[Federal Register Volume 78, Number 10 (Tuesday, January 15, 2013)]
[Rules and Regulations]
[Pages 3085-3287]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 2012-30946]



[[Page 3085]]

Vol. 78

Tuesday,

No. 10

January 15, 2013

Part II





Environmental Protection Agency





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40 CFR Parts 50, 51, 52 et al.





National Ambient Air Quality Standards for Particulate Matter; Final 
Rule

Federal Register / Vol. 78 , No. 10 / Tuesday, January 15, 2013 / 
Rules and Regulations

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 50, 51, 52, 53 and 58

[EPA-HQ-OAR-2007-0492; FRL-9761-8]
RIN 2060-AO47


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 the 
national ambient air quality standards (NAAQS) for particulate matter 
(PM), the EPA is making revisions to the suite of standards for PM to 
provide requisite protection of public health and welfare and to make 
corresponding revisions to the data handling conventions for PM and to 
the ambient air monitoring, reporting, and network design requirements. 
The EPA also is making revisions to the prevention of significant 
deterioration (PSD) permitting program with respect to the NAAQS 
revisions.
    With regard to primary (health-based) standards for fine particles 
(generally referring to particles less than or equal to 2.5 micrometers 
([mu]m) in diameter, PM2.5), the EPA is revising the annual 
PM2.5 standard by lowering the level to 12.0 micrograms per 
cubic meter ([mu]g/m\3\) so as to provide increased protection against 
health effects associated with long- and short-term exposures 
(including premature mortality, increased hospital admissions and 
emergency department visits, and development of chronic respiratory 
disease), and to retain the 24-hour PM2.5 standard at a 
level of 35 [mu]g/m\3\. The EPA is revising the Air Quality Index (AQI) 
for PM2.5 to be consistent with the revised primary 
PM2.5 standards. With regard to the primary standard for 
particles generally less than or equal to 10 [micro]m in diameter 
(PM10), the EPA is retaining the current 24-hour 
PM10 standard to continue to provide protection against 
effects associated with short-term exposure to thoracic coarse 
particles (i.e., PM10-2.5). With regard to the secondary 
(welfare-based) PM standards, the EPA is generally retaining the 
current suite of secondary standards (i.e., 24-hour and annual 
PM2.5 standards and a 24-hour PM10 standard). 
Non-visibility welfare effects are addressed by this suite of secondary 
standards, and PM-related visibility impairment is addressed by the 
secondary 24-hour PM2.5 standard.

DATES: The final rule is effective on March 18, 2013.

ADDRESSES: Section X.B requests comments on an information collection 
request regarding changes to the monitoring requirements. Submit your 
comments, identified by Docket ID No. EPA-HQ-OAR-2007-0492, to the EPA 
by one of the following methods:
     www.regulations.gov: Follow the on-line instructions for 
submitting comments.
     Email: a-and-r-Docket@epa.gov.
     Fax: 202-566-9744.
     Mail: Docket No. EPA-HQ-OAR-2007-0492, Environmental 
Protection Agency, Mail code 6102T, 1200 Pennsylvania Ave. NW., 
Washington, DC 20460. Please include a total of two copies.
     Hand Delivery: Docket No. EPA-HQ-OAR-2007-0492, 
Environmental Protection Agency, EPA West, Room 3334, 1301 Constitution 
Ave. NW., Washington, DC. Such deliveries are only accepted during the 
Docket's normal hours of operation, and special arrangements should be 
made for deliveries of boxed information.
    Instructions: Direct your comments to Docket ID No. EPA-HQ-OAR-
2007-0492. The EPA's policy is that all comments received will be 
included in the public docket without change and may be made available 
online at www.regulations.gov, including any personal information 
provided, unless the comment includes information claimed to be 
Confidential Business Information (CBI) or other information whose 
disclosure is restricted by statute. Do not submit information that you 
consider to be CBI or otherwise protected through www.regulations.gov 
or email. The www.regulations.gov Web site is an ``anonymous access'' 
system, which means the EPA will not know your identity or contact 
information unless you provide it in the body of your comment. If you 
send an email comment directly to the EPA without going through 
www.regulations.gov your email address will be automatically captured 
and included as part of the comment that is placed in the public docket 
and made available on the Internet. If you submit an electronic 
comment, the EPA recommends that you include your name and other 
contact information in the body of your comment and with any disk or 
CD-ROM you submit. If the EPA cannot read your comment due to technical 
difficulties and cannot contact you for clarification, the EPA may not 
be able to consider your comment. Electronic files should avoid the use 
of special characters, any form of encryption, and be free of any 
defects or viruses. For additional information about EPA's public 
docket visit the EPA Docket Center homepage at http://www.epa.gov/epahome/dockets.htm. Comments on this information collection request 
should also be sent to the Office of Management and Budget (OMB). See 
section X.B below for additional information regarding submitting 
comments to OMB.
    Docket: The EPA has established a docket for this action under 
Docket No. EPA-HQ-OAR-2007-0492. All documents in the docket are listed 
on the www.regulations.gov Web site. This includes documents in the 
rulemaking docket (Docket ID No. EPA-HQ-OAR-2007-0492) and a separate 
docket, established for 2009 Integrated Science Assessment (Docket No. 
EPA-HQ-ORD-2007-0517), that has have been incorporated by reference 
into the rulemaking docket. All documents in these dockets are listed 
on the www.regulations.gov Web site. Although listed in the index, some 
information is not publicly available, e.g., CBI or other information 
whose disclosure is restricted by statute. Certain other material, such 
as copyrighted material, is not placed on the Internet and may be 
viewed, with prior arrangement, at the EPA Docket Center. Publicly 
available docket materials are available either electronically in 
www.regulations.gov or in hard copy at the Air and Radiation Docket and 
Information Center, EPA/DC, EPA West, Room 3334, 1301 Constitution Ave. 
NW., Washington, DC. The Public Reading Room is open from 8:30 a.m. to 
4:30 p.m., Monday through Friday, excluding legal holidays. The 
telephone number for the Public Reading Room is (202) 566-1744 and the 
telephone number for the Air and Radiation Docket and Information 
Center is (202) 566-1742. For additional information about EPA's public 
docket visit the EPA Docket Center homepage at: http://www.epa.gov/epahome/dockets.htm.

FOR FURTHER INFORMATION CONTACT: Ms. Beth M. Hassett-Sipple, Health and 
Environmental Impacts Division, Office of Air Quality Planning and 
Standards, U.S. Environmental Protection Agency, Mail code C504-06, 
Research Triangle Park, NC 27711; telephone: (919) 541-4605; fax: (919) 
541-0237; email: hassett-sipple.beth@epa.gov.

SUPPLEMENTARY INFORMATION:

General Information

Availability of Related Information

    A number of the documents that are relevant to this rulemaking are 
available through the EPA's Office of Air Quality Planning and 
Standards (OAQPS) Technology Transfer Network (TTN) Web site at http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_index.html.

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These documents include the Plan for Review of the National Ambient Air 
Quality Standards for Particulate Matter (U.S. EPA, 2008a), available 
at http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_pd.html, the 
Integrated Science Assessment for Particulate Matter (U.S. EPA, 2009a), 
available at http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_isa.html, the Quantitative Health Risk Assessment for Particulate 
Matter (U.S. EPA, 2010a), available at http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_risk.html, the Particulate Matter Urban-
Focused Visibility Assessment (U.S. EPA 2010b), available at http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_risk.html, and the 
Policy Assessment for the Review of the Particulate Matter National 
Ambient Air Quality Standards (U.S. EPA, 2011a), available at http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_pa.html. These and 
other related documents are also available for inspection and copying 
in the EPA docket identified above.

Table of Contents

    The following topics are discussed in this preamble:
I. Executive Summary
    A. Purpose of This Regulatory Action
    B. Summary of Major Provisions
    C. Costs and Benefits
II. Background
    A. Legislative Requirements
    B. Review of the Air Quality Criteria and Standards for PM
    1. Previous PM NAAQS Reviews
    2. Litigation Related to the 2006 PM Standards
    3. Current PM NAAQS Review
    C. Related Control Programs To Implement PM Standards
    D. Summary of Proposed Revisions to the PM NAAQS
    E. Organization and Approach to Final PM NAAQS Decisions
III. Rationale for Final Decisions on the Primary PM2.5 
Standards
    A. Background
    1. General Approach Used in Previous Reviews
    2. Remand of Primary Annual PM2.5 Standard
    3. General Approach Used in the Policy Assessment for the 
Current Review
    B. Overview of Health Effects Evidence
    C. Overview of Quantitative Characterization of Health Risks
    D. Conclusions on the Adequacy of the Current Primary 
PM2.5 Standards
    1. Introduction
    a. Evidence- and Risk-based Considerations in the Policy 
Assessment
    b. CASAC Advice
    c. Administrator's Proposed Conclusions Concerning the Adequacy 
of the Current Primary PM2.5 Standards
    2. Comments on the Need for Revision
    3. Administrator's Final Conclusions Concerning the Adequacy of 
the Current Primary PM2.5 Standards
    E. Conclusions on the Elements of the Primary Fine Particle 
Standards
    1. Indicator
    2. Averaging Time
    3. Form
    a. Annual Standard
    b. 24-Hour Standard
    4. Level
    a. General Approach for Considering Standard Levels
    b. Proposed Decisions on Level
    i. Consideration of Alternative Standard Levels in the Policy 
Assessment
    ii. CASAC Advice
    iii. Administrator's Proposed Decisions on the Primary 
PM2.5 Standard Levels
    c. Comments on Standard Levels
    i. Annual Standard Level
    ii. 24-Hour Standard Level
    d. Administrator's Final Conclusions on the Primary 
PM2.5 Standard Levels
    F. Administrator's Final Decisions on the Primary 
PM2.5 Standards
IV. Rationale for Final Decision on Primary PM10 Standard
    A. Background
    1. Previous Reviews of the PM NAAQS
    a. Reviews Completed in 1987 and 1997
    b. Review Completed in 2006
    2. Litigation Related to the 2006 Primary PM10 
Standards
    3. General Approach Used in the Current Review
    B. Health Effects Related to Exposure to Thoracic Coarse 
Particles
    C. Consideration of the Current and Potential Alternative 
Standards in the Policy Assessment
    1. Consideration of the Current Standard in the Policy 
Assessment
    2. Consideration of Potential Alternative Standards in the 
Policy Assessment
    D. CASAC Advice
    E. Administrator's Proposed Conclusions Concerning the Adequacy 
of the Current Primary PM10 Standard
    F. Public Comments on the Administrator's Proposed Decision To 
Retain the Primary PM10 Standard
    G. Administrator's Final Decision on the Primary PM10 
Standard
V. Communication of Public Health Information
VI. Rationale for Final Decisions on the Secondary PM Standards
    A. Background
    1. Approaches Used in Previous Reviews
    2. Remand of 2006 Secondary PM2.5 Standards
    3. General Approach Used in the Policy Assessment for the 
Current Review
    B. Proposed Decisions on Secondary PM Standards
    1. PM-related Visibility Impairment
    a. Nature of PM-related Visibility Impairment
    i. Relationship Between Ambient PM and Visibility
    ii. Temporal Variations of Light Extinction
    iii. Periods During the Day of Interest for Assessment of 
Visibility
    iv. Exposure Durations of Interest
    v. Periods of Fog and Rain
    b. Public Perception of Visibility Impairment
    c. Summary of Proposed Conclusions
    i. Adequacy
    ii. Indicator
    iii. Averaging Time
    iv. Form
    v. Level
    vi. Administrator's Proposed Conclusions
    vii. Related Technical Analysis
    2. Other (Non-Visibility) PM-related Welfare Effects
    a. Evidence of Other Welfare Effects Related to PM
    b. CASAC Advice
    c. Summary of Proposed Decisions Regarding Other Welfare Effects
    C. Comments on Proposed Rule
    1. Comments on Proposed Secondary PM Standard for Visibility 
Protection
    a. Overview of Comments
    b. Indicator
    i. Comments on Calculated vs. Directly Measured Light Extinction
    ii. Comments on Specific Aspects of Calculated Light Extinction 
Indicator
    c. Averaging Time
    d. Form
    e. Level
    i. Comments on Visibility Preference Studies
    ii. Specific Comments on Level
    f. Need for a Distinct Secondary Standard
    g. Legal Issues
    h. Relationship With Regional Haze Program
    2. Comments on the Proposed Decision Regarding Non-Visibility 
Welfare Effects
    D. Conclusions on Secondary PM Standards
    1. Conclusions Regarding Secondary PM Standards To Address Non-
Visibility Welfare Effects
    2. Conclusions Regarding Secondary PM Standards for Visibility 
Protection
    E. Administrator's Final Decisions on Secondary PM Standards
VII. Interpretation of the NAAQS for PM
    A. Amendments to Appendix N: Interpretation of the NAAQS for 
PM2.5
    1. General
    2. Monitoring Considerations
    3. Requirements for Data Use and Reporting for Comparison With 
the NAAQS for PM2.5
    4. Comparisons with the PM2.5 NAAQS
    B. Exceptional Events
    C. Updates for Data Handling Procedures for Reporting the Air 
Quality Index
VIII. Amendments to Ambient Monitoring and Reporting Requirements
    A. Issues Related to 40 CFR Part 53 (Reference and Equivalent 
Methods)
    1. PM2.5 and PM10-2.5 Federal Equivalent 
Methods
    2. Use of Chemical Speciation Network (CSN) Methods to Support 
the Proposed New Secondary PM2.5 Visibility Index NAAQS
    B. Changes to 40 CFR Part 58 (Ambient Air Quality Surveillance)
    1. Terminology Changes
    2. Special Considerations for Comparability of PM2.5 
Ambient Air Monitoring Data to the NAAQS

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    a. Revoking Use of Population-Oriented as a Condition for 
Comparability of PM2.5 Monitoring Sites to the NAAQS
    b. Applicability of Micro- and Middle-scale Monitoring Sites to 
the Annual PM2.5 NAAQS
    3. Changes to Monitoring for the National Ambient Air Monitoring 
System
    a. Background
    b. Primary PM2.5 NAAQS
    i. Addition of a Near-road Component to the PM2.5 
Monitoring Network
    ii. Use of PM2.5 Continuous FEMs at SLAMS
    c. Revoking PM10-2.5 Speciation Requirements at NCore 
Sites
    d. Measurements for the Proposed New PM2.5 Visibility 
Index NAAQS
    4. Revisions to the Quality Assurance Requirements for SLAMS, 
SPMs, and PSD
    a. Quality Assurance Weight of Evidence
    b. Quality Assurance Requirements for the Chemical Speciation 
Network
    c. Waivers for Maximum Allowable Separation of Collocated 
PM2.5 Samplers and Monitors
    5. Revisions To Probe and Monitoring Path Siting Criteria
    a. Near-road Component to the PM2.5 Monitoring 
Network
    b. CSN Network
    c. Reinsertion of Table E-1 to Appendix E
    6. Additional Ambient Air Monitoring Topics
    a. Annual Monitoring Network Plans and Periodic Assessment
    b. Operating Schedules
    c. Data Reporting and Certification for CSN and IMPROVE Data
    d. Requirements for Archiving Filters
IX. Clean Air Act Implementation Requirements for the PM NAAQS
    A. Designation of Areas
    1. Overview of Clean Air Act Designations Requirements
    2. Proposed Designations Schedules
    3. Comments and Responses
    4. Final Intended Designations Schedules
    B. Section 110(a)(2) Infrastructure SIP Requirements
    C. Implementing the Revised Primary Annual PM2.5 NAAQS in 
Nonattainment Areas
    D. Prevention of Significant Deterioration and Nonattainment New 
Source Review Programs for the Revised Primary Annual PM2.5 NAAQS
    1. Prevention of Significant Deterioration
    a. Transition Provision (Grandfathering)
    i. Proposal
    ii. Comments and Responses
    iii. Final Action
    b. Modeling Tools and Guidance Applicable to the Revised Primary 
Annual PM2.5 NAAQS
    c. PSD Screening Tools: Significant Emissions Rates, Significant 
Impact Levels, and Significant Monitoring Concentration
    d. PSD Increments
    e. Other PSD Transition Issues
    2. Nonattainment New Source Review
    E. Transportation Conformity Program
    F. General Conformity Program
X. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review and 
Executive Order 13563: Improving Regulation and Regulatory Review
    B. Paperwork Reduction Act
    C. Regulatory Flexibility Act
    D. Unfunded Mandates Reform Act
    E. Executive Order 13132: Federalism
    F. Executive Order 13175: Consultation and Coordination With 
Indian Tribal Governments
    G. Executive Order 13045: Protection of Children From 
Environmental Health and Safety Risks
    H. Executive Order 13211: Actions that Significantly Affect 
Energy Supply, Distribution, or Use
    I. National Technology Transfer and Advancement Act
    J. Executive Order 12898: Federal Actions To Address 
Environmental Justice in Minority Populations and Low-Income 
Populations
    K. Congressional Review Act
References

I. Executive Summary

A. Purpose of This Regulatory Action

    Sections 108 and 109 of the Clean Air Act (CAA) govern the 
establishment, review, and revision, as appropriate, of the national 
ambient air quality standards (NAAQS) to protect public health and 
welfare. The CAA requires periodic review of the air quality criteria--
the science upon which the standards are based--and the standards 
themselves. This rulemaking is being done pursuant to these statutory 
requirements. The schedule for completing this review is established by 
a court order.
    In 2006, the EPA completed its last review of the PM NAAQS. In that 
review, the EPA took three principal actions: (1) With regard to fine 
particles (generally referring to particles less than or equal to 2.5 
micrometers ([mu]m) in diameter, PM2.5), at that time, the 
EPA revised the level of the primary 24-hour PM2.5 standard 
from 65 to 35 [mu]g/m\3\ and retained the level of the primary annual 
PM2.5 standard; (2) With regard to the primary standards for 
particles less than or equal to 10 [micro]m in diameter 
(PM10), the EPA retained the primary 24-hour PM10 
standard to continue to provide protection against effects associated 
with short-term exposure to thoracic coarse particles (i.e., 
PM10-2.5) and revoked the primary annual PM10 
standard; and (3) the EPA also revised the secondary standards to be 
identical in all respects to the primary standards.
    In subsequent litigation, the U.S. Court of Appeals for the 
District of Columbia Circuit remanded the primary annual 
PM2.5 standard to the EPA because the Agency had failed to 
explain adequately why the standard provided the requisite protection 
from both short- and long-term exposures to fine particles, including 
protection for at-risk populations such as children. The court remanded 
the secondary PM2.5 standards to the EPA because the Agency 
failed to explain adequately why setting the secondary standards 
identical to the primary standards provided the required protection for 
public welfare, including protection from PM-related visibility 
impairment.
    The EPA initiated this review in June 2007. Between 2007 and 2011, 
the EPA prepared draft and final Integrated Science Assessments, Risk 
and Exposure Assessments, and Policy Assessments. Multiple drafts of 
all of these documents were subject to review by the public and were 
peer reviewed by the EPA's Clean Air Scientific Advisory Committee 
(CASAC). The EPA proposed revisions to the primary and secondary PM 
NAAQS on June 29, 2012 (77 FR 38890). This final rulemaking is the 
final step in the review process.
    In this rulemaking, the EPA is revising the suite of standards for 
PM to provide requisite protection of public health and welfare. The 
EPA is revising the PSD permitting regulations to address the changes 
in the PM NAAQS. In addition, the EPA is updating the AQI for 
PM2.5 and making changes in the data handling conventions 
for PM and ambient air monitoring, reporting, and network design 
requirements to correspond with the changes to the PM NAAQS.

B. Summary of Major Provisions

    With regard to the primary standards for fine particles, the EPA is 
revising the annual PM2.5 standard by lowering the level 
from 15.0 to 12.0 [mu]g/m\3\ so as to provide increased protection 
against health effects associated with long-and short-term exposures. 
The EPA is retaining the level (35 [mu]g/m\3\) and the form (98th 
percentile) of the 24-hour PM2.5 standard to continue to 
provide supplemental protection against health effects associated with 
short-term exposures. This action provides increased protection for 
children, older adults, persons with pre-existing heart and lung 
disease, and other at-risk populations against an array of 
PM2.5-related adverse health effects that include premature 
mortality, increased hospital admissions and emergency department 
visits, and development of chronic respiratory disease. The EPA also is 
eliminating spatial averaging provisions as part of the form of the 
annual standard to avoid potential disproportionate impacts on at-risk 
populations.
    The final decisions for the primary annual and 24-hour 
PM2.5 standards are

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within the ranges that CASAC advised the Agency to consider. These 
decisions are based on an integrative assessment of an extensive body 
of new scientific evidence, which substantially strengthens what was 
known about PM2.5-related health effects in the last review, 
including extended analyses of key epidemiological studies, and 
evidence of health effects observed at lower ambient PM2.5 
concentrations, including effects in areas that likely met the current 
standards. The revised suite of PM2.5 standards also 
reflects consideration of a quantitative risk assessment that estimates 
public health risks likely to remain upon just meeting the current and 
various alternative standards. Based on this information, the 
Administrator concludes that the current primary PM2.5 
standards are not requisite to protect public health with an adequate 
margin of safety, as required by the CAA, and that these revisions are 
warranted to provide the appropriate degree of increased public health 
protection.
    With regard to the primary standard for thoracic coarse particles 
(PM10-2.5), the EPA is retaining the current 24-hour 
PM10 standard, with a level of 150 [mu]g/m\3\ and a one-
expected exceedance form, to continue to provide protection against 
effects associated with short-term exposure to PM10-2.5 
including premature mortality and increased hospital admissions and 
emergency department visits. In reaching this decision, the 
Administrator concludes that the available health evidence and air 
quality information for PM10-2.5, taken together with the 
considerable uncertainties and limitations associated with that 
information, suggests that a standard is needed to protect against 
short-term exposure to all types of PM10-2.5 and that the 
degree of public health protection provided against short-term 
exposures to PM10-2.5 does not need to be increased beyond 
that provided by the current PM10 standard.
    With regard to the secondary PM standards, the Administrator is 
retaining the current suite of secondary PM standards, except for a 
change to the form of the annual PM2.5 standard. 
Specifically, the EPA is retaining the current secondary 24-hour 
PM2.5 and PM10 standards, and is revising only 
the form of the secondary annual PM2.5 standard to remove 
the option for spatial averaging consistent with this change to the 
primary annual PM2.5 standard. This suite of secondary 
standards addresses PM-related non-visibility welfare effects including 
ecological effects, effects on materials, and climate impacts. With 
respect to PM-related visibility impairment, the Administrator has 
identified a target degree of protection, defined in terms of a 
PM2.5 visibility index (based on speciated PM2.5 
mass concentrations and relative humidity data to calculate 
PM2.5 light extinction), a 24-hour averaging time, and a 
90th percentile form, averaged over 3 years, and a level of 30 
deciviews (dv), which she judges to be requisite to protect public 
welfare with regard to visual air quality (VAQ). The EPA's analysis of 
monitoring data provides the basis for concluding that the current 
secondary 24-hour PM2.5 standard would provide sufficient 
protection, and in some areas greater protection, relative to this 
target protection level. Adding a distinct secondary standard to 
address visibility would not affect this protection. Since sufficient 
protection from visibility impairment will be provided for all areas of 
the country without adoption of a distinct secondary standard, and 
adoption of a distinct secondary standard will not change the degree of 
over-protection of VAQ provided for some areas of the country by the 
secondary 24-hour PM2.5 standard, the Administrator judges 
that adoption of a distinct secondary standard, in addition to the 
current suite of secondary standards, is not needed to provide 
requisite protection for both visibility and non-visibility related 
welfare effects.
    The revisions to the PM NAAQS trigger a process under which states 
(and tribes, if they choose) will make recommendations to the 
Administrator regarding designations, identifying areas of the country 
that either meet or do not meet the revised NAAQS. States will also 
review, modify and supplement their existing state implementation plans 
(SIPs), as needed. With regard to these implementation-related 
activities, the EPA intends to promulgate a separate implementation 
rule on a schedule that provides timely clarity to the states, tribes, 
and other parties responsible for NAAQS implementation. The NAAQS 
revisions also affect the applicable air permitting requirement, but 
cause no significant change to the transportation conformity and 
general conformity processes. The EPA is revising its PSD regulations 
to provide limited grandfathering from the requirements that result 
from the revised PM NAAQS.
    On other topics, the EPA is changing the AQI for PM2.5 
to be consistent with the revised primary PM2.5 NAAQS. The 
EPA also is revising the data handling procedures for PM2.5 
consistent with the revised PM2.5 NAAQS including the 
computations necessary for determining when the standards are met and 
the measurement data that are appropriate for comparison to the 
standards. With regard to monitoring-related activities, the EPA is 
updating several aspects of the monitoring regulations and specifically 
requiring that a small number of PM2.5 monitors be relocated 
to be collocated with measurements of other pollutants (e.g., nitrogen 
dioxide, carbon monoxide) in the near-road environment.

C. Costs and Benefits

    In setting the NAAQS, the EPA may not consider the costs of 
implementing the standards. This was confirmed by the United States 
Supreme Court in Whitman v. American Trucking Associations, 531 U.S. 
457, 465-472, 475-76 (2001), as noted in section II.A of this rule. As 
has traditionally been done in NAAQS rulemaking, the EPA has conducted 
a Regulatory Impact Analysis (RIA) to provide the public with 
information on the potential costs and benefits of attaining several 
alternative PM2.5 standards. In NAAQS rulemaking, the RIA is 
done for informational purposes only, and the final decisions on the 
NAAQS in this rulemaking are not in any way based on consideration of 
the information or analyses in the RIA. The RIA fulfills the 
requirements of Executive Orders 13563 and 12866. The summary of the 
RIA, which is discussed in more detail below in section X.A, estimates 
benefits ranging from $4,000 million to $9,100 million at a 3 percent 
discount rate and $3,600 million to $8,200 million at a 7 percent 
discount rate in 2020 and costs ranging from $53 million to $350 
million per year at a 7 percent discount rate.

II. Background

A. Legislative Requirements

    Two sections of the CAA govern the establishment, review and 
revision of the NAAQS. Section 108 (42 U.S.C. 7408) directs the 
Administrator to identify and list certain air pollutants and then to 
issue air quality criteria for those pollutants. The Administrator is 
to list those air pollutants that in her ``judgment, cause or 
contribute to air pollution which may reasonably be anticipated to 
endanger public health or welfare;'' ``the presence of which in the 
ambient air results from numerous or diverse mobile or stationary 
sources;'' and ``for which * * * [the Administrator] plans to issue air 
quality criteria * * *'' Air quality criteria are intended to 
``accurately reflect the latest scientific knowledge useful in 
indicating the kind and extent of all identifiable effects on public 
health or

[[Page 3090]]

welfare which may be expected from the presence of [a] pollutant in the 
ambient air * * *'' 42 U.S.C. 7408(b). Section 109 (42 U.S.C. 7409) 
directs the Administrator to propose and promulgate ``primary'' and 
``secondary'' NAAQS for pollutants for which air quality criteria are 
issued. Section 109(b)(1) defines a primary standard as one ``the 
attainment and maintenance of which in the judgment of the 
Administrator, based on such criteria and allowing an adequate margin 
of safety, are requisite to protect the public health.'' \1\ A 
secondary standard, as defined in section 109(b)(2), must ``specify a 
level of air quality the attainment and maintenance of which, in the 
judgment of the Administrator, based on such criteria, is requisite to 
protect the public welfare from any known or anticipated adverse 
effects associated with the presence of [the] pollutant in the ambient 
air.'' \2\
---------------------------------------------------------------------------

    \1\ The legislative history of section 109 indicates that a 
primary standard is to be set at ``the maximum permissible ambient 
air level * * * which will protect the health of any [sensitive] 
group of the population,'' and that for this purpose ``reference 
should be made to a representative sample of persons comprising the 
sensitive group rather than to a single person in such a group.'' S. 
Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970).
    \2\ Welfare effects as defined in section 302(h) (42 U.S.C. 
7602(h)) include, but are not limited to, ``effects on soils, water, 
crops, vegetation, man-made materials, animals, wildlife, weather, 
visibility and climate, damage to and deterioration of property, and 
hazards to transportation, as well as effects on economic values and 
on personal comfort and well-being.''
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    The requirement that primary standards provide an adequate margin 
of safety was intended to address uncertainties associated with 
inconclusive scientific and technical information available at the time 
of standard setting. It was also intended to provide a reasonable 
degree of protection against hazards that research has not yet 
identified. See Lead Industries Association v. EPA, 647 F.2d 1130, 1154 
(D.C. Cir 1980); American Petroleum Institute v. Costle, 665 F.2d 1176, 
1186 (D.C. Cir. 1981); American Farm Bureau Federation v. EPA, 559 F. 
3d 512, 533 (D.C. Cir. 2009); Association of Battery Recyclers v. EPA, 
604 F. 3d 613, 617-18 (D.C. Cir. 2010). Both kinds of uncertainties are 
components of the risk associated with pollution at levels below those 
at which human health effects can be said to occur with reasonable 
scientific certainty. Thus, in selecting primary standards that provide 
an adequate margin of safety, the Administrator is seeking not only to 
prevent pollution levels that have been demonstrated to be harmful but 
also to prevent lower pollutant levels that may pose an unacceptable 
risk of harm, even if the risk is not precisely identified as to nature 
or degree. The CAA does not require the Administrator to establish a 
primary NAAQS at a zero-risk level or at background concentration 
levels, see Lead Industries v. EPA, 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, the 
EPA considers such factors as the nature and severity of the health 
effects involved, the size of at-risk population(s), 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. See 
Lead Industries Association v. EPA, 647 F.2d at 1161-62; Whitman v. 
American Trucking Associations, 531 U.S. 457, 495 (2001).
    In setting standards that are ``requisite'' to protect public 
health and welfare, as provided in section 109(b), the EPA's task is to 
establish standards that are neither more nor less stringent than 
necessary for these purposes. In so doing, the EPA may not consider the 
costs of implementing the standards. See generally, Whitman v. American 
Trucking Associations, 531 U.S. 457, 465-472, 475-76 (2001). Likewise, 
``[a]ttainability and technological feasibility are not relevant 
considerations in the promulgation of national ambient air quality 
standards.'' American Petroleum Institute v. Costle, 665 F. 2d at 1185.
    Section 109(d)(1) requires that ``not later than December 31, 1980, 
and at 5-year intervals thereafter, the Administrator shall complete a 
thorough review of the criteria published under section 108 and the 
national ambient air quality standards * * * and shall make such 
revisions in such criteria and standards and promulgate such new 
standards as may be appropriate * * *'' Section 109(d)(2) requires that 
an independent scientific review committee ``shall complete a review of 
the criteria * * * and the national primary and secondary ambient air 
quality standards * * * and shall recommend to the Administrator any 
new * * * standards and revisions of existing criteria and standards as 
may be appropriate. * * *'' Since the early 1980's, this independent 
review function has been performed by the CASAC.\3\
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    \3\ 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. Lists 
of current CASAC members and review panels are available at: http://yosemite.epa.gov/sab/sabproduct.nsf/WebCASAC/CommitteesandMembership?OpenDocument. Members of the CASAC PM Review 
Panel are listed in the CASAC letters providing advice on draft 
assessment documents (Samet, 2009a-f, 2012a-d).
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B. Review of the Air Quality Criteria and Standards for PM

1. Previous PM NAAQS Reviews
    The EPA initially established NAAQS for PM under section 109 of the 
CAA in 1971. Since then, the Agency has made a number of changes to 
these standards to reflect continually expanding scientific 
information, particularly with respect to the selection of indicator\4\ 
and level. Table 1 provides a summary of the PM NAAQS that have been 
promulgated to date. These decisions are briefly discussed below.
---------------------------------------------------------------------------

    \4\ 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, 
such that the indicator for a PM NAAQS has historically been defined 
in terms of particle size ranges.

            Table 1--Summary of National Ambient Air Quality Standards Promulgated for PM 1971-2006 a
----------------------------------------------------------------------------------------------------------------
                                               Averaging
         Final rule             Indicator         time              Level                      Form
----------------------------------------------------------------------------------------------------------------
1971--36 FR 8186 April 30,    TSP..........  24-hour......  260 [mu]g/m\3\        Not to be exceeded more than
 1971.                                                       (primary).            once per year.
                                                            150 [mu]g/m\3\......
                                                            (secondary).........
                                             Annual.......  75 [mu]g/m\3\.......  Annual average.
                                                            (primary)...........
1987--52 FR 24634 July 1,     PM10.........  24-hour......  150 [mu]g/m\3\......  Not to be exceeded more than
 1987.                                                                             once per year on average over
                                                                                   a 3-year period.
                                             Annual.......  50 [mu]g/m\3\.......  Annual arithmetic mean,
                                                                                   averaged over 3 years.

[[Page 3091]]

 
1997--62 FR 38652 July 18,    PM2.5........  24-hour......  65 [mu]g/m\3\.......  98th percentile, averaged over
 1997.                                                                             3 years.\b\
                                             Annual.......  15.0 [mu]g/m\3\.....  Annual arithmetic mean,
                                                                                   averaged over 3 years.c d
                              PM10.........  24-hour......  150 [mu]g/m\3\......  Initially promulgated 99th
                                                                                   percentile, averaged over 3
                                                                                   years; when 1997 standards
                                                                                   for PM10 were vacated, the
                                                                                   form of 1987 standards
                                                                                   remained in place (not to be
                                                                                   exceeded more than once per
                                                                                   year on average over a 3-year
                                                                                   period).
                                             Annual.......  50 [mu]g/m\3\.......  Annual arithmetic mean,
                                                                                   averaged over 3 years.
2006--71 FR 61144 October     PM2.5........  24-hour......  35 [mu]g/m\3\.......  98th percentile, averaged over
 17, 2006.                                   Annual.......  15.0 [mu]g/m\3\.....   3 years.\b\
                                                                                  Annual arithmetic mean,
                                                                                   averaged over 3 years.c e
                              PM10.........  24-hour......  150 [mu]g/m\3\......  Not to be exceeded more than
                                                                                   once per year on average over
                                                                                   a 3-year period.
----------------------------------------------------------------------------------------------------------------
\a\ When not specified, primary and secondary standards are identical.
\b\ The level of the 24-hour standard is defined as an integer (zero decimal places) as determined by rounding.
  For example, a 3-year average 98th percentile concentration of 35.49 [mu]g/m\3\ would round to 35 [mu]g/m\3\
  and thus meet the 24-hour standard and a 3-year average of 35.50 [mu]g/m\3\ would round to 36 and, hence,
  violate the 24-hour standard (40 CFR part 50, appendix N).
\c\ The level of the annual standard is defined to one decimal place (i.e., 15.0 [mu]g/m\3\) as determined by
  rounding. For example, a 3-year average annual mean of 15.04 [mu]g/m\3\ would round to 15.0 [mu]g/m\3\ and,
  thus, meet the annual standard and a 3-year average of 15.05 [mu]g/m\3\ would round to 15.1 [mu]g/m\3\ and,
  hence, violate the annual standard (40 CFR part 50, appendix N).
\d\ The level of the standard was to be compared to measurements made at sites that represent ``community-wide
  air quality'' recording the highest level, or, if specific requirements were satisfied, to average
  measurements from multiple community-wide air quality monitoring sites (``spatial averaging'').
\e\ The EPA tightened the constraints on the spatial averaging criteria by further limiting the conditions under
  which some areas may average measurements from multiple community-oriented monitors to determine compliance
  (See 71 FR 61165 to 61167, October 17, 2006).

    In 1971, the EPA established NAAQS for PM based on the original air 
quality criteria document (DHEW, 1969; 36 FR 8186, April 30, 1971). The 
reference method specified for determining attainment of the original 
standards was the high-volume sampler, which collects PM up to a 
nominal size of 25 to 45 [mu]m (referred to as total suspended 
particles or TSP). The primary standards (measured by the indicator 
TSP) were 260 [mu]g/m\3\, 24-hour average, not to be exceeded more than 
once per year, and 75 [mu]g/m\3\, annual geometric mean. The secondary 
standard was 150 [mu]g/m\3\, 24-hour average, not to be exceeded more 
than once per year.
    In October 1979, the EPA announced the first periodic review of the 
criteria and NAAQS for PM, and significant revisions to the original 
standards were promulgated in 1987 (52 FR 24634, July 1, 1987). In that 
decision, the EPA changed the indicator for PM from TSP to 
PM10, the latter including particles with an aerodynamic 
diameter less than or equal to a nominal 10 [micro]m, which delineates 
thoracic particles (i.e., that subset of inhalable particles small 
enough to penetrate beyond the larynx to the thoracic region of the 
respiratory tract). The EPA also revised the primary standards by (1) 
replacing the 24-hour TSP standard with a 24-hour PM10 
standard of 150 [mu]g/m\3\ with no more than one expected exceedance 
per year and (2) replacing the annual TSP standard with a 
PM10 standard of 50 [mu]g/m\3\, annual arithmetic mean. The 
secondary standard was revised by replacing it with 24-hour and annual 
PM10 standards identical in all respects to the primary 
standards. The revisions also included a new reference method for the 
measurement of PM10 in the ambient air and rules for 
determining attainment of the new standards. On judicial review, the 
revised standards were upheld in all respects. Natural Resources 
Defense Council v. EPA, 902 F. 2d 962 (D.C. Cir. 1990).
    In April 1994, the EPA announced its plans for the second periodic 
review of the criteria and NAAQS for PM, and promulgated significant 
revisions to the NAAQS in 1997 (62 FR 38652, July 18, 1997). Most 
significantly, the EPA determined that although the PM NAAQS should 
continue to focus on thoracic particles (PM10), the fine and 
coarse fractions of PM10 should be considered separately. 
New standards were added, using PM2.5 as the indicator for 
fine particles. The PM10 standards were retained for the 
purpose of regulating the coarse fraction of PM10 (referred 
to as thoracic coarse particles or PM10-2.5).\5\ The EPA 
established two new PM2.5 standards: an annual standard of 
15.0 [mu]g/m\3\, based on the 3-year average of annual arithmetic mean 
PM2.5 concentrations from single or multiple monitors sited 
to represent community-wide air quality\6\ and a 24-hour standard of 65 
[mu]g/m\3\, based on the 3-year average of the 98th percentile of 24-
hour PM2.5 concentrations at each population-oriented 
monitor\7\ within an area. Also, the EPA established a new reference 
method for the measurement of PM2.5 in the ambient air and 
rules for determining attainment of the new standards. To continue to 
address thoracic coarse particles, the annual PM10 standard 
was retained, while the form, but not the level, of the 24-hour 
PM10 standard was revised 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.
---------------------------------------------------------------------------

    \5\ See 40 CFR parts 50, 53, and 58 for more information on 
reference and equivalent methods for measuring PM in ambient air.
    \6\ Monitoring stations sited to represent community-wide air 
quality would typically be at the neighborhood or urban-scale; 
however, where a population-oriented micro or middle-scale 
PM2.5 monitoring station represents many such locations 
throughout a metropolitan area, these smaller scales might also be 
considered to represent community-wide air quality [40 CFR part 58, 
appendix D, 4.7.1(b)].
    \7\ Population-oriented monitoring (or sites) means residential 
areas, commercial areas, recreational areas, industrial areas where 
workers from more than one company are located, and other areas 
where a substantial number of people may spend a significant 
fraction of their day (40 CFR 58.1).
---------------------------------------------------------------------------

    Following promulgation of the revised PM NAAQS in 1997, petitions 
for review were filed by a large number of

[[Page 3092]]

parties, addressing a broad range of issues. In May 1998, a three-judge 
panel of the U.S. Court of Appeals for the District of Columbia Circuit 
issued an initial decision that upheld the 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), rehearing granted in part and denied in 
part, 195 F. 3d 4 (D.C. Cir. 1999), affirmed in part and reversed in 
part, Whitman v. American Trucking Associations, 531 U.S. 457 (2001). 
The panel also found ``ample support'' for the EPA's decision to 
regulate coarse particle pollution, but vacated the 1997 
P.M.10 standards, concluding, in part, that PM10 
is a ``poorly matched indicator for coarse particulate pollution'' 
because it includes fine particles. Id. at 1053-55. Pursuant to the 
court's decision, the EPA removed the vacated 1997 P.M.10 
standards from the 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 P.M.10 standards to the 1997 
P.M.10 standards. The pre-existing 1987 P.M.10 
standards remained in place (65 FR 80776, December 22, 2000). The court 
also upheld the EPA's determination not to establish more stringent 
secondary standards for fine particles to address effects on visibility 
(175 F. 3d at 1027).
    More generally, the panel held (over a strong dissent) that the 
EPA's approach to establishing the level of the standards in 1997, both 
for the PM and for the 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 the EPA, stating that when the EPA considers these factors for 
potential non-threshold pollutants ``what EPA lacks is any determinate 
criterion for drawing lines'' to determine where the standards should 
be set. Consistent with the EPA's long-standing interpretation and D.C. 
Circuit precedent, the panel also reaffirmed its prior holdings that in 
setting NAAQS, the EPA is ``not permitted to consider the cost of 
implementing those standards.'' Id. at 1040-41.
    On EPA's petition for rehearing, the panel adhered to its position 
on these points. American Trucking Associations v. EPA, 195 F. 3d 4 
(D.C. Cir. 1999). The full Court of Appeals denied the EPA's request 
for rehearing en banc, with five judges dissenting. Id. at 13. Both 
sides filed cross appeals on these issues to the United States Supreme 
Court, which granted certiorari. In February 2001, the Supreme Court 
issued a unanimous decision upholding the EPA's position on both the 
constitutional and cost issues. Whitman v. American Trucking 
Associations, 531 U.S. 457, 464, 475-76. 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 cabined the EPA's discretion, affirming the 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 statutory standard of review that the 
EPA's PM2.5 standards were reasonably supported by the 
administrative record and were not ``arbitrary and capricious.'' 
American Trucking Association v. EPA, 283 F. 3d 355, 369-72 (D.C. Cir. 
2002).
    In October 1997, the EPA published its plans for the next periodic 
review of the air quality criteria and NAAQS for PM (62 FR 55201, 
October 23, 1997). After CASAC and public review of several drafts, the 
EPA's National Center for Environmental Assessment (NCEA) finalized the 
Air Quality Criteria Document for Particulate Matter (henceforth, AQCD 
or the ``Criteria Document'') in October 2004 (U.S. EPA, 2004) and 
OAQPS finalized an assessment document, Particulate Matter Health Risk 
Assessment for Selected Urban Areas (Abt Associates, 2005), and the 
Review of the National Ambient Air Quality Standards for Particulate 
Matter: Policy Assessment of Scientific and Technical Information, in 
December 2005 (henceforth, ``Staff Paper,'' U.S. EPA, 2005). In 
conjunction with its review of the Staff Paper, CASAC provided advice 
to the Administrator on revisions to the PM NAAQS (Henderson, 2005a). 
In particular, most CASAC PM Panel members favored revising the level 
of the primary 24-hour PM2.5 standard within the range of 35 
to 30 [mu]g/m\3\ with a 98th percentile form, in concert with revising 
the level of the primary annual PM2.5 standard within the 
range of 14 to 13 [mu]g/m\3\ (Henderson, 2005a, p.7). For thoracic 
coarse particles, the Panel had reservations in recommending a primary 
24-hour PM10-2.5 standard, and agreed that there was a need 
for more research on the health effects of thoracic coarse particles 
(Henderson, 2005b). With regard to secondary standards, most Panel 
members strongly supported establishing a new, distinct secondary 
PM2.5 standard to protect urban visibility (Henderson, 
2005a, p. 9).
    On January 17, 2006, the EPA proposed to revise the primary and 
secondary NAAQS for PM (71 FR 2620) and solicited comment on a broad 
range of options. Proposed revisions included: (1) Revising the level 
of the primary 24-hour PM2.5 standard to 35 [mu]g/m\3\; (2) 
revising the form, but not the level, of the primary annual 
PM2.5 standard by tightening the constraints on the use of 
spatial averaging; (3) replacing the primary 24-hour PM10 
standard with a 24-hour standard defined in terms of a new indicator, 
PM10-2.5, which was qualified so as to include any ambient 
mix of PM10-2.5 dominated by particles generated by high-
density traffic on paved roads, industrial sources, and construction 
sources, and to exclude any ambient mix of particles dominated by rural 
windblown dust and soils and agricultural and mining sources (71 FR 
2667 to 2668), set at a level of 70 [mu]g/m\3\ based on the 3-year 
average of the 98th percentile of 24-hour PM10-2.5 
concentrations; (4) revoking the primary annual PM10 
standard; and (5) revising the secondary standards by making them 
identical in all respects to the proposed suite of primary standards 
for fine and coarse particles.\8\ Subsequent to the proposal, CASAC 
provided additional advice to the 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 the 
standards for thoracic coarse particles (Henderson, 2006a).
---------------------------------------------------------------------------

    \8\ In recognition of an alternative view expressed by most 
members of the CASAC PM Panel, the Agency also solicited comments on 
a subdaily (4- to 8-hour averaging time) secondary PM2.5 
standard to address visibility impairment, considering alternative 
standard levels within a range of 20 to 30 [mu]g/m\3\ in conjunction 
with a form within a range of the 92nd to 98th percentile (71 FR 
2685, January 17, 2006).
---------------------------------------------------------------------------

    On October 17, 2006, the EPA published revisions to the PM NAAQS to 
provide increased protection of public health and welfare (71 FR 
61144). With regard to the primary and secondary standards for fine 
particles, the EPA revised the level of the primary 24-hour 
PM2.5 standard to 35 [mu]g/m\3\, retained the level of the 
primary annual PM2.5 standard at 15.0 [mu]g/m\3\, and

[[Page 3093]]

revised the form of the primary annual PM2.5 standard by 
adding further constraints on the optional use of spatial averaging. 
The EPA revised the secondary standards for fine particles by making 
them identical in all respects to the primary standards. With regard to 
the primary and secondary standards for thoracic coarse particles, the 
EPA retained the level and form of the 24-hour PM10 standard 
(such that the standard remained at a level of 150 [mu]g/m\3\ with a 
one-expected exceedance form and retained the PM10 
indicator) and revoked the annual PM10 standard. The EPA 
also established a new Federal Reference Method (FRM) for the 
measurement of PM10-2.5 in the ambient air (71 FR 61212 to 
13). Although the standards for thoracic coarse particles were not 
defined in terms of a PM10-2.5 indicator, the EPA adopted a 
new FRM for PM10-2.5 to facilitate consistent research on 
PM10-2.5 air quality and health effects and to promote 
commercial development of Federal Equivalent Methods (FEMs) to support 
future reviews of the PM NAAQS (71 FR 61212/2).
    Following issuance of the final rule, CASAC articulated its concern 
that the ``EPA's final rule on the NAAQS for PM does not reflect 
several important aspects of the CASAC's advice'' (Henderson et al., 
2006b, p. 1). With regard to the primary PM2.5 annual 
standard, CASAC expressed serious concerns regarding the decision to 
retain the level of the standard at 15 [mu]g/m\3\. Specifically, CASAC 
stated, ``It is the CASAC's consensus scientific opinion that the 
decision to retain without change the annual PM2.5 standard 
does not provide an `adequate margin of safety * * * requisite to 
protect the public health' (as required by the Clean Air Act), leaving 
parts of the population of this country at significant risk of adverse 
health effects from exposure to fine PM'' (Henderson et al., 2006b, p. 
2). Furthermore, CASAC pointed out that its recommendations ``were 
consistent with the mainstream scientific advice that EPA received from 
virtually every major medical association and public health 
organization that provided their input to the Agency'' (Henderson et 
al., 2006b, p. 2).\9\ With regard to EPA's final decision to retain the 
24-hour PM10 standard for thoracic coarse particles, CASAC 
had mixed views with regard to the decision to retain the 24-hour 
standard and the continued use of PM10 as the indicator of 
coarse particles, while also recognizing the need to have a standard in 
place to protect against effects associated with short-term exposures 
to thoracic coarse particles (Henderson et al., 2006b, p. 2). With 
regard to the EPA's final decision to revise the secondary 
PM2.5 standards to be identical in all respects to the 
revised primary PM2.5 standards, CASAC expressed concerns 
that its advice to establish a distinct secondary standard for fine 
particles to address visibility impairment was not followed and 
emphasized ``that continuing to rely on the primary standard to protect 
against all PM-related adverse environmental and welfare effects 
assures neglect, and will allow substantial continued degradation, of 
visual air quality over large areas of the country'' (Henderson et al, 
2006b, p. 2).
---------------------------------------------------------------------------

    \9\ CASAC specifically identified input provided by the American 
Medical Association, the American Thoracic Society, the American 
Lung Association, the American Academy of Pediatrics, the American 
College of Cardiology, the American Heart Association, the American 
Cancer Society, the American Public Health Association, and the 
National Association of Local Boards of Health (Henderson et al., 
2006b, p. 2).
---------------------------------------------------------------------------

2. Litigation Related to the 2006 PM Standards
    Several parties filed petitions for review following promulgation 
of the revised PM NAAQS in 2006. These petitions addressed the 
following issues: (1) Selecting the level of the primary annual 
PM2.5 standard; (2) retaining PM10 as the 
indicator of a standard for thoracic coarse particles, retaining the 
level and form of the 24-hour PM10 standard, and revoking 
the PM10 annual standard; and (3) setting the secondary 
PM2.5 standards identical to the primary standards. On 
February 24, 2009, the U.S. Court of Appeals for the District of 
Columbia Circuit issued its opinion in the case American Farm Bureau 
Federation v. EPA, 559 F. 3d 512 (D.C. Cir. 2009). The court remanded 
the primary annual PM2.5 NAAQS to the EPA because the EPA 
failed to adequately explain why the standard provided the requisite 
protection from both short- and long-term exposures to fine particles, 
including protection for at-risk populations such as children. American 
Farm Bureau Federation v. EPA, 559 F. 3d 512, 520-27 (D.C. Cir. 2009). 
With regard to the standards for PM10, the court upheld the 
EPA's decisions to retain the 24-hour PM10 standard to 
provide protection from thoracic coarse particle exposures and to 
revoke the annual PM10 standard. American Farm Bureau 
Federation v. EPA, 559 F. 2d at 533-38. With regard to the secondary 
PM2.5 standards, the court remanded the standards to the EPA 
because the Agency's decision was ``unreasonable and contrary to the 
requirements of section 109(b)(2)'' of the CAA. The court further 
concluded that the EPA failed to adequately explain why setting the 
secondary PM standards identical to the primary standards provided the 
required protection for public welfare, including protection from 
visibility impairment. American Farm Bureau Federation v. EPA, 559 F. 
2d at 528-32.
    The decisions of the court with regard to these three issues are 
discussed further in sections III.A.2, IV.A.2, and VI.A.2 below. The 
EPA is responding to the court's remands as part of the current review 
of the PM NAAQS.
3. Current PM NAAQS Review
    The EPA initiated the current review of the air quality criteria 
for PM in June 2007 with a general call for information (72 FR 35462, 
June 28, 2007). In July 2007, the EPA held two ``kick-off'' workshops 
on the primary and secondary PM NAAQS, respectively (72 FR 34003 to 
34004, June 20, 2007).\10\ These workshops provided an opportunity for 
a public discussion of the key policy-relevant issues around which the 
EPA would structure this PM NAAQS review and the most meaningful new 
science that would be available to inform our understanding of these 
issues.
---------------------------------------------------------------------------

    \10\ See workshop materials available at: http://www.regulations.gov/search/Regs/home.html#home Docket ID numbers 
EPA-HQ-OAR-2007-0492-008; EPA-HQ-OAR-2007-0492-009; EPA-HQ-OAR-2007-
0492-010; and EPA-HQ-OAR-2007-0492-012.
---------------------------------------------------------------------------

    Based in part on the workshop discussions, the EPA developed a 
draft Integrated Review Plan outlining the schedule, process, and key 
policy-relevant questions that would guide the evaluation of the air 
quality criteria for PM and the review of the primary and secondary PM 
NAAQS (U.S. EPA, 2007a). On November 30, 2007, the EPA held a 
consultation with CASAC on the draft Integrated Review Plan (72 FR 
63177, November 8, 2007), which included the opportunity for public 
comment. The final Integrated Review Plan (U.S. EPA, 2008a) 
incorporated comments from CASAC (Henderson, 2008) and the public on 
the draft plan as well as input from senior Agency 
managers.11 12
---------------------------------------------------------------------------

    \11\ The process followed in this review varies from the NAAQS 
review process described in section 1.1 of the Integrated Review 
Plan (U.S. EPA, 2008a). On May 21, 2009, Administrator Jackson 
called for key changes to the NAAQS review process including 
reinstating a policy assessment document that contains staff 
analyses of the scientific bases for alternative policy options for 
consideration by senior Agency management prior to rulemaking. In 
conjunction with this change, the EPA will no longer issue a policy 
assessment in the form of an advance notice of proposed rulemaking 
(ANPR) as discussed in the Integrated Review Plan (U.S. EPA, 2008a, 
p. 3). For more information on the overall process followed in this 
review including a description of the major elements of the process 
for reviewing NAAQS see Jackson (2009).
    \12\ All written comments submitted to the Agency are available 
in the docket for this PM NAAQS review (EPA-HQ-OAR-2007-0429). 
Transcripts of public meetings and teleconferences held in 
conjunction with CASAC's reviews are also included in the docket.

---------------------------------------------------------------------------

[[Page 3094]]

    A major element in the process for reviewing the NAAQS is the 
development of an Integrated Science Assessment. This document provides 
a concise evaluation and integration of the policy-relevant science, 
including key science judgments upon which the risk and exposure 
assessments build. As part of the process of preparing the PM 
Integrated Science Assessment, NCEA hosted a peer review workshop in 
June 2008 on preliminary drafts of key Integrated Science Assessment 
chapters (73 FR 30391, May 27, 2008). CASAC and the public reviewed the 
first external review draft Integrated Science Assessment (U.S. EPA, 
2008b; 73 FR 77686, December 19, 2008) at a meeting held on April 1 to 
2, 2009 (74 FR 2688, February 19, 2009). Based on CASAC (Samet, 2009e) 
and public comments, NCEA prepared a second draft Integrated Science 
Assessment (U.S. EPA, 2009b; 74 FR 38185, July 31, 2009), which was 
reviewed by CASAC and the public at a meeting held on October 5 and 6, 
2009 (74 FR 46586, September 10, 2009). Based on CASAC (Samet, 2009f) 
and public comments, NCEA prepared the final Integrated Science 
Assessment titled Integrated Science Assessment for Particulate Matter, 
December 2009 (U.S. EPA, 2009a; 74 FR 66353, December 15, 2009).
    Building upon the information presented in the PM Integrated 
Science Assessment, the EPA prepared Risk and Exposure Assessments that 
provide a concise presentation of the methods, key results, 
observations, and related uncertainties. In developing the Risk and 
Exposure Assessments for this PM NAAQS review, OAQPS released two 
planning documents: Particulate Matter National Ambient Air Quality 
Standards: Scope and Methods Plan for Health Risk and Exposure 
Assessment and Particulate Matter National Ambient Air Quality 
Standards: Scope and Methods Plan for Urban Visibility Impact 
Assessment (henceforth, Scope and Methods Plans, U.S. EPA, 2009c,d; 74 
FR 11580, March 18, 2009). These planning documents outlined the scope 
and approaches that staff planned to use in conducting quantitative 
assessments as well as key issues that would be addressed as part of 
the assessments. In designing and conducting the initial health risk 
and visibility impact assessments, the Agency considered CASAC comments 
(Samet 2009a,b) on the Scope and Methods Plans made during an April 
2009 consultation (74 FR 7688, February 19, 2009) as well as public 
comments. CASAC and the public reviewed two draft assessment documents, 
Risk Assessment to Support the Review of the PM2.5 Primary National 
Ambient Air Quality Standards: External Review Draft, September 2009 
(U.S. EPA, 2009e) and Particulate Matter Urban-Focused Visibility 
Assessment--External Review Draft, September 2009 (U.S. EPA, 2009f) at 
a meeting held on October 5 and 6, 2009 (74 FR 46586, September 10, 
2009). Based on CASAC (Samet 2009c,d) and public comments, OAQPS staff 
revised these draft documents and released second draft assessment 
documents (U.S. EPA, 2010d,e) in January and February 2010 (75 FR 4067, 
January 26, 2010) for CASAC and public review at a meeting held on 
March 10 and 11, 2010 (75 FR 8062, February 23, 2010). Based on CASAC 
(Samet, 2010a,b) and public comments on the second draft assessment 
documents, the EPA revised these documents and released final 
assessment documents titled Quantitative Health Risk Assessment for 
Particulate Matter, June 2010 (henceforth, ``Risk Assessment,'' U.S. 
EPA, 2010a) and Particulate Matter Urban-Focused Visibility 
Assessment--Final Document, July 2010 (henceforth, ``Visibility 
Assessment,'' U.S. EPA, 2010b) (75 FR 39252, July 8, 2010).
    Based on the scientific and technical information available in this 
review as assessed in the Integrated Science Assessment and Risk and 
Exposure Assessments, the EPA staff prepared a Policy Assessment. The 
Policy Assessment is intended to help ``bridge the gap'' between the 
relevant scientific information and assessments and the judgments 
required of the Administrator in reaching decisions on the NAAQS 
(Jackson, 2009, attachment, p. 2). American Farm Bureau Federation v. 
EPA, 559 F. 3d at 521. The Policy Assessment is not a decision 
document; rather it presents the EPA staff conclusions related to the 
broadest range of policy options that could be supported by the 
currently available information. A preliminary draft Policy Assessment 
(U.S. EPA, 2009g) was released in September 2009 for informational 
purposes and to facilitate discussion with CASAC at the October 5 and 
6, 2009 meeting on the overall structure, areas of focus, and level of 
detail to be included in the Policy Assessment. The EPA considered 
CASAC's comments on this preliminary draft in developing a first draft 
Policy Assessment (U.S. EPA, 2010c; 75 FR 4067, January 26, 2010) that 
built upon the information presented and assessed in the final 
Integrated Science Assessment and second draft Risk and Exposure 
Assessments. The EPA presented an overview of the first draft Policy 
Assessment at a CASAC meeting on March 10, 2010 (75 FR 8062, February 
23, 2010) and it was discussed during public CASAC teleconferences on 
April 8 and 9, 2010 (75 FR 8062, February 23, 2010) and May 7, 2010 (75 
FR 19971, April 16, 2010).
    The EPA developed a second draft Policy Assessment (U.S. EPA, 
2010f; 75 FR 39253, July 8, 2010) based on CASAC (Samet, 2010c) and 
public comments on the first draft Policy Assessment. CASAC reviewed 
the second draft document at a meeting on July 26 and 27, 2010 (75 FR 
32763, June 9, 2010). The EPA staff considered CASAC (Samet, 2010d) and 
public comments on the second draft Policy Assessment in preparing a 
final Policy Assessment titled Policy Assessment for the Review of the 
Particulate Matter National Ambient Air Quality Standards, April, 2011 
(U.S. EPA, 2011a; 76, FR 22665, April 22, 2011). This document includes 
final staff conclusions on the adequacy of the current PM standards and 
alternative standards for consideration.
    The schedule for the rulemaking in this review is subject to a 
court order in a lawsuit filed in February 2012 by a group of 
plaintiffs who alleged that the EPA had failed to perform its mandatory 
duty, under section 109(d)(1), to complete a review of the PM NAAQS 
within the period provided by statute. American Lung Association and 
National Parks Conservation Association v. EPA, D.D.C. No. 12-cv-00243 
(consol. with No. 12-cv-00531) Court orders in that case provide that 
the EPA sign a notice of proposed rulemaking concerning its review of 
the PM NAAQS no later than June 14, 2012 and a notice of final 
rulemaking no later than December 14, 2012.
    On June 14, 2012, the EPA issued its proposed decision to revise 
the NAAQS for PM (77 FR 38890, June 29, 2012) (henceforth 
``proposal''). In the proposal, the EPA identified revisions to the 
standards, based on the air quality criteria for PM, and to related 
data handling conventions and ambient air monitoring, reporting, and 
network design requirements. The EPA proposed revisions to the PSD 
permitting program with respect to the proposed NAAQS revisions. The 
Agency also proposed

[[Page 3095]]

changes to the AQI for PM2.5, consistent with the proposed 
primary PM2.5 standards. The proposal solicited public 
comments on alternative primary and secondary standards and related 
matters. The proposal is summarized in section II.D below.
    The EPA held two public hearings to receive public comment on the 
proposed revisions to the PM NAAQS (77 FR 39205, July 2, 2012). One 
hearing took place in Philadelphia, PA on July 17, 2012 and a second 
hearing took place in Sacramento, CA on July 19, 2012. At these public 
hearings, the EPA heard testimony from 168 individuals representing 
themselves or specific interested organizations.
    The EPA received more than 230,000 comments from members of the 
public and various interest groups on the proposed revisions to the PM 
NAAQS by the close of the public comment period on August 31, 2012. 
Major issues raised in the public comments are discussed throughout the 
preamble of this final action. A more detailed summary of all 
significant comments, along with the EPA's responses (henceforth 
``Response to Comments'') can be found in the docket for this 
rulemaking (Docket No. EPA-HQ-OAR-2007-0492) (U.S. EPA, 2012a).
    In the proposal, the EPA recognized that there were a number of new 
scientific studies on the health effects of PM that had been published 
since the mid-2009 cutoff date for inclusion in the Integrated Science 
Assessment.\13\ As in the last PM NAAQS review, the EPA committed to 
conduct a provisional review and assessment of any significant ``new'' 
studies published since the close of the Integrated Science Assessment, 
including studies submitted to the EPA during the public comment 
period. The purpose of the provisional science assessment was to ensure 
that the Administrator was fully aware of the ``new'' science that has 
developed since 2009 before making final decisions on whether to retain 
or revise the current PM NAAQS. The EPA screened and surveyed the 
recent health literature, including studies submitted during the public 
comment period, and conducted a provisional assessment (U.S. EPA, 
2012b) that places the results of those studies of potentially greatest 
policy relevance in the context of the findings of the Integrated 
Science Assessment (U.S. EPA, 2009a). This provisional assessment, 
including a summary of the key conclusions, can be found in the 
rulemaking docket (EPA-HQ-OAR-2007-0492).
---------------------------------------------------------------------------

    \13\ 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 2009 Integrated Science Assessment as 
``new'' studies is intended to clearly differentiate such studies 
from those that have been published since the last review and which 
are included in the Integrated Science Assessment (these studies are 
sometimes referred to as new (without quotation marks) or more 
recent studies, to indicate that they were not included in the 
Integrated Science Assessment and thus are newly available in this 
review).
---------------------------------------------------------------------------

    The provisional assessment found that the ``new'' studies expand 
the scientific information considered in the Integrated Science 
Assessment and provide important insights on the relationship between 
PM exposure and health effects. The provisional assessment also found 
that the ``new'' studies generally strengthen the evidence that long- 
and short-term exposures to fine particles are associated with a wide 
range of health effects. Some of the ``new'' epidemiological studies 
report effects in areas with lower PM2.5-concentrations than 
those in earlier studies considered in the Integrated Science 
Assessment. ``New'' toxicological and epidemiological studies continue 
to link various health effects with a range of fine particle sources 
and components. With regard to thoracic coarse particles, the 
provisional assessment recognized that a limited number of ``new'' 
studies provide evidence of an association with short-term 
PM10-2.5 exposures and increased asthma-related emergency 
department visits in children, but continue to provide no evidence of 
an association between long-term PM10-2.5 exposure and 
mortality. Further, the provisional assessment found that the results 
reported in ``new'' studies do not materially change any of the broad 
scientific conclusions regarding the health effects of PM exposure made 
in the Integrated Science Assessment.
    The EPA believes it was important to conduct a provisional 
assessment in this proceeding, so that the Administrator would be aware 
of the science that developed too recently for inclusion in the 
Integrated Science Assessment. However, it is also important to note 
that the 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, the EPA 
must decide whether to consider the ``new'' studies in this review and 
to 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, the EPA is basing its decision in this 
review on studies and related information included in the Integrated 
Science Assessment, Risk and Exposure Assessment, and Policy 
Assessment, which have undergone CASAC and public review. The studies 
assessed in the Integrated Science Assessment, and the integration of 
the scientific evidence presented in that document, have undergone 
extensive critical review by the EPA, CASAC, and the public during the 
development of the Integrated Science Assessment. 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. NAAQS decisions can have profound impacts on public health 
and welfare, and NAAQS decisions should be based on studies that have 
been rigorously assessed in an integrative manner not only by the EPA 
but also by the statutorily-mandated independent advisory committee, 
CASAC, and have been subject as well to 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 the 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) (the 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) (the 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 CAA 
concerning CASAC review of air quality criteria. The EPA has 
consistently followed this approach. 52 FR 24634, 24637 (July 1, 1987) 
(after review by CASAC, the EPA issued a post-proposal addendum to the 
PM Air Quality Criteria Document, to address certain new scientific 
studies not included in the 1982 Air Quality Criteria Document); 61 FR 
25566, 25568 (May 22, 1996) (after review by CASAC, the EPA issued a 
post-proposal supplement to the 1982 Air Quality Criteria Document to 
address certain new health studies not included in the 1982 Air Quality 
Criteria Document or 1986

[[Page 3096]]

Addendum). The EPA 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 and 2006 reviews. 58 FR 13008, 13013 to 13014 
(March 9, 1993) (ozone review); 62 FR 38652, 38662 (July 18, 1997) and 
71 FR 61141, 61148 to 61149 (October 17, 2006) (PM reviews) (The EPA 
conducted a provisional assessment but based the final PM decisions on 
studies and related information included in the air quality criteria 
that had been reviewed by CASAC).
    As discussed in the 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, 13013 to 13014, March 9, 1993). In this 
proceeding, 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 Integrated Science Assessment 
(U.S. EPA, 2012b). For this reason, reopening the air quality criteria 
review would not be warranted even if there were time to do so under 
the court order governing the schedule for this rulemaking. 
Accordingly, the EPA is basing the final decisions in this review on 
the studies and related information included in the PM air quality 
criteria that have undergone CASAC and public review. The EPA will 
consider the ``new'' 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 the EPA, CASAC, and the public.

C. Related Control Programs To Implement PM Standards

    States are primarily responsible for ensuring attainment and 
maintenance of NAAQS once the EPA has established them. Under section 
110 of the CAA and related provisions, states are to submit, for the 
EPA's approval, 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 the EPA, also 
administer the PSD permitting program (CAA sections 160 to 169). In 
addition, federal programs provide for nationwide reductions in 
emissions of PM and other air pollutants through the federal motor 
vehicle and motor vehicle fuel control program under title II of the 
Act (CAA sections 202 to 250) which involves controls for emissions 
from mobile sources and controls for the fuels used by these sources, 
and new source performance standards (NSPS) for stationary sources 
under section 111 of the CAA.
    Currently, there are 35 areas in the U.S. that are designated as 
nonattainment for the current annual PM2.5 standard and 32 
areas in the U.S. that are designated as nonattainment for the current 
24-hour PM2.5 standards. With the revisions to the PM NAAQS 
that are being finalized in this rule, the EPA will work with the 
states to conduct a new area designation process. Those states with new 
nonattainment areas will be required to develop SIPs to attain the 
standards. In developing their attainment plans, states will have to 
take into account projected emission reductions from federal and state 
rules that have already been adopted at the time of plan submittal. A 
number of significant emission reduction programs that will lead to 
reductions of PM and its precursors are in place today or are expected 
to be in place by the time any new SIPs will be due. Examples of such 
rules include regulations for onroad and nonroad engines and fuels, the 
utility and industrial boilers toxics rules, and various other programs 
already adopted by states to reduce emissions from key emissions 
sources. States will then evaluate the level of additional emission 
reductions needed for each nonattainment area to attain the standards 
``as expeditiously as practicable'' and adopt new state regulations, as 
appropriate. Section IX includes additional discussion of designation 
and implementation issues associated with the revised PM NAAQS.

D. Summary of Proposed Revisions to the PM NAAQS

    For reasons discussed in the proposal, the Administrator proposed 
to revise the current primary and secondary PM standards. With regard 
to the primary PM2.5 standards, the Administrator proposed 
to revise the level of the annual PM2.5 standard from 15.0 
[mu]g/m\3\ to a level within a range of 12.0 to 13.0 [mu]g/m\3\ and to 
retain the level of the 24-hour PM2.5 standard at 35 
[micro]g/m\3\. The Administrator also proposed to eliminate spatial 
averaging provisions as part of the form of the annual standard to 
avoid potential disproportionate impacts on at-risk populations. The 
EPA proposed to revise the AQI for PM2.5, consistent with 
the proposed primary PM2.5 standards.
    With regard to the primary coarse particle standard, the EPA 
proposed to retain the current 24-hour PM10 standard to 
continue to provide protection against effects associated with short-
term exposure to thoracic coarse particles (i.e., PM10-2.5).
    With regard to the secondary PM standards, the EPA proposed to 
revise the suite of secondary PM standards by adding a distinct 
standard for PM2.5 to address PM-related visibility 
impairment. The separate secondary standard was proposed to be defined 
in terms of a PM2.5 visibility index, which would use 
speciated PM2.5 mass concentrations and relative humidity 
data to calculate PM2.5 light extinction, translated to the 
deciview (dv) scale, similar to the Regional Haze Program; a 24-hour 
averaging time; a 90th percentile form averaged over 3 years; and a 
level set at one of two options--either 30 or 28 dv. The EPA also 
proposed to retain the current secondary standards generally to address 
non-visibility welfare effects.
    The EPA also proposed to revise the data handling procedures 
consistent with the revised primary and secondary standards for 
PM2.5 including the computations necessary for determining 
when these standards are met and the measurement data that are 
appropriate for comparison to the standards. With regard to monitoring-
related activities, the EPA proposed to update several aspects of the 
monitoring regulations and specifically to require that a small number 
of PM2.5 monitors be relocated to be collocated with 
measurements of other pollutants (e.g., nitrogen dioxide, carbon 
monoxide) in the near-road environment.

E. 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. Consistent with the decisions made by the 
EPA in the last review and with the conclusions in the Integrated 
Science Assessment and Policy Assessment, fine and thoracic coarse 
particles continue to be considered as separate subclasses of PM 
pollution. Primary standards for fine particles and for thoracic coarse 
particles are addressed in sections III and IV, respectively. Changes 
to the AQI for PM2.5, consistent with the revised primary 
PM2.5 standards, are addressed in section V. Secondary 
standards for fine and coarse particles are addressed in section VI. 
Related data handling conventions and exceptional events are addressed 
in section VII. Updates to the monitoring regulations are addressed in

[[Page 3097]]

section VIII. Implementation activities, including PSD-related actions, 
are addressed in section IX. Section X addresses applicable statutory 
and executive order reviews.
    Today's final decisions addressing standards for fine and coarse 
particles are based on a thorough review in the Integrated Science 
Assessment 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 Policy 
Assessment of the most policy-relevant information in the Integrated 
Science Assessment as well as a quantitative health risk assessment and 
urban-focused visibility assessment based on that information; (2) 
CASAC advice and recommendations, as reflected in its letters to the 
Administrator, its discussions of drafts of the Integrated Science 
Assessment, Risk and Exposure Assessments, and Policy Assessment at 
public meetings, and separate written comments prepared by individual 
members of the CASAC PM Review Panel; (3) public comments received 
during the development of these documents, both in connection with 
CASAC meetings and separately; and (4) extensive public comments 
received on the proposed rulemaking.

III. Rationale for Final Decisions on the Primary PM2.5 
Standards

    This section presents the Administrator's final decision regarding 
the need to revise the current primary PM2.5 standards and, 
more specifically, regarding revisions to the level and form of the 
existing primary annual PM2.5 standard in conjunction with 
retaining the existing primary 24-hour PM2.5 standard. As 
discussed more fully below, the rationale for the final decision is 
based on a thorough review, in the Integrated Science Assessment, of 
the latest scientific information, published through mid-2009, on human 
health effects associated with long- and short-term exposures to fine 
particles in the ambient air. The final decisions also take into 
account: (1) Staff assessments of the most policy-relevant information 
presented and assessed in the Integrated Science Assessment and staff 
analyses of air quality and human risks presented in the Risk 
Assessment and the Policy Assessment, upon which staff conclusions 
regarding appropriate considerations in this review are based; (2) 
CASAC advice and recommendations, as reflected in discussions of drafts 
of the Integrated Science Assessment, Risk Assessment, and Policy 
Assessment at public meetings, in separate written comments, and in 
CASAC's letters to the Administrator; (3) the multiple rounds of public 
comments received during the development of these documents, both in 
connection with CASAC meetings and separately; and (4) extensive public 
comments received on the proposal.
    In developing this final rule, the Administrator recognizes that 
the CAA requires her to reach a public health policy judgment as to 
what standards would be requisite--neither more nor less stringent than 
necessary--to protect public health with an adequate margin of safety, 
based on scientific evidence and technical assessments that have 
inherent uncertainties and limitations. This judgment requires making 
reasoned decisions as to what weight to place on various types of 
evidence and assessments, and on the related uncertainties and 
limitations. Thus, in selecting the final standards, the Administrator 
is seeking not only to prevent fine particle concentrations that have 
been demonstrated to be harmful but also to prevent lower fine particle 
concentrations that may pose an unacceptable risk of harm, even if the 
risk is not precisely identified as to nature or degree.
    As discussed below, as well as in more detail in the proposal, a 
substantial amount of new research has been conducted since the close 
of the science assessment in the last review of the PM2.5 
NAAQS (U.S. EPA, 2004), with important new information coming from 
epidemiological studies, in particular. This body of evidence includes 
hundreds of new epidemiological studies conducted in many countries 
around the world. In its assessment of the evidence judged to be most 
relevant to making decisions on elements of the primary 
PM2.5 standards, the EPA has placed greater weight on U.S. 
and Canadian studies using PM2.5 measurements, since studies 
conducted in other countries may reflect different demographic and air 
pollution characteristics.\14\
---------------------------------------------------------------------------

    \14\ Nonetheless, the Administrator recognizes the importance of 
all studies, including international studies, in the Integrated 
Science Assessment's considerations of the weight of the evidence 
that informs causality determinations.
---------------------------------------------------------------------------

    The newly available research studies as well as the earlier body of 
scientific evidence presented and assessed in the Integrated Science 
Assessment have undergone intensive scrutiny through multiple layers of 
peer review and opportunities for public review and comment. In 
developing this final rule, the EPA has drawn upon an integrative 
synthesis of the entire body of evidence concerning exposure to ambient 
fine particles and a broad range of health endpoints (U.S. EPA, 2009a, 
Chapters 2, 4, 5, 6, 7, and 8) focusing on those health endpoints for 
which the Integrated Science Assessment concludes that there is a 
causal or likely causal relationship with long- or short-term 
PM2.5 exposures. The EPA has also considered health 
endpoints for which the Integrated Science Assessment concludes there 
is evidence suggestive of a causal relationship with long-term 
PM2.5 exposures.
    The EPA has also drawn upon a quantitative risk assessment based 
upon the scientific evidence described and assessed in the Integrated 
Science Assessment. These analyses, discussed in the Risk Assessment 
(U.S. EPA, 2010a) and Policy Assessment (U.S. EPA, 2011a, chapter 2), 
have also undergone intensive scrutiny through multiple layers of peer 
review and multiple opportunities for public review and comment.
    Although important uncertainties remain in the qualitative and 
quantitative characterizations of health effects attributable to 
ambient fine particles, progress has been made in addressing these 
uncertainties in this review. The EPA's review of this information has 
been extensive and deliberate. This intensive evaluation of the 
scientific evidence and quantitative assessments has provided a 
comprehensive and adequate basis for regulatory decision making at this 
time.
    This section describes the integrative synthesis of the evidence 
and technical information contained in the Integrated Science 
Assessment, the Risk Assessment, and the Policy Assessment with regard 
to the current and alternative standards. The EPA notes that the final 
decision for retaining or revising the current primary PM2.5 
standards is a public health policy judgment made by the Administrator. 
The Administrator's final decision draws upon scientific information 
and analyses related to health effects and risks; judgments about 
uncertainties that are inherent in the scientific evidence and 
analyses; CASAC advice; and comments received in response to the 
proposal.
    In presenting the rationale for the final decisions on the primary 
PM2.5 standards, this section begins with a summary of the 
approaches used in setting the initial primary PM2.5 NAAQS 
in 1997 and in reviewing and revising those standards in 2006 (section 
III.A.1). The DC Circuit Court of Appeals remand of the primary annual 
PM2.5 standard in 2009 is discussed in section III.A.2. 
Taking into consideration this

[[Page 3098]]

history, section III.A.3 describes the EPA's general approach used in 
the current review for considering the need to retain or revise the 
current suite of fine particle standards, taking into account public 
comment on the proposed approach.
    The scientific evidence and quantitative risk assessment were 
presented in sections III.B and III.C of the proposal, respectively (77 
FR 38906 to 38917, June 29, 2012) and are outlined in sections III.B 
and III.C below. Subsequent sections of this preamble provide a more 
complete discussion of the Administrator's rationale, in light of key 
issues raised in public comments, for concluding that it is appropriate 
to revise the suite of current primary PM2.5 standards 
(section III.D), 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 III.E.1); averaging time (section III.E.2); form 
(section III.E.3); and level (section III.E.4). A summary of the final 
decisions to revise the suite of primary PM2.5 standards is 
presented in section III.F.

A. Background

    There are currently two primary PM2.5 standards 
providing public health protection from effects associated with fine 
particle exposures. The annual standard is set at a level of 15.0 
[mu]g/m\3\, based on the 3-year average of annual arithmetic mean 
PM2.5 concentrations from single or multiple monitors sited 
to represent community-wide air quality. The 24-hour standard is set at 
a level of 35 [mu]g/m\3\, based on the 3-year average of the 98th 
percentile of 24-hour PM2.5 concentrations at each 
population-oriented monitor within an area.
    The past and current approaches for reviewing the primary 
PM2.5 standards described below are all based most 
fundamentally on using information from epidemiological studies to 
inform the selection of PM2.5 standards that, in the 
Administrator's judgment, protect public health with an adequate margin 
of safety. Such information can be in the form of air quality 
distributions over which health effect associations have been observed 
in scientific studies or in the form of concentration-response 
functions that support quantitative risk assessment. However, evidence- 
and risk-based approaches using information from epidemiological 
studies to inform decisions on PM2.5 standards are 
complicated by the recognition that no population threshold, below 
which it can be concluded with confidence that PM2.5-related 
effects do not occur, can be discerned from the available evidence.\15\ 
As a result, any general approach to reaching decisions on what 
standards are appropriate necessarily requires judgments about how to 
translate the information available from the epidemiological studies 
into a basis for appropriate standards. This includes consideration of 
how to weigh the uncertainties in the reported associations across the 
distributions of PM2.5 concentrations in the studies and the 
uncertainties in quantitative estimates of risk, in the context of the 
entire body of evidence before the Agency. Such approaches are 
consistent with setting standards that are neither more nor less 
stringent than necessary, recognizing that a zero-risk standard is not 
required by the CAA.
---------------------------------------------------------------------------

    \15\ The term ``evidence-based'' approach or consideration 
generally refers to using the information in the scientific evidence 
to inform judgments on the need to retain or revise the NAAQS. The 
term ``risk-based'' generally refers to using the quantitative 
information in the Risk Assessment to inform such judgments.
---------------------------------------------------------------------------

1. General Approach Used in Previous Reviews
    The general approach used to translate scientific information into 
standards in the previous PM NAAQS reviews focused on consideration of 
alternative standard levels that were somewhat below the long-term mean 
PM2.5 concentrations reported in key epidemiological studies 
(U.S. EPA, 2011a, section 2.1.1). This approach recognized that the 
strongest evidence of PM2.5-related associations occurs 
where the bulk of the data exists, which is over a range of 
concentrations around the long-term (i.e., annual) mean.
    In setting primary PM2.5 annual and 24-hour standards 
for the first time in 1997, the Agency relied primarily on an evidence-
based approach that focused on epidemiological evidence, especially 
from short-term exposure studies of fine particles judged to be the 
strongest evidence at that time (U.S. EPA, 2011a, section 2.1.1.1). The 
EPA selected a level for the annual standard that was at or below the 
long-term mean PM2.5 concentrations in studies providing 
evidence of associations with short-term PM2.5 exposures, 
placing greatest weight on those short-term exposure studies that 
reported clearly statistically significant associations with mortality 
and morbidity effects. Further consideration of long-term mean 
PM2.5 concentrations associated with mortality and 
respiratory effects in children did not provide a basis for 
establishing a lower annual standard level. The EPA did not place much 
weight on quantitative risk estimates from the very limited risk 
assessment conducted, but did conclude that the risk assessment results 
confirmed the general conclusions drawn from the epidemiological 
evidence that a serious public health problem was associated with 
ambient PM levels allowed under the then current PM10 
standards (62 FR 38665/1, July 18, 1997).
    The EPA considered the epidemiological evidence and data on air 
quality relationships to set an annual PM2.5 standard that 
was intended to be the ``generally controlling'' standard; i.e., the 
primary means of lowering both long- and short-term ambient 
concentrations of PM2.5.\16\ In conjunction with the annual 
standard, the EPA also established a 24-hour PM2.5 standard 
to provide supplemental protection against days with high peak 
concentrations, localized ``hotspots,'' and risks arising from seasonal 
emissions that might not be well controlled by an annual standard (62 
FR 38669/3).
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    \16\ In so doing, the EPA noted that because an annual standard 
would focus control programs on annual average PM2.5 
concentrations, it would not only control long-term exposure levels, 
but would also generally control the overall distribution of 24-hour 
exposure levels, resulting 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, the 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, July 18, 1997). In the 
circumstances presented in that review, the EPA determined that it 
was appropriate to focus on the annual standard as the standard best 
suited to control both annual and daily air quality distributions 
(62 FR 38670).
---------------------------------------------------------------------------

    In 2006, the EPA used a different evidence-based approach to assess 
the appropriateness of the levels of the 24-hour and annual 
PM2.5 standards (U.S. EPA, 2011a, section 2.1.1.2). Based on 
an expanded body of epidemiological evidence that was stronger and more 
robust than that available in the 1997 review, including additional 
studies of both short- and long-term exposures, the EPA decided that 
using evidence of effects associated with periods of exposure that were 
most closely matched to the averaging time of each standard was the 
most appropriate public health policy approach for evaluating the 
scientific evidence to inform selecting the level of each standard. 
Thus, the EPA relied upon evidence from the short-term exposure studies 
as the principal basis for revising the level of the 24-hour 
PM2.5 standard from 65 to 35 [micro]g/m\3\ to protect 
against effects associated with short-term exposures. The EPA relied 
upon evidence from long-term exposure

[[Page 3099]]

studies as the principal basis for retaining the level of the annual 
PM2.5 standard at 15 [micro]g/m\3\ to protect against 
effects associated with long-term exposures. This approach essentially 
took the view that short-term studies were not appropriate to inform 
decisions relating to the level of the annual standard, and long-term 
studies were not appropriate to inform decisions relating to the level 
of the 24-hour standard. With respect to quantitative risk-based 
considerations, the EPA determined that the estimates of risks likely 
to remain upon attainment of the 1997 suite of PM2.5 
standards were indicative of risks that could be reasonably judged 
important from a public health perspective and, thus, supported 
revision of the standards. However, the EPA judged that the 
quantitative risk assessment had important limitations and did not 
provide an appropriate basis for selecting the levels of the revised 
standards in 2006 (71 FR 61174/1-2, October 17, 2006).
2. Remand of Primary Annual PM2.5 Standard
    As noted above in section II.B.2, several parties filed petitions 
for review in the U.S. Court of Appeals for the District of Columbia 
Circuit following promulgation of the revised PM NAAQS in 2006. These 
petitions challenged several aspects of the final rule including the 
level of the primary PM2.5 annual standard. The primary 24-
hour PM2.5 standard was not challenged by any of the 
litigants and, thus, was not considered in the court's review and 
decision.
    On judicial review, the D.C. Circuit remanded the primary annual 
PM2.5 NAAQS to the EPA on grounds that the Agency failed to 
adequately explain why the annual standard provided the requisite 
protection from both short- and long-term exposures to fine particles 
including protection for at-risk populations. American Farm Bureau 
Federation v. EPA, 559 F. 3d 512 (D.C. Cir. 2009). With respect to 
human health protection from short-term PM2.5 exposures, the 
court considered the different approaches used by the EPA in the 1997 
and 2006 p.m. NAAQS decisions, as summarized in section III.A.1 above. 
The court found that the EPA failed to adequately explain why a primary 
24-hour PM2.5 standard by itself would provide the 
protection needed from short-term exposures and remanded the primary 
annual PM2.5 standard to the EPA ``for further consideration 
of whether it is set at a level requisite to protect the public health 
while providing an adequate margin of safety from the risk of short-
term exposures to PM2.5.'' American Farm Bureau Federation 
v. EPA, 559 F. 3d at 520-24.
    With respect to protection from long-term exposure to fine 
particles, the court found that the EPA failed to adequately explain 
how the primary annual PM2.5 standard provided an adequate 
margin of safety for children and other at-risk populations. The court 
found that the EPA did not provide a reasonable explanation of why 
certain morbidity studies, including a study of children in Southern 
California showing lung damage associated with long-term 
PM2.5 exposure (Gauderman et al., 2000) and a multi-city 
study (24-Cities Study) evaluating decreased lung function in children 
associated with long-term PM2.5 exposures (Raizenne et al., 
1996), did not warrant a more stringent annual PM2.5 
standard. Id. at 522-23. Specifically, the court found that:

    EPA was unreasonably confident that, even though it relied 
solely upon long-term mortality studies, the revised standard would 
provide an adequate margin of safety with respect to morbidity among 
children. Notably absent from the final rule, moreover, is any 
indication of how the standard will adequately reduce risk to the 
elderly or to those with certain heart or lung diseases despite (a) 
the EPA's determination in its proposed rule that those 
subpopulations are at greater risk from exposure to fine particles 
and (b) the evidence in the record supporting that determination. 
Id. at 525.

