.5 exposures 
including minority populations, highly exposed groups, and other 
susceptible groups; (3) characterize and compare the influence of 
mass, composition, source characteristics and exposure estimates in 
different locations and differences in health outcomes, including 
comparisons in rural and urban areas; and (4) characterize the 
composition and variability of PM10-2.5 in towns, cities 
or metropolitan areas, including comparisons of rural and urban 
areas.
---------------------------------------------------------------------------

C. Impact of Decision on State Implementation Plans (SIPs) and Control 
Obligations

    The EPA's decision today to retain the PM10 NAAQS does 
not establish new legal obligations beyond those that already exist. 
Specifically, this final rule does not obligate States to revise SIPs 
or to create new obligations to control particular sources. In response 
to comments regarding potential impacts of any coarse particle standard 
on agricultural and mining sources, EPA notes that the NAAQS do not 
create emissions control obligations for individual sources or groups 
of sources. In this particular case, even if an individual source were 
shown to contribute to an exceedance of the 24-hour PM10 
standard, this would not necessarily result in regulation of that 
source. Decisions about which sources to control are generally made by 
the State in the context of developing or revising SIPs. Given that the 
available evidence regarding adverse health effects associated with 
exposure to thoracic coarse particles is strongest with respect to 
urban and industrial ambient mixes of those particles, EPA encourages 
States to focus control programs on urban and industrial sources to the 
extent that those sources are contributing to air quality violations. 
This would help to ensure that resources expended on implementing the 
24-hour PM10 standard realize the maximum public health and 
welfare benefits.
    With regard to emissions of thoracic coarse particles from 
agricultural sources, EPA recognizes that the United States Department 
of Agriculture (USDA) has been working with the agricultural community 
to develop conservation systems and activities to control coarse 
particle emissions. Based on current ambient monitoring information, 
these USDA-approved conservation systems and activities have proven to 
be effective in controlling these emissions in areas where coarse 
particles emitted from agricultural activities have been identified as 
a contributor to violation of the NAAQS. The EPA concludes that where 
USDA-approved conservation systems and activities have been 
implemented, these systems and activities have satisfied the Agency's 
reasonably available control measure and best available control measure 
requirements. The EPA believes that in the future, when properly 
implemented, USDA-approved conservation systems and activities should 
satisfy the requirements for reasonably available control measures or 
best available control measures. The EPA will work with States to 
identify appropriate measures to meet their RACM or BACM requirements, 
including site-specific conservation systems and activities. The EPA 
will continue to work with USDA to prioritize the development of new 
conservation systems and activities;

[[Page 61216]]

demonstrate and improve, where necessary, the control efficiencies of 
existing conservation systems and activities; and ensure that 
appropriate criteria are used for identifying the most effective 
application of conservation systems and activities.
    The EPA does not construe the Clean Air Act (CAA) to require that 
the Agency make an independent determination as to whether a PSD 
increment is violated in any specific State or Tribal reservation. The 
EPA has the discretion to inquire into these matters and call for 
revisions to a State's SIP if an EPA investigation concluded with EPA 
finding that the PSD increment is being exceeded. The EPA's regulations 
at 40 CFR 51.166(a)(3) directs a state to make revisions to its SIP if 
EPA or a State finds such an exceedance. However, this regulation does 
not require that EPA conduct its own investigation and make such a 
finding in all cases where a State has completed a periodic review and 
submitted its findings to EPA. Oversight of this nature is a matter 
within EPA's discretion. Likewise, section 110(k)(5) of the Clean Air 
Act does not require that EPA periodically investigate and determine 
whether a SIP is sufficient to protect the PSD increments. The EPA has 
the discretion to decide when it is appropriate to exercise its 
oversight authority and inquire into these issues in a specific State 
or Tribal reservation. When EPA exercises this discretion and finds an 
exceedance of the increments or another SIP deficiency, EPA is then 
required to issue a SIP call under section 110(k)(5) of the CAA. 
However, the CAA affords EPA discretion on whether to make a 
determination that a state SIP is deficient. See, New York Public 
Interest Research Group v. Whitman, 321 F.3d 316, 331 (2d Cir. 2003) 
(considering analogous provision of the CAA addressing EPA oversight of 
state Title V operating permit programs).

D. Consideration of Fugitive Emissions for New Source Review (NSR) 
Purposes

    Under the current NSR regulations, for purposes of determining 
whether a stationary source qualifies as a major stationary source, 
that source must include fugitive emissions in calculating the total 
amount of a pollutant directly emitted, or the potential to emit that 
pollutant, only if the source is associated with a source category 
listed by the Administrator pursuant to notice and comment rulemaking 
in accordance with Section 302(j) of the Clean Air Act (CAA). 
Agricultural and mining sources are generally not among those listed by 
the Administrator. Therefore, fugitive emissions from sources in these 
categories are generally not included in making major source 
determinations. However, the current NSR regulations require that once 
any source qualifies as a major stationary source, that source must 
count all fugitive emissions toward determining whether an emissions 
increase results in a major modification of that source regardless of 
whether the source is associated with a source category listed by the 
Administrator. On July 11, 2003, we received a petition for 
reconsideration of the current NSR regulations relating to whether 
fugitive emissions must be counted for purposes of determining whether 
a major modification occurs. In January 2004, we agreed to reconsider 
this issue, and we expect to propose changes to the existing 
regulations in the near future.

E. Handling of PM10 Exceedances Due to Exceptional Events

    The EPA recognizes that PM10 exceedances may be caused, 
in whole or in part, by exceptional events, including natural events 
such as windstorms. In some of these instances, the PM10 
exceedance(s) may also be associated with anthropogenic emissions that 
contribute to total PM10 concentrations. Under EPA's March 
2006 Proposed Rule on the Treatment of Data Influenced by Exceptional 
Events (71 FR 12592-12610), and consistent with historical practice, an 
exceedance may be treated as an exceptional event even though 
anthropogenic sources such as agriculture and mining emissions 
contribute to the exceedance. (EPA's Exceptional Events Rule will be 
finalized in March 2007 and will discuss this issue in more detail.)

VIII. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review

    Under section 3(f)(1) of Executive Order (EO) 12866 (58 FR 51735, 
October 4, 1993), this action is an ``economically significant 
regulatory action'' because it is likely to have an annual effect on 
the economy of $100 million or more. Accordingly, EPA submitted this 
action to the Office of Management and Budget (OMB) for review under EO 
12866 and any changes made in response to OMB recommendations have been 
documented in the docket for this action (Docket ID No. EPA-HQ-OAR-
2001-0017).
    In addition, EPA prepared a regulatory impact analysis (RIA) of the 
potential costs and benefits associated with this action, entitled 
``Regulatory Impact Analysis for Particulate Matter National Ambient 
Air Quality Standards'' (September 2006). The RIA estimates the 
nationwide costs and monetized human health and welfare benefits of 
attaining two alternatives to the current suite of PM2.5 
NAAQS (15 [mu]g/m3 annual, 65 [mu]g/m3 daily). 
Specifically, the RIA compares the current standards to the proposed 
alternative of 15 [mu]g/m3 annual, 35 [mu]g/m3 
daily and a tighter alternative of 14 [mu]g/m3 annual, 35 
[mu]g/m3 daily. The RIA contains illustrative analyses that 
consider a limited number of emissions control scenarios that States 
and Regional Planning Organizations might implement to achieve the 1997 
PM2.5 NAAQS and these alternative PM2.5 NAAQS. It 
calculates the incremental costs that might be incurred between the 
base year of 2015, which is the year by which States must all be in 
attainment with the 1997 PM2.5 standards (15 [mu]g/
m3 annual, 65 [mu]g/m3 daily), and 2020, which is 
the final date by which States would implement controls to attain the 
revised PM2.5 standards.
    As discussed above in section I.B, the Clean Air Act 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

    This action does not impose an information collection burden under 
the provisions of the Paperwork Reduction Act, 44 U.S.C. 3501 et seq. 
There are no information collection requirements directly associated 
with revisions to a NAAQS under section 109 of the CAA.
    Burden means the total time, effort, or financial resources 
expended by persons to generate, maintain, retain, or disclose or 
provide information to or for a Federal agency. This includes the time 
needed to review instructions; develop, acquire, install, and utilize 
technology and systems for the purposes of collecting, validating, and 
verifying information, processing and maintaining information, and 
disclosing and providing information; adjust the existing ways to 
comply with any previously applicable instructions and requirements; 
train personnel to be able to respond to a collection of information; 
search data sources; complete and review the collection of information; 
and transmit or otherwise disclose the information.

[[Page 61217]]

    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

    The EPA has determined that it is not necessary to prepare a 
regulatory flexibility analysis in connection with this final rule. For 
purposes of assessing the impacts of today's rule on small entities, 
small entity is defined as: (1) A small business that is a small 
industrial entity as defined by the Small Business Administration's 
(SBA) regulations at 13 CFR 121.201; (2) a small governmental 
jurisdiction that is a government of a city, county, town, school 
district or special district with a population of less than 50,000; and 
(3) a small organization that is any not-for-profit enterprise which is 
independently owned and operated and is not dominant in its field.
    After considering the economic impacts of today's final rule on 
small entities, EPA has concluded that this action will not have a 
significant economic impact on a substantial number of small entities. 
This rule will not impose any requirements on small entities. Rather, 
this rule establishes national standards for allowable concentrations 
of particulate matter in ambient air as required by section 109 of the 
CAA. See also ATA I at 1044-45 (NAAQS do not have significant impacts 
upon small entities because NAAQS themselves impose no regulations upon 
small entities).

D. Unfunded Mandates Reform Act

    Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), Public 
Law 104-4, establishes requirements for Federal agencies to assess the 
effects of their regulatory actions on State, local, and Tribal 
governments and the private sector. Under section 202 of the UMRA, EPA 
generally must prepare a written statement, including a cost-benefit 
analysis, for proposed and final rules with ``Federal mandates'' that 
may result in expenditures to State, local, and Tribal governments, in 
the aggregate, or to the private sector, of $100 million or more in any 
1 year. Before promulgating an EPA rule for which a written statement 
is needed, section 205 of the UMRA generally requires EPA to identify 
and consider a reasonable number of regulatory alternatives and adopt 
the least costly, most cost-effective or least burdensome alternative 
that achieves the objectives of the rule. The provisions of section 205 
do not apply when they are inconsistent with applicable law. Moreover, 
section 205 allows EPA to adopt an alternative other than the least 
costly, most cost-effective or least burdensome alternative if the 
Administrator publishes with the final rule an explanation why that 
alternative was not adopted. Before EPA establishes any regulatory 
requirements that may significantly or uniquely affect small 
governments, including Tribal governments, it must have developed under 
section 203 of the UMRA a small government agency plan. The plan must 
provide for notifying potentially affected small governments, enabling 
officials of affected small governments to have meaningful and timely 
input in the development of EPA regulatory proposals with significant 
Federal intergovernmental mandates, and informing, educating, and 
advising small governments on compliance with the regulatory 
requirements.
    Today's final rule contains no Federal mandates (under the 
regulatory provisions of Title II of the UMRA) for State, local, or 
Tribal governments or the private sector. The rule imposes no new 
expenditure or enforceable duty on any State, local or Tribal 
governments or the private sector, and EPA has determined that this 
rule contains no regulatory requirements that might significantly or 
uniquely affect small governments. Furthermore, as indicated 
previously, in setting a NAAQS 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 also ATA I at 1043 
(noting that because EPA is precluded from considering costs of 
implementation in establishing NAAQS, preparation of a Regulatory 
Impact Analysis pursuant to the Unfunded Mandates Reform Act would not 
furnish any information which the court could consider in reviewing the 
NAAQS). Accordingly, EPA has determined that the provisions of sections 
202, 203, and 205 of the UMRA do not apply to this final decision. The 
EPA acknowledges, however, that any corresponding revisions to 
associated SIP requirements and air quality surveillance requirements, 
40 CFR part 51 and 40 CFR part 58, respectively, might result in such 
effects. Accordingly, EPA has addressed unfunded mandates in the notice 
that announces the revisions to 40 CFR part 58, and will, as 
appropriate, address unfunded mandates when it proposes any revisions 
to 40 CFR part 51.

E. Executive Order 13132: Federalism

    Executive Order 13132, entitled ``Federalism'' (64 FR 43255, August 
10, 1999), requires EPA to develop an accountable process to ensure 
``meaningful and timely input by State and local officials in the 
development of regulatory policies that have federalism implications.'' 
``Policies that have federalism implications'' is defined in the 
Executive Order to include regulations that 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.''
    At the time of proposal, EPA concluded that the proposed rule would 
not have federalism implications. The EPA stated that the proposed rule 
would 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, as specified in Executive Order 13132. However, EPA 
recognized that States would have a substantial interest in this rule 
and any corresponding revisions to associated SIP requirements and air 
quality surveillance requirements, 40 CFR part 51 and 40 CFR part 58, 
respectively. Therefore, in the spirit of Executive Order 13132, and 
consistent with EPA policy to promote communications between EPA and 
State and local governments, EPA specifically solicited comment on the 
rule from State and local officials at the time of proposal.
    One commenter who opposed EPA's proposed decision on the standards 
for thoracic coarse particles stated that the decision violated E.O. 
13132. The commenter argued that EPA's proposal to replace the 
PM10 standards with a new 24-hour PM10-2.5 
standard based on a qualified indicator would substantially impact CAA 
section 107 which establishes that the States have primary 
responsibility for implementation of the NAAQS. Specifically, the 
commenter stated that the proposed rule language establishing that 
``agricultural sources, mining sources, and other similar sources of 
crustal material shall not be subject to control in meeting this 
standard'' was a clear infringement upon States' authority with regard 
to implementation of the NAAQS. The EPA notes that in light of the 
final decision to retain the PM10 indicator, and the 24-hour 
PM10 NAAQS, the concern voiced by this commenter is no 
longer relevant. The final rule does not exclude any sources

[[Page 61218]]

from control under the 24-hour PM10 standard.
    Therefore, EPA concludes that this final rule 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, as specified in Executive Order 13132. 
The rule does not alter the relationship between the Federal government 
and the States regarding the establishment and implementation of air 
quality improvement programs as codified in the CAA. Under section 109 
of the CAA, EPA is mandated to establish NAAQS; however, CAA section 
116 preserves the rights of States to establish more stringent 
requirements if deemed necessary by a State. Furthermore, this rule 
does not impact CAA section 107 which establishes that the States have 
primary responsibility for implementation of the NAAQS. Finally, as 
noted above in section E on UMRA, this rule does not impose significant 
costs on State, local, or Tribal governments or the private sector. 
Thus, Executive Order 13132 does not apply to this rule.

F. Executive Order 13175: Consultation and Coordination With Indian 
Tribal Governments

    Executive Order 13175, entitled ``Consultation and Coordination 
with Indian Tribal Governments'' (65 FR 67249, November 9, 2000), 
requires EPA to develop an accountable process to ensure ``meaningful 
and timely input by tribal officials in the development of regulatory 
policies that have tribal implications.'' This rule concerns the 
establishment of PM NAAQS. The Tribal Authority Rule gives Tribes the 
opportunity to develop and implement CAA programs such as the PM NAAQS, 
but it leaves to the discretion of the Tribe whether to develop these 
programs and which programs, or appropriate elements of a program, they 
will adopt.
    Although EPA determined at the time of proposal that Executive 
Order 13175 did not apply to this rule, EPA contacted tribal 
environmental professionals during the development of this rule. The 
EPA staff participated in the regularly scheduled Tribal Air call 
sponsored by the National Tribal Air Association during the summer and 
fall of 2005 as the proposal was under development, as well as the call 
in the spring of 2006 during the public comment period on the proposed 
rule. The EPA sent individual letters to all federally recognized 
Tribes within the lower 48 states and Alaska to give Tribal leaders the 
opportunity for consultation, and EPA staff also participated in Tribal 
public meetings, such as the National Tribal Forum meeting in April 
2006, where Tribes discussed their concerns regarding the proposed 
rule. Furthermore, the Administrator discussed the proposed PM NAAQS 
with members of the National Tribal Caucus and with leaders of 
individual Tribes during the spring and summer of 2006, in advance of 
his final decision.
    During the course of these meetings and in written comments 
submitted to the Agency, Tribal commenters expressed significant 
concerns about the implications of the proposed rule for Tribes. In 
particular, Tribes strongly opposed the proposed qualified 
PM10-2.5 indicator and the proposed monitor site-suitability 
requirements, especially the requirement that monitors used for 
comparison with the NAAQS be located within urbanized areas with a 
minimum population of 100,000. Tribal commenters pointed out that this 
would virtually exclude Tribes from applying the PM10-2.5 
standards because very few Tribal sites would meet this criterion. 
Tribes stated that EPA had violated its Trust Responsibility to Tribes 
in three ways. First, the commenters claimed that EPA had failed to 
engage in meaningful consultation with Tribal leaders regarding the 
proposed qualified PM10-2.5 indicator and other aspects of 
the proposed rule. Second, commenters claimed that the proposed 24-hour 
PM10-2.5 standard would have serious adverse impacts on the 
existing level of health protection for Tribes. Third, Tribal 
commenters objected to the proposed exclusion of ``agricultural 
sources, mining sources, and other similar sources of crustal 
material'' from the proposed PM10-2.5 indicator; like 
States, Tribes felt this provision was illegal and Tribal commenters 
argued this violated Tribal sovereignty. The EPA notes that its final 
decision to retain the current 24-hour PM10 standard, for 
the reasons noted above in Section III, without any qualifications or 
changes to the monitor siting requirements, effectively resolves the 
concerns raised by these commenters.
    EPA has determined that this final rule 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, since Tribes 
are not obligated to adopt or implement any NAAQS. Thus, Executive 
Order 13175 does not apply to this rule.

G. Executive Order 13045: Protection of Children From Environmental 
Health & Safety Risks

    Executive Order 13045, ``Protection of Children from Environmental 
Health Risks and Safety Risks'' (62 FR 19885, April 23, 1997) applies 
to any rule that: (1) Is determined to be ``economically significant'' 
as defined under Executive Order 12866, and (2) concerns an 
environmental health or safety risk that EPA has reason to believe may 
have a disproportionate effect on children. If the regulatory action 
meets both criteria, the Agency must evaluate the environmental health 
or safety effects of the rule on children, and explain why the 
regulation is preferable to other potentially effective and reasonably 
feasible alternatives considered by the Agency.
    This rule is subject to Executive Order 13045 because it is an 
economically significant regulatory action as defined by Executive 
Order 12866, and we believe that the environmental health risk 
addressed by this action may have a disproportionate effect on 
children. The NAAQS constitute uniform, national standards for PM 
pollution; 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, along with other sensitive population 
subgroups such as the elderly and people with existing heart or lung 
disease, are potentially susceptible to health effects resulting from 
PM exposure. Because children are considered a potentially susceptible 
population, we have carefully evaluated the environmental health 
effects of exposure to PM pollution among children. These effects and 
the size of the population affected are summarized in section 9.2.4 of 
the Criteria Document and section 3.5 of the Staff Paper, and the 
results of our evaluation of the effect of PM pollution on children are 
discussed in sections II and III of this preamble.