    In addition, the court held that the EPA had not adequately 
explained its decision to base the level of the annual standard 
essentially exclusively on the results of long-term studies and the 24-
hour standard level essentially exclusively on the results of short-
term studies. See 559 F. 3d at 522 (``[e]ven if the long-term studies 
available today are useful for setting an annual standard * * * it is 
not clear why the EPA no longer believes it useful to look as well to 
short-term studies in order to design the suite of standards that will 
most effectively reduce the risks associated with short-term 
exposure''); see also Id. at 523-24 (holding that the EPA had not 
adequately explained why a standard based on levels in short-term 
exposure studies alone provided appropriate protection from health 
effects associated with short-term PM2.5 exposures given the 
stated need to lower the entire air quality distribution, and not just 
peak concentrations, in order to control against short-term effects).
    In remanding the primary annual PM2.5 standard for 
reconsideration, the court did not vacate the standard, Id. at 530, so 
the standard remains in effect and is therefore the standard considered 
by the EPA in this review.
3. General Approach Used in the Policy Assessment for the Current 
Review
    This review is based on an assessment of a much expanded body of 
scientific evidence, more extensive air quality data and analyses, and 
a more comprehensive quantitative risk assessment relative to the 
information available in past reviews, as presented and assessed in the 
Integrated Science Assessment and Risk Assessment and discussed in the 
Policy Assessment. As a result, the EPA's general approach to reaching 
conclusions about the adequacy of the current suite of PM2.5 
standards and potential alternative standards that are appropriate to 
consider was broader and more integrative than in past reviews. Our 
general approach also reflected consideration of the issues raised by 
the court in its remand of the primary annual PM2.5 standard 
as discussed in section III.A.2 above, since decisions made in this 
review, and the rationales for those decisions, will comprise the 
Agency's response to the remand.
    The EPA's general approach took into account both evidence-based 
and risk-based considerations and the uncertainties related to both 
types of information, as well as advice from CASAC (Samet, 2010c,d) and 
public comments on the first and second draft Policy Assessments (U.S. 
EPA, 2010c,f). In so doing, the EPA staff developed a final Policy 
Assessment (U.S. EPA, 2011a) which provided as broad an array of policy 
options as was supported by the available information, recognizing that 
the selection of a specific approach to reaching final decisions on the 
primary PM2.5 standards will reflect the judgments of the 
Administrator as to what weight to place on the various approaches and 
types of information available in the current review.
    The Policy Assessment concluded it was most appropriate to consider 
the protection against PM2.5-related mortality and morbidity 
effects, associated with both long- and short-term exposures, afforded 
by the annual and 24-hour standards taken together, as was done in the 
1997 review, rather than to consider each standard separately, as was 
done in the 2006 review (U.S. EPA, 2011a, section 2.1.3).\17\ As the 
EPA recognized in 1997,

[[Page 3100]]

there are various ways to combine two standards to achieve an 
appropriate degree of public health protection. The extent to which 
these two standards are interrelated in any given area depends in large 
part on the relative levels of the standards, the peak-to-mean ratios 
that characterize air quality patterns in an area, and whether changes 
in air quality designed to meet a given suite of standards are likely 
to be of a more regional or more localized nature.
---------------------------------------------------------------------------

    \17\ By utilizing this approach, the Agency also is responsive 
to the remand of the 2006 standard. As noted in section III.A.2, the 
D.C. Circuit, in remanding the 2006 primary annual PM2.5 
standard, concluded that the Administrator had failed to adequately 
explain why an annual standard was sufficiently protective in the 
absence of consideration of the long-term mean PM2.5 
concentrations in short-term exposure studies as well, and likewise 
had failed to explain why a 24-hour standard was sufficiently 
protective in the absence of consideration of the effect of an 
annual standard on reducing the overall distribution of 24-hour 
average PM2.5 concentrations. 559 F. 3d at 520-24.
---------------------------------------------------------------------------

    In considering the combined effect of annual and 24-hour standards, 
the Policy Assessment recognized that changes in PM2.5 air 
quality designed to meet an annual standard would likely result not 
only in lower annual average PM2.5 concentrations but also 
in fewer and lower peak 24-hour PM2.5 concentrations. The 
Policy Assessment also recognized that changes designed to meet a 24-
hour standard would result not only in fewer and lower peak 24-hour 
PM2.5 concentrations but also in lower annual average 
PM2.5 concentrations. Thus, either standard could be viewed 
as providing protection from effects associated with both short- and 
long-term exposures, with the other standard serving to address 
situations where the daily peak and annual average concentrations are 
not consistently correlated.
    In considering the currently available evidence, the Policy 
Assessment recognized that the short-term exposure studies were 
primarily drawn from epidemiological studies that associated variations 
in area-wide health effects with monitor(s) that measured the variation 
in daily PM2.5 concentrations over the course of several 
years. The strength of the associations in these data was demonstrably 
in the numerous ``typical'' days within the air quality distribution, 
not in the peak days. See also 71 FR 61168, October 17, 2006 and 
American Farm Bureau Federation v. EPA, 559 F. 3d at 523, 524 (making 
the same point). The quantitative risk assessments conducted for this 
and previous reviews demonstrated the same point; that is, much, if not 
most of the aggregate risk associated with short-term exposures results 
from the large number of days during which the 24-hour average 
concentrations are in the low-to mid-range, below the peak 24-hour 
concentrations (U.S. EPA, 2011a, section 2.2.2; U.S. EPA, 2010a, 
section 3.1.2.2). In addition, there was no evidence suggesting that 
risks associated with long-term exposures were likely to be 
disproportionately driven by peak 24-hour concentrations.\18\
---------------------------------------------------------------------------

    \18\ In confirmation, a number of studies have presented 
analyses excluding higher PM concentration days and reported a 
limited effect on the magnitude of the effect estimates or 
statistical significance of the association (e.g., Dominici, 2006b; 
Schwartz et al., 1996; Pope and Dockery, 1992).
---------------------------------------------------------------------------

    For these reasons, the Policy Assessment concluded that strategies 
that focused primarily on reducing peak days were less likely to 
achieve reductions in the PM2.5 concentrations that were 
most strongly associated with the observed health effects. Furthermore, 
the Policy Assessment concluded that a policy approach that focused on 
reducing peak exposures would most likely result in more uneven public 
health protection across the U.S. by either providing inadequate 
protection in some areas or overprotecting in other areas (U.S. EPA, 
2011a, p. 2-9; U.S. EPA, 2010a, section 5.2.3). This is because, as 
discussed above, reductions based on control of peak days are less 
likely to control the bulk of the air quality distribution.
    The Policy Assessment concluded that a policy goal of setting a 
``generally controlling'' annual standard that will lower a wide range 
of ambient 24-hour PM2.5 concentrations, as opposed to 
focusing on control of peak 24-hour PM2.5 concentrations, 
was the most effective and efficient way to reduce total population 
risk and so provide appropriate protection. This approach, in contrast 
to one focusing on a generally controlling 24-hour standard, would 
likely reduce aggregate risks associated with both long- and short-term 
exposures with more consistency and would likely avoid setting national 
standards that could result in relatively uneven protection across the 
country, due to setting standards that are either more or less 
stringent than necessary in different geographical areas (U.S. EPA, 
2011a, p. 2-9).
    The Policy Assessment also concluded that an annual standard 
intended to serve as the primary means for providing protection from 
effects associated with both long- and short-term PM2.5 
exposures cannot be expected to offer sufficient protection against the 
effects of all short-term PM2.5 exposures. As a result, in 
conjunction with a generally controlling annual standard, the Policy 
Assessment concluded it was appropriate to consider setting a 24-hour 
standard to provide supplemental protection, particularly for areas 
with high peak-to-mean ratios possibly associated with strong local or 
seasonal sources, or PM2.5-related effects that may be 
associated with shorter-than-daily exposure periods (U.S. EPA, 2011a, 
p. 2-10).
    The Policy Assessment's consideration of the protection afforded by 
the current and alternative suites of standards focused on 
PM2.5-related health effects associated with long-term 
exposures for which the magnitude of quantitative estimates of risks to 
public health generated in the risk assessment was appreciably larger 
in terms of overall incidence and percent of total mortality or 
morbidity effects than for short-term PM2.5-related effects. 
Nonetheless, the EPA also considered health effects and estimated risks 
associated with short-term exposures. In both cases, the Policy 
Assessment placed greatest weight on health effects that had been 
judged in the Integrated Science Assessment to have a causal or likely 
causal relationship with PM2.5 exposures, while also 
considering health effects judged to be suggestive of a causal 
relationship or evidence that focused on specific at-risk populations. 
The Policy Assessment placed relatively greater weight on statistically 
significant associations that yielded relatively more precise effect 
estimates and that were judged to be robust to confounding by other air 
pollutants. In the case of short-term exposure studies, the Policy 
Assessment placed greatest weight on evidence from large multi-city 
studies, while also considering associations in single-city studies.
    In translating information from epidemiological studies into the 
basis for reaching staff conclusions on the adequacy of the current 
suite of standards, the Policy Assessment considered a number of 
factors. As an initial matter, the Policy Assessment considered the 
extent to which the currently available evidence and related 
uncertainties strengthens or calls into question conclusions from the 
last review regarding associations between fine particle exposures and 
health effects. The Policy Assessment also considered evidence of 
health effects in at-risk populations and the potential impacts on such 
populations. Further, the Policy Assessment explored the extent to 
which PM2.5-related health effects had been observed in 
areas where air quality distributions extend to lower concentrations 
than previously reported or in areas that would likely have met the 
current suite of standards.
    In translating information from epidemiological studies into the 
basis for reaching staff conclusions on

[[Page 3101]]

standard levels for consideration (U.S. EPA, 2011a, sections 2.1.3 and 
2.3.4), the Policy Assessment first recognized the absence of 
discernible thresholds in the concentration-response functions from 
long- and short-term PM2.5 exposure studies (U.S. EPA, 
2011a, section 2.4.3).\19\ In the absence of any discernible 
thresholds, the Agency's general approach for identifying appropriate 
standard levels for consideration involved characterizing the range of 
PM2.5 concentrations over which we have the most confidence 
in the associations reported in epidemiological studies. In so doing, 
the Policy Assessment recognized that there is no single factor or 
criterion that comprises the ``correct'' approach, but rather there are 
various approaches that are reasonable to consider for characterizing 
the confidence in the associations and the limitations and 
uncertainties in the evidence. Identifying the implications of various 
approaches for reaching conclusions on the range of alternative 
standard levels that is appropriate to consider can help inform the 
final decisions to either retain or revise the standards. Today's final 
decisions also take into account public health policy judgments as to 
the degree of health protection that is to be achieved.
---------------------------------------------------------------------------

    \19\ The epidemiological studies evaluated in the Integrated 
Science Assessment that examined the shape of concentration-response 
relationships and the potential presence of a threshold focused on 
cardiovascular-related hospital admissions and emergency department 
visits associated with short-term PM10 exposures and 
premature mortality associated with long-term PM2.5 
exposure (U.S. EPA, 2009a, sections 6.5, 6.2.10.10 and 7.6). 
Overall, the Integrated Science Assessment concluded that the 
studies evaluated support the use of a no-threshold, log-linear 
model but recognized that ``additional issues such as the influence 
of heterogeneity in estimates between cities, and the effect of 
seasonal and regional differences in PM on the concentration-
response relationship still require further investigation'' (U.S. 
EPA, 2009a, section 2.4.3).
---------------------------------------------------------------------------

    In reaching staff conclusions on the range of annual standard 
levels that was appropriate to consider, the Policy Assessment focused 
on identifying an annual standard that provided requisite protection 
from effects associated with both long- and short-term exposures. In so 
doing, the Policy Assessment explored different approaches for 
characterizing the range of PM2.5 concentrations over which 
our confidence in the nature of the associations for both long- and 
short-term exposures is greatest, as well as the extent to which our 
confidence is reduced at lower PM2.5 concentrations.
    First, the Policy Assessment recognized that the approach that most 
directly addressed this issue considered studies that analyzed 
confidence intervals around concentration-response relationships and in 
particular, analyses that averaged across multiple concentration-
response models rather than considering a single concentration-response 
model.\20\ The Policy Assessment explored the extent to which such 
analyses had been published for studies of health effects associated 
with long- or short-term PM2.5 exposures. Such analyses 
could potentially be used to characterize a concentration below which 
uncertainty in a concentration-response relationship substantially 
increases or is judged to be indicative of an unacceptable degree of 
uncertainty about the existence of a continuing concentration-response 
relationship. The Policy Assessment concluded that identifying this 
area of uncertainty in the concentration-response relationship could be 
used to inform identification of alternative standard levels that are 
appropriate to consider.
---------------------------------------------------------------------------

    \20\ This is distinct from confidence intervals around 
concentration-response relationships that are related to the 
magnitude of effect estimates generated at specific PM2.5 
concentrations (i.e., point-wise confidence intervals) and that are 
relevant to the precision of the effect estimate across the air 
quality distribution, rather than to our confidence in the existence 
of a continuing concentration-response relationship across the 
entire air quality distribution on which a reported association was 
based.
---------------------------------------------------------------------------

    Further, the Policy Assessment explored other approaches that 
considered different statistical metrics from epidemiological studies. 
The Policy Assessment first took into account the general approach used 
in previous PM reviews which focused on consideration of alternative 
standard levels that were somewhat below the long-term mean 
PM2.5 concentrations reported in epidemiological studies 
using air quality distributions based on composite monitor 
concentrations.\21\ This approach recognized that the strongest 
evidence of PM2.5-related associations occurs at 
concentrations around the long-term (i.e., annual) mean. In using this 
approach, the Policy Assessment placed greatest weight on those long- 
and short-term exposure studies that reported statistically significant 
associations with mortality and morbidity effects.
---------------------------------------------------------------------------

    \21\ Using the term ``composite monitor'' does not imply that 
the EPA can identify one monitor that represents the air quality 
evaluated in a specific study area. Rather, the composite monitor 
concentration represents the average concentration across monitors 
within each area with more than one monitor included in a given 
study as typically reported in epidemiological studies. For multi-
city studies, this metric reflects concentrations averaged across 
multiple monitors or from single monitors within each area and then 
averaged across study areas for an overall study mean 
PM2.5 concentration. This is consistent with the 
epidemiological evidence considered in other NAAQS reviews.
---------------------------------------------------------------------------

    In extending this approach, the Policy Assessment also considered 
information beyond a single statistical metric of PM2.5 
concentrations (i.e., the mean) to the extent such information was 
available. Pursuant to an express comment from CASAC (Samet 2010d, p. 
2), the Policy Assessment utilized distributional statistics (i.e., 
statistical characterization of an entire distribution of data) to 
identify the broader range of PM2.5 concentrations that had 
the most influence on the calculation of relative risk estimates in 
both long- and short-term exposure epidemiological studies. Thus, the 
Policy Assessment considered the part of the distribution of 
PM2.5 concentrations in which the data analyzed in the study 
(i.e., air quality and population-level data, as discussed below) were 
most concentrated, specifically, the range of PM2.5 
concentrations around the long-term mean over which our confidence in 
the magnitude and significance of associations observed in the 
epidemiological studies was greatest. The Policy Assessment then 
focused on the lower part of the distribution to characterize where the 
data became appreciably more sparse and, thus, where our understanding 
of the magnitude and significance of the associations correspondingly 
became more uncertain. The Policy Assessment recognized there was no 
single percentile value within a given distribution that was most 
appropriate or ``correct'' to use to characterize where our confidence 
in the associations becomes appreciably lower. The Policy Assessment 
concluded that the range from the 25th to 10th percentiles is a 
reasonable range to consider as a region where we had appreciably less 
confidence in the associations observed in epidemiological studies.\22\
---------------------------------------------------------------------------

    \22\ In the PM NAAQS review completed in 2006, the Staff Paper 
similarly recognized that the evidence of an association in any 
epidemiological study is ``strongest at and around the long-term 
average where the data in the study are most concentrated. For 
example, the interquartile range of long-term average concentrations 
within a study [with a lower bound of the 25th percentile] or a 
range within one standard deviation around the study mean, may 
reasonably be used to characterize the range over which the evidence 
of association is strongest'' (U.S. EPA, 2005, p. 5-22). A range of 
one standard deviation around the mean represents approximately 68 
percent of normally distributed data, and below the mean falls 
between the 25th and 10th percentiles.
---------------------------------------------------------------------------

    In considering distributional statistics from epidemiological 
studies, the final Policy Assessment focused on two types of 
population-level metrics that CASAC advised were most useful to 
consider in identifying the PM2.5 concentrations

[[Page 3102]]

most influential in generating the health effect estimates reported in 
the epidemiological studies.\23\ Consistent with CASAC advice, the most 
relevant information was the distribution of health events (e.g., 
deaths, hospitalizations) occurring within a study population in 
relation to the distribution of PM2.5 concentrations. 
However, in recognizing that access to health event data can be 
restricted, the Policy Assessment also considered the number of study 
participants within each study area as an appropriate surrogate for 
health event data.
---------------------------------------------------------------------------

    \23\ The second draft Policy Assessment focused on the 
distributions of ambient PM2.5 concentrations and 
associated population data across areas included in several multi-
city studies for which such data were available in seeking to 
identify the most influential range of concentrations (U.S. EPA, 
2010f, section 2.3.4.1). In its review of the second draft Policy 
Assessment, CASAC advised that it ``would be preferable to have 
information on the concentrations that were most influential in 
generating the health effect estimates in individual studies'' 
(Samet, 2010d, p.2). Therefore, in the final Policy Assessment, the 
EPA considered population-level data (i.e., area-specific health 
event data and study area population data) along with corresponding 
PM2.5 concentrations to generate a cumulative 
distribution of the population-level data relative to long-term mean 
PM2.5 concentrations to determine the most influential 
part of the air quality distribution (U.S. EPA, 2011a, Figure 2-7 
and associated text).
---------------------------------------------------------------------------

    The Policy Assessment recognized that an approach considering 
analyses of confidence intervals around concentration-response 
functions was intrinsically related to an approach that considered 
different distributional statistics. Both of these approaches could be 
employed to understand the broader distribution of PM2.5 
concentrations which correspond to the health events reported in 
epidemiological studies. In applying these approaches, the Policy 
Assessment, consistent with CASAC advice (Samet, 2010d, p. 3), 
considered PM2.5 concentrations from long- and short-term 
PM2.5 exposure studies using composite monitor 
distributions.
    In reaching staff conclusions on alternative standard levels that 
were appropriate to consider, the Policy Assessment also included a 
broader consideration of the uncertainties and limitations of the 
current scientific evidence. Most notably, these uncertainties are 
related to the heterogeneity observed in the epidemiological studies in 
the eastern versus western parts of the U.S., the relative toxicity of 
PM2.5 components, and the potential role of co-pollutants 
(U.S. EPA, 2011a, pp. 2-25 to 2-26). The limitations and uncertainties 
associated with the currently available scientific evidence, including 
the availability of fewer studies toward the lower range of alternative 
annual standard levels being considered in this proposal, are 
summarized in section III.B below and further discussed in section 
III.B.2 of the proposal.
    The Policy Assessment recognized that the level of protection 
afforded by the NAAQS relies both on the level and the form of the 
standard. The Policy Assessment concluded that a policy approach that 
used data based on composite monitor distributions to identify 
alternative standard levels, and then compared those levels to 
concentrations at maximum monitors to determine whether an area meets a 
given standard, inherently has the potential to build in some margin of 
safety (U.S. EPA, 2011a, p. 2-14).\24\ This conclusion was consistent 
with CASAC's comments on the second draft Policy Assessment, in which 
CASAC expressed its preference for focusing on an approach using 
composite monitor distributions ``because of its stability, and for the 
additional margin of safety it provides'' when ``compared to the 
maximum monitor perspective'' (Samet, et al., 2010d, pp. 2 to 3).
---------------------------------------------------------------------------

    \24\ Statistical metrics (e.g., means) based on composite 
monitor distributions may be identical to or below the same 
statistical metrics based on maximum monitor distributions. For 
example, some areas may have only one monitor, in which case the 
composite and maximum monitor distributions will be identical in 
those areas. Other areas may have multiple monitors that may be very 
close to the monitor measuring the highest concentrations, in which 
case the composite and maximum monitor distributions could be 
similar in those areas. As noted in Hassett-Sipple et al. (2010), 
for studies involving a large number of areas, the composite and 
maximum concentrations are generally within 5 percent of each other 
(77 FR 38905, fn. 30). Still other areas may have multiple monitors 
that may be separately impacted by local sources in which case the 
composite and maximum monitor distributions could be quite different 
(U.S. EPA, 2011a, p. 2-14). See further discussion of this issue in 
section III.E.4.c.i below.
---------------------------------------------------------------------------

    In reaching staff conclusions on alternative 24-hour standard 
levels that are appropriate to consider for setting a 24-hour standard 
intended to supplement the protection afforded by a generally 
controlling annual standard, the Policy Assessment considered currently 
available short-term PM2.5 exposure studies. The evidence 
from these studies informed our understanding of the protection 
afforded by the suite of standards against effects associated with 
short-term exposures. In considering the short-term exposure studies, 
the Policy Assessment evaluated both the distributions of 24-hour 
PM2.5 concentrations, with a focus on the 98th percentile 
concentrations (to the extent such data were available) to match the 
form of the current 24-hour PM2.5 standard, as well as the 
long-term mean PM2.5 concentrations reported in these 
studies. In addition to considering the epidemiological evidence, the 
Policy Assessment also considered air quality information based on 
county-level 24-hour and annual design values \25\ to understand the 
policy implications of the alternative standard levels supported by the 
underlying science. In particular, the Policy Assessment considered the 
extent to which different combinations of alternative annual and 24-
hour standards would support the policy goal of focusing on a generally 
controlling annual standard in conjunction with a 24-hour standard that 
would provide supplemental protection. In so doing, the Policy 
Assessment discussed the roles that each standard might be expected to 
play in the protection afforded by alternative suites of standards.
---------------------------------------------------------------------------

    \25\ Design values are the metrics (i.e., statistics) that are 
compared to the NAAQS levels to determine compliance.
---------------------------------------------------------------------------

    Beyond these evidence-based considerations, the Policy Assessment 
also considered the quantitative risk estimates and the key 
observations presented in the Risk Assessment. This assessment included 
an evaluation of 15 urban case study areas and estimated risk 
associated with a number of health endpoints associated with long- and 
short-term PM2.5 exposures (U.S. EPA, 2010a). As part of the 
risk-based considerations, the Policy Assessment considered estimates 
of the magnitude of PM2.5-related risks associated with 
recent air quality levels and air quality simulated to just meet the 
current and alternative suites of standards using alternative 
simulation approaches. The Policy Assessment also characterized the 
risk reductions, relative to the risks remaining upon just meeting the 
current standards, associated with just meeting alternative suites of 
standards. In so doing, the Policy Assessment recognized the 
uncertainties inherent in such risk estimates, and took such 
uncertainties into account by considering the sensitivity of the 
``core'' risk estimates to alternative assumptions and methods likely 
to have substantial impact on the estimates. In addition, the Policy 
Assessment considered additional analyses characterizing the 
representativeness of the urban study areas within a broader national 
context to understand the roles that the annual and 24-hour standards 
may play in affording protection against effects related to both long- 
and short-term PM2.5 exposures.
    Based on the approach discussed above, the Policy Assessment 
reached conclusions related to the primary PM2.5 standards 
that reflected an

[[Page 3103]]

understanding of both evidence-based and risk-based considerations to 
inform two overarching questions related to: (1) The adequacy of the 
current suite of PM2.5 standards and (2) revisions to the 
standards that were appropriate to consider in this review to protect 
against health effects associated with both long- and short-term 
exposures to fine particles. When evaluating the health protection 
afforded by the current or any alternative suites of standards 
considered, the Policy Assessment took into account the four basic 
elements of the NAAQS: The indicator, averaging time, form, and level.
    The general approach for reviewing the primary PM2.5 
standards described above provided a comprehensive basis that helped to 
inform the Administrator's judgments in reaching her proposed and final 
decisions to revise the current suite of primary fine particle NAAQS 
and in responding to the remand of the 2006 primary annual 
PM2.5 standard.

B. Overview of Health Effects Evidence

    This section outlines the key information presented in section 
III.B of the proposal (77 FR 38906 to 38911, June 29, 2012) and 
discussed more fully in the Integrated Science Assessment (Chapters 2, 
4, 5, 6, 7, and 8) and the Policy Assessment (Chapter 2) related to 
health effects associated with fine particle exposures. Section III.B. 
of the proposal discusses available information on the health effects 
associated with exposures to PM2.5, including the nature of 
such health effects (section III.B.1) and associated limitations and 
uncertainties (section III.B.2), at-risk populations (section III.B.3), 
and potential PM2.5-related impacts on public health 
(section III.B.4). As was true in the last two reviews, evidence from 
epidemiological, controlled human exposure and animal toxicological 
studies played a key role in the Integrated Science Assessment's 
evaluation of the scientific evidence.
    The 2006 PM NAAQS review concluded that there was ``strong 
epidemiological evidence'' for linking long-term PM2.5 
exposures with cardiovascular-related and lung cancer mortality and 
respiratory-related morbidity and for linking short-term 
PM2.5 exposures with cardiovascular-related and respiratory-
related mortality and morbidity (U.S. EPA, 2004, p. 9-46; U.S. EPA, 
2005, p. 5-4). Overall, the evidence from epidemiological, 
toxicological, and controlled human exposure studies supported ``likely 
causal associations'' between PM2.5 and both mortality and 
morbidity from cardiovascular and respiratory diseases, based on ``an 
assessment of strength, robustness, and consistency in results'' (U.S. 
EPA, 2004, p. 9-48).\26\
---------------------------------------------------------------------------

    \26\ The term ``likely causal association'' was used in the 2004 
Criteria Document to summarize the strength of the available 
evidence available in the last review for PM2.5. However, 
this terminology was not based on a formal framework for evaluating 
evidence for inferring causation. Since the last review, the EPA has 
developed a more formal framework for reaching causal determinations 
with standardized language to express evaluation of the evidence 
(U.S. EPA, 2009a, section 1.5).
---------------------------------------------------------------------------

    In this review, based on the expanded body of evidence, the EPA 
finds that:

    (1) In looking across the extensive new scientific evidence 
available in this review, our overall understanding of health 
effects associated with fine particle exposures has been greatly 
expanded. The currently available evidence is largely consistent 
with evidence available in the last review and substantially 
strengthens what is known about the effects associated with fine 
particle exposures.
    (2) A number of large multi-city epidemiological studies have 
been conducted throughout the U.S., including extended analyses of 
long-term exposure studies that were important to inform decision-
making in the last review. The body of currently available 
scientific evidence has also been expanded greatly by the 
publication of a number of new multi-city, time-series studies that 
have used uniform methodologies to investigate the effects of short-
term PM2.5 exposures on public health. This body of 
evidence provides a more expansive data base and considers multiple 
locations representing varying regions and seasons that provide 
evidence of the influence of different air pollution mixes on 
PM2.5-associated health effects. These studies provide 
more precise estimates of the magnitude of effects associated with 
short-term PM2.5 exposure than most smaller-scale single-
city studies that were more commonly available in the last review. 
These studies have reported consistent increases in morbidity and/or 
premature mortality related to ambient PM2.5 
concentrations, with the strongest evidence reported for 
cardiovascular-related effects.
    (3) In addition, the findings of new toxicological and 
controlled human exposure studies greatly expand and provide 
stronger support for a number of potential biological mechanisms or 
pathways for cardiovascular and respiratory effects associated with 
long- and short-term PM exposures. These studies provide coherence 
and biological plausibility for the effects observed in 
epidemiological studies.
    (4) Using a more formal framework for reaching causal 
determinations than used in prior reviews,\27\ the EPA concludes 
that a causal relationship exists between both long- and short-term 
exposures to PM2.5 and premature mortality and 
cardiovascular effects and a likely causal relationship exists 
between long- and short-term PM2.5 exposures and 
respiratory effects. Further, there is evidence suggestive of a 
causal relationship between long-term PM2.5 exposures and 
other health effects, including developmental and reproductive 
effects (e.g., low birth weight, infant mortality) and carcinogenic, 
mutagenic, and genotoxic effects (e.g., lung cancer mortality).\28\
---------------------------------------------------------------------------

    \27\ The causal framework draws upon the assessment and 
integration of evidence from across epidemiological, controlled 
human exposure, and toxicological studies, and the related 
uncertainties that ultimately influence our understanding of the 
evidence. This framework employs a five-level hierarchy that 
classifies the overall weight of evidence and causality using the 
following categorizations: causal relationship, likely to be causal 
relationship, suggestive of a causal relationship, inadequate to 
infer a causal relationship, and not likely to be a causal 
relationship (U.S. EPA, 2009a, Table 1-3). The development of the 
causal framework reflects considerable input from CASAC and the 
public, with CASAC concluding that, ``The five-level classification 
of strength of evidence for causal inference has been systemically 
applied [for PM]; this approach has provided transparency and a 
clear statement of the level of confidence with regard to causation, 
and we recommend its continued use in future ISAs'' (Samet, 2009f, 
p. 1).
    \28\ These causal inferences are based not only on the more 
expansive epidemiological evidence available in this review but also 
reflect consideration of important progress that has been made to 
advance our understanding of a number of potential biologic modes of 
action or pathways for PM-related cardiovascular and respiratory 
effects (U.S. EPA, 2009a, chapter 5).
---------------------------------------------------------------------------

    (5) The newly available evidence significantly strengthens the 
link between long- and short-term exposure to PM2.5 and 
premature mortality, while providing indications that the magnitude 
of the PM2.5-mortality association with long-term 
exposures may be larger than previously estimated. The strongest 
evidence comes from recent studies investigating long-term exposure 
to PM2.5 and cardiovascular-related mortality. The 
evidence supporting a causal relationship between long-term 
PM2.5 exposure and mortality also includes consideration 
of new studies that demonstrated an improvement in community health 
following reductions in ambient fine particles.
    (6) Several new studies have examined the association between 
cardiovascular effects and long-term PM2.5 exposures in 
multi-city studies conducted in the U.S. and Europe. While studies 
were not available in the last review with regard to long-term 
exposure and cardiovascular-related morbidity, recent studies have 
provided new evidence linking long-term exposure to PM2.5 
with an array of cardiovascular effects such as heart attacks, 
congestive heart failure, stroke, and mortality. This evidence is 
coherent with studies of short-term exposure to PM2.5 
that have observed associations with a continuum of effects ranging 
from subtle changes in indicators of cardiovascular health to 
serious clinical events, such as increased hospitalizations and 
emergency department visits due to cardiovascular disease and 
cardiovascular mortality.
    (7) Extended analyses of studies available in the last review as 
well as new epidemiological studies conducted in the U.S. and abroad 
provide stronger evidence of respiratory-related morbidity effects 
associated with long-term PM2.5 exposure. The strongest 
evidence for respiratory-related

[[Page 3104]]

effects is from studies that evaluated decrements in lung function 
growth, increased respiratory symptoms, and asthma development. The 
strongest evidence from short-term PM2.5 exposure studies 
has been observed for increased respiratory-related emergency 
department visits and hospital admissions for chronic obstructive 
pulmonary disease (COPD) and respiratory infections.
    (8) The body of scientific evidence is somewhat expanded from 
the 2006 review but is still limited with respect to associations 
between long-term PM2.5 exposures and developmental and 
reproductive effects as well as cancer, mutagenic, and genotoxic 
effects. The strongest evidence for an association between 
PM2.5 and developmental and reproductive effects comes 
from epidemiological studies of low birth weight and infant 
mortality, especially due to respiratory causes during the post-
neonatal period (i.e., 1 month-12 months of age). With regard to 
cancer effects, ``[m]ultiple epidemiologic studies have shown a 
consistent positive association between PM2.5 and lung 
cancer mortality, but studies have generally not reported 
associations between PM2.5 and lung cancer incidence'' 
(U.S. EPA 2009a p. 2-13).
    (9) Efforts to evaluate the relationships between PM composition 
and health effects continue to evolve. While many constituents of 
PM2.5 can be linked with differing health effects, the 
evidence is not yet sufficient to allow differentiation of those 
constituents or sources that may be more closely related to specific 
health outcomes nor to exclude any individual component or group of 
components associated with any source categories from the fine 
particle mixture of concern.
    (10) Specific groups within the general population are at 
increased risk for experiencing adverse health effects related to PM 
exposures. The currently available evidence expands our 
understanding of previously identified at-risk populations (i.e., 
children, older adults, and individuals with pre-existing heart and 
lung disease) and supports the identification of additional at-risk 
populations (e.g., persons with lower socioeconomic status, genetic 
differences). Evidence for PM-related effects in these at-risk 
populations has expanded and is stronger than previously observed. 
There is emerging, though still limited, evidence for additional 
potentially at-risk populations, such as those with diabetes, people 
who are obese, pregnant women, and the developing fetus.
    (11) The population potentially affected by PM2.5 is 
large. In addition, large subgroups of the U.S. population have been 
identified as at-risk populations. While individual effect estimates 
from epidemiological studies may be small in size, the public health 
impact of the mortality and morbidity associations can be quite 
large given the extent of exposure. Taken together, this suggests 
that exposure to ambient PM2.5 concentrations can have 
substantial public health impacts.
    (12) While the currently available scientific evidence is 
stronger and more consistent than in previous reviews, providing a 
strong basis for decision making in this review, the EPA recognizes 
that important uncertainties and limitations in the health effects 
evidence remain. Epidemiological studies evaluating health effects 
associated with long- and short-term PM2.5 exposures have 
reported heterogeneity in responses between cities and geographic 
regions within the U.S. This heterogeneity may be attributed, in 
part, to differences in the fine particle composition or related to 
exposure measurement error, which can introduce bias and increased 
uncertainty in associated health effect estimates. Variability in 
the associations observed across PM2.5 epidemiological 
studies may be due in part to exposure error related to measurement-
related issues, the use of central fixed-site monitors to represent 
population exposure to PM2.5, models used in lieu of or 
to supplement ambient measurements, and our limited understanding of 
factors that may influence exposures (e.g., topography, the built 
environment, weather, source characteristics, ventilation usage, 
personal activity patterns, photochemistry). In addition, where 
PM2.5 and other pollutants (e.g., ozone, nitrogen 
dioxide, and carbon monoxide) are correlated, it can be difficult to 
distinguish the effects of the various pollutants in the ambient 
mixture (i.e., co-pollutant confounding).\29\
---------------------------------------------------------------------------

    \29\ A copollutant meets the criteria for potential confounding 
in PM-health associations if: (1) It is a potential risk factor for 
the health effect under study; (2) it is correlated with PM; and (3) 
it does not act as an intermediate step in the pathway between PM 
exposure and the health effect under study (U.S. EPA, 2004, p. 8-
10).
---------------------------------------------------------------------------

    While uncertainties and limitations still remain in the available 
health effects evidence, the Administrator judges the currently 
available scientific data base to be stronger and more consistent than 
in previous reviews providing a strong basis for decision making in 
this review.

C. Overview of Quantitative Characterization of Health Risks

    In addition to a comprehensive evaluation of the health effects 
evidence available in this review, the EPA conducted an expanded 
quantitative risk assessment for selected health endpoints to provide 
additional information and insights to inform decisions on the primary 
PM2.5 NAAQS.\30\ As discussed in section III.C of the 
proposal, the approach used to develop quantitative risk estimates 
associated with PM2.5 exposures was built on the approach 
used and lessons learned in the last review and focused on improving 
the characterization of the overall confidence in the risk estimates, 
including related uncertainties, by incorporating a number of 
enhancements, in terms of both the methods and data used in the 
analyses.
---------------------------------------------------------------------------

    \30\ The quantitative risk assessment conducted for this review 
is more fully described and presented in the Risk Assessment (U.S. 
EPA, 2010a) and summarized in detail in the Policy Assessment (U.S. 
EPA, 2011a, sections 2.2.2. and 2.3.4.2). The scope and methodology 
for this risk assessment were developed over the last few years with 
considerable input from CASAC and the public as described in section 
II.B.3 above.
---------------------------------------------------------------------------

    The goals of this quantitative risk assessment were largely the 
same as those articulated in the risk assessment conducted for the last 
review. These goals included: (1) To provide estimates of the potential 
magnitude of premature mortality and/or selected morbidity effects in 
the population associated with recent ambient levels of 
PM2.5 and with simulating just meeting the current and 
alternative suites of PM2.5 standards in 15 selected urban 
study areas,\31\ including, where data were available, consideration of 
impacts on at-risk populations; (2) to develop a better understanding 
of the influence of various inputs and assumptions on the risk 
estimates to more clearly differentiate among alternative suites of 
standards; and (3) to gain insights into the distribution of risks and 
patterns of risk reductions and the variability and uncertainties in 
those risk estimates. In addition, the quantitative risk assessment 
included nationwide estimates of the potential magnitude of premature 
mortality associated with long-term exposure to recent ambient 
PM2.5 concentrations to more broadly characterize this risk 
on a national scale and to support the interpretation of the more 
detailed risk estimates generated for selected urban study areas.
---------------------------------------------------------------------------

    \31\ The Risk Assessment concluded that these 15 urban study 
areas were generally representative of urban areas in the U.S. 
likely to experience relatively elevated levels of risk related to 
ambient PM2.5 exposure with the potential for better 
characterization at the higher end of that distribution (U.S. EPA, 
2011a, p. 2-42; U.S. EPA, 2010a, section 4.4, Figure 4-17). The 
representativeness analysis also showed that the 15 urban study 
areas do not capture areas with the highest baseline morality risks 
or the oldest populations (both of which can result in higher 
PM2.5-related mortality estimates). However, some of the 
areas with the highest values for these attributes had relatively 
low PM2.5 concentrations (e.g., urban areas in Florida) 
and, consequently, the Risk Assessment concluded failure to include 
these areas in the set of urban study areas was unlikely to exclude 
high PM2.5-risk locations (U.S. EPA, 2010a, section 
4.4.1).
---------------------------------------------------------------------------

    The expanded and updated risk assessment conducted in this review 
included estimates of risk for: (1) All-cause, ischemic heart disease-
related, cardiopulmonary-related, and lung cancer-related mortality 
associated with long-term PM2.5 exposure; (2) non-
accidental, cardiovascular-related, and respiratory-related mortality 
associated with short-term PM2.5 exposure; and (3) 
cardiovascular-related and respiratory-related hospital admissions and 
asthma-related emergency department visits

[[Page 3105]]

associated with short-term PM2.5 exposure.\32\
---------------------------------------------------------------------------

    \32\ The evidence available for these selected health effect 
endpoints generally focused on the entire population, although some 
information was available to support analyses that considered 
differences in estimated risk for at-risk populations including 
older adults and persons with pre-existing cardiovascular and 
respiratory diseases.
---------------------------------------------------------------------------

    The Risk Assessment included a core set of risk estimates 
supplemented by an alternative set of risk results generated using 
single-factor and multi-factor sensitivity analyses. The core set of 
risk estimates was developed using the combination of modeling elements 
and input data sets identified in the Risk Assessment as having higher 
confidence relative to inputs used in the sensitivity analyses. The 
results of the sensitivity analyses provided information to evaluate 
and rank the potential impacts of key sources of uncertainty on the 
core risk estimates. In addition, the sensitivity analyses represented 
a set of reasonable alternatives to the core set of risk estimates that 
fell within an overall set of plausible risk estimates surrounding the 
core estimates.
    The EPA recognized that there were many sources of variability and 
uncertainty inherent in the inputs to its quantitative risk 
assessment.\33\ The design of the risk assessment included a number of 
elements to address these issues in order to increase the overall 
confidence in the risk estimates generated for the 15 urban study 
areas, including using guidance from the World Health Organization 
(WHO, 2008) as a framework for characterizing uncertainty in the 
analyses.\34\
---------------------------------------------------------------------------

    \33\ Variability refers to the heterogeneity of a variable of 
interest within a population or across different populations. 
Uncertainty refers to the lack of knowledge regarding the actual 
values of inputs to an analysis (U.S. EPA, 2010a, p. 3-63).
    \34\ The extent to which key sources of potential variability 
were (or were not) fully captured in the design of the risk 
assessment are discussed in section 3.5.2 of the Risk Assessment 
(U.S. EPA, 2010a, pp. 3-67 to 3-69).
---------------------------------------------------------------------------

    With respect to the sources of variability, the Risk Assessment 
considered those that contributed to differences in risk across urban 
study areas, but did not directly affect the degree of risk reduction 
associated with the simulation of just meeting current or alternative 
standard levels (e.g., differences in baseline incidence rates, 
demographics and population behavior). The Risk Assessment also focused 
on factors that not only introduced variability into risk estimates 
across study areas, but also played an important role in determining 
the magnitude of risk reductions upon simulation of just meeting 
current or alternative standard levels (e.g., peak-to-mean ratios of 
ambient PM2.5 concentrations within individual urban study 
areas and the nature of the rollback approach used to simulate just 
meeting the current or alternative standards). Key sources of potential 
variability that were likely to affect population risks included the 
following: (1) Intra-urban variability in ambient PM2.5 
concentrations, including PM2.5 composition; (2) variability 
in the patterns of reductions in PM2.5 concentrations 
associated with different rollback approaches when simulating just 
meeting the current or alternative standards; (3) co-pollutant 
exposures; (4) factors related to demographic and socioeconomic status; 
(5) behavioral differences across urban study areas (e.g., time spent 
outdoors); (6) baseline incidence rates; and (7) longer-term temporal 
variability in ambient PM2.5 concentrations reflecting 
meteorological trends as well as future changes in the mix of 
PM2.5 sources, including changes in air quality related to 
future regulatory actions.
    With regard to uncertainties, single and multi-factor sensitivity 
analyses were combined with a qualitative analysis to assess the impact 
of potential sources of uncertainty on the core risk estimates. Key 
sources of uncertainty included: (1) Characterizing intra-urban 
population exposure in the context of epidemiological studies linking 
PM2.5 to specific health effects; (2) statistical fit of the 
concentration-response functions for short-term exposure-related health 
endpoints; (3) shape of the concentration-response functions; (4) 
specifying the appropriate lag structure for short-term exposure 
studies; (5) transferability of concentration-response functions from 
study locations to urban study area locations for long-term exposure-
related health endpoints; (6) use of single-city versus multi-city 
studies in the derivation of concentration-response functions; (7) 
impact of historical air quality on estimates of health risk associated 
with long-term PM2.5 exposures; and (8) potential variation 
in effect estimates reflecting compositional differences in 
PM2.5.
    Beyond characterizing uncertainty and variability, a number of 
design elements were included in the risk assessment to increase the 
overall confidence in the risk estimates generated for the 15 urban 
study areas (U.S. EPA, 2011a, pp. 2-38 to 2-41). These elements 
included: (1) Use of a deliberative process for specifying components 
of the risk model that reflects consideration of the latest research on 
PM2.5 exposure and risk (U.S. EPA, 2010a, section 5.1.1); 
(2) integration of key sources of variability into the design as well 
as the interpretation of risk estimates (U.S. EPA, 2010a, section 
5.1.2); (3) assessment of the degree to which the urban study areas are 
representative of areas in the U.S. experiencing higher 
PM2.5-related risk (U.S. EPA, 2010a, section 5.1.3); and (4) 
identification and assessment of important sources of uncertainty and 
the impact of these uncertainties on the core risk estimates (U.S. EPA, 
2010a, section 5.1.4). Further, additional analyses examined potential 
bias and overall confidence in the risk estimates. Greater confidence 
is associated with risk estimates based on simulated annual mean 
PM2.5 concentrations that are within the region of the air 
quality distribution used in deriving the concentration-response 
functions where the bulk of the data reside (e.g., within one standard 
deviation around the long-term mean PM2.5 concentration) 
(U.S. EPA, 2011a, p. 2-38).
    Key observations and insights from the PM2.5 risk 
assessment, together with important caveats and limitations, were 
discussed in section III.C.3 of the proposal. In general, in 
considering the set of quantitative risk estimates and related 
uncertainties and limitations related to long- and short-term 
PM2.5 exposure together with consideration of the health 
endpoints which could not be quantified, the Policy Assessment 
concluded this information provided strong evidence that risks 
estimated to remain upon simulating just meeting the current suite of 
PM2.5 standards are important from a public health 
perspective, both in terms of severity and magnitude of effects. 
Patterns of increasing estimated risk reductions were generally 
observed as either the annual or 24-hour standard level, or both, were 
reduced over the ranges considered in the Risk Assessment.
    The magnitude of both long- and short-term exposure-related risk 
estimated to remain upon just meeting the current suite of standards as 
well as alternative standard levels was strongly associated with the 
simulated change in annual mean PM2.5 concentrations. 
Although long- and short-term exposure-related mortality rates have 
similar patterns in terms of the subset of urban study areas 
experiencing risk reductions for the current suite of standard levels, 
the magnitude of risk remaining is higher for long-term exposure-
related mortality and substantially lower for short-term exposure-
related mortality. Short-term exposure-related morbidity risk estimates 
were greater for cardiovascular-related than respiratory-related events 
and emergency

[[Page 3106]]

department visits for asthma-related events were significant: 
Furthermore, most of the aggregate risk associated with short-term 
exposures was not primarily driven by the small number of days with 
PM2.5 concentrations in the upper tail of the air quality 
distribution, but rather by the large number of days with 
PM2.5 concentrations at and around the mean of the 
distribution, that is, the 24-hour average concentrations that are in 
the low- to mid-range, well below the peak 24-hour concentrations (U.S. 
EPA, 2011a, p. 2-3).
    With regard to characterizing estimates of PM2.5-related 
risk associated with simulation of alternative standards, the Policy 
Assessment recognized that greater overall confidence was associated 
with estimates of risk reduction than for estimates of absolute risk 
remaining (U.S. EPA, 2011a, p. 2-94). Furthermore, the Policy 
Assessment recognized that estimates of absolute risk remaining for 
each of the alternative standard levels considered, particularly in the 
context of long-term exposure-related mortality, may be 
underestimated.\35\ In addition, the Policy Assessment observed that in 
considering the overall confidence associated with the quantitative 
analyses, the Risk Assessment recognized that: (1) Substantial 
variability existed in the magnitude of risk remaining across urban 
study areas and (2) in general, higher confidence was associated with 
risk estimates based on PM2.5 concentrations near the mean 
PM2.5 concentrations in the underlying epidemiological 
studies providing the concentration-response functions (e.g., within 
one standard deviation of the mean PM2.5 concentration 
reported). Furthermore, although the Risk Assessment estimated that the 
alternative 24-hour standard levels considered (when controlling) would 
result in additional estimated risk reductions beyond those estimated 
for alternative annual standard levels alone, these additional 
estimated reductions were highly variable. Conversely, the Risk 
Assessment recognized that alternative annual standard levels, when 
controlling, resulted in more consistent risk reductions across urban 
study areas, thereby potentially providing a more consistent degree of 
public health protection (U.S. EPA, 2010a, p. 5-17).
---------------------------------------------------------------------------

    \35\ Based on the consideration of both the qualitative and 
quantitative assessments of uncertainty, the Risk Assessment 
concluded that it is unlikely that the estimated risks are over-
stated, particularly for premature mortality related to long-term 
PM2.5 exposures. In fact, the Policy Assessment and the 
Risk Assessment concluded that the core risk estimates for this 
category of health effects may well be biased low based on 
consideration of alternative model specifications evaluated in the 
sensitivity analyses (U.S. EPA, 2011a, p. 2-41; U.S. EPA, 2010a, p. 
5-16; Figures 4-7 and 4-8). In addition, the Policy Assessment 
recognized that the currently available scientific information 
included evidence for a broader range of health endpoints and at-
risk populations beyond those included in the quantitative risk 
assessment, including decrements in lung function growth and 
respiratory symptoms in children as well as reproductive and 
developmental effects (U.S. EPA, 2011a, section 2.2.1).
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D. Conclusions on the Adequacy 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 and other information reflected in the 
Integrated Science Assessment, the Risk Assessment, and the Policy 
Assessment, the existing standards should be retained or revised. In 
considering the adequacy of the current suite of PM2.5 
standards, the Administrator has considered the large body of evidence 
presented and assessed in the Integrated Science Assessment (U.S. EPA, 
2009a), the quantitative assessment of risks, staff conclusions and 
associated rationales presented in the Policy Assessment, views 
expressed by CASAC, and public comments. The Administrator has taken 
into account both evidence- and risk-based considerations \36\ in 
developing final conclusions on the adequacy of the current primary 
PM2.5 standards.
---------------------------------------------------------------------------

    \36\ Evidence-based considerations include the assessment of 
epidemiological, toxicological, and controlled human exposure 
studies evaluating long- or short-term exposures to 
PM2.5, with supporting evidence related to dosimetry and 
potential pathways/modes of action, as well as the integration of 
evidence across each of these disciplines, as assessed in the 
Integrated Science Assessment (U.S. EPA, 2009a) and focus on the 
policy-relevant considerations as discussed in section III.B above 
and in the Policy Assessment (U.S. EPA, 2011a, section 2.2.1). Risk-
based considerations draw from the results of the quantitative 
analyses presented in the Risk Assessment (U.S. EPA, 2010a) and 
focus on the policy-relevant considerations as discussed in section 
III.C above and in the Policy Assessment (U.S. EPA, 2011a, section 
2.2.2).
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a. Evidence- and Risk-based Considerations in the Policy Assessment
    In considering the available epidemiological evidence in this 
review, the Policy Assessment took a broader approach than was used in 
the last review. This approach reflected the more extensive and 
stronger body of evidence available since the last review on health 
effects related to both long- and short-term exposure to 
PM2.5. As discussed in section III.A.3 above, this broader 
approach focused on setting the annual standard as the ``generally 
controlling'' standard for lowering both short- and long-term 
PM2.5 concentrations and so providing requisite protection 
to public health. In conjunction with such an annual standard, this 
approach focused on setting the 24-hour standard to provide 
supplemental protection against days with high peak PM2.5 
concentrations.
    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 Policy 
Assessment considered whether: (1) Statistically significant health 
effects associations with long- or short-term exposures to fine 
particles occur in areas that would likely have met the current 
PM2.5 standards [see American Trucking Associations, 283 F. 
3d at 369, 376 (revision of level of PM NAAQS justified when health 
effects are observed in areas meeting the existing standard)], and (2) 
associations with long-term exposures to fine particles extend down to 
lower air quality concentrations than had previously been observed. 
With regard to associations observed in long-term PM2.5 
exposure studies, the Policy Assessment recognized that extended 
follow-up analyses of the ACS and Harvard Six Cities studies provided 
consistent and stronger evidence of an association with mortality at 
lower air quality distributions than had previously been observed (U.S. 
EPA, 2011a, pp. 2-31 to 2-32). The original and reanalysis of the ACS 
study reported positive and statistically significant effects 
associated with a long-term mean PM2.5 concentration of 18.2 
[mu]g/m\3\ across 50 metropolitan areas for 1979 to 1983 (Pope et al., 
1995; Krewski et al., 2000).\37\ In extended analyses, positive and 
statistically significant effects of approximately similar magnitude 
were associated with declining PM2.5 concentrations, from an 
aggregate long-term mean in 58 metropolitan areas of 21.2 [micro]g/m\3\ 
in the original monitoring period (1979 to 1983) to 14.0 [micro]g/m\3\ 
for 116 metropolitan areas in the most recent years evaluated (1999-
2000), with an overall average across the two study periods in 51 
metropolitan areas of 17.7 [micro]g/m\3\ (Pope et al., 2002; Krewski et 
al., 2009). With regard to the Harvard Six Cities Study, the original 
and reanalysis reported positive and statistically significant effects 
associated

[[Page 3107]]

with a long-term mean PM2.5 concentration of 18.0 [mu]g/m\3\ 
for 1980 to 1985 (Dockery et al., 1993; Krewski et al., 2000). In an 
extended follow-up of this study, the aggregate long-term mean 
concentration across all years evaluated was 16.4 [mu]g/m\3\ for 1980 
to 1988 \38\ (Laden et al., 2006). In an additional analysis of the 
extended follow-up of the Harvard Six Cities study, investigators 
reported that the concentration-response relationship was linear and 
``clearly continuing below the level'' of the current annual standard 
(U.S. EPA, 2009a, p. 7-92; Schwartz et al., 2008).
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    \37\ The study periods referred to in the Policy Assessment 
(U.S. EPA, 2011a) and in this final rule reflect the years of air 
quality data that were included in the analyses, whereas the study 
periods identified in the Integrated Science Assessment (U.S. EPA, 
2009a) reflect the years of health event data that were included.
    \38\ Aggregate mean concentration provided by study author 
(personal communication from Dr. Francine Laden, 2009).
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    Cohort studies conducted since the last review provided additional 
evidence of mortality associated with air quality distributions that 
are generally lower than those reported in the ACS and Harvard Six 
Cities studies, with effect estimates that were similar or, in some 
studies, significantly greater in magnitude than in the ACS and Harvard 
Six Cities studies (see also, section III.D.1.a of the proposal, 77 FR 
38918 to 28919; U.S. EPA, 2011a, pp. 2-32 to 2-33). The Women's Health 
Initiative (WHI) study reported positive and most often statistically 
significant associations between long-term PM2.5 exposure 
and cardiovascular-related mortality as well as morbidity effects, with 
much larger relative risk estimates for mortality than in the ACS and 
Harvard Six Cities studies, at an aggregate long-term mean 
PM2.5 concentration of 12.9 [mu]g/m\3\ for 2000 (Miller et 
al., 2007).\39\
---------------------------------------------------------------------------

    \39\ The Policy Assessment noted that in comparison to other 
long-term exposure studies, the Miller et al. (2007) study was more 
limited in that it was based on only one year of air quality data 
(U.S. EPA, 2011a, p. 2-82). The proposal further noted that the air 
quality data considered were extrapolated from that one single year 
of air quality data (2000) to the whole study, and that the air 
quality data post-dated the years of health events considered (i.e., 
1994 to 1998) (77 FR 38918, fn 62).
---------------------------------------------------------------------------

    Using the Medicare cohort, Eftim et al. (2008) reported somewhat 
higher effect estimates than in the ACS and Harvard Six Cities studies 
with aggregate long-term mean concentrations of 13.6 [mu]g/m\3\ and 
14.1 [mu]g/m\3\, respectively, for 2000 to 2002. Zeger et al. (2008) 
reported associations between long-term PM2.5 exposure and 
mortality for the eastern region of the U.S. at an aggregated long-term 
PM2.5 median concentration of 14.0 [mu]g/m\3\, although no 
association was reported for the western region with an aggregate long-
term PM2.5 median concentration of 13.1 [mu]g/m\3\ (U.S. 
EPA, 2009a, p. 7-88).\40\
---------------------------------------------------------------------------

    \40\ Zeger et al. (2008) also reported positive and 
statistically significant effects for the central region, with an 
aggregate long-term mean PM2.5 concentration of 10.7 
[mu]g/m\3\. However, in contrast to the eastern and western risk 
estimates, the central risk estimate increased with adjustment for 
COPD (used as a proxy for smoking status). Due to the potential for 
confounding bias influencing the risk estimate for the central 
region, the Policy Assessment did not focus on the results reported 
in the central region to inform the adequacy of the current suite of 
standards or alternative annual standard levels (U.S. EPA, 2011a, p. 
2-32).
---------------------------------------------------------------------------

    Premature mortality in children reported in a national infant 
mortality study as well as mortality in a cystic fibrosis cohort 
including both children and adults reported positive but statistically 
nonsignificant effects associated with long-term aggregate mean 
concentrations of 14.8 [mu]g/m\3\ and 13.7 [mu]g/m\3\, respectively 
(Woodruff et al., 2008; Goss et al., 2004).
    With respect to respiratory morbidity effects associated with long-
term PM2.5 exposure, the across-city mean of 2-week average 
PM2.5 concentrations reported in the initial Southern 
California Children's Health Study was approximately 15.1 [micro]g/m\3\ 
(Peters et al., 1999). These results were found to be consistent with 
results of cross-sectional analyses of the 24-Cities Study (Dockery et 
al., 1996; Raizenne et al., 1996), which reported a long-term cross-
city mean PM2.5 concentration of 14.5 [mu]g/m\3\.\41\ In 
this review, extended analyses of the Southern California Children's 
Health Study provide stronger evidence of PM2.5-related 
respiratory effects, at lower air quality concentrations than had 
previously been reported, with a four-year aggregate mean concentration 
of 13.8 [mu]g/m\3\ across the 12 study communities (McConnell et al., 
2003; Gauderman et al., 2004, U.S. EPA, 2009a, Figure 7-4).
---------------------------------------------------------------------------

    \41\ See American Farm Bureau Federation v. EPA, 559 F. 3d at 
525 (noting the importance of these studies, as well as EPA's 
failure to properly take them into account).
---------------------------------------------------------------------------

    In also considering health effects for which the Integrated Science 
Assessment concluded evidence was suggestive of a causal relationship, 
the Policy Assessment noted a limited number of birth outcome studies 
that reported positive and statistically significant effects related to 
aggregate long-term mean PM2.5 concentrations down to 
approximately 12 [mu]g/m\3\ (U.S. EPA, 2011a, p. 2-33).
    Collectively, the Policy Assessment concluded that currently 
available evidence provided support for associations between long-term 
PM2.5 exposure and mortality and morbidity effects that 
extend to distributions of PM2.5 concentrations that are 
lower than those that had previously been associated with such effects, 
with aggregate long-term mean PM2.5 concentrations extending 
to well below the level of the current annual standard.
    The Policy Assessment also considered the long-term mean 
PM2.5 concentrations in short-term exposure studies in 
assessing the appropriateness of the level of the current annual 
standard. See American Farm Bureau Federation v. EPA, 559 F. 3d at 522, 
523-24 (remanding 2006 standard because the EPA had not adequately 
explained its choice not to consider long-term means of short-term 
exposure studies in assessing adequacy of primary annual 
PM2.5 standard). In light of the mixed findings reported in 
single-city, short-term exposure studies, the Policy Assessment placed 
comparatively greater weight on the results from multi-city studies in 
considering the adequacy of the current suite of standards (U.S. EPA, 
2011a, pp. 2-34 to 2-35).
    With regard to associations reported in short-term PM2.5 
exposure studies, the Policy Assessment recognized that long-term mean 
concentrations reported in new multi-city U.S. and Canadian studies 
provided evidence of associations between short-term PM2.5 
exposure and mortality at similar air quality distributions to those 
that had previously been observed in an 8-cities Canadian study 
(Burnett and Goldberg, 2003; aggregate long-term mean PM2.5 
concentration of 13.3 [mu]g/m\3\). In a multi-city time-series analysis 
of 112 U.S. cities, Zanobetti and Schwartz (2009) reported a positive 
and statistically significant association with all-cause, 
cardiovascular-related (e.g., heart attacks, stroke), and respiratory-
related mortality and short-term PM2.5 exposure, in which 
the aggregate long-term mean PM2.5 concentration was 13.2 
[mu]g/m\3\ (U.S. EPA, 2009a, Figure 6-24). Furthermore, city-specific 
effect estimates indicated the association between short-term exposure 
to PM2.5 and total mortality and cardiovascular- and 
respiratory-related mortality was consistently positive for an 
overwhelming majority (99 percent) of the 112 cities across a wide 
range of air quality concentrations (long-term mean concentrations 
ranging from 6.6 [mu]g/m\3\ to 24.7 [mu]g/m\3\; U.S. EPA, 2009a, Figure 
6-24, p. 6-178 to 179). The EPA staff noted that for all-cause 
mortality, city-specific effect estimates were statistically 
significant for 55 percent of the 112 cities, with long-term city-mean 
PM2.5 concentrations ranging from 7.8 [mu]g/m\3\ to 18.7 
[mu]g/m\3\ and 24-hour PM2.5 city-mean 98th percentile 
concentrations ranging from 18.4 to 64.9

[[Page 3108]]

[mu]g/m\3\ (personal communication with Dr. Antonella Zanobetti, 
2009).\42\
---------------------------------------------------------------------------

    \42\ Single-city Bayes-adjusted effect estimates for the 112 
cities analyzed in Zanobetti and Schwartz (2009) were provided by 
the study authors (personal communication with Dr. Antonella 
Zanobetti, 2009; see also U.S. EPA, 2009a, Figure 6-24).
---------------------------------------------------------------------------

    With regard to cardiovascular and respiratory morbidity effects, in 
the first analysis of the Medicare cohort conducted by Dominici et al. 
(2006a) across 204 U.S. counties, investigators reported a 
statistically significant association with hospitalizations for 
cardiovascular and respiratory diseases and short-term PM2.5 
exposure, in which the aggregate long-term mean PM2.5 
concentration was 13.4 [mu]g/m\3\. Furthermore, a sub-analysis 
restricted to days with 24-hour average concentrations of 
PM2.5 at or below 35 [mu]g/m\3\ indicated that, in spite of 
a reduced statistical power from a smaller number of study days, 
statistically significant associations were still observed between 
short-term exposure to PM2.5 and hospital admissions for 
cardiovascular and respiratory diseases (Dominici, 2006b).\43\ In an 
extended analysis of this cohort, Bell et al. (2008) reported a 
positive and statistically significant increase in cardiovascular 
hospitalizations associated with short-term PM2.5 exposure, 
in which the aggregate long-term mean PM2.5 concentration 
was 12.9 [mu]g/m\3\. These results, along with the observation that 
approximately 50 percent of the 204 county-specific mean 98th 
percentile PM2.5 concentrations in the study aggregated 
across all years were below the 24-hour standard of 35 [mu]g/m\3\, not 
only indicated that effects are occurring in areas that would meet the 
current standards but also suggested that the overall health effects 
observed across the U.S. are not primarily driven by the higher end of 
the PM2.5 air quality distribution (Bell, 2009a, personal 
communication from Dr. Michelle Bell regarding air quality data for 
Bell et al., 2008 and Dominici et al., 2006a).
---------------------------------------------------------------------------

    \43\ This sub-analysis was not included in the original 
publication (Dominici et al., 2006a). The study authors provided 
sub-analysis results for the Administrator's consideration as a 
letter to the docket following publication of the proposed rule in 
January 2006 (personal communication with Dr. Francesca Dominici, 
2006b). As noted in section III.A.3, this study is part of the basis 
for the conclusion that there is no evidence suggesting that risks 
associated with long-term exposures are likely to be 
disproportionately driven by peak 24-hour concentrations.
---------------------------------------------------------------------------

    Collectively, the Policy Assessment concluded that the findings 
from short-term PM2.5 exposure studies provided evidence of 
PM2.5-associated health effects occurring in areas that 
would likely have met the current suite of PM2.5 standards 
(U.S. EPA, 2011a, p. 2-35). These findings were further bolstered by 
evidence of statistically significant PM2.5-related health 
effects occurring in analyses restricted to days in which 24-hour 
average PM2.5 concentrations were below 35 [mu]g/m\3\ 
(Dominici, 2006b).
    In evaluating the currently available scientific evidence, as 
summarized in section III.B of the proposal, the Policy Assessment 
first concluded that there was stronger and more consistent and 
coherent support for associations between long- and short-term 
PM2.5 exposures and a broad range of health outcomes than 
was available in the last review, providing the basis for fine particle 
standards at least as protective as the current PM2.5 
standards (U.S. EPA, 2011a, p. 2-26). Having reached this initial 
conclusion, the Policy Assessment addressed the question of whether the 
available evidence supported consideration of standards that were more 
protective than the current standards. In so doing, the Policy 
Assessment considered whether there was now evidence that health effect 
associations have been observed in areas that likely met the current 
suite of PM2.5 standards. As discussed above, long- and 
short-term PM2.5 exposure studies provided evidence of 
associations with mortality and cardiovascular and respiratory effects 
both at lower ambient PM2.5 concentrations than had been 
observed in the previous review and at concentrations allowed by the 
current standards (U.S. EPA, 2011a, p. 2-35).
    In reviewing this information, the Policy Assessment recognized 
that important limitations and uncertainties associated with this 
expanded body of scientific evidence, as discussed in section III.B.2 
of the proposal, needed to be carefully considered in determining the 
weight to be placed on the body of studies available in this review. 
Taking these limitations and uncertainties into consideration, the 
Policy Assessment concluded that the currently available evidence 
clearly calls into question whether the current suite of primary 
PM2.5 standards protects public health with an adequate 
margin of safety from effects associated with long- and short-term 
exposures. Furthermore, the Policy Assessment concluded this evidence 
provides strong support for considering fine particle standards that 
would afford increased protection beyond that afforded by the current 
standards (U.S. EPA, 2011a, p. 2-35).
    In addition to evidence-based consideration, the Policy Assessment 
also considered the extent to which health risks estimated to occur 
upon simulating just meeting 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. In so doing, the Policy Assessment first noted that the 
quantitative risk assessment addresses: (1) The core PM2.5-
related risk estimates; (2) the related uncertainty and sensitivity 
analyses, including additional sets of reasonable risk estimates 
generated to supplement the core analysis; (3) an assessment of the 
representativeness of the urban study areas within a national context; 
\44\ and (4) consideration of patterns in design values and air quality 
monitoring data to inform interpretation of the risk estimates, as 
discussed in section III.C above.
---------------------------------------------------------------------------

    \44\ Based on analyses of the representativeness of the 15 urban 
study areas in the broader national context, the Policy Assessment 
concludes that these study areas are generally representative of 
urban areas in the U.S. likely to experience relatively elevated 
levels of risk related to ambient PM2.5 exposures (U.S. 
EPA, 2011a, p. 2-42).
---------------------------------------------------------------------------

    In considering the health risks estimated to remain upon simulation 
of just meeting the current suite of standards and considering both the 
qualitative and quantitative assessment of uncertainty completed as 
part of the assessment, the Policy Assessment concluded these risks are 
important from a public health standpoint and provided strong support 
for consideration of alternative standards that would provide increased 
protection beyond that afforded by the current PM2.5 (U.S. 
EPA, 2011a, pp. 2-47 to 2-48). This conclusion reflected consideration 
of both the severity and the magnitude of the effects. For example, the 
Risk Assessment indicated the possibility that premature deaths related 
to ischemic heart disease associated with long-term PM2.5 
exposure alone would likely be on the order of thousands of deaths per 
year in the 15 urban study areas upon simulating just meeting the 
current standards \45\ (U.S. EPA, 2011a, pp. 2-46 to 2-47). Moreover, 
additional risks were anticipated for premature mortality related to 
cardiopulmonary effects and lung cancer associated with long-term 
PM2.5 exposure as well as mortality and cardiovascular- and 
respiratory-related morbidity effects (e.g., hospital admissions, 
emergency department visits) associated with short-term 
PM2.5 exposures. Based on the consideration of both 
qualitative and

[[Page 3109]]

quantitative assessments of uncertainty completed as part of the 
quantitative risk assessment, the Risk Assessment concluded that it was 
unlikely that the estimated risks are over-stated, particularly for 
mortality related to long-term PM2.5 exposure, and may well 
be biased low based on consideration of alternative model 
specifications evaluated in the sensitivity analyses (U.S. EPA, 2010a, 
p. 5-16; U.S. EPA, 2011a, p. 2-41). Furthermore, the currently 
available scientific information summarized in section III.B of the 
proposal provided evidence for a broader range of health endpoints and 
at-risk populations beyond those included in the quantitative risk 
assessment (U.S. EPA, 2011a, p. 2-47).
---------------------------------------------------------------------------

    \45\ Premature mortality for all causes attributed to 
PM2.5 exposure was estimated to be on the order of tens 
of thousands of deaths per year on a national scale based on 2005 
air quality data (U.S. EPA, 2010a, Appendix G, Table G-1).
---------------------------------------------------------------------------

b. CASAC Advice
    The CASAC, based on its review of drafts of the Integrated Science 
Assessment, the Risk Assessment, and the Policy Assessment, provided an 
array of advice both with regard to interpreting the scientific 
evidence and quantitative risk assessment, as well as with regard to 
consideration of the adequacy of the current PM2.5 standards 
(Samet, 2009a,b,c,d,e,f; Samet 2010a,b,c,d). With regard to the 
adequacy of the current standards, CASAC concluded that the ``currently 
available information clearly calls into question the adequacy of the 
current standards'' (Samet, 2010d, p. i) and that the current standards 
are ``not protective'' (Samet, 2010d, p. 1). Further, in commenting on 
the first draft Policy Assessment, CASAC noted:

    With regard to the integration of evidence-based and risk-based 
considerations, CASAC concurs with EPA's conclusion that the new 
data strengthens the evidence available on associations previously 
considered in the last round of the assessment of the 
PM2.5 standard. CASAC also agrees that there are 
significant public health consequences at the current levels of the 
standard that justify consideration of lowering the PM2.5 
NAAQS further (Samet, 2010c, p. 12).

c. Administrator's Proposed Conclusions Concerning the Adequacy of the 
Current Primary PM2.5 Standards

    At the time of the proposal, in considering the body of scientific 
evidence, the Administrator concluded there was stronger and more 
consistent and coherent support for associations between long- and 
short-term PM2.5 exposure and a broader range of health 
outcomes than was available in the last review, providing the basis for 
fine particle standards at least as protective as the current 
PM2.5 standards. In particular, the Administrator recognized 
in section III.D.4 of the proposal that the Integrated Science 
Assessment concluded that the results of epidemiological and 
experimental studies form a plausible and coherent data set that 
supports a causal relationship between long- and short-term 
PM2.5 exposures and mortality and cardiovascular effects and 
a likely causal relationship between long- and short-term 
PM2.5 exposures and respiratory effects. Furthermore, the 
Administrator reflected that effects had been observed at lower ambient 
PM2.5 concentrations than what had been observed in the last 
review, including at ambient PM2.5 concentrations in areas 
that likely met the current PM2.5 NAAQS. With regard to the 
results of the quantitative risk assessment, the Administrator noted 
that the Risk Assessment concluded that the risks estimated to remain 
upon simulation of just meeting the current standards were important 
from a public health standpoint in terms of both the severity and 
magnitude of the effects.
    At the time of the proposal, in considering whether the current 
suite of PM2.5 standards should be revised to provide 
requisite public health protection, the Administrator carefully 
considered the staff conclusions and rationales presented in the Policy 
Assessment, 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 epidemiological 
studies and provisionally found the evidence of serious health effects 
reported in long- and short-term exposure studies conducted in areas 
that would have met 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 Integrated Science Assessment and Policy 
Assessment, 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- and short-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.\46\
---------------------------------------------------------------------------

    \46\ 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 III.E.4.
---------------------------------------------------------------------------

    Based on her consideration of these conclusions, as well as 
consideration of CASAC's conclusion that the evidence and risk 
assessment clearly called into question the adequacy of the public 
health protection provided by the current PM2.5 NAAQS and 
public comments on the proposal, the Administrator provisionally 
concluded that the current primary PM2.5 standards, taken 
together, were not requisite to protect public health with an adequate 
margin of safety and that revision was needed to provide increased 
public health protection. The Administrator provisionally concluded 
that the scientific evidence and information on risk provided strong 
support for consideration of alternative standards that would provide 
increased public health protection beyond that afforded by the current 
PM2.5 standards.
2. Comments on the Need for Revision
    This section addresses general comments based on relevant facts 
that either support or oppose any change to the current suite of 
primary PM2.5 standards. Comments on specific long- and 
short-term exposure studies that relate to consideration of the 
appropriate levels of the annual and 24-hour standards are addressed in 
section III.E.4 below. Many public comments asserted that the current 
PM2.5 standards are insufficient to protect public health 
with an adequate margin of safety and that revisions to the standards 
are therefore appropriate, indeed necessitated.
    Among those calling for revisions to the current standards were the 
Children's Health Protection Advisory Committee (CHPAC); major medical 
and public health groups including the American Heart Association 
(AHA), American Lung Association (ALA), American Public Health 
Association (APHA), American Thoracic Society (ATS); the Physicians for 
Social Responsibility (PSR); major environmental groups such as the 
Clean Air Council, Clean Air Task Force, Earthjustice, Environmental 
Defense Fund (EDF), National Resources Defense Council (NRDC), and 
Sierra Club; many environmental justice organizations as

[[Page 3110]]

well as medical doctors, academic researchers, health professionals, 
and many private citizens. For example, the American Heart Association 
and other major national public health and medical organizations stated 
that, ``[o]ur organizations are keenly aware of the public health and 
medical threats from particulate matter'' and called on the EPA to 
``significantly strengthen'' both the annual and 24-hour 
PM2.5 standards ``to help us protect the health of our 
patients and our nation'' (AHA et al., 2012, pp. 1 and 13). AHA et al. 
and ALA et al., as well as a group of more than 350 physicians, 
environmental health researchers, and public health and medical 
professionals articulated similar comments on the available evidence:

    Ample scientific evidence supports adopting tighter standards to 
protect the health of people who are most susceptible to the serious 
health effects of these pollutants. More than 10,000 peer-reviewed 
scientific studies have been published since 1997 when EPA adopted 
the current annual standard. These studies validate and extend 
earlier epidemiologic research linking both acute and chronic fine 
particle pollution with serious morbidity and mortality. The newer 
research has also expanded our understanding of the range of health 
outcomes associated with PM and has identified adverse respiratory 
and cardiovascular health effects at lower exposure levels than 
previously reported. As discussed and interpreted in the EPA's 2009 
Integrated Science Assessment for Particulate Matter, the new 
evidence reinforces already strong existing studies and supports the 
conclusion that PM2.5 is causally associated with 
numerous adverse health effects in humans at exposure levels far 
below the current standard. Such a conclusion demands prompt action 
to protect human health. (AHA et al., 2012, pp. 1 to 2; ALA et al., 
pp. 4 to 5; similar comment submitted by Rom et al., 2012, p. 1).

    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 the EPA are needed to 
adequately protect public health, particularly for at-risk populations. 
Many environmental justice organizations and individual commenters also 
expressed such views.
    The National Association of Clean Air Agencies (NACAA), the 
Northeast States for Coordinated Air Use Management (NESCAUM), and many 
State and local air agencies and health departments who commented on 
the PM2.5 standards supported revision of the suite of 
current PM2.5 standards, as did five state attorneys general 
(Schneiderman et al., 2012) and the National Tribal Air Association 
(NTAA).
    These commenters based their views chiefly on the body of evidence 
and technical analyses presented and discussed in the Integrated 
Science Assessment, the Risk Assessment, and the Policy Assessment 
finding the available scientific information to be stronger and more 
compelling than in the last review. These commenters generally placed 
much weight on CASAC's recommendation to revise the PM2.5 
standards to provide increased public health protection and on the EPA 
staff conclusions presented in the final Policy Assessment.
    Some of these commenters specifically mentioned extended analyses 
of seminal long-term exposure studies--the ACS (Krewski et al., 2009), 
Harvard Six Cities (Laden et al., 2006), and Southern California 
Children's Health (Gauderman et al., 2004) studies. These commenters 
also highlighted the availability of additional long-term exposure 
studies in this review, specifically a study of premature mortality in 
older adults (Eftim et al., 2008) and the WHI study of cardiovascular 
morbidity and mortality effects in women (Miller et al., 2007) 
providing stronger evidence of mortality and morbidity effects 
associated with long-term PM2.5 exposures at lower 
concentrations than had previously been observed, including studies of 
effects in at-risk populations. For example, some commenters asserted:

    Evidence during the last review showed clearly that the annual 
average standard needed to be much lower than the standard of 15 
[micro]g/m\3\ that was first set in 1997. The evidence has only 
grown since then. Multiple, multi-city studies over long periods of 
time have shown clear evidence of premature death, cardiovascular 
and respiratory harm as well as reproductive and developmental harm 
at contemporary concentrations far below the level of the current 
(annual) standard (ALA et al., 2012, p. 39; AHA et al., 2012, p. 
10).