H. Executive Order 13211: Actions That Significantly Affect Energy 
Supply, Distribution or Use

    This rule is not a ``significant energy action'' as defined in 
Executive Order 13211, ``Actions Concerning Regulations That 
Significantly Affect Energy Supply, Distribution, or Use'' (66 FR 28355 
(May 22, 2001)) 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 NAAQS for PM. The rule does not

[[Page 61219]]

prescribe specific pollution control strategies by which these ambient 
standards will be met. Such strategies will be developed by States on a 
case-by-case basis, and EPA cannot predict whether the control options 
selected by States will include regulations on energy suppliers, 
distributors, or users. Thus, 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 Advancement Act

    Section 12(d) of the National Technology Transfer Advancement Act 
of 1995 (NTTAA), Public Law 104-113, Section 12(d) (15 U.S.C. 272 note) 
directs EPA to use voluntary consensus standards in its regulatory 
activities unless to do so would be inconsistent with applicable law or 
otherwise impractical. Voluntary consensus standards are technical 
standards (e.g., materials specifications, test methods, sampling 
procedures, and business practices) that are developed or adopted by 
voluntary consensus standards bodies. The NTTAA directs EPA to provide 
Congress, through OMB, explanations when the Agency decides not to use 
available and applicable voluntary consensus standards.
    The final rule establishes requirements for environmental 
monitoring and measurement. Specifically, it establishes the FRM for 
PM10-2.5 measurement (and slightly amends the FRM for 
PM2.5). The FRM is the benchmark against which all ambient 
monitoring methods are measured. While the FRM is not a voluntary 
consensus standard, the equivalency criteria established in 40 CFR part 
53 do allow for the utilization of voluntary consensus standards if 
they meet the specified performance criteria.
    To the extent feasible, EPA employs a Performance-Based Measurement 
System (PBMS), which does not require the use of specific, prescribed 
analytic methods. The PBMS is defined as a set of processes wherein the 
data quality needs, mandates or limitations of a program or project are 
specified, and serve as criteria for selecting appropriate methods to 
meet those needs in a cost-effective manner. It 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. Though the FRM requirements utilize performance standards 
for some aspects of monitor design, multiple performance standards 
defined for many combinations of PM type, concentration, and 
environmental conditions would be required to be sure that monitors 
certified to purely performance-based standards actually performed 
similarly in the field, which would in turn require extensive testing 
of each candidate monitor design. Therefore, it is not practically 
possible to fully define the FRM in performance terms. Nevertheless, 
our approach in the past has resulted in multiple brands of monitors 
qualifying as FRM for PM, and we expect this to continue. Also, the FRM 
described in this final rule and the equivalency criteria contained in 
the revisions to 40 CFR part 53 do constitute performance based 
criteria for the instruments that will actually be deployed for 
monitoring PM10-2.5. Therefore, for most of the measurements 
that will be made and most of the measurement systems that make them, 
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

    Executive Order 12898, ``Federal Actions to Address Environmental 
Justice in Minority Populations and Low-Income Populations,'' requires 
Federal agencies to consider the impact of programs, policies, and 
activities on minority populations and low-income populations. 
According to EPA guidance, agencies are to assess whether minority or 
low-income populations face a risk or a rate of exposure to hazards 
that are significant and that ``appreciably exceeds or is likely to 
appreciably exceed the risk or rate to the general population or to the 
appropriate comparison group'' (EPA, 1998).
    In accordance with Executive Order 12898, the Agency has considered 
whether these decisions may have disproportionate negative impacts on 
minority or low-income populations. This rule establishes uniform, 
national ambient air quality standards for particulate matter, and is 
not expected to have disproportionate negative impacts on minority or 
low income populations. The EPA notes that some commenters expressed 
concerns that EPA had failed to adequately assess the environmental 
justice implications of its proposed decisions, and that the proposed 
revisions to both the fine particle and coarse particle standards would 
violate the principles of environmental justice. In particular, 
numerous commenters criticized the proposed qualified 
PM10-2.5 indicator, arguing that the exclusive urban focus 
of the indicator failed to protect large segments of the U.S. 
population (including Tribes and lower-income rural populations). The 
EPA believes that the final decision to retain the current nationally 
applicable 24-hour PM10 standard adequately addresses the 
concerns raised by these commenters, as discussed above in section III.
    Further, some commenters were concerned that the proposed 
PM2.5 standards would permit the continuation of 
disproportionate adverse health effects on minority and low-income 
populations because those populations are concentrated in urban areas 
where exposures are higher and are generally more susceptible (given 
lack of access to health care and prevalence of chronic conditions such 
as asthma). The EPA believes that the implications of the newly 
strengthened suite of PM2.5 standards will reduce health 
risks precisely in the areas subject to the highest fine particle 
concentrations. Furthermore, the PM2.5 NAAQS established in 
today's final rule are nationally uniform standards which in the 
Administrator's judgment protect public health with an adequate margin 
of safety. In making this determination, the Administrator expressly 
considered the available information regarding health effects among 
vulnerable and susceptible populations, such as those with preexisting 
conditions. Thus it remains EPA's conclusion that this rule is not 
expected to have disproportionate negative impacts on minority or low 
income populations.

K. Congressional Review Act

    The Congressional Review Act, 5 U.S.C. 801 et seq., as added by the 
Small Business Regulatory Enforcement Fairness Act of 1996, generally 
provides that before a rule may take effect, the agency promulgating 
the rule must submit a rule report, which includes a copy of the rule, 
to each House of the Congress and to the Comptroller General of the 
United States. EPA submitted a report containing this rule and other 
required information to the U.S. Senate, the U.S. House of 
Representatives, and the Comptroller General of the United States prior 
to publication of the rule in the Federal Register. A major rule cannot 
take effect until 60 days after it is published in the Federal 
Register. This action is a ``major rule'' as defined by 5 U.S.C. 
804(2). This rule will be effective December 18, 2006.

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List of Subjects in 40 CFR Part 50

    Environmental protection, Air pollution control, Carbon monoxide, 
Lead, Nitrogen dioxide, Ozone, Particulate matter, Sulfur oxides.

    Dated: September 21, 2006.
Stephen L. Johnson,
Administrator.


0
For the reasons set out in the preamble, title 40, chapter I 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. Section 50.3 is revised to read as follows:


Sec.  50.3  Reference conditions.

    All measurements of air quality that are expressed as mass per unit 
volume (e.g., micrograms per cubic meter) other than for the 
particulate matter (PM2.5) standards contained in Sec. Sec.  
50.7 and 50.13 shall be corrected to a reference temperature of 25 
(deg) C and a reference pressure of 760 millimeters of mercury (1,013.2 
millibars). Measurements of PM2.5 for purposes of comparison 
to the standards contained in Sec. Sec.  50.7 and 50.13 shall be 
reported based on actual ambient air volume measured at the actual 
ambient temperature and pressure at the monitoring site during the 
measurement period.


Sec.  50.6  [Amended]

0
3. Section 50.6 is amended by removing and reserving paragraph (b).

0
4. A new Sec.  50.13 is added to read as follows:


Sec.  50.13  National primary and secondary ambient air quality 
standards for PM2.5.

    (a) The national primary and secondary ambient air quality 
standards for particulate matter are 15.0 micrograms per cubic meter 
([mu]g/m\3\) annual arithmetic mean concentration, and 35 [mu]g/m\3\ 
24-hour average concentration measured in the ambient air as 
PM2.5 (particles with an aerodynamic diameter less than or 
equal to a nominal 2.5 micrometers) by either:
    (1) A reference method based on appendix L of this part and 
designated in accordance with part 53 of this chapter; or
    (2) An equivalent method designated in accordance with part 53 of 
this chapter.
    (b) The annual primary and secondary PM2.5 standards are 
met when the annual arithmetic mean concentration, as determined in 
accordance with appendix N of this part, is less than or equal to 15.0 
[mu]g/m3.
    (c) The 24-hour primary and secondary PM2.5 standards 
are met when the 98th percentile 24-hour concentration, as determined 
in accordance with appendix N of this part, is less than or equal to 35 
[mu]g/m3.

0
5. Appendix K to Part 50 is revised to read as follows:

Appendix K to Part 50--Interpretation of the National Ambient Air 
Quality Standards for Particulate Matter

1.0 General

    (a) This appendix explains the computations necessary for 
analyzing particulate matter data to determine attainment of the 24-
hour standards specified in 40 CFR 50.6. For the primary and 
secondary standards, particulate matter is measured in the ambient 
air as PM10 (particles with an aerodynamic diameter less 
than or equal to a nominal 10 micrometers) by a reference method 
based on appendix J of this part and designated in accordance with 
part 53 of this chapter, or by an equivalent method designated in 
accordance with part 53 of this chapter. The required frequency of 
measurements is specified in part 58 of this chapter.
    (b) The terms used in this appendix are defined as follows:
    Average refers to the arithmetic mean of the estimated number of 
exceedances per year, as per Section 3.1.
    Daily value for PM10 refers to the 24-hour average 
concentration of PM10 calculated or measured from 
midnight to midnight (local time).
    Exceedance means a daily value that is above the level of the 
24-hour standard after rounding to the nearest 10 [mu]g/
m3 (i.e., values ending in 5 or greater are to be rounded 
up).
    Expected annual value is the number approached when the annual 
values from an increasing number of years are averaged, in the 
absence of long-term trends in emissions or meteorological 
conditions.
    Year refers to a calendar year.
    (c) Although the discussion in this appendix focuses on 
monitored data, the same principles apply to modeling data, subject 
to EPA modeling guidelines.

2.0 Attainment Determinations

2.1 24-Hour Primary and Secondary Standards

    (a) Under 40 CFR 50.6(a) the 24-hour primary and secondary 
standards are attained when the expected number of exceedances per 
year at each monitoring site is less than or equal to one. In the 
simplest case, the number of expected exceedances at a site is 
determined by recording the number of exceedances in each calendar 
year and then averaging them over the past 3 calendar years. 
Situations in which 3 years of data are not available and possible 
adjustments for unusual events or trends are discussed in sections 
2.3 and 2.4 of this appendix. Further, when data for a year are 
incomplete, it is necessary to compute an estimated number of 
exceedances for that year by adjusting the observed number of 
exceedances. This procedure, performed by calendar quarter, is 
described in section 3.0 of this appendix. The expected number of 
exceedances is then estimated by averaging the individual annual 
estimates for the past 3 years.
    (b) The comparison with the allowable expected exceedance rate 
of one per year is made in terms of a number rounded to the nearest 
tenth (fractional values equal to or greater than 0.05 are to be 
rounded up; e.g., an exceedance rate of 1.05 would be rounded to 
1.1, which is the lowest rate for nonattainment).

2.2 Reserved

2.3 Data Requirements

    (a) 40 CFR 58.12 specifies the required minimum frequency of 
sampling for PM10. For the purposes of making comparisons 
with the particulate matter standards, all data produced by State 
and Local Air Monitoring Stations (SLAMS) and other sites submitted 
to EPA in accordance with the part 58 requirements must be used, and 
a minimum of 75 percent of the scheduled PM10 samples per 
quarter are required.
    (b) To demonstrate attainment of the 24-hour standards at a 
monitoring site, the monitor must provide sufficient data to perform 
the required calculations of sections 3.0 and 4.0 of this appendix. 
The amount of data required varies with the sampling frequency, data 
capture rate and the number of years of record. In all cases, 3 
years of representative monitoring data that meet the 75 percent 
criterion of the previous paragraph should be utilized, if 
available,

[[Page 61225]]

and would suffice. More than 3 years may be considered, if all 
additional representative years of data meeting the 75 percent 
criterion are utilized. Data not meeting these criteria may also 
suffice to show attainment; however, such exceptions will have to be 
approved by the appropriate Regional Administrator in accordance 
with EPA guidance.
    (c) There are less stringent data requirements for showing that 
a monitor has failed an attainment test and thus has recorded a 
violation of the particulate matter standards. Although it is 
generally necessary to meet the minimum 75 percent data capture 
requirement per quarter to use the computational equations described 
in section 3.0 of this appendix, this criterion does not apply when 
less data is sufficient to unambiguously establish nonattainment. 
The following examples illustrate how nonattainment can be 
demonstrated when a site fails to meet the completeness criteria. 
Nonattainment of the 24-hour primary standards can be established by 
the observed annual number of exceedances (e.g., four observed 
exceedances in a single year), or by the estimated number of 
exceedances derived from the observed number of exceedances and the 
required number of scheduled samples (e.g., two observed exceedances 
with every other day sampling). In both cases, expected annual 
values must exceed the levels allowed by the standards.

2.4 Adjustment for Exceptional Events and Trends

    (a) An exceptional event is an uncontrollable event caused by 
natural sources of particulate matter or an event that is not 
expected to recur at a given location. Inclusion of such a value in 
the computation of exceedances or averages could result in 
inappropriate estimates of their respective expected annual values. 
To reduce the effect of unusual events, more than 3 years of 
representative data may be used. Alternatively, other techniques, 
such as the use of statistical models or the use of historical data 
could be considered so that the event may be discounted or weighted 
according to the likelihood that it will recur. The use of such 
techniques is subject to the approval of the appropriate Regional 
Administrator in accordance with EPA guidance.
    (b) In cases where long-term trends in emissions and air quality 
are evident, mathematical techniques should be applied to account 
for the trends to ensure that the expected annual values are not 
inappropriately biased by unrepresentative data. In the simplest 
case, if 3 years of data are available under stable emission 
conditions, this data should be used. In the event of a trend or 
shift in emission patterns, either the most recent representative 
year(s) could be used or statistical techniques or models could be 
used in conjunction with previous years of data to adjust for 
trends. The use of less than 3 years of data, and any adjustments 
are subject to the approval of the appropriate Regional 
Administrator in accordance with EPA guidance.

3.0 Computational Equations for the 24-Hour Standards

3.1 Estimating Exceedances for a Year

    (a) If PM10 sampling is scheduled less frequently 
than every day, or if some scheduled samples are missed, a 
PM10 value will not be available for each day of the 
year. To account for the possible effect of incomplete data, an 
adjustment must be made to the data collected at each monitoring 
location to estimate the number of exceedances in a calendar year. 
In this adjustment, the assumption is made that the fraction of 
missing values that would have exceeded the standard level is 
identical to the fraction of measured values above this level. This 
computation is to be made for all sites that are scheduled to 
monitor throughout the entire year and meet the minimum data 
requirements of section 2.3 of this appendix. Because of possible 
seasonal imbalance, this adjustment shall be applied on a quarterly 
basis. The estimate of the expected number of exceedances for the 
quarter is equal to the observed number of exceedances plus an 
increment associated with the missing data. The following equation 
must be used for these computations:
[GRAPHIC] [TIFF OMITTED] TR17OC06.000

Where:

eq = the estimated number of exceedances for calendar 
quarter q;
vq = the observed number of exceedances for calendar 
quarter q;
Nq = the number of days in calendar quarter q;
nq = the number of days in calendar quarter q with 
PM10 data; and
q = the index for calendar quarter, q = 1, 2, 3 or 4.

    (b) The estimated number of exceedances for a calendar quarter 
must be rounded to the nearest hundredth (fractional values equal to 
or greater than 0.005 must be rounded up).
    (c) The estimated number of exceedances for the year, e, is the 
sum of the estimates for each calendar quarter.
[GRAPHIC] [TIFF OMITTED] TR17OC06.001

    (d) The estimated number of exceedances for a single year must 
be rounded to one decimal place (fractional values equal to or 
greater than 0.05 are to be rounded up). The expected number of 
exceedances is then estimated by averaging the individual annual 
estimates for the most recent 3 or more representative years of 
data. The expected number of exceedances must be rounded to one 
decimal place (fractional values equal to or greater than 0.05 are 
to be rounded up).
    (e) The adjustment for incomplete data will not be necessary for 
monitoring or modeling data which constitutes a complete record, 
i.e., 365 days per year.
    (f) To reduce the potential for overestimating the number of 
expected exceedances, the correction for missing data will not be 
required for a calendar quarter in which the first observed 
exceedance has occurred if:
    (1) There was only one exceedance in the calendar quarter;
    (2) Everyday sampling is subsequently initiated and maintained 
for 4 calendar quarters in accordance with 40 CFR 58.12; and
    (3) Data capture of 75 percent is achieved during the required 
period of everyday sampling. In addition, if the first exceedance is 
observed in a calendar quarter in which the monitor is already 
sampling every day, no adjustment for missing data will be made to 
the first exceedance if a 75 percent data capture rate was achieved 
in the quarter in which it was observed.

Example 1

    a. During a particular calendar quarter, 39 out of a possible 92 
samples were recorded, with one observed exceedance of the 24-hour 
standard. Using Equation 1, the estimated number of exceedances for 
the quarter is:

eq = 1 x 92/39 = 2.359 or 2.36.

    b. If the estimated exceedances for the other 3 calendar 
quarters in the year were 2.30, 0.0 and 0.0, then, using Equation 2, 
the estimated number of exceedances for the year is 2.36 + 2.30 + 
0.0 + 0.0 which equals 4.66 or 4.7. If no exceedances were observed 
for the 2 previous years, then the expected number of exceedances is 
estimated by: (\1/3\) x (4.7 + 0 + 0) = 1.57 or 1.6. Since 1.6 
exceeds the allowable number of expected exceedances, this 
monitoring site would fail the attainment test.

Example 2

    In this example, everyday sampling was initiated following the 
first observed exceedance as required by 40 CFR 58.12. Accordingly, 
the first observed exceedance would not be adjusted for incomplete 
sampling. During the next three quarters, 1.2 exceedances were 
estimated. In this case, the estimated exceedances for the year 
would be 1.0 + 1.2 + 0.0 + 0.0 which equals 2.2. If, as before, no 
exceedances were observed for the two previous years, then the 
estimated exceedances for the 3-year period would then be (\1/3\) x 
(2.2 + 0.0 + 0.0) = 0.7, and the monitoring site would not fail the 
attainment test.

3.2 Adjustments for Non-Scheduled Sampling Days

    (a) If a systematic sampling schedule is used and sampling is 
performed on days in addition to the days specified by the 
systematic sampling schedule, e.g., during episodes of high 
pollution, then an adjustment must be made in the equation for the 
estimation of exceedances. Such an adjustment is needed to eliminate 
the bias in the estimate of the quarterly and annual number of 
exceedances that would occur if the chance of an exceedance is 
different for scheduled than for non-scheduled days, as would be the 
case with episode sampling.
    (b) The required adjustment treats the systematic sampling 
schedule as a stratified sampling plan. If the period from one

[[Page 61226]]

scheduled sample until the day preceding the next scheduled sample 
is defined as a sampling stratum, then there is one stratum for each 
scheduled sampling day. An average number of observed exceedances is 
computed for each of these sampling strata. With nonscheduled 
sampling days, the estimated number of exceedances is defined as:
[GRAPHIC] [TIFF OMITTED] TR17OC06.002

Where:

eq = the estimated number of exceedances for the quarter;
Nq = the number of days in the quarter;
mq = the number of strata with samples during the 
quarter;
vj = the number of observed exceedances in stratum j; and
kj = the number of actual samples in stratum j.

    (c) Note that if only one sample value is recorded in each 
stratum, then Equation 3 reduces to Equation 1.

Example 3

    A monitoring site samples according to a systematic sampling 
schedule of one sample every 6 days, for a total of 15 scheduled 
samples in a quarter out of a total of 92 possible samples. During 
one 6-day period, potential episode levels of PM10 were 
suspected, so 5 additional samples were taken. One of the regular 
scheduled samples was missed, so a total of 19 samples in 14 
sampling strata were measured. The one 6-day sampling stratum with 6 
samples recorded 2 exceedances. The remainder of the quarter with 
one sample per stratum recorded zero exceedances. Using Equation 3, 
the estimated number of exceedances for the quarter is:

    Eq = (92/14) x (2/6 + 0 +. . .+ 0) = 2.19.