    These commenters also highlighted the availability of a number of 
short-term PM2.5 exposure studies as providing evidence of 
mortality and morbidity effects at concentrations below the level of 
the current 24-hour PM2.5 standard. Specifically, these 
commenters made note of multi-city studies of premature mortality 
(Zanobetti and Schwartz, 2009) and increased hospitalizations for 
cardiovascular and respiratory-related effects in older adults (Bell et 
al., 2008). These commenters also asserted the importance of many of 
the single-city studies, arguing that these studies ``provide valuable 
information regarding impacts on susceptible populations and on health 
risk in areas with high peak to mean concentration ratios'' (ALA, et 
al., 2012, p. 65). Collectively, considering the multi- and single-city 
short-term exposure studies, these commenters asserted ``the record 
clearly supports a more stringent 24-hour standard of 25 [micro]g/m\3\ 
to provide uniform protection in all regions of the country 
particularly from short-term spikes in pollution and from the sub-daily 
exposures that trigger heart attacks and strokes'' (ALA et al., 2012, 
p. 62). A group of more than 350 physicians, environmental health 
researchers, and public health and medical professionals argued, 
``[s]tudies of short-term exposure demonstrate that PM2.5 
air pollution increases the risk of hospital admissions for heart and 
lung problems even when you exclude days with pollution concentrations 
at or above the current daily standard of 35 [micro]g/m\3\. Daily 
concentrations must be capped at lower levels to protect against peak 
exposure days that occur due to local and seasonal sources of 
emissions'' (Rom et al., 2012, p. 2).
    In addition, many of these commenters generally concluded that 
progress had been made in reducing many of the uncertainties identified 
in the last review, in better understanding mechanisms by which 
PM2.5 may be causing the observed health effects, and in 
improving our understanding of at-risk populations. Further, a number 
of commenters argued that by making the standards more protective, the 
PM2.5 NAAQS would be more consistent with other existing 
standards (e.g., California's annual average standard of 12 [micro]g/
m\3\) (CARB, 2012; CA OEHHA, 2012). Other commenters argued that 
revising the primary PM2.5 standards would be more 
consistent with the recommendations of the World Health Organization 
(WHO) and/or Canada (e.g., ALA et al., 2012, p. 62; ISEE, 2012, p. 2; 
MOE-Ontario, 2012, p. 1).
    With regard to the scope of the literature reviewed for 
PM2.5-related health effects, some commenters asserted that 
the EPA inappropriately narrowed the scope of the review by excluding a 
number of categories of relevant studies, specifically related to 
studies of diesel pollution and traffic-related pollution (ALA, et al., 
2012, p. 17). These commenters argued that, based upon the exclusion of 
these types of studies, the Integrated Science Assessment ``came to the 
erroneous conclusion that the causal relationship between PM and cancer 
is merely suggestive. This conclusion does not square with the 
International Agency Research on Cancer (IARC) finding that diesel 
emissions are a known human carcinogen nor with the conclusions of

[[Page 3111]]

the extended analyses of the [Harvard] Six Cities and ACS cohort 
studies that report positive and statistically significant associations 
between PM2.5 and lung cancer.'' Id.
    Some of these commenters also noted the results of the EPA's 
quantitative 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. For example, ALA et al., noted that the Risk Assessment 
indicated the quantitative risk analyses likely underestimated 
PM2.5-related mortality (U.S. EPA, 2010a, p. 5-16) and 
argued that ``the measurements of risk should be treated 
conservatively'' (ALA, et al., 2012, p. 73). These commenters also 
summarized an expanded analysis of alternative PM2.5 
standard levels that they argued documented the need for more 
protective standards (McCubbin, 2011).
    In general, all of these commenters agreed on the importance of 
results from the large body of scientific studies reviewed in the 
Integrated Science Assessment and on the need to revise the suite of 
PM2.5 standards as articulated in the EPA's proposal, while 
generally differing with the EPA's proposed judgments about the extent 
to which the standards should be revised based on this evidence, 
specifically for providing protection for at-risk populations.
    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 Integrated Science 
Assessment and the Policy Assessment, and the EPA agrees that this 
evidence provides a strong basis for concluding that the current 
PM2.5 standards, taken together, are not requisite to 
protect public health with an adequate margin of safety, and they need 
to be revised to provide increased protection. For reasons discussed in 
section III.E.4.c below, however, the EPA disagrees with aspects of 
these commenters' views on the level of protection that is appropriate.
    The EPA disagrees with these commenters' views that diesel exhaust 
studies were excluded from the Integrated Science Assessment and were 
not considered when making the causality determination for cancer, 
mutagenicity, and genotoxicity. As discussed in section 7.5 of the 
Integrated Science Assessment, diesel exhaust studies were integrated 
within the broader body of scientific evidence that was considered in 
reaching the causality determination for these health endpoints. 
Additionally, as discussed in section 1.5.3 of the Integrated Science 
Assessment, the evidence from diesel exhaust studies was also 
considered as part of the collective evidence evaluated when making 
determinations for other, noncancer health outcomes (e.g., 
cardiovascular and respiratory effects).\47\ Specifically, when 
evaluating this evidence, the focus was on understanding the effects of 
diesel exhaust particles.
---------------------------------------------------------------------------

    \47\ In developing the second draft Integrated Science 
Assessment, the EPA reexamined the controlled human exposure and 
toxicological studies of fresh diesel and gasoline exhaust. This 
information, in addition to other considerations, supported a change 
in the causal determinations for ultrafine particles. Specifically, 
in reevaluating the causal determinations for short-term ultrafine 
particle exposures and cardiovascular and respiratory effects, the 
EPA changed the classification from ``inadequate'' to ``suggestive'' 
for both categories of health outcomes (Vandenberg, 2009, p. 3). 
CASAC agreed with the EPA's rationale for revising these causal 
determinations (Samet, 2009f, p. 10).
---------------------------------------------------------------------------

    It is important to recognize that the Integrated Science Assessment 
focused on experimental studies of diesel exhaust that evaluated 
exposures that were relevant to ambient concentrations, i.e., ``within 
one or two orders of magnitude of ambient PM concentrations'' (U.S. 
EPA, 2009a, section 1.3). The causal determination for cancer, 
mutagenicity, and genotoxicity presented in the Integrated Science 
Assessment represents an integration of experimental and observational 
evidence of exposures to ambient PM concentrations. The EPA fully 
considered the findings of studies that assessed these and other health 
effects associated with exposure to diesel particles in reaching 
causality determinations regarding health outcomes associated with 
PM2.5 exposures. Furthermore, CASAC supported the EPA's 
change to the causal determination for cancer and long-term 
PM2.5 concentrations from ``inadequate'' to ``suggestive'' 
(Samet, 2009f, p. 2).
    With regard to traffic studies, the EPA disagrees with the 
commenters' views that traffic studies that focused on exposure 
indicators such as distance to roadways should have been included in 
the Integrated Science Assessment. These studies were excluded from 
consideration because they did not measure ambient concentrations of 
specific air pollutants, including PM2.5, but instead were 
studies evaluating exposure to the undifferentiated ``traffic related 
air pollution'' mixture (ALA et al., 2012, p. 17) (U.S. EPA, 2009a, 
section 1.3). As a result, these studies do not add to the collective 
body of evidence on the relationship between long- or short-term 
exposure to ambient concentrations of PM2.5 and health 
effects.
    Some of these commenters also identified ``new'' studies that were 
not included in the Integrated Science Assessment as providing further 
support for the need to revise the primary PM2.5 standards. 
As discussed in section II.B.3 above, the 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 ``new'' studies for purposes of decision making in the next PM 
NAAQS review. Nonetheless, in provisionally evaluating commenters' 
arguments (see Response to Comments document), the EPA notes that its 
provisional assessment of ``new'' science found that such studies did 
not materially change the conclusions in the Integrated Science 
Assessment (U.S. EPA, 2012b).
    Another group of commenters opposed revising the current 
PM2.5 standards. These views were 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; the American Petroleum Institute (API) 
representing more than 500 oil and natural gas companies; the National 
Association of Manufacturers (NAM), the American Chemistry Council 
(ACC), the American Fuel & Petroleum Manufacturers (AFPM), the Alliance 
of Automobile Manufacturers (AAM), and other manufacturing 
associations; the Electric Power Research Institute (EPRI); and the 
Texas Commission on Environmental Quality (Texas CEQ). These commenters 
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. Furthermore, they asserted that the 
evidence that has become available since the last review does not 
establish 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, nor does the evidence demonstrate that the risk 
to public health upon attainment of the current standards would be 
greater than was

[[Page 3112]]

understood when the EPA established the current standards in 2006.
    These commenters generally expressed the view that the current 
standards provide the requisite degree of public health protection. In 
supporting their view, these commenters generally argued that the EPA's 
conclusions are inconsistent with the current state of the science and 
questioned the underlying scientific evidence including the causal 
determinations reached in the Integrated Science Assessment. More 
specifically, this group of commenters argued that: (1) The EPA did not 
apply its framework for causal determination consistently across 
studies or health outcomes and, in the process, the EPA relied on a 
selective group of long- and short-term exposure studies to reach 
conclusions regarding causality; (2) toxicological and controlled human 
exposure studies do not provide supportive evidence that the health 
effects observed in epidemiological studies are biologically plausible; 
(3) uncertainties in the underlying health science are as great or 
greater than in 2006; (4) there is no evidence of greater risk since 
the last review to justify tightening the current annual 
PM2.5 standard; and (5) ``new'' studies not included in the 
Integrated Science Assessment continue to increase uncertainty about 
possible health risks associated with exposure to PM2.5. 
These comments are discussed in turn below.
    (l) Some of these commenters asserted that the EPA did not apply 
its framework for causal determinations consistently across studies or 
health outcomes (e.g., ACC, 2012, Attachment A, pp. 1 to 2; API, 2012, 
Attachment 1, p. 30; NAM et al., 2012, pp. 22 to 25; Texas CEQ, 2012, 
pp 2 to 3).\48\ These commenters argued that the EPA downplayed 
epidemiological studies with null or inconsistent results, 
inappropriately used the Hill criteria when evaluating the 
epidemiological evidence, and used the same study and the same 
underlying database to conclude that there was a causal association 
between mortality and multiple criteria pollutants.
---------------------------------------------------------------------------

    \48\ The EPA notes that the same concerns about the causal 
determinations presented in the Integrated Science Assessment were 
raised in comments to CASAC on the draft Integrated Science 
Assessments (e.g., UARG, 2009; API, 2009; ACC, 2012, Appendix B). 
CASAC, therefore, had the opportunity to consider these comments in 
reaching consensus conclusions on this issue.
---------------------------------------------------------------------------

    The EPA disagrees with these commenters' views. First, the EPA 
recognizes that the evaluation of the scientific evidence and its 
application of the causal framework used in the current PM NAAQS review 
was the subject of exhaustive and detailed review by CASAC and the 
public. As summarized in section II.B.3 above, prior to finalizing the 
Integrated Science Assessment, two drafts were released for CASAC and 
public review to evaluate the scientific integrity of the documents. 
Evidence related to the substantive issues raised by CASAC and public 
commenters with regard to the content of the first and second draft 
Integrated Science Assessments were discussed at length during these 
public CASAC meetings and considered in developing the final Integrated 
Science Assessment. CASAC supported the development of the EPA's 
causality framework and its use in the current PM NAAQS review and 
concluded:

    The five-level classification of strength of evidence for causal 
inference has been systematically applied; this approach has 
provided transparency and a clear statement of the level of 
confidence with regard to causation, and we recommend its continued 
use in future Integrated Science Assessments (Samet 2009f, p. 1).

    These commenters asserted that during the application of the causal 
framework the EPA inappropriately relied on a selective group of long- 
and short-term exposure studies in reaching causal inferences (API, 
2012, pp 12 to 17; ACC, 2012, Attachment A, pp 1 to 2; NAM et al., 
2012, pp. 22 to 25; Texas CEQ, 2012, pp 2 to 3). Additionally, these 
commenters expressed the view that the EPA focused on a subset of 
epidemiological studies that reported positive and statistically 
significant results while ignoring other studies, especially those that 
reported no statistically significant associations, those that reported 
potential thresholds, or those that highlighted uncertainties and 
limitations in study design or results. Furthermore, some of these 
commenters argued that epidemiological studies are observational in 
nature and cannot provide evidence of a causal association.
    The EPA disagrees with these commenters' views on assessing the 
health effects evidence and on the conclusions regarding the causality 
determinations reached in the Integrated Science Assessment. In 
conducting a comprehensive evaluation of the evidence in the Integrated 
Science Assessment, the EPA recognized the distinction between the 
evaluation of the relative scientific quality of individual study 
results and the evaluation of the pattern of results within the broader 
body of scientific evidence and considered both in reaching causality 
determinations. The more detailed characterizations of individual 
studies included an assessment of the quality of the study based on 
specific criteria as described in the Integrated Science Assessment 
(U.S. EPA, 2009a, section 1.5.3).
    In developing an integrated assessment of the health effects 
evidence for PM, the EPA emphasized the importance of examining the 
pattern of results across various studies and did not focus solely on 
statistical significance \49\ as a criterion of study strength. This 
approach is consistent with views clearly articulated throughout the 
epidemiological and causal inference literature, specifically, that it 
is important not to focus on results of statistical tests to the 
exclusion of other information.\50\ The concepts underlying the EPA's 
approach to evaluating statistical associations have been discussed in 
numerous publications, including a report by the U.S. Surgeon General 
on the health consequences of smoking (Centers for Disease Control and 
Prevention, 2004). This report cautions against over-reliance on 
statistical significance in evaluating the overall evidence for an 
exposure-response relationship. Criteria characterized by Hill (1965) 
also addressed the value, or lack thereof, of statistical tests in the 
determination of cause:
---------------------------------------------------------------------------

    \49\ Statistical significance is an indicator of the precision 
of a study's results, which is influenced by a variety of factors 
including, but not limited to, the size of the study, exposure and 
measurement error, and statistical model specifications. Studies 
typically calculate ``p-values'' to determine whether the study 
results are statistically significant or whether the study results 
are likely to occur simply by chance. In general practice, effects 
are considered statistically significant if p values are less than 
0.05.
    \50\ For example, Rothman (1998) stated, ``Many data analysts 
appear to remain oblivious to the qualitative nature of significance 
testing [and that] * * * statistical significance is itself only a 
dichotomous indicator. As it has only two values, significant or not 
significant * * *. Nevertheless, p-values still confound effect size 
with study size, the two components of estimation that we believe 
need to be reported separately.'' As a result, Rothman recommended 
that p-values be omitted as long as point and interval estimates are 
available.

    No formal tests of significance can answer those [causal] 
questions. Such tests can, and should, remind us of the effects the 
play of chance can create, and they will instruct us in the likely 
magnitude of those effects. Beyond that, they contribute nothing to 
---------------------------------------------------------------------------
the `proof' of our hypothesis (Hill, 1965, p. 299).

    The statistical significance of individual study findings has 
played an important role in the EPA's evaluation of the study's results 
and the EPA has placed greater emphasis on studies reporting 
statistically significant results. However, in the broader evaluation 
of the evidence from many

[[Page 3113]]

epidemiological studies, and subsequently during the process of forming 
causality determinations in integrating evidence across 
epidemiological, controlled human exposure, and toxicological studies, 
the EPA has emphasized the pattern of results across epidemiological 
studies, and whether the effects observed were coherent across the 
scientific disciplines for drawing conclusions on the relationship 
between PM2.5 and different health outcomes. Thus, the EPA 
did not limit its focus or consideration to just studies that reported 
positive associations or where the results were statistically 
significant.
    In addition, some commenters asserted that the EPA inappropriately 
used the Hill criteria by failing to consider the limitations of 
studies with weak associations, thereby overstating the consistency of 
the observed associations (API, 2012, Attachment 1, pp. 30 to 35). 
These commenters argued that risk estimates greater than 3 to 4 reflect 
strong associations supportive of a causal link, while smaller risk 
estimates (i.e., 1.5 to 3) are considered to be weak and require other 
lines of evidence to demonstrate causality.
    As discussed in section 1.5.3 of the Integrated Science Assessment, 
the EPA thoroughly considered the limitations of all studies during its 
evaluation of the scientific literature (U.S. EPA,, 2009a, p. 1-14). 
This collective body of evidence, including known uncertainties and 
limitations of the studies evaluated, were considered during the 
process of forming causality determinations as discussed in chapters 6 
and 7 of the Integrated Science Assessment. For example, the EPA 
concluded that ``a causal relationship exists between short-term 
PM2.5 exposure and cardiovascular effects,'' however, in 
reaching this conclusion, the Agency recognized and considered 
limitations of the current evidence that still requires further 
examination (U.S. EPA, 2009a., in section 6.2.12.1). Therefore, the EPA 
disagrees with these commenters' views that the Hill criteria were 
inappropriately used in that the limitations of studies were not 
considered.
    The EPA also disagrees with the commenters' assertion that the 
magnitude of the association must be large to support a determination 
of causality. As discussed in the Integrated Science Assessment, the 
strength of the observed association is an important aspect to aid in 
judging causality and ``while large effects support causality, modest 
effects therefore do not preclude it'' (U.S. EPA, 2009a, Table 1-2, 
section 1.5.4). The weight of evidence approach used by the EPA 
encompasses a multitude of factors of which the magnitude of the 
association is only one component (U.S. EPA, 2009a, Table 1-3). An 
evaluation of the association across multiple investigators and 
locations supports the ``reproducibility of findings [which] 
constitutes one of the strongest arguments for causality'' (U.S. EPA, 
2009a, Table 1-2). Even though the risk estimates for air pollution 
studies may be modest, the associations are consistent across hundreds 
of studies as demonstrated in the Integrated Science Assessment. 
Furthermore, the causality determinations rely on different lines of 
evidence, by integrating evidence across disciplines, including animal 
toxicological studies and controlled human exposure studies.
    Furthermore, as summarized in section III.B above and discussed 
more fully in section III.B.3 of the proposal, the EPA recognizes that 
the population potentially affected by PM2.5 is 
considerable, including large subgroups of the U.S. population that 
have been identified as at-risk populations (e.g., children, older 
adults, persons with underlying cardiovascular or respiratory disease). 
While individual effect estimates from epidemiological studies may be 
modest in size, the public health impact of the mortality and morbidity 
associations can be quite large given that air pollution is ubiquitous. 
Indeed, with the large population exposed, exposure to a pollutant 
causally associated at a population level with mortality and serious 
illness has significant public health consequences, virtually 
regardless of the relative risk. Taken together, this information 
indicates that exposure to ambient PM2.5 concentrations has 
substantial public health impacts.
    In addition, these commenters believed that the EPA downplayed null 
or inconsistent findings in numerous long-term mortality studies with 
reported PM2.5 concentrations above and below the level of 
the current annual standard. The EPA disagrees that studies with null 
or inconsistent findings were not accurately presented and considered 
in the Integrated Science Assessment. For example, as discussed 
throughout section 7.6 and depicted in Figures 7-6 and 7-7 of the 
Integrated Science Assessment, the EPA presented the collective 
evidence from all studies that examined the association between long-
term PM2.5 exposure and mortality. Overall, across these 
studies there was evidence of consistent positive associations in 
different cohorts. That evidence, in combination with the biological 
plausibility provided by experimental and toxicological studies 
evaluated in sections 7.1 and 7.2 of the Integrated Science Assessment, 
supported a causal relationship exists between long-term 
PM2.5 exposure and mortality.
    Lastly, some of these commenters argued that in some cases, the EPA 
used the same study and the same underlying database to conclude that 
there is a causal association between mortality and multiple criteria 
pollutants. These commenters argued, ``[i]n doing so, EPA attributes 
the cause of the mortality effects observed to whichever criteria 
pollutant it is reviewing at the time'' (API, 2012, pp. 14 to 16).
    The EPA strongly disagrees that the Agency ``attributes the cause 
of mortality effects observed to whichever criteria pollutant it is 
reviewing at the time.'' The EPA consistently recognizes that other 
pollutants are also associated with health outcomes, as is reflected in 
the fact that the EPA has established regulations to limit emissions of 
particulate criteria pollutants as well as other gaseous criteria 
pollutants. Epidemiological studies often examine the association 
between short- and long-term exposures to multiple air pollutants and 
mortality within a common dataset in an attempt to identify the air 
pollutant(s) of the complex mixture most strongly associated with 
mortality. In evaluating these studies, the EPA employs specific study 
selection criteria to identify those studies most relevant to the 
review of the NAAQS. In its assessment of the health evidence regarding 
PM2.5, the EPA has carefully evaluated the potential for 
confounding, effect measure modification, and the role of 
PM2.5 as a component of a complex mixture of air pollutants 
(U.S. EPA, 2009a, p. 1-9). The EPA used a rigorous weight of evidence 
approach to inform causality that evaluated consistency across studies 
within a discipline, evidence for coherence across disciplines, and 
biological plausibility. Additionally, during this process, the EPA 
assessed the limitations of each study in the context of the collective 
body of evidence. It was the collective evidence, not one individual 
study that ultimately determined whether a causal relationship exists 
between a pollutant and health outcome. In the Integrated Science 
Assessment, the combination of epidemiological and experimental 
evidence formed the basis for the Agency concluding for the first time 
that a causal relationship exists between short- or long-term exposure 
to a criteria pollutant and mortality (U.S. EPA, 2009, sections 2.3.1.1 
and 2.3.1.2).

[[Page 3114]]

    Additionally, while the EPA has evaluated some of the studies used 
to inform the causality determination for PM in the Integrated Science 
Assessments for other criteria air pollutants, the Agency has done so 
in the context of examining the collective body of evidence for each of 
the respective criteria air pollutants. As such, the body of evidence 
to inform causality has varied from pollutant to pollutant resulting in 
the association between each criteria air pollutant and mortality being 
classified at a different level of the five-level hierarchy used to 
inform causation (e.g., U.S. EPA, 2008e, U.S. EPA, 2008f, U.S. EPA, 
2010k).
    The EPA notes that the final causality determinations presented in 
the Integrated Science Assessment reflected CASAC's recommendations on 
the second draft Integrated Science Assessment (Samet, 2009f, pp. 2 to 
3). Specifically, CASAC supported the EPA's changes (in the second 
versus first draft Integrated Science Assessment) from ``likely 
causal'' to ``causal'' for long-term exposure to PM2.5 and 
cardiovascular effects and for cancer and PM2.5 (from 
``inadequate'' to ``suggestive''). Id. Furthermore, CASAC recommended 
``upgrading'' the causal classification for PM2.5 and total 
mortality to ``causal'' for both the short- and long-term timeframes. 
Id. With regard to mortality, the ``EPA carefully reevaluated the body 
of evidence, including the collective evidence for biological 
plausibility for mortality effects, and determined that a causal 
relationship exists for short- and long-term exposure to 
PM2.5 and mortality, consistent with the CASAC comments'' 
(Jackson, 2010).
    (2) With regard to toxicological and controlled human exposure 
studies, these commenters argued that the available evidence does not 
provide coherence or biological plausibility for health effects 
observed in epidemiological studies (API, 2012, pp. 21 to 22, 
Attachment 1, pp. 25 to 29; AAM, 2012, pp. 15 to 16; Texas CEQ, 2012, 
p. 3). With regard to the issue of mechanisms, these commenters noted 
that although the EPA recognizes that new evidence is now available on 
potential mechanisms and plausible biological pathways, the evidence 
provided by toxicological and controlled human exposure studies still 
does not resolve all questions about how PM2.5 at ambient 
concentrations could produce the mortality and morbidity effects 
observed in epidemiological studies. More specifically, for example, 
some of these commenters argued that:

    A review of the Integrated Science Assessment, however, suggests 
that the experimental evidence is inconsistent and not coherent with 
findings in epidemiology studies. Specifically, the findings of mild 
and reversible effects in most experimental studies conducted at 
elevated exposures are not consistent with the more serious 
associations described in epidemiology studies (e.g., hospital 
admissions and mortality). Also, both animal studies and controlled 
human exposure studies have identified no effect levels for acute 
and chronic exposure to PM and PM constituents at concentrations 
considerably above ambient levels. EPA should consider the 
experimental findings in light of these higher exposure levels and 
what the relevance may be for ambient exposures (API, 2012, 
Attachment 1, p. 25).

    The EPA notes that in the review completed in 1997, the Agency 
considered the lack of demonstrated biological mechanisms for the 
varying effects observed in epidemiological studies to be an important 
caution in its integrated assessment of the health evidence upon which 
the standards were based (71 FR 61157, October 17, 2006). In the review 
completed in 2006, the EPA recognized the findings from additional 
research that indicated that different health responses were linked 
with different particle characteristics and that both individual 
components and complex particle mixtures appeared to be responsible for 
many biologic responses relevant to fine particle exposures. Id. Since 
that 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 (U.S. 
EPA, 2011a, section 2.5), it has provided important insights as 
discussed in section III.B.1 of the proposal (77 FR at 38906 to 38909) 
and discussed more fully in the Integrated Science Assessment (U.S. 
EPA, 2009a, Chapter 5).
    As noted in the proposal, toxicological studies provide evidence to 
support the biological plausibility of cardiovascular and respiratory 
effects associated with long- and short-term PM exposures observed in 
epidemiological studies (77 FR 38906) and provide supportive 
mechanistic evidence that the cardiovascular morbidity effects observed 
in long-term exposure epidemiological studies are coherent with studies 
of cardiovascular-related mortality (77 FR 38907). The Integrated 
Science Assessment concluded that the new evidence available in this 
review ``greatly expands'' upon the evidence available in the last 
review ``particularly in providing greater understanding of the 
underlying mechanisms for PM2.5 induced cardiovascular and 
respiratory effects for both short- and long-term exposures'' (U.S. 
EPA, 2009a, p. 2-17). The mechanistic evidence now available, taken 
together with newly available epidemiological evidence, increases the 
Agency's confidence that a causal relationship exists between long- and 
short-term exposure to PM2.5 and cardiovascular effects and 
mortality.\51\ In addition, CASAC supported the Integrated Science 
Assessment approach and characterization of potential mechanisms or 
modes of action (Samet, 2009e, pp. 7 to 8; Samet, 2009f, p. 11), as 
well as the findings of a causal relationship at the population level 
between exposure to PM2.5 and mortality and cardiovascular 
effects (Samet, 2009f, pp. 2 to 3).\52\
---------------------------------------------------------------------------

    \51\ See American Trucking Associations v. EPA, 175 F. 3d 1027, 
1055-56 (DC Cir. 1999) reversed in part and affirmed in part sub 
nom, Whitman v. American Trucking Associations, 531 U.S. 457 (2001) 
holding that the EPA could establish NAAQS without identifying a 
biological mechanism (``To begin with, the statute itself requires 
no such proof. The Administrator may regulate air pollutants 
``emissions of which, in his judgment, cause or contribute to air 
pollution which may reasonably be anticipated to endanger public 
health or welfare.'' (emphasis added by the court). Moreover, this 
court has never required the type of explanation petitioners seek 
from EPA. In fact, we have expressly held that EPA's decision to 
adopt and set air quality standards need only be based on 
`reasonable extrapolations from some reliable evidence'* * *. 
Indeed, were we to accept petitioners' view, EPA (or any agency for 
that matter) would be powerless to act whenever it first recognizes 
clear trends of mortality or morbidity in areas dominated by a 
particular pathogen.'').
---------------------------------------------------------------------------

    Additionally, the EPA disagrees with commenters that the mild and 
reversible effects observed in controlled human exposure studies are 
inconsistent with the more serious effects observed in epidemiological 
studies. Ethical considerations regarding the types of studies that can 
be performed with human subjects generally limit the effects that can 
be evaluated to those that are transient, reversible, and of limited 
short-term consequence. The relatively small number of subjects 
recruited for controlled exposure studies should also be expected to 
have less variability in health status and risk factors than occurring 
in the general population.\53\ Consequently, the severity

[[Page 3115]]

of health effects observed in controlled human exposure studies 
evaluating the effects of PM should be expected to be less than 
observed in epidemiologic studies. Nonetheless, that effects are 
observed in relatively healthy individuals participating in controlled 
exposure studies serves as an indicator that PM is initiating health 
responses and that more severe responses may reasonably be expected in 
a more diverse population.
---------------------------------------------------------------------------

    \53\ For example, the EPA excludes from its controlled human 
exposure studies involving exposure to PM2.5 any 
individual with a significant risk factor for experiencing adverse 
effects from such exposure. Thus, the EPA excludes a priori the 
following categories of persons: those with a history of angina, 
cardiac arrhythmias, and ischemic myocardial infarction or coronary 
bypass surgery; those with a cardiac pacemaker; those with 
uncontrolled hypertension (greater than 150 systolic and 90 
diastolic); those with neurogenetive diseases; those with a history 
of bleeding diathesis; those taking beta-blockers; those using oral 
anticoagulants; those who are pregnant, attempting to become 
pregnant, or breastfeeding; those who have experienced a respiratory 
infection within four weeks of exposure; those experiencing eye or 
abdominal surgery within six weeks of exposure; those with active 
allergies; those with a history of chronic illnesses such as 
diabetes, cancer, rheumatologic diseases, immunodeficiency state, 
known cardiovascular disease, or chronic respiratory diseases; 
smokers. The EPA ``Application for Independent Review Board Approval 
of Human Subjects Research: Cardiopulmonary Effects of healthy Older 
GSTM1 Null and Sufficient individuals to Concentrated Ambient Air 
Particles (CAPTAIN)'', Nov. 9, 2011, p. 9.
---------------------------------------------------------------------------

    It should also be noted that there is a small body of toxicological 
evidence demonstrating mortality in rodents exposed to PM (e.g., 
Killingsworth et al. 1997). Overall it is not surprising that lethality 
is not induced in more toxicological research, as these types of 
studies do not readily lend themselves to this endpoint. 
Epidemiological studies have observed associations between PM and 
mortality in communities with populations in the range of many 
thousands to millions of people. Clearly, it is not feasible to expose 
hundreds (if not thousands) of animals to ambient PM (potentially over 
many years) in a laboratory setting to induce enough lethalities to 
distinguish between natural deaths and those attributable to PM. 
Furthermore, the heterogeneous human populations sampled in 
epidemiological studies are comprised of individuals with different 
physical, genetic, health, and socioeconomic backgrounds which may 
impact the outcome. However, in toxicological studies, the rodent 
groups are typically inbred, such that inter-individual variability is 
minimized. Thus, if the rodent strain used is quite robust, PM-induced 
effects may not be observed at low exposure concentrations.
    (3) In asserting that the uncertainties in the underlying health 
science are as great or greater than in the last review and therefore 
do not support revision to the standards at this time, commenters in 
this group variously discussed a number of issues related to: (a) 
Confounding, (b) heterogeneity in risk estimates, (c) exposure 
measurement error, (d) model specification, (e) the shape of the 
concentration-response relationship, and (f) understanding the relative 
toxicity of components within the mixture of fine particles. Each of 
these issues is addressed below and some are discussed in more detail 
in the Response to Comments document.
    In summary, these commenters concluded that the substantial 
uncertainties present in the last review have not been resolved and/or 
that the uncertainty about the possible health risks associated with 
PM2.5 exposure has not diminished. As discussed below, the 
EPA believes that the overall uncertainty about possible health risks 
associated with both long- and short-term PM2.5 exposure has 
diminished to an important degree since the last review. While the EPA 
agrees that important uncertainties remain, and that future research 
directed toward addressing these uncertainties is warranted, the EPA 
disagrees with commenters' views that the remaining uncertainties in 
the scientific evidence are too great to warrant revising the current 
PM2.5 NAAQS.
(a) Confounding
    Some commenters have criticized the EPA for not adequately 
addressing the issue of confounding in both long- and short-term 
exposure studies of mortality and morbidity. This includes confounding 
due to copollutants, as well as unmeasured confounding.\54\
---------------------------------------------------------------------------

    \54\ The Integrated Science Assessment defines confounding as 
``a confusion of effects. Specifically, the apparent effect of the 
exposure of interest is distorted because the effect of an 
extraneous factor is mistaken for or mixed with the actual exposure 
effect (which may be null) (Rothman and Greenland, 1998)'' (U.S. 
EPA, 2009a, p. 1-16). Epidemiological analyses attempt to adjust or 
control for these characteristics (i.e., potential confounders) that 
differ between exposed and non-exposed individuals (U.S. EPA, 2009a, 
section 1.5.3). Not all risk factors can be controlled for within a 
study design/model and are termed ``unmeasured confounders.'' An 
unmeasured confounder is a confounder that has not previously been 
measured and therefore is not included in the study design/model.
---------------------------------------------------------------------------

    With regard to copollutant confounding, these commenters asserted 
that the EPA has not adequately interpreted the results from studies 
that examined the effect of copollutants on the relationship between 
long- and short-term PM2.5 exposures and mortality and 
morbidity outcomes. These commenters contend that the EPA has 
inappropriately concluded that PM2.5-related mortality and 
morbidity associations are generally robust to confounding. The 
commenters stated that statistically significant PM2.5 
associations in single-pollutant models in epidemiological studies do 
not remain statistically significant in copollutant models.
    The loss of statistical significance or the reduction in the 
magnitude of the effect estimate when a co-pollutant model is used may 
be the result of factors other than confounding. These changes do not 
prove either the existence or absence of confounding. These impacts 
must be evaluated in a broader context that considers the entire body 
of evidence. The broader examination of this issue in the Integrated 
Science Assessment included a focus on evaluating the stability of the 
size of the effect estimates in epidemiological studies conducted by a 
number of research groups using single- and copollutant models (U.S. 
EPA, 2009a, sections 6.2.10.9, 6.3.8.5, and 6.5, Figures 6-5, 6-9, and 
6-15). This examination found that, for most epidemiological studies, 
there was little change in effect estimates based on single- and 
copollutant models, although the Integrated Science Assessment 
recognized that in some cases, the PM2.5 effect estimates 
were markedly reduced in size and lost statistical significance. 
Additionally, the EPA notes that these comments do not adequately 
reflect the complexities inherent in assessing the issue of copollutant 
confounding. As discussed in the proposal (77 FR 38907, 38909, and 
38910) and more fully in the Integrated Science Assessment (U.S.EPA, 
2009a, sections 6.2, 6.3, and 6.5), although copollutant models may be 
useful tools for assessing whether gaseous copollutants may be 
potential confounders, such models alone cannot determine whether 
copollutants are in fact confounders. Interpretation of the results of 
copollutant 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 statistical power of the studies in question 
inherent in the study methodology. For example, the every-third or 
sixth-day sampling schedule often employed for PM2.5 
measurements compared to daily measurements of gaseous copollutants 
drastically reduces the overall sample size to assess the effect of 
copollutants on the PM2.5-morbidity or mortality 
relationship, such that the reduced sample size can lead to less 
precise effect estimates (e.g., wider confidence intervals).
    The EPA recognizes that when PM2.5 is correlated with 
gaseous pollutants it can be difficult to identify the effect of 
individual pollutants in the ambient mixture (77 FR 38910). However, 
based on the available evidence, the EPA

[[Page 3116]]

concludes epidemiological studies continue to support the conclusion 
that PM2.5 associations with mortality and morbidity 
outcomes are robust to the inclusion of gaseous copollutants in 
statistical models. The EPA evaluated the potential confounding effects 
of gaseous copollutants and, although it is recognized that 
uncertainties and limitations still remain, the Agency concluded the 
collective body of scientific evidence is ``stronger and more 
consistent than in previous reviews providing a strong basis for 
decision making in this review'' (77 FR 38910/1).
    Several commenters offered detailed comments on the long-term 
PM2.5 exposure studies arguing that associations from 
mortality studies are subjected to unmeasured confounding and as a 
result are not appropriately characterized as providing evidence of a 
causal relationship between long-term PM2.5 exposure and 
mortality (e.g., UARG, 2012, pp. 10 to 11, Attachment A, pp. 17 to 23; 
API, 2012, pp. 13 to 14, Attachment 1, pp. 11 to 14, Attachment 7, pp. 
2-10; ACC, 2012, p. 18 to 21; AFPM, 2012, p. 8; Texas CEQ, 2012, p. 4). 
Specifically, commenters cited two studies (i.e., Janes et al., 2007 
and Greven et al., 2011) that used a new type of statistical analysis 
to examine associations between annual (long-term) and monthly (sub-
chronic) PM2.5 exposure and mortality. The commenters 
interpreted the results of these analyses as evidence of unmeasured 
confounding in the long-term PM2.5 exposure-mortality 
relationship. These commenters interpreted these studies as raising 
fundamental questions regarding the EPA's determination that a causal 
relationship exists between long-term PM2.5 exposure and 
mortality. In addition to the commenters mentioned above, all of the 
authors of the publications by Janes et al. (2007) and Greven et al. 
(2011) (i.e., Francesca Dominici, Scott Zeger, Holly Janes, and Sonja 
Greven) submitted a joint comment to the public docket in order to 
clarify specific points regarding these two studies (Dominici et al., 
2012).
    The first study, Janes et al. (2007), was evaluated in the 
Integrated Science Assessment (U.S. EPA, 2009a, p. 7-88). The second 
study, Greven et al. (2011), an extension of the Janes et al. (2007) 
study adding three more years of data, is a ``new'' study discussed in 
the Provisional Science Assessment (U.S. EPA, 2012). Both studies used 
nationwide Medicare mortality data to examine the association between 
monthly average PM2.5 concentrations over the preceding 12 
months and monthly mortality rates in 113 U.S. counties and examined 
whether community-specific trends in monthly PM2.5 
concentrations and mortality declined at the same rate as the national 
rate. The investigators examined this by decomposing the association 
between PM2.5 and mortality into two components: (1) 
National trends, defined as the association between the national 
average trend in monthly PM2.5 concentrations averaged over 
the previous 12 months and the national average trend in monthly 
mortality rates, and (2) local trends, defined as county-specific 
deviations in monthly PM2.5 concentrations and monthly 
mortality rates from national trends.
    The EPA does not question the results of the national trends 
analyses conducted by Janes et al. (2007) and Greven et al. (2011).\55\ 
Both Janes et al. (2007) and Greven et al. (2011) observed positive and 
statistically significant associations between long-term exposure to 
PM2.5 and mortality in their national analyses. However, 
Janes et al. (2007) and Greven et al. (2011) eliminated all of the 
spatial variation in air pollution and mortality in their data set when 
estimating the national effect, focusing instead on both chronic 
(yearly) and sub-chronic (monthly) temporal differences in the data 
(Dominici et al. 2012). Janes et al. (2007) (Table 1) highlighted that 
over 90 percent of the variance in the data set used for the analyses 
conducted by both Janes et al. (2007) and Greven et al. (2011) was 
attributable to spatial variability, which the authors chose to 
discard. As noted above, the focus of the analyses by Janes et al. 
(2007) and Greven et al. (2011) was on two components: (1) A temporal 
or time component, i.e., the ``national'' trends analysis, which 
examined the association between the national average trend in monthly 
PM2.5 concentrations averaged over the previous 12 months 
and the national average trend in monthly mortality rates and (2) a 
space-by-time component, i.e., the ``local'' trends analysis, which 
examined county-specific deviations in monthly PM2.5 
concentrations and monthly mortality rates from national trends. These 
two components combined comprised less than 10 percent of the variance 
in the data set. The authors included a focus on the space-by-time 
component, which represented approximately 5 percent of the variance in 
the data set, in an attempt to identify, absent confounding, if 
PM2.5 was associated with mortality at this unique exposure 
window. Thus, these studies are not directly comparable to other cohort 
studies investigating the relationship between long-term exposure to 
PM2.5 and mortality, which make use of spatial variability 
in air pollution and mortality data.\56\ This point was highlighted by 
the study authors who stated that ``when one considers that this wealth 
of information is not accounted for in [Janes 2007], it is not as 
surprising that * * * vastly different estimates of the 
PM2.5/mortality relationship [were observed] than in other 
studies that do exploit that variability'' (Dominici et al., 2012, p. 
2).
---------------------------------------------------------------------------

    \55\ In its evaluation of Janes et al. (2007) in the Integrated 
Science Assessment, the EPA did not identify limitations in the 
statistical methods used per se (U.S. EPA, 2009a, p. 7-88) and 
included the results of the national-scale analyses in that study in 
the body of evidence that supported the determination that there is 
a causal relationship between long-term PM2.5 exposure 
and mortality.
    \56\ Though not directly comparable, the national effect 
estimates for mortality reported by Janes et al. (2007) and Greven 
et al. (2011) are coincidentally similar in magnitude to those 
previously reported. It is important to note that previous cohort 
studies have focused on identifying spatial differences in 
PM2.5 concentrations between cities, while Janes et al. 
(2007) and Greven et al. (2011) focus primarily on temporal 
differences in PM2.5 concentrations. In fact, Greven et 
al. (2011) state ``We do not focus here on a third type [of 
statistical approach] used in cohort studies, measuring the 
association between average PM2.5 levels and average age-
adjusted mortality rates across cities (purely spatial or cross-
sectional association).''
---------------------------------------------------------------------------

    The EPA notes that the results of the local trends analyses 
conducted by Janes et al. (2007) and Greven et al. (2011) are limited 
by the monthly timescale used in these analyses. This view is 
consistent with comments on the Janes et al. (2007) study articulated 
in Pope and Burnett (2007),\57\ which noted that an important 
limitation of the local scale analysis conducted by Janes et al. (2007) 
and subsequently by Greven et al. (2011) was the subchronic exposure 
window considered in these analyses. Both studies used annual average 
PM2.5 concentrations to characterize long-term national 
trends which was consistent with exposure windows considered in other 
studies of long-term exposure to PM2.5 and mortality.\58\ 
However, the local scale analyses used monthly average PM2.5 
concentrations to characterize county-specific deviations from national 
trends (the local scale). The use of monthly average data likely does 
not provide

[[Page 3117]]

enough exposure contrast to observe temporal changes in mortality at 
the local scale. It also represents a different exposure window than 
considered in the large body of evidence of health effects related to 
short-term (from less than one day to up to several days) and chronic 
(one or more years) measures of PM2.5.
---------------------------------------------------------------------------

    \57\ Some commenters argued that there were flaws in the 
criticisms offered by Pope and Burnett (2007) on the paper by Janes 
et al. (2007) (UARG, 2012, Attachment A, pp. 19 to 23). The EPA 
responds to each of these specific comments in the Response to 
Comments document.
    \58\ As noted above, however, Janes et al. (2007) and Greven et 
al. (2011) focused on temporal variability and other studies of 
long-term exposure to PM2.5 and mortality focus on 
spatial variability.
---------------------------------------------------------------------------

    Furthermore, the EPA disagrees with commenters that studies by 
Janes et al. (2007) and Greven et al. (2011) provide evidence that 
other studies of long-term exposure to PM2.5 and mortality 
are affected by unmeasured confounding. As noted above, the design of 
the studies conducted by Janes et al. (2007) and Greven et al. (2011) 
are fundamentally different than those used in other studies of long-
term exposure to PM2.5 and mortality, including the ACS 
cohort and the Harvard Six Cities study. Studies, such as the ACS and 
Harvard Six Cities studies, used the spatial variation between cities 
to measure the effect of long-term (annual) exposures to 
PM2.5 on mortality risk, and did not conduct any analyses 
relying on the temporal variation in PM2.5. The opposite is 
true of the Janes et al. (2007) and Greven et al. (2011) studies which 
first removed the spatial variability in PM2.5 and then 
examined the temporal variation at both the national and local scale to 
measure the effects of temporal differences in PM2.5 on 
mortality risk. Janes et al. (2007) and Greven et al. (2011) focus on 
changes in PM2.5 concentrations over time and, therefore, 
control for confounders would be based on including variables that vary 
over time rather than over space. As a result, any evidence of 
potential confounding of the PM2.5-mortality risk 
relationship derived from Janes et al. (2007) and Greven et al. (2011) 
cannot be extrapolated to draw conclusions related to potential spatial 
confounding in studies based on the spatial variation in 
PM2.5 concentrations.
    As detailed in the Integrated Science Assessment (U.S. EPA, 2009a, 
section 7.6), and recognized by the authors of Janes et al. (2007) and 
Greven et al. (2011), the cohort studies that informed the causality 
determination for long-term PM2.5 exposure and mortality 
``have developed approaches to adjust for measured and unmeasured 
confounders'' (Dominici et al., 2012, p. 2). These approaches were 
specifically designed to adjust for spatial confounding. The hypothesis 
that the authors of Janes et al. (2007) and Greven et al. (2011) chose 
to examine was that differences in the local and national effects 
indicated unmeasured temporal confounding in either the local or 
national effect estimate. This hypothesis was specific to these two 
studies that examined temporal variability in exposure to air pollution 
and did not include known potential confounders at either the national 
or local scale as time-varying covariates in the statistical model. The 
authors acknowledged that the interpretation of either the national or 
local estimates needs to occur with an appreciation of the potential 
confounding effects of national and local scale covariates that were 
omitted from the model (Dominici et al., 2012).
    It is important to recognize that because Janes et al. (2007) and 
Greven et al. (2011) focused on variations in PM2.5 over 
time and not space, the results from these two studies do not provide 
any indication that other studies of long-term exposure to 
PM2.5 and mortality exhibit spatial confounding, or that 
PM2.5 does not cause mortality.\59\ The authors of Janes et 
al. (2007) and Greven et al. (2011) recognized that ``it is entirely 
possible that these papers are looking for an association at a 
timescale for which no association truly exists'' (Dominici et al., 
2012, p. 3). Furthermore, as highlighted in the Integrated Science 
Assessment and discussed by Pope and Burnett (2007), the conclusions of 
Janes et al. (2007) ``are overstated * * * [T]heir analysis tells us 
little or nothing about unmeasured confounding in those and related 
studies because the methodology of Janes et al. largely excludes the 
sources of variability that are exploited in those other studies. By 
using monthly mortality counts and lagged 12-month average pollution 
concentrations, the authors eliminate the opportunity to exploit short-
term or day-to-day variability.''
---------------------------------------------------------------------------

    \59\ Further, the EPA notes that Janes et al. (2007) and Greven 
et al. (2011) provide no information relevant to examining 
confounding in studies of short-term exposure to PM2.5.
---------------------------------------------------------------------------

    In conclusion, the EPA interprets the results of the analyses 
conducted by Janes et al. (2007) and Greven et al. (2011) as being 
consistent with prior knowledge of examining associations with long-
term exposure to PM2.5 at the national scale using long-term 
average PM2.5 concentrations. For the reasons presented 
above and discussed in more detail in the Response to Comments 
document, the Agency disagrees with the commenters' assumption that the 
results of Janes et al. (2007) and Greven et al. (2011) indicate 
unmeasured confounding in the results of other cohort studies of long-
term exposure to PM2.5 and mortality. Therefore, the EPA 
concludes that these studies do not invalidate the large body of 
epidemiological evidence that supports the EPA's determination that a 
causal relationship exists between long-term PM2.5 exposure 
and mortality.\60\
---------------------------------------------------------------------------

    \60\ The EPA notes that the EPA's conclusion with regard to 
interpretation of the results from Janes et al. (2007) and Greven et 
al. (2012) is supported by the study authors' conclusion that 
``[o]ur results do not invalidate previous epidemiological studies'' 
(Dominici, 2012, p. 1 (emphasis original)).
---------------------------------------------------------------------------

(b) Heterogeneity in Risk Estimates
    Some commenters argued that the heterogeneity in risk estimates 
observed in multi-city epidemiological studies and the lack of 
statistical significance in many regional or seasonal estimates 
highlights a potential bias associated with combined multi-city 
epidemiological study results (e.g., API, 2012, Attachment 1, pp. 15 to 
19). These commenters further argued that more refined intra-urban 
exposure estimates conducted for two of the largest cities included in 
the ACS study, Los Angeles and New York City, based on land-use 
regression models and/or kriging methods (Krewski et al., 2009) 
``underscore the importance of considering city-specific health 
estimates, which may account for heterogeneity in PM2.5 
concentrations or other differences among cities, rather than relying 
on pooled nationwide results from multi-city studies'' (API, 2012, 
Attachment 1, p. 17).
    With respect to understanding the nature and magnitude of 
PM2.5-related risks, the EPA agrees that epidemiological 
studies evaluating health effects associated with long- and short-term 
PM2.5 exposures have reported heterogeneity in responses 
between cities and effect estimates across geographic regions of the 
U.S. (U.S. EPA, 2009a, sections 6.2.12.1, 6.3.8.1, 6.5.2, and 7.6.1; 
U.S. EPA, 2011a, p. 2-25). For example, when focusing on short-term 
PM2.5 exposure, the Integrated Science Assessment found that 
multi-city studies that examined associations with mortality and 
cardiovascular and respiratory hospital admissions and emergency 
department visits demonstrated greater cardiovascular effects in the 
eastern versus the western U.S. (Dominici, et al., 2006a; Bell et al., 
2008; Franklin et al. (2007, 2008)).
    In addition, the Integrated Science Assessment evaluated studies 
that provided some evidence for seasonal differences in 
PM2.5 risk estimates, specifically in the northeast. The 
Integrated Science Assessment found evidence indicating that 
individuals may be at greater risk of dying from higher exposures to 
PM2.5 in the warmer months, and at greater risk of 
PM2.5 associated hospitalization for

[[Page 3118]]

cardiovascular and respiratory diseases during colder months of the 
year. The limited influence of seasonality on PM risk estimates in 
other regions of the U.S. may be due to a number of factors including 
varying PM composition by season, exposure misclassification due to 
regional tendencies to spend more or less time outdoors and air 
conditioning usage, and the prevalence of infectious diseases during 
the winter months (U.S. EPA, 2009a, p. 3-182).
    Overall, the EPA took note in the proposal that uncertainties still 
remain regarding various factors that contribute to heterogeneity 
observed in epidemiological studies (77 FR 38909/3). Nonetheless, the 
EPA recognizes that this heterogeneity could be attributed, at least in 
part, to differences in PM2.5 composition across the U.S., 
as well as to exposure differences that vary regionally such as 
personal activity patterns, microenvironmental characteristics, and the 
spatial variability of PM2.5 concentrations in urban areas 
(U.S. EPA, 2009a, section 2.3.2; 77 FR 38910).
    As recognized in the Policy Assessment, the current epidemiological 
evidence and the limited amount of city-specific speciated 
PM2.5 data do not allow conclusions to be drawn that 
specifically differentiate effects of PM2.5 in different 
locations (U.S. EPA, 2011a, p. 2-25). Furthermore, the Integrated 
Science Assessment concluded ``that many constituents of 
PM2.5 can be linked with multiple health effects, and the 
evidence is not yet sufficient to allow differentiation of those 
constituents or sources that are more closely related to specific 
health outcomes'' (U.S. EPA, 2009a, p. 2-17). CASAC thoroughly reviewed 
the EPA's presentation of the scientific evidence indicating 
heterogeneity in PM2.5 effect estimates in epidemiological 
studies and concurred with the overall conclusions presented in the 
Integrated Science Assessment.
(c) Exposure Measurement Error
    Some commenters argued that the EPA did not adequately consider 
exposure measurement error, which they asserted is an important source 
of bias in epidemiological studies that can bias effect estimates in 
either direction (e.g., API, 2012, Attachment 1, pp. 19 to 20).
    The EPA agrees that exposure measurement error is an important 
source of uncertainty and that the variability in risk estimates 
observed in multi-city studies could be attributed, in part, to 
exposure error due to measurement-related issues (77 FR 38910). 
However, the Agency disagrees with the commenters' assertion that 
exposure measurement error was not adequately considered in this 
review. The Integrated Science Assessment included an extensive 
discussion that addresses issues of exposure measurement error (U.S. 
EPA, 2009a, sections 2.3.2 and 3.8.6). Exposure measurement error may 
lead to bias in effect estimates in epidemiological studies. A number 
of studies evaluated in the last review (U.S. EPA, 2004, section 8.4.5) 
and in the current review (U.S. EPA, 2009a, section 3.8.6) have 
discussed the direction and magnitude of bias resulting from specified 
patterns of exposure measurement error (Armstrong 1998; Thomas et al. 
1993; Carroll et al. 1995) and have generally concluded ``classical'' 
(i.e., random, within-person) exposure measurement error can bias 
effect estimates towards the null. Therefore, consistent with 
conclusions reached in the last review, the Integrated Science 
Assessment concluded ``in most circumstances, exposure error tends to 
bias a health effect estimate downward'' (U.S. EPA, 2009a, sections 
2.3.2 and 3.8.6) (emphasis added). Thus, the EPA has both considered 
and accounted for the possibility of exposure measurement error, and 
the possible bias would make it more difficult to detect true 
associations, not less difficult.
(d) Model Specification
    Commenters contended that the EPA did not account for the fact that 
``selecting an appropriate statistical model for epidemiologic studies 
of air pollution involves several choices that involve much ambiguity, 
scant biological evidence, and a profound impact on analytic results, 
given that many estimated associations are weak'' (ACC, 2012, p. 5). 
For short-term exposure studies, the EPA recognizes, as summarized in 
the HEI review panel commentary that selecting a level of control to 
adjust for time-varying factors, such as temperature, in time-series 
epidemiological studies involves a trade-off (HEI, 2003). 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 
(U.S. EPA, 2004, 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. As demonstrated in section 6.5 of the Integrated Science 
Assessment, the EPA thoroughly considered each of these issues and the 
overall effect of different model specifications on the association 
between short-term PM2.5 exposure and mortality. Regardless 
of the model employed, consistent positive associations were observed 
across studies that controlled for the potential confounding effects of 
time and weather using different approaches (U.S. EPA 2009a, Figure 6-
27). The EPA also considered the influence of model specification in 
the examination of long-term PM2.5 exposure studies. For 
example, in section 7.6 of the Integrated Science Assessment, Figures 
7-6 and 7-7 summarize the collective evidence that evaluated the 
association between long-term PM2.5 exposure and mortality. 
Regardless of the model used, these studies collectively found evidence 
of consistent positive associations between long-term PM2.5 
exposure and mortality.
    The EPA, therefore, disagrees with commenters that model 
specification was not considered when evaluating the epidemiological 
evidence used to form causality determinations. The EPA specifically 
points out that the process of assessing the scientific quality and 
relevance of epidemiological studies includes examining ``important 
methodological issues (e.g., lag or time period between exposure and 
effects, model specifications, thresholds, mortality displacement) 
related to interpretation of the health evidence (U.S. EPA, 2009, p. 1-
9).'' Consistent with the conclusions of the 2004 PM Air Quality 
Criteria Document, the EPA recognizes that there is still no clear 
consensus at this time as to what constitutes appropriate control of 
weather and temporal trends in short-term exposure studies, and that no 
single statistical modeling approach is likely to be most appropriate 
in all cases (U.S. EPA, 2004, p. 8-238). However, the 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.
(e) Concentration-Response Relationship
    Additionally, commenters questioned the interpretation of the shape 
of the

[[Page 3119]]

concentration-response relationship, specifically stating that multiple 
studies have demonstrated that there is a threshold in the PM-health 
effect relationship and that the log-linear model is not biologically 
plausible (API, 2012, Attachment 9; ACC, 2012, Appendix A, pp. 7 to 8). 
The EPA disagrees with this assertion due to the number of studies 
evaluated in the Integrated Science Assessment that continue to support 
the use of a no-threshold, log-linear model to most appropriately 
represent the PM concentration-response relationship (U.S. EPA, 2009a, 
section 2.4.3). While recognizing that uncertainties remain, the EPA 
believes that our understanding of this issue for both long- and short-
term exposure studies has advanced since the last review. As discussed 
in the Integrated Science Assessment, both long- and short-term 
exposure studies have employed a variety of statistical approaches to 
examine the shape of the concentration-response function and whether a 
threshold exists. While the EPA recognizes that there likely are 
individual biological thresholds for specific health responses, the 
Integrated Science Assessment concluded the overall evidence from 
existing epidemiological studies does not support the existence of 
thresholds at the population level, for effects associated with either 
long-term or short-term PM exposures within the ranges of air quality 
observed in these studies (U.S. EPA, 2009a, section 2.4.3).\61\ The 
Integrated Science Assessment concluded that this evidence collectively 
supported the conclusion that a no-threshold, log-linear model is most 
appropriate (U.S. EPA, 2009a, sections 6.2.10.10, 6.5.2.7, and 7.6.4). 
CASAC likewise advised that ``[a]lthough there is increasing 
uncertainty at lower levels, there is no evidence of a threshold'' 
(Samet, 2010d, p. ii).
---------------------------------------------------------------------------

    \61\ While epidemiological analyses have not identified a 
population threshold in the range of air quality concentrations 
evaluated in these studies, the EPA recognizes that it is possible 
that such thresholds exist towards the lower end of these ranges (or 
below these ranges).
---------------------------------------------------------------------------

    The EPA recognizes that some short-term exposure studies have 
examined the PM2.5 concentration-response relationship in 
individual cities or on a city-to-city basis and observed heterogeneity 
in the shape of the concentration-response curve across cities. As 
discussed in (b) above, these findings are a source of uncertainty that 
the EPA agrees requires further investigation. Nonetheless, the 
Integrated Science Assessment concluded that ``the studies evaluated 
further support the use of a no-threshold, log-linear model, but 
additional issues such as the influence of heterogeneity in estimates 
between cities and the effects of seasonal and regional differences in 
PM on the concentration-response-relationship still require further 
investigation'' (U.S. EPA, 2009a, p. 2-25).
(f) Relative Toxicity of PM2.5 Components
    Some commenters highlighted uncertainties in understanding the role 
of individual constituents within the mix of fine particles. These 
commenters asserted that a mass-based standard may not be appropriate 
due to the growing body of evidence indicating that certain 
PM2.5 components may be more closely related to specific 
health outcomes (e.g., EC and OC) (EPRI, 2012, p. 2).
    With regard to questions about the role of individual constituents 
within the mix of fine particles, as a general matter, the EPA 
recognizes 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. At the time of the last review, the Agency determined that it 
was appropriate to continue to control fine particles as a group, as 
opposed to singling out any particular component or class of fine 
particles (71 FR 61162 to 61164, October 17, 2006). This distinction 
was based largely on epidemiological 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 the proposal (77 FR 38922 to 38923) 
and in section III.E.1 below, while most epidemiological studies 
continue to be indexed by PM2.5 mass, several recent 
epidemiological studies included in the Integrated Science Assessment 
have used PM2.5 speciation data to evaluate health effects 
associated with fine particle exposures. In the Integrated Science 
Assessment, the EPA thoroughly evaluated the scientific evidence that 
examined the effect of different PM2.5 components and 
sources on a variety of health outcomes (U.S. EPA, 2009a, section 6.6) 
and observed that 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. The 
Integrated Science Assessment concluded that the current body of 
scientific evidence indicated that ``many constituents of PM can be 
linked with differing health effects and the evidence is not yet 
sufficient to allow differentiation of those constituents or sources 
that are more closely related to specific health outcomes'' (U.S. EPA, 
2009a, p. 2-26 and 6-212). Furthermore, the Policy Assessment concluded 
that the evidence is not sufficient to support eliminating any 
component or group of components associated with any specific source 
categories from the mix of fine particles included in the 
PM2.5 indicator (U.S. EPA, 2009a, p. 2-56). CASAC agreed 
that it was reasonable to retain PM2.5 as an indicator for 
fine particles in this review as ``[t]here was insufficient peer-
reviewed literature to support any other indicator at this time'' 
(Samet, 2010c, p. 12).
    This information is relevant to the Agency's decision to retain 
PM2.5 as the indicator for fine particles as discussed in 
section III.E.1 below. The EPA also believes that it is relevant to the 
Agency's conclusion as to whether revision of the suite of primary 
PM2.5 standards is appropriate. While there remain 
uncertainties 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.
    In summary, in considering the above issues related to 
uncertainties in the underlying health science, on balance, the 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 long- and short-
term PM2.5 exposures and mortality and morbidity effects, 
alone and in combination with other pollutants, and 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 epidemiological studies of associations observed in areas 
meeting the current primary PM2.5 standards, specifically in 
areas meeting the current primary annual PM2.5 standard, 
adds support to its conclusion that the current suite of 
PM2.5 standards needs to be revised to provide increased 
public health protection.
    (4) In asserting that there is no evidence of greater risk since 
the 2006

[[Page 3120]]

review to justify lowering the current annual PM2.5 
standard, some commenters argued that, ``if the current primary 
PM2.5 annual standard of 15 [mu]g/m\3\ was considered to be 
adequately protective of public health in 2006, given relative risk 
estimates that EPA was using at that time, then that standard would 
surely still be adequately protective of the public health if relative 
risk estimates remain at the same level (or lower)'' (UARG, 2012, 
Attachment 1, p. 24). These commenters compared risk coefficients used 
for mortality in the EPA's risk assessment done in the last review with 
those from the Agency's core risk assessment done as part of this 
review, and they concluded that ``the entire range of the core relative 
risk for long-term mortality is lower now than it was in the prior 
review'' (UARG, 2012, Attachment 1, p. 24). These commenters used this 
conclusion as the basis for a claim that there is no reason to revise 
the current annual PM2.5 standard.
    The EPA believes that this claim is fundamentally flawed. 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 concentrations), but also the degree of 
confidence that the Agency has that the observed health effects are 
causally linked to PM2.5 exposure at those concentrations. 
As documented in the Integrated Science Assessment and in the 
recommendations and conclusions of CASAC, the EPA recognizes 
significant advances in our understanding of the health effects of 
PM2.5, based on evidence that is stronger than in the last 
review. As a result of these advances, the EPA is now more certain that 
fine particles, alone or in combination with other pollutants, present 
a significant risk to public health at concentrations allowed by the 
current primary PM2.5 standards. From this more 
comprehensive perspective, since the risks presented by 
PM2.5 are more certain, similar or even somewhat lower 
relative risk estimates 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. This also 
ignores that the relative risk estimate is only one factor considered 
by the Administrator, e.g. it ignores that epidemiological studies 
since the last review indicate associations between PM2.5 
and mortality and morbidity in areas meeting the current annual 
standard.
    In any case, the commenters' reliance on the flawed 2006 review is 
misplaced. As discussed in section III.A.2 above, the D.C. Circuit 
remanded Administrator Johnson's 2006 decision to retain the primary 
annual PM2.5 standard because the Agency failed to 
adequately explain why the annual standard provided the requisite 
protection from both short- and long-term exposure to fine particles 
including protection for at-risk populations. The 2006 standard was 
also at sharp odds with CASAC advice and recommendations as to the 
requisite level of protection (Henderson, 2006a,b). In other words, the 
2006 primary annual PM2.5 standard is not an appropriate 
benchmark for comparison.
    (5) Some of these commenters also identified ``new'' as well as 
older studies that had been included in prior reviews as providing 
additional evidence that the causality determinations presented in the 
Integrated Science Assessment did not consider the totality of the 
scientific literature, further supporting their view that a revision of 
the PM2.5 is unwarranted. As discussed in section II.B.3 
above, the 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 Integrated Science Assessment that have 
undergone CASAC and public review, and will consider newly published 
studies for purposes of decisionmaking in the next PM NAAQS review. In 
provisionally evaluating commenters' arguments (see Response to 
Comments document), the EPA notes that its provisional assessment of 
``new'' science found that such studies did not materially change the 
conclusions reached in the Integrated Science Assessment (U.S. EPA, 
2012b).
3. Administrator's Final Conclusions Concerning the Adequacy of the 
Current Primary PM2.5 Standards
    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 
Integrated Science Assessment, and discussed in the Policy Assessment, 
are 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 epidemiological 
studies. The Administrator believes that this literature, combined with 
the other scientific evidence discussed in the Integrated Science 
Assessment, collectively represents a strong and generally robust body 
of evidence of serious health effects associated with both long- and 
short-term exposures to PM2.5. As discussed in the 
Integrated Science Assessment and Policy Assessment, the EPA 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. In that 
context, the Administrator finds the evidence of serious health effects 
reported in exposure studies conducted in areas with long-term mean 
concentrations ranging from approximately at or above the level of the 
annual standard to long-term mean concentrations significantly below 
the level of the annual standard 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 information in the quantitative risk 
assessment lends support to this conclusion.
    There has been extensive critical review of this body of evidence, 
the quantitative risk assessment, and related uncertainties, including 
review by CASAC and the public. The public comments on the basis for 
the EPA's proposed decision to revise the suite of primary 
PM2.5 standards have identified a number of issues about 
which different parties disagree including issues for which additional 
research is warranted. Having weighed all comments and the advice of 
CASAC, the Administrator believes that since the last review the 
overall uncertainty about the public health risks associated with both 
long- and short-term exposure to PM2.5 has been diminished 
to an important degree. The remaining uncertainties in the available 
evidence do not diminish confidence in the associations between 
exposure to fine particles and mortality and serious morbidity effects. 
Based on her increased confidence in the association between exposure 
to PM2.5 and serious public health effects, combined with 
evidence of such an association in areas that would meet the current 
standards, the Administrator agrees with CASAC that revision of the 
current suite of PM2.5 standards to provide increased public 
health protection is necessary. Based on these considerations, the 
Administrator concludes that the current suite of primary 
PM2.5 standards is not sufficient, and thus not requisite, 
to protect public health with an

[[Page 3121]]

adequate margin of safety, and that revision is needed to increase 
public health protection.
    It is important to note that this conclusion, and the reasoning on 
which it is based, do not resolve the question of what specific 
revisions are appropriate. That requires looking specifically at the 
current 24-hour and annual PM2.5 standards, including their 
indicator, averaging times, forms, and levels, and evaluating the 
scientific evidence and other information relevant to determining the 
appropriate revision of the suite of standards.

E. Conclusions on the Elements of the Primary Fine Particle Standards

1. Indicator
    In initially setting standards for fine particles in 1997, the EPA 
concluded it was appropriate to control fine particles as a group, 
rather than singling out any particular component or class of fine 
particles. The EPA noted that community health studies had found 
significant associations between various indicators of fine particles, 
and that health effects in a large number of areas had significant mass 
contributions of differing components or sources of fine particles. In 
addition, a number of toxicological and controlled human exposure 
studies had reported health effects associations with high 
concentrations of numerous fine particle components. It was also not 
possible to rule out any component within the mix of fine particles as 
not contributing to the fine particle effects found in the 
epidemiologic studies (62 FR 38667, July 18, 1977). In establishing a 
size-based indicator in 1977 to distinguish fine particles from 
particles in the coarse mode, the EPA noted that the available 
epidemiological studies of fine particles were based largely on 
PM2.5 and also considered monitoring technology that was 
generally available. The selection of a 2.5 [micro]m size cut reflected 
the regulatory importance of defining an indicator that would more 
completely capture fine particles under all conditions likely to be 
encountered across the U.S., especially when fine particle 
concentrations and humidity are likely to be high, while recognizing 
that some small coarse particles would also be captured by current 
methods to monitor PM2.5 (62 FR 38666 to 38668, July 18, 
1997). In the last review, based on the same considerations, the EPA 
again recognized that the available information supported retaining the 
PM2.5 indicator and remained too limited to support a 
distinct standard for any specific PM2.5 component or group 
of components associated with any source categories of fine particles 
(71 FR 61162 to 61164, October 17, 2006).
    In this current review, the same considerations continue to apply 
for selection of an appropriate indicator for fine particles. As an 
initial matter, the Policy Assessment recognizes that the available 
epidemiological studies linking mortality and morbidity effects with 
long- and short-term exposures to fine particles continue to be largely 
indexed by PM2.5. For the same reasons discussed in the last 
two reviews, the Policy Assessment concluded that it was appropriate to 
consider retaining a PM2.5 indicator to provide protection 
from effects associated with long- and short-term fine particle 
exposures (U.S. EPA, 2011a, p. 2-50).
    The Policy Assessment also considered the expanded body of evidence 
available in this review to consider whether there was sufficient 
evidence to support a separate standard for ultrafine particles \62\ or 
whether there was sufficient evidence to establish distinct standards 
focused on regulating specific PM2.5 components or a group 
of components associated with any source categories of fine particles 
(U.S. EPA, 2011a, section 2.3.1).
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    \62\ Ultrafine particles, generally including particles with a 
mobility diameter less than or equal to 0.1 [micro]m, are emitted 
directly to the atmosphere or are formed by nucleation of gaseous 
constituents in the atmosphere (U.S. EPA, 2009a, p. 3-3).
---------------------------------------------------------------------------

    A number of studies available in this review have evaluated 
potential health effects associated with short-term exposures to 
ultrafine particles. As noted in the Integrated Science Assessment, the 
enormous number and larger, collective surface area of ultrafine 
particles are important considerations for focusing on this particle 
size fraction in assessing potential public health impacts (U.S. EPA, 
2009a, p. 6-83). Per unit mass, ultrafine particles may have more 
opportunity to interact with cell surfaces due to their greater surface 
area and their greater particle number compared with larger particles 
(U.S. EPA, 2009a, p. 5-3). Greater surface area also increases the 
potential for soluble components (e.g., transition metals, organics) to 
adsorb to ultrafine particles and potentially cross cell membranes and 
epithelial barriers (U.S. EPA, 2009a, p. 6-83). In addition, evidence 
available in this review suggests that the ability of particles to 
enhance allergic sensitization is associated more strongly with 
particle number and surface area than with particle mass (U.S. EPA, 
2009a, p. 6-127).
    New evidence, primarily from controlled human exposure and 
toxicological studies, expands our understanding of cardiovascular and 
respiratory effects related to short-term ultrafine particle exposures. 
However, the Policy Assessment concluded that this evidence was still 
very limited and largely focused on exposure to diesel exhaust, for 
which the Integrated Science Assessment concluded it was unclear 
whether the effects observed are due to ultrafine particles, larger 
particles within the PM2.5 mixture, or the gaseous 
components of diesel exhaust (U.S. EPA, 2009a, p. 2-22). In addition, 
the Integrated Science Assessment noted uncertainties associated with 
the controlled human exposure studies using concentrated ambient 
particle systems which have been shown to modify the composition of 
ultrafine particles (U.S. EPA, 2009a, p. 2-22, see also section 1.5.3).
    The Policy Assessment recognized that there are relatively few 
epidemiological studies that have examined potential cardiovascular and 
respiratory effects associated with short-term exposures to ultrafine 
particles (U.S. EPA, 2011a, p. 2-51). These studies have reported 
inconsistent and mixed results (U.S. EPA, 2009a, section 2.3.5).
    Collectively, in considering the body of scientific evidence 
available in this review, the Integrated Science Assessment concluded 
that the currently available evidence was suggestive of a causal 
relationship between short-term exposures to ultrafine particles and 
cardiovascular and respiratory effects. Furthermore, the Integrated 
Science Assessment concluded that evidence was inadequate to infer a 
causal relationship between short-term exposure to ultrafine particles 
and mortality as well as long-term exposure to ultrafine particles and 
all outcomes evaluated (U.S. EPA, 2009a, sections 2.3.5, 6.2.12.3, 
6.3.10.3, 6.5.3.3, 7.2.11.3, 7.3.9, 7.4.3.3, 7.5.4.3, and 7.6.5.3; 
Table 2-6).
    With respect to our understanding of ambient ultrafine particle 
concentrations, at present, there is no national network of ultrafine 
particle samplers; thus, only episodic and/or site-specific data sets 
exist (U.S. EPA, 2009a, p. 2-2). Therefore, the Policy Assessment 
recognized a national characterization of concentrations, temporal and 
spatial patterns, and trends was not possible at this time, and the 
availability of ambient ultrafine measurements to support health 
studies was extremely limited (U.S. EPA, 2011a, p. 2-51). In general, 
measurements of ultrafine particles are highly dependent on monitor 
location and, therefore, more subject to exposure error than

[[Page 3122]]

accumulation mode particles (U.S. EPA, 2009a, p. 2-22). Furthermore, 
the number of ultrafine particles generally decreases sharply downwind 
from sources, as ultrafine particles may grow into the accumulation 
mode by coagulation or condensation (U.S. EPA, 2009a, p. 3-89). Limited 
studies of ambient ultrafine particle measurements have suggested that 
these particles exhibit a high degree of spatial and temporal 
heterogeneity driven primarily by differences in nearby source 
characteristics (U.S. EPA, 2009a, p. 3-84). Internal combustion engines 
and, therefore, roadways are a notable source of ultrafine particles, 
so concentrations of these particles near roadways are generally 
expected to be elevated (U.S. EPA, 2009a, p. 2-3). Concentrations of 
ultrafine particles have been reported to drop off much more quickly 
with distance from roadways than fine particles (U.S. EPA, 2009a, p. 3-
84).
    In considering both the currently available health effects evidence 
and the air quality data, the Policy Assessment concluded that this 
information was still too limited to provide support for consideration 
of a distinct PM standard for ultrafine particles (U.S. EPA, 2011a, p. 
2-52).
    In addressing the issue of particle composition, the Integrated 
Science Assessment concluded that, ``[f]rom a mechanistic perspective, 
it is highly plausible that the chemical composition of PM would be a 
better predictor of health effects than particle size'' (U.S. EPA, 
2009a, p. 6-202). Heterogeneity of ambient concentrations of 
PM2.5 constituents (e.g., elemental carbon, organic carbon, 
sulfates, nitrates) observed in different geographical regions as well 
as regional heterogeneity in PM2.5-related health effects 
reported in a number of epidemiological studies are consistent with 
this hypothesis (U.S. EPA, 2009a, section 6.6).
    With respect to the availability of ambient measurement data for 
fine particle components in this review, the Policy Assessment noted 
that there were now more extensive ambient PM2.5 speciation 
measurement data available through the Chemical Speciation Network 
(CSN) than in previous reviews (U.S. EPA, 2011a, section 1.3.2 and 
Appendix B, section B.1.3). The Integrated Science Assessment observed 
that data from the CSN provided further evidence of spatial and 
seasonal variation in both PM2.5 mass and composition among 
cities and geographic regions (U.S. EPA, 2009a, pp. 3-50 to 3-60; 
Figures 3-12 to 3-18; Figure 3-47). Some of this variation may be 
related to regional differences in meteorology, sources, and topography 
(U.S. EPA, 2009a, p. 2-3).
    The currently available epidemiological, toxicological, and 
controlled human exposure studies evaluated in the Integrated Science 
Assessment on the health effects associated with ambient 
PM2.5 constituents and categories of fine particle sources 
used a variety of quantitative methods applied to a broad set of 
PM2.5 constituents, rather than selecting a few constituents 
a priori (U.S. EPA, 2009a, p. 2-26). Epidemiological studies have used 
measured ambient PM2.5 speciation data, including monitoring 
data from the CSN, while all of the controlled human exposure and most 
of the toxicological studies have used concentrated ambient particles 
and analyzed the constituents therein (U.S. EPA, 2009a, p. 6-203).\63\ 
The CSN provides PM2.5 speciation measurements generally on 
a one-in-three or one-in-six day sampling schedule and, thus, does not 
capture data every day at most sites.\64\
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    \63\ Most studies considered between 7 to 20 ambient 
PM2.5 constituents, with elemental carbon, organic 
carbon, sulfates, nitrates, and metals most commonly measured. Many 
of the studies grouped the constituents with various factorization 
or source apportionment techniques to examine the relationship 
between the grouped constituents and various health effects. 
However, not all studies labeled the constituent groupings according 
to their presumed source and a small number of controlled human 
exposure and toxicological studies did not use any constituent 
grouping. These differences across studies substantially limit any 
integrative interpretation of these studies (U.S. EPA, 2009a, p. 6-
203).
    \64\ To expand our understanding of the role of specific 
PM2.5 components and sources with respect to the observed 
health effects, researchers have expressed a strong interest in 
having access to PM2.5 speciation measurements collected 
more frequently (U.S. EPA, 2011a, p. 2-53, including footnote 47).
---------------------------------------------------------------------------

    The Policy Assessment recognized that several new multi-city 
studies evaluating short-term exposures to fine particle constituents 
are now available. These studies continued to show an association 
between mortality and cardiovascular and/or respiratory morbidity 
effects and short-term exposures to various PM2.5 components 
including nickel, vanadium, elemental carbon, organic carbon, nitrates, 
and sulfates (U.S. EPA, 2011a, section 2.3.1; U.S. EPA, 2009a, sections 
6.5.2.5 and 6.6).
    Limited evidence is available to evaluate the health effects 
associated with long-term exposures to PM2.5 components 
(U.S. EPA, 2009a, section 7.6.2). The Policy Assessment noted the most 
significant new evidence was provided by a study that evaluated 
multiple PM2.5 components and an indicator of traffic 
density in an assessment of health effects related to long-term 
exposure to PM2.5 (Lipfert et al., 2006a). Using health data 
from a cohort of U.S. military veterans and PM2.5 
measurement data from the CSN, Lipfert et al. (2006a) reported positive 
associations between mortality and long-term exposures to nitrates, 
elemental carbon, nickel, and vanadium as well as traffic density and 
peak ozone concentrations (U.S. EPA, 2011a, p. 2-54; U.S. EPA, 2009a, 
pp. 7-89 to 7-90).
    With respect to source categories of fine particles potentially 
associated with a range of health endpoints, the Integrated Science 
Assessment reported that the currently available evidence suggests 
associations between cardiovascular effects and a number of specific 
PM2.5-related source categories, including oil combustion, 
wood or biomass burning, motor vehicle emissions, and crustal or road 
dust sources (U.S. EPA, 2009a, section 6.6; Table 6-18). In addition, a 
few studies have evaluated associations between PM2.5-
related source categories and mortality. For example, one study 
reported an association between mortality and a PM2.5 coal 
combustion factor (Laden et al., 2000), while other studies linked 
mortality to a secondary sulfate long-range transport PM2.5 
source (Ito et al., 2006; Mar et al., 2006) (U.S. EPA, 2009a, section 
6.6.2.1). Other studies have looked at different components of 
particulate matter. There was less consistency in associations observed 
between selected sources of fine particles and respiratory health 
endpoints, which may be partially due to the fact that fewer studies 
have evaluated respiratory-related outcomes and measures. However, 
there was some evidence for PM2.5-related associations with 
secondary sulfate and decrements in lung function in asthmatic and 
healthy adults (U.S. EPA, 2009a, p. 6-211; Gong et al., 2005; Lanki et 
al., 2006). A couple of studies have observed an association between 
respiratory endpoints in children and adults with asthma and surrogates 
for the crustal/soil/road dust and traffic sources of PM (U.S. EPA, 
2009a, p. 6-205; Gent et al., 2009; Penttinen et al., 2006).
    Recent studies have shown that source apportionment methods have 
the potential to add useful insights into which sources and/or PM 
constituents may contribute to different health effects. Of particular 
interest are several epidemiological studies that compared source 
apportionment methods and reported consistent results across research 
groups (U.S. EPA, 2009a, p. 6-211; Hopke et al., 2006; Ito et al., 
2006; Mar et al., 2006; Thurston et al., 2005).

[[Page 3123]]

These studies reported associations between total mortality and 
secondary sulfate in two cities for two different lag times. The 
sulfate effect was stronger for total mortality in Washington, DC and 
for cardiovascular-related mortality in Phoenix (U.S. EPA, 2009a, p. 6-
204). These studies also found some evidence for associations with 
mortality and a number of source categories (e.g., biomass/wood 
combustion, traffic, copper smelter, coal combustion, sea salt) at 
various lag times (U.S. EPA, 2009a, p. 6-204). Sarnat et al. (2008) 
compared three different source apportionment methods and reported 
consistent associations between emergency department visits for 
cardiovascular diseases with mobile sources and biomass combustion as 
well as increased respiratory-related emergency department visits 
associated with secondary sulfate (U.S. EPA, 2009a, pp. 6-204 and 6-
211).
    Collectively, in considering the currently available evidence for 
health effects associated with specific PM2.5 components or 
groups of components associated with any source categories of fine 
particles as presented in the Integrated Science Assessment, the Policy 
Assessment concluded that additional information available in this 
review continues to provide evidence that many different constituents 
of the fine particle mixture as well as groups of components associated 
with specific source categories of fine particles are linked to adverse 
health effects (U.S. EPA, 2011a, p. 2-55). However, as noted in the 
Integrated Science Assessment, while ``[t]here is some evidence for 
trends and patterns that link particular ambient PM constituents or 
sources with specific health outcomes * * * there is insufficient 
evidence to determine whether these patterns are consistent or robust'' 
(U.S. EPA, 2009a, p. 6-210). Assessing this information, the Integrated 
Science Assessment concluded that ``the evidence is not yet sufficient 
to allow differentiation of those constituents or sources that are more 
closely related to specific health outcomes'' (U.S. EPA, 2009a, pp. 2-
26 and 6-212). Therefore, the Policy Assessment concluded that the 
currently available evidence is not sufficient to support consideration 
of a separate indicator for a specific PM2.5 component or 
group of components associated with any source category of fine 
particles. Furthermore, the Policy Assessment concluded that the 
evidence is not sufficient to support eliminating any component or 
group of components associated with any source categories of fine 
particles from the mix of fine particles included in the 
PM2.5 indicator (U.S. EPA, 2011a, p. 2-56).
    The CASAC agreed with the EPA staff conclusions presented in the 
Policy Assessment and concluded that it is appropriate to consider 
retaining PM2.5 as the indicator for fine particles and 
further asserted, ``There [is] insufficient peer-reviewed literature to 
support any other indicator at this time'' (Samet, 2010c, p. 12). CASAC 
expressed a strong desire for the EPA to ``look ahead to future review 
cycles and reinvigorate support for the development of evidence that 
might lead to newer indicators that may correlate better with the 
health effects associated with ambient air concentrations of PM * * *'' 
(Samet, 2010c, p 2).
    Consistent with the staff conclusions presented in the Policy 
Assessment and CASAC advice, the Administrator proposed to retain 
PM2.5 as the indicator for fine particles. Further, the 
Administrator provisionally concluded that currently available 
scientific information does not provide a sufficient basis for 
supplementing mass-based, primary fine particle standards with 
standards using a separate indicator for ultrafine particles or a 
separate indicator for a specific PM2.5 component or group 
of components associated with any source categories of fine particles. 
In addition, the Administrator also provisionally concluded that the 
currently available scientific information did not provide a sufficient 
basis for eliminating any individual component or group of components 
associated with any source categories from the mix of fine particles 
included in the PM2.5 mass-based indicator.
    The EPA received comparatively few public comments on issues 
related to the indicator for fine particles.\65\ Some commenters 
emphasized the need to conduct additional research to more fully 
understand the effect of specific PM2.5 components and/or 
sources on public health. These commenters 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) may be 
appropriate to consider in future PM NAAQS reviews for standards 
intended to protect against the array of health effects that have been 
associated with fine particles as indexed by PM2.5. For 
example, the PSR encouraged more research and monitoring related to 
PM2.5 components and noted the importance of components 
associated with coal combustion (PSR, 2012, pp. 5 to 6). EPRI asserted 
that ``new'' studies support focusing on EC and OC and encouraged the 
EPA to seriously consider the mass-based approach (EPRI, 2012, p. 2). 
Likewise, Georgia Mining Association supported additional monitoring 
and research efforts related to PM2.5 composition and 
specifically encouraged the evaluation of using particle number (e.g., 
particle count) (GMA, 2012, pp. 2 to 3).
---------------------------------------------------------------------------

    \65\ No public comments were submitted regarding the use of a 
different size cut for fine particles.
---------------------------------------------------------------------------

    The Administrator agrees with CASAC as well as these commenters 
that the results of additional research and monitoring efforts will be 
helpful for informing future PM NAAQS reviews. 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. However, based 
upon the scientific information considered in the Integrated Science 
Assessment as well as the public comments summarized above, the 
Administrator continues to take note there is evidence that many 
different constituents of the fine particle mixture as well as groups 
of components associated with specific sources of fine particles are 
linked to adverse health effects. Furthermore, she recognizes that the 
evidence is not yet sufficient to differentiate those constituents or 
sources that are most closely related to specific health outcomes nor 
to exclude any PM2.5 components or sources of fine particles 
from the mix of particles included in the PM2.5 indicator.
    Having considered the public comments on this issue, the 
Administrator concurs with the Policy Assessment conclusions and CASAC 
recommendations and concludes that it is appropriate to retain 
PM2.5 as the indicator for fine particles.
2. Averaging Time
    In 1997, the EPA initially set both an annual standard, to provide 
protection from health effects associated with both long- and short-
term exposures to PM2.5, and a 24-hour standard to 
supplement the protection afforded by the annual standard (62 FR 38667 
to 38668, July, 18, 1997). In the last review, the EPA retained both 
annual and 24-hour averaging times (71 FR 61164, October 17, 2006). 
These decisions were based, in part, on evidence of health effects 
related to both long-term (from a year to several years) and short-term 
(from less than one day to up to several days) measures of 
PM2.5.

[[Page 3124]]

    The overwhelming majority of studies conducted since the last 
review continue to utilize annual (or multi-year) and 24-hour averaging 
times, reflecting the averaging times of the current PM2.5 
standards. These studies continue to provide evidence that health 
effects are associated with annual and 24-hour averaging times. 
Therefore, the Policy Assessment concluded it is appropriate to retain 
the current annual and 24-hour averaging times to provide protection 
from effects associated with both long- and short-term PM2.5 
exposures (U.S. EPA, 2011a, p. 2-57).
    In considering whether the information available in this review 
supports consideration of different averaging times for 
PM2.5 standards specifically with regard to considering a 
standard with an averaging time less than 24 hours to address health 
effects associated with sub-daily PM2.5 exposures, the 
Policy Assessment noted there continues to be a growing body of studies 
that provide additional evidence of effects associated with exposure 
periods less than 24-hours (U.S. EPA, 2011a, p. 2-57). Relative to 
information available in the last review, recent studies provide 
additional evidence for cardiovascular effects associated with sub-
daily (e.g., one to several hours) exposure to PM, especially effects 
related to cardiac ischemia, vasomotor function, and more subtle 
changes in markers of systemic inflammation, hemostasis, thrombosis and 
coagulation (U.S. EPA, 2009a, section 6.2). Because these studies have 
used different indicators (e.g., PM2.5, PM10, 
PM10-2.5, ultrafine particles), averaging times (e.g., 1, 2, 
and 4 hours), and health outcomes, it is difficult to draw conclusions 
about cardiovascular effects associated specifically with sub-daily 
exposures to PM2.5.
    With regard to respiratory effects associated with sub-daily 
PM2.5 exposures, the currently available evidence was much 
sparser than for cardiovascular effects and continues to be very 
limited. The Integrated Science Assessment concluded that for several 
studies of hospital admissions or medical visits for respiratory 
diseases, the strongest associations were observed with 24-hour average 
or longer exposures, not with less than 24-hour exposures (U.S. EPA, 
2009a, section 6.3).
    Collectively, the Policy Assessment concluded that this 
information, when viewed as a whole, is too unclear, with respect to 
the indicator, averaging time and health outcome, to serve as a basis 
for consideration of establishing a primary PM2.5 standard 
with an averaging time shorter than 24-hours at this time (U.S. EPA, 
2011a, p. 2-57).
    With regard to health effects associated with PM2.5 
exposure across varying seasons in this review, Bell et al. (2008) 
reported higher PM2.5 risk estimates for hospitalization for 
cardiovascular and respiratory diseases in the winter compared to other 
seasons. In comparison to the winter season, smaller statistically 
significant associations were also reported between PM2.5 
and cardiovascular morbidity for spring and autumn, and a positive, but 
statistically non-significant association was observed for the summer 
months. In the case of mortality, Zanobetti and Schwartz (2009) 
reported a 4-fold higher effect estimate for PM2.5-
associated mortality for the spring as compared to the winter. Taken 
together, these results provided emerging but limited evidence that 
individuals may be at greater risk of dying from higher exposures to 
PM2.5 in the warmer months and may be at greater risk of 
PM2.5-associated hospitalization for cardiovascular and 
respiratory diseases during colder months of the year (U.S. EPA, 2011a, 
p. 2-58).
    Overall, the Policy Assessment observed that there are few studies 
presently available to deduce a general pattern in PM2.5-
related risk across seasons. In addition, these studies utilized 24-
hour exposure periods within each season to assess the 
PM2.5-associated health effects and do not provide 
information on health effects associated with a season-long exposure to 
PM2.5. Due to these limitations in the currently available 
evidence, the Policy Assessment concluded that there was no basis to 
consider a seasonal averaging time separate from a 24-hour averaging 
time.
    Based on the above considerations, the Policy Assessment concluded 
that the currently available information provided strong support for 
consideration of retaining the current annual and 24-hour averaging 
times but does not provide support for considering alternative 
averaging times (U.S. EPA, 2011a, p. 2-58). In addition, CASAC 
considered it appropriate to retain the current annual and 24-hour 
averaging times for the primary PM2.5 standards (Samet, 
2010c, pp. 2 to 3). At the time of the proposal, the Administrator 
concurred with the staff conclusions and CASAC advice and proposed that 
the averaging times for the primary PM2.5 standards should 
continue to include annual and 24-hour averages to protect against 
health effects associated with long- and short-term exposures. 
Furthermore, the Administrator provisionally concluded, consistent with 
conclusions reached in the Policy Assessment and by CASAC, that the 
currently available information was too limited to support 
consideration of alternative averaging times to establish a national 
standard with a shorter-than 24-hour averaging time or with a seasonal 
averaging time.
    The EPA received no significant public comments on the issue of 
averaging time for the PM2.5 primary standards. The 
Administrator concurs with recommendations made by CASAC and the staff 
conclusions presented in the Policy Assessment 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 long- and short-term 
exposure periods.
3. Form
    The ``form'' of a standard defines the air quality statistic that 
is to be compared to the level of the standard in determining whether 
an area attains the standard. In this review, the EPA considers whether 
currently available information supports retaining or revising the 
forms for the annual or 24-hour PM2.5 standards.
a. Annual Standard
    In 1997, the 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 
was intended to represent a relatively stable measure of air quality 
and to characterize longer-term area-wide PM2.5 
concentrations, in conjunction with a 24-hour standard designed to 
provide adequate protection against localized peak or seasonal 
PM2.5 concentrations. The level of the standard was to be 
compared to measurements made at each community-oriented monitoring 
site, or, if specific criteria were met, measurements from multiple 
community-oriented monitoring sites could be averaged (i.e., spatial 
averaging) \66\ (62 FR 38671 to 38672, July 18, 1997). The constraints 
were intended to ensure that spatial averaging would not result in 
inequities in the level of protection provided by the standard (62 FR 
38672, July 18, 1997). This approach was consistent with the 
epidemiological studies on which the PM2.5 standard was 
primarily based, in which air quality data were generally averaged 
across multiple monitors in an

[[Page 3125]]

area or were taken from a single monitor that was selected to represent 
community-wide exposures.
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    \66\ Spatial averaging as part of the form of the annual 
PM2.5 standard is unique to this standard and is not used 
with other PM standards nor with other NAAQS.
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    In the last review, the EPA tightened the criteria for use of 
spatial averaging to provide increased protection for vulnerable 
populations exposed to PM2.5. This change was based in part 
on an analysis of the potential for disproportionate impacts on 
potentially at-risk populations, which found 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 (71 FR 61166/2, October 17, 2006; U.S. EPA, 2005, section 
5.3.6.1).
    In this review, as outlined in section III.B above and discussed 
more fully in section III.B.3 of the proposal, there now exist more 
health data such that the Integrated Science Assessment has identified 
persons from lower socioeconomic strata as an at-risk population (U.S. 
EPA, 2009a, section 8.1.7; U.S. EPA, 2011a, section 2.2.1). Moreover, 
there now exist more years of PM2.5 air quality data than 
were available in the last review. Consideration in the Policy 
Assessment of the spatial variability across urban areas that was 
revealed by this expanded data base has raised questions as to whether 
an annual standard that allows for spatial averaging, even within 
specified constraints as narrowed in 2006 (71 FR 61165 to 61167, 
October 17, 2006), would provide appropriate public health protection.
    In considering the potential for disproportionate impacts on at-
risk populations, the Policy Assessment considered an update of an air 
quality analysis conducted for the last review (U.S. EPA, 2011a, pp. 2-
59 to 60; Schmidt, 2011, Analysis A). This analysis focused on 
determining whether the spatial averaging provisions, as modified in 
2006, could introduce inequities in protection for at-risk populations 
exposed to PM2.5. Specifically, the Policy Assessment 
considered whether persons of lower socioeconomic status, minority 
groups, or different age groups (i.e., children or older adults) are 
more likely than the general population to live in areas 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 Block or Census Block Group level, including 
percent minority population, percent minority subgroup population, 
percent of persons living below the poverty level, percent of persons 
18 years of age or older, and percent of persons 65 years of age and 
older. In each candidate geographic area, data from the Census Block(s) 
or Census Block Group(s) surrounding the location of the monitoring 
site (as delineated by radii buffers of 0.5, 1.0, 2.0, and 3.0 miles) 
in which the highest air quality value was monitored were compared to 
the average of monitored values in the area. This analysis looked 
beyond areas that would meet the current spatial averaging criteria and 
considered all urban areas (i.e., Core Based Statistical Areas or 
CBSAs) with at least two valid annual design value monitors (Schmidt, 
2011, Analysis A). Recognizing the limitations of such cross-sectional 
analyses, the Policy Assessment observed that the highest 
concentrations in an area tend to be measured at monitors located in 
areas where the surrounding populations are more likely to live below 
the poverty line and to have higher percentage of minorities (U.S. EPA, 
2011a, p. 2-60).
    Based upon the analysis described above, the Policy Assessment 
concluded that the existing constraints on spatial averaging, as 
modified in 2006, may be inadequate to avoid substantially greater 
exposures in some areas, potentially resulting in disproportionate 
impacts on at-risk populations of persons with lower SES levels as well 
as minorities. Therefore, the Policy Assessment concluded that it was 
appropriate to consider revising the form of the annual 
PM2.5 standard such that it did not allow for the use of 
spatial averaging across monitors. In doing so, the level of the annual 
PM2.5 standard would be compared to measurements made at the 
monitoring site that represents area-wide air quality recording the 
highest PM2.5 concentrations \67\ (U.S. EPA, 2011a, p. 2-
60).
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    \67\ As discussed in section VIII.B.1 below, the EPA is revising 
several terms associated with PM2.5 monitor placement. 
Specifically, the EPA is revoking the term ``community-oriented'' 
and replacing it with the term ``area-wide'' monitoring.
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    The CASAC agreed with staff conclusions that it was ``reasonable'' 
for the EPA to eliminate the spatial averaging provisions (Samet, 
2010d, p. 2). Further, in CASAC's comments on the first draft Policy 
Assessment, it noted, ``Given mounting evidence showing that persons 
with lower SES levels are a susceptible group for PM-related health 
risks, CASAC recommends that the provisions that allow for spatial 
averaging across monitors be eliminated for the reasons cited in the 
(first draft) Policy Assessment'' (Samet, 2010c, p. 13). In its review 
of the second draft Policy Assessment, CASAC recognized ``although much 
of the epidemiological research has been conducted using community-wide 
averages, several key studies reference the nearest measurement site, 
so that some risk estimates are not necessarily biased by the averaging 
process. Further, the number of such studies is likely to expand in the 
future'' (Samet, 2010d, pp. 1 to 2).
    Only two areas in the country used the initial spatial averaging 
provisions for demonstrating attainment with the primary annual 
PM2.5 standard set in 1997 (70 FR 19847, April 14, 2005; 
U.S. EPA, 2006c). Since these provisions were tightened in 2006, no 
area has used spatial averaging to demonstrate attainment. No areas in 
the country are currently using the spatial averaging provisions to 
demonstrate attainment with the current primary annual PM2.5 
standard.
    In considering the Policy Assessment's conclusions based on the 
results of the analysis discussed above and concern over the evidence 
of potential disproportionate impacts on at-risk populations as well as 
CASAC advice, the Administrator proposed to revise the form of the 
annual PM2.5 standard to eliminate the use of spatial 
averaging. Thus, the Administrator proposed revising the form of the 
annual PM2.5 standard to compare the level of the standard 
with measurements from each ``appropriate'' monitor in an area \68\ 
with no allowance for spatial averaging. Thus, for an area with 
multiple monitors, the appropriate reporting monitor with the highest 
design value would determine the attainment status for that area.
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    \68\ As discussed in section VIII.B.2.b below, the EPA concludes 
that PM2.5 monitoring sites at micro- and middle-scale 
locations are comparable to the annual standard if the monitoring 
site has been approved by the Regional Administrator as representing 
an area-wide location.
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    Of the commenters noted in section III.D.2 above who supported a 
more stringent annual PM2.5 standard, those who commented on 
the form of the annual PM2.5 standard supported the EPA's 
proposal to eliminate the spatial averaging provisions. These 
commenters contended that the EPA's analyses of the potential impacts 
of spatial averaging, discussed above and in the proposal (77 FR 
38924), demonstrated that the current form results in uneven public 
health protection leading to disproportionate impacts on at-risk 
populations. Specifically, the ALA and other environmental and public 
health commenters contended that ``spatial averaging allows exposure of 
people to unhealthy levels of pollution at specific locales even within 
an area meeting the standard'' (ALA et al., 2012, p. 23).

[[Page 3126]]

These commenters particularly focused on the importance for low-income 
and minority populations of eliminating the spatial averaging 
provisions. They concluded that spatial averaging ``is an environmental 
justice concern because poor people are more likely to live near roads, 
depots, factories, ports, and other pollution sources.'' Id. p. 24.
    Other commenters (e.g., AAM, 2012; Dow, 2012) also supported the 
elimination of spatial averaging in order to ``avoid potential 
disproportionate impacts on at-risk populations'' and to maximize ``the 
benefits to public health of reducing the annual PM2.5 
standard.'' However, these groups expressed concern that the 
elimination of spatial averaging, in combination with the requirement 
for near road monitors (as discussed in section VIII.B.3.b.i of the 
proposal), would effectively and inappropriately increase the 
stringency of the annual PM2.5 standard.
    This concern was also shared by other commenters who disagreed with 
the elimination of spatial averaging. For example, the Class of '85 RRG 
emphasized concerns about increasing the stringency of the standard 
while providing few health benefits if spatial averaging is eliminated, 
particularly in combination with the requirement for near-road 
monitors. These commenters contended that ``[b]ecause EPA proposes to 
use the readings from the highest single worst case monitor (rather 
than the average of all community area monitors), and since roadway 
monitoring locations will likely be worst case monitors, the proposed 
NAAQS will become more stringent without targeting the PM2.5 
species most harmful to human health'' (Class of '85 RRG, 2012, p. 6).
    Several commenters also maintained that because spatial averaging 
is consistent with how air quality data are considered in the 
underlying epidemiological studies, such averaging should not be 
eliminated. Specifically, commenters including NAM et al., AFPM, and 
ACC pointed out that PM2.5 epidemiological studies use 
spatially averaged multi-monitor concentrations, rather than the single 
highest monitor, when evaluating health effects. Therefore, these 
commenters contended that allowing spatial averaging would make the 
PM2.5 standard more consistent with the approaches used in 
the epidemiological studies upon which the standard is based. In 
addition, some commenters also contended that the EPA failed to 
consider whether modifying, rather than eliminating, the constraints on 
spatial averaging would have been sufficient to protect the public 
health. If so, these commenters argued that ``elimination of spatial 
averaging would go beyond what is requisite to protect the public 
health'' (NAM et al., 2012, p. 20).
    In considering the public comments on the form of the annual 
standard, the EPA recognizes a number of commenters agreed with the 
basis for the EPA's proposal to eliminate spatial averaging. While 
other commenters expressed disagreement or concern with the proposed 
decision to eliminate the spatial averaging provisions, the Agency 
notes that these commenters did not challenge the analyses or 
considerations that provided the fundamental basis for the 
Administrator's proposed decision. Rather, these commenters generally 
raised concerns that eliminating the option for spatial averaging would 
increase the stringency of the standard, especially in light of 
additional monitoring sites in near-road environments (as discussed in 
section VIII.B.3.b.1 below).
    The EPA does not agree with the comment that siting some monitors 
in near roadway environments makes the standard more stringent or 
impermissibly more stringent. As discussed in section VIII.B.3.b.i 
below, a significant fraction of the population lives in proximity to 
major roads, and these exposures occur in locations that represent 
ambient air. Monitoring in such areas does not make the standard more 
stringent than warranted, but rather affords the intended protection to 
the exposed populations, among them at-risk populations, exposed to 
fine particles in these areas. Thus, in cases where monitors in near 
roadway environments are deemed to be representative of area-wide air 
quality they would be compared to the annual standard (as discussed 
more fully in section VIII below). The 24-hour and annual NAAQS are 
designed to protect the public with an adequate margin of safety, and 
this siting provision is fully consistent with providing the protection 
the standard is designed to provide and does not make the standard more 
stringent or more stringent than necessary.
    Monitors that are representative of area-wide air quality may be 
compared to the annual standard. This is consistent with the use of 
monitoring data in the epidemiological studies that provide the primary 
basis for determining the level of the annual standard. In addition, 
the EPA notes that the annual standard is designed to protect against 
both long- and short-term exposures through controlling the broad 
distribution of air quality across an area over time.\69\ It is fully 
consistent with the protection the standard is designed to provide for 
near road monitors to be compared to the annual standard if the monitor 
is representative of area-wide air quality. This does not make the 
standard either more stringent or impermissibly more stringent.
---------------------------------------------------------------------------

    \69\ This is in contrast to the 24-hour standard which is 
designed to provide supplemental protection, addressing peak 
exposures that might not otherwise be addressed by the annual 
standard. Consistent with this, monitors are not required to be 
representative of area-wide air quality to be compared to the 24-
hour standard.
---------------------------------------------------------------------------

    In further considering these comments, the EPA notes that the 
stringency or level of protection provided by each NAAQS is not based 
solely on the form of the standard; rather, the four elements of the 
standard that together serve to define each standard (i.e., indicator, 
averaging time, form, and level) must be considered collectively in 
evaluating the protection afforded by each standard. Therefore, the EPA 
considers these comments are also appropriate to discuss collectively 
with other issues related to the appropriate level for annual standard, 
and are discussed below in sections III.E.4.c-d.
    In reaching a final decision on the form of the annual standard, 
the Administrator considers the available analyses, CASAC advice, and 
public comments on form as discussed above. She also considers related 
issues in the public comments on the level of the annual standard as 
discussed in section III.E.4.c below. She notes that even when the 
annual PM2.5 standard was first set in 1997, the spatial 
averaging provisions included constraints intended to ensure that 
inequities in the level of protection would not result. These 
constraints on spatial averaging were tightened in the last review, 
based on an analysis showing the potential for spatial averaging to 
allow higher PM2.5 concentrations in locations where 
subgroups within the general population were potentially 
disproportionately exposed and hence, at disproportionate risk (e.g., 
low income and minority communities). The Administrator notes that in 
proposing to eliminate spatial averaging altogether in this review, she 
has relied on further analyses in the current review (Schmidt, 2011, 
Analysis A). As discussed above and in the proposal (77 FR 38924), 
these analyses showed that the current constraints on spatial averaging 
may be inadequate in some areas to avoid substantially greater 
exposures for people living near monitors recording the highest 
PM2.5 concentrations. Such exposures could result in

[[Page 3127]]

disproportionate impacts to at-risk populations, including low-income 
populations as well as minority groups.
    On this basis, the Administrator concludes that public health would 
not be protected with an adequate margin of safety in all locations, as 
required by law, if disproportionately higher exposure concentrations 
in at-risk populations such as low income communities as well as 
minority communities were averaged together with lower concentrations 
measured at other sites in a large urban area. See ALA v. EPA, 134 F. 
3d 388, 389 (D.C. Cir., 1998) (``this court has held that `NAAQS must 
protect not only average healthy individuals, but also sensitive 
citizens such as children,' and `if a pollutant adversely affects the 
health of these sensitive individuals, EPA must strengthen the entire 
national standard''') and Coalition of Battery Recyclers Association v. 
EPA, 604 F 3d. 613, 617 (D.C. Cir., 2010) (``Petitioners' assertion 
that the revised lead NAAQS is overprotective because it is more 
stringent than necessary to protect the entire population of young U.S. 
children ignores that the Clean Air Act allows protection of sensitive 
subpopulations.'') In reaching this conclusion, the Administrator 
further notes that her concern over possible disproportionate 
PM2.5-related health impacts in at-risk populations extends 
to populations living near important sources of PM2.5, 
including the large populations that live near major roadways.\70\
---------------------------------------------------------------------------

    \70\ Section VIII.B.3.b.i below discusses public comments 
specifically related to the proposed requirement for near-road 
monitors.
---------------------------------------------------------------------------

    In light of all of the above considerations, including 
consideration of available analyses, CASAC advice, and public comments, 
the Administrator concludes that the current form of the annual 
PM2.5 standard should be revised to eliminate spatial 
averaging provisions. Thus, the level of the revised annual 
PM2.5 standard established with this rule will be compared 
with measurements from each appropriate monitor in an area, with no 
allowance for spatial averaging. The Administrator's conclusions with 
regard to the appropriate level of the annual PM2.5 standard 
to set in conjunction with this form are discussed below in section 
III.E.4.d.
b. 24-Hour Standard
    In 1997, the EPA established the form of the 24-hour 
PM2.5 standard as the 98th percentile of 24-hour 
concentrations at each population-oriented monitor within an area, 
averaged over three years (62 FR at 38671 to 38674, July 18, 1997). The 
Agency selected the 98th percentile as an appropriate balance between 
adequately limiting the occurrence of peak concentrations and providing 
increased stability which, when averaged over 3 years, facilitated 
effective health protection through the development of more stable 
implementation programs. By basing the form of the standard on 
concentrations measured at population-oriented monitoring sites, the 
EPA intended to provide protection for people residing in or near 
localized areas of elevated concentrations. In the last review, in 
conjunction with lowering the level of the 24-hour standard, the EPA 
retained this form based in part on a comparison with the 99th 
percentile form.\71\
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    \71\ In reaching this final decision, the EPA recognized a 
technical problem associated with a potential bias in the method 
used to calculate the 98th percentile concentration for this form. 
The EPA adjusted the sampling frequency requirement in order to 
reduce this bias. Accordingly, the Agency modified the final 
monitoring requirements such that areas that are within 5 percent of 
the standards are required to increase the sampling frequency to 
every day (71 FR 61164 to 61165, October 17, 2006).
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    In revisiting the stability of a 98th versus 99th percentile form 
for a 24-hour standard intended to provide supplemental protection for 
a generally controlling annual standard, an analysis presented in the 
Policy Assessment considered air quality data reported in 2000 to 2008 
to update our understanding of the ratio between peak-to-mean 
PM2.5 concentrations. This analysis provided evidence that 
the 98th percentile value was a more stable metric than the 99th 
percentile (U.S. EPA, 2011a, Figure 2-2, p. 2-62).
    At the time of the proposal, the Agency recognized that the 
selection of the appropriate form of the 24-hour standard includes 
maintaining adequate protection against peak 24-hour concentrations 
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.\72\ As in previous reviews, the EPA recognized that a 
concentration-based form, compared to an exceedance-based form, was 
more reflective of the health risks posed by elevated pollutant 
concentrations because such a form 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 level of the 
standard. Further, the Agency provisionally concluded that a 
concentration-based form, when averaged over three years, provided an 
appropriate balance between limiting peak pollutant concentrations and 
providing a stable regulatory target, thus facilitating the development 
of more stable implementation programs.
---------------------------------------------------------------------------

    \72\ See ATA III, 283 F.3d at 374-376 which concludes that it is 
legitimate for the EPA to consider overall stability of the standard 
and its resulting promotion of overall effectiveness of NAAQS 
control programs in setting a standard that is requisite to protect 
the public health.
---------------------------------------------------------------------------

    In considering the information provided in the Policy Assessment 
and 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, the Administrator proposed to retain the 
98th percentile form of the 24-hour standard. The Administrator 
provisionally concluded that the 98th percentile form represents an 
appropriate balance between adequately limiting the occurrence of peak 
concentrations and providing increased stability relative to an 
alternative 99th percentile form.
    Few public commenters commented specifically on the form of the 24-
hour 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 in section III.D.2 
above, 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. Other public commenters representing state and local air 
agencies and industry groups generally supported retaining the current 
98th percentile form. In most cases, these groups expressed the overall 
view that the current 24-hour PM2.5 standard, including the 
form of the current standard, should be retained.
    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

[[Page 3128]]

health protection in the long run.\73\ 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 staff conclusions presented in the Policy Assessment and 
CASAC recommendations and concludes that it is appropriate to retain 
the 98th percentile form for the 24-hour PM2.5 standard.
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    \73\ As just noted above, it is legitimate for the EPA to 
consider promotion of overall effectiveness of risk management 
programs designed to attain the NAAQS, including their overall 
stability, in setting a standard that is requisite to protect the 
public health. The context for the court's discussion in ATA III is 
identical to that here; whether to adopt a 98th percentile form for 
a 24-hour standard intended to provide supplemental protection for a 
generally controlling annual standard.
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4. Level
    In the last review, the EPA selected levels for the annual and the 
24-hour PM2.5 standards using evidence of effects associated 
with periods of exposure that were most closely matched to the 
averaging time of each standard. Thus, as discussed in section III.A.1, 
the EPA relied upon evidence from long-term exposure studies as the 
principal basis for selecting the level of the annual PM2.5 
standard that would protect against effects associated with long-term 
exposures. The EPA relied upon evidence from the short-term exposure 
studies as the principal basis for selecting the level of the 24-hour 
PM2.5 standard that would protect against effects associated 
with short-term exposures. As summarized in section III.A.2 above, the 
2006 decision to retain the level of the annual PM2.5 
standard at 15 [mu]g/m\3\ \74\ was challenged and on judicial review, 
the DC Circuit remanded the primary annual PM2.5 standard to 
the EPA, finding that EPA's explanation for its approach to setting the 
level of the annual standard was inadequate.
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    \74\ Throughout this section, the annual standard levels are 
denoted as integer values for simplicity, although, as noted above 
in section II.B.1, Table 1, the annual standard level is defined to 
one decimal place, such that the current annual standard level is 
15.0 [mu]g/m\3\. Alternative annual standard levels discussed in 
this section are similarly defined to one decimal place.
---------------------------------------------------------------------------

a. General Approach for Considering Standard Levels
    Building upon the lessons learned in the previous PM NAAQS reviews, 
in considering alternative standard levels supported by the currently 
available scientific information, the Policy Assessment used an 
approach that integrated evidence-based and risk-based considerations, 
took into account CASAC advice, and considered the issues raised by the 
court in remanding the primary annual PM2.5 standard. 
Following the general approach outlined in section III.A.3 above, for 
the reasons discussed below, the Policy Assessment concluded it was 
appropriate to consider the protection afforded by the annual and 24-
hour standards taken together against mortality and morbidity effects 
associated with both long- and short-term PM2.5 exposures. 
This was consistent with the approach taken in the review completed in 
1997 rather than considering each standard separately, as was done in 
the review completed in 2006.
    Beyond looking directly at the relevant epidemiologic evidence, the 
Policy Assessment considered the extent to which specific alternative 
PM2.5 standard levels were likely to reduce the nature and 
magnitude of both long-term exposure-related mortality risk and short-
term exposure-related mortality and morbidity risk (U.S. EPA, 2011a, 
section 2.3.4.2; U.S.EPA, 2010a, section 4.2.2). As noted in section 
III.C above, patterns of increasing estimated risk reductions were 
generally observed as either the annual or 24-hour standard, or both, 
were reduced below the level of the current standards (U.S. 2011a, 
Figures 2-11 and 2-12; U.S. EPA, 2010a, sections 4.2.2, 5.2.2, and 
5.2.3).
    Based on the quantitative risk assessment, the Policy Assessment 
observed, as discussed in section III.A.3, that analyses conducted for 
this and previous reviews demonstrated that much, if not most, of the 
aggregate risk associated with short-term exposures results from the 
large number of days during which the 24-hour average concentrations 
are in the low-to mid-range, below the peak 24-hour concentrations 
(U.S. EPA, 2011a, p. 2-9). Furthermore, as discussed in section III.C 
above and in section III.C.3 of the proposal, the Risk Assessment 
observed that alternative annual standard levels, when controlling, 
resulted in more consistent risk reductions across urban study areas, 
thereby potentially providing a more consistent degree of public health 
protection (U.S. EPA, 2010a, pp. 5-15 to 5-16). In contrast, the Risk 
Assessment noted that the results of simulating alternative suites of 
PM2.5 standards including different combinations of 
alternative annual and 24-hour standard levels suggested that an 
alternative 24-hour standard level can produce additional estimated 
risk reductions beyond that provided by an alternative annual standard 
alone. However, the degree of estimated risk reduction provided by 
alternative 24-hour standard levels was highly variable, in part due to 
the choice of rollback approached used (U.S. EPA, 2010a, p. 5-17).
    Based on its review of the second draft Policy Assessment, CASAC 
agreed with the EPA staff's general approach for translating the 
available epidemiological evidence, risk information, and air quality 
information into the basis for reaching conclusions on alternative 
standards for consideration. Furthermore, CASAC agreed ``that it is 
appropriate to return to the strategy used in 1997 that considers the 
annual and the short-term standards together, with the annual standard 
as the controlling standard, and the short-term standard supplementing 
the protection afforded by the annual standard'' and ``considers it 
appropriate to place the greatest emphasis'' on health effects judged 
to have evidence supportive of a causal or likely causal relationship 
as presented in the Integrated Science Assessment (Samet, 2010d, p. 1).
    Therefore, the Policy Assessment concluded, consistent with 
specific CASAC advice, that it was appropriate to set a ``generally 
controlling'' annual standard that will lower a wide range of ambient 
24-hour concentrations. The Policy Assessment concluded this approach 
would likely reduce aggregate risks associated with both long- and 
short-term exposures with more consistency than a generally controlling 
24-hour standard and would be the most effective and efficient way to 
reduce total PM2.5-related population risk and so provide 
appropriate protection. The staff believed this approach, in contrast 
to one focusing on a generally controlling 24-hour standard, would 
likely reduce aggregate risks associated with both long- and short-term 
exposures with more consistency and would likely avoid setting national 
standards that could result in relatively uneven protection across the 
country due to setting standards that were either more or less 
stringent than necessary in different geographical areas.
    The Policy Assessment recognized that an annual standard intended 
to serve as the primary means for providing protection against effects 
associated with both long- and short-term PM2.5 exposures 
cannot be expected to offer an adequate margin of safety against the 
effects of all short-term PM2.5 exposures. As a result, in 
conjunction with a generally controlling annual standard, the Policy 
Assessment concluded it was appropriate to

[[Page 3129]]

consider setting a 24-hour standard to provide supplemental protection, 
particularly for areas with high peak-to-mean ratios possibly 
associated with strong local or seasonal sources, or PM2.5-
related effects that may be associated with shorter-than-daily exposure 
periods.
    At the time of the proposal, the Administrator agreed with the 
approach discussed in the Policy Assessment as summarized in section 
III.A.3 above, and supported by CASAC, of considering the protection 
afforded by the annual and 24-hour standards taken together for 
mortality and morbidity effects associated with both long- and short-
term exposures to PM2.5. Furthermore, based on the evidence 
and quantitative risk assessment, the Administrator provisionally 
concluded it was appropriate to set a ``generally controlling'' annual 
standard that will lower a wide range of ambient 24-hour 
concentrations, with a 24-hour standard focused on providing 
supplemental protection, particularly for areas with high peak-to-mean 
ratios possibly associated with strong local or seasonal sources, or 
PM2.5-related effects that may be associated with shorter-
than daily exposure periods. The Administrator provisionally concluded 
this approach would likely reduce aggregate risks associated with both 
long- and short-term exposures more consistently than a generally 
controlling 24-hour standard and would be the most effective and 
efficient way to reduce total PM2-5-related population risk.
    The Administrator is mindful that considering what standards are 
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. At the time of 
the proposal, in considering how to translate the available information 
into appropriate standard levels, the Administrator weighed the 
available scientific information and associated uncertainties and 
limitations. For the purpose of determining what standard levels were 
appropriate to propose, the Administrator recognized, as did the EPA 
staff in the Policy Assessment, that there was no single factor or 
criterion that comprised the ``correct'' approach to weighing the 
various types of available evidence and information, but rather there 
were various approaches that were appropriate to consider. The 
Administrator further recognized that different evaluations of the 
evidence and other information before the Administrator could reflect 
placing different weight on the relative strengths and limitations of 
the scientific information, and different judgments could be made as to 
how such information should appropriately be used in making public 
health policy decisions on standard levels. This recognition led the 
Administrator to consider various approaches to weighing the evidence 
so as to identify appropriate standard levels to propose. In so doing, 
the Administrator encouraged extensive public comment on alternative 
approaches to weighing the evidence and other information so as to 
inform her public health policy judgments before reaching final 
decisions on appropriate standard levels.
b. Proposed Decisions on Standard Levels
i. Consideration of the Alternative Standard Levels in the Policy 
Assessment
    In recognizing the absence of a discernible population threshold 
below which effects would not occur, the Policy Assessment's general 
approach for identifying alternative annual standard levels that were 
appropriate to consider focused on characterizing the part of the 
distribution of PM2.5 concentrations in which we had the 
most confidence in the associations reported in the epidemiological 
studies and conversely where our confidence in the association became 
appreciably lower. The most direct approach to address this issue, 
consistent with CASAC advice (Samet, 2010c, p. 10), was to consider 
epidemiological studies reporting confidence intervals around 
concentration-response relationships (U.S. EPA, 2011a, p. 2-63). Based 
on a thorough search of the available evidence, the Policy Assessment 
identified only one study (Schwartz et al., 2008) that conducted a 
multi-model analysis to characterize confidence intervals around the 
estimated concentration-response relationship. The Policy Assessment 
concluded that this single relevant analysis was too limited to serve 
as the principal basis for identifying alternative standard levels in 
this review (U.S. EPA, 2011a, p. 2-70).
    The Policy Assessment explored other approaches to characterize the 
part of the distributions of long-term mean PM2.5 
concentrations that were most influential in generating health effect 
estimates in long- and short-term epidemiological studies, and placed 
greatest weight on those studies that reported positive and 
statistically significant associations (U.S. EPA, 2011a, p. 2-63). 
First, as discussed in section III.A.3 above, the Policy Assessment 
considered the statistical metric used in previous reviews. This 
approach recognized the EPA's views that the strongest evidence of 
associations occurs at concentrations around the long-term mean 
concentration. Thus, in earlier reviews, the EPA focused on identifying 
standard levels that were somewhat below the long-term mean 
concentrations reported in PM2.5 epidemiological studies. 
The long-term mean concentrations represented air quality data 
typically used in epidemiological analyses and provided a direct link 
between PM2.5 concentrations and the observed health 
effects. Further, these data were available for all long- and short-
term exposure studies analyzed and, therefore, represented the data set 
available for the broadest set of epidemiological studies.

[[Page 3130]]

    However, consistent with CASAC's comments on the second draft 
Policy Assessment \75\ (Samet, 2010d, p. 2), in preparing the final 
Policy Assessment, the EPA staff explored ways to take into account 
additional information from epidemiological studies, when available 
(Rajan et al., 2011). These analyses focused on evaluating different 
statistical metrics, beyond the long-term mean concentration, to 
characterize the part of the distribution of PM2.5 
concentrations in which staff continued to have confidence in the 
associations observed in epidemiological studies and below which there 
was a comparative lack of data such that the staff's confidence in the 
relationship was appreciably less. This would also be the part of the 
distribution of PM2.5 concentrations which had the most 
influence on generating the health effect estimates reported in 
epidemiological studies. As discussed in section III.A.3 above, the 
Policy Assessment recognized there was no one percentile value within a 
given distribution that was the most appropriate or ``correct'' way to 
characterize where our confidence in the associations becomes 
appreciably lower. The Policy Assessment concluded that focusing on 
concentrations within the lower quartile of a distribution, such as the 
range from the 25th to the 10th percentile, was reasonable to consider 
as a region within which we begin to have appreciably less confidence 
in the associations observed in epidemiological studies.\76\ In the EPA 
staff's view, considering lower PM2.5 concentrations, down 
to the lowest concentration observed in a study, would be a highly 
uncertain basis for selecting alternative standard levels (U.S. EPA, 
2009a, p. 2-71).
---------------------------------------------------------------------------

    \75\ While CASAC expressed the view that it would be most 
desirable to have information on concentration-response 
relationships, they recognized that it would also be ``preferable to 
have information on the concentrations that were most influential in 
generating the health effect estimates in individual studies'' 
(Samet, 2010d, p. 2).
    \76\ In the last review, staff believed it was appropriate to 
consider a level for an annual PM2.5 standard that was 
somewhat below the averages of the long-term concentrations across 
the cities in each of the key long-term exposures studies, 
recognizing that the evidence of an association in any such study 
was strongest at and around the long-term average where the data in 
the study are most concentrated. For example, the interquartile 
range of long-term average concentrations within a study and a range 
within one standard deviation around the study mean were considered 
reasonable approaches for characterizing the range over which the 
evidence of association is strongest (U.S. EPA, 2005, pp. 5-22 to 5-
23). In this review, the Policy Assessment noted the 
interrelatedness of the distributional statistics and a range of one 
standard deviation around the mean which contains approximately 68 
percent of normally distributed data, in that one standard deviation 
below the mean falls between the 25th and 10th percentiles (U.S. 
EPA, 2011a, p. 2-71).
---------------------------------------------------------------------------

    As outlined in section III.A.3 above, the Policy Assessment 
recognized that there were two types of population-level information to 
consider in identifying the range of PM2.5 concentrations 
which have the most influence on generating the health effect estimates 
reported in epidemiological studies. The most relevant information to 
consider was the number of health events (e.g., deaths, 
hospitalizations) occurring within a study population in relation to 
the distribution of PM2.5 concentrations likely experienced 
by study participants. However, in recognizing that access to health 
event data may be restricted, and consistent with advice from CASAC 
(Samet 2010d, p. 2), EPA staff also considered the number of 
participants within each study area, in relation to the distribution of 
PM2.5 concentrations (i.e., study population data), as a 
surrogate for health event data.
    In applying this approach, the Policy Assessment focused on 
identifying the part of the distribution of PM2.5 
concentrations which had the most influence on generating health effect 
estimates in epidemiological studies, as discussed in section III.A.3 
above. As discussed below, in working with study investigators, the EPA 
staff was able to obtain health event data for three large multi-city 
studies (Krewski et al., 2009; Zanobetti and Schwartz, 2009; Bell et 
al., 2008) and population data for the same three studies and one 
additional long-term exposure study (Miller et al., 2007), as 
documented in a staff memorandum (Rajan et al., 2011).\77\ For the 
three studies for which both health event and study population data 
were available, the EPA staff analyzed the reliability of using study 
population data as a surrogate for health event data. Based on these 
analyses, the EPA staff recognized that the 10th and 25th percentiles 
of the health event and study population distributions are nearly 
identical and concluded that the distribution of population data can be 
a useful surrogate for event data, providing support for consideration 
of the study population data for Miller et al. (2007), for which health 
event data were not available (Rajan et al., 2011, Analysis 1 and 
Analysis 2, in particular, Table 1 and Figures 1 and 2).
---------------------------------------------------------------------------

    \77\ The distributional statistical analysis of population-level 
data built upon an earlier analysis that evaluated the distributions 
of air quality and associated population data for three long-term 
exposure studies and three short-term exposure studies (Schmidt et 
al., 2010, Analysis 2).
---------------------------------------------------------------------------

    With regard to the long-term mean PM2.5 concentrations 
which are relevant to the first approach, Figures 1 through 3 (U.S. 
EPA, 2011a, Figures 2-4, 2-5, 2-6, and 2-8) summarize data available 
for multi-city, long- and short-term exposure studies that evaluated 
endpoints classified in the Integrated Science Assessment as having 
evidence of a causal or likely causal relationship or evidence 
suggestive of a causal relationship, showing the studies with long-term 
mean PM2.5 concentrations below 17 [mu]g/m\3\.\78\ As 
discussed in more detail in section III.E.4.b of the proposal, Figures 
1 and 3 summarize the health outcomes evaluated, relative risk 
estimates, air quality data, and geographic scope for long- and short-
term exposure studies, respectively, that evaluated mortality (evidence 
of a causal relationship); cardiovascular effects (evidence of a causal 
relationship); and respiratory effects (evidence of a likely causal 
relationship) in the general population, as well as in older adults, an 
at-risk population. Figure 2 provides this same summary information for 
long-term exposure studies that evaluated respiratory effects (evidence 
of a likely causal relationship) in children, an at-risk population, as 
well as developmental effects (evidence suggestive of a causal 
relationship).
---------------------------------------------------------------------------

    \78\ Additional studies presented and assessed in the Integrated 
Science Assessment report effects at higher long-term mean 
PM2.5 concentrations (e.g., U.S. EPA, 2009a, Figures 2-1, 
2-2, 7-6, and 7-7).
    \79\ The long-term mean PM2.5 concentrations reported 
by the study authors for the Miller et al. (2007) and Lipfert et al. 
(2006a) studies are discussed more fully in the Response to Comments 
document (U.S. EPA, 2012a).

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[[Page 3132]]


[GRAPHIC] [TIFF OMITTED] TR15JA13.001


[[Page 3133]]


[GRAPHIC] [TIFF OMITTED] TR15JA13.002

    With regard to consideration of additional information from 
epidemiological studies which was relevant to the second approach, the 
EPA staff compiled a summary of the range of PM2.5 
concentrations

[[Page 3134]]

corresponding with the 25th to 10th percentiles of health event or 
study population data from the four multi-city studies, for which 
distributional statistics are available \80\ (U.S. EPA, 2011a, Figure 
2-7; Rajan et al., 2011, Table 1). By considering this approach, one 
could focus on the range of PM2.5 concentrations below the 
long-term mean ambient concentrations over which we continue to have 
confidence in the associations observed in epidemiological studies 
(e.g., above the 25th percentile) where commensurate public health 
protection could be obtained for PM2.5-related effects and, 
conversely, identify the range in the distribution below which our 
confidence in the associations is appreciably less, to identify 
alternative annual standard levels.
---------------------------------------------------------------------------

    \80\ The EPA staff obtained health event data (e.g., number of 
deaths, hospitalizations) occurring in a study population for three 
multi-city studies (Krewski et al., 2009; Zanobetti and Schwartz, 
2009; Bell et al., 2008) and study population data were obtained for 
the same three studies and one additional study (Miller et al., 
2007) (U.S. EPA, 2011a, p. 2-71). If health event or study 
population data were available for additional studies, the EPA could 
employ distributional statistics to identify the broader range of 
PM2.5 concentrations that were most influential in 
generating health effect estimates in those studies.
---------------------------------------------------------------------------

    The mean PM2.5 concentrations associated with the 
studies summarized in Figures 1, 2, and 3 and with the distributional 
statistics analyses (Rajan et al., 2011) are based on concentrations 
averaged across ambient monitors within each area included in a given 
study and then averaged across study areas to calculate an overall 
study mean concentration, as discussed above. Figure 4, discussed in 
more detail in section III.E.4.a of the proposal, summarizes 
statistical metrics for those key studies \81\ included in Figures 1, 
2, and 3 that provide evidence of positive and generally statistically 
significant PM2.5-related effects, which are relevant to the 
two approaches for translating epidemiological evidence into potential 
standard levels as discussed above. The top of Figure 4 includes 
information for long-term exposure studies evaluating health outcomes 
classified as having evidence of a causal or likely causal relationship 
with PM2.5 exposures (long-term mean PM2.5 
concentrations indicated by diamond symbols). The middle of Figure 4 
includes information for short-term exposure studies evaluating health 
outcomes classified as having evidence of a causal or likely causal 
relationship with PM2.5 exposures (long-term mean 
PM2.5 concentrations indicated by triangle symbols). The 
bottom of Figure 4 includes information for long-term exposures studies 
evaluating health outcomes classified as having evidence suggestive of 
a causal relationship (long-term mean PM2.5 concentrations 
indicated by square symbols). Figure 4 also summarizes the range of 
PM2.5 concentrations corresponding with the 25th (indicated 
by solid circles) to 10th (indicated by open circles) percentiles of 
the health event or study population data from the four multi-city 
studies (highlighted in bold text) for which distributional statistics 
are available.
---------------------------------------------------------------------------

    \81\ Long- and short-term exposure studies considered ``key'' 
studies for consideration are summarized in Figure 4 and include 
those studies observing effects for which the evidence supported a 
causal or likely causal association. This figure represents the 
subset of multi-city studies included in Figures 1 through 3 that 
provided evidence of positive and generally statistically 
significant effects associated in whole or in part with more recent 
air quality data, generally representing health effects associated 
with lower PM2.5 concentrations than had previously been 
considered in the last review. The EPA notes that many of these 
studies evaluated multiple health endpoints, and not all of the 
effects evaluated provided evidence of positive and statistically 
significant effects. For purposes of informing the Administrator's 
decision on the appropriate standard levels, the Agency considers 
the full body of scientific evidence and focuses on those aspects of 
the key studies that provided evidence of positive and generally 
statistically significant effects.
    \82\ The long-term mean PM2.5 concentrations reported 
by the study authors for the Miller et al. (2007) and Lipfert et al. 
(2006a) studies are discussed more fully in the Response to Comments 
document (U.S. EPA, 2012a).

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[[Page 3135]]

[GRAPHIC] [TIFF OMITTED] TR15JA13.003

    In considering the evidence, the Policy Assessment recognized that 
NAAQS are standards set so as to provide requisite protection, neither 
more nor less stringent than necessary to protect public health, with 
an adequate margin of safety. This judgment, ultimately made by the 
Administrator, involves weighing the strength of the evidence and the 
inherent uncertainties and limitations of that evidence. Therefore, 
depending on

[[Page 3136]]

the weight placed on different aspects of the evidence and inherent 
uncertainties, consideration of different alternative standard levels 
could be supported.
    Given the currently available evidence discussed in more detail in 
section III.E.4.b of the proposal and considering the various 
approaches discussed above, the Policy Assessment concluded it was 
appropriate to focus on an annual standard level within a range of 
about 12 to 11 [mu]g/m\3\ (U.S. EPA, 2011a, pp. 2-82, 2-101, and 2-
106). As illustrated in Figure 4, the Policy Assessment recognized that 
a standard level of 12 [mu]g/m\3\, at the upper end of this range, was 
somewhat below the long-term mean PM2.5 concentrations 
reported in all the multi-city, long- and short-term exposure studies 
that provided evidence of positive and statistically significant 
associations with health effects classified as having evidence of a 
causal or likely causal relationship, including premature mortality and 
hospitalizations and emergency department visits for cardiovascular and 
respiratory effects as well as respiratory effects in children. 
Further, a level of 12 [mu]g/m\3\ would reflect consideration of 
additional population-level information from such epidemiological 
studies in that it generally corresponded with approximately the 25th 
percentile of the available distributions of health events data in the 
studies for which population-level information was available. In 
addition, a level of 12 [mu]g/m\3\ would reflect some consideration of 
studies that provided more limited evidence of reproductive and 
developmental effects, which were suggestive of a causal relationship, 
in that it was about at the same level as the lowest long-term mean 
PM2.5 concentrations reported in such studies (see Figure 
4).
    Alternatively the Policy Assessment recognized that an annual 
standard level of 11 [mu]g/m\3\, at the lower end of this range, was 
well below the lowest long-term mean PM2.5 concentrations 
reported in all multi-city long- and short-term exposure studies that 
provide evidence of positive and statistically significant associations 
with health effects classified as having evidence of a causal or likely 
causal relationship. A level of 11 [mu]g/m\3\ would reflect placing 
more weight on the distributions of health event and population data, 
in that this level was within the range of PM2.5 
concentrations corresponding to the 25th and 10th percentiles of all 
the available distributions of such data. In addition, a level of 11 
[mu]g/m\3\ was somewhat below the lowest long-term mean 
PM2.5 concentrations reported in reproductive and 
developmental effects studies that are suggestive of a causal 
relationship. Thus, a level of 11 [mu]g/m\3\ would reflect an approach 
to translating the available evidence that places relatively more 
emphasis on margin of safety considerations and less certain causal 
relationships than would a standard set at a higher level. Such a 
policy approach would tend to weigh uncertainties in the evidence in 
such a way as to avoid potentially underestimating PM2.5-
related risks to public health. Further, recognizing the uncertainties 
inherent in identifying any particular point at which our confidence in 
reported associations becomes appreciably less, the Policy Assessment 
concluded that the available evidence did not provide a sufficient 
basis to consider alternative annual standard levels below 11 [mu]g/
m\3\ (U.S. EPA, 2011a, p. 2-81).
    The Policy Assessment also considered the extent to which the 
available evidence provided a basis for considering alternative annual 
standard levels above 12 [mu]g/m\3\. As discussed below, the Policy 
Assessment concluded that it could be reasonable to consider a standard 
level up to 13 [mu]g/m\3\ based on a policy approach that weighed 
uncertainties in the evidence in such a way as to avoid potentially 
overestimating PM2.5-related risks to public health, 
especially to the extent that primary emphasis was placed on long-term 
exposure studies as a basis for an annual standard level. A level of 13 
[mu]g/m\3\ was somewhat below the long-term mean PM2.5 
concentrations reported in all but one of the long-term exposure 
studies providing evidence of positive and statistically significant 
associations with PM2.5-related health effects classified as 
having a causal or likely causal relationship. As shown in Figure 4, 
the one long-term exposure study with a long-term mean PM2.5 
concentration just below 13 [mu]g/m\3\ was the Miller et al., (2007) 
study. However, as noted in section III.D.1.a of the proposal and 
discussed in more detail in the Response to Comments document, the 
Policy Assessment observed that in comparison to other long-term 
exposure studies, the Miller et al. study was more limited in that it 
was based on only one year of air quality data and the one year was 
after the health outcomes were reported (U.S. EPA, 2011a, pp. 2-81 to 
2-82). Thus, to the extent that less weight was placed on the Miller et 
al. study than on other long-term exposure studies with more robust air 
quality data, a level of 13 [mu]g/m\3\ could be considered as being 
protective of long-term exposure related effects classified as having a 
causal or likely causal relationship. In also considering short-term 
exposure studies, however, the Policy Assessment noted that a level of 
13 [mu]g/m\3\ was below the long-term mean PM2.5 
concentrations reported in most but not all such studies. In 
particular, two studies--Burnett et al. (2004) and Bell et al. (2008)--
reported long-term mean PM2.5 concentrations of 12.8 and 
12.9 [mu]g/m\3\, respectively. In considering these studies, the Policy 
Assessment found no basis to conclude that these two studies were any 
more limited or uncertain than the other short-term exposure studies 
shown in Figures 3 and 4 (U.S. EPA, 2011a, p. 2-82). On this basis, as 
discussed below, the Policy Assessment concluded that consideration of 
an annual standard level of 13 [mu]g/m\3\ would have implications for 
the degree of protection that would need to be provided by the 24-hour 
standard, in order that the suite of PM2.5 standards, taken 
together, would provide appropriate protection from effects on public 
health related to short-term exposure to PM2.5 (U.S. EPA, 
2011a, p. 2-82).
    The Policy Assessment also noted that a standard level of 13 [mu]g/
m\3\ would reflect a judgment that the uncertainties in the 
epidemiological evidence as summarized in section III.B above and 
discussed in more detail in section III.B.2 of the proposal, including 
uncertainties related to the heterogeneity observed in the 
epidemiological studies in the eastern versus western parts of the 
U.S., the relative toxicity of PM2.5 components, and the 
potential role of co-pollutants, are too great to warrant placing any 
weight on the distributions of health event and population data that 
extend down below the long-term mean concentrations into the lower 
quartile of the data. This level would also reflect a judgment that the 
evidence from reproductive and developmental effects studies that is 
suggestive of a causal relationship was too uncertain to support 
consideration of any lower level.
    Beyond evidence-based considerations, the Policy Assessment also 
considered the extent to which the quantitative risk assessment 
supported consideration of these alternative standard levels or 
provided support for lower levels. In considering simulations of just 
meeting alternative annual standard levels within the range of 13 to 11 
[mu]g/m\3\ (in conjunction with the current 24-hour standard level of 
35 [mu]g/m\3\), the Policy Assessment concluded that important public 
health improvements are associated with risk

[[Page 3137]]

reductions estimated for standard levels of 13 and 12 [mu]g/m\3\ and 
noted that the level of 11 [mu]g/m\3\ was not included in the 
quantitative risk assessment. The Policy Assessment noted that the 
overall confidence in the quantitative risk estimates varied for the 
different alternative standard levels evaluated and was stronger for 
the higher levels and substantially lower for the lowest level 
evaluated (i.e., 10 [mu]g/m\3\). Based on the above considerations, the 
Policy Assessment concluded that the quantitative risk assessment 
provided support for considering alternative annual standard levels 
within a range of 13 to 11 [mu]g/m\3\, but did not provide strong 
support for considering lower alternative standard levels (U.S. EPA, 
2011a, pp. 2-102 to 2-103).
    Taken together, the Policy Assessment concluded that consideration 
of alternative annual standard levels in the range of 13 to 11 [mu]g/
m\3\ may be appropriate. Furthermore, the Policy Assessment concluded 
that the currently available evidence most strongly supported 
consideration of an alternative annual standard level in the range of 
12 to 11 [mu]g/m\3\ (U.S. EPA, 2011a, p. 2-82). The Policy Assessment 
concluded that an alternative level within the range of 12 to 11 [mu]g/
m\3\ would more fully take into consideration the available information 
from all long- and short-term PM2.5 exposure studies, 
including studies of at-risk populations, than would a higher level. 
This range also reflected placing weight on information from studies 
that helped to characterize the range of PM2.5 
concentrations over which we continue to have confidence in the 
associations observed in epidemiological studies, as well as the extent 
to which our confidence in the associations was appreciably less at 
lower concentrations.
    As recognized in sections III.A.3 and III.E.4.a above, an annual 
standard intended to serve as the primary means for providing 
protection from effects associated with both long- and short-term 
PM2.5 exposures is not expected to provide appropriate 
protection against the effects of all short-term PM2.5 
exposures (unless established at a level so low as to undoubtedly 
provide more protection than necessary for long-term exposures). Of 
particular concern are areas with high peak-to-mean ratios possibly 
associated with strong local or seasonal sources, or PM2.5-
related effects that may be associated with shorter-than-daily exposure 
periods. As a result, the Policy Assessment concluded that it was 
appropriate to consider alternative 24-hour PM2.5 standard 
levels that would supplement the protection provided by an annual 
standard.
    As outlined in section III.A.3 above, the Policy Assessment 
considered the available evidence from short-term PM2.5 
exposure studies, as well as the uncertainties and limitations in that 
evidence, to assess the degree to which alternative annual and 24-hour 
PM2.5 standards can be expected to reduce the estimated 
risks attributed to short-term fine particle exposures. In considering 
the available epidemiological evidence, the Policy Assessment took into 
account information from multi-city studies as well as single-city 
studies. The Policy Assessment considered the distributions of 24-hour 
PM2.5 concentrations reported in short-term exposure 
studies, focusing on the 98th percentile concentrations to match the 
form of the 24-hour standard as discussed in section III.E.3.b above. 
In recognizing that the annual and 24-hour standards work together to 
provide protection from effects associated with short-term 
PM2.5 exposures, the Policy Assessment also considered 
information on the long-term mean PM2.5 concentrations from 
these studies.
    In addition to considering the epidemiological evidence, the Policy 
Assessment considered air quality information, specifically peak-to-
mean ratios using county-level 24-hour and annual design values, to 
characterize air quality patterns in areas possibly associated with 
strong local or seasonal sources. These patterns helped in 
understanding the extent to which different combinations of annual and 
24-hour standards would be consistent with the policy goal of setting a 
generally controlling annual standard with a 24-hour standard that 
provides supplemental protection especially for areas with high peak-
to-mean ratios (U.S. EPA, 2011a, p. 2-14).
    In considering the information provided by the short-term exposure 
studies, the Policy Assessment recognized that to the extent these 
studies were conducted in areas that likely did not meet one or both of 
the current standards, such studies did not help inform the 
characterization of the potential public health improvements of 
alternative standards set at lower levels. Therefore, in considering 
the short-term exposure studies to inform staff conclusions regarding 
levels of the 24-hour standard that are appropriate to consider, the 
Policy Assessment placed greatest weight on studies conducted in areas 
that likely met both the current annual and 24-hour standards.
    With regard to multi-city studies that evaluated effects associated 
with short-term PM2.5 exposures, as summarized in Figure 3 
above and discussed in more detail in section III.E.4.c of the 
proposal, the Policy Assessment noted that, to the extent air quality 
distributions were reduced to reflect just meeting the current 24-hour 
standard, additional protection would be anticipated for the effects 
observed in the three multi-city studies with 98th percentile values 
greater than 35 [mu]g/m\3\ (Burnett et al., 2004; Burnett and Goldberg, 
2003; Franklin et al., 2008). In the three additional studies with 98th 
percentile values below 35 [mu]g/m\3\, specifically 98th percentile 
concentrations of 34.2, 34.3, and 34.8 [mu]g/m\3\, the Policy 
Assessment noted that these studies reported long-term mean 
PM2.5 concentrations of 12.9, 13.2, and 13.4 [mu]g/m\3\, 
respectively (Bell et al., 2008; Zanobetti and Schwartz, 2009; Dominici 
et al., 2006a). To the extent that consideration was given to revising 
the level of the annual standard, as discussed in section III.E.4.b of 
the proposal, the Policy Assessment recognized that potential changes 
associated with meeting such an alternative annual standard would 
result in lowering risks associated with both long- and short-term 
PM2.5 exposures. Consequently, in considering a 24-hour 
standard that would operate in conjunction with an annual standard to 
provide appropriate public health protection, the Policy Assessment 
noted that to the extent that the level of the annual standard was 
revised to within a range of 13 to 11 [mu]g/m\3\, in particular in the 
range of 12 to 11 [mu]g/m\3\, additional protection would be provided 
for the long-term effects observed in these multi-city studies (U.S. 
EPA, 2011a, p. 2-84).
    Based on this information, the Policy Assessment concluded that the 
multi-city, short-term exposure studies generally provided support for 
retaining the 24-hour standard level at 35 [mu]g/m\3\ so long as the 
standard is in conjunction with an annual standard level revised to 
within a range of 12 to 11 [mu]g/m\3\ (U.S. EPA, 2011a, p. 2-84). 
Alternatively, in conjunction with an annual standard level of 13 
[mu]g/m\3\, the Policy Assessment concluded that the multi-city studies 
provided limited support for revising the 24-hour standard level 
somewhat below 35 [mu]g/m\3\, such as down to 30 [mu]g/m\3\, based on 
one study (Bell et al., 2008) that reported positive and statistically 
significant effects with an overall 98th percentile value below the 
level of the current 24-hour standard and an overall long-term mean 
concentration slightly less than 13 [mu]g/m\3\ (Figure 3; U.S. EPA, 
2011a, p. 2-84).
    In reaching staff conclusions regarding alternative 24-hour 
standard levels that were appropriate to consider,

[[Page 3138]]

the Policy Assessment also took into account relevant information from 
single-city studies that evaluated effects associated with short-term 
PM2.5 exposures. The Policy Assessment recognized that these 
studies may provide additional insights regarding impacts on at-risk 
populations and/or on areas with isolated peak concentrations.
    As discussed in more detail in section III.E.4.c of the proposal, 
although a number of single-city studies reported effects at 
appreciably lower PM2.5 concentrations than multi-city 
short-term exposure studies, the uncertainties and limitations 
associated with the single-city studies were considerably greater than 
those associated with the multi-city studies and, thus, the Policy 
Assessment concluded there was less confidence in using these studies 
as a basis for setting the level of a standard. Therefore, the Policy 
Assessment concluded that the multi-city short-term exposure studies 
provided the strongest evidence to inform decisions on the level of the 
24-hour standard, and the single-city studies did not warrant 
consideration of 24-hour standard levels different from those supported 
by the multi-city studies (U.S. EPA, 2011a, p. 2-88).
    In addition to considering the epidemiological evidence, the Policy 
Assessment took into account air quality information based on county-
level 24-hour and annual design values to understand the public health 
implications of the alternative standard levels supported by the 
currently available scientific evidence, as discussed in this section. 
Consistent with the general approach discussed in section III.A.3 
above, the Policy Assessment considered the extent to which different 
combinations of alternative annual and 24-hour standard levels based on 
the evidence would support the policy goal of lowering annual and 24-
hour air quality distributions by using the annual standard to be the 
``generally controlling'' standard in conjunction with setting the 24-
hour standard to provide supplemental protection (U.S. EPA, 2011a, pp 
2-88 to 2-91, Figure 2-10).
    Using information on the relationship of the 24-hour and annual 
design values, the Policy Assessment examined the implications of three 
alternative suites of PM2.5 standards identified as 
appropriate to consider based on the currently available scientific 
evidence, as discussed above. The Policy Assessment concluded that an 
alternative suite of PM2.5 standards that would include an 
annual standard level of 11 or 12 [mu]g/m\3\ and a 24-hour standard 
with a level of 35 [mu]g/m\3\ (i.e., 11/35 or 12/35) would result in 
the annual standard being the generally controlling standard in most 
areas although the 24-hour standard would continue to be the generally 
controlling standard in the Northwest (U.S. EPA, 2011a, pp. 2-89 to 2-
91 and Figure 2-10). These Northwest counties generally represented 
areas where the annual mean PM2.5 concentrations have 
historically been low but where relatively high 24-hour concentrations 
occur, often related to seasonal wood smoke emissions. Alternatively, 
combining an alternative annual standard of 13 [mu]g/m\3\ with a 24-
hour standard of 30 [mu]g/m\3\ would result in many more areas across 
the country in which the 24-hour standard would likely become the 
controlling standard (the standard driving air quality distributions 
lower) than if an alternative annual standard of 12 or 11 [mu]g/m\3\ 
were paired with the current level of the 24-hour standard (i.e., 35 
[mu]g/m\3\).
    The Policy Assessment concluded that consideration of retaining the 
24-hour standard level at 35 [mu]g/m\3\ would reflect placing greatest 
weight on evidence from multi-city studies that reported positive and 
statistically significant associations with health effects classified 
as having a causal or likely causal relationship. In conjunction with 
lowering the annual standard level, especially within a range of 12 to 
11 [mu]g/m\3\, this alternative recognized additional public health 
protection against effects associated with short-term PM2.5 
exposures which would be provided by lowering the annual standard such 
that revision to the 24-hour standard would not be warranted (U.S. EPA, 
2011a, p. 2-91).
    Beyond evidence-based considerations, the Policy Assessment also 
considered the extent to which the quantitative risk assessment 
supported consideration of retaining the current 24-hour standard level 
or provided support for lower standard levels. In considering 
simulations of just meeting the current 24-hour standard level of 35 
[mu]g/m\3\ or alternative levels of 30 or 25 [mu]g/m\3\ (in conjunction 
with alternative annual standard levels within a range of 13 to 11 
[mu]g/m\3\), the Policy Assessment noted that the overall confidence in 
the quantitative risk estimates varied for the different standard 
levels evaluated and was stronger for the higher levels and 
substantially lower for the lowest level evaluated (i.e., 25 [mu]g/
m\3\). Based on this information, the Policy Assessment concluded that 
the quantitative risk assessment provided support for considering a 24-
hour standard level of 35 or 30 [mu]g/m\3\ (in conjunction with an 
alternative standard level within a range of 13 to 11 [mu]g/m\3\) but 
did not provide strong support for considering lower alternative 24-
hour standard levels (U.S. EPA, 2011a, pp. 2-102 to 2-103).
    Taken together, the Policy Assessment concluded that while it was 
appropriate to consider an alternative 24-hour standard level within a 
range of 35 to 30 [mu]g/m\3\, the currently available evidence most 
strongly supported consideration for retaining the current 24-hour 
standard level at 35 [mu]g/m\3\ in conjunction with lowering the level 
of the annual standard within a range of 12 to 11 [mu]g/m\3\ (U.S. EPA, 
2011a, p. 2-92).
ii. CASAC Advice
    Based on its review of the second draft Policy Assessment, CASAC 
agreed with the general approach for translating the available 
epidemiological evidence, risk information, and air quality information 
into the basis for reaching conclusions on alternative standards for 
consideration. Furthermore, CASAC agreed ``that it is appropriate to 
return to the strategy used in 1997 that considers the annual and the 
short-term standards together, with the annual standard as the 
controlling standard, and the short-term standard supplementing the 
protection afforded by the annual standard'' and ``considers it 
appropriate to place the greatest emphasis'' on health effects judged 
to have evidence supportive of a causal or likely causal relationship 
as presented in the Integrated Science Assessment (Samet, 2010d, p. 1).
    CASAC concluded that the range of levels presented in the second 
draft Policy Assessment (i.e., alternative annual standard levels 
within a range of 13 to 11 [mu]g/m\3\ and alternative 24-hour standard 
levels within a range of 35 to 30 [mu]g/m\3\) ``are supported by the 
epidemiological and toxicological evidence, as well as by the risk and 
air quality information compiled'' in the Integrated Science 
Assessment, Risk Assessment, and second draft Policy Assessment. CASAC 
further noted that ``[a]lthough there is increasing uncertainty at 
lower levels, there is no evidence of a threshold (i.e., a level below 
which there is no risk for adverse health effects)'' (Samet, 2010d, p. 
ii).
    Although CASAC supported the alternative standard level ranges 
presented in the second draft Policy Assessment, it did not express 
support for any specific levels or combinations of standards. Rather, 
CASAC encouraged the EPA to develop a clearer rationale in the final 
Policy Assessment for staff conclusions regarding annual

[[Page 3139]]

and 24-hour standards that were appropriate to consider, including 
consideration of the combination of these standards supported by the 
available information (Samet, 2010d, p. ii). Specifically, in 
commenting on a distributional statistical analysis of air quality and 
associated population data presented in the second draft Policy 
Assessment, CASAC encouraged staff to focus on information related to 
the concentrations that were most influential in generating the health 
effect estimates in individual studies to inform alternative standard 
levels. CASAC urged that the EPA redo that analysis using health event 
or study population data (Samet, 2010d, p. 2). CASAC also commented 
that the approach presented in the second draft Policy Assessment to 
identify alternative 24-hour standard levels which focused on peak-to-
mean ratios was not relevant for informing the actual level (Samet 
2010d, p. 4). Further, they expressed the concern that the combinations 
of annual and 24-hour standard levels discussed in the second draft 
Policy Assessment (i.e., in the range of 13 to 11 [mu]g/m\3\ for the 
annual standard, in conjunction with retaining the current 24-hour 
PM2.5 standard level of 35 [mu]g/m\3\; alternatively, 
revising the level of the 24-hour standard to 30 [mu]g/m\3\ in 
conjunction with an annual standard level of 11 [mu]g/m\3\) ``may not 
be adequately inclusive'' and ``[i]t was not clear why, for example a 
daily standard of 30 [mu]g/m\3\ should only be considered in 
combination with an annual level of 11 [mu]g/m\3\'' (Samet, 2010d, p. 
ii). CASAC encouraged the EPA to more clearly explain its rationale for 
identifying the 24-hour/annual combinations that are appropriate for 
consideration (Samet 2010d, p. ii).
    In considering CASAC's advice as well as public comment on the 
second draft Policy Assessment, the EPA staff conducted additional 
analyses and modified their conclusions regarding alternative standard 
levels that were appropriate to consider. The staff conclusions in the 
final Policy Assessment (U.S. EPA, 2011a, section 2.3.4.4) differed 
somewhat from the alternative standard levels discussed in the second 
draft Policy Assessment (U.S. EPA, 2010f, section 2.3.4.3), upon which 
CASAC based its advice. Changes made in the final Policy Assessment 
were primarily focused on improving and clarifying the approach for 
translating the epidemiological evidence into a basis for staff 
conclusions on the broadest range of alternative standard levels 
supported by the available scientific information and more clearly 
articulating the rationale for the staff's conclusions (Wegman, 2011, 
pp. 1 to 2). Consistent with CASAC's advice to consider more 
information from epidemiological studies, as discussed in section 
III.E.4.b.1 above, the EPA analyzed additional population-level data 
obtained from several study authors (Rajan et al., 2011). In 
transmitting the final Policy Assessment to CASAC, the Agency notified 
CASAC that the final staff conclusions reflected consideration of 
CASAC's advice and that those staff conclusions were based, in part, on 
the specific distributional analysis that CASAC had urged the EPA to 
conduct (Wegman, 2011, p.2). Thus, CASAC had an opportunity to comment 
on the final Policy Assessment, but chose not to provide any additional 
comments or advice after receiving it.
iii. Administrator's Proposed Decisions on the Primary PM2.5 
Standard Levels
    In reaching her conclusions regarding appropriate alternative 
standard levels to consider, the Administrator considered the 
epidemiological and other scientific evidence, estimates of risk 
reductions associated with just meeting alternative annual and/or 24-
hour standards, air quality analyses, related limitations and 
uncertainties, staff conclusions as presented in the Policy Assessment, 
and the advice of CASAC. As an initial matter, the Administrator agreed 
with the general approach discussed in the Policy Assessment as 
summarized in sections III.A.3 and III.E.4.a above, and supported by 
CASAC, of considering the protection afforded by the annual and 24-hour 
standards taken together for mortality and morbidity effects associated 
with both long- and short-term exposures to PM2.5 (77 FR 
38939). Furthermore, based on the evidence and quantitative risk 
assessment, the Administrator provisionally concluded it is appropriate 
to set a ``generally controlling'' annual standard that will lower a 
wide range of ambient 24-hour concentrations, with a 24-hour standard 
focused on providing supplemental protection, particularly for areas 
with high peak-to-mean ratios possibly associated with strong local or 
seasonal sources, or PM2.5-related effects that may be 
associated with shorter-than daily exposure periods. The Administrator 
provisionally concluded this approach would likely reduce aggregate 
risks associated with both long- and short-term exposures more 
consistently than a generally controlling 24-hour standard and would be 
the most effective and efficient way to reduce total PM2.5-
related population risk. Id.
    In reaching decisions on alternative standard levels to propose, 
the Administrator judged that it was most appropriate to examine where 
the evidence of associations observed in the epidemiological studies 
was strongest and, conversely, where she had appreciably less 
confidence in the associations observed in the epidemiological studies. 
Based on the characterization and assessment of the epidemiological and 
other studies presented and assessed in the Integrated Science 
Assessment and the Policy Assessment, the Administrator recognized the 
substantial increase in the number and diversity of studies available 
in this review including extended analyses of the seminal studies of 
long-term PM2.5 exposures (i.e., ACS and Harvard Six Cities 
studies) as well as important new long-term exposure studies (as 
summarized in Figures 1 and 2). Collectively, the Administrator noted 
that these studies, along with evidence available in the last review, 
provided consistent and stronger evidence of an association with 
premature mortality, with the strongest evidence related to 
cardiovascular-related mortality, at lower ambient concentrations than 
previously observed. The Administrator also recognized the availability 
of stronger evidence of morbidity effects associated with long-term 
PM2.5 exposures, including evidence of cardiovascular 
effects from the WHI study and respiratory effects, including decreased 
lung function growth, from the extended analyses for the Southern 
California Children's Health Study. Furthermore, the Administrator 
recognized new U.S. multi-city studies that greatly expanded and 
reinforced our understanding of mortality and morbidity effects 
associated with short-term PM2.5 exposures, providing 
stronger evidence of associations at ambient concentrations similar to 
those previously observed (as summarized in Figure 3). Id. at 38939-40.
    The newly available scientific evidence built upon the previous 
scientific data base to provide evidence of generally robust 
associations and to provide a basis for greater confidence in the 
reported associations than in the last review. The Administrator 
recognized that the weight of evidence, as evaluated in the Integrated 
Science Assessment, was strongest for health endpoints classified as 
having evidence of a causal relationship. These relationships included 
those between long- and short-term PM2.5 exposures and 
mortality and cardiovascular effects. She recognized that the weight of 
evidence was also

[[Page 3140]]

strong for health endpoints classified as having evidence of a likely 
causal relationship, which included those between long- and short-term 
PM2.5 exposures and respiratory effects. In addition, the 
Administrator made note of the much more limited evidence for health 
endpoints classified as having evidence suggestive of a causal 
relationship, including developmental, reproductive and carcinogenic 
effects. Id. at 38940.
    Based on information discussed and presented in the Integrated 
Science Assessment, the Administrator recognized that health effects 
may occur over the full range of concentrations observed in the long- 
and short-term epidemiological studies and that no discernible 
threshold for any effects can be identified based on the currently 
available evidence (U.S. EPA, 2009a, section 2.4.3). She also 
recognized, in taking note of CASAC advice and the distributional 
statistics analysis discussed in section III.E.4.b.i above and in the 
Policy Assessment, that there was significantly greater confidence in 
observed associations over certain parts of the air quality 
distributions in the studies, and conversely, that there was 
significantly diminished confidence in ascribing effects to 
concentrations toward the lower part of the distributions.
    Consistent with the general approach summarized in section III.A.3 
above, and supported by CASAC as discussed in section III.E.4.a above, 
the Administrator generally agreed that it was appropriate to consider 
a level for an annual standard that was somewhat below the long-term 
mean PM2.5 concentrations reported in long- and short-term 
exposure studies. In recognizing that the evidence of an association in 
any such study was strongest at and around the long-term average where 
the data in the study are most concentrated, she understood that this 
approach did not provide a bright line for reaching decisions about 
appropriate standard levels. The Administrator noted that long-term 
mean PM2.5 concentrations were available for each study 
considered and, therefore, represented the most robust data set to 
inform her decisions on appropriate annual standard levels. She also 
noted that the overall study mean PM2.5 concentrations were 
generally calculated based on monitored concentrations averaged across 
monitors in each study area with multiple monitors, referred to as a 
composite monitor concentration, in contrast to the highest 
concentration monitored in each study area, referred to as a maximum 
monitor concentration, which are used to determine whether an area 
meets a given standard. In considering such long-term mean 
concentrations, the Administrator understood that it was appropriate to 
consider the weight of evidence for the health endpoints evaluated in 
such studies in giving weight to this information. Id.
    Based on the information summarized in Figure 4 above and presented 
in more detail in the Policy Assessment (U.S. EPA, 2011a, chapter 2) 
for effects classified in the Integrated Science Assessment as having a 
causal or likely causal relationship with PM2.5 exposures, 
the Administrator observed an overall pattern of statistically 
significant associations reported in studies of long-term 
PM2.5 exposures with long-term mean concentrations ranging 
from somewhat above the current standard level of 15 [mu]g/m\3\ down to 
the lowest mean concentration in such studies of 12.9 [mu]g/m\3\ (in 
Miller et al., 2007).\83\ She observed a similar pattern of 
statistically significant associations in studies of short-term 
PM2.5 exposures with long-term mean concentrations ranging 
from around 15 [mu]g/m\3\ down to 12.8 [mu]g/m\3\ (in Burnett et al., 
2004). With regard to effects classified as providing evidence 
suggestive of a causal relationship, the Administrator observed a small 
number of long-term exposure studies related to developmental and 
reproductive effects that reported statistically significant 
associations with overall study mean PM2.5 concentrations 
down to 11.9 [mu]g/m\3\ (in Bell et al., 2007).\84\ Id.
---------------------------------------------------------------------------

    \83\ The EPA notes that the Miller et al., (2007) study provides 
strong evidence of cardiovascular related effects associated with 
long-term PM2.5 exposures. At the time of the proposal, 
the EPA recognized the limited nature of the air quality data 
considered in this study (77 FR 38918, fn. 62). The EPA has reviewed 
those limitations, in conjunction with consideration of public 
comments received on the proposal as discussed in section III.E.4.c, 
in conjunction with reaching a final decision on the level of the 
annual standard.
    \84\ With respect to suggestive evidence related to cancer, 
mutagenic, and genotoxic effects, the PM2.5 
concentrations reported in studies generally included ambient 
concentrations that are equal to or greater than ambient 
concentrations observed in studies that reported mortality and 
cardiovascular and respiratory effects (U.S. EPA, 2009a, section 
7.5), such that in selecting alternative standard levels that 
provide protection from mortality and cardiovascular and respiratory 
effects, it is reasonable to anticipate that protection will also be 
provided for carcinogenic effects.
---------------------------------------------------------------------------

    The Administrator also considered additional information from 
epidemiological studies, consistent with CASAC advice, to take into 
account the broader distribution of PM2.5 concentrations and 
the degree of confidence in the observed associations over the broader 
air quality distribution. In considering this additional information, 
she understood that the Policy Assessment presented information on the 
25th and 10th percentiles of the distributions of PM2.5 
concentrations available from four multi-city studies to provide a 
general frame of reference as to the part of the distribution in which 
the data become appreciably more sparse and, thus, where her confidence 
in the associations observed in epidemiological studies would become 
appreciably less.
    As summarized in Figure 4 above, the Administrator took note of 
additional population-level data that were available for four studies 
(Krewski et al., 2009; Miller et al., 2007; Bell et al., 2008; 
Zanobetti and Schwartz, 2009), each of which reported statistically 
significant associations with health endpoints classified as having 
evidence of a causal relationship. In considering the long-term 
PM2.5 concentrations associated with the 25th percentile 
values of the population-level data for these four studies, she 
observed that these values ranged from somewhat above to somewhat below 
12 [mu]g/m\3\. The Administrator recognized that these studies include 
some of the strongest evidence available within the overall body of 
scientific evidence and noted that three of these studies (Krewski et 
al., 2009; Bell et al., 2008; Zanobetti and Schwartz, 2009) were used 
as the basis for concentration-response functions used in the 
quantitative risk assessment (U.S. EPA, 2010a, section 3.3.3).
    In considering this information, the Administrator noted that CASAC 
advised that information about the long-term PM2.5 
concentrations that were most influential in generating the health 
effect estimates in epidemiological studies can help to inform 
selection of an appropriate annual standard level. However, the 
Administrator also recognized that additional population-level data 
were available for only these four studies and, therefore, she believed 
that these studies comprised a more limited data set than one based on 
long-term mean PM2.5 concentrations for which data were 
available for all studies considered, as discussed above.
    The Administrator recognized, as summarized in section III.B above, 
that important uncertainties remain in the evidence and information 
considered in this review of the primary fine particle standards. These 
uncertainties are generally related to understanding the relative 
toxicity of the different components in the fine particle mixture, the 
role of PM2.5 in the complex ambient mixture, exposure 
measurement errors inherent in epidemiological studies based on 
concentrations measured at

[[Page 3141]]

fixed monitor sites, and the nature, magnitude, and confidence in 
estimated risks related to increasingly lower ambient PM2.5 
concentrations. Furthermore, the Administrator noted that 
epidemiological studies have reported heterogeneity in responses both 
within and between cities and geographic regions across the U.S. She 
recognized that this heterogeneity may be attributed, in part, to 
differences in fine particle composition in different regions and 
cities. The Administrator also recognized that there are additional 
limitations associated with evidence for reproductive and developmental 
effects, identified as being suggestive of a causal relationship with 
long-term PM2.5 exposures, including: the limited number of 
studies evaluating such effects; uncertainties related to identifying 
the relevant exposure time periods of concern; and limited 
toxicological evidence providing little information on the mode of 
action(s) or biological plausibility for an association between long-
term PM2.5 exposures and adverse birth outcomes. Id. at 
38941.
    The Administrator was mindful that considering what standards were 
requisite to protect public health with an adequate margin of safety 
required public health policy judgments that neither overstated nor 
understated the strength and limitations of the evidence or the 
appropriate inferences to be drawn from the evidence. In considering 
how to translate the available information into appropriate standard 
levels, the Administrator weighed the available scientific information 
and associated uncertainties and limitations. For the purpose of 
determining what standard levels were appropriate to propose, the 
Administrator recognized, as did EPA staff in the Policy Assessment, 
that there was no single factor or criterion that comprised the sole 
``correct'' approach to weighing the various types of available 
evidence and information, but rather there were various approaches that 
are appropriate to consider. The Administrator further recognized that 
different evaluations of the evidence and other information before the 
Administrator could reflect placing different weight on the relative 
strengths and limitations of the scientific information, and different 
judgments could be made as to how such information should appropriately 
be used in making public health policy decisions on standard levels. 
This recognition led the Administrator to consider various approaches 
to weighing the evidence so as to identify appropriate standard levels 
to propose. In so doing, the Administrator encouraged extensive public 
comment on alternative approaches to weighing the evidence and other 
information so as to inform her public health policy judgments before 
reaching final decisions on appropriate standard levels.
    In considering the available information, the Administrator noted 
the advice of CASAC that the currently available scientific 
information, including epidemiological and toxicological evidence as 
well as risk and air quality information, provided support for 
considering an annual standard level within a range of 13 to 11 [mu]g/
m\3\ and a 24-hour standard level within a range of 35 to 30 [mu]g/
m\3\. In addition, the Administrator recognized that the Policy 
Assessment concluded that the available evidence and risk-based 
information support consideration of annual standard levels in the 
range of 13 to 11 [mu]g/m\3\, and that the Policy Assessment also 
concluded that the evidence most strongly supported consideration of an 
annual standard level in the range of 12 to 11 [mu]g/m\3\. In 
considering how the annual and 24-hour standards work together to 
provide appropriate public health protection, the Administrator 
observed that CASAC did not express support for any specific levels or 
combinations of standards within these ranges. Nor did CASAC choose to 
comment on additional information and analyses presented in the final 
Policy Assessment prepared in response to CASAC's recommendations on 
the second draft Policy Assessment (Wegman, 2011).
    In considering the extent to which the currently available evidence 
and information provided support for specific standard levels within 
the ranges identified by CASAC and the Policy Assessment as appropriate 
for consideration, the Administrator initially considered standard 
levels within the range of 13 to 11 [mu]g/m\3\ for the annual standard. 
In so doing, the Administrator first considered the long-term mean 
PM2.5 concentrations reported in studies of effects 
classified as having evidence of a causal or likely causal 
relationship, as summarized in Figure 4 above and discussed more 
broadly above. She noted that a level at the upper end of this range 
would be below most but not all the overall study mean concentrations 
from the multi-city studies of long- and short-term exposures, whereas 
somewhat lower levels within this range would be below all such overall 
study mean concentrations. In considering the appropriate weight to 
place on this information, the Administrator again noted that the 
evidence of an association in any such study was strongest at and 
around the long-term average where the data in the study are most 
concentrated, and that long-term mean PM2.5 concentrations 
were available for each study considered and, therefore, represented 
the most robust data set to inform her decisions on appropriate annual 
standard levels. Further, she was mindful that this approach did not 
provide a bright line for reaching decisions about appropriate standard 
levels. Id.
    In considering the long-term mean PM2.5 concentrations 
reported in studies of effects classified as having evidence suggestive 
of a causal relationship, as summarized in Figure 4 for reproductive 
and developmental effects, the Administrator noted that a level at the 
upper end of this range would be below the overall study mean 
concentration in one of the three studies, while levels in the mid- to 
lower part of this range would be below the overall study mean 
concentrations in two or three of these studies. In considering the 
appropriate weight to place on this information, the Administrator 
noted the very limited nature of this evidence of such effects and the 
additional uncertainties in these epidemiological studies relative to 
the studies that provide evidence of causal or likely causal 
relationships.
    The Administrator also considered the distributional analyses of 
population-level information that were available from four of the 
epidemiological studies that provide evidence of effects identified as 
having a causal relationship with long- or short-term PM2.5 
concentrations for annual standard levels within the same range of 13 
to 11 [mu]g/m\3\. In so doing, the Administrator first noted that a 
level in the mid-part of this range generally corresponds with 
approximately the 25th percentile of the distributions of health events 
data available in three of these studies. The Administrator also noted 
that standard levels toward the upper part of this range would reflect 
placing substantially less weight on this information, whereas standard 
levels toward the lower part of this range would reflect placing 
substantially more weight on this information. In considering this 
information, the Administrator noted that there was no bright line that 
delineates the part of the distribution of PM2.5 
concentrations within which the data become appreciably more sparse 
and, thus, where her confidence in the associations observed in 
epidemiological studies became appreciably less.
    In considering mean PM2.5 concentrations and 
distributional

[[Page 3142]]

analyses from the various sets of epidemiological studies noted above, 
the Administrator was mindful, as noted above, that such studies 
typically report concentrations based on composite monitor 
distributions, in which concentrations may be averaged across multiple 
ambient monitors that may be present within each area included in a 
given study. Thus, a policy approach that used data based on composite 
monitors to identify potential alternative standard levels would 
inherently build in a margin of safety of some degree relative to an 
alternative standard level based on measurements at the monitor within 
an area that records the highest concentration, or the maximum monitor, 
since once a standard was set, concentrations at appropriate maximum 
monitors within an area were generally used to determine whether an 
area meets a given standard.
    The Administrator also recognized that judgments about the 
appropriate weight to place on any of the factors discussed above 
should reflect consideration not only of the relative strength of the 
evidence but also on the important uncertainties that remained in the 
evidence and information being considered in this review. The 
Administrator noted that the extent to which these uncertainties 
influenced judgments about appropriate annual standard levels within 
the range of 13 to 11 [mu]g/m\3\ would likely be greater for standard 
levels in the lower part of this range which would necessarily be based 
on fewer available studies than would higher levels within this range.
    Based on the above considerations, the Administrator concluded that 
it was appropriate to propose to set a level for the primary annual 
PM2.5 standard within the range of 12 to 13 [mu]g/m\3\. The 
Administrator provisionally concluded that a standard set within this 
range would reflect alternative approaches to appropriately placing the 
most weight on the strongest available evidence, while placing less 
weight on much more limited evidence and on more uncertain analyses of 
information available from a relatively small number of studies. 
Further, she provisionally concluded that a standard level within this 
range would reflect alternative approaches to appropriately providing 
an adequate margin of safety for the populations at risk for the 
serious health effects classified as having evidence of a causal or 
likely causal relationship, depending in part on the emphasis placed on 
margin of safety considerations. The Administrator recognized that 
setting an annual standard level at the lower end of this range would 
reflect an approach that placed more emphasis on the entire body of the 
evidence, including the analysis of the distribution of air quality 
concentrations most influential in generating health effect estimates 
in the studies, and on margin of safety considerations, than would 
setting a level at the upper end of the range. Conversely, an approach 
that would support a level at the upper end of this range would 
generally support a view that the uncertainties remaining in the 
evidence are such that the evidence does not warrant setting a lower 
annual standard level. Id. at 38942.
    At the time of the proposal, while the Administrator recognized 
that CASAC advised, and the Policy Assessment concluded, that the 
available scientific information provided support for considering a 
range that extended down to 11 [mu]g/m\3\, she concluded that proposing 
such an extended range would reflect a public health policy approach 
that placed more weight on relatively limited evidence and more 
uncertain information and analyses than she considered appropriate at 
this time. Nonetheless, the Administrator solicited comment on a level 
down to 11 [mu]g/m\3\ as well as on approaches for translating 
scientific evidence and rationales that would support such a level. 
Such an approach might reflect a view that the uncertainties associated 
with the available scientific information warrant a highly 
precautionary public health policy response that would incorporate a 
large margin of safety.
    The Administrator recognized that potential air quality changes 
associated with meeting an annual standard set at a level within the 
range of 12 to 13 [mu]g/m\3\ will result in lowering risks associated 
with both long- and short-term PM2.5 exposures. However, the 
Administrator recognized that such an annual standard intended to serve 
as the primary means for providing protection from effects associated 
with both long- and short-term PM2.5 exposures would not by 
itself be expected to offer requisite protection with an adequate 
margin of safety against the effects of all short-term PM2.5 
exposures. As a result, in conjunction with proposing an annual 
standard level in the range of 12 to 13 [mu]g/m\3\, the Administrator 
provisionally concluded that it was appropriate to continue to provide 
supplemental protection by means of a 24-hour standard set at the 
appropriate level, particularly for areas with high peak-to-mean ratios 
possibly associated with strong local or seasonal sources, or for 
PM2.5-related effects that may be associated with shorter-
than-daily exposure periods.
    Based on the approach discussed in section III.A.3 above, at the 
time of the proposal the Administrator relied upon evidence from the 
short-term exposure studies as the principal basis for selecting the 
level of the 24-hour standard. In considering these studies as a basis 
for the level of a 24-hour standard, and having selected a 98th 
percentile form for the standard, the Administrator agreed with the 
focus in the Policy Assessment of looking at the 98th percentile 
values, as well as at the long-term mean PM2.5 
concentrations in these studies.
    In considering the information provided by the short-term exposure 
studies, the Administrator recognized that to the extent these studies 
were conducted in areas that likely did not meet one or both of the 
current standards, such studies did not help inform the 
characterization of the potential public health improvements of 
alternative standards set at lower levels. By reducing the 
PM2.5 concentrations in such areas to just meet the current 
standards, the Administrator anticipated that additional public health 
protection would occur. Therefore, the Administrator focused on studies 
that reported positive and statistically significant associations in 
areas that would likely have met both the current 24-hour and annual 
standards. She also considered whether or not these studies were 
conducted in areas that would likely have met an annual standard level 
of 12 to 13 [mu]g/m\3\ to inform her decision regarding an appropriate 
24-hour standard level. As discussed in section III.E.4.a, consistent 
with the Policy Assessment, the Administrator concluded that multi-
city, short-term exposure studies provided the strongest data set for 
informing her decisions on appropriate 24-hour standard levels. The 
Administrator viewed the single-city, short-term exposure studies as a 
much more limited data set providing mixed results and, therefore, she 
had less confidence in using those studies as a basis for setting the 
level of a 24-hour standard. With regard to the limited number of 
single-city studies that reported positive and statistically 
significant associations for a range of health endpoints related to 
short-term PM2.5 concentrations in areas that would likely 
have met the current suite of PM2.5 standards, the 
Administrator recognized that many of those studies had significant 
limitations (e.g., limited statistical power, limited exposure data) or 
equivocal results (mixed results within the same study area) that made 
them unsuitable to form the basis for setting the level of a 24-hour 
standard.