0
6. Appendix L to part 50 is amended by:
0
a. Revising section 1.1;
0
b. Revising the heading of section 7.3.4 and adding introductory text;
0
c. Revising paragraph (a) of section 7.3.4.3:
0
d. Adding section 7.3.4.4;
0
e. Revising Table L-1 in section 7.4.19;
0
f. Revising section 8.3.6;
0
g. Revising the first sentence in section 10.10 and revising section 
10.13; and
0
h. Revising reference 2 in section 13.0 to read as follows:

Appendix L to Part 50--Reference Method for the Determination of Fine 
Particulate Matter as PM2.5 in the Atmosphere

1.0 Applicability.

    1.1 This method provides for the measurement of the mass 
concentration of fine particulate matter having an aerodynamic 
diameter less than or equal to a nominal 2.5 micrometers 
(PM2.5) in ambient air over a 24-hour period for purposes 
of determining whether the primary and secondary national ambient 
air quality standards for fine particulate matter specified in Sec.  
50.7 and Sec.  50.13 of this part are met. The measurement process 
is considered to be nondestructive, and the PM2.5 sample 
obtained can be subjected to subsequent physical or chemical 
analyses. Quality assessment procedures are provided in part 58, 
appendix A of this chapter, and quality assurance guidance are 
provided in references 1, 2, and 3 in section 13.0 of this appendix.
* * * * *
    7.3.4 Particle size separator. The sampler shall be configured 
with either one of the two alternative particle size separators 
described in this section 7.3.4. One separator is an impactor-type 
separator (WINS impactor) described in sections 7.3.4.1, 7.3.4.2, 
and 7.3.4.3 of this appendix. The alternative separator is a 
cyclone-type separator (VSCCTM) described in section 
7.3.4.4 of this appendix.
* * * * *
    7.3.4.3 * * *
    (a) Composition. Dioctyl sebacate (DOS), single-compound 
diffusion oil.
* * * * *
    7.3.4.4 The cyclone-type separator is identified as a BGI 
VSCCTM Very Sharp Cut Cyclone particle size separator 
specified as part of EPA-designated equivalent method EQPM-0202-142 
(67 FR 15567, April 2, 2002) and as manufactured by BGI 
Incorporated, 58 Guinan Street, Waltham, Massachusetts 20451.
* * * * *
    7.4.19 * * *

                                Table L-1 to Appendix L of Part 50.--Summary of Information To Be Provided by the Sampler
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                  Availability                                  Format
                                                       Appendix L  -------------------------------------------------------------------------------------
             Information to be provided                  section                     End of        Visual     Data  output     Digital
                                                        reference    Anytime \1\   period \2\    display \3\       \4\       reading \5\       Units
--------------------------------------------------------------------------------------------------------------------------------------------------------
Flow rate, 30-second maximum interval...............      7.4.5.1       [check]   ............      [check]             *          XX.X            L/min
Flow rate, average for the sample period............      7.4.5.2             *       [check]             *       [check]          XX.X            L/min
Flow rate, CV, for sample period....................      7.4.5.2             *       [check]             *       [check]          XX.X                %
Flow rate, 5-min. average out of spec. (FLAG \6\)...      7.4.5.2       [check]       [check]       [check]   [check][ssbo       On/Off   ..............
                                                                                                                       x]
Sample volume, total................................      7.4.5.2             *       [check]       [check]       [check]          XX.X               m3
Temperature, ambient, 30-second interval............        7.4.8       [check]   ............      [check]   ............         XX.X           [deg]C
Temperature, ambient, min., max., average for the           7.4.8             *       [check]       [check]      {[ssbox]          XX.X           [deg]C
 sample period......................................
Baro. pressure, ambient, 30-second interval.........        7.4.9       [check]   ............      [check]   ............          XXX            mm Hg
Baro. pressure, ambient, min., max., average for the        7.4.9             *       [check]       [check]   [check][ssbo          XXX            mm Hg
 sample period......................................                                                                   x]
Filter temperature, 30-second interval..............       7.4.11       [check]   ............      [check]   ............         XX.X           [deg]C
Filter temp. differential, 30-second interval, out         7.4.11             *       [check]       [check]   [check][ssbo       On/Off   ..............
 of spec. (FLAG \6\)................................                                                                   x]
Filter temp., maximum differential from ambient,           7.4.11             *             *             *             *   X.X, YY/MM/   [deg]C, Yr/Mon/
 date, time of occurrence...........................                                                                           DD HH.mm     Day Hrs. min
Date and Time.......................................       7.4.12       [check]   ............      [check]   ............     YY/MM/DD       Yr/Mon/Day
                                                                                                                                  HH.mm         Hrs. min
Sample start and stop time settings.................       7.4.12       [check]       [check]       [check]       [check]      YY/MM/DD       Yr/Mon/Day
                                                                                                                                  HH.mm         Hrs. min
Sample period start time............................       7.4.12   ............      [check]       [check]       [check]      YY/MM/DD       Yr/Mon/Day
                                                                                                                                  HH.mm         Hrs. min

[[Page 61227]]

 
Elapsed sample time.................................       7.4.13             *       [check]       [check]       [check]         HH.mm         Hrs. min
Elapsed sample time, out of spec. (FLAG \6\)........       7.4.13   ............      [check]       [check]   [check][ssbo       On/Off   ..............
                                                                                                                       x]
Power interruptions <=1 min., start time of first 10     7.4.15.5             *       [check]             *       [check]       1HH.mm,         Hrs. min
                                                                                                                            2HH.mm, etc.
User-entered information, such as sampler and site         7.4.16       [check]       [check]       [check]   [check][ssbo  As entered
 identification.....................................                                                                   x]
--------------------------------------------------------------------------------------------------------------------------------------------------------
[check] Provision of this information is required.
* Provision of this information is optional. If information related to the entire sample period is optionally provided prior to the end of the sample
  period, the value provided should be the value calculated for the portion of the sampler period completed up to the time the information is provided.
[ssbox] Indicates that this information is also required to be provided to the Air Quality System (AQS) data bank; see Sec.   58.16 of this chapter. For
  ambient temperature and barometric pressure, only the average for the sample period must be reported.
1. Information is required to be available to the operator at any time the sampler is operating, whether sampling or not.
2. Information relates to the entire sampler period and must be provided following the end of the sample period until reset manually by the operator or
  automatically by the sampler upon the start of a new sample period.
3. Information shall be available to the operator visually.
4. Information is to be available as digital data at the sampler's data output port specified in section 7.4.16 of this appendix following the end of
  the sample period until reset manually by the operator or automatically by the sampler upon the start of a new sample period.
5. Digital readings, both visual and data output, shall have not less than the number of significant digits and resolution specified.
6. Flag warnings may be displayed to the operator by a single flag indicator or each flag may be displayed individually. Only a set (on) flag warning
  must be indicated; an off (unset) flag may be indicated by the absence of a flag warning. Sampler users should refer to section 10.12 of this appendix
  regarding the validity of samples for which the sampler provided an associated flag warning.

* * * * *
    8.3.6 The post-sampling conditioning and weighing shall be 
completed within 240 hours (10 days) after the end of the sample 
period, unless the filter sample is maintained at temperatures below 
the average ambient temperature during sampling (or 4 [deg]C or 
below for average sampling temperatures less than 4 [deg]C) during 
the time between retrieval from the sampler and the start of the 
conditioning, in which case the period shall not exceed 30 days. 
Reference 2 in section 13.0 of this appendix has additional guidance 
on transport of cooled filters.
* * * * *
    10.10 Within 177 hours (7 days, 9 hours) of the end of the 
sample collection period, the filter, while still contained in the 
filter cassette, shall be carefully removed from the sampler, 
following the procedure provided in the sampler operation or 
instruction manual and the quality assurance program, and placed in 
a protective container. * * *
* * * * *
    10.13 After retrieval from the sampler, the exposed filter 
containing the PM2.5 sample should be transported to the 
filter conditioning environment as soon as possible, ideally to 
arrive at the conditioning environment within 24 hours for 
conditioning and subsequent weighing. During the period between 
filter retrieval from the sampler and the start of the conditioning, 
the filter shall be maintained as cool as practical and continuously 
protected from exposure to temperatures over 25 [deg]C to protect 
the integrity of the sample and minimize loss of volatile components 
during transport and storage. See section 8.3.6 of this appendix 
regarding time limits for completing the post-sampling weighing. See 
reference 2 in section 13.0 of this appendix for additional guidance 
on transporting filter samplers to the conditioning and weighing 
laboratory.
* * * * *

13.0 References

* * * * *
    2. Quality Assurance Guidance Document 2.12. Monitoring 
PM2.5 in Ambient Air Using Designated Reference or Class 
I Equivalent Methods. U.S. EPA, National Exposure Research 
Laboratory. Research Triangle Park, NC, November 1988 or later 
edition. Currently available at: http://www.epa.gov/ttn/amtic/
pmqainf.html.
* * * * *

0
7. Appendix N to part 50 is revised to read as follows:

Appendix N to Part 50--Interpretation of the National Ambient Air 
Quality Standards for PM2.5

1. General

    (a) This appendix explains the data handling conventions and 
computations necessary for determining when the annual and 24-hour 
primary and secondary national ambient air quality standards (NAAQS) 
for PM2.5 specified in Sec.  50.7 and Sec.  50.13 of this 
part are met. PM2.5, defined as particles with an 
aerodynamic diameter less than or equal to a nominal 2.5 
micrometers, is measured in the ambient air by a Federal reference 
method (FRM) based on appendix L of this part, as applicable, and 
designated in accordance with part 53 of this chapter, or by a 
Federal equivalent method (FEM) designated in accordance with part 
53 of this chapter, or by an Approved Regional Method (ARM) 
designated in accordance with part 58 of this chapter. Data handling 
and computation procedures to be used in making comparisons between 
reported PM2.5 concentrations and the levels of the 
PM2.5 NAAQS are specified in the following sections.
    (b) Data resulting from exceptional events, for example 
structural fires or high winds, may be given special consideration. 
In some cases, it may be appropriate to exclude these data in whole 
or part because they could result in inappropriate values to compare 
with the levels of the PM2.5 NAAQS. In other cases, it 
may be more appropriate to retain the data for comparison with the 
levels of the PM2.5 NAAQS and then for EPA to formulate 
the appropriate regulatory response.
    (c) The terms used in this appendix are defined as follows:
    Annual mean refers to a weighted arithmetic mean, based on 
quarterly means, as defined in section 4.4 of this appendix.
    Creditable samples are samples that are given credit for data 
completeness. They include valid samples collected on required 
sampling days and valid ``make-up'' samples taken for missed or 
invalidated samples on required sampling days.
    Daily values for PM2.5 refers to the 24-hour average 
concentrations of PM2.5 calculated (averaged from hourly 
measurements) or measured from midnight to midnight (local standard 
time) that are used in NAAQS computations.
    Designated monitors are those monitoring sites designated in a 
State or local agency PM Monitoring Network Description in 
accordance with part 58 of this chapter.
    Design values are the metrics (i.e., statistics) that are 
compared to the NAAQS levels to determine compliance, calculated as 
shown in section 4 of this appendix:
    (1) The 3-year average of annual means for a single monitoring 
site or a group of monitoring sites (referred to as the ``annual 
standard design value''). If spatial averaging