[[Page 3143]]

    With regard to multi-city studies that evaluated effects associated 
with short-term PM2.5 exposures, the Administrator observed 
an overall pattern of positive and statistically significant 
associations in studies with 98th percentile values averaged across 
study areas in the range of 45.8 to 34.2 [mu]g/m\3\ (Burnett et al., 
2004; Zanobetti and Schwartz, 2009; Bell et al., 2008; Dominici et al., 
2006a, Burnett and Goldberg, 2003; Franklin et al., 2008). The 
Administrator noted that, to the extent air quality distributions were 
reduced to reflect just meeting the current 24-hour standard, 
additional protection would be anticipated for the effects observed in 
the three multi-city studies with 98th percentile values greater than 
35 [mu]g/m\3\ (Burnett et al., 2004; Burnett and Goldberg, 2003; 
Franklin et al., 2008). In the three additional studies with 98th 
percentile values below 35 [mu]g/m\3\, specifically 98th percentile 
concentrations of 34.2, 34.3, and 34.8 [mu]g/m\3\, the Administrator 
noted that these studies reported long-term mean PM2.5 
concentrations of 12.9, 13.2, and 13.4 [mu]g/m\3\, respectively (Bell 
et al., 2008; Zanobetti and Schwartz, 2009; Dominici et al., 2006a).
    In proposing to revise the level of the annual standard to within 
the range of 12 to 13 [mu]g/m\3\, as discussed above, the Administrator 
recognized that additional protection would be provided for the short-
term effects observed in these multi-city studies in conjunction with 
an annual standard level of 12 [mu]g/m\3\, and in two of these three 
studies in conjunction with an annual standard level of 13 [mu]g/m\3\. 
She noted that the study-wide mean concentrations were based on 
averaging across monitors within study areas and that compliance with 
the standard would be based on concentrations measured at the monitor 
reporting the highest concentration within each area. The Administrator 
believed it would be reasonable to conclude that revision to the 24-
hour standard would not be appropriate in conjunction with an annual 
standard within this range. Based on the above considerations related 
to the epidemiological evidence, the Administrator provisionally 
concluded that it was appropriate to retain the level of the 24-hour 
standard at 35 [mu]g/m\3\, in conjunction with a revised annual 
standard level in the proposed range of 12 to 13 [mu]g/m\3\.
    In addition to considering the epidemiological evidence, the 
Administrator also took into account air quality information based on 
county-level 24-hour and annual design values to understand the public 
health implications of retaining the 24-hour standard level at 35 
[mu]g/m\3\ in conjunction with an annual standard level within the 
proposed range of 12 to 13 [mu]g/m\3\. She considered whether these 
suites of standards would meet a public health policy goal which 
included setting the annual standard to be the ``generally 
controlling'' standard in conjunction with setting the 24-hour standard 
to provide supplemental protection to the extent that additional 
protection is warranted. As discussed above, the Administrator 
provisionally concluded that this approach was the most effective and 
efficient way to reduce total population risk associated with both 
long- and short-term PM2.5 exposures, resulting in more 
uniform protection across the U.S. than the alternative of setting the 
24-hour standard to be the controlling standard.
    In considering the air quality information, the Administrator first 
recognized that there was no annual standard within the proposed range 
of levels, when combined with a 24-hour standard at the proposed level 
of 35 [mu]g/m\3\, for which the annual standard would be the generally 
controlling standard in all areas of the country. She further observed 
that such a suite of PM2.5 standards with an annual standard 
level of 12 [mu]g/m\3\ would result in the annual standard as the 
generally controlling standard in most regions across the country, 
except for certain areas in the Northwest, where the annual mean 
PM2.5 concentrations have historically been low but where 
relatively high 24-hour concentrations occur, often related to seasonal 
wood smoke emissions (U.S. EPA, 2011a, pp. 2-89 to 2-91, Figure 2-10). 
Although not explicitly delineated on Figure 2-10 in the Policy 
Assessment, an annual standard of 13 [mu]g/m\3\ would be somewhat less 
likely to be the generally controlling standard in some regions of the 
U.S. outside the Northwest in conjunction with a 24-hour standard level 
of 35 [mu]g/m\3\.
    Taking the above considerations into account, the Administrator 
proposed to revise the level of the primary annual PM2.5 
standard from 15.0 [mu]g/m\3\ to within the range of 12.0 to 13.0 
[mu]g/m\3\ and to retain the 24-hour standard level at 35 [mu]g/m\3\. 
In the Administrator's judgment, such a suite of primary 
PM2.5 standards and the rationale supporting such levels 
could reasonably be judged to reflect alternative approaches to the 
appropriate consideration of the strength of the available evidence and 
other information and their associated uncertainties and the advice of 
CASAC.
    The Administrator recognized that the final suite of standards 
selected from within the proposed range of annual standard levels, or 
the broader range of annual standard levels on which public comment was 
solicited, must be clearly responsive to the issues raised by the DC 
Circuit's remand of the 2006 primary annual PM2.5 standard. 
Furthermore, at the time of the proposal, she recognized that the final 
suite of standards will reflect her ultimate judgment in the final 
rulemaking as to the suite of primary PM2.5 standards that 
would be requisite to protect the public health with an adequate margin 
of safety from effects associated with fine particle exposures. The 
final judgment to be made by the Administrator will appropriately 
consider the requirement for a standard that is neither more nor less 
stringent than necessary and will recognize 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 the proposal, having reached her provisional 
judgment to propose revising the annual standard level from 15.0 to 
within a range of 12.0 to 13.0 [mu]g/m\3\ and to propose retaining the 
24-hour standard level at 35 [mu]g/m\3\, the Administrator solicited 
public comment on this range of levels and on approaches to considering 
the available evidence and information that would support the choice of 
levels within this range. The Administrator also solicited public 
comment on alternative annual standard levels down to 11 [mu]g/m\3\ and 
on the combination of annual and 24-hour standards that commenters may 
believe is appropriate, along with the approaches and rationales used 
to support such levels. In addition, given the importance the evidence 
from epidemiologic studies played in considering the appropriate annual 
and 24-hour levels, the Administrator solicited public comment on 
issues related to translating epidemiological evidence into standards, 
including approaches for addressing the uncertainties and limitations 
associated with this evidence.
c. Comments on Standard Levels
    This section addresses comments that relate to consideration of the 
appropriate levels of the primary annual and 24-hour PM2.5 
standards, including comments on the general approach used by the EPA 
to translate the available scientific information into standard levels 
and how specific PM2.5 exposure studies should be considered 
as a basis for the standard levels. These comments on standard levels 
expand upon the more general comments that either supported or opposed 
any change to the current suite of primary PM2.5

[[Page 3144]]

standards, which are addressed above in section III.D.2.\85\ As 
explained there, one group of commenters generally opposed any change 
to the current primary PM2.5 standards and more specifically 
disagreed with the basis for the EPA's proposal to revise the annual 
standard level. Another group of commenters supported revising the 
current suite of primary PM2.5 standards to provide 
increased public health protection. Some commenters in this second 
group argued that both the annual and 24-hour standard levels should be 
lowered while other commenters in this group agreed with the EPA's 
proposal to retain the level of the 24-hour standard in conjunction 
with revising the level of the annual standard. While generally 
supporting the EPA's proposal to lower the level of the annual 
standard, many commenters in this group disagreed that a level within 
the EPA's proposed range was adequately protective and supported a 
level of 11 [mu]g/m\3\ or below.
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    \85\ Specific comments on the forms of the annual and 24-hour 
standards are addressed in section III.E.3.a and III.E.3.b, 
respectively.
---------------------------------------------------------------------------

i. Annual Standard Level
    The group of commenters opposed to any change to the current suite 
of primary PM2.5 standards generally raised questions 
regarding the underlying scientific evidence, including the causal 
determinations reached in the Integrated Science Assessment, and 
focused strongly on the uncertainties they saw in the scientific 
evidence as a basis for their conclusion that no changes to the current 
standard levels were warranted. In commenting on the proposed standard 
levels, these commenters typically relied on the arguments summarized 
and addressed above in section III.D.2 as to why they believed it was 
inappropriate for the EPA to make any revisions to the suite of primary 
PM2.5 standards. That is, they asserted that the EPA's 
causal determinations were not adequately supported by the underlying 
scientific information; the biological plausibility of health effects 
observed in epidemiological studies has not been demonstrated in 
controlled human exposure and toxicological studies; uncertainties in 
the underlying health science are as great or greater than in 2006; 
there is no evidence of greater risk since the last review to justify 
tightening the current annual PM2.5 standard; and ``new'' 
studies not included in the Integrated Science Assessment continue to 
increase uncertainty about possible health risks associated with 
exposure to PM2.5.
    With regard to the level of the annual standard, these commenters 
strongly disagreed with the Agency's proposed decision to revise the 
level to within a range of 12 to 13 [mu]g/m\3\ and argued that the 
current standard level of 15 [mu]g/m\3\ should be retained. For 
example, UARG, API, and other commenters in this group raised a number 
of issues that they asserted called into question the EPA's 
interpretation of the epidemiological evidence to support revising the 
annual standard level. These commenters raised specific questions 
related to the general approach used by the EPA to translate the air 
quality and other information from specific epidemiological studies 
into standard levels, including: (1) The EPA's approach for using 
composite monitor air quality distributions reported in epidemiological 
studies to select a standard level that would be compared to 
measurements at the monitor recording the highest value in an area to 
determine compliance with the standard; (2) the appropriate exposure 
period for effects observed in long-term exposure mortality studies; 
and (3) the use of the EPA's analysis of distributions of underlying 
population-level data (i.e., health event and study population data) 
for those epidemiological studies for which such information was 
available. These commenters also raised questions regarding the EPA's 
consideration of specific scientific evidence as a basis for setting a 
standard level, including: (4) evidence of respiratory morbidity 
effects in long-term exposure studies and (5) more limited evidence of 
health effects which have been categorized in the Integrated Science 
Assessment as suggestive of a causal relationship (i.e., developmental 
and reproductive outcomes). These comments are discussed in turn below.
    (1) Some commenters in this group argued that one reason why they 
believe there is no basis for setting a standard level below 15 [mu]g/
m\3\ is that the air quality metric from epidemiological studies that 
the EPA relied on in the proposal is not the same metric that will be 
compared to the level of the standard to determine compliance with the 
standard. That is, commenters noted that the long-term mean 
PM2.5 concentrations that the EPA considered, shown in 
Figure 4 above, are composite monitor mean concentrations (i.e., 
concentrations averaged across multiple monitors within areas with more 
than one monitor), whereas the PM2.5 concentrations that 
will be compared to the level of the standard are maximum monitor 
concentrations (i.e., the concentration measured by the monitor within 
an area reporting the highest concentration). This comment was 
presented most specifically in UARG's comments (UARG, 2012, Attachment 
1, pp. 2 to 6), which raised two overarching issues as discussed below.
    First, the commenter noted that the EPA's approach of considering 
composite monitor mean PM2.5 concentrations in selecting a 
standard level, and then comparing the maximum monitor mean 
PM2.5 concentration in each area to the standard level when 
the standard is implemented, was characterized in the proposal as 
inherently having the potential to build in a margin of safety (UARG, 
2012, Attachment 1, p. 4, citing 77 FR 38905). The commenter asserted 
that the Administrator is ignoring this distinction between composite 
and maximum monitor concentrations, and that this approach creates an 
unwarranted case for lowering the standard level, since in the 
commenter's view, it would result in a margin of safety that would be 
arbitrary, not based on evidence, and unquantified (UARG, 2012, 
Attachment 1, p. 4). In support of this view, the commenter asserted 
that there is a significant difference between composite monitor mean 
PM2.5 concentrations and maximum monitor mean 
PM2.5 concentrations. The commenter asserted that the 
maximum monitor value will always be higher than the composite monitor 
value (except in areas that contain only a single monitor), such that 
when an area just attains the NAAQS, that area's composite monitor 
long-term mean PM2.5 concentration will be lower than the 
level of the standard (UARG, 2012, Attachment 1, p. 3).
    Second, the commenter asserted that a more ``reasoned and 
consistent approach would be to decide on a mean composite monitor 
PM2.5 level that should be achieved and then identify the 
maximum monitor level that would result in that composite value'' 
(UARG, 2012, Attachment 1, p. 4). The commenter conducted an analysis 
of maximum monitor versus composite monitor annual mean 
PM2.5 concentrations using monitoring data \86\ from 2006 to 
2008 and presented results averaged across areas within two groups 
(i.e., those with design values \87\ above the current standard level 
and those with design values just below the

[[Page 3145]]

current standard level) to illustrate their suggested alternative 
approach. The commenter interpreted this analysis as showing that the 
composite monitor long-term mean PM2.5 concentrations from 
the subset of the epidemiological studies shown in Figure 4 (of the 
proposal and above) that the commenter considered to be an appropriate 
focus for this analysis would be achieved across the U.S. if the 
current annual NAAQS of 15 [mu]g/m\3\ is retained and attained. The 
commenter considered the subset of epidemiological studies that 
included only long-term exposures studies of effects for which the 
evidence is categorized as causal or likely causal, but did not 
consider short-term exposure studies. On this basis, the commenter 
asserted that attaining the current annual PM2.5 standard 
would result in composite monitor long-term mean concentrations in all 
areas that would be generally within or below the range of the 
composite monitor long-term mean concentrations from such studies and, 
as a result, there is no reason to lower the level of the current 
annual NAAQS.
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    \86\ The commenter indicated that this analysis was based on 
monitoring data for every core based statistical area (CBSA) in the 
EPA's Air Quality System (AQS) database.
    \87\ The design value is the air quality statistic that is 
compared to the level of the NAAQS to determine the attainment 
status of a given area.
---------------------------------------------------------------------------

    In considering the first issue related to the EPA's approach, the 
EPA notes that in proposing to revise both the form and level of the 
annual standard, the Administrator clearly took into account the 
distinction between the composite monitor long-term mean 
PM2.5 concentrations from the epidemiological studies, 
considered as a basis for selecting an annual standard level, and 
maximum monitor long-term mean PM2.5 concentrations. In 
deciding to focus on the composite monitor long-term mean 
concentrations in selecting the standard level, and on the maximum 
monitor concentrations in selecting the form of the standard (i.e., 
consistent with proposing to eliminate the option for spatial averaging 
across monitors within an area when implementing the standard \88\), 
the Administrator reasonably considered the distinction between these 
metrics in a manner that was consistent with advice from CASAC (Samet 
et al., 2010d, pp. 2 to 3).
---------------------------------------------------------------------------

    \88\ As discussed above in section III.E.3.a.
---------------------------------------------------------------------------

    As noted above in section III.A.3, the EPA recognizes that a 
statistical metric (e.g., the mean of a distribution) based on maximum 
monitor concentrations may be identical to or above the same 
statistical metric based on composite monitor concentrations. More 
specifically, many areas have only one monitor, in which case the 
composite and maximum monitor concentrations are identical. Based on 
the most recent data from the EPA's AQS from 2009 to 2011 in the 331 
CBSAs in which valid PM2.5 data are available, as discussed 
in Frank (2012a, Table 5), there were 208 such areas (with design 
values ranging up to about 15 [mu]g/m\3\). Frank (2012a) also observed 
that other areas have multiple monitors with composite and maximum 
monitor mean PM2.5 concentrations that were the same or 
relatively close, with 57 areas in which the maximum monitor mean 
concentration was no more than 0.5 [mu]g/m\3\ higher than the composite 
monitor mean concentration and 56 areas in which the difference was 
between 0.6 and 2 [mu]g/m\3\. Further, there were only a few other 
areas in which the maximum monitor mean concentration was appreciably 
higher than the composite monitor mean concentration, such as areas in 
which some monitors may be separately impacted by local sources. There 
were only 10 such areas in the country in which the maximum monitor 
mean concentration was between 2 to 6 [mu]g/m\3\ higher than the 
composite monitor concentration (Frank, 2012a, Table 4).\89\ Thus, the 
EPA does not agree that there is a significant difference between 
composite monitor mean PM2.5 concentrations and maximum 
monitor mean PM2.5 concentrations in the large majority of 
areas across the country.
---------------------------------------------------------------------------

    \89\ The average difference between the maximum and composite 
design value among the 123 CBSAs with two or more monitors is 0.8 
[mu]g/m\3\ and the median difference is 0.6 [mu]g/m\3\. The 25th and 
75th percentiles are 0.3 and 1.0 [mu]g/m\3\, respectively (Frank, 
2012a, p. 4).
---------------------------------------------------------------------------

    In proposing to revise the form of the annual PM2.5 
standard, as discussed above in section III.E.3.a, the EPA noted that 
when an annual PM2.5 standard was first set in 1997, the 
form of the standard included the option for averaging across 
measurements at appropriate monitoring sites within an area, generally 
consistent with the composite monitor approach used in epidemiological 
studies, with some constraints intended to ensure that spatial 
averaging would not result in inequities in the level of protection for 
communities within large metropolitan areas. In the last review the EPA 
tightened the constraints on spatial averaging, and in this review has 
eliminated the option altogether, on the basis of analyses in each 
review that showed that such constraints may be inadequate to avoid 
substantially greater exposures for people living in locations around 
the monitors recording the highest PM2.5 concentrations in 
some areas, potentially resulting in disproportionate impacts on at-
risk populations of persons with lower SES levels as well as 
minorities. In light of these analyses, and consistent with the 
Administrator's decision to revise the form of the annual 
PM2.5 standard by eliminating the option for spatial 
averaging, the EPA continues to conclude that a standard level based on 
consideration of long-term mean concentrations from composite monitors, 
and applied at each monitor within an area including the monitor 
measuring the highest concentration, is the appropriate approach to use 
in setting a standard that will protect public health, including the 
health of at-risk populations, with an adequate margin of safety, as 
required by the CAA.
    The EPA acknowledges that at proposal, the Agency characterized the 
approach of using maximum monitor concentrations to determine 
compliance with the standard, while selecting the standard level based 
on consideration of composite monitor concentrations, as one that 
inherently had the potential to build in a margin of safety (77 FR 
38905), and CASAC reiterated that view in supporting the EPA's approach 
(Samet, 2010d, p. 3). Nonetheless, in light of the discussion above, 
the EPA more specifically recognizes that this approach does not build 
in any margin of safety in the large number of areas across the country 
with only one monitor. Further, based on the analyses done to inform 
consideration of the form of the standard (Schmidt, 2011, Analysis A), 
the EPA concludes that this approach does not provide a margin of 
safety for the at-risk populations that live around the monitor 
measuring the highest concentration, such as in those few areas in 
which the maximum monitor concentration is appreciably higher than the 
composite monitor concentration. Rather, this approach properly treats 
those at-risk populations the same way it does the broader populations 
that live in areas with only one monitor, by providing the same degree 
of protection for those at-risk populations that would otherwise be 
disproportionately impacted as it does for the broader populations in 
other areas, While the EPA recognizes that this approach can result in 
some additional margin of safety for the subset of areas with multiple 
monitors in which at-risk populations may not be disproportionately 
represented in areas around the maximum monitor, which may be the case 
in areas with relatively small differences between the maximum and 
composite monitor concentrations, the EPA notes that this margin would 
be relatively small in such areas.
    Based on the above considerations, the EPA does not agree that the 
Agency's approach of using maximum monitor concentrations to determine 
compliance with the standard, while

[[Page 3146]]

selecting the standard level based on consideration of composite 
monitor concentrations creates an unwarranted case for lowering the 
standard level based on a margin of safety that would be arbitrary, not 
based on evidence, or lack quantification. The EPA recognizes that 
setting a standard to protect public health, including the health of 
at-risk populations, with an adequate margin of safety, depends upon 
selecting a standard level sufficiently below where the EPA has found 
the strongest evidence of health effects so as to provide such 
protection, and that the EPA's approach regarding consideration of 
composite and maximum monitor concentrations is intended to, and does, 
serve to address this requirement as part of and not separate from the 
selection of an appropriate standard level based on the health effects 
evidence.
    In considering the second issue related to the commenter's 
suggested alternative approach, the EPA strongly disagrees with the 
commenter's view that a more ``reasoned and consistent approach would 
be to decide on a mean composite monitor PM2.5 level that 
should be achieved and then identify the maximum monitor level that 
would result in that composite value'' (UARG, 2012, Attachment 1, p. 
4). As discussed above, the EPA notes that for areas with only one 
monitor, or with multiple monitors that measure concentrations that are 
very close in magnitude, the maximum monitor level that would limit the 
composite monitor PM2.5 level to be no greater than the 
level that should be achieved to protect public health with an adequate 
margin of safety, would essentially be the same as that composite 
monitor level. Further, as discussed above, even for areas in which the 
maximum monitor concentration is appreciably higher than other monitor 
concentrations within the same area, public health would not be 
protected with an adequate margin of safety if the disproportionately 
higher exposures of at-risk, susceptible populations around the monitor 
measuring the highest concentration were in essence averaged away with 
measurements from monitors in other locations within large urban areas. 
Further, the commenter's suggested approach would be based on annual 
average PM2.5 concentrations that have been measured over 
some past time period. Such an approach would reflect the air quality 
that existed in the past, but it would not necessarily provide 
appropriate constraints on the range of concentrations that would be 
allowed by such a standard in the future, when relationships between 
maximum and composite monitor concentrations in areas across the 
country may be different. For these reasons, the EPA fundamentally 
rejects the commenter's suggested approach because in the EPA's view it 
would not protect public health, including providing protection for at-
risk populations, with an adequate margin of safety in areas across the 
country.
    More specifically, in further considering the commenter's analysis 
of design values based on maximum versus composite monitor annual mean 
PM2.5 concentrations using monitoring data from 2006 to 2008 
which they assert supports retaining the current standard level of 15 
[mu]g/m\3\, the EPA finds flaws with the numerical results and the 
scope of the analysis, as well as flaws in the commenter's translation 
of the analysis results into the basis for selecting an annual standard 
level.
    In considering the commenter's analysis, the EPA notes that the 
analysis compared maximum versus composite monitor annual mean 
PM2.5 concentrations, averaged over 3 years, for two groups 
of areas: (1) Areas with design values that exceed the current annual 
standard level (i.e., greater than 15.0 [mu]g/m\3\) and (2) areas with 
design values that are just attaining the current annual standard 
(i.e., between 14.5 and 15.0 [mu]g/m\3\).\90\ The commenter indicated 
that they used the full body of PM2.5 monitoring data from 
the EPA's AQS database (UARG, 2012, Attachment 1, p. 4), In attempting 
to reproduce the commenter's results, the EPA repeated the calculations 
using only valid air quality data (i.e., data that meet data 
completeness and monitor siting criteria) from the AQS database for the 
same time period (Frank, 2012a).\91\ Based on this corrected analysis, 
the EPA finds that the composite monitor concentrations averaged across 
the areas within each group are somewhat higher than those calculated 
by the commenter, and the average differences between the maximum and 
composite monitor concentrations are somewhat smaller (Frank, 2012a, 
Table 3).\92\ Notably, the difference between the maximum and composite 
monitor average concentrations for the second group of areas is 
substantially reduced in the corrected analysis, such that the 
difference (averaged across the 10 areas with valid data in the second 
group) is approximately 0.5 [mu]g/m\3\, not 1.2 [mu]g/m\3\ as in the 
commenter's analysis. In addition, the commenter's analysis compared 
the average of the composite monitors to the average of the maximum 
monitors for each subset of areas. This comparison of averages across 
all the areas in each subset masks the fact that the large majority of 
areas across the country have only one monitor, with the composite 
monitor and maximum monitor values the same for such areas, and many 
other areas have a maximum monitor value that is close to the composite 
monitor value. As discussed above, these circumstances have a major 
impact on the protection that would be achieved by the approach 
suggested by the commenter.
---------------------------------------------------------------------------

    \90\ For the first group of areas (which included 33 areas), 
this analysis calculated an average across the areas of maximum 
monitor annual mean PM2.5 concentrations, averaged over 3 
years, of 17.2 [mu]g/m\3\ compared to an average of composite 
monitor concentrations of 14.3 [mu]g/m\3\. For the second group of 
areas (which included 11 areas), this analysis calculated an average 
across the areas of maximum monitor annual mean concentrations, 
averaged over 3 years, of 14.8 [mu]g/m\3\ compared to an average of 
composite monitor concentrations of 13.6 [mu]g/m\3\ (UARG, 2012, 
Attachment 1, Table 1).
    \91\ The EPA notes that the Frank (2012a) analysis is similar to 
an earlier EPA staff analysis (Hassett-Sipple et al., 2010), which 
used air quality data from EPA's AQS database to compare maximum 
versus composite monitor long-term mean PM2.5 
concentrations across the study areas in six selected multi-city 
epidemiological studies.
    \92\ The EPA's analysis was intended to repeat the commenter's 
analysis, but using only valid air quality data (from 2006 to 2008). 
For the first group of areas (which included 21 areas with valid 
data), the EPA's analysis calculated an average across the areas of 
maximum monitor annual mean concentrations, averaged of 3 years, of 
16.8 [mu]g/m\3\ compared to an average of composite monitor 
concentrations of 14.8 [mu]g/m\3\. For the second group of areas 
(which included 10 areas with valid data), the EPA's analysis 
calculated an average across the areas of maximum monitor annual 
mean concentrations, averaged over 3 years, of 14.8 [mu]g/m\3\ 
compared to an average of composite monitor concentrations of 14.2 
[mu]g/m\3\ (Frank, 2012a, Table 3).
---------------------------------------------------------------------------

    With regard to the scope of the commenter's analysis, the EPA finds 
that by limiting the scope to a small subset of areas with design 
values above or just below the current annual standard level of 15 
[mu]g/m\3\, the analysis ignores the large number of areas across the 
country with lower design values that are relevant to consider in light 
of the epidemiological evidence of serious health effects at lower 
concentrations, well below the level of the current standard.
    In translating the analysis results into the basis for selecting an 
annual standard level, the commenter's translation is premised on the 
view that the ``natural focal point'' for setting an annual 
PM2.5 standard level should be somewhere within the range of 
the long-term mean PM2.5 concentrations from the subset of 
epidemiological studies that included only long-term exposure studies 
of effects for which the evidence is categorized as causal or likely 
causal, but not for effects categorized as suggestive of causality, nor 
did it

[[Page 3147]]

include short-term exposure studies (which are included in Figure 4 of 
the proposal notice and above). Such a view is not consistent with 
setting a standard that would provide sufficient protection from the 
serious health effects reported even in the limited subset of studies 
considered by the commenter, including protecting public health with an 
adequate margin of safety. As discussed below, the EPA does not agree 
with the commenter's view as to the appropriate focal point for 
selecting the level of an annual PM2.5 standard, or with the 
limited set of studies considered by the commenter as a basis for 
selecting the level of the annual PM2.5 standard.
    Regarding an appropriate focal point for selecting the level of the 
annual standard, as discussed in the proposal and as advised by CASAC, 
the EPA has focused on PM2.5 concentrations somewhat below 
the lowest long-term mean concentrations from each of the key studies 
of both long- and short-term exposures of effects for which the 
evidence is causal or likely causal, as considered by the EPA (i.e., 
the first two sets of studies shown in Figure 4). If the level of the 
annual standard was set just somewhere within the range of the long-
term mean concentrations from the various long-term exposure studies, 
then one or more of the studies would have a long-term mean 
concentration below the selected level of the standard. Absent some 
reason to ignore or discount these studies, which the commenter does 
not provide (and of which the EPA is unaware), setting such a standard 
would allow that level of air quality, where the evidence of health 
effects is strongest, and its associated risk of PM2.5-
related mortality and/or morbidity effects to continue. Selecting such 
a standard level could not be considered sufficient to protect the 
public health with an adequate margin of safety.
    Further, focusing on just the long-term mean PM2.5 
concentrations in the key epidemiological studies--even the lowest 
long-term mean concentration from the set of key studies--is not 
appropriate. Concentrations at and around the long-term mean 
concentrations represent the part of the air quality distribution where 
the data in any given study are most concentrated and, thus, where the 
confidence in the magnitude and significance of an association in such 
study is strongest. However, the evidence of an association with 
adverse health effects in the studies is not limited to the 
PM2.5 concentrations just at and around the long-term mean, 
but rather extends more broadly to a lower part of the distribution, 
recognizing that no discernible population-level threshold for any such 
effects can be identified based on the available evidence. This broader 
region of the distribution of PM2.5 concentrations should be 
considered to the extent relevant information is available, recognizing 
that the degree of confidence in the association identified in a study 
would become lower as one moves below concentrations at and around the 
long-term mean concentration in any given study. The commenter's 
approach ignores this fundamental consideration.
    Regarding the set of studies that is appropriate to inform the 
selection of the level of the annual PM2.5 standard, the EPA 
finds that limiting consideration only to the long-term exposure 
studies, as this commenter suggests, would be tantamount to ignoring 
the short-term exposure studies,\93\ which provide some of the 
strongest evidence from the entire body of epidemiological studies. 
Thus, selecting an annual standard level using the limited set of 
studies suggested by the commenter would fail to provide a degree of 
protection that would be sufficient to protect public health with an 
adequate margin of safety.
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    \93\ The commenter suggests that the EPA should not place 
significant reliance on the long-term mean concentrations from 
short-term exposure studies because ``[T]he short[hyphen]term 
studies did not use the annual average of PM2.5 to 
develop their associations; they used the daily 24-hour averages of 
PM2.5. Thus, short-term studies do not provide a natural 
indicator for the appropriate level of an annual standard * * *.'' 
(UARG, 2012, Attachment 1, p. 3). The EPA finds this argument 
unpersuasive. Quite simply, effects were observed in these studies 
with an air quality distribution that can meaningfully be 
characterized by these long-term mean concentrations. Indeed, in 
remanding the 2006 standard, the D.C. Circuit discussed at length 
the interrelationship of the long- and short-term standards and 
studies, and remanded the 2006 standard to the EPA, in part, for 
ignoring those relationships without adequate explanation. American 
Farm Bureau Federation v. EPA. 559 F. 3d at 522-24.
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    For all the reasons discussed above, the EPA finds the commenter's 
concerns with the EPA's approach to considering composite and maximum 
monitor PM2.5 concentrations in selecting the level of the 
annual PM2.5 standard to be without merit. Further, the EPA 
finds no support in the commenter's analysis for their suggested 
alternative approach.
    (2) With respect to the appropriate exposure period for mortality 
effects observed in long-term exposure studies, some commenters in this 
group generally expressed views consistent with comments from UARG that 
argued that these studies ``are most likely detecting health risk from 
earlier, higher PM2.5 levels and misattributing those risks 
to more recent, lower PM2.5 levels'' (UARG, 2012, Attachment 
1, p 7). Further, this commenter asserted that ``there is no knowledge 
or evidence indicating whether premature deaths are the result of 
PM2.5 exposures in the most recent year; or due to physical 
damages incurred from PM2.5 exposures much earlier in life 
(with the impact on lifespan only emerging later in life); or due to 
total accumulated PM2.5 exposure over many years.'' Id. In 
addition, the commenter asserted that the long-term exposure studies of 
mortality are central to the EPA's basis for proposing to set a lower 
annual standard level, since most of the estimated benefits associated 
with a lower annual PM2.5 standard are based on reductions 
in mortality related to long-term exposures to PM2.5.
    As an initial matter, the EPA has recognized the challenge in 
distinguishing between PM2.5-associated effects due to past 
and recent long-term exposures, and in identifying the relevant latency 
period for long-term exposure to PM and resultant health effects (U.S. 
EPA, 2009a, section 7.6.4; 77 FR 38941/1). While the EPA has 
acknowledged that there remain important uncertainties related to 
characterizing the most relevant exposure periods in long-term exposure 
studies, the assertion that there is ``no knowledge or evidence'' that 
helps to inform this issue is not correct, as discussed below.
    Both in the last review and in the current review, the EPA has 
assessed studies that used different air quality periods for estimating 
long-term exposure and tested associations with mortality for the 
different exposure periods (U.S. EPA, 2004, section 8.2.3.5; U.S. EPA 
2009a, section 7.6.4). In this review, the Integrated Science 
Assessment discussed studies available since the last review that have 
assessed the relationship between long-term exposure to 
PM2.5 and mortality to explore the issue of the latency 
period between exposure to PM2.5 and death (U.S. EPA, 2009a, 
section 7.6.4).
    Notably, in a recent analysis of the extended Harvard Six Cities 
Study, Schwartz et al. (2008) used model averaging (i.e., multiple 
models were averaged and weighted by probability of accuracy) to assess 
exposure periods prospectively (77 FR 38907/1-2). The exposure periods 
were estimated across a range of unconstrained distributed lag models 
(i.e., same year, one year prior, two years prior to death). In 
comparing lags, the authors reported that the effects of changes in 
exposure to PM2.5 on mortality were strongest within a 2-
year period prior to death (U.S. EPA, 2009a, p. 7-92, Figure 7-9). 
Similarly, a large

[[Page 3148]]

multi-city study of the elderly found that the mortality risk 
associated with long-term exposure to PM10 reported 
cumulative effects that extended over the years that deaths were 
observed in the study population (i.e., the follow-up period) and for 
the 3-year period prior to death (Zanobetti et al., 2008).
    Further, in a study of two locations that experienced an abrupt 
decline in PM2.5 concentrations (i.e., Utah Steel Strike, 
coal ban in Ireland), R[ouml][ouml]sli et al. (2005) reported that 
approximately 75 percent of health benefits were observed in the first 
5 years (U.S. EPA, 2009a, Table 7-9). Schwartz et al. (2008) and Puett 
et al. (2008) found, in a comparison of exposure periods ranging from 1 
month to 48 months prior to death, that exposure to PM10 24 
months prior to death exhibited the strongest association, and the 
weakest association was reported for exposure in the time period of 1 
month prior to death.
    Overall, the EPA notes that the available evidence for determining 
the exposure period that is causally related to the mortality effects 
of long-term PM2.5 exposures, as discussed above, cannot 
specifically disentangle the effects observed in long-term exposure 
studies associated with more recent air quality measurements from 
effects that may have been associated with earlier, and most likely 
higher, PM2.5 exposures. While the evidence suggests that a 
latency period of up to five years would account for the majority of 
deaths, it does not provide a basis for concluding that it is solely 
recent PM2.5 concentrations that account for the mortality 
risk observed in such studies. Nonetheless, the more recent air quality 
data does well at explaining the relationships observed between long-
term exposures to PM2.5 and mortality, with the strongest 
association observed in the two years prior to death. Further, the EPA 
recognizes that there is no discernible population-level threshold 
below which effects would not occur, such that it is reasonable to 
consider that health effects may occur over the full range of 
concentrations observed in the epidemiological studies, including the 
lower concentrations in the latter years.
    In light of this evidence and these considerations, the EPA 
concludes that it is appropriate to consider air quality concentrations 
that are generally contemporaneous with the collection of health event 
data (i.e., collected over the same time period) as being causally 
associated with at least some proportion of the deaths assessed in a 
long-term exposure study. This would include long-term mean 
PM2.5 concentrations from most of the key long-term exposure 
studies of effects with causal or likely causal evidence shown in 
Figure 4 above, which reported long-term mean PM2.5 
concentrations ranging from 13.6 [micro]g/m\3\ to 14.3 [micro]g/m\3\. 
These studies include studies of mortality by Eftim et al. (2008), 
which separately analyzed the ACS and Harvard Six City sites, Zeger et 
al. (2008), and Lipfert et al. (2006a), as well as studies of morbidity 
endpoints by Goss et al. (2004), McConnell et al. (2003) and Gauderman 
et al. (2004), and Dockery et al. (1996) and Razienne et al. (1996). 
The EPA acknowledges that uncertainty in the relevant exposure period 
is most notable in two other long-term exposure studies of mortality. 
The Miller et al. (2007) reported a long-term mean PM2.5 
concentration for a 1-year exposure period that post-dated the follow-
up period in which health event data were collected by two years. Also, 
the Krewski et al. (2009) study reported a long-term mean 
PM2.5 concentration for an exposure period that included 
only the last two years of the 18-year follow-up period. Based on these 
considerations, the EPA does not now consider it appropriate to put 
weight on the reported long-term mean concentrations from these two 
studies for the purpose of translating the information from the long-
term mortality studies into a basis for selecting the level of the 
annual PM2.5 standard.\94\
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    \94\ Nonetheless, the EPA notes that the Krewski et al. (2009) 
and Miller et al. (2007) studies provide strong evidence of 
mortality and cardiovascular-related effects associated with long-
term PM2.5 exposures to inform causality determinations 
reached in the Integrated Science Assessment (U.S. EPA, 2009a, 
sections 7.2.11 and 7.6).
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    In addition, the EPA acknowledges that exposure periods that extend 
at least a couple years prior to the follow-up period in which health 
event data were collected would likely more fully capture the PM-
related deaths in such studies. To explore how much higher the long-
term mean PM2.5 concentrations would likely have been had 
air quality data prior to the follow-up years of the studies been 
included, the EPA conducted a sensitivity analysis of long-term mean 
PM2.5 concentrations (Schmidt, 2012a) particularly 
considering studies that only included deaths from a relatively recent 
follow-up period. As examples of such studies, this analysis considered 
the Eftim et al. (2008) study of mortality in the ACS sites and the 
Harvard Six Cities sites, as well as sites in the eastern region in the 
Zeger et al. (2008) study. Using data from the EPA's AQS database, the 
analysis added the two years of air quality data just prior to the 
follow-up period in each study, which was 2000 to 2002 in Eftim et al. 
(2008) and 2000 to 2005 in Zeger et al. (2008). The analysis then 
calculated the extended long-term mean PM2.5 concentration 
for each study. As discussed in Schmidt (2012a), in each case the long-
term mean PM2.5 concentration averaged over the extended 
exposure period was less than 0.4 [micro]g/m\3\ higher than the long-
term mean PM2.5 concentration averaged over the follow-up 
period. The EPA finds it reasonable to conclude that such a relatively 
small difference in long-term mean PM2.5 concentrations 
would likely apply for other long-term exposure studies that used 
similarly recent follow-up periods as well (e.g., Goss et al., 2004; 
Lipfert et al., 2006a).
    Based on the above considerations, the EPA concludes that it is 
appropriate to consider the available air quality information from the 
long-term exposure studies, while taking into account the uncertainties 
in the relevant long-term exposure periods in weighing the information 
from these studies. The EPA recognizes that considering such 
information in selecting an appropriate annual standard level has the 
potential to build in some margin of safety. The EPA further concludes 
that it is appropriate to consider the air quality information from the 
set of long-term exposure studies discussed above in the context of the 
broader array of epidemiological studies that inform the EPA's 
consideration of the level of the annual PM2.5 standard.
    The EPA also notes that while the long-term exposure studies are an 
important component of the epidemiological evidence that informs the 
Agency's consideration of the level of the annual standard, they do not 
provide the only relevant information, nor are they the set of studies 
for which the relevant long-term mean PM2.5 concentrations 
are the lowest. As discussed in the proposal, the EPA also considers 
the long-term mean PM2.5 concentrations from the short-term 
mortality and morbidity studies as providing important information in 
considering the level of the annual standard. As discussed above, a 
large proportion of the aggregate risk associated with short-term 
exposures results from the large number of days during which the 24-
hour average concentrations are in the low- to mid-range of the 
concentrations observed in the studies. Thus, setting the level of the 
annual standard based on long-term mean concentrations, as well as the 
distribution of concentrations below the mean, in the short-term 
exposure studies is the most effective and efficient way to reduce 
total PM2.5-

[[Page 3149]]

related risk from the broad array of mortality and morbidity effects 
associated with short-term exposures.
    Further, the EPA notes that the relevant exposure period for the 
short-term exposure studies is the period contemporaneous with the 
collection of health event data, and that this exposure period is not 
subject to the uncertainties discussed above related to the long-term 
exposure studies. Recognizing that the long-term mean PM2.5 
concentrations from several of the multi-city short-term exposure 
studies shown in Figure 4 are below the long-term mean PM2.5 
concentrations from the long-term exposure studies (with the exception 
of Miller et al., 2007).\95\ It is reasonable that in selecting the 
level of the annual standard primary consideration should be given to 
the information from this set of short-term exposure studies. There is 
no reasonable basis to discount the long-term mean concentrations of 
the short-term exposure studies for purposes of setting the level of 
the annual standard. Thus, the commenter is incorrect in asserting that 
the long-term exposure studies, not the short-term exposure studies, 
would be central in the Administrator's decision on the level of the 
annual standard. The standard is ultimately intended to protect not 
just against the single type of effect that contributes the most to 
quantitative estimates of risk to public health, but rather to the 
broad array of effects, including mortality and morbidity effects from 
long- and short-term exposures across the range of at-risk populations 
impacted by PM2.5-related effects.
---------------------------------------------------------------------------

    \95\ As noted above, the EPA is not placing weight on the 
reported long-term mean concentrations from the Miller et al. (2007) 
study for the purpose of translating the information from the long-
term mortality studies into a basis for selecting the level of the 
annual PM2.5 standard.
---------------------------------------------------------------------------

    (3) With regard to the EPA's analysis of distributions of 
underlying population-level data (i.e., health event and study 
population data) and corresponding air quality data from each study 
area in certain key multi-city epidemiological studies (Rajan et al., 
2011), some commenters in this group raised a number of issues related 
to this analysis (API, 2012, Attachment 1 pp. 5 to 6; McClellan, 2012, 
pp.2 to 4). Some commenters noted the limited number of studies for 
which health event and study population data were available, and 
questioned whether these distributions would apply to other studies. 
Commenters expressed concerns that this analysis had not been formally 
reviewed by CASAC and was not published in the peer-review literature. 
Based on such concerns, some commenters asserted that the EPA should 
not consider this information as a basis for selecting a standard 
level.
    As an initial matter, as discussed in section III.E.4.b above, the 
EPA agrees with CASAC's advice that it is appropriate to consider 
additional data beyond the mean PM2.5 concentrations in key 
multi-city studies to help inform selection of the level of the annual 
PM2.5 standard. As both the EPA and CASAC recognize, in the 
absence of a discernible threshold, health effects may occur over the 
full range of concentrations observed in the epidemiological studies. 
Nonetheless, the EPA recognizes that confidence in the magnitude and 
significance of an association is highest at and around the long-term 
mean PM2.5 concentrations reported in the studies and the 
degree of confidence becomes lower at lower concentrations within any 
given study. Following CASAC's advice (Samet, 2010d, p.2), the EPA used 
additional population-level and air quality data made available by 
study authors to conduct an analysis of the distributions of such data, 
to help inform consideration of how the degree of confidence in the 
magnitude and significance of observed associations varies across the 
range of long-term mean PM2.5 concentrations in study areas 
within key multi-city epidemiological studies. In the EPA's view, such 
consideration is important in selecting a level for an annual standard 
that will protect public health with an adequate margin of safety.
    With regard to the number of multi-city studies for which an 
analysis of the distributions of population-level data across the study 
areas and the corresponding annual mean PM2.5 concentrations 
was done, the EPA noted at proposal that data for such an analysis were 
made available from study authors for four studies, including two long-
term exposure studies and two short-term exposure studies.\96\ The EPA 
recognized that access to health event data can be restricted due to 
confidentiality issues, such that it is not reasonable to expect that 
such information could be made available from all studies. In 
considering the information from these four studies, the EPA has 
further taken into consideration uncertainties discussed in response to 
the above comment related to the appropriate exposure period for long-
term exposure studies. Based on these considerations, as noted above, 
the EPA concludes that such uncertainties are an important factor in 
evaluating the usefulness of the air quality information from the two 
long-term exposure studies in this analysis (Krewski et al., 2009; 
Miller et al., 2007) and that it would not be appropriate to place 
weight on the distributional analysis of health event and air quality 
data from these two studies specifically for the purpose of translating 
the information from the long-term mortality studies into a basis for 
selecting the level of the annual PM2.5 standard. Such 
uncertainties are not relevant to the short-term exposure studies, and 
thus, the Agency focuses on the two short-term exposure studies in this 
analysis (Bell et al., 2008; Zanobetti and Schwartz, (2009).
---------------------------------------------------------------------------

    \96\ Health event data and study population data were available 
from two short-term exposure studies (Bell et al. 2008; Zanobetti 
and Schwartz, 2009) and one long-term exposure study (Krewski et 
al., 2009). Only study population data were available from another 
long-term exposure study (Miller et al., 2007).
---------------------------------------------------------------------------

    In focusing on these two short-term exposure studies, the EPA first 
notes that these studies are key multi-city studies that reported 
positive and statistically significant associations between mortality 
and cardiovascular-related hospital admissions across a large number of 
areas throughout the U.S. (112 U.S. cities in Zanobetti and Schwartz, 
2009; 202 U.S. counties in Bell et al., 2008) using relatively recent 
air quality and health event data (i.e., 1999 through 2005 in both 
studies). The EPA considers this to be a modest but important data set 
to use for this distributional analysis to help inform consideration of 
how much below the long-term mean PM2.5 concentrations in 
key multi-city long- and short-term exposure studies the annual 
PM2.5 standard level should be set. While the EPA 
acknowledges that having such data available from more studies would 
have been useful, the Agency finds the information from this limited 
set of studies to be an important consideration in selecting an annual 
standard level, consistent with CASAC advice to consider such 
information.
    In considering the results of this distributional analysis, as 
discussed more fully in the Response to Comments document, the EPA 
considers PM2.5 concentrations between the 25th and 10th 
percentiles of the distribution of health events to be a reasonable 
range for providing a general frame of reference for that part of the 
distribution in which confidence in the magnitude and significance of 
the association may be appreciably lower than confidence at and around 
the long-term mean concentration. For the two short-term exposure 
studies included in this analysis, the EPA notes that the 
PM2.5 concentrations corresponding to the 25th percentiles 
of the distributions of

[[Page 3150]]

health events were 12.5 [micro]g/m\3\ and 11.5 [micro]g/m\3\, 
respectively, for Zanobetti and Schwartz (2009) and for Bell et al. 
(2008), with the 10th percentiles being lower by approximately 2 
[micro]g/m\3\ in each study (Rajan et al., 2011, Table 1). In 
considering this information, the EPA recognizes, however, that there 
is no clear dividing line or single percentile within a given 
distribution (including both above and below the 25th percentile) 
provided by the scientific evidence that is most appropriate or 
`correct' to use to characterize where the degree of confidence in the 
associations warrants setting the annual standard level. The decision 
as to the appropriate standard level below the long-term mean 
concentrations of the key studies is largely a public health policy 
judgment to be made by the Administrator, taking into account all of 
the evidence and its related uncertainties, as discussed in section 
III.E.4.d below.
    In response to concerns that this analysis was not reviewed by 
CASAC nor published in the peer-reviewed literature, the EPA notes that 
this analysis was conducted to directly respond to advice from CASAC, 
as discussed in section III.E.4.b.i above, in conjunction with their 
review of the Policy Assessment. The EPA notes that the same type of 
distributional analysis was presented in the second draft Policy 
Assessment based on air quality data, as well as population-weighted 
air quality data, rather than health event or study population data. In 
considering that distributional information, CASAC urged that the EPA 
redo the analysis using health event or study population data, which is 
exactly what the EPA did and presented in the final Policy Assessment. 
The EPA provided CASAC with the final Policy Assessment and 
communicated how the final staff conclusions reflected consideration of 
its advice and that those staff conclusions were based in part on the 
specific distributional analysis that CASAC had urged the EPA to 
conduct (Wegman, 2011, Attachment p. 2). CASAC did not choose to 
provide any additional comments or advice after receiving the final 
Policy Assessment. The EPA considers this distributional analysis to be 
the product of the peer review conducted by CASAC of the Policy 
Assessment, and thus does not agree with commenters' characterization 
that the analysis lacked appropriate peer review. The EPA's final 
analysis was based on the comments provided by CASAC, the peer review 
committee established pursuant to the CAA, on the draft analysis, such 
that the final analysis stems directly from CASAC's advice and the 
EPA's response to its comments.
    Based on the above considerations, the EPA continues to conclude 
that its analysis of distributions of health event and air quality data 
from two key multi-city epidemiological studies provides important 
information related to understanding the associations between health 
events observed in each city (e.g., deaths, hospitalizations) and the 
corresponding long-term mean PM2.5 concentrations observed 
in the studies. While recognizing that this is a relatively modest data 
set, the EPA further concludes that such information can appropriately 
help to inform the selection of the level of an annual standard that 
will protect public health with an adequate margin of safety from these 
types of health effects which are causally related to long- and short-
term exposures to PM2.5.
    (4) Some commenters in this group asserted there were limitations 
in the long-term exposure studies of morbidity, including studies 
evaluating respiratory effects in children. For example, one commenter 
(UARG, 2012, p. 12, Attachment 1, pp. 14 to 16) asserted there were 
serious limitations in the long-term exposure studies of respiratory 
morbidity in each of the studies considered by the EPA (including 
McConnell et al., 2003; Gauderman et al., 2004; Dockery et al., 1996; 
Raizenne et al., 1996; and Goss et al., 2004) and argued that this 
evidence provides only a ``weak association'' with PM2.5 
exposures. This commenter asserted that many of these long-term 
exposure studies evaluating respiratory effects were considered at the 
time the EPA reaffirmed the current annual standard level of 15 
[micro]g/m\3\ in 2006, that the Administrator in the last review 
determined that the information they provided ``was too limited to 
serve as the basis for setting a level of a national standard,'' and 
that they should be given little weight in setting the level of the 
annual standard in this review (UARG, 2012, Attachment 1, p. 14).
    More specifically, this commenter asserted that the McConnell et 
al. (2003) and Gauderman et al. (2004) studies reported mixed results 
for associations with PM2.5 and stronger associations with 
NO2 (API, 2012, Attachment 1, pp. 14 to 15). Similarly, this 
commenter argued that the Dockery et al. (1996) and Raizenne et al. 
(1996) studies showed stronger associations with acidity than with fine 
particles (measured as PM2.1). Id. pp. 15 to 16. With regard 
to the cystic fibrosis study, this commenter noted that the association 
between pulmonary exacerbations and PM2.5 in this study was 
no longer statistically significant when the model adjusted for each 
individual's baseline lung function. The commenters referred to the 
data on lung function as an ``important explanatory variable,'' and 
suggested that the EPA should rely on results from the model that 
included individual baseline lung function information. Id. p. 16. For 
the reasons discussed below and in more detail in the Response to 
Comments document, the EPA disagrees with the commenters' 
interpretation of these studies.
    As an initial matter, the EPA notes that three of these studies 
(McConnell et al., 2003; Dockery et al., 1996; Raizenne et al., 1996) 
as well as the initial studies from the Southern California Children's 
Health Study (Peters et al., 1999; McConnell et al., 1999; Gauderman et 
al., 2000, 2002; Avol et al., 2001) were discussed and considered in 
the 2004 Air Quality Criteria Document (U.S. EPA, 2004) and, thus, 
considered within the air quality criteria supporting the EPA's final 
decisions in the review completed in 2006. Two additional studies 
(Gauderman et al., 2004; Goss et al., 2004) were discussed and 
considered in the provisional science assessment conducted for the last 
review (U.S. EPA, 2006a). The EPA concluded that ``new'' studies 
considered in the provisional assessment completed in 2006 did not 
materially change any of the broad scientific conclusions regarding the 
health effects of PM exposure made in the Criteria Document (71 FR 
61148 to 61149, October 17, 2006). All of these studies were considered 
in the Integrated Science Assessment that informs the current review 
(U.S. EPA, 2009a).
    With regard to the Southern California Children's Health Study, 
extended analyses considered in the Integrated Science Assessment 
provided evidence that clinically important deficits in lung function 
\97\ associated with long-term exposure to PM2.5 persist 
into early adulthood (U.S. EPA, 2009a, p. 7-27; Gauderman et al., 
2004). These effects remained positive in copollutant models.\98\ 
Additional analyses of the

[[Page 3151]]

Southern California Children's Health Study cohort reported an 
association between long-term PM2.5 exposure and bronchitic 
symptoms (U.S. EPA, 2009a, p. 7-23 to 7-24; McConnell et al., 2003, 
long-term mean concentration of 13.8 [micro]g/m\3\) that remained 
positive in co-pollutant models, with the PM2.5 effect 
estimates increasing in magnitude in some models and decreasing in 
others, and a strong modifying effect of PM2.5 on the 
association between lung function and asthma incidence (U.S. EPA, 
2009a, 7-24; Islam et al., 2007). The outcomes observed in the more 
recent reports from the Southern California Children's Health Study, 
including evaluation of a broader range of endpoints and longer follow-
up periods, were larger in magnitude and more precise than reported in 
the initial version of the study. Supporting these results were new 
longitudinal cohort studies conducted by other researchers in varying 
locations using different methods (U.S. EPA, 2009a, section 7.3.9.1). 
The EPA, therefore, disagrees with the commenters that the studies by 
McConnell et al. (2003) and Gauderman et al. (2004) are flawed and 
should not be used in the PM NAAQS review process.
---------------------------------------------------------------------------

    \97\ Clinical significance was defined as an FEV1 
below 80 percent of the predicted value, a criterion commonly used 
in clinical settings to identify persons at increased risk for 
adverse respiratory conditions (U.S. EPA, 2009a, p. 7-29 to 7-30). 
The primary NAAQS for sulfur dioxide (SO2) also included 
this interpretation for FEV1 (75 FR 35525, June 22, 
2010).
    \98\ Gauderman et al. (2004) clearly stated throughout their 
analysis that NO2 was one component of a highly 
correlated mixture that contains PM2.5. Gauderman et al. 
(2004) did not present the results from copollutants models but 
stated ``two-pollutant models for any pair of pollutants did not 
provide a significantly better fit to the data than the 
corresponding single-pollutant models.''
---------------------------------------------------------------------------

    The 24-City study \99\ by Dockery et al. (1996) (long-term mean 
concentration of 14.5 [micro]g/m\3\) was considered in the current as 
well as two previous reviews (U.S. EPA, 2009a; U.S. EPA, 2004; U.S. 
EPA, 1996). This study observed that PM, specifically ``particle strong 
acidity'' and sulfate particles (indicators of fine particles), were 
associated with reports of bronchitis in the previous year. Similarly, 
the magnitude of the associations between bronchitis and 
PM10 and PM2.1 were similar to those for acidic 
aerosols and sulfate particles, though the confidence intervals for the 
PM10 and PM2.1 associations were slightly wider 
and the associations were not statistically significant. Acid aerosols, 
sulfate, and fine particles are formed in secondary reactions of the 
emissions from incomplete combustion and these pollutants have similar 
regional and temporal distributions. As noted by the study authors, 
``the strong correlations of several pollutants in this study, 
especially particle strong acidity with sulfate (r=0.90) and 
PM2.1 (r=0.82), make it difficult to distinguish the agent 
of interest'' (Dockery et al., 1996, p. 505). Overall, Dockery et al. 
(1996) (and, similarly, Raizenne et al., 1996) observed similar 
associations between respiratory health effects and acid aerosols, 
sulfate, PM10 and PM2.1 concentrations. The 
commenters noted that the associations with particle acidity were 
sensitive to the inclusion of the six Canadian sites. The EPA notes 
that none of these Canadian cities were in the ``sulfate belt'' where 
particle strong acidity was highest. Thus, the change in the effect 
estimate when the six Canadian cities were excluded from the analysis 
is likely due to the lower prevalence of bronchitis and the lower 
concentrations of acid aerosols in these cities, and not due to some 
difference in susceptibility to bronchitis between the U.S. and 
Canadian populations that is not due to air pollution, as suggested by 
the commenters (UARG, 2012, Attachment 1, p. 15). In fact, contrary to 
the statements made by the commenters, the authors did not observe any 
subgroups that appeared to be markedly more susceptible to the risk of 
bronchitis.
---------------------------------------------------------------------------

    \99\ The 24-City study conducted by Dockery et al. (1996) 
included 18 sites in the U.S. and 6 sites in Canada. The Raizenne et 
al. (1996) study considered 22 of these 24 study areas. Athens, OH 
and South Brunswick, NJ were not included in this study.
---------------------------------------------------------------------------

    The Goss et al. (2004) study considered a U.S. cohort of cystic 
fibrosis patients and provided evidence of association between long-
term PM2.5 exposures and exacerbations of respiratory 
symptoms resulting in hospital admissions or use of home intravenous 
antibiotics (U.S. EPA, 2009a, p. 7-25; long-term mean concentration of 
13.7 [micro]g/m\3\). The commenters noted that the association between 
pulmonary exacerbations and PM2.5 in this study was no 
longer statistically significant when the model adjusted for each 
individual's baseline lung function. The commenters referred to the 
data on lung function as an ``important explanatory variable,'' and 
suggested that the EPA should rely on results from the model that 
included individual baseline lung function information. The EPA 
disagrees with the commenters' interpretation of this study. The Agency 
concludes it is unlikely that lung function is a potential confounder 
or an important explanatory variable in this study. In fact, the 
authors noted that ``it is more likely that lung function decline may 
be intimately associated with chronic exposure to air pollutants and 
may be part of the causal pathway in worsening prognosis in CF [cystic 
fibrosis]; in support of this explanation, we found both cross-
sectional and longitudinal strong inverse relationships between 
FEV1 and PM levels'' (Goss et al., 2004, p. 819). The EPA 
notes that adjusting for a variable that is on the causal pathway can 
lead to overadjustment bias, which is likely to attenuate the 
association (Schisterman et al. 2009); this is likely what was observed 
by the authors. Thus, the EPA continues to believe it is appropriate to 
focus on the results reported in Goss et al. (2004) that did not 
include individual baseline lung function in the model.
    In addition, the EPA disagrees with commenters' reliance solely on 
statistical significance when interpreting the study results from 
individual study results and the collective evidence across studies. As 
discussed in section III.D.2 above, statistical significance of 
individual study findings has played an important role in the EPA's 
evaluation of the study's results and the EPA has placed greater 
emphasis on studies reporting statistically significant results. 
However, in the broader evaluation of the evidence from many 
epidemiological studies, and subsequently during the process of forming 
causality determinations in the Integrated Science Assessment by 
integrating evidence from across epidemiological, controlled human 
exposure, and toxicological studies, the EPA has emphasized the pattern 
of results across epidemiological studies and whether the effects 
observed were coherent across the scientific disciplines for drawing 
conclusions on the relationship between PM2.5 and different 
health outcomes.
    As noted in section III.B.1.a of the proposal, with regard to 
respiratory effects, the Integrated Science Assessment concluded that 
extended analyses of studies available in the last review as well as 
new epidemiological studies conducted in the U.S. and abroad provided 
stronger evidence of respiratory-related morbidity associated with 
long-term PM2.5 exposure (77 FR 38918). The strongest 
evidence for respiratory-related effects available in this review was 
from epidemiological studies that evaluated decrements in lung function 
growth in children and increased respiratory symptoms and disease 
incidence in adults (U.S. EPA, 2009a, sections 2.3.1.2, 7.3.1.1, and 
7.3.2.1).
    In considering the collective evidence from epidemiological, 
toxicological, and controlled human exposure studies, including the 
studies discussed above, the EPA recognizes that the Integrated Science 
Assessment concluded that a causal relationship is likely to exist 
between long-term PM2.5 exposures and respiratory effects 
(U.S. EPA, 2009a, p. 2-12, pp. 7-42 to 7-43). CASAC concurred with this 
causality determination (Samet, 2009f, p.9).

[[Page 3152]]

    The commenter's assertion that the EPA should adhere to its 
assessment of these studies as it did in the review completed in 2006 
is significantly mistaken. Most obviously, the EPA's final decision in 
the last review was held to be deficient by the DC Circuit in remanding 
the 2006 primary annual PM2.5 standard. As discussed in 
section III.A.2 above, the DC Circuit specifically held that the EPA 
did not provide a reasonable explanation of why certain morbidity 
studies, including an earlier study from the Southern California 
Children's Health Study (Gauderman et al., 2000, long-term mean 
PM2.5 concentration approximately 15 [mu]g/m\3\) and the 24-
Cities Study (Raizenne et al., 1996, long-term mean concentrations 
approximately 14.5 [micro]g/m\3\) did not warrant a more stringent 
annual PM2.5 standard when the long-term mean 
PM2.5 concentrations reported in those studies were at or 
lower than the level of the annual standard. American Farm Bureau 
Federation v. EPA. 559 F. 3d at 525. Indeed, the court found that, 
viewed together, the Gauderman et al. (2000) and Raizenne et al., 
(1996) studies ``are related and together indicate a significant public 
health risk * * * On this record, therefore, it appears the EPA too 
hastily discounted the Gauderman and 24-Cities studies as lacking in 
significance.'' Id.
    In this review, the EPA recognizes a significant amount of evidence 
beyond these two studies that expands our understanding of respiratory 
effects associated with long-term PM2.5 exposures. This body 
of scientific evidence includes an extended and new analyses from the 
Southern California Children's Health Study (Gauderman et al., 2004; 
Islam et al., 2007; Stanojevic et al., 2008) as well as additional 
studies that examined these health effects (Kim et al., 2004; Goss et 
al., 2004). Thus, even more so than in the last review, the evidence 
indicates a ``significant public health risk'' to children from long-
term PM2.5 exposures at concentrations below the level of 
the current annual standard. A standard that does not reflect 
appropriate consideration of this evidence would not be requisite to 
protect public health with an adequate margin of safety.
    (5) With regard to the use of studies of health effects for which 
the EPA finds the evidence to be ``suggestive'' of a causal 
relationship, some commenters argued that such studies ``do not merit 
any weight in the setting of the annual NAAQS'' (e.g., UARG, 2012, 
Appendix 1, p. 3).
    The EPA disagrees with the commenter's view that studies of health 
effects for which the evidence is suggestive of a causal relationship, 
rather than studies of health effects for which the evidence supports a 
causal or likely causal relationship, merit no weight at all in setting 
the NAAQS. To place no weight at all on such evidence would in essence 
treat such evidence as though it had been categorized as ``not likely 
to be a causal relationship.'' To do so would ignore the important 
distinctions in the nature of the evidence supporting these different 
causality determinations in the Integrated Science Assessment. It would 
also ignore the CAA requirement that primary standards are to be set to 
provide protection with an adequate margin of safety, including 
providing protection for at-risk populations. Thus, ignoring this 
information in making decisions on the appropriate standard level would 
not be appropriate.\100\ Nonetheless, in considering studies of health 
effects for which the evidence is suggestive of a causal relationship, 
the EPA does believe that it is appropriate to place less weight on 
such studies than on studies of health effects for which there is 
evidence of a causal or likely causal relationship.
---------------------------------------------------------------------------

    \100\ As discussed in section II.A above, the requirement that 
primary standards provide an adequate margin of safety was intended 
to address uncertainties associated with inconclusive scientific and 
technical information available at the time of standard setting. I 
was also intended to provide a reasonable degree of protection 
against hazards that research has not yet identified. This certainly 
encompasses consideration of effects for which there is evidence 
suggestive of a causal relationship.
---------------------------------------------------------------------------

    A second group of commenters supported revising the suite of 
primary PM2.5 standards to provide increased public health 
protection. These commenters found the available scientific information 
and technical analyses to be stronger and more compelling than in the 
last review. These commenters generally placed substantial weight on 
CASAC advice and on the EPA staff analyses presented in the final 
Policy Assessment, which concluded that the evidence most strongly 
supported an annual standard level within a range of 11 to 12 [mu]g/
m\3\ (U.S. EPA, 2011a, p. 2-206). While some of these commenters felt 
that the level should be set within the proposed range (12 to 13 [mu]g/
m\3\), most of these commenters advocated a level of 11 [mu]g/
m\3\.\101\ For example, ALA et al., asserted:
---------------------------------------------------------------------------

    \101\ As discussed in section III.E.4.c.ii, many of these 
commenters also supported lowering the level of the primary 24-hour 
PM2.5 standard.

    The EPA's proposed PM2.5 standards, while a step in 
the right direction are insufficient to protect public health, 
including the health of susceptible populations, with an adequate 
margin of safety as required by the Clean Air Act * * *we will 
discuss the enormous gap in public health protection afforded by an 
annual standard of 13 [micro]g/m\3\, at the upper end of the 
proposed range, compared to the more protective 11 [micro]g/m\3\, as 
---------------------------------------------------------------------------
advocated by our organizations (ALA et al., 2012, p. 6).

    In general, these commenters expressed the view that given the 
strength of the available scientific evidence, the serious nature of 
the health effects associated with PM2.5 exposures, the 
large size of the at-risk populations, the risks associated with long- 
and short-term PM2.5 exposures, and the important 
uncertainties inherently present in the evidence, the EPA should follow 
a highly precautionary policy response by selecting an annual standard 
level that incorporates a large margin of safety.
    More specifically, these commenters offered a range of comments 
related to the general approach used by the EPA to select standard 
levels, including: (1) The EPA's approach for setting a generally 
controlling annual standard; (2) the importance of the greatly expanded 
and stronger overall scientific data base; (3) consideration of the 
distributional statistical analysis conducted by the EPA and other 
approaches for translating the air quality information from specific 
epidemiological studies into standard levels; and (4) the significance 
of the PM2.5-related public health impacts, especially 
potential impacts on at-risk populations, including children, in 
reaching judgments on setting standards that provide protection with an 
adequate margin of safety. These comments are discussed in turn below.
    (1) Some of these commenters disagreed with the EPA's approach for 
setting a ``generally controlling'' annual standard in conjunction with 
a 24-hour standard providing supplemental protection particularly for 
areas with high peak-to-mean ratios. These commenters argued this 
approach would lead to ``regional inequities'' as demonstrated in the 
EPA's analyses contained in Appendix C of the Policy Assessment (ALA et 
al., pp. 26 to 27). Specifically, these commenters argued:

    There is no basis in the Clean Air Act for such a determination. 
The Clean Air Act requires only that the NAAQS achieve public health 
protection with an adequate margin of safety. It is well-documented 
that both long- and short-term exposures to PM2.5 have 
serious and sometimes irreversible health impacts. There is no 
health protection reason to argue that one standard should be 
``controlling'' as a matter of policy without regard to the health 
consequences of such a policy. To adopt such a policy ignores the 
obligation to provide equal protection under

[[Page 3153]]

the law to all Americans because it would result in uneven 
protection from air pollution in different localities and regions of 
the country (ALA et al., 2012, p. 26).

    The EPA believes these commenters misunderstood the basis for the 
EPA's policy goal of setting a ``generally controlling'' annual 
standard. This approach relates exclusively to setting standards that 
will provide requisite protection against effects associated with both 
long- and short-term PM2.5 exposures. It does so by lowering 
the overall air quality distributions across an area, recognizing that 
changes in PM2.5 air quality designed to meet an annual 
standard would likely result not only in lower annual mean 
PM2.5 concentrations but also in fewer and lower peak 24-
hour PM2.5 concentrations. As discussed in section III.A.3 
in the proposal and above, the EPA recognizes that there are various 
ways to combine the two primary PM2.5 standards to achieve 
an appropriate degree of public health protection. Furthermore, the 
extent to which these two standards are interrelated in any given area 
depends in large part on the relative levels of the standards, the 
peak-to-mean ratios that characterize air quality patterns in an area, 
and whether changes in air quality designed to meet a given suite of 
standards are likely to be of a more regional or more localized nature.
    In focusing on an approach of setting a generally controlling 
annual standard, the EPA's intent is in fact to avoid the potential 
``regional inequities'' that are of concern to the commenters. The EPA 
judges that the most appropriate way to set standards that provide more 
consistent public health protection is by using the approach of setting 
a generally controlling annual standard. This judgment builds upon 
information presented in the Policy Assessment as discussed in section 
III.A.3 above. More specifically, the Policy Assessment recognized that 
the short-term exposure studies primarily evaluated daily variations in 
health effects with monitor(s) that measured the variation in daily 
PM2.5 concentrations over the course of several years. The 
strength of the associations observed in these epidemiological studies 
was demonstrably in the numerous ``typical'' days within the air 
quality distribution, not in the peak days (U.S. EPA, 2011a, p. 2-9). 
In addition, the quantitative risk assessments conducted for this and 
previous reviews demonstrated the same point, that is, much, if not 
most, of the aggregate risk associated with short-term PM2.5 
exposures results from the large number of days during which the 24-
hour average concentrations are in the low-to mid-range, below the peak 
24-hour concentrations (U.S. EPA, 2011a, section 2.2.2; U.S. EPA, 
2010a, section 3.1.2.2). In addition, there was no evidence suggesting 
that risks associated with long-term exposures were likely to be 
disproportionately driven by peak 24-hour concentrations.\102\
---------------------------------------------------------------------------

    \102\ In confirmation, a number of studies have presented 
analyses excluding higher PM concentration days and reported a 
limited effect on the magnitude of the effect estimates or 
statistical significance of the association (e.g., Dominici, 2006b; 
Schwartz et al., 1996; Pope and Dockery, 1992).
---------------------------------------------------------------------------

    For these reasons, the Policy Assessment concluded that strategies 
that focused primarily on reducing peak days were less likely to 
achieve reductions in the PM2.5 concentrations that were 
most strongly associated with the observed health effects. Furthermore, 
the Policy Assessment concluded that an approach that focused on 
reducing peak exposures would most likely result in more uneven public 
health protection across the U.S. by either providing inadequate 
protection in some areas or overprotecting in other areas (U.S. EPA, 
2011a, p. 2-9; U.S. EPA, 2010a, section 5.2.3). This is because 
reductions based on control of peak days are less likely to control the 
bulk of the air quality distribution.
    As a result, the EPA believes an approach that focuses on a 
generally controlling annual standard would likely reduce aggregate 
risks associated with both long- and short-term exposures more 
consistently than a generally controlling 24-hour standard and, 
therefore, would be the most effective and efficient way to reduce 
total PM2.5-related population risk. The CASAC agreed with 
this approach and considered it was ``appropriate to return to the 
strategy used in 1997 that considers the annual and the short-term 
standards together, with the annual standard as the controlling 
standard, and the short-term standard supplementing the protection 
afforded by the annual standard'' (Samet, 2010d, p. 1). For the reasons 
discussed above, the EPA disagrees with the comments that this approach 
will result in the concerns raised by the commenters; rather the EPA 
concludes that this approach will help to address these concerns.
    (2) Many of these commenters asserted that the currently available 
scientific information is greatly expanded and stronger compared to the 
last review. Some of these commenters highlighted the availability of 
multiple, multi-city long- and short-term exposure studies providing 
``repeated, consistent evidence of effects below the current annual 
standard level'' (ALA et al., 2012, pp. 39 to 49) and, more 
specifically, ``significant evidence of harm with strong confidence 
well below EPA's proposed annual standard range of 12-13 [mu]g/m\3\'' 
(AHA et al., 2012, pp. 10 to 12).
    The EPA recognizes that in setting standards that are requisite to 
protect public health with an adequate margin of safety, the 
Administrator must weigh the various types of available scientific 
information in reaching public health policy judgments that neither 
overstate nor understate the strength and limitations of this 
information or the appropriate inferences to be drawn from the 
available science.
    In general, the EPA agrees with these commenters' views that the 
currently available scientific evidence is stronger ``because of its 
breadth and the substantiation of previously observed health effects'' 
(77 FR 38906/2) and provides ``greater confidence in the reported 
associations than in the last review'' (77 FR 38940/1). The EPA also 
agrees with the commenters' position that it is appropriate to consider 
the regions within the broader air quality distributions where we have 
the strongest confidence in the associations reported in 
epidemiological studies in setting the level of the annual standard. 
However, as discussed in section III.E.4.d below, in weighing the 
available evidence and technical analyses, as well as the associated 
uncertainties and limitations in that information, the EPA disagrees 
with the commenters' views regarding the extent to which the available 
scientific information provides support for considering an annual 
standard level below the proposed range (i.e., below 12 to 13 [mu]g/
m\3\). In particular, the EPA disagrees with the degree to which these 
commenters place more weight on the relatively more uncertain evidence 
that is suggestive of a causal relationship (e.g., low birth weight). 
Consistent with CASAC advice (Samet, 2010d, p. 1), the Agency concludes 
it is appropriate and reasonable to place the greatest emphasis on 
health effects for which the Integrated Science Assessment concluded 
there is evidence of a causal or likely causal relationship and to 
place less weight on the health effects that provide evidence that is 
only suggestive of a causal relationship.
    (3) With regard to using the air quality information from 
epidemiological studies to inform decisions on standard levels, 
commenters in this group generally supported the EPA's efforts to 
explore different statistical metrics from

[[Page 3154]]

epidemiological studies to inform the Administrator's decisions. These 
commenters argued that by considering different analytic measures--
either concentrations one standard deviation below the long-term means 
reported in the epidemiological studies or the EPA's distributional 
statistical analysis of population-level data that extends the approach 
used in previous PM NAAQS reviews to consider information beyond a 
single statistical metric--``the annual standard must be significantly 
lower than EPA has proposed'' (ALA et al., 2012, pp. 50 to 61). 
Furthermore, with regard to characterizing the PM2.5 air 
quality at which associations have been observed, some of these 
commenters highlighted CASAC's recommendation that ``[f]urther 
consideration should be given to using the 10th percentile as a level 
for assessing various scenarios of levels for the PM NAAQS'' (Samet, 
2010c, p. 11) (ALA et al., 2012, p. 55). Other commenters urged that 
the EPA extend the distributional analysis to include additional 
studies. For example, CHPAC urged the EPA to also conduct 
distributional analysis for children's health studies to better inform 
standards that would protect both children and adults from adverse 
health outcomes (CHPAC, 2012, p. 3).
    The EPA agrees with these commenters' views that it is appropriate 
to take into account different statistical metrics from epidemiological 
studies to inform the decisions on standard levels that are appropriate 
to consider in setting a standard that will protect public health with 
an adequate margin of safety. In the development of the Policy 
Assessment, the EPA staff explored various approaches for using 
information from epidemiological studies in setting the standards. The 
general approach used in the final Policy Assessment, discussed in 
sections III.A.3 and III.E.4.a above, reflects consideration of CASAC 
advice (Samet, 2010c, d) and public comments on multiple drafts of the 
Policy Assessment.
    With regard to using the distributional statistical analysis to 
characterize the confidence in the associations, the EPA emphasizes 
that there is no clear dividing line provided by the scientific 
evidence, and that choosing how far below the long-term mean 
concentrations from the epidemiological studies is appropriate to 
identify a standard level that will provide protection for the public 
health with an adequate margin of safety is largely a public health 
policy judgment. The EPA considers the region from approximately the 
25th to 10th percentiles to be a reasonable range for providing a 
general frame of reference as to the part of the distribution over 
which our confidence in the magnitude and significance of the 
associations observed in epidemiological studies is appreciably lower. 
Based on these considerations, the EPA concludes that it is not 
appropriate to place as much confidence in the magnitude and 
significance of the associations over the lower percentiles of the 
distributions in each study as at and around the long-term mean 
concentrations. Thus, the EPA disagrees with the commenters' views that 
this analysis compels placing more emphasis on the lower part of this 
range in selecting a level for an annual standard that will protect 
public health with an adequate margin of safety. The EPA recognizes 
that this information comes primarily from two short-term exposure 
studies, a relatively modest data set. In light of the limited nature 
of this information, and in recognition of more general uncertainties 
inherent in the epidemiological evidence, the Administrator deems it 
reasonable not to place more emphasis on concentrations in the lower 
part of this range, as discussed below in section III.E.4.d.
    With regard to the scope of the distributional statistical 
analysis, the EPA requested additional population-level data from the 
study authors for a group of six multi-city studies for which previous 
air quality analyses had been conducted (Hassett-Sipple et al., 2010; 
Schmidt et al., 2010, Analysis 2). These six studies were originally 
selected because they considered multiple locations representing 
varying geographic regions across multiple years. Thus, these studies 
provided evidence on the influence of different particle mixtures on 
health effects associated with long- and short-term PM2.5 
exposures. In addition, these multi-city studies considered relatively 
more recent health events and air quality conditions (1999 to 2005). As 
discussed in section III.E.4.b.i above, the EPA received and analyzed 
population-level data for four of the six studies (Rajan et al., 2011). 
Three of these four studies (Krewski et al., 2009; Bell et al., 2008; 
Zanobetti and Schwartz, 2009) served as the basis for the 
concentration-response functions used to develop the core risk 
estimates (U.S. EPA, 2010a, section 3.3.3). While, the EPA agrees that 
it would be useful to have such data from more studies, the Agency 
believes that the additional data that was requested and received from 
study authors provide useful information to help inform the 
Administrator's selection of the annual standard level.
    (4) Many commenters in this group highlighted PM2.5-
related impacts on at-risk populations, including potential impacts on 
children, older adults, persons with pre-existing heart and lung 
disease, and low-income populations, to support their views that the 
annual standard should be revised to a level of 11 [mu]g/m\3\ or lower 
(CHPAC, 2012; AHA et al., 2012; ALA, 2012, pp. 29 to 38; Rom et al., 
2012; Air Alliance Houston, et al., 2012). These commenters urged the 
EPA to adopt a policy approach that placed less weight on the remaining 
uncertainties and limitations in the evidence and placed more emphasis 
on margin of safety considerations, including providing protection 
against effects for which there is more limited scientific evidence. 
For example, CHPAC urged the EPA ``to place the same weight on studies 
examining impacts on children's health as that of adult studies. * * * 
The fact that there may be stronger evidence from adult studies does 
not mean that standards based on adult studies will be protective for 
children and consequently will meet the standard requisite to protect 
public health with an adequate margin of safety'' (CHPAC, 2012 p. 3). 
Furthermore, with regard to the EPA's approach for weighing 
uncertainties, some of these commenters stated that ``we find no 
justification in the preamble for an annual standard level as high as 
13 [mu]g/m\3\, other than the vague assertion that uncertainties 
increase at lower concentrations. Further, the final proposal 
completely failed to address the Policy Assessment recommendations that 
if 13 [mu]g/m\3\ was proposed, the 24-hour standard should be 
strengthened as well'' (ALA et al., p. 7).
    The EPA has carefully evaluated and considered evidence of effects 
in at-risk populations. With regard to effects classified as having 
evidence of a causal or likely causal relationship with long- or short-
term PM2.5 exposures (i.e., premature mortality, 
cardiovascular effects, and respiratory effects), the Agency takes note 
that it considered the full range of studies evaluating these effects, 
including studies of at-risk populations, to inform its review of the 
primary PM2.5 standards. Specific multi-city studies 
summarized in Figures 1, 2, and 3 above highlight evidence of effects 
observed in two different lifestages--children and older adults--that 
have been identified as at-risk populations. Thus, the EPA places as 
much weight on studies that explored effects in children for which the 
evidence is causal or likely causal in

[[Page 3155]]

nature as on studies of such effects in adults, including older adults. 
As discussed above in responses to commenters supporting the retention 
of the current standards, in setting the standard, the EPA has focused 
on considering PM2.5 concentrations somewhat below the 
lowest long-term mean concentrations from each of the key studies of 
both long- and short-term exposures of effects for which the evidence 
supports a causal or likely causal relationship (i.e., the first two 
sets of studies shown in Figure 4). Absent some reason to ignore or 
discount these studies, which the commenter does not provide (and of 
which the EPA is unaware), the EPA considers the available evidence of 
effects in children as well as other at-risk populations.
    With respect to the EPA's consideration of more limited studies 
providing evidence suggestive of a causal relationship (e.g., 
developmental and reproductive effects), as noted above in responding 
to comments from the first group of commenters, the Agency agrees that 
it is important to place some weight on this body of evidence in 
setting standards that provide protection for at-risk populations, as 
required by the CAA. However, the Agency does not agree that the same 
weight must be placed on this information as on the body of scientific 
information for which there is evidence of a causal or likely causal 
relationship. To do so would ignore the difference in the breadth and 
strength of the evidence supporting the different causality 
determinations reached in the Integrated Science Assessment.
    With regard to weighing the uncertainties and limitations remaining 
in the evidence and technical analyses, as discussed in section II.A 
above, the EPA recognizes that in setting a primary NAAQS that provides 
an adequate margin of safety, the Administrator must consider a number 
of factors including the nature and severity of the health effects 
involved, the size of sensitive population(s) at risk, and the kind and 
degree of the uncertainties that remain. As discussed in section 
III.E.4.d below, the Agency agrees with these commenters that, in 
weighing the available evidence and technical analyses including the 
uncertainties and limitations in this scientific information, there is 
no justification for setting a primary PM2.5 annual standard 
level as high as 13 [mu]g/m\3\.
    Finally, some commenters in both groups also identified ``new'' 
studies that were not included in the Integrated Science Assessment as 
providing further support for their views on the level of the annual 
standard. As discussed in section II.B.3 above, the EPA completed a 
provisional review and assessment of ``new'' studies published since 
the close of the Integrated Science Assessment, including ``new'' 
studies submitted by commenters (U.S. EPA, 2012b). The provisional 
assessment found that the ``new'' studies expand the scientific 
information considered in the Integrated Science Assessment and provide 
important insights on the relationship between PM2.5 
exposure and health effects of PM (U.S. EPA, 2012b). However, the EPA 
notes that the provisional assessment found that the ``new'' science 
did not materially change the conclusions reached in the Integrated 
Science Assessment. The 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 Integrated Science Assessment that 
have undergone CASAC and public review, and will consider newly 
published studies for purposes of decision making in the next PM NAAQS 
review.
ii. 24-Hour Standard Level
    With respect to the level of the 24-hour standard, the EPA received 
comments on the proposal from two distinct groups of commenters. One 
group that included virtually all commenters representing industry 
associations, businesses, and many States agreed with the Agency's 
proposed decision to retain the level of the 24-hour PM2.5 
standard. The other group of commenters included many medical groups, 
numerous physicians and academic researchers, many public health 
organizations, some State and local agencies, five State Attorneys 
General, and a large number of individual commenters. These commenters 
disagreed with the Agency's proposed decision and argued that EPA 
should lower the level of the 24-hour standard to 30 or 25 [mu]g/m\3\. 
Comments from these groups on the level of the 24-hour PM2.5 
standard are addressed below and in the Response to Comments Document.
    As noted above, of the public commenters who addressed the level of 
the 24-hour PM2.5 standard, all industry commenters and most 
State and local commenters supported the proposed decision to retain 
the current level of 35 [mu]g/m\3\. In many cases, these groups agreed 
with the rationale supporting the Administrator's proposed decision to 
retain the current 24-hour PM2.5 standard, including her 
emphasis on the annual standard as the generally controlling standard 
with the 24-hour standard providing supplementary protection, and her 
conclusion that multi-city, short-term exposure studies provide the 
strongest data set for informing decisions on the appropriate 24-hour 
standard level. Many of these commenters agreed with the 
Administrator's view that the single-city, short-term studies provided 
a much more limited data set (e.g., limited statistical power, limited 
exposure data) and more equivocal results (e.g., mixed results within 
the same study area), making them an unsuitable basis for setting the 
level of the 24-hour standard.
    While these commenters agreed with the EPA's proposed decision to 
retain the current 24-hour PM2.5 standard, some did not 
agree with the EPA's approach to considering the evidence from short-
term multi-city studies. For example, a commenter representing UARG 
pointed out that the 98th percentile concentrations reported in the 
proposal for multi-city studies reflect the averages of 98th percentile 
concentrations across the cities included in those studies (UARG, 2012; 
Attachment 1; p. 25). This commenter contended that such averaged 98th 
percentile PM2.5 concentrations do not provide information 
that can appropriately inform a decision on the adequacy of the public 
health protection provided by the current or alternative 24-hour 
standards.
    While the EPA agrees that there is uncertainty in linking effects 
reported in multi-city studies to specific air quality concentrations 
(U.S. EPA, 2011a, section 2.3.4.1), the EPA disagrees with this 
commenter's view that such uncertainty precludes the use of averaged 
98th percentile PM2.5 concentrations to inform a decision on 
the appropriateness of the protection provided by the 24-hour 
PM2.5 standard. In particular, the EPA notes that averaged 
98th percentile concentrations do provide information on the extent to 
which study cities contributing to reported associations would likely 
have met or violated the current 24-hour PM2.5 standard 
during the study period. As evidence of this, the EPA notes the three 
multi-city studies specifically highlighted by this commenter as having 
averaged 98th percentile 24-hour PM2.5 concentrations below 
35 [mu]g/m\3\ (Dominici et al., 2006a; Bell et al., 2008; Zanobetti and 
Schwartz, 2009). Based on the 98th percentiles of 24-hour 
PM2.5 concentrations in the individual cities evaluated in 
these studies, the EPA notes that the majority of these study cities 
would likely have met the current standard during the study periods 
(Hassett-Sipple et al., 2010). Therefore, regardless of whether the 
averaged 98th percentile concentrations or the 98th

[[Page 3156]]

percentile concentrations in each city are considered, these studies 
provide evidence for associations between short-term PM2.5 
and mortality or morbidity across a large number of U.S. cities, the 
majority of which would likely have met the current 24-hour 
PM2.5 standard during study periods. In their review of the 
PM Policy Assessment, CASAC endorsed the conclusions drawn from 
analyses of averaged 98th percentile 24-hour PM2.5 
concentrations, and the EPA continues to conclude that this type of 
information can appropriately inform the Administrator's decision on 
the current 24-hour PM2.5 standard.\103\
---------------------------------------------------------------------------

    \103\ This is not to say that the EPA's decision on whether to 
revise the 24-hour PM2.5 standard should be based on or 
only be informed by considerations of whether studies reported 
associations with mortality or morbidity in areas with averaged 98th 
percentile PM2.5 concentrations less than 35 mg/m\3\. As 
discussed below, in reaching a decision in this final notice on the 
most appropriate approach to strengthen the suite of 
PM2.5 standards, the Administrator considers the degree 
of public health protection provided by the combination of the 
annual and 24-hour standards together.
---------------------------------------------------------------------------

    Another group of commenters argued that the 24-hour standard level 
should be lowered. Many of these commenters supported setting the level 
of the 24-hour PM2.5 standard at either 25 or 30 [mu]g/m\3\. 
In support of their position, the ALA et. al., AHA et al., five state 
Attorneys General, and a number of additional groups pointed to 98th 
percentile PM2.5 concentrations in locations of multi-city 
and single-city epidemiological studies. For example, the ALA and 
others pointed to multi-city studies by Dominici et al. (2006a), 
Zanobetti and Schwartz (2009), Burnett et al. (2000), and Bell et al. 
(2008) as providing evidence for associations with mortality and 
morbidity in study locations with averaged (i.e., averaged across 
cities) 98th percentile 24-hour PM2.5 concentrations below 
35 [mu]g/m\3\. These commenters also pointed to several single-city and 
panel studies reporting associations between short-term 
PM2.5 and mortality or morbidity in locations with 
relatively low 24-hour PM2.5 concentrations. Because some of 
these multi- and single-city studies have reported associations with 
health effects in locations with 98th percentile PM2.5 
concentrations below 35 [mu]g/m\3\, commenters maintained that the 
current 24-hour PM2.5 standard (i.e., with its level of 35 
[mu]g/m\3\) does not provide an appropriate degree of protection in all 
areas.
    In further support of their position that the level of the current 
24-hour standard should be lowered, these commenters pointed out the 
variability across the U.S. in ratios of 24-hour to annual 
PM2.5 concentrations. They noted that some locations, 
including parts of the northwestern U.S., experience relatively low 
annual PM2.5 concentrations but can experience relatively 
high 24-hour concentrations at certain times of the year. In order to 
provide protection against effects associated with short-term 
PM2.5 exposures, especially in locations with high ratios of 
24-hour to annual PM2.5 concentrations, these commenters 
advocated setting a lower level for the 24-hour standard.
    The EPA agrees with these commenters that it is appropriate to 
maintain a 24-hour PM2.5 standard in order to supplement the 
protection provided by the revised annual standard, particularly in 
locations with relatively high ratios of 24-hour to annual 
PM2.5 concentrations. However, in highlighting 98th 
percentile PM2.5 concentrations in study locations without 
also considering the impact of a revised annual standard on short-term 
concentrations, these commenters ignore the fact that many areas would 
be expected to experience decreasing short- and long-term 
PM2.5 concentrations in response to a revised annual 
standard.
    In considering the specific multi-city studies highlighted by 
public commenters who advocated a more stringent 24-hour standard, the 
EPA notes that such studies have reported consistently positive and 
statistically significant associations with short-term PM2.5 
exposures in locations with averaged 98th percentile PM2.5 
concentrations ranging from 45.8 to 34.2 [mu]g/m\3\ and long-term mean 
PM2.5 concentrations ranging from 13.4 to 12.9 (Burnett and 
Goldberg, 2003; Burnett et al., 2004; Dominici et al., 2006a; Bell et 
al., 2008; Franklin et al., 2008; Zanobetti and Schwartz, 2009).\104\ 
The EPA notes that to the extent air quality distributions are reduced 
to meet the current 24-hour standard with its level of 35 [mu]g/m\3\ 
and/or the revised annual standard with its level of 12 [mu]g/m\3\, 
additional protection would be anticipated against the effects reported 
in these short-term, multi-city studies. Put another way, to attain an 
annual standard with a level below the long-term means in the locations 
of these short-term studies (as EPA is adopting here), the overall air 
quality distributions in the majority of study cities will necessarily 
be reduced, resulting in lower daily PM2.5 ambient 
concentrations. We therefore expect that the revised annual standard 
will result in 98th percentile PM2.5 concentrations in these 
cities that are lower than those measured in the studies, and that the 
overall distributions of PM2.5 concentrations will be lower 
than those reported to be associated with health effects. Thus, even 
for effects reported in multi-city studies with averaged 98th 
percentile concentrations below 35 [mu]g/m\3\, additional protection 
from the risks associated with short-term exposures is anticipated from 
the revised annual standard, without revising the 24-hour standard, 
because long-term average PM2.5 concentrations in multi-city 
study locations were above the level of the revised annual standard 
(i.e., 12 [mu]g/m\3\).\105\ As discussed above, reducing the annual 
standard is the most efficient way to reduce the risks from short-term 
exposures identified in these studies, as the bulk of the risk comes 
from the large number of days across the bulk of the air quality 
distribution, not the relatively small number of days with peak 
concentrations.
---------------------------------------------------------------------------

    \104\ Commenters also highlighted associations with short-term 
PM2.5 concentrations reported in sub-analyses restricted 
to days with 24-hour concentrations at or below 35 [mu]g/m\3\ 
(Dominici, 2006b). These sub-analyses were not included in the 
original publication by Dominici et al. (2006a). Authors provided 
results of sub-analyses for the Administrator's consideration in a 
letter to the docket following publication of the proposed rule in 
January 2006 (personal communication with Dr. Francesca Dominici, 
2006b). As noted in section III.A.3, these sub-analyses are part of 
the basis for the conclusion that there is no evidence suggesting 
that risks associated with long-term exposures are likely to be 
disproportionately driven by peak 24-hour concentrations. Because 
the sub-analyses did not present long-term average PM2.5 
concentrations, it is not clear whether they reflected 
PM2.5 air quality that would have been allowed by the 
revised annual PM2.5 standard being established in this 
rule.
    \105\ It is also the case that additional protection is 
anticipated in locations with 98th percentile 24-hour 
PM2.5 concentrations above 35 [mu]g/m\3\, even if long-
term concentrations are below 12 [mu]g/m\3\. As noted in the 
proposal and in the Policy Assessment (U.S. EPA, 2011a, Figure 2-
10), parts of the northwestern U.S. are more likely than other parts 
of the country to violate the 24-hour standard and meet the revised 
annual standard.
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    In considering the single-city studies highlighted by public 
commenters who advocated a more stringent 24-hour standard, the EPA 
first notes that, overall, these single-city studies reported mixed 
results. Specifically, some studies reported positive and statistically 
significant associations with PM2.5, some studies reported 
positive but non-significant associations, and several studies reported 
negative associations or a mix of positive and negative associations 
with PM2.5. In light of these inconsistent results, the 
proposal noted that the overall body of evidence from single-city 
studies is mixed, particularly in locations with 98th percentiles of 
24-hour concentrations below 35 [mu]g/m\3\. Therefore, although some 
single-city

[[Page 3157]]

studies reported effects at appreciably lower PM2.5 
concentrations than short-term multi-city studies, the uncertainties 
and limitations associated with the single-city studies were noted to 
be greater. In light of these greater uncertainties and limitations, 
the Administrator concluded in the proposal that she had less 
confidence in using these studies as a basis for setting the level of 
the standard (77 FR 38943).
    Given the considerations and conclusions noted above, in the 
proposal the Administrator concluded that the short-term multi-city 
studies provide the strongest evidence to inform decisions on the level 
of the 24-hour standard. Further, she viewed single-city, short-term 
exposure studies as a much more limited data set providing mixed 
results, and she had less confidence in using these studies as a basis 
for setting the level of a 24-hour standard (77 FR 38942). In 
highlighting specific single-city studies, public health, 
environmental, and State and local commenters appear to have 
selectively focused on studies reporting associations with 
PM2.5 and to have overlooked studies that reported more 
equivocal results (e.g., Ostro et al., 2003; Rabinovitch et al., 2004; 
Slaughter et al., 2005; Villeneuve et al., 2006) (U.S. EPA, 2011, 
Figure 2-9). As such, these commenters have not presented new 
information that causes the EPA to reconsider its decision to emphasize 
multi-city studies over single-city studies when identifying the 
appropriate level of the 24-hour PM2.5 standard.
    In further considering the single-city studies highlighted by 
public commenters, the EPA notes that some commenters advocating for a 
lower level for the 24-hour PM2.5 standard also discussed 
short-term studies that have been published since the close of the 
Integrated Science Assessment. These recent studies were conducted in 
single cities or in small panels of volunteers. As in prior NAAQS 
reviews and as discussed above in more detail (section II.B.3), the EPA 
is basing its decisions in this review on studies and related 
information assessed in the Integrated Science Assessment. The studies 
assessed in the Integrated Science Assessment, and the conclusions 
based on those studies, have undergone extensive critical review by the 
EPA, CASAC, and the public. The rigor of that review makes the studies 
assessed in the Integrated Science Assessment, and the conclusions 
based on those studies, the most reliable source of scientific 
information on which to base decisions on the NAAQS.
    However, as discussed above (section II.B.3), the EPA recognizes 
that ``new studies'' may sometimes be of such significance that it is 
appropriate to delay a decision on revision of a NAAQS and to 
supplement the pertinent air quality criteria so the studies can be 
taken into account. In the present case, the EPA's provisional 
consideration of ``new studies'' concludes that, taken in context, the 
``new'' information and findings do not materially change any of the 
broad scientific conclusions made in the air quality criteria regarding 
the health effects of PM2.5 (U.S. EPA, 2012b).
    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 completing this review. Accordingly, 
the EPA is basing its final decisions in this review on the studies and 
related information included in the PM Integrated Science Assessment 
(i.e., the air quality criteria) that has undergone CASAC and public 
review. The EPA will consider the ``new studies'' in the next periodic 
review of the PM NAAQS, which will provide an opportunity to fully 
assess these studies through a more rigorous review process involving 
the EPA, CASAC, and the public.
    Some public health, medical, and environmental commenters also 
criticized the EPA's interpretation of PM2.5 risk results. 
These commenters presented risk estimates for combinations of annual 
and 24-hour standards using more recent air quality data than that used 
in the EPA's Risk Assessment (U.S. EPA, 2010a). Based on these 
additional risk analyses, the ALA and other commenters contended that 
public health benefits could continue to increase as annual and 24-hour 
standard levels decrease below 13 [mu]g/m\3\ and 35 [mu]g/m\3\, 
respectively.
    The EPA agrees with commenters that important public health 
benefits are expected as a result of revising the level of the annual 
standard to 12 [mu]g/m\3\, as is done in this rule, rather than 13 
[mu]g/m\3\. The Agency also acknowledges that estimated 
PM2.5-associated health risks continue to decrease with 
annual standard levels below 12 [mu]g/m\3\ and/or with 24-hour standard 
levels below 35 [mu]g/m\3\. However, the EPA disagrees with the 
commenters' views regarding the extent to which risk estimates support 
setting standard levels below 12 [mu]g/m\3\ (annual standard) and 35 
[mu]g/m\3\ (24-hour standard).\106\
---------------------------------------------------------------------------

    \106\ This section focuses on the 24-hour standard. Section 
III.E.4.c.i above also discusses these commenters' recommendations 
within the context of the annual PM2.5 standard.
---------------------------------------------------------------------------

    The CAA charges the Administrator with setting NAAQS that are 
``requisite'' (i.e., neither more nor less stringent than necessary) to 
protect public health with an adequate margin of safety. In setting 
such standards the Administrator must weigh the available scientific 
evidence and information, including associated uncertainties and 
limitations. As described above, in reaching her proposed decisions on 
the PM2.5 standards that would provide ``requisite'' 
protection, the Administrator carefully considered the available 
scientific evidence and risk information, making public health policy 
judgments that, in her view, neither overstated nor understated the 
strengths and limitations of that evidence and information. In 
contrast, as discussed more fully above, public health, medical, and 
environmental commenters who recommended levels below 35 [mu]g/m\3\ for 
the 24-hour PM2.5 standard have not provided new information 
or analyses to suggest that such standard levels are appropriate, given 
the uncertainties and limitations in the available health evidence, 
particularly uncertainties in studies conducted in locations with 98th 
percentile 24-hour PM2.5 concentrations below 35 [mu]g/m\3\ 
and long-term average concentrations below 12 [mu]g/m\3\.
d. Administrator's Final Conclusions on the Primary PM2.5 
Standard Levels
    In reaching her conclusions regarding appropriate standard levels, 
the Administrator has considered the epidemiological and other 
scientific evidence, estimates of risk reductions associated with just 
meeting alternative annual and/or 24-hour standards, air quality 
analyses, related limitations and uncertainties, the advice of CASAC, 
and extensive public comments on the proposal. After careful 
consideration of all of these, the Administrator has decided to revise 
the level of the primary annual PM2.5 standard from 15.0 
[mu]g/m\3\ to 12.0 [mu]g/m\3\ and to retain the level of the primary 
24-hour standard at 35 [mu]g/m\3\.
    As an initial matter, the Administrator agrees with the approach 
supported by CASAC and discussed in the Policy Assessment as summarized 
in sections III.A.3 and III.E.4.a above, of considering the annual and 
24-hour standards together in determining the protection afforded 
against mortality and morbidity effects associated with both long- and 
short-term exposures to PM2.5. This approach is consistent 
with the approach taken in the review

[[Page 3158]]

completed in 1997, in contrast to the approach used in the review 
completed in 2006 where each standard was considered independently of 
the other (i.e., only data from long-term exposure studies were used to 
inform the level of the annual standard and only data from short-term 
exposure studies were used to inform the level of the 24-hour 
standard).\107\
---------------------------------------------------------------------------

    \107\ See 71 FR 61148 and 61168, October 17, 2006.
---------------------------------------------------------------------------

    Based on the evidence and quantitative risk assessment, the 
Administrator concludes that it is appropriate to set an annual 
standard that is generally controlling, which will lower the broad 
distribution of 24-hour average concentrations in an area as well as 
the annual average concentration, so as to provide protection from both 
long- and short-term PM2.5 exposures. In conjunction with 
this, it is appropriate to set a 24-hour standard focused on providing 
supplemental protection, particularly for areas with high peak-to-mean 
ratios of 24-hour concentrations, possibly associated with strong local 
or seasonal sources, and for PM2.5-related effects that may 
be associated with shorter-than daily exposure periods. The 
Administrator concludes this approach will reduce aggregate risks 
associated with both long- and short-term exposures more consistently 
than a generally controlling 24-hour standard and is the most effective 
and efficient way to reduce total PM2.5-related population 
risk and to protect public health with an adequate margin of safety.
    In selecting the level of the annual PM2.5 standard, 
based on the characterization and assessment of the epidemiological and 
other studies presented and assessed in the Integrated Science 
Assessment and the Policy Assessment, the Administrator recognizes the 
substantial increase in the number and diversity of studies available 
in this review. This expanded body of evidence includes extended 
analyses of the seminal studies of long-term PM2.5 exposures 
(i.e., ACS and Harvard Six Cities studies) as well as important new 
long-term exposure studies (as summarized in Figures 1 and 2). 
Collectively, the Administrator notes that these studies, along with 
evidence available in the last review, provide consistent and stronger 
evidence than previously observed of an association between long-term 
PM2.5 exposures and premature mortality in areas with lower 
long-term ambient concentrations than previously observed, with the 
strongest evidence related to cardiovascular-related mortality. The 
Administrator also recognizes the availability of stronger evidence of 
morbidity effects associated with long-term PM2.5 exposures, 
including evidence of respiratory effects such as decreased lung 
function growth, from the extended analyses for the Southern California 
Children's Health Study and evidence of cardiovascular effects from the 
WHI study. Furthermore, the Administrator recognizes new U.S. multi-
city studies that greatly expand and reinforce our understanding of 
mortality and morbidity effects associated with short-term 
PM2.5 exposures, providing stronger evidence of associations 
in areas with ambient concentrations similar to those previously 
observed in short-term exposure studies considered in the previous 
review (as summarized in Figure 3).
    The Administrator recognizes the strength of the scientific 
evidence for evaluating health effects associated with fine particles, 
noting that the newly available scientific evidence builds upon the 
previous scientific data base to provide evidence of generally robust 
associations and a basis for greater confidence in the reported 
associations than in the last review. She notes the conclusion of the 
Integrated Science Assessment that this body of evidence supports a 
causal relationship between long- and short-term PM2.5 
exposures and mortality and cardiovascular effects and a likely causal 
relationship between long- and short-term PM2.5 exposures 
and respiratory effects. In addition, the Administrator notes 
additional, but more limited evidence, for a broader range of health 
endpoints including evidence suggestive of a causal relationship for 
developmental and reproductive effects as well as for carcinogenic 
effects.
    Based on information discussed and presented in the Integrated 
Science Assessment, the Administrator recognizes that health effects 
may occur over the full range of concentrations observed in the 
epidemiological studies of both long-term and short-term exposures, 
since no discernible population-level threshold for any such effects 
can be identified based on the currently available evidence (U.S. EPA, 
2009a, section 2.4.3). To inform her decisions on an appropriate level 
for the annual standard that will protect public health with an 
adequate margin of safety, in the absence of any discernible 
population-level thresholds, the Administrator judges that it is 
appropriate to consider the relative degree of confidence in the 
magnitude and significance of the associations observed in 
epidemiological studies across the range of long-term PM2.5 
concentrations in such studies. Further, she recognizes, in taking note 
of CASAC advice and the distributional statistics analysis discussed in 
the Policy Assessment and in section III.E.4.a above, that there is 
significantly greater confidence in the magnitude and significance of 
observed associations for the part of the air quality distribution 
corresponding to where the bulk of the health events evaluated in each 
study have been observed, generally at and around the long-term mean 
concentrations. Conversely, she also recognizes that there is 
significantly diminished confidence in the magnitude and significance 
of observed associations in the lower part of the air quality 
distribution corresponding to where a relatively small proportion of 
the health events were observed. Further, the Administrator recognizes 
that the long-term mean concentrations, or any other specific point in 
the air quality distribution of each study, do not represent a ``bright 
line'' at and above which effects have been observed and below which 
effects have not been observed.
    In considering the long-term mean concentrations reported in 
epidemiological studies, the Administrator recognizes that in selecting 
a level of the annual standard that will protect public health with an 
adequate margin of safety, it is not sufficient to focus on a 
concentration generally somewhere within the range of long-term mean 
concentrations from the key long-term and short-term exposure studies 
that reported lower concentrations than had been observed in earlier 
reviews. These key studies provide information for various types of 
serious health endpoints (including mortality and morbidity effects), 
different study populations (which may include at-risk populations such 
as children and older adults), and different air quality distributions 
that are specific to each study. A level somewhere within the range of 
long-term mean concentrations of the full set of key studies would be 
higher than the long-term mean of at least one of the studies being 
considered and therefore would not provide a sufficient degree of 
protection against the health effects observed in that study. Absent 
some reasoned basis to place less weight on the evidence in the 
epidemiological study with the lowest long-term mean concentration 
among these key studies, this approach would not be consistent with the 
requirement to set a standard that will protect public health with an

[[Page 3159]]

adequate margin of safety.\108\ Thus, the Administrator recognizes it 
is important to protect against the serious effects observed in each of 
these studies so as to protect public health with an adequate margin of 
safety. In so doing, she looks to identify the study with the lowest 
long-term mean concentration within the full set of key studies to help 
inform her decision of the appropriate standard level which will 
provide protection for the broad array of health outcomes observed in 
all of the studies, including effects observed in at-risk populations.
---------------------------------------------------------------------------

    \108\ See American Farm Bureau Federation v. EPA, 559 F. 3d 512, 
525-26 (D.C. Cir. 2009).
---------------------------------------------------------------------------

    Further, consistent with the general approach summarized in section 
III.E.4.a above and supported by CASAC as discussed in section 
III.E.4.b.ii above, the Administrator recognizes that it is appropriate 
to consider a level for an annual standard that is not just at but 
rather is somewhat below the long-term mean PM2.5 
concentrations reported in each of the key long- and short-term 
exposure studies. In so doing, she focuses especially on multi-city 
studies that evaluated health endpoints for which the associations are 
causal or likely causal (i.e., mortality and cardiovascular and 
respiratory effects associated with both long- and short-term 
PM2.5 exposures). As discussed above, the importance of 
considering a level somewhat below the lowest long-term mean 
concentrations in this set of key studies is to establish a standard 
that would be protective against the observed effects in all of the 
studies, and that takes into account the relative degree of confidence 
in the magnitude and significance of observed associations across the 
air quality distributions in these studies.
    The Administrator recognizes that there is no clear way to identify 
how much below the long-term mean concentrations of key studies to set 
a standard that would provide requisite protection with an adequate 
margin of safety. She therefore must use her judgment to weigh the 
available scientific and technical information, and associated 
uncertainties, to reach a final decision on the appropriate standard 
level. In considering the information in Figures 1-4 for effects 
classified as having evidence of a causal or likely causal relationship 
with long- or short-term PM2.5 exposures, she observes a 
cluster of short-term exposure studies with long-term mean 
concentrations within a range of 13.4 [mu]g/m\3\ down to 12.8 [mu]g/
m\3\ (Dominici et al., 2006a; Burnett and Goldberg, 2003; Zanobetti and 
Schwartz, 2009; Bell et al., 2008; Burnett et al., 2004). She also 
observes a cluster of long-term exposure studies with long-term mean 
concentrations within a range of 14.5 [mu]g/m\3\ to 13.6 [mu]g/m\3\ 
(Dockery et al., 1996; Lipfert et al., 2006a; Zeger et al., 2008; 
McConnell et al., 2003; Goss et al., 2004; Eftim et al., 2008). For the 
reasons discussed in response to public comments in section III.E.4.c 
above, the Administrator is less influenced by the long-term mean 
PM2.5 concentrations from the Miller et al. (2007) and 
Krewski et al. (2009) studies with reported long-term mean 
PM2.5 concentrations of 12.9 and 14.0 [mu]g/m\3\, 
respectively. In each case, the most relevant exposure periods would 
likely have had higher mean PM2.5 concentrations than those 
reported in the studies.\109\ Thus, the Administrator considers the 
long-term mean PM2.5 concentrations from these two studies 
to be a highly uncertain basis for informing her selection of the 
annual standard level.\110\
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    \109\ In the case of Miller et al. (2007), the mean 
concentration is based on a single year of air quality data which 
post-dated by two years the period for which the health events data 
were collected. In the case of Krewski et al. (2009), the air 
quality data were based on the last two years of the 18-year period 
for which the health event data were collected.
    \110\ Nonetheless, as noted above, the EPA notes that the 
Krewski et al. (2009) and Miller et al. (2007) studies provide 
strong evidence of mortality and cardiovascular-related effects 
associated with long-term PM2.5 exposures to inform 
causality determinations reached in the Integrated Science 
Assessment (U.S. EPA, 2009a, sections 7.2.11 and 7.6).
---------------------------------------------------------------------------

    To help guide her judgment of the appropriate level below the long-
term mean concentrations in the epidemiological studies at which to set 
the standard, the Administrator considered additional information from 
epidemiological studies concerning the broader distribution of 
PM2.5 concentrations which correspond to the health events 
observed in these studies (e.g., deaths, hospitalizations). The 
Administrator observes that the development and use of this information 
in considering standard levels is consistent with CASAC's advice, as 
discussed in section III.E.4.b.ii above, to focus on understanding the 
concentrations that were most influential in generating the health 
effect estimates in individual studies (Samet, 2010d, p. 2).
    In considering this additional population-level information, the 
Administrator recognizes that, in general, the confidence in the 
magnitude and significance of an association identified in a study is 
strongest at and around the long-term mean concentration for the air 
quality distribution, as this represents the part of the distribution 
in which the data in any given study are generally most concentrated. 
She also recognizes that the degree of confidence decreases as one 
moves towards the lower part of the distribution. Consistent with the 
approach used in the Policy Assessment, the Administrator believes that 
the range from approximately the 25th to 10th percentiles is a 
reasonable range for providing a general frame of reference as to the 
part of the distribution in which her confidence in the associations 
observed in epidemiological studies is appreciably lower. However, as 
noted above, it is important to emphasize that there is no clear 
dividing line or single percentile within a given distribution provided 
by the scientific evidence that is most appropriate or `correct' to use 
to characterize where the degree of confidence in the associations 
warrants setting the annual standard level. The decision of the 
appropriate standard level below the long-term mean concentrations of 
the key studies, which in conjunction with the other elements of the 
standard would protect public health with an adequate margin of safety, 
is largely a public health policy judgment, taking into account all of 
the evidence and its related uncertainties.
    As discussed in section III.E.4.b, the Administrator takes note of 
additional population-level data that were made available to the EPA by 
study authors.\111\ In considering this information, the Administrator 
particularly focuses on the analysis of the distributions of the health 
event data for each area within these studies and the corresponding air 
quality data for the two short-term exposure studies (Zanobetti and 
Schwartz, 2009; Bell et al., 2008). These short-term exposure studies 
evaluate the relationship between daily changes (one or more days) in 
PM2.5 concentrations and daily changes in health events 
(e.g., deaths, hospitalizations), such that the air quality 
concentrations that comprise the most relevant exposure periods in 
these

[[Page 3160]]

studies are contemporaneous with the health event data. In addition, 
these studies considered more recent air quality data, representing 
generally lower PM2.5 concentrations, in a large number of 
study areas across the U.S. Thus, such studies provide the most useful 
evidence for an analysis evaluating the distribution of health event 
data and the corresponding long-term mean PM2.5 
concentrations across the areas included in each multi-city study.
---------------------------------------------------------------------------

    \111\ As summarized in section III.E.4.a, population-level data 
were provided to the EPA for four studies. These four studies 
represent some of the strongest evidence showing associations 
between health effects and PM2.5 within the overall body 
of scientific evidence and include three studies (Krewski et al., 
2009; Bell et al., 2008; and Zanobetti and Schwartz, 2009) that were 
used as the basis for concentration-response functions in the 
quantitative risk assessment (U.S. EPA, 2010a, section 3.3.3). The 
Administrator recognizes that the additional population-level data 
available for these four multi-city studies represents a more 
limited data set compared to the set of long-term mean 
PM2.5 concentrations which were available in the 
published literature for all studies considered in the Integrated 
Science Assessment.
---------------------------------------------------------------------------

    The Administrator also considered the additional population-level 
data that were made available to EPA for two long-term exposure studies 
(Krewski et al., 2009; Miller et al., 2007). She recognizes that in 
long-term exposure studies investigators follow a specific group of 
study participants (i.e., cohort) over time and across urban study 
areas, and evaluate how PM2.5 concentrations averaged over a 
period of years are associated with specific health endpoints (e.g., 
deaths) across cities. As discussed in response to public comments in 
section III.E.4.c, disentangling the effects observed in long-term 
exposure studies associated with more recent air quality measurements 
from effects that may have been associated with earlier, and most 
likely higher, PM2.5 exposures introduces some uncertainty 
with regard to understanding the appropriate exposure window associated 
with the observed effects. This is in contrast to the short-term 
exposure studies where the relevant exposure period is contemporaneous 
to the period for which the health data were collected. In light of 
these considerations, as noted above, the Administrator considers the 
analysis of air quality concentrations that correspond to the 
distribution of population-level data in these two studies to be a 
highly uncertain basis for informing her selection of the annual 
standard level.
    Based on the above considerations, the Administrator views the 
additional population-level data for the two short-term exposure 
studies as appropriate to help inform her judgment of how much below 
the long-term mean concentrations to set the level of the annual 
standard. The Administrator notes that the long-term mean 
PM2.5 concentrations corresponding with study areas 
contributing to the 25th percentiles of the distribution of deaths and 
cardiovascular-related hospitalizations in these two short-term 
exposure studies were 12.5 [micro]g/m\3\ and 11.5 [micro]g/m\3\, 
respectively, for Zanobetti and Schwartz (2009) and for Bell et al. 
(2008), with the 10th percentiles being lower by approximately 2 
[micro]g/m\3\ in each study.
    The Administrator recognizes, as summarized in section III.B above 
and discussed more fully in section III.B.2 of the proposal, that 
important uncertainties remain in the evidence and information 
considered in this review of the primary fine particle standards. These 
uncertainties are generally related to understanding the relative 
toxicity of the different components in the fine particle mixture, the 
role of PM2.5 in the complex ambient mixture, exposure 
measurement errors, and the nature and magnitude of estimated risks 
related to increasingly lower ambient PM2.5 concentrations. 
Furthermore, the Administrator notes that epidemiological studies have 
reported heterogeneity in responses both within and between cities and 
geographic regions across the U.S. She recognizes that this 
heterogeneity may be attributed, in part, to differences in fine 
particle composition in different regions and cities.\112\
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    \112\ Nonetheless, as explained in section III.E.1, the 
currently available evidence is not sufficient to support replacing 
or supplementing the PM2.5 indicator with any other 
indicator defined in terms of a specific fine particle component or 
group of components associated with any source categories of fine 
particles. Furthermore, the evidence is not sufficient to support 
eliminating any component or group of components associated with any 
source categories of fine particles from the mix of fine particles 
included in the PM2.5 indicator.
---------------------------------------------------------------------------

    With regard to evidence for reproductive and developmental effects 
identified as being suggestive of a causal relationship with long-term 
PM2.5 exposures, the Administrator recognizes that there are 
a number of limitations associated with this body of evidence 
including: the limited number of studies evaluating such effects; 
uncertainties related to identifying the relevant exposure time periods 
of concern; and limited toxicological evidence providing little 
information on the mode of action(s) or biological plausibility for an 
association between long-term PM2.5 exposures and adverse 
birth outcomes. Nonetheless, the Administrator believes that this more 
limited body of evidence provides some support for considering that 
serious effects may be occurring in a susceptible population at 
concentrations lower than those associated with effects classified as 
having a causal or likely causal relationship with long-term 
PM2.5 exposures (i.e., mortality, cardiovascular, and 
respiratory effects).
    Overall, the Administrator believes that the available evidence 
interpreted in light of the remaining uncertainties, as summarized 
above and discussed more fully in the Integrated Science Assessment and 
the Policy Assessment, provides increased confidence relative to 
information available in the last review and provides a strong basis 
for informing her final decisions in the current review. The 
Administrator is mindful that considering what standards are 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. In considering how to 
translate the available information into appropriate standard levels, 
the Administrator weighs the available scientific information and 
associated uncertainties and limitations. For the purpose of 
determining what annual standard level is appropriate the Administrator 
recognizes that there is no single factor or criterion that comprises 
the ``correct'' approach to weighing the various types of available 
evidence and information.
    In considering this information, the Administrator notes the advice 
of CASAC that ``there are significant public health consequences at the 
current levels of the standards that justify consideration of lowering 
the PM2.5 NAAQS further'' (Samet, 2010c, p. 12). In 
addition, she recognizes that CASAC concluded, ``although there is 
increasing uncertainty at lower levels, there is no evidence of a 
threshold (i.e., a level below which there is no risk for adverse 
effects)'' (Samet, 2010d, p.ii) and that the final decisions on 
standard levels must reflect a judgment of the available scientific 
information with respect to her interpretation of the CAA's requirement 
to set primary standards that provide requisite protection to public 
health with an adequate margin of safety (Samet, 2010d, p. 4). The 
Administrator recognizes CASAC's advice that the currently available 
scientific information provided support for considering an annual 
standard level within a range of 13 to 11 [mu]g/m\3\ and a 24-hour 
standard level within a range of 35 to 30 [mu]g/m\3\. In considering 
how the annual and 24-hour standards work together to provide 
appropriate public health protection, the Administrator observes that 
CASAC did not express support for any specific levels or combinations 
of standards within these ranges. She also notes that CASAC encouraged 
the EPA staff to consider additional data from epidemiological studies 
to help quantify the characterization of the PM2.5 
concentrations that were most influential in generating the health

[[Page 3161]]

effect estimates in these studies (Samet, 2010d, p. 2).
    In response to CASAC's advice, the Administrator recognizes that 
the EPA staff acquired additional data from authors of key 
epidemiological studies and analyzed these data to characterize the 
distribution of PM2.5 concentrations in relation to health 
events data to better understand the degree of confidence in the 
associations observed in the studies as discussed above. The 
Administrator recognizes that the final Policy Assessment included 
consideration of these additional analyses in reaching final staff 
conclusions with regard to the broadest range of alternative standard 
levels supported by the science. She takes note that the final Policy 
Assessment concluded that while alternative standard levels within the 
range of 13 to 11 [mu]g/m\3\ were appropriate to consider, the evidence 
most strongly supported consideration of an annual standard level in 
the range of 12 to 11 [mu]g/m\3\. The Administrator is aware that, in 
transmitting the final Policy Assessment to CASAC, the Agency notified 
CASAC that the final staff conclusions reflected consideration of 
CASAC's advice and that those staff conclusions were based, in part, on 
the specific distributional analysis that CASAC had urged the EPA to 
conduct (Wegman, 2011). Thus, CASAC had an opportunity to comment on 
the final Policy Assessment, but chose not to provide any additional 
comments or advice after receiving it.
    In selecting the annual standard level, the Administrator has 
considered many factors including the nature and severity of the health 
effects involved, the strength of the overall body of scientific 
evidence as considered in reaching causality determinations, the size 
of the at-risk populations, and the estimated public health impacts. 
She has also considered the kind and degree of the uncertainties that 
remain in the available scientific information. She recognizes that the 
association between PM2.5 and serious health effects is well 
established, including at concentrations below those allowed by the 
current standard. Further, she recognizes the CAA requirement that 
requires primary standards to provide an adequate margin of safety was 
intended to address uncertainties associated with inconclusive 
scientific and technical information as well as to provide a reasonable 
degree of protection against hazards that research has not yet 
identified. In considering the currently available evidence, as 
summarized and discussed more broadly above, the information on risk, 
CASAC advice, the conclusions of the Policy Assessment, and public 
comments on the proposal, the Administrator strongly believes that a 
lower annual standard level is needed to protect public health with an 
adequate margin of safety.
    In reaching her final decision on the appropriate annual standard 
level to set, the Administrator is mindful 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, including the health of at-risk populations, with an adequate 
margin of safety. On balance, the Administrator concludes that an 
annual standard level of 12 [mu]g/m\3\ would be requisite to protect 
the public health with an adequate margin of safety from effects 
associated with long- and short-term PM2.5 exposures, while 
still recognizing that uncertainties remain in the scientific 
information.
    In the Administrator's judgment, an annual standard of 12 [mu]g/
m\3\ appropriately reflects placing greatest weight on evidence of 
effects for which the Integrated Science Assessment determined there is 
a causal or likely causal relationship with long- and short-term 
PM2.5 exposures. An annual standard level of 12 [mu]g/m\3\ 
is below the long-term mean PM2.5 concentrations reported in 
each of the key multi-city, long- and short-term exposures studies 
providing evidence of an array of serious health effects (e.g., 
premature mortality, increased hospitalization for cardiovascular and 
respiratory effects). As noted above, the importance of considering a 
level somewhat below the lowest long-term mean concentration in the 
full set of studies considered is to set a standard that would provide 
appropriate protection against the observed effects in all such 
studies.
    In reaching her decision, the Administrator has taken into account 
that at and around the mean PM2.5 concentration in any given 
study represents a part of the air quality distribution in which the 
health event data in that study are generally most concentrated. 
Furthermore, in identifying an appropriate annual standard level below 
the long-term mean concentrations, she recognizes that there is no 
evidence to support the existence of any discernible threshold, and, 
therefore, she has a high degree of confidence that the observed 
effects are associated with concentrations not just at but extending 
somewhat below the long-term mean concentration. To further inform her 
judgment in setting the annual standard level so as to protect public 
health with an adequate margin of safety, the Administrator has placed 
weight on additional population-level information available from a 
subset of these epidemiological studies, consistent with CASAC advice. 
In particular, she has drawn from two short-term exposure studies, 
which provide the most relevant information for evaluating the 
distribution of health events and corresponding long-term 
PM2.5 concentrations. As explained above, this helps inform 
her judgment as to the degree of confidence in the observed 
associations in the epidemiological studies. In this regard, the 
Administrator generally judges the region around the 25th percentile as 
a reasonable part of the distribution to help guide her decision on the 
appropriate standard level. Since this evidence comes primarily from 
two studies, a relatively modest data set, the Administrator deems it 
reasonable not to draw further inferences from air quality and health 
event data in the lower part of the distribution for the purpose of 
setting a standard level. The Administrator notes that the long-term 
mean PM2.5 concentrations around the 25th percentile of the 
distributions of deaths and cardiovascular-related hospitalizations 
were approximately around 12 [mu]g/m\3\ in these two studies. The 
Administrator views this information as helpful in guiding her 
determination as to where her confidence in the magnitude and 
significance of the associations is reduced to such a degree that a 
standard set at a lower level would not be warranted to provide 
requisite protection that is neither more nor less than needed to 
provide an adequate margin of safety.
    The Administrator also recognizes that a level of 12 [mu]g/m\3\ 
places some weight on studies which provide evidence of reproductive 
and developmental effects (e.g., infant mortality, low birth weight). 
These studies were identified in the Integrated Science Assessment as 
having evidence suggestive of a causal relationship with long-term 
PM2.5 concentrations. A level of 12 [mu]g/m\3\ is 
approximately the same level as the lowest long-term mean concentration 
reported in such studies (Figures 2 and 4; 11.9 [mu]g/m\3\ for Bell et 
al., 2007).\113\ While the Administrator

[[Page 3162]]

acknowledges that this evidence is limited, she believes it is 
appropriate to place some weight on these studies in order to set a 
standard that provides protection with an adequate margin of safety, 
including providing protection for at-risk populations, as required by 
the CAA. Due to the limited nature of this evidence, she has determined 
it is not necessary to set a standard below the lowest long-term mean 
concentration in these studies.
---------------------------------------------------------------------------

    \113\ With respect to cancer, mutagenic, and genotoxic effects, 
the Administrator observes that the PM2.5 concentrations 
reported in studies evaluating these effects generally included 
ambient concentrations that are equal to or greater than ambient 
concentrations observed in studies that reported mortality and 
cardiovascular and respiratory effects (U.S. EPA, 2009a, section 
7.5). Therefore, the Administrator concludes that in selecting 
alternative standard levels that provide protection from mortality 
and cardiovascular and respiratory effects, it is reasonable to 
anticipate that protection will also be provided for carcinogenic 
effects.
---------------------------------------------------------------------------

    In reflecting on extensive public comments received on the proposal 
as discussed in section III.E.4.c above, the Administrator recognizes 
that some commenters have offered different evaluations of the evidence 
and other information available in this review and would make different 
judgments about the weight to place on the relative strengths and 
limitations of the scientific information and about how such 
information could be used in making public health policy decisions on 
the annual standard level. One group of such commenters who supported a 
higher annual standard level (e.g., above 13 [mu]g/m\3\) would place 
greater weight on the remaining uncertainties in the evidence as a 
basis for supporting a higher standard level than the Administrator 
judges to be appropriate. Such an approach is based on these 
commenters' judgment that the uncertainties remaining in the evidence 
are too great to warrant setting an annual standard below the current 
level. The Administrator does not agree.
    As an initial matter, an annual standard level of 13 [mu]g/m\3\ or 
higher would be above the long-term mean concentrations reported in two 
well-conducted, multi-city short-term exposure studies reporting 
positive and statistically significant associations of serious effects 
(Burnett et al., 2004 and Bell et al., 2008). These important studies 
are fully consistent with the pattern of evidence presented by the 
large body of evidence in this review. As the Administrator recognized 
in the proposal, and as advised by CASAC, the appropriate focus for 
selecting the level of the annual PM2.5 standard is on 
concentrations somewhat below the lowest long-term mean concentrations 
from the set of key studies of both long-term and short-term 
PM2.5 exposures considered by the EPA (i.e., as shown in 
Figure 4). Thus, a standard level set at 13 [mu]g/m\3\ or higher would 
clearly not provide protection for the effects observed in the full set 
epidemiological studies and, therefore, this standard level could not 
be judged to be requisite with an adequate margin of safety.\114\
---------------------------------------------------------------------------

    \114\ The Administrator is mindful that, in reviewing the 2006 
final PM NAAQS decisions, the D.C. Circuit Court of Appeals 
concluded that the EPA failed to adequately explain why that annual 
standard provided requisite protection from effects associated with 
both long- and short-term exposures or from morbidity effects in 
children and other at-risk populations when long-term means of 
important short-term studies were below the level the Administrator 
selected for the annual standard. See American Farm Bureau v. EPA. 
559 F. 3d 512, 524-26. There is no reasonable basis to discount 
these two studies for purposes of setting the level of the annual 
standard.
---------------------------------------------------------------------------

    In addition, as noted above, in recognizing that there is no 
evidence to support the existence of a discernible threshold below 
which an effect would not occur, the Administrator is mindful that 
effects occur around and below the long-term mean concentrations 
reported in both the short-term and long-term the epidemiological 
studies. A standard level of 13 [mu]g/m\3\ or higher would not 
appropriately take into account evidence from the two well-conducted, 
multi-city, short-term exposure studies reporting serious effects with 
long-term mean concentrations below 13 [mu]g/m\3\ noted above (Burnett 
et al, 2004; Bell et al., 2008). Such a standard level would also not 
appropriately take into account additional population-level data from a 
limited number of epidemiological studies. This approach would ignore 
CASAC's advice to consider such information in order to better 
understand the concentrations over which there is a high degree of 
confidence regarding the magnitude and significance of the associations 
observed in individual epidemiological studies and where there is 
appreciably less confidence.
    Furthermore, a standard level of 13 [mu]g/m\3\ or higher would not 
appropriately take into account the more limited evidence of effects in 
some at-risk populations (e.g., low birth weight). In the 
Administrator's view, a standard set at this level would not provide 
protection with an adequate margin of safety, including providing 
protection for at-risk populations. The Administrator is mindful that 
the CAA requirement that primary standards provide an adequate margin 
of safety, discussed in section II.A above, was intended to address 
uncertainties associated with inconclusive scientific and technical 
information available at the time of standard setting as well as to 
provide a reasonable degree of protection against hazards that research 
has not yet identified.
    In light of the entire body of evidence as discussed above, the 
Administrator judges that an annual standard level set above 12 [mu]g/
m\3\ would not be sufficient to protect public health with an adequate 
margin of safety from the serious health effects associated with long- 
and short-term exposure to PM2.5.
    The Administrator also recognizes that a second group of commenters 
supported a lower annual standard level (e.g., no higher than 11 [mu]g/
m\3\). Such a standard level would reflect placing essentially as much 
weight on the relatively more limited data providing evidence 
suggestive of a causal relationship for effects observed in some at-
risk populations (e.g., low birth weight) as on more certain evidence 
of effects classified as having a causal or likely causal relationship 
with PM2.5 exposures. In the Administrator's view, while it 
is important to place some weight on such suggestive evidence, it would 
not be appropriate to place as much weight on it as the commenters 
would do.
    An annual standard level of 11 [mu]g/m\3\ would also reflect these 
commenters' judgment that it is appropriate to focus on a lower part of 
the distributions of health event data from the small number of 
epidemiological studies for which this information was made available 
than the Administrator believes is warranted. In the Administrator's 
view, using this type of information to set a standard level of 11 
[mu]g/m\3\ or below would assume too high a degree of confidence in the 
magnitude and significance of the associations observed in the lower 
part of the distributions of health events observed in these studies. 
Given the uncertainties in the evidence and the limited set of studies 
for which the EPA has information on the distribution of health event 
data and corresponding air quality data, the Administrator believes it 
is not appropriate to focus on the lower part of the distributions of 
health events data.
    On balance, the Administrator finds that the available evidence 
interpreted in light of the remaining uncertainties does not justify a 
standard level set below 12 [mu]g/m\3\ as necessary to protect public 
health with an adequate margin of safety.
    After carefully considering the above considerations and the public 
comments summarized in section III.E.4.c above, the Administrator has 
decided to set the level of the primary annual PM2.5 
standard at 12 [mu]g/m\3\. In her judgment, a standard set at this 
level provides the requisite degree of public health protection, 
including the health of at-risk populations, with an adequate margin of 
safety and is neither more nor less stringent than necessary for this 
purpose.

[[Page 3163]]

    As discussed above, the Administrator concludes that an approach 
that focuses on setting a generally controlling annual standard is the 
most effective and efficient way to reduce total population risk 
associated with both long- and short-term PM2.5 exposures. 
Such an approach would result in more uniform protection across the 
U.S. than the alternative of setting the levels of the 24-hour and 
annual standard such that the 24-hour standard would generally be the 
controlling standard in areas across the country (see section III.A.3).
    The Administrator recognizes that potential air quality changes 
associated with meeting an annual standard level of 12.[mu]g/m\3\ will 
result in lowering risks associated with both long- and short-term 
PM2.5 exposures by lowering the overall air quality 
distribution. However, the Administrator recognizes that such an annual 
standard alone would not be expected to offer sufficient protection 
with an adequate margin of safety against the effects of short-term 
PM2.5 exposures in all parts of the country. As a result, in 
conjunction with an annual standard level of 12 [mu]g/m\3\, the 
Administrator concludes that it is appropriate to continue to provide 
supplemental protection by means of a 24-hour standard set at the 
appropriate level, particularly for areas with high peak-to-mean ratios 
possibly associated with strong local or seasonal sources and for areas 
with PM2.5-related effects that may be associated with 
shorter-than-daily exposure periods.
    In selecting the level of a 24-hour standard meant to provide such 
supplemental protection, the Administrator relies upon evidence and air 
quality information from key short-term exposure studies. In 
considering these studies, the Administrator notes that to the extent 
air quality distributions in the study areas considered are reduced to 
meet the current 24-hour standard (at a level of 35 [mu]g/m\3\) or to 
meet the revised annual standard discussed above (at a level of 12 
[mu]g/m\3\), additional protection would be anticipated against the 
effects observed in these studies. In light of this, when selecting the 
appropriate level for the 24-hour standard, the Administrator considers 
both the 98th percentiles of 24-hour PM2.5 concentrations 
and the long-term mean PM2.5 concentrations in the locations 
of the short-term exposure studies. She notes that such consideration 
of both short- and long-term PM2.5 concentrations can inform 
her decision on the extent to which a given 24-hour standard, in 
combination with the revised annual standard established in this rule, 
would provide protection against the health effects reported in short-
term studies.
    As discussed in section III.E.4.a above, the Administrator 
concludes that multi-city short-term exposure studies provide the 
strongest data set for informing her decisions on appropriate 24-hour 
standard levels. With regard to the limited number of single-city 
studies that reported positive and statistically significant 
associations for a range of health endpoints related to short-term 
PM2.5 concentrations in areas that would likely have met the 
current suite of PM2.5 standards, the Administrator 
recognizes that many of these studies had significant limitations 
(e.g., limited statistical power, limited exposure data) or equivocal 
results (mixed results within the same study area) that make them 
unsuitable to form the basis for setting the level of a 24-hour 
standard.
    With regard to multi-city studies that evaluated effects associated 
with short-term PM2.5 exposures, the Administrator observes 
an overall pattern of positive and statistically significant 
associations in studies with 98th percentile 24-hour values averaged 
across study areas within the range of 45.8 to 34.2 [mu]g/m\3\ (Burnett 
et al., 2004; Zanobetti and Schwartz, 2009; Bell et al., 2008; Dominici 
et al., 2006a, Burnett and Goldberg, 2003; Franklin et al., 2008). The 
Administrator notes that, to the extent air quality distributions are 
reduced to reflect just meeting the current 24-hour standard, 
additional protection would be provided for the effects observed in the 
three multi-city studies with 98th percentile values greater than 35 
[mu]g/m\3\ (Burnett et al., 2004; Burnett and Goldberg, 2003; Franklin 
et al., 2008). In the three additional multi-city studies with 98th 
percentile values below 35 [mu]g/m\3\, specifically 98th percentile 
concentrations of 34.2, 34.3, and 34.8 [mu]g/m\3\, the Administrator 
notes that these studies reported long-term mean PM2.5 
concentrations of 12.9, 13.2, and 13.4 [mu]g/m\3\, respectively (Bell 
et al., 2008; Zanobetti and Schwartz, 2009; Dominici et al., 2006a). In 
revising the level of the annual standard to 12 [mu]g/m\3\, as 
discussed above, the Administrator recognizes that additional 
protection would be provided for the short-term effects observed in 
these multi-city studies such that revision to the 24-hour standard 
would not be warranted. That is, by lowering the level of the annual 
standard to 12 [mu]g/m\3\, the 98th percentile of the distribution 
would be lowered as well such that additional protection from effects 
associated with short-term exposures would be afforded. Therefore, the 
epidemiological evidence supports a conclusion that it is appropriate 
to retain the level of the 24-hour standard at 35 [mu]g/m\3\, in 
conjunction with a revised annual standard level of 12 [mu]g/m\3\.
    In addition to considering the epidemiological evidence, the 
Administrator also has taken into account air quality information based 
on county-level 24-hour and annual design values to understand the 
implications of revising the annual standard level from 15 to 12 [mu]g/
m\3\ in conjunction with retaining the 24-hour standard level at 35 
[mu]g/m\3\. She has considered this information to evaluate the public 
health protection provided by the two standards in combination and to 
evaluate the most appropriate means of developing a suite of standards 
providing requisite public health protection with an adequate margin of 
safety.
    In considering the air quality information, the Administrator 
observes that a suite of PM2.5 standards that includes an 
annual standard level of 12 [mu]g/m\3\ and a 24-hour standard level of 
35 [mu]g/m\3\ would result in the annual standard as the generally 
controlling standard in most regions across the country, except for 
certain areas in the Northwest, where the annual mean PM2.5 
concentrations have historically been low but where relatively high 24-
hour concentrations occur, often related to seasonal wood smoke 
emissions (U.S. EPA, 2011a, pp. 2-89 to 2-91, Figure 2-10). In fact, 
these are the type of areas for which the supplemental protection 
afforded by the 24-hour standard is intended, such that the two 
standards together provide the requisite degree of protection. The 
Administrator concludes the current 24-hour standard at a level of 35 
[mu]g/m\3\, in conjunction with a revised annual standard level of 12 
[mu]g/m\3\, will provide appropriate protection from effects observed 
in studies in such areas in which the long-term mean concentrations 
were below 12 [mu]g/m\3\ and the 98th percentile 24-hour concentrations 
were above 35 [mu]g/m\3\ (e.g., areas in the Northwest U.S.).
    After carefully taking the public comments and above considerations 
into account, the Administrator has decided to retain the current level 
of the primary PM2.5 24-hour standard at 35 [mu]g/m\3\ in 
conjunction with revising the annual standard level from 15.0 [mu]g/
m\3\ to 12.0 [mu]g/m\3\.\115\ In the Administrator's

[[Page 3164]]

judgment, this suite of primary PM2.5 standards and the 
rationale supporting these levels appropriately reflects consideration 
of the strength of the available evidence and other information and its 
associated uncertainties as well as the advice of CASAC and 
consideration of public comments. In the Administrator's judgment, this 
suite of primary PM2.5 standards is sufficient but not more 
protective than necessary to protect the public health, including at-
risk populations, with an adequate margin of safety from effects 
associated with long- and short-term exposures to fine particles. This 
suite of standards will provide significant protection from serious 
health effects including premature mortality and cardiovascular and 
respiratory morbidity effects that are causally or likely causally 
related to long- and short-term PM2.5 exposures. These 
standards will also provide an appropriate degree of protection against 
other health effects for which there is more limited evidence of 
effects and causality, such as reproductive and developmental effects. 
This judgment by the Administrator appropriately considers the 
requirement for a standard that is requisite to protect public health 
but is neither more nor less stringent than necessary.\116\
---------------------------------------------------------------------------

    \115\ As noted in section II.B.1, Table 1 and section III.E.4.a 
above, the annual standard level is defined to one decimal place. 
Throughout this section, the annual standard levels discussed have 
been denoted as integer values (e.g., 12 [mu]g/m\3\) for simplicity.
    \116\ The Administrator also judges that this suite of standards 
addresses the issues raised by the D.C. Circuit's remand of the 2006 
primary annual PM2.5 standard by appropriately revising 
that standard.
---------------------------------------------------------------------------

D. Administrator's Final Decisions on Primary PM2.5 Standards

    For the reasons discussed above, and taking into account the 
information and assessments presented in the Integrated Science 
Assessment, Risk Assessment, and Policy Assessment, the advice and 
recommendations of CASAC, and public comments to date, the 
Administrator revises the current suite of primary PM2.5 
standards. Specifically, the Administrator revises: (1) The level of 
the primary annual PM2.5 standard to 12.0 [mu]g/m\3\ and (2) 
the form of the primary annual PM2.5 standard to one based 
on the highest appropriate area-wide monitor in an area, with no option 
for spatial averaging. In conjunction with revising the primary annual 
PM2.5 standard to provide protection from effects associated 
with long- and short-term PM2.5 exposures, the Administrator 
retains the level of 35 [mu]g/m\3\ and the 98th percentile form of the 
primary 24-hour PM2.5 standard to continue to provide 
supplemental protection for areas with high peak PM2.5 
concentrations. The Administrator is not revising the current 
PM2.5 indicator or the annual and 24-hour averaging times 
for the primary PM2.5 standards. The Administrator concludes 
that this suite of standards would be requisite to protect public 
health with an adequate margin of safety against health effects 
potentially associated with long- and short-term PM2.5 
exposures.

IV. Rationale for Final Decision on Primary PM10 Standard

    This section presents the rationale for the Administrator's final 
decision to retain the current 24-hour primary PM10 standard 
in order to continue to provide public health protection against short-
term exposures to inhalable particles in the size range of 2.5 to 10 
[mu]m (i.e., PM10-2.5 or thoracic coarse particles). These 
are particles capable of reaching the most sensitive areas of the lung, 
including the trachea, bronchi, and deep lungs. The current standard 
uses PM10 as the indicator for thoracic coarse particles, 
and thus is referred to as a PM10 standard.\117\
---------------------------------------------------------------------------

    \117\ Throughout this section of the preamble, we are using the 
terms ``thoracic coarse particles'', ``inhalable coarse particles'', 
and ``PM10-2.5'' synonymously.
---------------------------------------------------------------------------

    As discussed more fully in the proposal and below, this rationale 
is based on a thorough review of the latest scientific evidence, 
published through mid-2009 and assessed in the Integrated Science 
Assessment (U.S. EPA, 2009a), evaluating human health effects 
associated with long- and short-term exposures to thoracic coarse 
particles. The Administrator's final decision also takes into account: 
(1) The EPA staff analyses of air quality information and health 
evidence and staff conclusions regarding the current and potential 
alternative standards, as presented in the Policy Assessment for the PM 
NAAQS (U.S. EPA, 2011a); (2) CASAC advice and recommendations, as 
reflected in discussions at public meetings of drafts of the Integrated 
Science Assessment and Policy Assessment, and in CASAC's letters to the 
Administrator; (3) the multiple rounds of public comments received 
during the development of the Integrated Science Assessment and Policy 
Assessment, both in connection with CASAC meetings and separately; and 
(4) public comments (including testimony at the public hearings) 
received on the proposal.
    In presenting the rationale for the final decision to retain the 
current primary PM10 standard, this section discusses the 
EPA's past reviews of the PM NAAQS and the general approach taken to 
review the current standard (section IV.A), the health effects 
associated with exposures to ambient PM10-2.5 (section 
IV.B), the consideration of the current and potential alternative 
standards in the Policy Assessment (section IV.C), CASAC 
recommendations regarding the current and potential alternative 
standards (section IV.D), the Administrator's proposed decision to 
retain the current primary PM10 standard (section IV.E), 
public comments received in response to the Administrator's proposed 
decision (section IV.F), and the Administrator's final decision to 
retain the current primary PM10 standard (section IV.G).

A. Background

    The following sections discuss previous reviews of the PM NAAQS 
(section IV.A.1), the litigation of the EPA's 2006 decision on the 
PM10 standards (section IV.A.2), and the general approach 
taken to review the primary PM10 standard in the current 
review (section IV.A.3).
1. Previous Reviews of the PM NAAQS
a. Reviews Completed in 1987 and 1997
    The PM NAAQS have always included some type of a primary standard 
to protect against effects associated with exposures to thoracic coarse 
particles. In 1987, when the EPA first revised the PM NAAQS, the EPA 
changed the indicator for PM from TSP to focus on inhalable particles, 
those which can penetrate into the trachea, bronchi, and deep lungs (52 
FR 24634, July 1, 1987). In that review, the EPA changed the PM 
indicator to PM10 based on evidence that the risk of adverse 
health effects associated with particles with a nominal mean 
aerodynamic diameter less than or equal to 10 [mu]m was significantly 
greater than risks associated with larger particles (52 FR 24639, July 
1, 1987).
    In the 1997 review, in conjunction with establishing new fine 
particle (i.e., PM2.5) standards (discussed above in 
sections II.B.1 and III.A.1), the EPA concluded that continued 
protection was warranted against potential effects associated with 
thoracic coarse particles in the size range of 2.5 to 10 [mu]m. This 
conclusion was based on particle dosimetry, toxicological information, 
and on limited epidemiological evidence from studies that measured 
PM10 in areas where the coarse fraction was likely to 
dominate PM10 mass (62 FR 38677, July 18, 1997). The EPA 
concluded there that a PM10 standard could provide requisite 
protection against effects associated with particles

[[Page 3165]]

in the size range of 2.5 to 10 [mu]m.\118\ Although the EPA considered 
a more narrowly defined indicator for thoracic coarse particles in that 
review (i.e., PM10-2.5), the EPA concluded that it was more 
appropriate, based on existing evidence, to continue to use 
PM10 as the indicator. 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. These were two studies conducted in areas where 
the coarse fraction was the dominant fraction of PM10, and 
which substantially exceeded the 24-hour PM10 standard (62 
FR 38679). In addition, there were only very limited ambient air 
quality data then available specifically for PM10-2.5, in 
contrast to the extensive monitoring network already in place for 
PM10. Therefore, the EPA considered it more administratively 
feasible to use PM10 as an indicator. The EPA also stated 
that the PM10 standards would work in conjunction with the 
PM2.5 standards by regulating the portion of particulate 
pollution not regulated by the then newly adopted PM2.5 
standards.
---------------------------------------------------------------------------

    \118\ With regard to the 24-hour PM10 standard, the 
EPA retained the indicator, averaging time, and level (150 [mu]g/
m\3\), but revised the form (i.e., from one-expected-exceedance to 
the 99th percentile).
---------------------------------------------------------------------------

    In May 1998, a three-judge panel of the U.S. Court of Appeals for 
the District of Columbia Circuit found ``ample support'' for the EPA's 
decision to regulate coarse particle pollution, but vacated the 1997 
PM10 standards, concluding that the EPA had failed to 
adequately explain its choice of PM10 as the indicator for 
thoracic coarse particles American Trucking Associations v. EPA, 175 F. 
3d 1027, 1054-56 (D.C. Cir. 1999). In particular, the court held that 
the EPA had not explained the use of an indicator under which the 
allowable level of coarse particles varied according to the amount of 
PM2.5 present, and which, moreover, potentially double 
regulated PM2.5. The court also rejected considerations of 
administrative feasibility as justification for use of PM10 
as the indicator for thoracic coarse PM, since NAAQS (and their 
elements) are to be based exclusively on health and welfare 
considerations. Id. at 1054. Pursuant to the court's decision, the EPA 
removed the vacated 1997 PM10 standards from the 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 thus remained in place. Id. at 80777.
b. Review Completed in 2006
    In the review of the PM NAAQS that concluded in 2006, the EPA 
considered the growing, but still limited, body of evidence supporting 
associations between health effects and thoracic coarse particles 
measured as PM10-2.5.\119\ The new studies available in the 
2006 review included epidemiological studies that reported associations 
with health effects using direct measurements of PM10-2.5, 
as well as dosimetric and toxicological studies. In considering this 
growing body of PM10-2.5 evidence, as well as evidence from 
studies that measured PM10 in locations where the majority 
of PM10 was in the PM10-2.5 fraction (U.S. EPA, 
2005, section 5.4.1), staff concluded that the level of protection 
afforded by the existing 1987 PM10 standard remained 
appropriate (U.S. EPA, 2005, p. 5-67) but recommended that the 
indicator for the standard be revised. Specifically, staff recommended 
replacing the PM10 indicator with an indicator of urban 
thoracic coarse particles in the size range of 10-2.5 [mu]m (U.S. EPA, 
2005, pp. 5-70 to 5-71). The agency proposed to retain a standard for a 
subset of thoracic coarse particles, proposing a qualified 
PM10-2.5 indicator to focus on the mix of thoracic coarse 
particles generally present in urban environments. More specifically, 
the proposed revised thoracic coarse particle standard would have 
applied only to an ambient mix of PM10-2.5 dominated by 
resuspended dust from high-density traffic on paved roads and/or by 
industrial and construction sources. The proposed revised standard 
would not have applied to any ambient mix of PM10-2.5 
dominated by rural windblown dust and soils. In addition, agricultural 
sources, mining sources, and other similar sources of crustal material 
would not have been subject to control in meeting the standard (71 FR 
2667 to 2668, January 17, 2006).
---------------------------------------------------------------------------

    \119\ The PM Staff Paper (U.S. EPA, 2005) also presented results 
of a quantitative assessment of health risks for 
PM10-2.5. However, staff concluded that the nature and 
magnitude of the uncertainties and concerns associated with this 
risk assessment weighed against its use as a basis for recommending 
specific levels for a thoracic coarse particle standard (U.S. EPA, 
2005, p. 5-69).
---------------------------------------------------------------------------

    The Agency received a large number of comments overwhelmingly and 
persuasively opposed to the proposed qualified PM10-2.5 
indicator (71 FR 61188 to 61197, October 17, 2006). After careful 
consideration of the scientific evidence and the recommendations 
contained in the 2005 Staff Paper, the advice and recommendations from 
CASAC, and the public comments received regarding the appropriate 
indicator for coarse particles, and after extensive evaluation of the 
alternatives available to the Agency, the Administrator decided it 
would not be appropriate to adopt the proposed qualified 
PM10-2.5 indicator, or any qualified indicator. Underlying 
this determination was the Administrator's decision that it was 
requisite to provide protection from exposure to all thoracic coarse 
PM, regardless of its origin. The Administrator thus rejected arguments 
that there are no health effects from community-level exposures to 
coarse PM in non-urban areas (71 FR 61189). The EPA concluded that 
dosimetric, toxicological, occupational and epidemiological evidence 
supported retention of a primary standard for short-term exposures that 
included all thoracic coarse particles (i.e., particles of both urban 
and non-urban origin), consistent with the Act's requirement that 
primary NAAQS must be requisite to protect the public health and 
provide an adequate margin of safety. At the same time, the Agency 
concluded that the standard should target protection toward urban 
areas, where the evidence of health effects from exposure to 
PM10-2.5 was strongest (71 FR at 61193, 61197). The proposed 
indicator was not suitable for that purpose. Not only did it 
inappropriately provide no protection at all to many areas, but it 
failed to identify many areas where the ambient particle mix was 
dominated by coarse particles contaminated with urban/industrial types 
of coarse particles for which evidence of health effects was strongest 
(71 FR 61193).
    The Agency ultimately concluded that the existing indicator, 
PM10, was most consistent with the evidence. Although 
PM10 includes both coarse and fine PM, the Agency concluded 
that it remained an appropriate indicator for thoracic coarse particles 
because, as discussed in the PM Staff Paper (U.S. EPA, 2005, p. 2-54, 
Figures 2-23 and 2-24), fine particle levels are generally higher in 
urban areas and, therefore, a PM10 standard set at a single 
unvarying level will generally result in lower allowable concentrations 
of thoracic coarse particles in urban areas than in non-urban areas (71 
FR 61195-96). The EPA considered this to be an appropriate targeting of 
protection given that the strongest evidence for effects associated 
with thoracic coarse particles came from epidemiological studies 
conducted in urban areas and that elevated fine particle concentrations 
in urban areas could result in increased contamination of coarse 
fraction particles by PM2.5,

[[Page 3166]]

potentially increasing the toxicity of thoracic coarse particles in 
urban areas (id.). Given the evidence that the existing (i.e., 1987) 
PM10 standard was established at a level and form which 
afforded requisite protection with an adequate margin of safety, the 
Agency retained the level and form of the 24-hour PM10 
standard.\120\
---------------------------------------------------------------------------

    \120\ Thus, the standard is met when a 24-hour average 
PM10 concentration of 150 [mu]g/m\3\ is not exceeded more 
than one day per year, on average over a three-year period. As noted 
above, the 1987 PM10 standard was not adopted solely to 
control thoracic coarse particles. However, when reviewing this 
standard in the 2006 review, EPA determined that the level and form 
of the standard being reviewed (i.e., the 1987 PM10 
standard) provided requisite protection with an adequate margin of 
safety from short-term exposures to thoracic coarse particles.
---------------------------------------------------------------------------

    The Agency also revoked the annual PM10 standard, in 
light of the conclusion in the PM Criteria Document (U.S. EPA, 2004, p. 
9-79) that the available evidence does not suggest an association with 
long-term exposure to PM10-2.5 and the conclusion in the 
Staff Paper (U.S. EPA, 2005, p. 5-61) that there is no quantitative 
evidence that directly supports retention of an annual standard. This 
decision was consistent with CASAC advice and recommendations 
(Henderson, 2005a,b).
    In the same rulemaking, the EPA also included a new FRM for the 
measurement of PM10-2.5 in the ambient air (71 FR 61212 to 
61213, October 17, 2006). Although the standard for thoracic coarse 
particles does not use a PM10-2.5 indicator, the new FRM for 
PM10-2.5 was established to provide a basis for approving 
FEMs and to promote the gathering of scientific data to support future 
reviews of the PM NAAQS (71 FR 61202/3, October 17, 2006).\121\
---------------------------------------------------------------------------

    \121\ As noted below, however, with this rule the EPA is 
revoking the requirement for PM10-2.5 speciation at NCore 
monitoring sites due to technical issues related to the development 
of appropriate monitoring methods (section VIII.B.3.c). The 
requirement for PM10-2.5 mass measurements at NCore sites 
is being retained.
---------------------------------------------------------------------------

2. Litigation Related to the 2006 Primary PM10 Standards
    A number of groups filed suit in response to the final decisions 
made in the 2006 review. See American Farm Bureau Federation v. EPA, 
559 F. 3d 512 (D.C. Cir. 2009). Among the petitions for review were 
challenges from industry groups on the decision to retain the 
PM10 indicator and the level of the PM10 standard 
and from environmental and public health groups on the decision to 
revoke the annual PM10 standard. The court upheld both the 
decision to retain the 24-hour PM10 standard and the 
decision to revoke the annual standard.
    First, the court upheld the EPA's decision for a standard to 
encompass all thoracic coarse PM, both of urban and non-urban origin. 
The court rejected arguments that the evidence showed there are no 
risks from exposure to non-urban coarse PM. The court further found 
that the EPA had a reasonable basis not to set separate standards for 
urban and non-urban coarse PM, namely the inability to reasonably 
define what ambient mixes would be included under either `urban' or 
`non-urban;' and the evidence in the record that supported the EPA's 
appropriately cautious decision to provide ``some protection from 
exposure to thoracic coarse particles * * * in all areas.'' 559 F. 3d 
at 532-33. Specifically, the court stated,

    Although the evidence of danger from coarse PM is, as EPA 
recognizes, ``inconclusive,'' (71 FR 61193, October 17, 2006), the 
agency need not wait for conclusive findings before regulating a 
pollutant it reasonably believes may pose a significant risk to 
public health. The evidence in the record supports the EPA's 
cautious decision that ``some protection from exposure to thoracic 
coarse particles is warranted in all areas.'' Id. As the court has 
consistently reaffirmed, the CAA permits the Administrator to ``err 
on the side of caution'' in setting NAAQS. 559 F. 3d at 533.

    The court also upheld the EPA's decision to retain the level of the 
standard at 150 [mu]g/m\3\ and to use PM10 as the indicator 
for thoracic coarse particles. In upholding the level of the standard, 
the court referred to the conclusion in the Staff Paper that there is 
``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.'' 559 F. 3d at 534. The court also rejected arguments that a 
PM10 standard established at an unvarying level will result 
in arbitrarily varying levels of protection given that the level of 
coarse PM would vary based on the amount of fine PM present. The court 
agreed that the variation in allowable coarse PM was in accord with the 
strength of the evidence: Typically less coarse PM would be allowed in 
urban areas (where levels of fine PM are typically higher), in accord 
with the strongest evidence of health effects from coarse particles. 
559 F. 3d at 535-36. In addition, such regulation would not 
impermissibly double regulate fine particles, since any additional 
control of fine particles (beyond that afforded by the primary 
PM2.5 standard) would be for a different purpose: To prevent 
contamination of coarse particles by fine particles. 559 F. 3d at 535, 
536. These same explanations justified the choice of PM10 as 
an indicator and provided the reasoned explanation for that choice 
lacking in the record for the 1997 standard. 559 F. 3d at 536.
    With regard to the challenge from environmental and public health 
groups, the court upheld the EPA's decision to revoke the annual 
PM10 standard. The court rejected the argument that the EPA 
is required by law to have an annual PM10 standard, holding 
that section 109(d)(1) of the Act allows the EPA to revoke a standard 
no longer warranted by the current scientific understanding. 559 F. 3d 
at 538. The court further held that the EPA's decision to revoke the 
annual standard was supported by the science:

    The EPA reasonably decided that an annual coarse PM standard is 
not necessary because, as the Criteria Document and the Staff Paper 
make clear, the latest scientific data do not indicate that long-
term exposure to coarse particles poses a health risk. The CASAC 
also agreed that an annual coarse PM standard is unnecessary. 559 F. 
3d at 538-39.
3. General Approach Used in the Current Review
    The approach taken to considering the existing and potential 
alternative primary PM10 standards in the current review 
builds upon the approaches used in previous PM NAAQS reviews. This 
approach is based most fundamentally on using information from 
epidemiological studies and air quality analyses to inform the 
identification of a range of policy options for consideration by the 
Administrator. The Administrator considers the appropriateness of the 
current and potential alternative standards, taking into account the 
four elements of the NAAQS: Indicator, averaging time, form, and level.
    Evidence-based approaches to using information from epidemiological 
studies to inform decisions on PM standards are complicated by the 
recognition that no population threshold, below which it can be 
concluded with confidence that PM-related effects do not occur, can be 
discerned from the available evidence (U.S. EPA, 2009a, sections 2.4.3 
and 6.5.2.7).\122\ As a result, any approach to

[[Page 3167]]

reaching decisions on what standards are appropriate requires judgments 
about how to translate the information available from the 
epidemiological studies into a basis for appropriate standards, which 
includes consideration of how to weigh the uncertainties in reported 
associations across the distributions of PM concentrations in the 
studies. The approach taken to informing these decisions in the current 
review recognizes that the available health effects evidence reflects a 
continuum consisting of ambient levels at which scientists generally 
agree that health effects are likely to occur through lower levels at 
which the likelihood and magnitude of the response become increasingly 
uncertain. Such an approach is consistent with setting standards that 
are neither more nor less stringent than necessary, recognizing that a 
zero-risk standard is not required by the CAA.
---------------------------------------------------------------------------

    \122\ Studies that have characterized the concentration-response 
relationships for PM exposures have evaluated PM10, which 
includes both coarse and fine particles, and PM2.5 (U.S. 
EPA, 2009a, sections 2.4.3 and 6.5.2.7).
---------------------------------------------------------------------------

    Because the purpose of the PM10 standard is to protect 
against exposures to PM10-2.5, it is most appropriate to 
focus on PM10-2.5 health studies when considering the degree 
of public health protection provided by the current PM10 
standard. Compared to health studies of PM10, studies that 
evaluate associations with PM10-2.5 provide clearer evidence 
for health effects following exposures to thoracic coarse particles. In 
contrast, it is difficult to interpret PM10 studies within 
the context of a standard meant to protect against exposures to 
PM10-2.5 because PM10 is comprised of both fine 
and coarse particles, even in locations with the highest concentrations 
of PM10-2.5 (U.S. EPA, 2011a, Figure 3-4). Therefore, the 
extent to which PM10 effect estimates reflect associations 
with PM10-2.5 versus PM2.5 can be highly 
uncertain. In light of this uncertainty, it is preferable to consider 
PM10-2.5 studies when such studies are available. Given the 
availability in this review of a number of studies that evaluated 
associations with PM10-2.5, and given that the Integrated 
Science Assessment weight-of-evidence conclusions for thoracic coarse 
particles were based on studies of PM10-2.5, in this review 
the EPA focuses primarily on studies that have specifically evaluated 
PM10-2.5.\123\
---------------------------------------------------------------------------

    \123\ It should also be noted that CASAC endorsed the approach 
adopted in the Integrated Science Assessment, which draws weight-of-
evidence conclusions for PM2.5 and PM10-2.5, 
but not for PM10 (Samet, 2009f).
---------------------------------------------------------------------------

    As discussed in more detail in the Risk Assessment (U.S. EPA, 
2010a, Appendix H), the EPA did not conduct a quantitative assessment 
of health risks associated with PM10-2.5. The Risk 
Assessment concluded that limitations in the monitoring network and in 
the health studies that rely on that monitoring network, which would be 
the basis for estimating PM10-2.5 health risks, would 
introduce significant uncertainty into a PM10-2.5 risk 
assessment such that the risk estimates generated would be of limited 
value in informing review of the standard. Therefore, it was judged 
that a quantitative assessment of PM10-2.5 risks is not 
supportable at this time (U.S. EPA, 2010a, p. 2-6). This decision does 
not indicate that health effects are not associated with exposure to 
thoracic coarse particles. Rather, as noted above, it reflects the 
conclusion that limitations in the available health studies and air 
quality information would introduce significant uncertainty into a 
quantitative assessment of PM10-2.5 risks such that the risk 
estimates generated would be of limited value in informing review of 
the standard.

B. Health Effects Related to Exposure to Thoracic Coarse Particles

    This section briefly outlines the key information presented in 
section IV.B of the proposal (77 FR 38947 to 38951, June 29, 2012), and 
discussed more fully in the Integrated Science Assessment (U.S. EPA, 
2009a, Chapters 2, 4, 5, 6, 7, and 8) and the Policy Assessment (U.S. 
EPA, 2011a, Chapter 3), related to health effects associated with 
thoracic coarse particle exposures. In looking across the new 
scientific evidence available in this review, our overall understanding 
of health effects associated with thoracic coarse particle exposures 
has been expanded, though important uncertainties remain. Some 
highlights of the key policy-relevant scientific evidence available in 
this review include the following:

    (1) A number of multi-city and single-city epidemiological 
studies have evaluated associations between short-term 
PM10-2.5 and mortality, cardiovascular effects (e.g., 
including hospital admissions and emergency department visits), and/
or respiratory effects. Despite differences in the approaches used 
to estimate ambient PM10-2.5 concentrations, the majority 
of these studies have reported positive, though often not 
statistically significant, associations with short-term 
PM10-2.5 concentrations. Most PM10-2.5 effect 
estimates remained positive in co-pollutant models that included 
either gaseous or particulate co-pollutants. In U.S. study locations 
likely to have met the current PM10 standard during the 
study period, a few PM10-2.5 effect estimates were 
statistically significant and remained so in co-pollutant 
models.\124\
---------------------------------------------------------------------------

    \124\ The statistical significance of effect estimates provides 
important information on their statistical precision. However, when 
a group of studies report effect estimates that are similar in 
direction and magnitude, such a pattern of results warrants 
consideration of those studies even if not all reported 
statistically significant associations in single- or co-pollutant 
models (section III.D.2, above). In considering the 
PM10-2.5 epidemiologic studies below, the Administrator 
considers both the pattern of results across studies and the 
statistical significance of those results.
---------------------------------------------------------------------------

    (2) A small number of controlled human exposure studies have 
reported alterations in heart rate variability or increased 
pulmonary inflammation following short-term exposure to 
PM10-2.5, providing some support for the associations 
reported in epidemiological studies. Toxicological studies that have 
examined the effects of PM10-2.5 have used intratracheal 
instillation and, because these studies do not directly mirror any 
real-world mode of exposure, they provide only limited evidence for 
the biological plausibility of PM10-2.5-induced effects.
    (3) Using a more formal framework for reaching causal 
determinations than used in previous reviews, the Integrated Science 
Assessment concluded that the existing evidence is ``suggestive'' of 
a causal relationship between short-term PM10-2.5 
exposures and mortality, cardiovascular effects, and respiratory 
effects (U.S. EPA, 2009a, section 2.3.3).\125\ In contrast, the 
Integrated Science Assessment concluded that available evidence is 
``inadequate'' to infer a causal relationship between long-term 
PM10-2.5 exposures and various health effects.
---------------------------------------------------------------------------

    \125\ The causal framework draws upon the assessment and 
integration of evidence from across epidemiological, controlled 
human exposure, and toxicological studies, and the related 
uncertainties that ultimately influence our understanding of the 
evidence. This framework employs a five-level hierarchy that 
classifies the overall weight-of-evidence using the following 
categorizations: Causal relationship, likely to be causal 
relationship, suggestive of a causal relationship, inadequate to 
infer a causal relationship, and not likely to be a causal 
relationship (U.S. EPA, 2009a, Table 1-3). In the case of a 
``suggestive'' determination, ``the evidence is suggestive of a 
causal relationship with relevant pollutant exposures, but is 
limited because chance, bias and confounding cannot be ruled out. 
For example, at least one high-quality epidemiologic study shows an 
association with a given health outcome but the results of other 
studies are inconsistent'' (U.S. EPA, 2009a, Table 1-3).
---------------------------------------------------------------------------

    (4) There are several at-risk populations that may be especially 
susceptible or vulnerable to PM-related effects, including effects 
associated with exposures to coarse particles. These groups include 
those with preexisting heart and lung diseases, specific genetic 
differences, and lower socioeconomic status as well as the 
lifestages of childhood and older adulthood. Evidence for PM-related 
effects in these at-risk populations has expanded and is stronger 
than previously observed. There is emerging, though still limited, 
evidence for additional potentially at-risk populations, such as 
those with diabetes, people who are obese, pregnant women, and the 
developing fetus.
    (5) The Integrated Science Assessment concludes that currently 
available evidence is insufficient to draw distinctions in particle 
toxicity based on composition and notes that recent studies have 
reported that PM (both PM2.5 and PM10-2.5) 
from a variety of sources,

[[Page 3168]]

including sources likely to be present in urban and non-urban 
locations, is associated with adverse health effects.