[[Page 61228]]

has been approved by EPA for a group of sites which meet the 
criteria specified in section 2(b) of this appendix and section 
4.7.5 of appendix D of 40 CFR part 58, then 3 years of spatially 
averaged annual means will be averaged to derive the annual standard 
design value for that group of sites (further referred to as the 
``spatially averaged annual standard design value''). Otherwise, the 
annual standard design value will represent the 3-year average of 
annual means for a single site (further referred to as the ``single 
site annual standard design value'').
    (2) The 3-year average of annual 98th percentile 24-hour average 
values recorded at each monitoring site (referred to as the ``24-
hour standard design value'').
    Extra samples are non-creditable samples. They are daily values 
that do not occur on scheduled sampling days and that can not be 
used as make-ups for missed or invalidated scheduled samples. Extra 
samples are used in mean calculations and are subject to selection 
as a 98th percentile.
    Make-up samples are samples taken to supplant missed or 
invalidated required scheduled samples. Make-ups can be made by 
either the primary or the collocated instruments. Make-up samples 
are either taken before the next required sampling day or exactly 
one week after the missed (or voided) sampling day. Also, to be 
considered a valid make-up, the sampling must be administered 
according to EPA guidance.
    98th percentile is the daily value out of a year of 
PM2.5 monitoring data below which 98 percent of all daily 
values fall.
    Year refers to a calendar year.

2.0 Monitoring Considerations.

    (a) Section 58.30 of this chapter specifies which monitoring 
locations are eligible for making comparisons with the 
PM2.5 standards.
    (b) To qualify for spatial averaging, monitoring sites must meet 
the criterion specified in section 4.7.5 of appendix D of 40 CFR 
part 58 as well as the following requirements:
    (1) The annual mean concentration at each site shall be within 
10 percent of the spatially averaged annual mean.
    (2) The daily values for each site pair among the 3-year period 
shall yield a correlation coefficient of at least 0.9 for each 
calendar quarter.
    (3) All of the monitoring sites should principally be affected 
by the same major emission sources of PM2.5. For example, 
this could be demonstrated by site-specific chemical speciation 
profiles confirming all major component concentration averages to be 
within 10 percent for each calendar quarter.
    (4) The requirements in paragraphs (b)(1) through (3) of this 
section shall be met for 3 consecutive years in order to produce a 
valid spatially averaged annual standard design value. Otherwise, 
the individual (single) site annual standard design values shall be 
compared directly to the level of the annual NAAQS.
    (c) Section 58.12 of this chapter specifies the required minimum 
frequency of sampling for PM2.5. Exceptions to the 
specified sampling frequencies, such as a reduced frequency during a 
season of expected low concentrations (i.e., ``seasonal sampling''), 
are subject to the approval of EPA. Annual 98th percentile values 
are to be calculated according to equation 6 in section 4.5 of this 
appendix when a site operates on a ``seasonal sampling'' schedule.

3.0 Requirements for Data Used for Comparisons With the 
PM2.5 NAAQS and Data Reporting Considerations.

    (a) Except as otherwise provided in this appendix, only valid 
FRM/FEM/ARM PM2.5 data required to be submitted to EPA's 
Air Quality System (AQS) shall be used in the design value 
calculations.
    (b) PM2.5 measurement data (typically hourly for 
continuous instruments and daily for filter-based instruments) shall 
be reported to AQS in micrograms per cubic meter ([mu]g/m\3\) to one 
decimal place, with additional digits to the right being truncated.
    (c) Block 24-hour averages shall be computed from available 
hourly PM2.5 concentration data for each corresponding 
day of the year and the result shall be stored in the first, or 
start, hour (i.e., midnight, hour `0') of the 24-hour period. A 24-
hour average shall be considered valid if at least 75 percent (i.e., 
18) of the hourly averages for the 24-hour period are available. In 
the event that less than all 24 hourly averages are available (i.e., 
less than 24, but at least 18), the 24-hour average shall be 
computed on the basis of the hours available using the number of 
available hours as the divisor (e.g., 19). 24-hour periods with 
seven or more missing hours shall be considered valid if, after 
substituting zero for all missing hourly concentrations, the 24-hour 
average concentration is greater than the level of the standard. The 
computed 24-hour average PM2.5 concentrations shall be 
reported to one decimal place (the additional digits to the right of 
the first decimal place are truncated, consistent with the data 
handling procedures for the reported data).
    (d) Except for calculation of spatially averaged annual means 
and spatially averaged annual standard design values, all other 
calculations shown in this appendix shall be implemented on a site-
level basis. Site level data shall be processed as follows:
    (1) The default dataset for a site shall consist of the measured 
concentrations recorded from the designated primary FRM/FEM/ARM 
monitor. The primary monitor shall be designated in the appropriate 
State or local agency PM Monitoring Network Description. All daily 
values produced by the primary sampler are considered part of the 
site record (i.e., that site's daily value); this includes all 
creditable samples and all extra samples.
    (2) Data for the primary monitor shall be augmented as much as 
possible with data from collocated FRM/FEM/ARM monitors. If a valid 
24-hour measurement is not produced from the primary monitor for a 
particular day (scheduled or otherwise), but a valid sample is 
generated by a collocated FRM/FEM/ARM instrument (and recorded in 
AQS), then that collocated value shall be considered part of the 
site data record (i.e., that site's daily value). If more than one 
valid collocated FRM/FEM/ARM value is available, the average of 
those valid collocated values shall be used as the daily value.
    (e) All daily values in the composite site record are used in 
annual mean and 98th percentile calculations, however, not all daily 
values are give credit towards data completeness requirements. Only 
``creditable'' samples are given credit for data completeness. 
Creditable samples include valid samples on scheduled sampling days 
and valid make-up samples. All other types of daily values are 
referred to as ``extra'' samples.

4.0 Comparisons With the PM2.5 NAAQS.

4.1 Annual PM2.5 NAAQS.

    (a) The annual PM2.5 NAAQS is met when the annual 
standard design value is less than or equal to 15.0 micrograms per 
cubic meter ([mu]g/m\3\).
    (b) For single site comparisons, 3 years of valid annual means 
are required to produce a valid annual standard design value. In the 
case of spatial averaging, 3 years of valid spatially averaged 
annual means are required to produce a valid annual standard design 
value. Designated sites with less than 3 years of data shall be 
included in annual spatial averages for those years that data 
completeness requirements are met. A year meets data completeness 
requirements when at least 75 percent of the scheduled sampling days 
for each quarter have valid data. [Quarterly data capture rates 
(expressed as a percentage) are specifically calculated as the 
number of creditable samples for the quarter divided by the number 
of scheduled samples for the quarter, the result then multiplied by 
100 and rounded to the nearest integer.] However, years with at 
least 11 samples in each quarter shall be considered valid, 
notwithstanding quarters with less than complete data, if the 
resulting annual mean, spatially averaged annual mean concentration, 
or resulting annual standard design value concentration (rounded 
according to the conventions of section 4.3 of this appendix) is 
greater than the level of the standard. Furthermore, where the 
explicit 11 sample per quarter requirement is not met, the site 
annual mean shall still be considered valid if, by substituting a 
low value (described below) for the missing data in the deficient 
quarters (substituting enough to meet the 11 sample minimum), the 
computation still yields a recalculated annual mean, spatially 
averaged annual mean concentration, or annual standard design value 
concentration over the level of the standard. The low value used for 
this substitution test shall be the lowest reported daily value in 
the site data record for that calendar quarter over the most recent 
3-year period. If an annual mean is deemed complete using this test, 
the original annual mean (without substituted low values) shall be 
considered the official mean value for this site, not the result of 
the recalculated test using the low values.
    (c) The use of less than complete data is subject to the 
approval of EPA, which may consider factors such as monitoring site 
closures/moves, monitoring diligence, and nearby concentrations in 
determining whether to use such data.

[[Page 61229]]

    (d) The equations for calculating the annual standard design 
values are given in section 4.4 of this appendix.

4.2 24-Hour PM2.5 NAAQS.

    (a) The 24-hour PM2.5 NAAQS is met when the 24-hour 
standard design value at each monitoring site is less than or equal 
to 35 [mu]g/m3. This comparison shall be based on 3 
consecutive, complete years of air quality data. A year meets data 
completeness requirements when at least 75 percent of the scheduled 
sampling days for each quarter have valid data. However, years shall 
be considered valid, notwithstanding quarters with less than 
complete data (even quarters with less than 11 samples), if the 
resulting annual 98th percentile value or resulting 24-hour standard 
design value (rounded according to the conventions of section 4.3 of 
this appendix) is greater than the level of the standard.
    (b) The use of less than complete data is subject to the 
approval of EPA which may consider factors such as monitoring site 
closures/moves, monitoring diligence, and nearby concentrations in 
determining whether to use such data for comparisons to the NAAQS.
    (c) The equations for calculating the 24-hour standard design 
values are given in section 4.5 of this appendix.
    4.3 Rounding Conventions. For the purposes of comparing 
calculated values to the applicable level of the standard, it is 
necessary to round the final results of the calculations described 
in sections 4.4 and 4.5 of this appendix. Results for all 
intermediate calculations shall not be rounded.
    (a) Annual PM2.5 standard design values shall be 
rounded to the nearest 0.1 [mu]g/m3 (decimals 0.05 and 
greater are rounded up to the next 0.1, and any decimal lower than 
0.05 is rounded down to the nearest 0.1).
    (b) 24-hour PM2.5 standard design values shall be 
rounded to the nearest 1 [mu]g/m3 (decimals 0.5 and 
greater are rounded up to the nearest whole number, and any decimal 
lower than 0.5 is rounded down to the nearest whole number).