    Although new PM10-2.5 scientific studies have become 
available since the last review and have expanded our understanding of 
the association between PM10-2.5 and adverse health effects 
(see above and U.S. EPA, 2009a, Chapter 6), important uncertainties 
remain. These uncertainties, and their implications for interpreting 
the scientific evidence, include the following:

    (1) The potential for confounding by co-occurring pollutants, 
especially PM2.5, has been addressed with co-pollutant 
models in only a relatively small number of PM10-2.5 
epidemiological studies (U.S. EPA, 2009a, section 2.3.3). This is a 
particularly important limitation given the relatively small body of 
experimental evidence (i.e., controlled human exposure and animal 
toxicological studies) available to support the associations between 
PM10-2.5 and adverse health effects. The net impact of 
such limitations is to increase uncertainty in characterizations of 
the extent to which PM10-2.5 itself, rather than one or 
more co-occurring pollutants, is responsible for the mortality and 
morbidity effects reported in epidemiological studies.
    (2) There is greater spatial variability in PM10-2.5 
concentrations than PM2.5 concentrations, resulting in 
increased exposure error for PM10-2.5 (U.S. EPA, 2009a, 
p. 2-8). Available measurements do not provide sufficient 
information to adequately characterize the spatial distribution of 
PM10-2.5 concentrations (U.S. EPA, 2009a, section 
3.5.1.1). The net effect of these uncertainties on 
PM10-2.5 epidemiological studies is to bias the results 
of such studies toward the null hypothesis. That is, as noted in the 
Integrated Science Assessment, these limitations in estimates of 
ambient PM10-2.5 concentrations ``would tend to increase 
uncertainty and make it more difficult to detect effects of 
PM10-2.5 in epidemiologic studies'' (U.S. EPA, 2009a, p. 
2-21).
    (3) Only a relatively small number of PM10-2.5 
monitoring sites are currently operating and such sites have been in 
operation for a relatively short period of time, limiting the 
spatial and temporal coverage for routine measurement of 
PM10-2.5 concentrations. Given these limitations in 
routine monitoring, epidemiological studies have employed different 
approaches for estimating PM10-2.5 concentrations. Given 
the relatively small number of PM10-2.5 monitoring sites, 
the relatively large spatial variability in ambient 
PM10-2.5 concentrations (see above), the use of different 
approaches to estimating ambient PM10-2.5 concentrations 
across epidemiological studies, and the limitations inherent in such 
estimates, the distributions of thoracic coarse particle 
concentrations over which reported health outcomes occur remain 
highly uncertain (U.S. EPA, 2009a, sections 2.2.3, 2.3.3, 2.3.4, and 
3.5.1.1).
    (4) There is relatively little information on the chemical and 
biological composition of PM10-2.5 and the effects 
associated with the various components (U.S. EPA, 2009a, section 
2.3.4). Without more information on the chemical speciation of 
PM10-2.5, the apparent variability in associations with 
health effects across locations is difficult to characterize (U.S. 
EPA, 2009a, section 6.5.2.3).
    (5) One of the implications of the uncertainties and limitations 
discussed above is that the Risk Assessment concluded it would not 
be appropriate to conduct a quantitative assessment of health risks 
associated with PM10-2.5. The lack of a quantitative 
PM10-2.5 risk assessment in the current review adds to 
the uncertainty in any conclusions about the extent to which 
revision of the current PM10 standard would be expected 
to improve the protection of public health, beyond the protection 
provided by the current standard.\126\
---------------------------------------------------------------------------

    \126\ As noted above, the EPA's decision not to conduct a 
quantitative risk assessment reflects uncertainty regarding the 
value of such an assessment, but does not indicate that health 
effects are not associated with exposure to thoracic coarse 
particles.
---------------------------------------------------------------------------

C. Consideration of the Current and Potential Alternative Standards in 
the Policy Assessment

    The following sections discuss the Policy Assessment's 
consideration of the current and potential alternative standards to 
protect against exposures to thoracic coarse particles (U.S. EPA, 
2011a, chapter 3). Section IV.C.1 discusses the consideration of the 
current standard while section IV.C.2 discusses the consideration of 
potential alternative standards in terms of the basic elements of a 
standard: Indicator, averaging time, form, and level.
1. Consideration of the Current Standard in the Policy Assessment
    As discussed above the 24-hour PM10 standard is meant to 
protect the public health against exposures to thoracic coarse 
particles (i.e., PM10-2.5). In considering the adequacy of 
the current PM10 standard, the Policy Assessment considered 
the health effects evidence linking short-term PM10-2.5 
exposures with mortality and morbidity (U.S. EPA, 2009a, chapters 2 and 
6), the ambient PM10 concentrations in PM10-2.5 
study locations (U.S. EPA, 2011a, section 3.2.1), the uncertainties and 
limitations associated with this health evidence (U.S. EPA, 2011a, 
section 3.2.1), and the consideration of these uncertainties and 
limitations as part of the weight of evidence conclusions in the 
Integrated Science Assessment (U.S. EPA, 2009a).
    In considering the health evidence, air quality information, and 
associated uncertainties as they relate to the current PM10 
standard, the Policy Assessment noted that a decision on the adequacy 
of the public health protection provided by that standard is a public 
health policy judgment in which the Administrator weighs the evidence 
and information, as well as its uncertainties. Therefore, depending on 
the emphasis placed on different aspects of the evidence, information, 
and uncertainties, consideration of different conclusions on the 
adequacy of the current standard could be supported. For example, the 
Policy Assessment noted that one approach to considering the evidence, 
information, and its associated uncertainties would be to place 
emphasis on the following (U.S. EPA, 2011a, section 3.2.3):

    (1) While most of PM10-2.5 effect estimates reported 
for mortality and morbidity were positive, many were not 
statistically significant, even in single-pollutant models. This 
includes effect estimates reported in study locations with 
PM10 concentrations above those allowed by the current 
24-hour PM10 standard.
    (2) The number of epidemiological studies that have employed co-
pollutant models to address the potential for confounding, 
particularly by PM2.5, remains limited. Therefore, the 
extent to which PM10-2.5 itself, rather than one or more 
co-pollutants, contributes to reported health effects remains 
uncertain.
    (3) Only a limited number of experimental studies provide 
support for the associations reported in epidemiological studies, 
resulting in further uncertainty regarding the plausibility of a 
causal link between PM10-2.5 and mortality and morbidity.
    (4) Limitations in PM10-2.5 monitoring and the 
different approaches used to estimate PM10-2.5 
concentrations across epidemiological studies result in uncertainty 
as to the ambient PM10-2.5 concentrations at which the 
reported effects occur.
    (5) The chemical and biological composition of 
PM10-2.5, and the effects associated with the various 
components, remains uncertain. Without more information on the 
chemical speciation of PM10-2.5, the apparent variability 
in associations across locations is difficult to interpret.
    (6) In considering the available evidence and its associated 
uncertainties, the Integrated Science Assessment concluded that the 
evidence is ``suggestive'' of a causal relationship between short-
term PM10-2.5 exposures and mortality, cardiovascular 
effects, and respiratory effects. These weight-of-evidence 
conclusions contrast with those for the relationships between 
PM2.5 exposures and adverse health effects, which were 
judged in the Integrated Science Assessment to be either ``causal'' 
or ``likely causal'' for mortality, cardiovascular effects, and 
respiratory effects.

    The Policy Assessment concluded that, to the extent a decision on 
the adequacy of the current 24-hour PM10 standard were to 
place emphasis on the considerations noted above, it could be judged 
that, although it remains appropriate to maintain a standard to protect 
against short-term exposures to

[[Page 3169]]

thoracic coarse particles, the available evidence suggests that the 
current 24-hour PM10 standard appropriately protects public 
health and provides an adequate margin of safety against effects that 
have been associated with PM10-2.5 exposures. Although such 
an approach to considering the adequacy of the current standard would 
recognize the positive, and in some cases statistically significant, 
associations between all types of PM10-2.5 and mortality and 
morbidity, it would place relatively greater emphasis on the 
limitations and uncertainties noted above, which tend to complicate the 
interpretation of that evidence.
    In addition, the Policy Assessment noted the judgment that, given 
the uncertainties and limitations in the PM10-2.5 health 
evidence and air quality information, it would not have been 
appropriate to conduct a quantitative assessment of health risks 
associated with PM10-2.5 (U.S. EPA, 2011a, p. 3-6; U.S. EPA, 
2010a, pp. 2-6 to 2-7, Appendix H). As discussed above, the lack of a 
quantitative PM10-2.5 risk assessment adds to the 
uncertainty associated with any characterization of potential public 
health improvements that would be realized with a revised standard.
    The Policy Assessment also noted an alternative approach to 
considering the evidence and its uncertainties would place emphasis on 
the following (U.S. EPA, 2011a, section 3.2.3):

    (1) Several multi-city epidemiological studies conducted in the 
U.S., Canada, and Europe, as well as a number of single-city 
studies, have reported generally positive, and in some cases 
statistically significant, associations between short-term 
PM10-2.5 concentrations and adverse health endpoints 
including mortality and cardiovascular-related and respiratory-
related hospital admissions and emergency department visits.
    (2) Both single-city and multi-city analyses, using different 
approaches to estimate ambient PM10-2.5 concentrations, 
have reported positive PM10-2.5 effect estimates in 
locations that would likely have met the current 24-hour 
PM10 standard. In a few cases, these PM10-2.5 
effect estimates were statistically significant.
    (3) While limited in number, studies that have evaluated co-
pollutant models have generally reported that PM10-2.5 
effect estimates remain positive, and in a few cases statistically 
significant, when these models include gaseous pollutants or fine 
particles.
    (4) Support for the plausibility of the associations reported in 
epidemiological studies is provided by a small number of controlled 
human exposure studies reporting that short-term (i.e., 2-hour) 
exposures to PM10-2.5 decrease heart rate variability and 
increase markers of pulmonary inflammation.

    This approach to considering the health evidence, air quality 
information, and the associated uncertainties would place substantial 
weight on the generally positive PM10-2.5 effect estimates 
that have been reported for mortality and morbidity, even those effect 
estimates that are not statistically significant. The Policy Assessment 
concluded that this could be judged appropriate given that consistent 
results have been reported across multiple studies using different 
approaches to estimate ambient PM10-2.5 concentrations and 
that exposure measurement error, which is likely to be larger for 
PM10-2.5 than for PM2.5, tends to bias the 
results of epidemiological studies toward the null hypothesis, making 
it less likely that associations will be detected. Such an approach 
would place less weight on the uncertainties and limitations in the 
evidence that resulted in the Integrated Science Assessment conclusions 
that the evidence is only suggestive of a causal relationship.
    Given all of the above, the Policy Assessment concluded that it 
would be appropriate to consider either retaining or revising the 
current 24-hour PM10 standard, depending on the approach 
taken to considering the available evidence, air quality information, 
and the uncertainties and limitations associated with that evidence and 
information (U.S. EPA, 2011a, section 3.2.3).

2. Consideration of Potential Alternative Standards in the Policy 
Assessment

    Given the conclusion that it would be appropriate to consider 
either retaining or revising the current PM10 standard, the 
Policy Assessment also considered what potential alternative standards, 
if any, could be supported by the available scientific evidence in 
order to increase public health protection against exposures to 
PM10-2.5. The Policy Assessment considered such potential 
alternative standards defined in terms of the elements of a standard 
(i.e., indicator, averaging time, form, and level). Key conclusions 
from the Policy Assessment regarding indicator, averaging time, and 
form included the following:

    (1) A PM10 indicator would continue to appropriately 
target protection against thoracic coarse particle exposures to 
those locations where the evidence is strongest for associations 
with adverse health effects (i.e., urban areas).
    (2) The available evidence supports the importance of 
maintaining a standard that protects against short-term exposures to 
all thoracic coarse particles. Given that the majority of this 
evidence is based on 24-hour average thoracic coarse particle 
concentrations, consideration of a 24-hour averaging time remains 
appropriate.
    (3) Given the limited body of evidence supporting 
PM10-2.5-related effects following long-term exposures, 
which resulted in the Integrated Science Assessment conclusion that 
the available evidence is ``inadequate'' to infer a causal 
relationship between long-term PM10-2.5 exposures and a 
variety of health effects, consideration of an annual thoracic 
coarse particle standard is not supported at this time.
    (4) To the extent it is judged appropriate to revise the current 
24-hour PM10 standard, it would be appropriate to 
consider revising the form to the 3-year average of the 98th 
percentile of the annual distribution of 24-hour PM10 
concentrations.

    In considering the available evidence and air quality information 
within the context of identifying potential alternative standard levels 
for consideration (assuming a decision were made that it is appropriate 
to amend the standard), the Policy Assessment first noted that a 
standard level as high as about 85 [mu]g/m\3\, for a 24-hour 
PM10 standard with a 98th percentile form, could be 
supported. Based on considering air quality concentrations in study 
locations, the Policy Assessment noted that such a standard level would 
be expected to maintain PM10 and PM10-2.5 
concentrations below those present in U.S. locations of single-city 
studies where PM10-2.5 effect estimates have been reported 
to be positive and statistically significant and below those present in 
some locations where single-city studies reported PM10-2.5 
effect estimates that were positive, but not statistically significant. 
These include some locations likely to have met the current 
PM10 standard during the study periods (U.S. EPA, 2011a, 
section 3.3.4).
    The Policy Assessment also noted that, based on analysis of the 
number of people living in counties that could violate the current and 
potential alternative PM10 standards, a 24-hour 
PM10 standard with a 98th percentile form and a level 
between 75 and 80 [mu]g/m\3\ would provide a level of public health 
protection that is generally equivalent, across the U.S., to that 
provided by the current standard. Given this, the Policy Assessment 
concluded that it would be appropriate to consider standard levels in 
the range of approximately 75 to 80 [mu]g/m\3\ (with a 98th percentile 
form), to the extent population counts were emphasized in comparing the 
public health protection provided by the current and potential 
alternative standards and to the extent it was judged appropriate to 
set a revised standard providing at least the level of public health 
protection that is provided by the current standard, based on such 
population counts (U.S. EPA, 2011a, section 3.3.4).

[[Page 3170]]

    The Policy Assessment also concluded that alternative approaches to 
considering the evidence could lead to consideration of standard levels 
below 75 [mu]g/m\3\ for a standard with a 98th percentile form. For 
example, a number of single-city epidemiological studies have reported 
positive, though not statistically significant, PM10-2.5 
effect estimates in locations with 98th percentile PM10 
concentrations below 75 [mu]g/m\3\. Given that exposure error is 
particularly important for PM10-2.5 epidemiological studies 
and can bias the results of these studies toward the null hypothesis 
(see section IV.B above), the Policy Assessment noted that it could be 
judged appropriate to place more weight on positive associations 
reported in these epidemiological studies, even when those associations 
are not statistically significant. In addition, the Policy Assessment 
noted that multi-city averages of 98th percentile PM10 
concentrations in the locations evaluated by U.S. multi-city studies of 
thoracic coarse particles (Zanobetti and Schwartz, 2009; Peng et al., 
2008) were near or below 75 ppb. Despite uncertainties in the extent to 
which effects reported in multi-city studies are associated with the 
short-term air quality in any particular location, the Policy 
Assessment noted that emphasis could be placed on these multi-city 
averaged concentrations. The Policy Assessment concluded that, to the 
extent more weight is placed on single-city studies reporting positive, 
but not statistically significant, PM10-2.5 effect estimates 
and on multi-city studies, it could be appropriate to consider standard 
levels as low as 65 [mu]g/m\3\ with a 98th percentile form (U.S. EPA, 
2011a, section 3.3.4).
    In considering potential alternative standard levels below 65 
[mu]g/m\3\, the Policy Assessment noted that the overall body of 
PM10-2.5 health evidence is relatively uncertain, with 
somewhat stronger support in U.S. studies for associations with 
PM10-2.5 in locations with 98th percentile PM10 
concentrations above 85 [mu]g/m\3\ than in locations with 98th 
percentile PM10 concentrations below 65 [mu]g/m\3\. In light 
of the limitations in the evidence for a relationship between 
PM10-2.5 and adverse health effects in locations with 
relatively low PM10 concentrations, along with the overall 
uncertainties in the body of PM10-2.5 health evidence as 
described above and in the Integrated Science Assessment, the Policy 
Assessment concluded that consideration of standard levels below 65 
[mu]g/m\3\ was not appropriate (U.S. EPA, 2011a, section 3.3.4).

D. CASAC Advice

    Following their review of the first and second draft Policy 
Assessments, CASAC provided advice and recommendations regarding the 
current and potential alternative standards for thoracic coarse 
particles (Samet, 2010c,d). With regard to the existing PM10 
standard, CASAC concluded that ``the current data, while limited, is 
sufficient to call into question the level of protection afforded the 
American people by the current standard'' (Samet, 2010d, p. 7). In 
drawing this conclusion, CASAC noted the positive associations in 
multi-city and single-city studies, including in locations with 
PM10 concentrations below those allowed by the current 
standard. In addition, CASAC gave ``significant weight to studies that 
have generally reported that PM10-2.5 effect estimates 
remain positive when evaluated in co-pollutant models'' and concluded 
that ``controlled human exposure PM10-2.5 studies showing 
decreases in heart rate variability and increases in markers of 
pulmonary inflammation are deemed adequate to support the plausibility 
of the associations reported in epidemiologic studies'' (Samet, 2010d, 
p. 7).\127\ Given all of the above conclusions CASAC recommended that 
``the primary standard for PM10 should be revised'' (Samet, 
2010d, p. ii and p. 7). In discussing potential revisions, while CASAC 
noted that the scientific evidence supports adoption of a standard at 
least as stringent as the current standard, they recommended revising 
the current standard in order to increase public health protection. In 
considering potential alternative standards, CASAC drew conclusions and 
made recommendations in terms of the major elements of a standard: 
indicator, averaging time, form, and level.
---------------------------------------------------------------------------

    \127\ Nonetheless, CASAC endorsed the Integrated Science 
Assessment weight of evidence conclusions for PM10-2.5 
(i.e., that the evidence is only ``suggestive'' of a causal 
relationship between short-term exposures and mortality, respiratory 
effects, and cardiovascular effects) (Samet, 2009e; Samet, 2009f).
---------------------------------------------------------------------------

    The CASAC agreed with the EPA staff's conclusions that the 
available evidence supports consideration in the current review of 
retaining the current PM10 indicator and the current 24-hour 
averaging time (Samet, 2010c, Samet, 2010d). Specifically, with regard 
to indicator, CASAC concluded that ``[w]hile it would be preferable to 
use an indicator that reflects the coarse PM directly linked to health 
risks (PM10-2.5), CASAC recognizes that there is not yet 
sufficient data to permit a change in the indicator from 
PM10 to one that directly measures thoracic coarse 
particles'' (Samet, 2010d, p. ii). In addition, CASAC ``vigorously 
recommends the implementation of plans for the deployment of a network 
of PM10-2.5 sampling systems so that future epidemiological 
studies will be able to more thoroughly explore the use of 
PM10-2.5 as a more appropriate indicator for thoracic coarse 
particles'' (Samet, 2010d, p. 7).
    The CASAC also agreed that the evidence supports consideration of a 
potential alternative form. Specifically, CASAC ``felt strongly that it 
is appropriate to change the statistical form of the PM10 
standard to a 98th percentile'' (Samet, 2010d, p.7). In reaching this 
conclusion, CASAC noted that ``[p]ublished work has shown that the 
percentile form has greater power to identify non-attainment and a 
smaller probability of misclassification relative to the expected 
exceedance form of the standard'' (Samet, 2010d. p. 7).
    With regard to standard level, in conjunction with a 98th 
percentile form, CASAC concluded that ``alternative standard levels of 
85 and 65 [mu]g/m\3\ (based on consideration of 98th percentile 
PM10 concentration) could be justified'' (Samet, 2010d, 
p.8). However, in considering the evidence and uncertainties, CASAC 
recommended a standard level from the lower part of the range discussed 
in the Policy Assessment, recommending a level ``somewhere in the range 
of 75 to 65 [mu]g/m\3\'' (Samet, 2010d, p. ii).
    In making this recommendation, CASAC noted that the number of 
people living in counties with air quality not meeting the current 
standard is approximately equal to the number living in counties that 
would not meet a 98th percentile standard with a level between 75 and 
80 [mu]g/m\3\. CASAC used this information as the basis for their 
conclusion that a 98th percentile standard between 75 and 80 [mu]g/m\3\ 
would be ``comparable to the degree of protection afforded to the 
current PM10 standard'' (Samet, 2010d, p. ii). Given this 
conclusion regarding the comparability of the current and potential 
alternative standards, as well as their conclusion on the public health 
protection provided by the current standard (i.e., that available 
evidence is sufficient to call it into question), CASAC recommended a 
level within a range of 75 to 65 [mu]g/m\3\ in order to increase public 
health protection, relative to that provided by the current standard 
(Samet 2010d, p. ii).

[[Page 3171]]

E. Administrator's Proposed Conclusions Concerning the Adequacy of the 
Current Primary PM10 Standard

    In considering the evidence and information as they relate to the 
adequacy of the current 24-hour PM10 standard, the 
Administrator first noted in the proposal that this standard is meant 
to protect the public health against effects associated with short-term 
exposures to PM10-2.5. In the last review, it was judged 
appropriate to maintain such a standard given the ``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'' (71 FR 61185, October 17, 2006). Given the continued 
expansion in the body of scientific evidence linking short-term 
PM10-2.5 to health outcomes such as premature death and 
hospital visits, discussed in detail in the Integrated Science 
Assessment (U.S. EPA, 2009a, Chapter 6) and summarized in the proposal, 
the Administrator provisionally concluded that the available evidence 
continued to support the appropriateness of maintaining a standard to 
protect the public health against effects associated with short-term 
(e.g., 24-hour) exposures to all PM10-2.5. In drawing 
provisional conclusions in the proposal as to whether the current 
PM10 standard remains requisite (i.e., neither more nor less 
stringent than necessary) to protect public health with an adequate 
margin of safety against such exposures, the Administrator considered 
the following:

    (1) The extent to which it is appropriate to maintain a standard 
that provides some measure of protection against all 
PM10-2.5, regardless of composition or source of origin;
    (2) The extent to which it is appropriate to retain a 
PM10 indicator for a standard meant to protect against 
exposures to ambient PM10-2.5; and
    (3) The extent to which the current PM10 standard 
provides an appropriate degree of public health protection.

    With regard to the first point, the proposal noted the conclusion 
from the last review that dosimetric, toxicological, occupational, and 
epidemiological evidence supported retention of a primary standard to 
provide some measure of protection against short-term exposures to all 
thoracic coarse particles, regardless of their source of origin or 
location, consistent with the Act's requirement that primary NAAQS 
provide requisite protection with an adequate margin of safety (71 FR 
61197). In that review, the EPA concluded that PM from a number of 
source types, including motor vehicle emissions, coal combustion, oil 
burning, and vegetative burning, are associated with health effects 
(U.S. EPA, 2004). This information formed part of the basis for the 
D.C. Circuit's holding that it was appropriate for the thoracic coarse 
particle standard to provide ``some protection from exposure to 
thoracic coarse particles * * * in all areas'' (American Farm Bureau 
Federation v. EPA, 559 F. 3d at 532-33).
    In considering this issue in the proposal, the Administrator judged 
that the expanded body of scientific evidence in this review provides 
even more support for a standard that protects against exposures to all 
thoracic coarse particles, regardless of their location or source of 
origin. Specifically, the Administrator noted that epidemiological 
studies have reported positive associations between PM10-2.5 
and mortality or morbidity in a large number of cities across North 
America, Europe, and Asia, encompassing a variety of environments where 
PM10-2.5 sources and composition are expected to vary 
widely. See 77 FR 38959. In considering this evidence, the Integrated 
Science Assessment concluded that ``many constituents of PM can be 
linked with differing health effects'' (U.S. EPA, 2009a, p. 2-26). 
While PM10-2.5 in most of these study areas is of largely 
urban origin, the Administrator noted that some recent studies have 
also linked mortality and morbidity with relatively high ambient 
concentrations of thoracic coarse particles of non-urban crustal 
origin. In considering these studies, she noted the Integrated Science 
Assessment's conclusion that ``PM (both PM2.5 and 
PM10-2.5) from crustal, soil or road dust sources or PM 
tracers linked to these sources are associated with cardiovascular 
effects'' (U.S. EPA, 2009a, p. 2-26).
    In light of this body of available evidence reporting 
PM10-2.5-associated health effects across different 
locations with a variety of sources, as well as the Integrated Science 
Assessment's conclusions regarding the links between adverse health 
effects and PM sources and composition, the Administrator provisionally 
concluded in the proposal that it is appropriate to maintain a standard 
that provides some measure of protection against exposures to all 
thoracic coarse particles, regardless of their location, source of 
origin, or composition (77 FR 38959-60).
    With regard to the second point, in considering the appropriateness 
of a PM10 indicator for a standard meant to provide such 
public health protection, the Administrator noted that the rationale 
used in the last review to support the unqualified PM10 
indicator (see above) remains relevant in the current review. 
Specifically, as an initial consideration, she noted that 
PM10 mass includes both coarse PM (PM10-2.5) and 
fine PM (PM2.5). As a result, the concentration of 
PM10-2.5 allowed by a PM10 standard set at a 
single level declines as the concentration of PM2.5 
increases. At the same time, the Administrator noted that 
PM2.5 concentrations tend to be higher in urban areas than 
in rural areas (U.S. EPA, 2005, p. 2-54, and Figures 2-23 and 2-24) 
and, therefore, a PM10 standard will generally allow lower 
PM10-2.5 concentrations in urban areas than in rural areas. 
77 FR 38960.
    In considering the appropriateness of this variation in allowable 
PM10-2.5 concentrations, the Administrator considered the 
relative strength of the evidence for health effects associated with 
PM10-2.5 of urban origin versus non-urban origin. She 
specifically noted that, as described above and similar to the 
scientific evidence available in the last review, the large majority of 
the available evidence for thoracic coarse particle health effects 
comes from studies conducted in locations with sources more typical of 
urban and industrial areas than of rural areas. Although as just noted, 
associations with adverse health effects have been reported in some 
study locations where PM10-2.5 is largely non-urban in 
origin (i.e., in dust storm studies), particle concentrations in these 
study areas are typically much higher than reported in study locations 
where the PM10-2.5 is of urban origin. Therefore, the 
Administrator noted that the strongest evidence for a link between 
PM10-2.5 and adverse health impacts, particularly for such a 
link at relatively low particle concentrations, comes from studies 
where exposure is to PM10-2.5 of urban or industrial origin. 
77 FR 38960.
    The Administrator also noted that chemical constituents present at 
higher levels in urban or industrial areas, including byproducts of 
incomplete combustion (e.g. polycyclic aromatic hydrocarbons) emitted 
as PM2.5 from motor vehicles as well as metals and other 
contaminants emitted from anthropogenic sources, can contaminate 
PM10-2.5 (U.S. EPA, 2004, p. 8-344; 71 FR 2665). While the 
Administrator acknowledged the uncertainty expressed in the Integrated 
Science Assessment regarding the extent to which, based on available 
evidence, particle composition can be linked to health outcomes, she 
also considered the possibility that PM10-2.5 contaminants 
typical of urban or industrial areas could increase the

[[Page 3172]]

toxicity of thoracic coarse particles in urban locations (77 FR 38960).
    Given that the large majority of the evidence for 
PM10-2.5 toxicity, particularly at relatively low particle 
concentrations, comes from study locations where thoracic coarse 
particles are of urban origin, and given the possibility that 
PM10-2.5 contaminants in urban areas could increase particle 
toxicity, the Administrator provisionally concluded in the proposal 
that it remains appropriate to maintain a standard that targets public 
health protection to urban locations. Specifically, she concluded at 
proposal that it is appropriate to maintain a standard that allows 
lower ambient concentrations of PM10-2.5 in urban areas, 
where the evidence is strongest that thoracic coarse particles are 
linked to mortality and morbidity, and higher concentrations in non-
urban areas, where the public health concerns are less certain. Id.
    Given all of the above considerations and conclusions, the 
Administrator judged that the available evidence supported retaining a 
PM10 indicator for a standard that is meant to protect 
against exposure to thoracic coarse particles. In reaching this initial 
judgment, she noted that, to the extent a PM10 indicator 
results in lower allowable concentrations of thoracic coarse particles 
in some areas compared to others, lower concentrations will be allowed 
in those locations (i.e., urban or industrial areas) where the science 
has shown the strongest evidence of adverse health effects associated 
with exposure to thoracic coarse particles and where we have the most 
concern regarding PM10-2.5 toxicity. Therefore, the 
Administrator provisionally concluded that 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 (77 FR 38960).
    In reaching this provisional conclusion, the Administrator also 
noted that, in their review of the second draft Policy Assessment, 
CASAC concluded that ``[w]hile it would be preferable to use an 
indicator that reflects the coarse PM directly linked to health risks 
(PM10-2.5), CASAC recognizes that there is not yet 
sufficient data to permit a change in the indicator from 
PM10 to one that directly measures thoracic coarse 
particles'' (Samet, 2010d, p. ii). In addition, CASAC ``vigorously 
recommends the implementation of plans for the deployment of a network 
of PM10-2.5 sampling systems so that future epidemiological 
studies will be able to more thoroughly explore the use of 
PM10-2.5 as a more appropriate indicator for thoracic coarse 
particles'' (Samet, 2010d, p. 7). Given this recommendation, the 
Administrator further judged that, although current evidence is not 
sufficient to identify a standard based on an alternative indicator 
that would be requisite to protect public health with an adequate 
margin of safety across the United States, consideration of alternative 
indicators (e.g., PM10-2.5) in future reviews is desirable 
and could be informed by additional research, as described in the 
Policy Assessment (U.S. EPA, 2011a, section 3.5).
    With regard to the third point, in evaluating the degree of public 
health protection provided by the current PM10 standard, the 
Administrator noted that the Policy Assessment discussed two different 
approaches to considering the scientific evidence and air quality 
information (U.S. EPA, 2011a, section 3.2.3). These different 
approaches, which are described above (section IV.C.1), lead to 
different conclusions regarding the appropriateness of the degree of 
public health protection provided by the current PM10 
standard. The Administrator further noted that the primary difference 
between the two approaches lies in the extent to which weight is placed 
on the following (U.S. EPA, 2011a, section 3.2.3):

    (1) The PM10-2.5 weight-of-evidence classifications 
presented in the Integrated Science Assessment concluding that the 
existing evidence is suggestive of a causal relationship between 
short-term PM10-2.5 exposures and mortality, 
cardiovascular effects, and respiratory effects (a classification 
supported by CASAC);
    (2) Individual PM10-2.5 epidemiological studies 
reporting associations in locations that meet the current 
PM10 standard, including associations that are not 
statistically significant;
    (3) The limited number of PM10-2.5 epidemiological 
studies that have evaluated co-pollutant models;
    (4) The limited number of PM10-2.5 controlled human 
exposure studies;
    (5) Uncertainties in the PM10-2.5 air quality 
concentrations reported in epidemiological studies, given 
limitations in PM10-2.5 monitoring data and the different 
approaches used across studies to estimate ambient 
PM10-2.5 concentrations; and
    (6) Uncertainties and limitations in the evidence that tend to 
call into question the presence of a causal relationship between 
PM10-2.5 exposures and mortality/morbidity.

    In evaluating the different possible approaches to considering the 
public health protection provided by the current PM10 
standard, the Administrator first noted that when the available 
PM10-2.5 scientific evidence and its associated 
uncertainties are considered, the Integrated Science Assessment 
concluded that the evidence is suggestive of a causal relationship 
between short-term PM10-2.5 exposures and mortality, 
cardiovascular effects, and respiratory effects. As discussed in 
section IV.B.1 above and in more detail in the Integrated Science 
Assessment (U.S. EPA, 2009a, section 1.5), a suggestive determination 
is made when the ``[e]vidence is suggestive of a causal relationship 
with relevant pollutant exposures, but is limited because chance, bias 
and confounding cannot be ruled out.'' In contrast, the Administrator 
noted that she proposed to strengthen the annual fine particle standard 
based on a body of scientific evidence judged sufficient to conclude 
that a causal relationship exists (i.e., mortality, cardiovascular 
effects) or is likely to exist (i.e., respiratory effects) (section 
III.B). 77 FR 38961. The suggestive judgment for PM10-2.5 
reflects the greater degree of uncertainty associated with this body of 
evidence, as discussed above (sections IV.B and IV.C) and summarized 
below.
    In the proposal (77 FR 38961), the Administrator noted that the 
important uncertainties and limitations associated with the scientific 
evidence and air quality information raise questions as to whether 
public health benefits would be achieved by revising the existing 
PM10 standard. Such uncertainties and limitations include 
the following:

    (1) While PM10-2.5 effect estimates reported for 
mortality and morbidity were generally positive, most were not 
statistically significant, even in single-pollutant models. This 
includes effect estimates reported in some study locations with 
PM10 concentrations above those allowed by the current 
24-hour PM10 standard.
    (2) The number of epidemiological studies that have employed co-
pollutant models to address the potential for confounding, 
particularly by PM2.5, remains limited. Therefore, the 
extent to which PM10-2.5 itself, rather than one or more 
co-pollutants, contributes to reported health effects is less 
certain.
    (3) Only a limited number of experimental studies (i.e., 
controlled human exposure and animal toxicological) provide support 
for the associations reported in epidemiological studies, resulting 
in further uncertainty regarding the plausibility of the 
associations between PM10-2.5 and mortality and morbidity 
reported in epidemiological studies.
    (4) Limitations in PM10-2.5 monitoring data and the 
different approaches used by epidemiological study researchers to 
estimate PM10-2.5 concentrations across epidemiological 
studies result in uncertainty in the ambient PM10-2.5 
concentrations at which the reported effects occur, increasing 
uncertainty in estimates of the extent to

[[Page 3173]]

which changes in ambient PM10-2.5 concentrations would 
likely impact public health.
    (5) The lack of a quantitative PM10-2.5 risk 
assessment further contributes to uncertainty regarding the extent 
to which any revisions to the current PM10 standard would 
be expected to improve the protection of public health, beyond the 
protection provided by the current standard (see section III.B.5 
above).
    (6) The chemical and biological composition of 
PM10-2.5, and the effects associated with the various 
components, remains uncertain. Without more information on the 
chemical speciation of PM10-2.5, the apparent variability 
in associations across locations is difficult to interpret.

    In considering these uncertainties and limitations, the 
Administrator noted in particular the considerable degree of 
uncertainty in the extent to which health effects reported in 
epidemiological studies are due to PM10-2.5 itself, as 
opposed to one or more co-occurring pollutants. As discussed above, 
this uncertainty reflects the fact that there are a relatively small 
number of PM10-2.5 studies that have utilized co-pollutant 
models, particularly co-pollutant models that have included 
PM2.5, and a very limited body of controlled human exposure 
evidence supporting the biological plausibility of a causal 
relationship between PM10-2.5 and mortality and morbidity at 
ambient concentrations. The Administrator noted that these important 
limitations in the overall body of health evidence introduce 
uncertainty into the interpretation of individual epidemiological 
studies, particularly those studies reporting associations with 
PM10-2.5 that are not statistically significant. Given this, 
the Administrator reached the provisional conclusion in the proposal 
that it is appropriate to place relatively little weight on 
epidemiological studies reporting associations with PM10-2.5 
that are not statistically significant in single-pollutant and/or co-
pollutant models. Id.
    With regard to this provisional conclusion, the Administrator noted 
that, for single-city mortality studies conducted in the United States 
where ambient PM10 concentration data were available for 
comparison to the current standard, positive and statistically 
significant PM10-2.5 effect estimates were only reported in 
study locations that would likely have violated the current 
PM10 standard during the study period (U.S. EPA, 2011a, 
Figure 3-2). In U.S. study locations that would likely have met the 
current standard, PM10-2.5 effect estimates for mortality 
were positive, but not statistically significant (U.S. EPA, 2011a, 
Figure 3-2). In considering U.S. study loc`ations where single-city 
morbidity studies were conducted, and which would likely have met the 
current PM10 standard during the study period, the 
Administrator noted that PM10-2.5 effect estimates were both 
positive and negative, with most not statistically significant (U.S. 
EPA, 2011a, Figure 3-3).
    In addition, in considering single-city analyses for the locations 
evaluated in a large U.S. multi-city mortality study (Zanobetti and 
Schwartz, 2009), the Administrator noted that associations in most of 
the study locations were not statistically significant and that this 
was the only study to estimate ambient PM10-2.5 
concentrations as the difference between county-wide PM10 
and PM2.5 mass. As discussed in the Policy Assessment and in 
the proposal, it is not clear how computed PM10-2.5 
measurements, such as those used by Zanobetti and Schwartz (2009), 
compare with the PM10-2.5 concentrations obtained in other 
studies either by direct measurement or by calculating the difference 
using co-located samplers (U.S. EPA, 2009a, section 6.5.2.3). For these 
reasons, in the proposal the Administrator noted that ``there is 
considerable uncertainty in interpreting the associations in these 
single-city analyses'' (77 FR 38961-62).
    The Administrator acknowledged that an approach to considering the 
available scientific evidence and air quality information that 
emphasizes the above considerations differs from the approach taken by 
CASAC. Specifically, in its review of the draft Policy Assessment CASAC 
placed a substantial amount of weight on individual studies, 
particularly those reporting positive health effects associations for 
PM10-2.5 in locations that met the current PM10 
standard during the study period. In emphasizing these studies, as well 
as the limited number of supporting studies that have evaluated co-
pollutant models and the small number of supporting experimental 
studies, CASAC concluded that ``the current data, while limited, is 
sufficient to call into question the level of protection afforded the 
American people by the current standard'' (Samet, 2010d, p. 7) and 
recommended revising the current PM10 standard (Samet, 
2010d).
    The Administrator carefully considered CASAC's advice and 
recommendations. She noted that in making its recommendation on the 
current PM10 standard, CASAC did not discuss its approach to 
considering the important uncertainties and limitations in the health 
evidence, and did not discuss how these uncertainties and limitations 
were reflected in its recommendation. Nor did CASAC discuss 
uncertainties in the reported concentrations of PM10-2.5 in 
the epidemiological studies, or how reported concentrations in the 
various studies relate to one another when differing measurement 
methodologies are used. As discussed above, such uncertainties and 
limitations contributed to the conclusions in the Integrated Science 
Assessment that the PM10-2.5 evidence is only suggestive of 
a causal relationship, a conclusion that CASAC endorsed (Samet, 
2009e,f). Given the importance of these uncertainties and limitations 
to the interpretation of the evidence, as reflected in the weight of 
evidence conclusions in the Integrated Science Assessment and as 
discussed above, the Administrator judged it appropriate to consider 
and account for them when drawing conclusions about the potential 
implications of individual PM10-2.5 health studies for the 
current standard.
    In light of the above approach to considering the scientific 
evidence, air quality information, and associated uncertainties, the 
Administrator reached the following provisional conclusions in the 
proposal:

    (1) When viewed as a whole the available evidence and 
information suggests that the degree of public health protection 
provided against short-term exposures to PM10-2.5 does 
not need to be increased beyond that provided by the current 
PM10 standard. This provisional conclusion noted the 
important uncertainties and limitations associated with the overall 
body of health evidence and air quality information for 
PM10-2.5, as discussed above and as reflected in the 
Integrated Science Assessment weight-of-evidence conclusions; that 
PM10-2.5 effect estimates for the most serious health 
effect, mortality, were not statistically significant in U.S. 
locations that met the current PM10 standard and where 
coarse particle concentrations were either directly measured or 
estimated based on co-located samplers; and that PM10-2.5 
effect estimates for morbidity endpoints were both positive and 
negative in locations that met the current standard, with most not 
statistically significant.
    (2) The degree of public health protection provided by the 
current standard is not greater than warranted. This provisional 
conclusion noted that positive and statistically significant 
associations with mortality were reported in single-city U.S. study 
locations likely to have violated the current PM10 
standard.\128\
---------------------------------------------------------------------------

    \128\ There are similarities with the conclusions drawn by the 
Administrator in the last review. There, the Administrator concluded 
that there was no 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 (71 FR 61202). In addition, the 
Administrator concluded that there was a high degree of uncertainty 
in the relevant population exposures implied by the morbidity 
studies suggesting that there is little basis for concluding that a 
greater degree of protection is warranted. Id. The D.C. Circuit in 
American Farm Bureau Federation v EPA explicitly endorsed this 
reasoning. 559 F. 3d at 534.


[[Page 3174]]


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    In reaching these provisional conclusions, the Administrator noted 
that the Policy Assessment also discussed the potential for a revised 
PM10 standard (i.e., with a revised form and level) to be 
``generally equivalent'' to the current standard, but to better target 
public health protection to locations where there is greater concern 
regarding PM10-2.5-associated health effects (U.S. EPA, 
2011a, sections 3.3.3 and 3.3.4). In considering such a potential 
revised standard, the Policy Assessment discussed the large amount of 
variability in PM10 air quality correlations across 
monitoring locations and over time (U.S. EPA, 2011a, Figure 3-7) and 
the regional variability in the relative degree of public health 
protection that could be provided by the current and potential 
alternative standards (U.S. EPA, 2011a, Table 3-2). In light of this 
variability, the Administrator noted the Policy Assessment conclusion 
that no single revised PM10 standard (i.e., with a revised 
form and level) would provide public health protection equivalent to 
that provided by the current standard, consistently over time and 
across locations (U.S. EPA, 2011a, section 3.3.4). That is, a revised 
standard, even one that is meant to be ``generally equivalent'' to the 
current PM10 standard, could increase protection in some 
locations while decreasing protection in others (77 FR 38962).
    In considering the appropriateness of revising the current 
PM10 standard in this way, the Administrator noted the 
following:

    (1) Positive PM10-2.5 effect estimates for mortality 
were not statistically significant in U.S. locations that met the 
current PM10 standard and where coarse particle 
concentrations were either directly measured or estimated based on 
co-located samplers, while positive and statistically significant 
associations with mortality were reported in locations likely to 
have violated the current PM10 standard.
    (2) Effect estimates for morbidity endpoints in locations that 
met the current standard were both positive and negative, with most 
not statistically significant.
    (3) Important uncertainties and limitations associated with the 
overall body of health evidence and air quality information for 
PM10-2.5, as discussed above and as reflected in the 
Integrated Science Assessment weight-of-evidence conclusions, call 
into question the extent to which the type of quantified and refined 
targeting of public health protection envisioned under a revised 
standard could be reliably accomplished.

    Given all of the above considerations, the Administrator noted that 
there is a large amount of uncertainty in the extent to which public 
health would be improved by changing the locations to which the 
PM10 standard targets protection. Therefore, she reached the 
provisional conclusion that the current PM10 standard should 
not be revised in order to change that targeting of protection.
    In considering all of the above, including the scientific evidence, 
the air quality information, the associated uncertainties, and CASAC's 
advice, the Administrator reached the provisional conclusion that the 
current 24-hour PM10 standard is requisite (i.e., neither 
more protective nor less protective than necessary) to protect public 
health with an adequate margin of safety against effects that have been 
associated with PM10-2.5. In light of this provisional 
conclusion, the Administrator proposed to retain the current 
PM10 standard in order to protect against health effects 
associated with short-term exposures to PM10-2.5 (77 FR 
38963).
    The Administrator recognized that her proposed conclusions and 
decision to retain the current PM10 standard differed from 
CASAC's recommendations, stemming from the differences in how the 
Administrator and CASAC considered and accounted for the evidence and 
its limitations and uncertainties. In light of CASAC's views and 
recommendation to revise the current PM10 standard, the 
Administrator welcomed the public's views on these different approaches 
to considering and accounting for the evidence and its limitations and 
uncertainties, as well as on the appropriateness of revising the 
primary PM10 standard, including revising the form and level 
of the standard. In doing so, the Administrator solicited comment on 
all aspects of the proposed decision, including her rationale for 
reaching the provisional conclusion that the current PM10 
standard is requisite to protect public health with an adequate margin 
of safety and the provisional conclusion that it is not appropriate to 
revise the current PM10 standard by setting a ``generally 
equivalent'' standard with the goal of better targeting public health 
protection.

F. Public Comments on the Administrator's Proposed Decision To Retain 
the Primary PM10 Standard

    This section discusses the major public comments received on the 
Administrator's proposed decision to retain the primary PM10 standard. 
Additional comments are addressed in the Response to Comments Document 
(U.S. EPA, 2012a).
    Many public commenters agreed with the Administrator's proposed 
decision to retain the current 24-hour primary PM10 
standard. Among those expressing a position on this proposed decision, 
industry groups and most State and Local commenters endorsed the 
Administrator's proposed rationale for retaining the current primary 
PM10 standard, including her consideration of the available 
scientific evidence and associated uncertainties and her consideration 
of CASAC recommendations.
    Although industry commenters generally agreed with the 
Administrator's proposed decision to retain the current primary 
PM10 standard, some also contended that the current standard 
is ``excessively precautionary'' (NMA and NCBA, 2012, p. 4) and a few 
expressed support for a less stringent standard for coarse particles 
that are comprised largely of crustal material. For example, the Coarse 
Particulate Matter Coalition (CPMC) (2012) and several other industry 
commenters recommended that the final decision allow application of a 
98th percentile form for the current standard (i.e. with its level of 
150 [mu]g/m\3\) in cases where coarse particles consist primarily of 
crustal material. Such an approach would allow more yearly exceedances 
of the existing standard level than are allowed with the current one-
expected-exceedance form. These industry commenters contended that a 
98th percentile form applied in this way would provide appropriate 
regulatory relief for areas where the evidence for coarse particle-
related health effects is relatively uncertain.
    In reaching her conclusion that the current primary PM10 
standard is requisite to protect public health with an adequate margin 
of safety, the Administrator considered the degree of public health 
protection provided by the current standard as a whole, including all 
elements of that standard (i.e., indicator, averaging time, form, 
level). As discussed above and in the following section, this 
conclusion reflects the Administrator's judgments that (1) the current 
standard appropriately provides some measure of protection against 
exposures to all thoracic coarse

[[Page 3175]]

particles, regardless of their location, source of origin, or 
composition and (2) the current standard appropriately allows lower 
ambient concentrations of PM10-2.5 in urban areas, where the 
evidence is strongest that thoracic coarse particles are linked to 
mortality and morbidity, and higher concentrations in non-urban areas, 
where the public health concerns are less certain.
    Because the considerations that led to these judgments, and to the 
conclusion that the current primary PM10 standard is 
requisite to protect public health, took into account the degree of 
public health protection provided by the standard as a whole, it would 
not be appropriate to consider revising one element of the standard 
(e.g., the form, as suggested by commenters in this case) without 
considering the extent to which the other elements of the standard 
should also be revised. The change in form requested by industry 
commenters, without also lowering the level of the standard, would 
markedly reduce the public health protection provided against exposures 
to thoracic coarse particles.\129\ However, industry commenters have 
not presented new evidence or analyses to support their conclusion that 
an appropriate degree of public health protection could be achieved by 
allowing the use of an alternative form (i.e., 98th percentile) for 
some coarse particles, while retaining the other elements of the 
current standard. Nor have these commenters presented new evidence or 
analyses challenging the basis for the conclusion in the proposal that 
the varying amounts of coarse particles allowed in urban versus non-
urban areas under the current 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. In light of this, EPA 
does not believe that a reduction in public health protection, such as 
that requested by industry commenters, is warranted.
---------------------------------------------------------------------------

    \129\ Based on regression analyses presented in the PA (U.S. 
EPA, 2011a, Figures 3-7 and 3-8), PM10 one-expected-
exceedance concentration-equivalent design values were between 
approximately 175 and 300 [mu]g/m\3\ at monitoring locations 
recording 3-year averages of 98th percentile 24-hour PM10 
concentrations around 150 [mu]g/m\3\ (i.e., the level of the current 
standard). This suggests that, depending on the location, a 24-hour 
PM10 standard with a 98th percentile form in conjunction 
with the current level (i.e., as recommended by these commenters) 
could be ``generally equivalent'' to a 24-hour PM10 
standard with a one-expected-exceedance form and a level as high as 
approximately 300 [mu]g/m\3\. Based on this analysis, a 24-hour 
PM10 standard with a 98th percentile form and a level of 
150 [mu]g/m\3\ would be markedly less health protective than the 
current standard.
---------------------------------------------------------------------------

    In further considering these comments, it is to be remembered that 
epidemiologic studies have not demonstrated that coarse particles of 
non-urban origin do not cause health effects, and commenters have not 
provided additional evidence on this point. While there are fewer 
studies of non-urban coarse particles than of urban coarse particles, 
several studies have reported positive and statistically significant 
associations between coarse particles of crustal, non-urban origin and 
mortality or morbidity (Ostro et al., 2003; Bell et al., 2008; Chan et 
al., 2008; Middleton et al., 2008; Perez et al., 2008). These studies 
formed part of the basis for the PM Integrated Science Assessment 
conclusion that ``recent studies have suggested that PM (both 
PM2.5 and PM10-2.5) from crustal, soil or road 
dust sources or PM tracers linked to these sources are associated with 
cardiovascular effects'' (U.S. EPA, 2009a, p. 2-26). Moreover, crustal 
coarse particles may be contaminated with toxic trace elements and 
other components from previously deposited fine PM from ubiquitous 
sources such as mobile source engine exhaust, as well as by toxic 
metals from smelters or other industrial activities, animal waste, or 
pesticides (U.S. EPA, 2004, p. 8-344). In the proposal, the 
Administrator acknowledged the potential for this type of contamination 
to increase the toxicity of coarse particles of crustal, non-urban 
origin (77 FR 38960; see also 71 FR 61190).
    In suggesting a change in the form of the current standard, 
industry commenters also did not address the manifold difficulties 
noted above, and in the last review, associated with developing an 
indicator that could reliably identify ambient mixes dominated by 
particular types of sources of coarse particles. See above and 71 FR 
61193. Yet such an indicator would be a prerequisite of the type of 
standard these commenters request.
    For all of the reasons discussed above, the EPA does not agree with 
industry commenters who recommended allowing the application of a 98th 
percentile form for the current standard in cases where coarse 
particles consist primarily of crustal material.
    Some industry commenters contended that the uncertainties and 
limitations that precluded a quantitative risk assessment also preclude 
revising the PM10 standard. Although the EPA agrees that 
there are important uncertainties and limitations in the extent to 
which the quantitative relationships between ambient 
PM10-2.5 and health outcomes can be characterized in risk 
models, the Agency does not agree that such limitations alone preclude 
the option of revising a NAAQS. As noted above, the lack of a 
quantitative PM10-2.5 risk assessment in the current review 
adds uncertainty to conclusions about the extent to which revision of 
the current PM10 standard would be expected to improve the 
protection of public health, beyond the protection provided by the 
current standard. However, the EPA does not agree that such 
uncertainties necessarily preclude revision of a NAAQS. Indeed, with 
respect to thoracic coarse particles, the DC Circuit noted that 
``[a]lthough the evidence of danger from coarse PM is, as the EPA 
recognizes, `inconclusive', the agency need not wait for conclusive 
findings before regulating a pollutant it reasonably believes may pose 
a significant risk to public health.'' 559 F. 3d at 533. Thus, the 
Administrator's conclusion that the current 24-hour PM10 
standard provides requisite protection of public health relies on her 
consideration of the broad body of evidence, rather than solely on the 
uncertainties that led to the decision not to conduct a quantitative 
assessment of PM10-2.5 health risks.
    Commenters representing a number of environmental groups and 
medical organizations disagreed with the Administrator's proposal to 
retain the current primary PM10 standard. These commenters 
generally requested that the EPA revise the PM10 standard to 
increase public health protection, consistent with the recommendations 
from CASAC.
    As discussed above and in the proposal, in reaching provisional 
conclusions in the proposal regarding the current standard, the 
Administrator carefully considered CASAC's advice and recommendations. 
She specifically noted that in making its recommendation on the current 
PM10 standard, CASAC did not discuss its approach to 
considering the important uncertainties and limitations in the health 
evidence, and did not discuss how these uncertainties and limitations 
were reflected in its recommendations. Such uncertainties and 
limitations contributed to the conclusions in the Integrated Science 
Assessment that the PM10-2.5 evidence is only suggestive of 
a causal relationship, a conclusion that CASAC endorsed (Samet, 
2009e,f). These commenters also did not address the important 
uncertainties in the epidemiologic studies on which their comments are 
based. Given the importance of these uncertainties and limitations to 
the interpretation of the

[[Page 3176]]

evidence, as reflected in the weight of evidence conclusions in the 
Integrated Science Assessment and as discussed in the proposal, the 
Administrator judges that it is appropriate to consider and account for 
them when drawing conclusions about the implications of individual 
PM10-2.5 health studies for the current standard. Commenters 
have not provided new information that would change the Administrator's 
views on the evidence and uncertainties.
    In recommending that the PM10 standard be revised, some 
commenters supported their conclusions by referencing studies that 
evaluated PM10, rather than PM10-2.5. These 
commenters contended that ``[t]he most relevant studies to the setting 
of a PM10 standard are the thousands of studies that have 
reported adverse effects associated with PM10 pollution'' 
(ALA et al., 2012).
    As discussed in the Policy Assessment, the proposal, and above, 
since the establishment of the primary PM2.5 standards, the 
purpose of the primary PM10 standard has been to protect 
against health effects associated with exposures to 
PM10-2.5. PM10 is the indicator, not the target 
pollutant. With regard to the appropriateness of considering 
PM10 health studies for the purpose of reaching conclusions 
on a standard meant to protect against exposures to 
PM10-2.5, the proposal noted that PM10 includes 
both fine and coarse particles, even in locations with the highest 
concentrations of PM10-2.5. Therefore, the extent to which 
PM10 effect estimates reflect associations with 
PM10-2.5 versus PM2.5 can be highly uncertain and 
it is often unclear how PM10 health studies should be 
interpreted when considering a standard meant to protect against 
exposures to PM10-2.5. Given this uncertainty and the 
availability of a number of PM10-2.5 health studies in this 
review, the Integrated Science Assessment considered 
PM10-2.5 studies, but not PM10 studies, when 
drawing weight-of-evidence conclusions regarding the coarse 
fraction.\130\ In light of the uncertainty in ascribing 
PM10-related health effects to the coarse or fine fractions, 
indicating that the best evidence for effects associated with exposures 
to PM10-2.5 comes from studies evaluating 
PM10-2.5 itself, and given CASAC's support for the approach 
adopted in the Integrated Science Assessment, which draws weight-of-
evidence conclusions for PM2.5 and PM10-2.5 but 
not for PM10 (Samet, 2009f), the EPA continues to conclude 
that it is appropriate to focus on PM10-2.5 health studies 
when considering the degree of public health protection provided by the 
current primary PM10 standard, a standard intended 
exclusively to provide protection against exposures to 
PM10-2.5.
---------------------------------------------------------------------------

    \130\ Although EPA relied in the 1997 review on evidence from 
PM10 studies, EPA did so out of necessity (i.e., there 
were as yet no reliable studies measuring PM10-2.5). In 
the 2006 review, EPA placed primary reliance on epidemiologic 
studies measuring or estimating PM10-2.5, although there 
were comparatively few such studies. In this review, a larger body 
of PM10-2.5 studies are available. EPA regards these 
studies as the evidence to be given principal weight in reviewing 
the adequacy of the PM10 standard.
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G. Administrator's Final Decision on the Primary PM10 Standard

    In reaching a final decision on the primary PM10 
standard, the Administrator takes into account the available scientific 
evidence, and the assessment of that evidence, in the Integrated 
Science Assessment; the analyses and staff conclusions presented in the 
Policy Assessment; the advice and recommendations of CASAC; and public 
comments on the proposal. In particular, as in the proposal, the 
Administrator places emphasis on her consideration of the following 
issues:

    (1) The extent to which it is appropriate to maintain a standard 
that provides some measure of protection against all 
PM10-2.5, regardless of composition or source of origin;
    (2) The extent to which it is appropriate to retain a 
PM10 indicator for a standard meant to protect against 
exposures to ambient PM10-2.5; and
    (3) The extent to which the current PM10 standard 
provides an appropriate degree of public health protection.

    Each of these issues is discussed below.
    With regard to the first issue, as in the proposal the 
Administrator judges that the expanded body of scientific evidence 
available in this review provides ample support for a standard that 
protects against exposures to all thoracic coarse particles, regardless 
of their location or source of origin. There was already ample evidence 
for this position in the previous review,\131\ and that evidence has 
since increased. Specifically, the Administrator notes that 
epidemiological studies have reported positive associations between 
PM10-2.5 and mortality or morbidity in a large number of 
cities across North America, Europe, and Asia, encompassing a variety 
of environments where PM10-2.5 sources and composition are 
expected to vary widely. In considering this evidence, the Integrated 
Science Assessment concludes that ``many constituents of PM can be 
linked with differing health effects'' (U.S. EPA, 2009a, p. 2-26). 
Although PM10-2.5 in most of these study areas is of largely 
urban origin, the Administrator notes that some recent studies have 
also linked mortality and morbidity with relatively high ambient 
concentrations of particles of non-urban crustal origin. In considering 
these studies, she notes the Integrated Science Assessment's conclusion 
that ``PM (both PM2.5 and PM10-2.5) from crustal, 
soil or road dust sources or PM tracers linked to these sources are 
associated with cardiovascular effects'' (U.S. EPA, 2009a, p. 2-26). 
The Administrator likewise notes CASAC's emphatic advice that a 
standard remains needed for all types of thoracic coarse PM.\132\ In 
light of this body of available evidence reporting PM10-2.5-
associated health effects across different locations with a variety of 
sources, the Integrated Science Assessment's conclusions regarding the 
links between adverse health effects and PM sources and composition, 
and CASAC's advice, the Administrator concludes in the current review 
that it is appropriate to maintain a standard that provides some 
measure of protection against exposures to all thoracic coarse 
particles, regardless of their location, source of origin, or 
composition.
---------------------------------------------------------------------------

    \131\ The D.C. Circuit agreed. See 559 F. 3d at 532-33.
    \132\ Indeed, CASAC recommended making the standard for all 
types of thoracic coarse PM more stringent (Samet, 2010d).
---------------------------------------------------------------------------

    With regard to the second issue, in considering the appropriateness 
of a PM10 indicator for a standard meant to provide such 
public health protection, the Administrator notes that the rationale 
used in the last review to support the unqualified PM10 
indicator remains relevant in the current review. Specifically, as an 
initial consideration, she notes that PM10 mass includes 
both coarse PM (PM10-2.5) and fine PM (PM2.5). As 
a result, the concentration of PM10-2.5 allowed by a 
PM10 standard set at a single level declines as the 
concentration of PM2.5 increases. At the same time, the 
Administrator notes that PM2.5 concentrations tend to be 
higher in urban areas than rural areas (U.S. EPA, 2005, p. 2-54, and 
Figures 2-23 and 2-24) and, therefore, a PM10 standard will 
generally allow lower PM10-2.5 concentrations in urban areas 
than in rural areas.
    In considering the appropriateness of this variation in allowable 
PM10-2.5 concentrations, the Administrator considers the 
relative strength of the evidence for health effects associated with 
PM10-2.5 of urban origin versus non-urban origin. She 
specifically notes that, as discussed in the proposal, the large 
majority of the available evidence for

[[Page 3177]]

thoracic coarse particle health effects comes from studies conducted in 
locations with sources more typical of urban and industrial areas than 
rural areas. While associations with adverse health effects have been 
reported in some study locations where PM10-2.5 is largely 
non-urban in origin (i.e., in dust storm studies), particle 
concentrations in these study areas are typically much higher than 
reported in study locations where the PM is of urban origin. Therefore, 
the Administrator notes that the strongest evidence for a link between 
PM10-2.5 and adverse health impacts, particularly for such a 
link at relatively low particle concentrations, comes from studies of 
urban or industrial PM10-2.5.
    The Administrator also notes that chemical constituents present at 
higher levels in urban or industrial areas, including byproducts of 
incomplete combustion (e.g. polycyclic aromatic hydrocarbons) emitted 
as PM2.5 from motor vehicles as well as metals and other 
contaminants emitted from anthropogenic sources, can contaminate 
PM10-2.5 (U.S. EPA, 2004, p. 8-344; 71 FR 2665, January 17, 
2006). While the Administrator acknowledges the uncertainty expressed 
in the Integrated Science Assessment regarding the extent to which 
particle composition can be linked to health outcomes based on 
available evidence, she also considers the possibility that 
PM10-2.5 contaminants typical of urban or industrial areas 
could increase the toxicity of thoracic coarse particles in urban 
locations.
    Given that the large majority of the evidence for 
PM10-2.5 toxicity, particularly at relatively low particle 
concentrations, comes from study locations where thoracic coarse 
particles are of urban origin, and given the possibility that 
PM10-2.5 contaminants in urban areas could increase particle 
toxicity, the Administrator concludes that it remains appropriate to 
maintain a standard that provides some protection in all areas but 
targets public health protection to urban locations. Specifically, she 
concludes that it is appropriate to maintain a standard that allows 
lower ambient concentrations of PM10-2.5 in urban areas, 
where the evidence is strongest that thoracic coarse particles are 
linked to mortality and morbidity, and higher concentrations in non-
urban areas, where the public health concerns are less certain.
    Given all of the above considerations and conclusions, the 
Administrator judges that the available evidence supports retaining a 
PM10 indicator for a standard that is meant to protect 
against exposures to thoracic coarse particles. In reaching this 
judgment, she notes that, to the extent a PM10 indicator 
results in lower allowable concentrations of thoracic coarse particles 
in some areas compared to others, lower concentrations will be allowed 
in those locations (i.e., urban or industrial areas) where the science 
has shown the strongest evidence of adverse health effects associated 
with exposure to thoracic coarse particles and where we have the most 
concern regarding PM10-2.5 toxicity. Therefore, the 
Administrator concludes that 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.133 134
---------------------------------------------------------------------------

    \133\ As discussed in the proposal, the Administrator 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 (insofar as that variability is 
understood). Although currently available information does not allow 
any more precise adjustment for relative toxicity, the Administrator 
believes the standard will generally ensure that the coarse particle 
levels allowed will be lower in urban areas and higher in non-urban 
areas. Addressing this qualitative relationship, the DC Circuit held 
that ``[i]t is true that the EPA relies on a qualitative analysis to 
describe the protection the coarse PM NAAQS will provide. But the 
fact that the EPA's analysis is qualitative rather than quantitative 
does not undermine its validity as an acceptable rationale for the 
EPA's decision.'' 559 F. 3d at 535.
    \134\ The D.C. Circuit agreed with similar conclusions in the 
last review and held that this rationale reasonably supported use of 
an unqualified PM10 indicator for thoracic coarse 
particles. American Farm Bureau Federation v. EPA, 559 F. 3d at 535-
36.
---------------------------------------------------------------------------

    In reaching this conclusion, the Administrator also notes that, in 
their review of the second draft Policy Assessment, CASAC concluded 
that ``[w]hile it would be preferable to use an indicator that reflects 
the coarse PM directly linked to health risks (PM10-2.5), 
CASAC recognizes that there is not yet sufficient data to permit a 
change in the indicator from PM10 to one that directly 
measures thoracic coarse particles'' (Samet, 2010d, p. ii). Thus, 
consistent the considerations presented above and with CASAC advice, 
the Administrator concludes that it is appropriate to retain 
PM10 as the indicator for thoracic coarse particles.\135\
---------------------------------------------------------------------------

    \135\ In addition, CASAC ``vigorously recommends the 
implementation of plans for the deployment of a network of 
PM10-2.5 sampling systems so that future epidemiological 
studies will be able to more thoroughly explore the use of 
PM10-2.5 as a more appropriate indicator for thoracic 
coarse particles'' (Samet, 2010d, p. 7). Consideration of 
alternative indicators (e.g., PM10-2.5) in future reviews 
could be informed by additional research, as described in the Policy 
Assessment (U.S. EPA, 2011a, section 3.5).
---------------------------------------------------------------------------

    With regard to the third issue, in evaluating the degree of public 
health protection provided by the current PM10 standard, the 
Administrator first notes that when the available PM10-2.5 
scientific evidence and its associated uncertainties were considered, 
the Integrated Science Assessment concluded that the evidence is 
suggestive of a causal relationship between short-term 
PM10-2.5 exposures and mortality, cardiovascular effects, 
and respiratory effects. As discussed above and in more detail in the 
Integrated Science Assessment (U.S. EPA, 2009a, section 1.5), a 
suggestive determination is made when the ``[e]vidence is suggestive of 
a causal relationship with relevant pollutant exposures, but is limited 
because chance, bias and confounding cannot be ruled out.'' In 
contrast, the Administrator notes that she is strengthening the annual 
fine particle standard based on a body of scientific evidence judged 
sufficient to conclude that a causal relationship exists (i.e., 
mortality, cardiovascular effects) or is likely to exist (i.e., 
respiratory effects). The suggestive judgment for PM10-2.5 
reflects the greater degree of uncertainty associated with this body of 
evidence, as discussed above and in more detail in the proposal, and as 
summarized below.
    The Administrator notes that the important uncertainties and 
limitations associated with the scientific evidence and air quality 
information raise questions as to whether public health benefits would 
be achieved by revising the existing PM10 standard. Such 
uncertainties and limitations include the following:

    (1) While PM10-2.5 effect estimates reported for 
mortality and morbidity were generally positive, most were not 
statistically significant, even in single-pollutant models. This 
includes effect estimates reported in some study locations with 
PM10 concentrations above those allowed by the current 
24-hour PM10 standard.
    (2) The number of epidemiological studies that have employed co-
pollutant models to address the potential for confounding, 
particularly by PM2.5, remains limited. Therefore, the 
extent to which PM10-2.5 itself, rather than one or more 
co-pollutants, contributes to reported health effects remains 
uncertain.
    (3) Only a limited number of experimental studies provide 
support for the associations reported in epidemiological studies, 
resulting in further uncertainty regarding the plausibility of the 
associations between PM10-2.5 and mortality and morbidity 
reported in epidemiological studies.

[[Page 3178]]

    (4) Limitations in PM10-2.5 monitoring data and the 
different approaches used to estimate PM10-2.5 
concentrations across epidemiological studies result in uncertainty 
in the ambient PM10-2.5 concentrations at which the 
reported effects occur, increasing uncertainty in estimates of the 
extent to which changes in ambient PM10-2.5 
concentrations would likely impact public health.
    (5) The lack of a quantitative PM10-2.5 risk 
assessment further contributes to uncertainty regarding the extent 
to which any revisions to the current PM10 standard would 
be expected to improve the protection of public health, beyond the 
protection provided by the current standard (see section III.B.5 
above).
    (6) The chemical and biological composition of 
PM10-2.5, and the effects associated with the various 
components, remains uncertain. Without more information on the 
chemical speciation of PM10-2.5, the apparent variability 
in associations across locations is difficult to characterize.

    In considering these uncertainties and limitations, the 
Administrator notes in particular the considerable degree of 
uncertainty in the extent to which health effects reported in 
epidemiological studies are due to PM10-2.5 itself, as 
opposed to one or more co-occurring pollutants. As discussed above, 
this uncertainty reflects the fact that there are a relatively small 
number of PM10-2.5 studies that have evaluated co-pollutant 
models, particularly co-pollutant models that have included 
PM2.5, and a very limited body of controlled human exposure 
evidence supporting the plausibility of a causal relationship between 
PM10-2.5 and mortality and morbidity at ambient 
concentrations. The Administrator notes that these important 
limitations in the overall body of health evidence introduce 
uncertainty into the interpretation of individual epidemiological 
studies, particularly those studies reporting associations with 
PM10-2.5 that are not statistically significant. Given this, 
the Administrator reaches the conclusion that it is appropriate to 
place relatively little weight on epidemiological studies reporting 
associations with PM10-2.5 that are not statistically 
significant in single-pollutant and/or co-pollutant models.\136\
---------------------------------------------------------------------------

    \136\ The Administrator acknowledges that this approach to 
interpreting the evidence differs in emphasis from the approach she 
has adopted for the evidence relating to PM2.5. As 
discussed above in section III.E.4, for fine particles the 
Administrator has considered not only whether study results are 
statistically significant (or remain so after application of co-
pollutant models), but she also places emphasis on the overall 
pattern of results across the epidemiological literature. This 
includes giving some credence to studies that reported statistically 
non-significant associations. This difference in emphasis stems from 
the much stronger overall body of evidence available for fine 
particles, compared to coarse particles. As discussed above, when 
the available PM2.5 scientific evidence and its 
associated uncertainties were considered, the Integrated Science 
Assessment concluded that the evidence was sufficient to conclude 
that causal relationships exist with mortality and cardiovascular 
effects, and that a causal relationship is likely to exist with 
respiratory effects. In contrast, the Integrated Science Assessment 
concluded that the evidence is suggestive of a causal relationship 
between short-term PM10-2.5 exposures and mortality, 
cardiovascular effects, and respiratory effects. A suggestive 
determination is made when the ``[e]vidence is suggestive of a 
causal relationship with relevant pollutant exposures, but is 
limited because chance, bias and confounding cannot be ruled out'' 
(U.S. EPA, 2009a, section 1.5). The suggestive judgment for 
PM10-2.5 reflects the greater degree of uncertainty 
associated with this body of evidence.
---------------------------------------------------------------------------

    With regard to this conclusion, the Administrator notes that, for 
single-city mortality studies conducted in the United States where 
ambient PM10 concentration data were available for 
comparison to the current standard, positive and statistically 
significant PM10-2.5 effect estimates were only reported in 
study locations that would likely have violated the current 
PM10 standard during the study period (U.S. EPA, 2011a, 
Figure 3-2). In U.S. study locations that would likely have met the 
current standard, PM10-2.5 effect estimates for mortality 
were positive, but not statistically significant (U.S. EPA, 2011a, 
Figure 3-2). In considering U.S. study locations where single-city 
morbidity studies were conducted, and which would likely have met the 
current PM10 standard during the study period, the 
Administrator notes that PM10-2.5 effect estimates were both 
positive and negative, with most not statistically significant (U.S. 
EPA, 2011a, Figure 3-3).
    In addition, in considering single-city analyses for the locations 
evaluated in a large U.S. multi-city mortality study (Zanobetti and 
Schwartz, 2009), the Administrator notes that associations in most of 
the study locations were not statistically significant and that this 
was the only study to estimate ambient PM10-2.5 
concentrations as the difference between county-wide PM10 
and PM2.5 mass. As discussed in the proposal, the 
Administrator notes that it is not clear how computed 
PM10-2.5 measurements, such as those used by Zanobetti and 
Schwartz (2009), compare with the PM10-2.5 concentrations 
obtained in other studies either by direct measurement by calculating 
the difference using co-located samplers (U.S. EPA, 2009a, section 
6.5.2.3). For these reasons, as in the proposal, the Administrator 
notes that there is considerable uncertainty in interpreting the 
associations, and especially the concentrations at which such 
associations may have occurred, in these single-city analyses.
    The Administrator acknowledges that an approach to considering the 
available scientific evidence and air quality information that 
emphasizes the above considerations differs from the approach taken by 
CASAC. Specifically, CASAC placed a substantial amount of weight on 
individual studies, particularly those reporting positive health 
effects associations in locations that met the current PM10 
standard during the study period. In emphasizing these studies, as well 
as the limited number of supporting studies that have evaluated co-
pollutant models and the small number of supporting experimental 
studies, CASAC concluded that ``the current data, while limited, is 
sufficient to call into question the level of protection afforded the 
American people by the current standard'' (Samet, 2010d, p. 7) and 
recommended revising the current PM10 standard (Samet, 
2010d).
    The Administrator has carefully considered CASAC's advice and 
recommendations. She notes that in making its recommendation on the 
current PM10 standard, CASAC did not discuss its approach to 
considering the important uncertainties and limitations in the health 
evidence, and did not discuss how these uncertainties and limitations 
are reflected in its recommendation. As discussed above, such 
uncertainties and limitations contributed to the conclusions in the 
Integrated Science Assessment that the PM10-2.5 evidence is 
only suggestive of a causal relationship, a conclusion that CASAC 
endorsed (Samet, 2009e,f). Given the importance of these uncertainties 
and limitations to the interpretation of the evidence, as reflected in 
the weight of evidence conclusions in the Integrated Science Assessment 
and as discussed above, the Administrator judges that it is appropriate 
to consider and account for them when drawing conclusions about the 
potential implications of individual PM10-2.5 health studies 
for the current standard.
    In light of the above approach to considering the scientific 
evidence, air quality information, and associated uncertainties, the 
Administrator reaches the following conclusions:

    (1) When viewed as a whole the available evidence and 
information suggests that the degree of public health protection 
provided against short-term exposures to PM10-2.5 should 
be maintained but does not need to be increased beyond that provided 
by the current PM10 standard. This conclusion emphasizes 
the important uncertainties and limitations associated with the 
overall body

[[Page 3179]]

of health evidence and air quality information for 
PM10-2.5, as discussed above and as reflected in the 
Integrated Science Assessment weight-of-evidence conclusions; that 
PM10-2.5 effect estimates for the most serious health 
effect, mortality, were not statistically significant in U.S. 
locations that met the current PM10 standard and where 
coarse particle concentrations were either directly measured or 
estimated based on co-located samplers; and that PM10-2.5 
effect estimates for morbidity endpoints were both positive and 
negative in locations that met the current standard, with most not 
statistically significant.\137\
---------------------------------------------------------------------------

    \137\ This is not to say that the EPA could not adopt or revise 
a standard for a pollutant for which the evidence is suggestive of a 
causal relationship. Indeed, with respect to thoracic coarse 
particles itself, the DC Circuit noted that ``[a]lthough the 
evidence of danger from coarse PM is, as the EPA recognizes, 
`inconclusive', the agency need not wait for conclusive findings 
before regulating a pollutant it reasonably believes may pose a 
significant risk to public health.'' American Farm Bureau Federation 
v EPA 559 F. 3d at 533. As explained in the text above, it is the 
Administrator's judgment that significant uncertainties presented by 
the evidence and information before her in this review, both as to 
causality and as to concentrations at which effects may be 
occurring, best support a decision to retain rather than revise the 
current primary 24-hour PM10 standard.
---------------------------------------------------------------------------

    (2) The degree of public health protection provided by the 
current standard is not greater than warranted. This conclusion 
notes that positive and statistically significant associations with 
mortality were reported in single-city U.S. study locations likely 
to have violated the current PM10 standard.\138\
---------------------------------------------------------------------------

    \138\ There are similarities with the conclusions drawn by the 
Administrator in the last review. There, the Administrator concluded 
that there was no 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. 71 FR 61202. In addition, the 
Administrator concluded that there was a high degree of uncertainty 
in the relevant population exposures implied by the morbidity 
studies suggesting that there is little basis for concluding that a 
greater degree of protection is warranted. Id. The D.C. Circuit in 
American Farm Bureau Federation v EPA explicitly endorsed this 
reasoning. 559 F. 3d at 534.

    In reaching these conclusions, the Administrator notes that the 
Policy Assessment also discussed the potential for a revised 
PM10 standard (i.e., with a revised form and level) to be 
``generally equivalent'' to the current standard, but to better target 
public health protection to locations where there is greater concern 
regarding PM10-2.5-associated health effects (U.S. EPA, 
2011a, sections 3.3.3 and 3.3.4).\139\ In considering such a potential 
revised standard, the Policy Assessment discusses the large amount of 
variability in PM10 air quality correlations across 
monitoring locations and over time (U.S. EPA, 2011a, Figure 3-7) and 
the regional variability in the relative degree of public health 
protection that could be provided by the current and potential 
alternative standards (U.S. EPA, 2011a, Table 3-2). In light of this 
variability, the Administrator notes the Policy Assessment conclusion 
that no single revised PM10 standard (i.e., with a revised 
form and level) would provide public health protection equivalent to 
that provided by the current standard, consistently over time and 
across locations (U.S. EPA, 2011a, section 3.3.4). That is, a revised 
standard, even one that is meant to be ``generally equivalent'' to the 
current PM10 standard, could increase protection in some 
locations while decreasing protection in other locations.
---------------------------------------------------------------------------

    \139\ As discussed in detail above (section IV.C.2.d) and in the 
Policy Assessment (U.S. EPA, 2011a, sections 3.3.3 and 3.3.4), a 
revised standard that is generally equivalent to the current 
PM10 standard could provide a degree of public health 
protection that is similar to the degree of protection provided by 
the current standard, across the United States as a whole. However, 
compared to the current PM10 standard, such a generally 
equivalent standard would change the degree of public health 
protection provided in some specific areas, providing increased 
protection in some locations and decreased protection in other 
locations.
---------------------------------------------------------------------------

    In considering the appropriateness of revising the current 
PM10 standard in this way, the Administrator notes the 
following:

    (1) As discussed above, positive PM10-2.5 effect 
estimates for mortality were not statistically significant in U.S. 
locations that met the current PM10 standard and where 
coarse particle concentrations were either directly measured or 
estimated based on co-located samplers, while positive and 
statistically significant associations with mortality were reported 
in locations likely to have violated the current PM10 
standard.
    (2) Also as discussed above, effect estimates for morbidity 
endpoints in locations that met the current standard were both 
positive and negative, with most not statistically significant.
    (3) Important uncertainties and limitations associated with the 
overall body of health evidence and air quality information for 
PM10-2.5, as discussed above and as reflected in the 
Integrated Science Assessment weight-of-evidence conclusions, call 
into question the extent to which the type of quantified and refined 
targeting of public health protection envisioned under a revised 
standard could be reliably accomplished.

    Given all of the above considerations, the Administrator notes that 
there is a large amount of uncertainty in the extent to which public 
health would be improved by changing the locations to which the 
PM10 standard targets protection. Therefore, she reaches the 
conclusion that the current PM10 standard should not be 
revised in order to change that targeting of protection.
    In considering all of the above, including the scientific evidence, 
the air quality information, the associated uncertainties, CASAC's 
advice, and public comments received on the proposed rule, the 
Administrator reaches the conclusion in the current review that the 
existing 24-hour PM10 standard, with its one-expected 
exceedance form and a level of 150 [mu]g/m\3\, is requisite (i.e., 
neither more protective nor less protective than necessary) to protect 
public health with an adequate margin of safety against effects that 
have been associated with PM10-2.5. In light of this 
conclusion, with this rule the Administrator retains the current 
PM10 standard.