4.4 Equations for the Annual PM2.5 NAAQS.

    (a) An annual mean value for PM2.5 is determined by 
first averaging the daily values of a calendar quarter using 
equation 1 of this appendix:
[GRAPHIC] [TIFF OMITTED] TR17OC06.003

Where:

Xq,y,s = the mean for quarter q of the year y for site s;
nq = the number of daily values in the quarter; and
xi q,y,s = the ith value in quarter q for year 
y for site s.

    (b) Equation 2 of this appendix is then used to calculate the 
site annual mean:
[GRAPHIC] [TIFF OMITTED] TR17OC06.004

Where:

Xy,s = the annual mean concentration for year y (y = 1, 
2, or 3) and for site s; and
Xq,y,s = the mean for quarter q of year y for site s.

    (c) If spatial averaging is utilized, the site-based annual 
means will then be averaged together to derive the spatially 
averaged annual mean using equation 3 of this appendix. Otherwise 
(i.e., for single site comparisons), skip to equation 4.B of this 
appendix.
[GRAPHIC] [TIFF OMITTED] TR17OC06.005

Where:

xy = the spatially averaged mean for year y,
xy,s = the annual mean for year y and site s for sites 
designated to be averaged that meet completeness criteria , and
ns = the number of sites designated to be averaged that 
meet completeness criteria.

    (d) The annual standard design value is calculated using 
equation 4A of this appendix when spatial averaging and equation 4B 
of this appendix when not spatial averaging:
[GRAPHIC] [TIFF OMITTED] TR17OC06.006

[GRAPHIC] [TIFF OMITTED] TR17OC06.007

Where:

x = the annual standard design value (the spatially averaged annual 
standard design value for equation 4A of this appendix and the 
single site annual standard design value for equation 4B of this 
appendix); and
xy = the spatially averaged annual mean for year y 
(result of equation 3 of this appendix) when spatial averaging is 
used, or
xy,s the annual mean for year y and site s (result of 
equation 2 of this appendix) when spatial averaging is not used.
    (e) The annual standard design value is rounded according to the 
conventions in section 4.3 of this appendix before a comparison with 
the standard is made.

4.5 Equations for the 24-Hour PM2.5 NAAQS

    (a) When the data for a particular site and year meet the data 
completeness requirements in section 4.2 of this appendix, 
calculation of the 98th percentile is accomplished by the steps 
provided in this subsection. Equation 5 of this appendix shall be 
used to compute annual 98th percentile values, except that where a 
site operates on an approved seasonal sampling schedule, equation 6 
of this appendix shall be used instead.
    (1) Regular formula for computing annual 98th percentile values. 
Calculation of annual 98th percentile values using the regular 
formula (equation 5) will be based on the creditable number of 
samples (as described below), rather than on the actual number of 
samples. Credit will not be granted for extra (non-creditable) 
samples. Extra samples, however, are candidates for selection as the 
annual 98th percentile. [The creditable number of samples will 
determine how deep to go into the data distribution, but all samples 
(creditable and extra) will be considered when making the percentile 
assignment.] The annual creditable number of samples is the sum of 
the four quarterly creditable number of samples. Sort all the daily 
values from a particular site and year by ascending value. (For 
example: (x[1], x[2], x[3], * * *, x[n]). In this case, x[1] is the 
smallest number and x[n] is the largest value.) The 98th percentile 
is determined from this sorted series of daily values which is 
ordered from the lowest to the highest number. Compute (0.98) x (cn) 
as the number ``i.d,'' where `cn' is the annual creditable number of 
samples, ``i'' is the integer part of the result, and ``d'' is the 
decimal part of the result. The 98th percentile value for year y, 
P0.98,!y, is calculated using equation 5 of this 
appendix:
[GRAPHIC] [TIFF OMITTED] TR17OC06.008

Where:

    P0.98,!y = 98th percentile for year y;
x[i+1] = the (i+1)\th\ number in the ordered series of 
numbers;
i = the integer part of the product of 0.98 and cn.

    (2) Formula for computing annual 98th percentile values when 
sampling frequencies are seasonal. Calculate the annual 98th 
percentiles by determining the smallest measured concentration, x, 
that makes W(x) greater than 0.98 using equation 6 of this appendix:

[[Page 61230]]

[GRAPHIC] [TIFF OMITTED] TR17OC06.009

Where:

dHigh = number of calendar days in the ``High'' season;
dLow = number of calendar days in the ``Low'' season;
dHigh+ = days in a year; and
dLow
[GRAPHIC] [TIFF OMITTED] TR17OC06.010

Such that ``a'' can be either ``High'' or ``Low''; ``x'' is 
the measured concentration; and ``dHigh/(dHigh 
+ dLow) and dLow/(dHigh + 
dLow)'' are constant and are called seasonal ``weights.''
    (b) The 24-hour standard design value is then calculated by 
averaging the annual 98th percentiles using equation 7 of this 
appendix:
[GRAPHIC] [TIFF OMITTED] TR17OC06.011

    (c) The 24-hour standard design value (3-year average 98th 
percentile) is rounded according to the conventions in section 4.3 
of this appendix before a comparison with the standard is made.

0
8. Appendix O is added to part 50 to read as follows:

Appendix O to Part 50--Reference Method for the Determination of Coarse 
Particulate Matter as PM10-2.5 in the Atmosphere

1.0 Applicability and Definition

    1.1 This method provides for the measurement of the mass 
concentration of coarse particulate matter (PM10-2.5) in 
ambient air over a 24-hour period. In conjunction with additional 
analysis, this method may be used to develop speciated data.
    1.2 For the purpose of this method, PM10-2.5 is 
defined as particulate matter having an aerodynamic diameter in the 
nominal range of 2.5 to 10 micrometers, inclusive.
    1.3 For this reference method, PM10-2.5 
concentrations shall be measured as the arithmetic difference 
between separate but concurrent, collocated measurements of 
PM10 and PM2.5, where the PM10 
measurements are obtained with a specially approved sampler, 
identified as a ``PM10c sampler,'' that meets more 
demanding performance requirements than conventional PM10 
samplers described in appendix J of this part. Measurements obtained 
with a PM10c sampler are identified as ``PM10c 
measurements'' to distinguish them from conventional PM10 
measurements obtained with conventional PM10 samplers. 
Thus, PM10-2.5 = PM10c - PM2.5.
    1.4 The PM10c and PM2.5 gravimetric 
measurement processes are considered to be nondestructive, and the 
PM10c and PM2.5 samples obtained in the 
PM10-2.5 measurement process can be subjected to 
subsequent physical or chemical analyses.
    1.5 Quality assessment procedures are provided in part 58, 
appendix A of this chapter. The quality assurance procedures and 
guidance provided in reference 1 in section 13 of this appendix, 
although written specifically for PM2.5, are generally 
applicable for PM10c, and, hence, PM10-2.5 
measurements under this method, as well.
    1.6 A method based on specific model PM10c and 
PM2.5 samplers will be considered a reference method for 
purposes of part 58 of this chapter only if:
    (a) The PM10c and PM2.5 samplers and the 
associated operational procedures meet the requirements specified in 
this appendix and all applicable requirements in part 53 of this 
chapter, and
    (b) The method based on the specific samplers and associated 
operational procedures have been designated as a reference method in 
accordance with part 53 of this chapter.
    1.7 PM10-2.5 methods based on samplers that meet 
nearly all specifications set forth in this method but have one or 
more significant but minor deviations or modifications from those 
specifications may be designated as ``Class I'' equivalent methods 
for PM10-2.5 in accordance with part 53 of this chapter.
    1.8 PM2.5 measurements obtained incidental to the 
PM10-2.5 measurements by this method shall be considered 
to have been obtained with a reference method for PM2.5 
in accordance with appendix L of this part.
    1.9 PM10c measurements obtained incidental to the 
PM10-2.5 measurements by this method shall be considered 
to have been obtained with a reference method for PM10 in 
accordance with appendix J of this part, provided that:
    (a) The PM10c measurements are adjusted to EPA 
reference conditions (25 [deg]C and 760 millimeters of mercury), and
    (b) Such PM10c measurements are appropriately 
identified to differentiate them from PM10 measurements 
obtained with other (conventional) methods for PM10 
designated in accordance with part 53 of this chapter as reference 
or equivalent methods for PM10.

2.0 Principle

    2.1 Separate, collocated, electrically powered air samplers for 
PM10c and PM2.5 concurrently draw ambient air 
at identical, constant volumetric flow rates into specially shaped 
inlets and through one or more inertial particle size separators 
where the suspended particulate matter in the PM10 or 
PM2.5 size range, as applicable, is separated for 
collection on a polytetrafluoroethylene (PTFE) filter over the 
specified sampling period. The air samplers and other aspects of 
this PM10-2.5 reference method are specified either 
explicitly in this appendix or by reference to other applicable 
regulations or quality assurance guidance.
    2.2 Each PM10c and PM2.5 sample collection 
filter is weighed (after moisture and temperature conditioning) 
before and after sample collection to determine the net weight 
(mass) gain due to collected PM10c or PM2.5. 
The total volume of air sampled by each sampler is determined by the 
sampler from the measured flow rate at local ambient temperature and 
pressure and the sampling time. The mass concentrations of both 
PM10c and PM2.5 in the ambient air are 
computed as the total mass of collected particles in the 
PM10 or PM2.5 size range, as appropriate, 
divided by the total volume of air sampled by the respective 
samplers, and expressed in micrograms per cubic meter ([mu]g/
m3)at local temperature and pressure conditions. The mass 
concentration of PM10-2.5 is determined as the 
PM10c concentration value less the corresponding, 
concurrently measured PM2.5 concentration value.
    2.3 Most requirements for PM10-2.5 reference methods 
are similar or identical to the requirements for PM2.5 
reference methods as set forth in appendix L to this part. To insure 
uniformity, applicable appendix L requirements are incorporated 
herein by reference in the sections where indicated rather than 
repeated in this appendix.

3.0 PM10	2.5 Measurement Range

    3.1 Lower concentration limit. The lower detection limit of the 
mass concentration measurement range is estimated to be 
approximately 3 [mu]g/m3, based on the observed precision 
of PM2.5 measurements in the national PM2.5 
monitoring network, the probable similar level of precision for the 
matched PM10c measurements, and the additional 
variability arising from the differential nature of the measurement 
process. This value is provided merely as a guide to the 
significance of low PM10-2.5 concentration measurements.
    3.2 Upper concentration limit. The upper limit of the mass 
concentration range is determined principally by the 
PM10c filter

[[Page 61231]]

mass loading beyond which the sampler can no longer maintain the 
operating flow rate within specified limits due to increased 
pressure drop across the loaded filter. This upper limit cannot be 
specified precisely because it is a complex function of the ambient 
particle size distribution and type, humidity, the individual filter 
used, the capacity of the sampler flow rate control system, and 
perhaps other factors. All PM10c samplers are estimated 
to be capable of measuring 24-hour mass concentrations of at least 
200 [mu]g/m3 while maintaining the operating flow rate 
within the specified limits. The upper limit for the 
PM10-2.5 measurement is likely to be somewhat lower 
because the PM10-2.5 concentration represents only a 
fraction of the PM10 concentration.
    3.3 Sample period. The required sample period for 
PM10-2.5 concentration measurements by this method shall 
be at least 1,380 minutes but not more than 1,500 minutes (23 to 25 
hours), and the start times of the PM2.5 and 
PM10c samples are within 10 minutes and the stop times of 
the samples are also within 10 minutes (see section 10.4 of this 
appendix).

4.0 Accuracy (bias)

    4.1 Because the size, density, and volatility of the particles 
making up ambient particulate matter vary over wide ranges and the 
mass concentration of particles varies with particle size, it is 
difficult to define the accuracy of PM10-2.5 measurements 
in an absolute sense. Furthermore, generation of credible 
PM10-2.5 concentration standards at field monitoring 
sites and presenting or introducing such standards reliably to 
samplers or monitors to assess accuracy is still generally 
impractical. The accuracy of PM10-2.5 measurements is 
therefore defined in a relative sense as bias, referenced to 
measurements provided by other reference method samplers or based on 
flow rate verification audits or checks, or on other performance 
evaluation procedures.
    4.2 Measurement system bias for monitoring data is assessed 
according to the procedures and schedule set forth in part 58, 
appendix A of this chapter. The goal for the measurement uncertainty 
(as bias) for monitoring data is defined in part 58, appendix A of 
this chapter as an upper 95 percent confidence limit for the 
absolute bias of 15 percent. Reference 1 in section 13 of this 
appendix provides additional information and guidance on flow rate 
accuracy audits and assessment of bias.

5.0 Precision

    5.1 Tests to establish initial measurement precision for each 
sampler of the reference method sampler pair are specified as a part 
of the requirements for designation as a reference method under part 
53 of this chapter.
    5.2 Measurement system precision is assessed according to the 
procedures and schedule set forth in appendix A to part 58 of this 
chapter. The goal for acceptable measurement uncertainty, as 
precision, of monitoring data is defined in part 58, appendix A of 
this chapter as an upper 95 percent confidence limit for the 
coefficient of variation (CV) of 15 percent. Reference 1 in section 
13 of this appendix provides additional information and guidance on 
this requirement.
    6.0 Filters for PM10c and PM2.5 Sample 
Collection. Sample collection filters for both PM10c and 
PM2.5 measurements shall be identical and as specified in 
section 6 of appendix L to this part.
    7.0 Sampler. The PM10-2.5 sampler shall consist of a 
PM10c sampler and a PM2.5 sampler, as follows:
    7.1 The PM2.5 sampler shall be as specified in 
section 7 of appendix L to this part.
    7.2 The PM10c sampler shall be of like manufacturer, 
design, configuration, and fabrication to that of the 
PM2.5 sampler and as specified in section 7 of appendix L 
to this part, except as follows:
    7.2.1 The particle size separator specified in section 7.3.4 of 
appendix L to this part shall be eliminated and replaced by a 
downtube extension fabricated as specified in Figure O-1 of this 
appendix.
    7.2.2 The sampler shall be identified as a PM10c 
sampler on its identification label required under Sec.  53.9(d) of 
this chapter.
    7.2.3 The average temperature and average barometric pressure 
measured by the sampler during the sample period, as described in 
Table L-1 of appendix L to this part, need not be reported to EPA's 
AQS data base, as required by section 7.4.19 and Table L-1 of 
appendix L to this part, provided such measurements for the sample 
period determined by the associated PM2.5 sampler are 
reported as required.
    7.3 In addition to the operation/instruction manual required by 
section 7.4.18 of appendix L to this part for each sampler, 
supplemental operational instructions shall be provided for the 
simultaneous operation of the samplers as a pair to collect 
concurrent PM10c and PM2.5 samples. The 
supplemental instructions shall cover any special procedures or 
guidance for installation and setup of the samplers for 
PM10-2.5 measurements, such as synchronization of the 
samplers' clocks or timers, proper programming for collection of 
concurrent samples, and any other pertinent issues related to the 
simultaneous, coordinated operation of the two samplers.
    7.4 Capability for electrical interconnection of the samplers to 
simplify sample period programming and further ensure simultaneous 
operation is encouraged but not required. Any such capability for 
interconnection shall not supplant each sampler's capability to 
operate independently, as required by section 7 of appendix L of 
this part.

8.0 Filter Weighing

    8.1 Conditioning and weighing for both PM10c and 
PM2.5 sample filters shall be as specified in section 8 
of appendix L to this part. See reference 1 of section 13 of this 
appendix for additional, more detailed guidance.
    8.2 Handling, conditioning, and weighing for both 
PM10c and PM2.5 sample filters shall be 
matched such that the corresponding PM10c and 
PM2.5 filters of each filter pair receive uniform 
treatment. The PM10c and PM2.5 sample filters 
should be weighed on the same balance, preferably in the same 
weighing session and by the same analyst.
    8.3 Due care shall be exercised to accurately maintain the 
paired relationship of each set of concurrently collected 
PM10c and PM2.5 sample filters and their net 
weight gain data and to avoid misidentification or reversal of the 
filter samples or weight data. See Reference 1 of section 13 of this 
appendix for additional guidance.
    9.0 Calibration. Calibration of the flow rate, temperature 
measurement, and pressure measurement systems for both the 
PM10c and PM2.5 samplers shall be as specified 
in section 9 of appendix L to this part.

10.0 PM10	2.5 Measurement Procedure

    10.1 The PM10c and PM2.5 samplers shall be 
installed at the monitoring site such that their ambient air inlets 
differ in vertical height by not more than 0.2 meter, if possible, 
but in any case not more than 1 meter, and the vertical axes of 
their inlets are separated by at least 1 meter but not more than 4 
meters, horizontally.
    10.2 The measurement procedure for PM10c shall be as 
specified in section 10 of appendix L to this part, with 
``PM10c'' substituted for ``PM2.5'' wherever 
it occurs in that section.
    10.3 The measurement procedure for PM2.5 shall be as 
specified in section 10 of appendix L to this part.
    10.4 For the PM10-2.5 measurement, the 
PM10c and PM2.5 samplers shall be programmed 
to operate on the same schedule and such that the sample period 
start times are within 5 minutes and the sample duration times are 
within 5 minutes.
    10.5 Retrieval, transport, and storage of each PM10c 
and PM2.5 sample pair following sample collection shall 
be matched to the extent practical such that both samples experience 
uniform conditions.
    11.0 Sampler Maintenance. Both PM10c and 
PM2.5 samplers shall be maintained as described in 
section 11 of appendix L to this part.

12.0 Calculations

    12.1 Both concurrent PM10c and PM2.5 
measurements must be available, valid, and meet the conditions of 
section 10.4 of this appendix to determine the PM10-2.5 
mass concentration.
    12.2 The PM10c mass concentration is calculated using 
equation 1 of this section:
[GRAPHIC] [TIFF OMITTED] TR17OC06.012

Where:

PM10c = mass concentration of PM10c, [mu]g/
m3;
Wf, Wi = final and initial masses (weights), 
respectively, of the filter used to collect the PM10c 
particle sample, [mu]g;
Va = total air volume sampled by the PM10c 
sampler in actual volume units measured at local conditions of 
temperature and pressure, as provided by the sampler, m3.

    Note: Total sample time must be between 1,380 and 1,500 minutes 
(23 and 25 hrs) for a fully valid PM10c sample; however, 
see also section 3.3 of this appendix.


[[Page 61232]]


    12.3 The PM2.5 mass concentration is calculated as 
specified in section 12 of appendix L to this part.
    12.4 The PM10-2.5 mass concentration, in [mu]g/
m3, is calculated using Equation 2 of this section:
[GRAPHIC] [TIFF OMITTED] TR17OC06.013

13.0 Reference

    1. Quality Assurance Guidance Document 2.12. Monitoring 
PM2.5 in Ambient Air Using Designated Reference or Class 
I Equivalent Methods. Draft, November 1998 (or later version or 
supplement, if available). Available at: www.epa.gov/ttn/
amtic/pgqa.html.

14.0 Figures

    Figure O-1 is included as part of this appendix O.
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[GRAPHIC] [TIFF OMITTED] TR17OC06.014

[FR Doc. 06-8477 Filed 10-16-06; 8:45 am]
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