V. Communication of Public Health Information

    Sections 319(a)(1) and (3) of the CAA require the EPA to establish 
a uniform air quality index for reporting of air quality. These 
sections specifically direct the Administrator to ``promulgate 
regulations establishing an air quality monitoring system throughout 
the United States which utilizes uniform air quality monitoring 
criteria and methodology and measures such air quality according to a 
uniform air quality index'' and ``provides for daily analysis and 
reporting of air quality based upon such uniform air quality index * * 
*'' In 1979, the EPA established requirements for index reporting (44 
FR 27598, May 10, 1979). The requirement for State and local agencies 
to report the AQI appears in 40 CFR 58.50, and the specific 
requirements (e.g., what to report, how to report, reporting frequency, 
calculations) are in appendix G to 40 CFR part 58.
    Information on the public health implications of ambient 
concentrations of criteria pollutants is currently made available 
primarily by AQI reporting through EPA's AIRNow Web site.\140\ The 
current AQI has been in use since its inception in 1999.\141\ It 
provides accurate, timely, and easily understandable information about 
daily levels of pollution (40 CFR 58.50). The AQI establishes a 
nationally uniform system of indexing pollution levels for ozone, 
carbon monoxide, nitrogen

[[Page 3180]]

dioxide, PM, and sulfur dioxide. The AQI is also recognized 
internationally as a proven tool to effectively communicate air quality 
information to the public.
---------------------------------------------------------------------------

    \140\ See http://www.airnow.gov/.
    \141\ In 1976, the EPA established a nationally uniform air 
quality index, then called the Pollutant Standard Index (PSI), for 
use by State and local agencies on a voluntary basis (41 FR 37660, 
September 7, 1976). In August 1999, the EPA adopted revisions to 
this air quality index (64 FR 42530, August 4, 1999) and renamed the 
index the AQI.
---------------------------------------------------------------------------

    The AQI converts pollutant concentrations in a community's air to a 
number on a scale from 0 to 500. Reported AQI values enable the public 
to know whether air pollution levels in a particular location are 
characterized as good (0-50), moderate (51-100), unhealthy for 
sensitive groups (101- 150), unhealthy (151-200), very unhealthy (201-
300), or hazardous (301-500). The AQI index value of 100 typically 
corresponds to the level of the short-term (e.g., daily or hourly 
standard) NAAQS for each pollutant. Below an index value of 100, an 
intermediate value of 50 was defined either as the level of the annual 
standard if an annual standard has been established (e.g., 
PM2.5, nitrogen dioxide), or as a concentration equal to 
one-half the value of the short-term standard used to define an index 
value of 100 (e.g., carbon monoxide). An AQI value greater than 100 
means that a pollutant is in one of the unhealthy categories (i.e., 
unhealthy for sensitive groups, unhealthy, very unhealthy, or 
hazardous) on a given day. An AQI value at or below 100 means that a 
pollutant concentration is in one of the satisfactory categories (i.e., 
moderate or good). The underlying health information that supports the 
NAAQS review also supports the selection of the AQI ``breakpoints''--
the ambient concentrations that delineate the various AQI categories 
for each pollutant.
    Historically, state and local agencies have primarily used the AQI 
to provide general information to the public about air quality and its 
relationship to public health. For more than a decade, many states and 
local agencies, as well as the EPA and other Federal agencies, have 
been developing new and innovative programs and initiatives to provide 
more information to the public in a more timely way. These initiatives, 
including air quality forecasting, real-time data reporting through the 
AirNow Web site, and state and local air quality action day programs, 
can serve to provide useful, up-to-date, and timely information to the 
public about air pollution and its effects. Such information will help 
individuals take actions to avoid or to reduce exposures to ambient 
pollution at levels of concern to them. Thus, these programs have 
significantly broadened the ways in which state and local agencies can 
meet the nationally uniform AQI reporting requirements and contribute 
to state and local efforts to provide community health protection.
    With respect to an AQI value of 50, the historical approach is to 
set it at the same level of the annual primary standard, if there is 
one. This is consistent with the previous AQI sub-index for 
PM2.5, in which the AQI value of 50 was set at 15 [micro]g/
m\3\ in 1999, consistent with the level of the annual PM2.5 
standard at that time. In recognition of the proposed change to the 
annual PM2.5 standard summarized in section III.F of the 
proposal, the EPA proposed a conforming change to the PM2.5 
sub-index of the AQI to be consistent with the proposed change to the 
annual standard. As discussed below, no state or local agencies, or 
their organizations (e.g., NACAA), that commented on the proposed 
changes to the AQI disagreed with our proposed approach. Based on these 
comments, the EPA continues to see no basis for deviating from this 
approach in this review. Thus, the EPA is taking final action to set an 
AQI value of 50 at 12.0 [mu]g/m\3\, 24-hour average, consistent with 
the final decision on the annual PM2.5 standard level 
(section III.F).
    With respect to an AQI value of 100, which is the basis for 
advisories to individuals in sensitive groups, in the proposal we 
described two general approaches that could be used to select the 
associated PM2.5 level. By far the most common approach, 
which has been used with all of the other sub-indices, is to set an AQI 
value of 100 at the same level as the short-term standard. In the 
proposal, the EPA recognized that some state and local air quality 
agencies have expressed a strong preference that the Agency set an AQI 
value of 100 equal to any short-term standard (77 FR 38964). These 
agencies typically express the view that this linkage is useful for the 
purpose of communicating with the public about the standard, as well as 
providing consistent messages about the health impacts associated with 
daily air quality. The EPA proposed to use this approach to set the AQI 
value of 100 at 35 [mu]g/m\3\, 24-hour average, consistent with the 
proposed decision to retain the current 24-hour PM2.5 
standard. Id.
    An alternative approach discussed in the proposal (77 FR 38964), 
was to directly evaluate the health effects evidence to select the 
level for an AQI value of 100. This was the approach used in the 1999 
rulemaking to set the AQI value of 100 at a level of 40 [mu]g/m\3\, 24-
hour average,\142\ when the 24-hour standard level was 65 [mu]g/m\3\. 
This alternative approach was used in the case of the PM2.5 
sub-index, because the annual and 24-hour PM2.5 standards 
set in 1997 were designed to work together, and the intended degree of 
health protection against short-term risks was not defined by the 24-
hour standard alone, but rather by the combination of the two standards 
working in concert. Indeed, at that time, the 24-hour standard was set 
to provide supplemental protection relative to the principal protection 
provided by the annual standard. In the proposal, the EPA solicited 
comment on this alternative approach in recognition that, as proposed, 
the 24-hour PM2.5 standard is intended to continue to 
provide supplemental protection against effects associated with short-
term exposures of PM2.5 by working in conjunction with the 
annual standard to reduce 24-hour exposures to PM2.5. The 
EPA recognized that in the past, some state and local air quality 
agencies have expressed support for this alternative approach. Using 
this alternative approach could have resulted in consideration of a 
lower level for an AQI value of 100, based on the discussion of the 
health information pertaining to the level of the 24-hour standard in 
section III.E.4 of the proposal. The EPA encouraged state and local air 
quality agencies to comment on both the approach and the level at which 
to set an AQI value of 100 together with any supporting rationale. Of 
the state or local agencies, or their organizations (e.g., NACAA), that 
commented on the proposed changes to the AQI, only one organization, 
NESCAUM, expressed some support for this approach. In its comments, 
NESCAUM expressed support for a 24-hour standard set at 30 [mu]g/m\3\, 
24-hour average. NESCAUM also expressed the view that EPA should 
carefully consider how to set the breakpoint for an AQI value of 100. 
NESCAUM expressed the view that if the EPA were to keep the 24-hour 
PM2.5 standard at 35 [mu]g/m\3\, the annual standard would 
be controlling, and a 24-hour breakpoint at that level (35 [mu]g/m\3\) 
would not be very effective for the purposes of public health 
messaging. However, other agencies, such as Georgia Department of 
Natural Resources (Georgia DNR), expressed the view that linkage 
between the short-term standard and the AQI of 100 is useful for the 
purpose of communicating with the public about the standard as well as 
providing consistent messages about the health

[[Page 3181]]

impacts associated with the daily air quality. Based on these comments, 
the EPA sees no basis for deviating from the approach proposed in this 
review. Thus, the EPA is taking final action to set an AQI value of 100 
at 35 [mu]g/m\3\, 24-hour average, consistent with the final decision 
on the 24-hour PM2.5 standard level (section III.F).
---------------------------------------------------------------------------

    \142\ Currently, we are cautioning members of sensitive groups 
at the AQI value of 100 at 35 [mu]g/m\3\, 24-hour average, 
consistent with more recent guidance from the EPA with regard to the 
development of State emergency episode contingency plans (Harnett, 
2009, Attachment B).
---------------------------------------------------------------------------

    With respect to an AQI value of 150, this level is based upon the 
same health effects information that informs the selection of the level 
of the 24-hour standard and the AQI value of 100. The AQI value of 150 
was set in the 1999 rulemaking at a level of 65 [mu]g/m\3\, 24-hour 
average. In considering what level to propose for an AQI value of 150, 
we stated the view that the health effects evidence indicates that the 
level of 55 [mu]g/m\3\, 24-hour average, is appropriate to use \143\ in 
conjunction with an AQI value of 100 set at the level of 35 [mu]g/m\3\. 
The Agency's approach to selecting the levels at which to set the AQI 
values of 100 and 150 inherently recognizes that the epidemiological 
evidence upon which these decisions are based provides no evidence of 
discernible thresholds, below which effects do not occur in either 
sensitive groups or in the general population, at which to set these 
two breakpoints. Therefore, the EPA concluded the use of a proportional 
adjustment would be appropriate. Commenters did not comment on this 
proposed approach to revising the AQI value of 150; thus, the EPA is 
taking final action to set an AQI value of 150 at 55 [mu]g/m\3\, 24-
hour average.
---------------------------------------------------------------------------

    \143\ We note that this level is consistent with the level 
recommended in the more recent EPA guidance (Harnett, 2009, 
Attachment B), which is in use by many State and local agencies.
---------------------------------------------------------------------------

    Based on the air quality and health considerations discussed in 
section V of the proposal, the EPA concluded that it was appropriate to 
propose to retain the current level of 500 [mu]g/m\3\, 24-hour average, 
for the AQI value of 500. In addition, the EPA solicited comment on 
alternative levels and approaches to setting a level for the AQI value 
of 500, as well as supporting information and rationales for such 
alternative levels. The EPA also solicited any additional information, 
data, research or analyses that may be useful to inform a final 
decision on the appropriate level to set the AQI value of 500. 
Receiving no information with which to inform alternative approaches to 
setting an AQI value of 500, the EPA is taking final action to retain 
the current level of 500 [mu]g/m\3\, 24-hour average, for the AQI value 
of 500.
    For the intermediate breakpoints in the AQI between the values of 
150 and 500, the EPA proposed PM2.5 concentrations that 
generally reflected a linear relationship between increasing index 
values and increasing PM2.5 values (77 FR 38965). The 
available scientific evidence of health effects related to population 
exposures to PM2.5 concentrations between the level of the 
24-hour standard and an AQI value of 500 suggested a continuum of 
effects in this range, with increasing PM2.5 concentrations 
being associated with increasingly larger numbers of people likely to 
experience such effects. The generally linear relationship between AQI 
values and PM2.5 concentrations in this range is consistent 
with the health evidence. This also is consistent with the Agency's 
practice of setting breakpoints in symmetrical fashion where health 
effects information does not suggest particular levels.
    Table 2 below summarizes the finalized breakpoints for the 
PM2.5 sub-index.\144\ Table 2 shows the intermediate 
breakpoints for AQI values of 200, 300 and 400 based on a linear 
interpolation between the proposed levels for AQI values of 150 and 
500. If a different level were to be set for an AQI value of 150 or 
500, intermediate levels would be calculated based on a linear 
relationship between the selected levels for AQI values of 150 and 500.
---------------------------------------------------------------------------

    \144\ As discussed in section VII.C below, the EPA is also 
updating the data handling procedures for reporting the AQI and 
corresponding updates for other AQI-sub-indices presented in Table 2 
of appendix G of 40 CFR part 58.

                                    Table 2--Breakpoints for PM2.5 Sub-Index
----------------------------------------------------------------------------------------------------------------
                                                                                           Proposed breakpoints
                            AQI category                                 Index values      ([mu]g/m\3\, 24-hour
                                                                                                 average)
----------------------------------------------------------------------------------------------------------------
Good................................................................               0-50               0.0-(12.0)
Moderate............................................................             51-100              (12.1)-35.4
Unhealthy for Sensitive Groups......................................            101-150                35.5-55.4
Unhealthy...........................................................            151-200               55.5-150.4
Very Unhealthy......................................................            201-300              150.5-250.4
Hazardous...........................................................            301-400              250.5-350.4
                                                                                401-500              350.5-500.4
----------------------------------------------------------------------------------------------------------------

    In retaining the 500 level for the AQI as described above, we note 
that the EPA is not establishing a Significant Harm Level (SHL) for 
PM2.5. The SHL is an important part of air pollution 
Emergency Episode Plans, which are required for certain areas by CAA 
section 110(a)(2)(G) and associated regulations at 40 CFR 51.150, under 
the Prevention of Air Pollution Emergency Episodes program. The Agency 
believes that air quality responses established through an Emergency 
Episode Plan should be developed through a collaborative process 
working with State and Tribal air quality, forestry and agricultural 
agencies, Federal land management agencies, private land managers and 
the public. Therefore, if in future rulemaking the EPA proposes 
revisions to the Prevention of Air Pollution Emergency Episodes 
program, the proposal will include a SHL for PM2.5 that is 
developed in collaboration with these organizations. As discussed in 
the 1999 Air Quality Index Reporting Rule (64 FR 42530), if a future 
rulemaking results in a SHL that is different from the 500 value of the 
AQI for PM2.5, the AQI will be revised accordingly.
    The EPA also received more general comments on AQI reporting, 
comments that did not pertain to setting specific breakpoints. One set 
of commenters (e.g., API and UARG), expressed the view that changes to 
the AQI are not appropriate. They noted that air quality is getting 
better, and in fact is better than when EPA established the AQI. These 
commenters stated that the proposed changes to the annual standard and 
the AQI would mean that the public would hear less often that air 
quality is good, and thereby would receive apparently inconsistent or 
misleading messages that air quality is

[[Page 3182]]

worse. The AQI has been revised several times in conjunction with 
revisions to the standards. State and local air quality agencies and 
organizations are proficient at communicating with the public about the 
reasons for changes to the AQI. Therefore, the EPA strongly disagrees 
with these commenters that the public will receive inconsistent or 
misleading messages. Recognizing the importance of the AQI as a 
communication tool that allows the public to take exposure reduction 
measures when air quality may pose health risks, the EPA agrees with 
state and local air quality agencies and organizations that favored 
revising the AQI at the same time as the primary standard.
    A few state and local air quality agencies and organizations 
recommended against using near-roadway PM2.5 monitors for 
AQI reporting. In support of this comment, they expressed the following 
views, that near-roadway monitors are source-oriented, represent micro-
scale conditions, and the agencies don't have experience using them for 
AQI reporting. The EPA disagrees with the comment in that these 
monitors will be sited at existing near-road stations sited to be 
representative of area-wide PM2.5 concentrations indicative 
of general population exposure. Accordingly, data from these near-road 
monitors should be included in the AQI since they provide information 
about PM2.5 levels that millions of people, who work, live 
and go to school near busy roadways, are exposed to. The stations are 
representative of somewhat elevated concentrations in near-road 
environments, but since these stations represent many such locations 
throughout a metropolitan area, they are appropriate for characterizing 
exposure in typical portions of major urban areas. The EPA is committed 
to helping air quality agencies develop appropriate ways to report 
PM2.5 levels from these monitors using the AQI.

VI. Rationale for Final Decisions on the Secondary PM Standards

    This section presents the Administrator's final decisions regarding 
the need to revise the current suite of secondary PM2.5 and 
PM10 standards to address visibility impairment and other 
welfare effects considered in this review. Specifically, this section 
describes the Administrator's final decision to retain the current 
suite of secondary PM standards to address PM-related visibility 
impairment as well as other PM-related welfare effects, including 
ecological effects, effects on materials, and climate impacts. This 
suite of standards includes an annual PM2.5 standard of 15 
[mu]g/m\3\, a 24-hour PM2.5 standard of 35 [mu]g/m\3\, and a 
24-hour PM10 standard of 150 [mu]g/m\3\. The Administrator 
is revising only the form of the secondary annual PM2.5 
standard to remove the option for spatial averaging consistent with 
this change to the primary annual PM2.5 standard. Contrary 
to what was proposed, the Administrator has decided not to establish a 
distinct standard to address PM-related visibility impairment. The 
rationale for this decision is presented below.
    The Administrator's final decisions on the secondary standards are 
based on a thorough review of the latest scientific information 
published through mid-2009 on welfare effects associated with fine and 
coarse particles in the ambient air, as presented in the Integrated 
Science Assessment. The final decisions also take into account: (1) 
Staff assessments of the most policy-relevant information presented and 
assessed in the Integrated Science Assessment and staff analyses of air 
quality and visibility effects presented in the Visibility Assessment 
and the Policy Assessment, upon which staff conclusions regarding 
appropriate considerations in this review are based; (2) CASAC advice 
and recommendations, as reflected in discussions of drafts of the 
Integrated Science Assessment, Visibility Assessment, and Policy 
Assessment at public meetings, in separate written comments, and in 
CASAC's letters to the Administrator; (3) the multiple rounds of public 
comments received during the development of these documents, both in 
connection with CASAC meetings and separately; and (4) public comments 
received on the proposal.
    In particular, this section presents background information on the 
EPA's previous and current reviews of the secondary PM standards 
(section VI.A), a summary of the proposed decisions regarding the 
secondary PM standards (section VI.B), a discussion of significant 
public comments received on those proposed decisions (section VI.C), 
and the Administrator's final decisions on the secondary PM standards 
(section VI.D).

A. Background

    The current suite of secondary PM standards is identical to the 
suite of primary PM standards set in 2006, including 24-hour and annual 
PM2.5 standards and a 24-hour PM10 standard. The 
current secondary PM2.5 standards are intended to provide 
protection from PM-related visibility impairment, whereas the entire 
suite of secondary PM standards is intended to provide protection from 
other PM-related effects on public welfare, including effects on 
sensitive ecosystems, materials damage and soiling, and climatic and 
radiative processes.
    The approach used for reviewing the current suite of secondary PM 
standards built upon and broadened the approaches used in previous PM 
NAAQS reviews. The following discussion focuses particularly on the 
current secondary PM2.5 standards related to visibility 
impairment and provides a summary of the approaches used to review and 
establish secondary PM2.5 standards in the last two reviews 
(section VI.A.1); judicial review of the 2006 standards that resulted 
in the remand of the secondary annual and 24-hour PM2.5 
NAAQS to the EPA (section VI.A.2); and the approach used in this review 
for evaluating the secondary PM2.5 standards (section 
VI.A.3).
1. Approaches Used in Previous Reviews
    The original secondary PM2.5 standards were established 
in 1997, and a revision to the 24-hour standard was made in 2006. The 
approaches used in making final decisions on secondary standards in 
those reviews, as well as the current review, utilized different ways 
to consider the underlying body of scientific evidence. They also 
reflected an evolution in EPA's understanding of the nature of the 
effect on public welfare from PM-related visibility impairment, from an 
approach that focused only on Federal Class I area visibility impacts 
to a more multifaceted approach that also considered PM-related impacts 
on visibility in non-Federal Class I areas, such as in urban areas. 
This evolution occurred in conjunction with the expansion of available 
PM data and information from visibility-related studies of public 
perception, valuation, and personal comfort and well-being.
    In 1997, the EPA revised the PM NAAQS in part by establishing new 
identical primary and secondary PM2.5 standards. In revising 
the secondary standards, the EPA recognized that PM produces adverse 
effects on visibility and that impairment of visibility was being 
experienced throughout the U.S., in multi-state regions, urban areas, 
and remote mandatory Federal Class I areas alike. However, in 
considering an appropriate level for a secondary standard to address 
adverse effects of PM2.5 on visibility, the EPA concluded 
that the determination of a single national level was complicated by 
important regional differences influenced by factors such as

[[Page 3183]]

background and current levels of PM2.5, composition of 
PM2.5, and average relative humidity. Variations in these 
factors across regions could thus result in situations where attaining 
an appropriately protective concentration of fine particles in one 
region might or might not provide adequate protection in a different 
region. The EPA also determined that there was insufficient information 
at that time to establish a level for a national secondary standard 
that would represent a threshold above which visibility conditions 
would always be adverse and below which visibility conditions would 
always be acceptable.
    Based on an assessment of the potential visibility improvements 
that would result from reaching attainment with the new primary 
standards for PM2.5, the EPA concluded that attainment of 
the annual and 24-hour PM2.5 primary standards would lead to 
visibility improvements in the eastern U.S. at both urban and regional 
scales, but little or no change in the western U.S., except in and near 
certain urban areas.
    The EPA also considered the potential effectiveness of a regional 
haze program, required by sections 169A and 169B of the CAA \145\ to 
address those effects of PM on visibility that would not be addressed 
through attainment of the primary PM2.5 standards. The 
regional haze program would be designed to address the widespread, 
regionally uniform type of haze caused by a multitude of sources. The 
structure and requirements of sections 169A and 169B of the CAA provide 
for visibility protection programs that can be more responsive to the 
factors contributing to regional differences in visibility than can 
programs addressing the kinds of nationally applicable secondary NAAQS 
considered in the 1997 review. The regional haze visibility goal is 
more protective than a secondary NAAQS since the goal is to eliminate 
any anthropogenic impairment rather than to provide a level of 
protection from visibility impairment that is requisite to protect the 
public welfare. Thus, an important factor considered in the 1997 review 
was whether a regional haze program, in conjunction with secondary 
standards set identical to the suite of PM2.5 primary 
standards, would provide appropriate protection for visibility in non-
Federal Class I areas. The EPA concluded that the two programs and 
associated control strategies should provide such protection due to the 
regional approaches needed to manage emissions of pollutants that 
impair visibility in many of these areas.
---------------------------------------------------------------------------

    \145\ In 1977, Congress established as a national goal ``the 
prevention of any future, and the remedying of any existing, 
impairment of visibility in mandatory Federal Class I areas which 
impairment results from manmade air pollution,'' section 169A(a)(1) 
of the CAA. The EPA is required by section 169A(a)(4) of the CAA to 
promulgate regulations to ensure that ``reasonable progress'' is 
achieved toward meeting the national goal.
---------------------------------------------------------------------------

    For these reasons, in 1997 the EPA concluded that a national 
regional haze program, combined with a nationally applicable level of 
protection achieved through secondary PM2.5 standards set 
identical to the primary PM2.5 standards, would be more 
effective for addressing regional variations in the adverse effects of 
PM2.5 on visibility than would be national secondary 
standards for PM with levels lower than the primary PM2.5 
standards. The EPA further recognized that people living in certain 
urban areas may place a high value on unique scenic resources in or 
near these areas and as a result might experience visibility problems 
attributable to sources that would not necessarily be addressed by the 
combined effects of a regional haze program and PM2.5 
secondary standards. The EPA concluded that in such cases, state or 
local regulatory approaches, such as past action in Colorado to 
establish a local visibility standard for the City of Denver, would be 
more appropriate and effective in addressing these special situations 
because of the localized and unique characteristics of the problems 
involved. Visibility in an urban area located near a mandatory Federal 
Class I area could also be improved through state implementation of the 
then-current visibility regulations, by which emission limitations can 
be imposed on a source or group of sources found to be contributing to 
``reasonably attributable'' impairment in the mandatory Federal Class I 
area.
    Based on these considerations, in 1997 the EPA set secondary 
PM2.5 standards identical to the primary PM2.5 
standards, that would work in conjunction with the Regional Haze 
Program to be established under sections 169A and 169B of the CAA, as 
the most appropriate and effective means of addressing the public 
welfare effects associated with visibility impairment. Together, the 
two programs and associated control strategies were expected to provide 
appropriate protection against PM-related visibility impairment and 
enable all regions of the country to make reasonable progress toward 
the national visibility goal.
    In 2006, the EPA revised the suite of secondary PM2.5 
standards to address visibility impairment by making the suite of 
secondary standards identical to the revised suite of primary 
PM2.5 standards. The EPA's decision regarding the need to 
revise the suite of secondary PM2.5 standards reflected a 
number of new developments that had occurred and sources of information 
that had become available following the 1997 review. First, the EPA 
promulgated a Regional Haze Program in 1999 (65 FR 35713, July 1, 1999) 
which required states to establish goals for improving visibility in 
Federal Class I areas and to adopt control strategies to achieve these 
goals. Second, extensive new information from visibility and fine 
particle monitoring networks had become available, allowing for updated 
characterizations of visibility trends and PM concentrations in urban 
areas, as well as Federal Class I areas. These new data allowed the EPA 
to better characterize visibility impairment in urban areas and the 
relationship between visibility and PM2.5 concentrations. 
Finally, additional studies in the U.S. and abroad provided the basis 
for the establishment of standards and programs to address specific 
visibility concerns in a number of local areas. These studies (Denver, 
Phoenix, and British Columbia) utilized photographic representations of 
visibility impairment and produced reasonably consistent results in 
terms of the visual ranges found to be generally acceptable by study 
participants. The EPA considered the information generated by these 
studies useful in characterizing the nature of particle-induced haze 
and for informing judgments about the acceptability of various levels 
of visual air quality in urban areas across the U.S. Based largely on 
this information, the Administrator concluded that it was appropriate 
to revise the secondary PM2.5 standards to provide increased 
protection from visibility impairment principally in urban areas, in 
conjunction with the regional haze program for protection of visual air 
quality in Federal Class I areas.
    In so doing, the Administrator recognized that PM-related 
visibility impairment is principally related to fine particle 
concentrations and that perception of visibility impairment is most 
directly related to short-term, nearly instantaneous levels of visual 
air quality. Thus, in considering whether the then-current suite of 
secondary standards would provide the appropriate degree of protection, 
he concluded that it was appropriate to focus on just the 24-hour 
secondary PM2.5 standard to provide requisite protection.
    The Administrator then considered whether PM2.5 mass 
remained the appropriate indicator for a secondary

[[Page 3184]]

standard to protect visibility, primarily in urban areas. The 
Administrator noted that PM-related visibility impairment is 
principally related to fine particle levels. 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. 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 the 2006 review found that, in urban areas, 
visibility levels showed 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. In analyzing how 
well PM2.5 concentrations correlated with visibility in 
urban locations across the U.S., the 2005 Staff Paper concluded that 
clear correlations existed between 24-hour average PM2.5 
concentrations and calculated (i.e., reconstructed) light extinction, 
which is directly related to visual range (U.S. EPA, 2005, p. 7-6). 
These correlations were similar in the eastern and western regions of 
the U.S. These correlations were less influenced by relative humidity 
and more consistent across regions when PM2.5 concentrations 
were averaged over shorter, daylight time periods (e.g., 4 to 8 hours) 
when relative humidity in eastern urban areas was generally lower and 
thus more similar to relative humidity in western urban areas. The 2005 
Staff Paper noted that a standard set at any specific PM2.5 
concentration would 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. The 2005 Staff Paper concluded that it was 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 (e.g., 4 to 8 hours) of daylight 
hours (U.S. EPA, 2005, p. 7-6). Based on their review of the Staff 
Paper, most CASAC Panel members also endorsed such a PM2.5 
indicator for a secondary standard to address visibility impairment 
(Henderson, 2005a, p. 9). Based on the above considerations, the 
Administrator concluded that PM2.5 should be retained as the 
indicator for fine particles as part of a secondary standard to address 
visibility protection, in conjunction with averaging times from 4 to 24 
hours.
    In considering what level of protection against PM-related 
visibility impairment would be appropriate, the Administrator took into 
account the results of the public perception and attitude surveys 
regarding the acceptability of various degrees of visibility impairment 
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. In the Administrator's judgment, these 
sources provided 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. Based on 
photographic representations of varying levels of visual air quality, 
public perception studies, and local and state visibility standards, 
the 2005 Staff Paper had concluded that 30 to 20 [mu]g/m\3\ 
PM2.5 represented a reasonable range for a national 
visibility standard primarily for urban areas, based on a sub-daily 
averaging time (U.S. EPA, 2005, p. 7-13). The upper end of this range 
was below the levels at which illustrative scenic views are 
significantly obscured, and the lower end was around the level at which 
visual air quality generally appeared to be good based on observation 
of the illustrative views. This concentration range generally 
corresponded to median visual ranges in urban areas within regions 
across the U.S. of approximately 25 to 35 km, a range that was bounded 
above by the visual range targets selected in specific areas where 
state or local agencies placed particular emphasis on protecting visual 
air quality. In considering a reasonable range of forms for a 
PM2.5 standard within this range of levels, the 2005 Staff 
Paper had concluded that a concentration-based percentile form was 
appropriate, and that the upper end of the range of concentration 
percentiles for consideration should be consistent with the 98th 
percentile used for the primary standard and that the lower end of the 
range should be the 92nd percentile, which represented the mean of the 
distribution of the 20 percent most impaired days, as targeted in the 
regional haze program (U.S. EPA, 2005 pp. 7-11 to 7-13). While 
recognizing that it was difficult to select any specific level and form 
based on then-currently available information (Henderson, 2005a, p. 9), 
the CASAC Panel was generally in agreement with the ranges of levels 
and forms presented in the 2005 Staff Paper.
    The Administrator also considered the level of protection that 
would be afforded by the proposed suite of primary PM2.5 
standards (71 FR 2681, January 17, 2006), on the basis that although 
significantly more information was available than in the 1997 review 
concerning the relationship between fine PM levels and visibility 
across the country, there was 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. In so doing, the Administrator compared 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. Based on such an analysis, the Administrator observed 
that the predicted percent of counties with monitors not likely to meet 
the proposed suite of primary PM2.5 standards was actually 
somewhat greater 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 2005 Staff Paper, and a form within the recommended 
range. Based on this comparison, the Administrator tentatively 
concluded that revising the secondary 24-hour PM2.5 standard 
to be identical to the proposed revised primary PM2.5 
standard (and retaining the then-current annual secondary 
PM2.5 standard) was a reasonable policy approach to 
addressing visibility protection primarily in urban areas. In proposing 
this approach, the Administrator also solicited comment on a sub-daily 
(4- to 8-hour averaging time) secondary PM2.5 standard (71 
FR 2675 to 2781, January 17, 2006).
    In commenting on the proposed decision, the CASAC requested that a 
sub-daily standard to protect visibility ``be favorably reconsidered'' 
(Henderson, 2006a, p.6). The CASAC noted three cautions regarding the 
proposed reliance on a secondary PM2.5 standard identical to 
the proposed 24-hour primary PM2.5 standard: (1) 
PM2.5 mass measurement is a better indicator of visibility 
impairment during daylight hours, when relative humidity is generally 
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 PM2.5 standard as a 
proxy introduces error and

[[Page 3185]]

uncertainty in protecting visibility; and sub-daily standards are used 
for other NAAQS and should be the focus for visibility; (2) CASAC and 
its monitoring subcommittees had repeatedly commended EPA's initiatives 
promoting the introduction of continuous and near-continuous PM 
monitoring and recognized that an expanded deployment of continuous 
PM2.5 monitors would be consistent with setting a sub-daily 
standard to protect visibility; and (3) the analysis showing a 
similarity between percentages of counties not likely to meet what the 
CASAC Panel considered to be a lenient 4- to 8-hour secondary standard 
and a secondary standard identical to the proposed 24-hour primary 
standard was a numerical coincidence that was 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 ``[i]t 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'' (Henderson, 2006a, pp. 5 to 6).
    In reaching a final decision, the Administrator focused on the 
relative protection provided by the proposed primary standards based on 
the above-mentioned similarities in percentages of counties meeting 
alternative standards and on the limitations in the information 
available concerning studies of public perception and attitudes 
regarding the acceptability of various degrees of visibility impairment 
in urban areas, as well as on the subjective nature of the judgment 
required. In so doing, the Administrator concluded that caution was 
warranted in establishing a distinct secondary standard for visibility 
impairment and that the available information did not 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.
2. Remand of 2006 Secondary PM2.5 Standards
    As noted above in section II.B.2 above, several parties filed 
petitions for review challenging EPA's decision to set the secondary 
NAAQS for fine PM identical to the primary NAAQS. On judicial review, 
the D.C. Circuit remanded to the EPA for reconsideration the secondary 
NAAQS for fine PM because the Agency's decision was unreasonable and 
contrary to the requirements of section 109(b)(2). American Farm Bureau 
Federation v. EPA, 559 F. 3d 512 (D.C. Cir., 2009).
    The petitioners argued that the EPA's decision lacked a reasoned 
basis. First, they asserted that the EPA never determined what level of 
visibility was ``requisite to protect the public welfare.'' They argued 
that the EPA unreasonably rejected the target level of protection 
recommended by its staff, while failing to provide a target level of 
its own. The court agreed, stating that ``the EPA's failure to identify 
such a level when deciding where to set the level of air quality 
required by the revised secondary fine PM NAAQS is contrary to the 
statute and therefore unlawful. Furthermore, the failure to set any 
target level of visibility protection deprived the EPA's decision-
making of a reasoned basis.'' 559 F. 3d at 530.
    Second, the petitioners challenged EPA's method of comparing the 
protection expected from potential standards. They contended that the 
EPA relied on a meaningless numerical comparison, ignored the effect of 
humidity on the usefulness of a standard using a daily averaging time, 
and unreasonably concluded that the primary standards would achieve a 
level of visibility roughly equivalent to the level the EPA staff and 
CASAC deemed ``requisite to protect the public welfare.'' The court 
found that the EPA's equivalency analysis based on the percentages of 
counties exceeding alternative standards ``failed on its own terms.'' 
The same table showing the percentages of counties exceeding 
alternative secondary standards, used for comparison to the percentages 
of counties exceeding alternative primary standards to show 
equivalency, also included six other alternative secondary standards 
within the recommended CASAC range that would be more ``protective'' 
under EPA's definition than the adopted primary standards. Two-thirds 
of the potential secondary standards within the CASAC's recommended 
range would be substantially more protective than the adopted primary 
standards. The court found that the EPA failed to explain why it looked 
only at one of the few potential secondary standards that would be less 
protective, and only slightly less so, than the primary standards. More 
fundamentally, however, the court found that the EPA's equivalency 
analysis based on percentages of counties demonstrated nothing about 
the relative protection offered by the different standards, and that 
the tables offered no valid information about the relative visibility 
protection provided by the standards. 559 F. 3d at 530-31.
    Finally, the Staff Paper had made clear that a visibility standard 
using PM2.5 mass as the indicator in conjunction with a 
daily averaging time would be confounded by regional differences in 
humidity. The court noted that the EPA acknowledged this problem, yet 
did not address this issue in concluding that the primary standards 
would be sufficiently protective of visibility. 559 F. 3d at 530. 
Therefore, the court granted the petition for review and remanded for 
reconsideration the secondary PM2.5 NAAQS.
3. General Approach Used in the Policy Assessment for the Current 
Review
    The approach used in this review broadened the general approaches 
used in the last two PM NAAQS reviews by utilizing, to the extent 
available, enhanced tools, methods, and data to more comprehensively 
characterize visibility impacts. As such, the EPA took into account 
considerations based on both the scientific evidence (``evidence-
based'') and a quantitative analysis of PM-related impacts on 
visibility (``impact-based'') to inform conclusions related to the 
adequacy of the current secondary PM2.5 standards and 
alternative standards that were appropriate for consideration in this 
review. As in past reviews, the EPA also considered that the secondary 
NAAQS should address PM-related visibility impairment in conjunction 
with the Regional Haze Program, such that the secondary NAAQS would 
focus on protection from visibility impairment principally in urban 
areas in conjunction with the Regional Haze Program that is focused on 
improving visibility in Federal Class I areas. The EPA again recognized 
that such an approach remains the most appropriate and effective means 
of addressing the public welfare effects associated with visibility 
impairment in areas across the country.
    The Policy Assessment drew from the qualitative evaluation of all 
studies discussed in the Integrated Science Assessment (U.S. EPA, 
2009a). Specifically, the Policy Assessment considered the extensive 
new air quality and source apportionment information available from the 
regional planning organizations, long-standing evidence of PM effects 
on visibility, and limited public preference study information from 
four urban areas (U.S. EPA, 2009a, chapter 9), as well as the 
integration of evidence across disciplines (U.S. EPA, 2009a, chapter 
2). In addition, limited information that had become available 
regarding the characterization of public preferences in urban areas 
provided

[[Page 3186]]

some new perspectives on the usefulness of this information in 
informing the selection of target levels of urban visibility 
protection. On these bases, the Policy Assessment again focused 
assessments on visibility conditions in urban areas.
    The conclusions in the Policy Assessment reflected EPA staff's 
understanding of both evidence-based and impact-based considerations to 
inform two overarching questions related to (1) the adequacy of the 
current suite of PM2.5 standards and (2) what potential 
alternative standards, if any, should be considered in this review to 
provide appropriate protection from PM-related visibility impairment. 
In addressing these broad questions, the discussions in the Policy 
Assessment were organized around a series of more specific questions 
reflecting different aspects of each overarching question (U.S. EPA, 
2011a, Figure 4-1). When evaluating the visibility protection afforded 
by the current or any alternative standards considered, the Policy 
Assessment took into account the four basic elements of the NAAQS: 
indicator, averaging time, level, and form.

B. Proposed Decisions on Secondary PM Standards

    At the time of proposal, the Administrator proposed to revise the 
suite of secondary PM standards by adding a distinct standard for 
PM2.5 to address PM-related visibility impairment, focused 
primarily on visibility in urban areas. This proposed standard was to 
be defined in terms of a PM2.5 visibility index, which would 
use measured PM2.5 mass concentration, in combination with 
speciation and relative humidity data, to calculate PM2.5 
light extinction, translated into the deciview (dv) scale; a 24-hour 
averaging time; a 90th percentile form, averaged over 3 years; and a 
level of 28-30 dv. To address other non-visibility welfare effects, the 
Administrator proposed to retain the current suite of secondary PM 
standards generally, while revising only the form of the secondary 
annual PM2.5 standard to remove the option for spatial 
averaging consistent with this proposed change to the primary annual 
PM2.5 standard. Each of these proposed decisions is 
described in more detail in the proposal and below.
1. PM-Related Visibility Impairment
    As discussed in Section VI.B of the proposal, the Administrator's 
proposed decision regarding a distinct secondary standard to provide 
protection from visibility impairment reflected careful consideration 
of the following: (1) The latest scientific information on visibility 
effects associated with PM as described in the Integrated Science 
Assessment (U.S. EPA, 2009a); (2) insights gained from assessments of 
correlations between ambient PM2.5 and visibility impairment 
prepared by EPA staff in the Visibility Assessment (U.S. EPA, 2010b); 
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 on visual air quality. This section summarizes key 
information from the proposal regarding the nature of visibility 
impairment, including the relationship between ambient PM and 
visibility, temporal variations in light extinction, periods during the 
day of interest for assessing visibility conditions, and exposure 
durations of interest (section VI.B.1.a); limited public perceptions 
and attitudes about visibility impairment and the impacts of visibility 
impairment on public welfare (section VI.B.1.b); CASAC advice regarding 
the need for, and design of, secondary standards to protect visibility 
(section VI.B.1.c); and the Administrator's proposed conclusions 
regarding setting a distinct standard to address visibility impairment 
(section VI.B.1.d).
a. Nature of PM-Related Visibility Impairment
    As noted at the time of proposal, the fundamental science 
characterizing the contribution of PM, especially fine particles, to 
visibility impairment is well understood. This science provides the 
basis for the Integrated Science Assessment designation of the 
relationship between PM and visibility impairment as causal. New 
research available in this review, discussed in chapter 9 of the 
Integrated Science Assessment, continues to support and refine EPA's 
understanding of the effect of PM on visibility and the source 
contributions to that effect in rural and remote locations. This 
research provides new insights regarding the regional source 
contributions to urban visibility impairment and better 
characterization of the increment in PM concentrations and visibility 
impairment that occur in many cities (i.e., the urban excess) relative 
to conditions in the surrounding rural areas (i.e., regional 
background). Ongoing urban PM2.5 speciated and aggregated 
mass monitoring has produced new information that has allowed for 
updated characterization of current visibility levels in urban areas.
i. Relationship Between Ambient PM and Visibility
    Visibility impairment is caused by the scattering and absorption of 
light by suspended particles and gases in the atmosphere. When PM is 
present in the air, its contribution to light extinction typically 
greatly exceeds that of gases. The combined effect of light scattering 
and absorption by both particles and gases is characterized as light 
extinction, i.e., the fraction of light that is scattered or absorbed 
in the atmosphere. Light extinction can be quantified by a light 
extinction coefficient with units of 1/distance, which is often 
expressed as 1/(1 million meters) or inverse megameters (abbreviated 
Mm-1) or in terms of an alternative scale known as the 
deciview scale, defined by the following equation: \146\
---------------------------------------------------------------------------

    \146\ As used in the Regional Haze Program, the term 
bext refers to light extinction due to PM2.5, 
PM10-2.5, and ``clean'' atmospheric gases. In the Policy 
Assessment, in focusing on light extinction due to PM2.5, 
the deciview values include only the effects of PM2.5 and 
the gases. The ``Rayleigh'' term associated with clean atmospheric 
gases is represented by the constant value of 10 Mm-\1\. 
Omission of the Rayleigh term would create the possibility of 
negative deciview values when the PM2.5 concentration is 
very low.
---------------------------------------------------------------------------

Deciview (dv) = 10 ln (bext/ 10 Mm-1)

The deciview scale is frequently used in the scientific literature on 
visibility, as well as in the Regional Haze Program. In particular, the 
deciview scale is used in the public perception studies that were 
considered in the past and current reviews to inform judgments about an 
appropriate degree of protection to be provided by a secondary NAAQS.
    The amount of light extinction contributed by PM depends on the 
particle concentration as well as on the particle size distribution and 
composition and also on the relative humidity. As described in detail 
in section VI.B.1.a of the proposal, visibility scientists have 
developed an algorithm, known as the IMPROVE algorithm,\147\ to 
estimate light extinction using routinely monitored fine particle 
(PM2.5) speciation and coarse particle mass 
(PM10-2.5) data, as well as data on relative humidity. There 
is both an original and a revised version of the IMPROVE algorithm 
(Pitchford et al., 2007). The revised version was developed to address 
observed biases in the predictions using the original algorithm under 
very low and very high

[[Page 3187]]

light extinction conditions.\148\ These IMPROVE algorithms are 
routinely used to calculate light extinction levels on a 24-hour basis 
in Federal Class I areas under the Regional Haze Program.
---------------------------------------------------------------------------

    \147\ The algorithm is referred to as the IMPROVE algorithm 
because it was developed specifically to use the aerosol monitoring 
data generated at network sites and with equipment specifically 
designed to support the IMPROVE program and was evaluated using 
IMPROVE optical measurements at the subset of sites that make those 
measurements (Malm et al., 1994).
    \148\ These biases were detected by comparing light extinction 
estimates generated from the IMPROVE algorithm to direct optical 
measurements in a number of rural Federal Class I areas.
---------------------------------------------------------------------------

    In either version of the IMPROVE algorithm, the concentration of 
each of the major aerosol components is multiplied by a dry extinction 
efficiency value and, for the hygroscopic components (i.e., ammoniated 
sulfate and ammonium nitrate), also multiplied by an additional factor 
to account for the water growth to estimate these components' 
contribution to light extinction. Summing the contribution of each 
component gives the estimate of total light extinction per unit 
distance denoted as the light extinction coefficient (bext), as shown 
below for the original IMPROVE algorithm.

bext [ap] 3 x f(RH) x [Sulfate]
    + 3 x f(RH) x [Nitrate]
    + 4 x [Organic Mass]
    + 10 x [Elemental Carbon]
    + 1 x [Fine Soil]
    + 0.6 x [Coarse Mass]
    + 10

    Light extinction (bext) is in units of Mm-1, 
the mass concentrations of the components indicated in brackets are in 
units of [mu]g/m\3\, and f(RH) is the unitless water growth term that 
depends on relative humidity. The final term of 10 Mm-1 is 
known as the Rayleigh scattering term and accounts for light scattering 
by the natural gases in unpolluted air. Despite the simplicity of this 
algorithm, it performs reasonably well and permits the contributions to 
light extinction from each of the major components (including the water 
associated with the sulfate and nitrate compounds) to be separately 
approximated. Inspection of the PM component-specific terms in the 
simple original IMPROVE algorithm shows that most of the 
PM2.5 components contribute 5 times or more light extinction 
than a similar concentration of PM10-2.5.
    The f(RH) term in the original algorithm reflects the increase in 
light scattering caused by particulate sulfate and nitrate under 
conditions of high relative humidity. Particles with hygroscopic 
components (e.g., particulate sulfate and nitrate) contribute more 
light extinction at higher relative humidity than at lower relative 
humidity because they change size in the atmosphere in response to 
ambient relative humidity conditions. For relative humidity below 40 
percent the f(RH) value is 1, but it increases to 2 at approximately 66 
percent, 3 at approximately 83 percent, 4 at approximately 90 percent, 
5 at approximately 93 percent, and 6 at approximately 95 percent 
relative humidity. The result is that both particulate sulfate and 
nitrate are more efficient per unit mass in light extinction than any 
other aerosol component for relative humidity above approximately 85 
percent where their total light extinction efficiency exceeds the 10 
m\2\/g associated with elemental carbon (EC). PM containing elemental 
or black carbon (BC) absorbs light as well as scattering it, making it 
the component with the greatest light extinction contributions per unit 
of mass concentration, except for the hygroscopic components under 
these high relative humidity conditions.\149\
---------------------------------------------------------------------------

    \149\ The IMPROVE algorithm does not explicitly separate the 
light-scattering and light-absorbing effects of elemental carbon.
---------------------------------------------------------------------------

    As noted above, subsequent to the development of the original 
IMPROVE algorithm, an alternative algorithm (variously referred to as 
the ``revised algorithm'' or the ``new algorithm'' in the literature) 
was developed. The revised IMPROVE algorithm is different from the 
original algorithm in several important ways. First, the revised 
algorithm employs a more complex split-component mass extinction 
efficiency to correct biases believed to be related to particle size 
distributions.\150\ Specifically, the revised algorithm incorporates 
terms to account for particles representing the different dry 
extinction and water uptake from two size modes of sulfate, nitrate and 
organic mass.\151\ Second, the revised algorithm uses a different 
multiplier for organic carbon for purposes of estimating organic 
carbonaceous material to better represent aged aerosol found in remote 
areas.\152\ In addition, the revised algorithm includes a term for 
hygroscopic sea salt that can be important for remote coastal areas, 
and site-specific Rayleigh light scattering terms in place of a 
universal Rayleigh light scattering value. As noted in section VI.B.1.a 
of the proposal, the revised IMPROVE algorithm can yield higher 
estimates of current light extinction levels in urban areas on days 
with relatively poor visibility as compared to the original algorithm 
(Pitchford, 2010). This difference is primarily attributable to the 
split-component mass extinction efficiency treatment in the revised 
algorithm. This revised algorithm was evaluated at 21 remote locations 
and is generally used by RPOs and States for implementation of the 
Regional Haze Rule.
---------------------------------------------------------------------------

    \150\ In either version of the IMPROVE algorithm, the 
concentration of each of the major aerosol components is multiplied 
by a dry extinction efficiency value and, for the hygroscopic 
components (i.e., ammoniated sulfate and ammonium nitrate), also 
multiplied by an additional factor to account for the water growth 
to estimate these components' contribution to light extinction. Both 
the dry extinction efficiency and water growth terms have been 
developed by a combination of empirical assessment and theoretical 
calculation using typical particle size distributions associated 
with each of the major aerosol components.
    \151\ The relative contributions of sulfate, nitrate, and 
organic mass concentrations to visibility impairment with the 
revised algorithm are different than with the original algorithm due 
to the combination of the dry extinction coefficient and f(RH) 
functions for derived concentrations of small and large particles. 
The apportionment of the total fine particle concentration of each 
of the three PM2.5 components into the concentrations of 
the small and large size fractions was empirically developed for 
remote areas. The fraction of the fine particle component that is in 
the large mode is estimated by dividing the total concentration of 
the component by 20 [mu]g/m\3\. If the total concentration of a 
component exceeds 20 [mu]g/m\3\, all of it is assumed to be in the 
large mode.
    \152\ The revised IMPROVE algorithm uses a multiplier of 1.8 for 
rural areas instead of 1.4 as used in the original algorithm for the 
mean ratio of organic mass to organic carbon.
---------------------------------------------------------------------------

ii. Temporal Variations of Light Extinction
    Particulate matter concentrations and light extinction in urban 
environments vary from hour to hour throughout the 24-hour day due to a 
combination of diurnal changes in meteorological conditions and 
systematic changes in emissions activity (e.g., rush hour traffic). 
Various factors combine to make early morning the most likely time for 
peak urban light extinction; although the net effects of the systematic 
urban- and larger-scale variations mean that peak daytime PM light 
extinction levels can occur any time of day, in many areas they occur 
most often in early morning hours (U.S. EPA, 2010b, sections 3.4.2 and 
3.4.3; Figures 3-9, 3-10, and 3-12). This temporal pattern in urban 
areas contrasts with the general lack of a strong diurnal pattern in PM 
concentrations and light extinction in most Federal Class I areas, 
reflective of a relative lack of local sources as compared to urban 
areas. The use in the Regional Haze Program of 24-hour average 
concentrations in the IMPROVE algorithm is consistent with this general 
lack of a strong diurnal pattern in Federal Class I areas.
iii. Periods During the Day of Interest for Assessment of Visibility
    As noted in sections VI.B.1.b and VI.B.1.c of the proposal, daytime 
visibility has dominated the attention of

[[Page 3188]]

those who have studied the visibility effects of air pollution, 
particularly in urban areas. The EPA recognizes, however, that 
physically PM light extinction behaves the same at night as during the 
day and can contribute to nighttime visibility effects by enhancing the 
scattering of anthropogenic light, contributing to the ``skyglow'' 
within and over populated areas, adding to the total sky brightness, 
and contributing to the reduction in contrast of stars against the 
background. However, little research has been conducted on nighttime 
visibility, and the state of the science is not comparable to that 
associated with daytime visibility impairment, particularly in terms of 
the impact on human welfare. The Policy Assessment notes that the 
science is not available at this time to support adequate 
characterization specifically of nighttime PM light extinction 
conditions and the related effects on public welfare (U.S. EPA, 2011a, 
p. 4-18). Therefore the EPA has focused its assessments of PM 
visibility impacts in urban areas on daylight hours during this review.
iv. Exposure Durations of Interest
    As noted in section VI.B.1.d of the proposal, the roles that 
exposure duration and variations in visual air quality within any given 
exposure period play in determining the acceptability or 
unacceptability of a given level of visual air quality have not been 
investigated via preference studies. In the preference studies 
available for this review, subjects were simply asked to rate the 
acceptability or unacceptability of each image of a haze-obscured 
scene, without being provided any suggestion of assumed duration or of 
assumed conditions before or after the occurrence of the scene 
presented. Preference and/or valuation studies show that atmospheric 
visibility conditions can be quickly assessed and preferences 
determined. The EPA is unaware of any studies that characterize the 
extent to which different frequencies and durations of exposure to 
visibility conditions contribute to the degree of public welfare impact 
that occurs.
    The Policy Assessment considered a variety of circumstances that 
are commonly expected to occur in evaluating the potential impact of 
visibility impairment on the public welfare based on available 
information (U.S. EPA, 2011a, pp. 4-19 to 4-20). In some circumstances, 
such as infrequent visits to scenic vistas in natural or urban 
environments, people are motivated specifically to take the opportunity 
to view a valued scene and are likely to do so for many minutes to 
hours to appreciate various aspects of the vista they choose to view. 
However, the public has many more opportunities to notice visibility 
conditions on a daily basis in settings associated with performing 
daily routines (e.g., during commutes and while working, exercising, or 
recreating outdoors). As noted in the Policy Assessment, information 
regarding the fraction of the public that has only one or a few 
opportunities to experience visibility during the day, or on the role 
the duration of the observed visibility conditions has on wellbeing 
effects associated with those visibility conditions, is not available 
(U.S. EPA, 2011a, p. 4-20). However, it is possible that people with 
limited opportunities to experience visibility conditions on a daily 
basis would receive the entire impact of the day's visual air quality 
based on the visibility conditions that occur during the short time 
period when they can see it. Since this group could be affected on the 
basis of observing visual air quality conditions for periods as short 
as one hour or less, and because during each daylight hour there are 
some people outdoors, commuting, or near windows, the Policy Assessment 
judged that it would be appropriate to use the maximum hourly value of 
PM light extinction during daylight hours for each day for purposes of 
evaluating the adequacy of the current suite of secondary standards. 
Other observers may have access to visibility conditions throughout the 
day. For this group, it might be that an hour with poor or 
``unacceptable'' visibility can be offset by one or more other hours 
with clearer conditions. Therefore, the proposal acknowledged that it 
might also be appropriate to consider a multi-hour daylight exposure 
period.
v. Periods of Fog and Rain
    As discussed in section VI.C of the proposal, the EPA also 
recognized that it is appropriate to give special treatment to periods 
of fog and rain when considering whether current PM2.5 
standards adequately protect public welfare from PM-related visibility 
impairment. Visibility impairment occurs during periods with fog or 
precipitation irrespective of the presence or absence of PM. Therefore, 
it is logical that periods with naturally impaired visibility due to 
fog or precipitation should not be treated as having PM-impaired 
visibility. There are multiple ways to adjust visibility data to reduce 
the effects of fog and precipitation. In the Visibility Assessment, 
following the advice of CASAC, the EPA evaluated the effect of 
excluding daylight hours for which relative humidity was greater than 
90 percent from analyses in order to avoid precipitation and fog 
confounding estimates of PM visibility impairment. For the 15 urban 
areas included in the Visibility Assessment, the EPA found that a 90 
percent relative humidity cutoff criterion was effective in that on 
average less than 6 percent of the daylight hours were removed from 
consideration, yet those hours had on average ten times the likelihood 
of rain, six times the likelihood of snow/sleet, and 34 times the 
likelihood of fog compared with hours with 90 percent or lower relative 
humidity. In the Regional Haze program, the EPA utilizes monthly 
average relative humidity values based on 10 years of climatological 
data to reduce the effect of fog and precipitation. This approach 
focuses on longer-term averages for each monitoring site and thereby 
eliminates the effect of very high humidity conditions on visibility at 
those locations.
b. Public Perception of Visibility Impairment
    As described in section VI.B.2 of the proposal, there are two main 
types of studies that evaluate the public perception of urban 
visibility impairment: urban visibility preference studies and urban 
visibility valuation studies. As noted in the Integrated Science 
Assessment, ``[b]oth types of studies are designed to evaluate 
individuals' desire (or demand) for good visual air quality (VAQ) where 
they live, using different metrics to evaluate demand. Urban visibility 
preference studies examine individuals' demand by investigating what 
amount of visibility degradation is unacceptable while economic studies 
examine demand by investigating how much one would be willing to pay to 
improve visibility'' (U.S. EPA, 2009a, p. 9-66). Because of the limited 
number of new studies on urban visibility valuation, the Integrated 
Science Assessment cites to the discussion in the 2004 Criteria 
Document of the various methods one can use to determine the economic 
valuation of changes in visibility, which include hedonic valuation, 
contingent valuation and contingent choice, and travel cost.
    Contingent valuation studies are a type of stated preference study 
that measures the strength of preferences and expresses that preference 
in dollar values. Contingent valuation studies often include payment 
vehicles that require respondents to consider implementation costs and 
their ability to pay for visibility improvements in their responses. 
This study design

[[Page 3189]]

aspect is critical because the EPA cannot consider implementations 
costs in setting either primary or secondary NAAQS. Therefore in 
considering the information available to help inform the standard-
setting process, the EPA has focused on the public perception studies 
that do not embed consideration of implementation costs. Nonetheless, 
the EPA recognizes that valuation studies do provide additional 
evidence that the public is experiencing losses in welfare due to 
visibility impairment.\153\ The public perception studies are described 
in detail below.
---------------------------------------------------------------------------

    \153\ In the regulatory impact analysis (RIA) accompanying this 
rulemaking, the EPA describes a revised approach to estimate urban 
residential visibility benefits that applies the results of several 
contingent valuation studies. The EPA is unable to apply the public 
perception studies to estimate benefits because they do not provide 
sufficient information on which to develop monetized benefits 
estimates. Specifically, the public perception studies do not 
provide preferences expressed in dollar values, even though they do 
provide additional evidence that the benefits associated with 
improving residential visibility are not zero. As previously noted 
in this preamble, the RIA is done for informational purposes only, 
and the proposed decisions on the NAAQS in this rulemaking are not 
in any way based on consideration of the information or analyses in 
the RIA.
---------------------------------------------------------------------------

    In order to identify levels of visibility impairment appropriate 
for consideration in setting secondary PM NAAQS to protect the public 
welfare, the Visibility Assessment comprehensively examined information 
that was available in this review regarding people's stated preferences 
regarding acceptable and unacceptable visual air quality.
    Light extinction is an atmospheric property that by itself does not 
directly translate into a public welfare effect. Instead, light 
extinction becomes meaningful in the context of the impact of 
differences in visibility on the human observer. This has been studied 
in terms of the acceptability or unacceptability expressed for the 
visibility impact of a given level of light extinction by a human 
observer. The perception of the visibility impact of a given level of 
light extinction occurs in conjunction with the associated 
characteristics and lighting conditions of the viewed scene.\154\ Thus, 
a given level of light extinction may be perceived differently by 
observers looking at different scenes or the same scene with different 
lighting characteristics. Likewise, different observers looking at the 
same scene with the same lighting may have different preferences 
regarding the associated visual air quality. When scene and lighting 
characteristics are held constant, the perceived appearance of a scene 
(i.e., how well the scenic features can be seen and the amount of 
visible haze) depends only on changes in light extinction. This has 
been demonstrated using the WinHaze model (Molenar et al., 1994) that 
uses image processing technology to apply user-specified changes in 
light extinction values to the same base photograph with set scene and 
lighting characteristics.
---------------------------------------------------------------------------

    \154\ By ``characteristics of the scene'' the EPA means the 
distance(s) between the viewer and the object(s) of interest, the 
shapes and colors of the objects, the contrast between objects and 
the sky or other background, and the inherent interest of the 
objects to the viewer. Distance is particularly important because at 
a given value of light extinction, which is a property of air at a 
given point(s) in space, more light is actually absorbed and 
scattered when light passes through more air between the object and 
the viewer.
---------------------------------------------------------------------------

    Much of what is known about the acceptability of levels of 
visibility comes from survey studies in which participants were asked 
questions about their preference or the value they place on various 
visibility levels as displayed to them in scenic photographs and/or 
WinHaze images with a range of known light extinction levels. The 
Visibility Assessment (U.S. EPA, 2010b, chapter 2) reviewed the limited 
number of urban visibility preference studies currently available 
(i.e., four studies) to assess the light extinction levels judged by 
the participant to have acceptable visibility for those particular 
scenes.
    The reanalysis of urban preference studies conducted in the 
Visibility Assessment for this review included three completed western 
urban visibility preference survey studies plus a pair of smaller focus 
studies designed to explore and further develop urban visibility survey 
instruments. The three western studies included one in Denver, Colorado 
(Ely et al., 1991), one in the lower Fraser River valley near 
Vancouver, British Columbia (BC), Canada (Pryor, 1996), and one in 
Phoenix, Arizona (BBC Research & Consulting, 2003). A pilot focus group 
study was also conducted for Washington, DC (Abt Associates Inc., 
2001). In response to an EPA request for public comment on the Scope 
and Methods Plan (74 FR 11580, March 18, 2009), comments were received 
(Smith, 2009) about the results of a new focus group study of scenes 
from Washington, DC, that had been conducted on subjects from both 
Houston, Texas, and Washington, DC, using scenes, methods and 
approaches similar to the method and approach employed in the EPA pilot 
study (Smith and Howell, 2009). When taken together, these studies from 
the four different urban areas included a total of 852 individuals, 
with each individual responding to a series of questions while viewing 
a set of images of various urban visual air quality conditions.
    The approaches used in the four studies were similar and were all 
derived from the method first developed for the Denver urban visibility 
study. In particular, the studies all used a similar group interview 
type of survey to investigate the level of visibility impairment that 
participants described as ``acceptable.'' In each preference study, 
participants were initially given a set of ``warm up'' exercises to 
familiarize them with how the scene in the photograph or image appears 
under different VAQ conditions. The participants next were shown 25 
randomly ordered photographs (images), and asked to rate each one based 
on a scale of 1 (poor) to 7 (excellent). They were then shown the same 
photographs or images again, in the same order, and asked to judge 
whether each of the photographs (images) would violate what they would 
consider to be an appropriate urban visibility standard (i.e. whether 
the level of impairment was ``acceptable'' or ``unacceptable''). The 
term ``acceptable'' was not defined, so that each person's response was 
based on his/her own values and preferences for VAQ. However, when 
answering this question, participants were instructed to consider the 
following three factors: (1) The standard would be for their own urban 
area, not a pristine national park area where the standards might be 
stricter; (2) The level of an urban visibility standard violation 
should be set at a VAQ level considered to be unreasonable, 
objectionable, and unacceptable visually; and (3) Judgments of 
standards violations should be based on visibility only, not on health 
effects. While the results differed among the four urban areas, results 
from a rating exercise show that within each preference study, 
individual survey participants consistently distinguish between photos 
or images representing different levels of light extinction, and that 
more participants rate as acceptable images representing lower levels 
of light extinction than they do images representing higher levels.
    Given the similarities in the approaches used, the EPA staff 
concluded that it was reasonable to compare the results to identify 
overall trends in the study findings and to conclude that this 
comparison can usefully inform the selection of a range of levels for 
use in further analyses. However, the staff also noted that variations 
in the specific materials and methods used in each study introduce 
uncertainties that should also be considered when interpreting the 
results

[[Page 3190]]

of these comparisons. Key differences between the studies include the 
following: (1) Scene characteristics; (2) image presentation methods 
(e.g., projected slides of actual photos, projected images generated 
using WinHaze (a significant technical advance in the method of 
presenting visual air quality conditions), or use of a computer monitor 
screen; (3) number of participants in each study; (4) participant 
representativeness of the general population of the relevant 
metropolitan area; and (5) specific wording used to frame the questions 
used in the group interview process.
    In the Visibility Assessment, each study was evaluated separately 
and figures developed to display the percentage of participants that 
rated the visual air quality depicted in each photograph as 
``acceptable.'' Ely et al. (1991) introduced a ``50% acceptability'' 
criterion analysis of the Denver preference study results. The 50 
percent acceptability criterion is designed to identify the visual air 
quality level (defined in terms of deciviews or light extinction) that 
best divides the photographs into two groups: Those with a visual air 
quality rated as acceptable by the majority of the participants, and 
those rated not acceptable by the majority of participants. The 
Visibility Assessment adopted this criterion as a useful index for 
comparison between studies. The results of each analysis were then 
combined graphically to allow for visual comparison. This information 
was then carried forward into the Policy Assessment. Figure 5 presents 
the graphical summary of the results of the studies in the four cities 
and draws on results previously presented in Figures 2-3, 2-5, 2-7, and 
2-11 of chapter 2 in the Visibility Assessment. Figure 5 also contains 
lines at 20 dv and 30 dv that generally identify a range where the 50 
percent acceptance criteria occur across all four of the urban 
preference studies (U.S. EPA, 2011a, p. 4-24). Out of the 114 data 
points shown in Figure 5, only one photograph (or image) with a visual 
air quality below 20 dv was rated as acceptable by less than 50 percent 
of the participants who rated that photograph.\155\ Similarly, only one 
image with a visual air quality above 30 dv was rated acceptable by 
more than 50 percent of the participants who viewed it.\156\
---------------------------------------------------------------------------

    \155\ Only 47 percent of the British Columbia participants rated 
a 19.2 dv photograph as acceptable.
    \156\ In the 2001 Washington, DC study, a 30.9 dv image was used 
as a repeated slide. The first time it was shown 56 percent of the 
participants rated it as acceptable, but only 11 percent rated it as 
acceptable the second time it was shown. The same visual air quality 
level was rated as acceptable by 4 percent of the participants in 
the 2009 study (Test 1). All three points are shown in Figure 5.
    \157\ Top scale shows light extinction in inverse megameter 
units; bottom scale in deciviews. Logit analysis estimated response 
functions are shown as the color-coded curved lines for each of the 
four urban areas.
[GRAPHIC] [TIFF OMITTED] TR15JA13.004

    As Figure 5 above shows, each urban area has a separate and unique 
response curve that appears to indicate that it is distinct from the 
others.\158\ These curves are the result of a logistical regression 
analysis using a logit model of the greater than 19,000 ratings of haze 
images as acceptable or unacceptable. The model results can be used to 
estimate the visual air quality in terms of dv values where the 
estimated response functions cross the 50 percent acceptability level, 
as well as any alternative criteria levels. Selected examples of these 
are shown in Table 4-

[[Page 3191]]

1 of the Policy Assessment (U.S. EPA, 2011a; U.S. EPA, 2010b, Table 2-
4). This table shows that the logit model results also support the 
upper and lower ends of the range of 50th percentile acceptability 
values (e.g., near 20 dv for Denver and near 30 dv for Washington, DC) 
already identified in Figure 5.
---------------------------------------------------------------------------

    \158\ At present, data is only available for four urban areas, 
as presented in Figure 5 and discussed throughout this section. 
Additional research could help inform whether the range identified 
by combining the results of the studies depicted in Figure 5 is more 
broadly representative.
---------------------------------------------------------------------------

    Based on the composite results and the effective range of 50th 
percentile acceptability across the four urban preference studies shown 
in Figure 5 and Table 4-1 of the Policy Assessment, benchmark levels of 
(total) light extinction were selected in a range from 20 dv to 30 dv 
(75 to 200 Mm-1) \159\ for the purpose of provisionally 
assessing whether visibility conditions would be considered acceptable 
(i.e., less than the low end of the range), unacceptable (i.e., greater 
than the high end of the range), or potentially acceptable (within the 
range) based on the very limited public preference information. A 
midpoint of 25 dv (120 Mm-1) was also selected for use in 
the assessment. This level is also very near to the 50th percentile 
criterion value from the Phoenix study (i.e., 24.2 dv), which is by far 
the best of the four studies in terms of the fit of the data to the 
response curve and the representativeness of study participants. Based 
on the currently available information, the Policy Assessment concluded 
that the use of 25 dv to represent the middle of the distribution of 
results seemed well supported (U.S. EPA, 2011a, p. 4-25).
---------------------------------------------------------------------------

    \159\ These values were rounded from 74 Mm-1 and 201 
Mm-1 to avoid an implication of greater precision than is 
warranted. Note that the middle value of 25 dv when converted to 
light extinction is 122 Mm-1 is rounded to 120 
Mm-1 for the same reason. Assessments conducted for the 
Visibility Assessment and the first and second drafts of the Policy 
Assessment used the unrounded values. The Policy Assessment 
considered the results of assessment using unrounded values to be 
sufficiently representative of what would result if the rounded 
values were used that it was unnecessary to redo the assessments. 
That is why some tables and figures in the Policy Assessment 
reflected the unrounded values.
---------------------------------------------------------------------------

    These three benchmark values provide a low, middle, and high set of 
light extinction conditions that are used to provisionally define 
daylight hours with urban haze conditions that have been judged 
unacceptable by at least 50 percent of the participants in one or more 
of these preference studies. As discussed above, PM light extinction is 
taken to be (total) light extinction minus the Rayleigh scatter,\160\ 
such that the low, middle, and high levels correspond to PM light 
extinction levels of about 65 Mm-1, 110 Mm-1, and 
190 Mm-1. In the Visibility Assessment, these three light 
extinction levels were called Candidate Protection Levels (CPLs). This 
term was also used in the Policy Assessment and in the proposal notice. 
It is important to note, however, that the degree of protection 
provided by a secondary NAAQS is not determined solely by any one 
component of the standard but by all the components (i.e., indicator, 
averaging time, form, and level) being applied together. Therefore, the 
Policy Assessment noted that the term CPL is meant only to indicate 
target levels of visibility within a range that the EPA staff felt 
appropriate for consideration that could, in conjunction with other 
elements of the standard, including indicator, averaging time, and 
form, potentially provide an appropriate degree of visibility 
protection.
---------------------------------------------------------------------------

    \160\ Rayleigh scatter is light scattering by atmospheric gases 
which is on average about 10 Mm-1.
---------------------------------------------------------------------------

    In characterizing the Policy Assessment's confidence in each CPL 
and across the range, a number of issues were considered (U.S. EPA, 
2011a, p. 4-26). Looking first at the two studies that define the upper 
and lower bounds of the range, the Policy Assessment considered whether 
they represent a true regional distinction in preferences for urban 
visibility conditions between western and eastern U.S. There was little 
information available to help evaluate the possibility of a regional 
distinction especially given that there have been preference studies in 
only one eastern urban area. Smith and Howell (2009) found little 
difference in preference response to Washington, DC, haze photographs 
between the study participants from Washington, DC, and those from 
Houston, Texas.\161\ This provides some limited evidence that the value 
judgment of the public in different areas of the country may not be an 
important factor in explaining the differences in these study results.
---------------------------------------------------------------------------

    \161\ The first preference study using WinHaze images of a 
scenic vista from Washington, DC was conducted in 2001 using 
subjects who were residents of Washington, DC. More recently, Smith 
and Howell (2009) interviewed additional subjects using the same 
images and interview procedure. The additional subjects included 
some residents of the Washington, DC area and some residents of the 
Houston, Texas area.
---------------------------------------------------------------------------

    In further considering what factors could explain the observed 
differences in preferences across the four urban areas, the Policy 
Assessment noted that the urban scenes used in each study had different 
characteristics (U.S. EPA, 2011a, p. 4-26). For example, each of the 
western urban visibility preference study scenes included mountains in 
the background while the single eastern urban study did not. It is also 
true that each of the western scenes included objects at greater 
distances from the camera location than in the eastern study. There is 
no question that objects at a greater distance have a greater 
sensitivity to perceived visibility changes as light extinction is 
changed compared to otherwise similar scenes with objects at a shorter 
range. This alone might explain the difference between the results of 
the eastern study and those from the western urban studies. Having 
scenes with the object of greatest intrinsic value nearer and hence 
less sensitive in the eastern urban area compared with more distant 
objects of greatest intrinsic value in the western urban areas could 
further explain the difference in preference results.
    Another question considered was whether the high CPL value that is 
based on the eastern preference results is likely to be generally 
representative of urban areas that do not have associated mountains or 
other valued objects visible in the distant background. Such areas 
would include the middle of the country, many areas in the eastern 
U.S., and possibly some areas in the western U.S. as well.\162\ Based 
on the currently available information, the Policy Assessment concluded 
that the high end of the CPL range (30 dv) is an appropriate level to 
consider (U.S. EPA, 2011a, p. 4-27).
---------------------------------------------------------------------------

    \162\ In order to examine this issue, an effort would have to be 
made to see if scenes in such areas could be found that would be 
generally comparable to the western scenes (e.g., scenes that 
contain valued scenic elements at more sensitive distances than that 
used in the eastern study). This is only one of a family of issues 
concerning how exposure to urban scenes of varying sensitivity 
affects public perception for which no preference study information 
is currently available.
---------------------------------------------------------------------------

    With respect to the low end of the range, the Policy Assessment 
considered factors that might further refine its understanding of the 
robustness of this level. The Policy Assessment concluded that 
additional urban preference studies, especially with a greater variety 
in types of scenes, could help evaluate whether the lower CPL value of 
20 dv is generally supportable (U.S. EPA, 2011a, p. 4-27). Further, the 
reason for the noisiness in data points around the curves apparent in 
both the Denver and British Columbia results compared to the smoother 
curve fit of Phoenix study results could be explored. One possible 
explanation discussed in the Policy Assessment is that these older 
studies use photographs taken at different times of day and on 
different days to capture the range of light extinction levels needed 
for the preference studies. In contrast, the use of WinHaze in the 
Phoenix (and Washington, DC) study reduced variations that affect scene 
appearance preference rating and avoided the uncertainty inherent in 
using ambient measurements to

[[Page 3192]]

represent sight path-averaged light extinction values. Reducing these 
sources of noisiness and uncertainty in the results of future studies 
of sensitive urban scenes could provide more confidence in the 
selection of a low CPL value.
    Based on the above considerations, and recognizing the limitations 
in the currently available information, the Policy Assessment concluded 
that it is reasonable to consider a range of CPL values including a 
high value of 30 dv, a mid-range value of 25 dv, and a low value of 20 
dv (U.S. EPA, 2011a, p. 4-27). Based on its review of the second draft 
Policy Assessment, CASAC also supported this set of CPLs for 
consideration by the EPA in this review. CASAC noted that these CPL 
values were based on all available visibility preference data and that 
they bound the study results as represented by the 50 percent 
acceptability criteria. While recommending that further visibility 
preference studies be conducted to reduce remaining uncertainties,\163\ 
CASAC concluded that this range of levels was ``adequately supported by 
the evidence presented'' (Samet, 2010d, p. iii).
---------------------------------------------------------------------------

    \163\ ``CASAC has also identified needs for the next review 
cycle in terms of further research on a number of topics related to 
urban visibility; * * *. In particular, there is a need for the 
Agency to conduct additional urban visibility preference studies 
over a broad range of urban areas and viewing conditions, to further 
evaluate and refine the range of visibility levels considered to be 
acceptable in the current assessment.'' (Samet, 2010a)
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c. Summary of Proposed Conclusions
i. Adequacy of the Current Standards for PM-Related Visibility 
Impairment
    At the time of proposal, the Administrator provisionally concluded 
that the current suite of secondary PM standards is not sufficiently 
protective of visual air quality, and that consideration should be 
given to an alternative secondary standard that would provide 
additional protection against PM-related visibility impairment, with a 
focus primarily in urban areas. This proposed conclusion was based on 
the information presented in the proposal with regard to the nature of 
PM-related visibility impairment, the results of public perception 
surveys on the acceptability of varying degrees of visibility 
impairment in urban areas, analyses of the number of days that are 
estimated to exceed a range of candidate protection levels under 
conditions simulated to just meet the current standards, and the advice 
of CASAC. This section summarizes key points from section VI.C of the 
proposal regarding visibility under current conditions, the degree of 
protection afforded by the current standards, and CASAC's advice 
regarding the adequacy of the current standards.
    As discussed in section VI.C.1 of the proposal, to evaluate 
visibility under current conditions the Visibility Assessment and 
Policy Assessment estimated PM-related light extinction\164\ levels for 
15 urban areas\165\ in the United States. Consistent with the emphasis 
in this review on the hourly or multi-hour time periods that might 
reasonably characterize the visibility effects experienced by various 
segments of the population, these analyses focused on using maximum 1-
hour and 4-hour values of PM light extinction during daylight hours for 
purposes of evaluating the degree of visibility impairment. Hourly 
average PM-related light extinction was analyzed in terms of both 
PM10 and PM2.5 light extinction. For reasons 
discussed above, hours with relative humidity greater than 90 percent 
were excluded from consideration. Recent visibility conditions in these 
urban areas were then compared to the CPLs identified above. The 
Visibility Assessment, which focused on PM10 light 
extinction in 14 of the 15 urban areas during the 2005 to 2007 time 
period,\166\ found that all 14 areas had daily maximum hourly 
PM10 light extinction values estimated to exceed even the 
highest CPL some of the days. Except for the two Texas areas and the 
non-California western urban areas, all of the other urban areas were 
estimated to have maximum hourly PM10 concentrations that 
exceeded the high CPL on about 20 percent to over 60 percent of the 
days. All 14 of the urban areas were estimated to have maximum hourly 
PM10 concentrations that exceeded the low CPL on about 40 
percent to over 90 percent of the days. In general, areas in the East 
and in California tend to have a higher frequency of hourly visibility 
conditions estimated to be above the high CPL compared with those in 
the western U.S.
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    \164\ PM-related light extinction is used here to refer to the 
light extinction caused by PM regardless of particle size; 
PM10 light extinction refers to the contribution by 
particles sampled through an inlet with a particle size 50 percent 
cutpoint of 10 [mu]m diameter; and PM2.5 light extinction 
refers to the contribution by particles sampled through an inlet 
with a particle size 50 percent cutpoint of 2.5 [mu]m diameter.
    \165\ The 15 urban areas are Tacoma, Fresno, Los Angeles, 
Phoenix, Salt Lake City, Dallas, Houston, St. Louis, Birmingham, 
Atlanta, Detroit, Pittsburgh, Baltimore, Philadelphia, and New York.
    \166\ Comments on the second draft Visibility Assessment from 
those familiar with the monitoring sites in St. Louis indicated that 
the site selected to provide continuous PM10 monitoring, 
although less than a mile from the site of the PM2.5 
data, was not representative of the urban area and resulted in 
unrealistically large PM10-2.5 values. The EPA staff 
considered these comments credible and set aside the St. Louis 
assessment results for PM10 light extinction. Thus, 
results and statements in the Policy Assessment regarding 
PM10 light extinction applied to only the other 14 areas. 
However, results regarding PM2.5 light extinction in most 
cases applied to all 15 study areas because the St. Louis estimates 
for PM2.5 light extinction were not affected by the 
PM10 monitoring issue.
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    The Policy Assessment repeated the Visibility Assessment-type 
modeling based on PM2.5 light extinction and data from the 
more recent 2007 to 2009 time period for the same 15 study areas 
(including St. Louis). While the estimates of the percentage of daily 
maximum hourly PM2.5 light extinction values exceeding the 
CPLs were somewhat lower than for PM10 light extinction, the 
patterns of these estimates across the study areas was found to be 
similar. More specifically, except for the two Texas and the non-
California western urban areas, all of the other urban areas were 
estimated to have maximum hourly PM2.5 concentrations that 
exceeded the high CPL on about 10 percent up to about 50 percent of the 
days based on PM2.5 light extinction, while all 15 areas 
were estimated to have maximum hourly PM2.5 concentrations 
that exceeded the low CPL on over 10 percent to over 90 percent of the 
days.
    To evaluate how PM-related visibility would be affected by just 
meeting the current suite of PM2.5 secondary standards, the 
Policy Assessment applied the proportional rollback approach described 
in section VI.C.2 of the proposal to all the PM2.5 
monitoring sites in each study area.\167\ After adjusting for 
composition, the Policy Assessment applied the original IMPROVE 
algorithm to calculate the PM10 light extinction, using 
``rolled back'' PM2.5 component concentrations, the current 
conditions PM10-2.5 concentration for the day and hour, and 
relative humidity for the day and hour.
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    \167\ Phoenix and Salt Lake City met the current 
PM2.5 NAAQS under current conditions and required no 
reduction.
---------------------------------------------------------------------------

    In these analyses, the Policy Assessment estimated both 
PM2.5 and PM10 light extinction in terms of both 
daily maximum 1-hour average values and multi-hour (i.e., 4-hour) 
average values for daylight hours. Figure 4-7 and Table 4-6 of the 
Policy Assessment displayed the results of the rollback procedures as a 
box and whisker plot of daily maximum daylight 1-hour PM2.5 
light extinction and the percentage of daily maximum hourly 
PM2.5 light extinction values estimated to exceed the CPLs 
when just meeting the current

[[Page 3193]]

suite of PM2.5 secondary standards for all 15 areas 
considered in the Visibility Assessment (including St. Louis) 
(excluding hours with relative humidity greater than 90 percent). These 
displays showed that the daily maximum 1-hour average PM2.5 
light extinction values in all of the study areas other than the three 
western non-California areas were estimated to exceed the high CPL on 
about 8 percent up to over 30 percent of the days and to exceed the 
middle CPL on about 30 percent up to about 70 percent of the days, 
while all areas except Phoenix were estimated to have daily maximum 1-
hour average PM2.5 light extinction values that exceeded the 
low CPL on over 15 percent to about 90 percent of the days. Figure 4-8 
and Table 4-7 of the Policy Assessment present results based on daily 
maximum 4-hour average values. These displays show that the daily 
maximum 4-hour average PM2.5 light extinction values in all 
of the study areas other than the three western non-California areas 
and the two areas in Texas were estimated to exceed the high CPL on 
about 4 percent up to over 15 percent of the days and to exceed the 
middle CPL on about 15 percent up to about 45 percent of the days, 
while all areas except Phoenix were estimated to have daily maximum 4-
hour average PM2.5 light extinction values that exceeded the 
low CPL on over 10 percent to about 75 percent of the days. A similar 
set of figures and tables were developed in terms of PM10 
light extinction (U.S. EPA, 2011a, Figures 4-5 and 4-6, Tables 4-4 and 
4-5).
    Taking the results of these analyses focusing on 1-hour and 4-hour 
maximum light extinction values into account, the Policy Assessment 
concluded that the available information in this review clearly called 
into question the adequacy of the current suite of PM2.5 
standards in the context of public welfare protection from visibility 
impairment, primarily in urban areas, and supported consideration of 
alternative standards to provide appropriate protection (U.S. EPA, 
2011a, p. 4-39). This conclusion was based in part on the large 
percentage of days, in many urban areas, that were estimated to have 
maximum 1-hour or 4-hour light extinction values that exceed the range 
of CPLs identified for consideration under simulations of conditions 
that would just meet the current suite of PM2.5 secondary 
standards. In particular, for air quality that was simulated to just 
meet the current PM2.5 standards, greater than 10 percent of 
the days were estimated to have peak light extinction values that 
exceed the highest, least protective CPL of 30 dv in terms of 
PM2.5 light extinction for 9 of the 15 urban areas, based on 
1-hour average values, and would thus likely fail to meet a 90th 
percentile-based standard at that level. For these areas, the percent 
of days estimated to have maximum 1-hour values that exceed the highest 
CPL ranged from over 10 percent to over 30 percent. Similarly, when the 
middle CPL of 25 dv was considered, greater than 30 percent up to 
approximately 70 percent of the days were estimated to have peak light 
extinction that exceeded that CPL in terms of PM2.5 light 
extinction, for 11 of the 15 urban areas, based on 1-hour average 
values. Based on a 4-hour averaging time, 5 of the areas were estimated 
to have at least 10 percent of the days with peak light extinction 
exceeding the highest CPL in terms of PM2.5 light 
extinction, and 8 of the areas were estimated to have at least 30 
percent of the days with peak light extinction exceeding the middle CPL 
in terms of PM2.5 light extinction. For the lowest CPL of 20 
dv, the percentages of days with 4-hour maximum light extinction 
estimated to exceed that CPL are even higher for all cases considered. 
Based on all of the above, the Policy Assessment concluded that PM 
light extinction estimated to be associated with just meeting the 
current suite of PM2.5 secondary standards in many areas 
across the country exceeded levels and percentages of days that could 
reasonably be considered to be important from a public welfare 
perspective (U.S. EPA, 2011a, p. 4-40).
    Further, the Policy Assessment concluded that use of the current 
indicator of PM2.5 mass, in conjunction with the current 24-
hour and annual averaging times, is clearly called into question for a 
national standard intended to protect public welfare from PM-related 
visibility impairment (U.S. EPA, 2011a, p. 4-40). This is because such 
a standard is inherently variable in the degree of protection provided 
because of regional differences in relative humidity and species 
composition of PM2.5, which are critical factors in the 
relationship between the mix of fine particles in the ambient air and 
the associated impairment of visibility. The Policy Assessment noted 
that this concern was one of the important elements in the court's 
decision to remand the PM2.5 secondary standards set in 2006 
to the Agency.
    Thus, in addition to concluding that the available information 
clearly calls into question the adequacy of the protection against PM-
related visibility impairment afforded by the current suite of 
PM2.5 standards, the Policy Assessment also concluded that 
it clearly calls into question the appropriateness of each of the 
current standard elements: indicator, averaging time, form, and level 
(U.S. EPA, 2011a, p. 4-40).
    After reviewing the information and analysis in the second draft 
Policy Assessment, CASAC concluded that the ``currently available 
information clearly calls into question the adequacy of the current 
standards and that consideration should be given to revising the suite 
of standards to provide increased public welfare protection'' (Samet, 
2010d, p. iii). CASAC noted that the detailed estimates of hourly PM 
light extinction associated with just meeting the current standards 
``clearly demonstrate that current standards do not protect against 
levels of visual air quality which have been judged to be unacceptable 
in all of the available urban visibility preference studies.'' Further, 
CASAC stated, with respect to the current suite of secondary 
PM2.5 standards, that ``[T]he levels are too high, the 
averaging times are too long, and the PM2.5 mass indicator 
could be improved to correspond more closely to the light scattering 
and absorption properties of suspended particles in the ambient air'' 
(Samet, 2010d, p. 9).
    After considering the available evidence and the advice of CASAC, 
the Administrator concluded at the time of proposal that such 
information did provide an appropriate basis to inform a conclusion as 
to whether the current standards afford adequate protection against PM-
related visibility impairment in urban areas. The Administrator took 
into account the information discussed above with regard to the nature 
of PM-related visibility impairment, the results of public perception 
surveys on the acceptability of varying degrees of visibility 
impairment in urban areas, analyses of the number of days on which peak 
1-hour or 4-hour light extinction values are estimated to exceed a 
range of candidate protection levels under conditions simulated to just 
meet the current standards, and the advice of CASAC. She noted the 
clear causal relationship between PM in the ambient air and impairment 
of visibility, the evidence from the visibility preference studies, and 
the rationale for determining a range of candidate protection levels 
based on those studies. She also noted the relatively large number of 
days when maximum 1-hour or 4-hour light extinction values were 
estimated to exceed the three candidate protection levels, including 
the highest level of 30 dv, under the current standards. While 
recognizing the limitations in the available information on public

[[Page 3194]]

perceptions of the acceptability of varying degree of visibility 
impairment and the information on the number of days estimated to 
exceed the CPLs, she concluded that such information provided an 
appropriate basis to inform a conclusion as to whether the current 
standards provide adequate protection against PM-related visibility 
impairment in urban areas. Based on these considerations, and placing 
great importance on the advice of CASAC, the Administrator 
provisionally concluded that the current standards are not sufficiently 
protective of visual air quality, and that consideration should be 
given to an alternative secondary standard that would provide 
additional protection against PM-related visibility impairment, with a 
focus primarily in urban areas.
    Having reached this conclusion, the Administrator also stated at 
the time of proposal that the current indicator of PM2.5 
mass, in conjunction with the current 24-hour and annual averaging 
times, is not well suited for a national standard intended to protect 
public welfare from PM-related visibility impairment. As noted in the 
proposal, the current standards do not incorporate information on the 
concentrations of various species within the mix of ambient particles, 
nor do they incorporate information on relative humidity, both of which 
play a central role in determining the relationship between the mix of 
PM in the ambient air and impairment of visibility. Such considerations 
were reflected both in CASAC's advice to set a distinct secondary 
standard that would more directly reflect the relationship between 
ambient PM and visibility impairment and in the court's remand of the 
current secondary PM2.5 standards. Based on the above 
considerations, at the time of proposal the Administrator provisionally 
concluded that the current secondary PM2.5 standards, taken 
together, are neither sufficiently protective nor suitably structured 
to provide an appropriate degree of public welfare protection from PM-
related visibility impairment, primarily in urban areas. This led the 
EPA to consider alternative standards by looking at each of the 
elements of the standards--indicator, averaging time, form, and level--
as discussed below.
ii. Indicator
    At the time of proposal, the EPA considered three alternative 
indicators for a PM2.5 standard designed to protect against 
visibility impairment: The current PM2.5 mass indicator; 
directly measured PM2.5 light extinction; and calculated 
PM2.5 light extinction. Directly measured PM2.5 
light extinction is a measurement (or combination of measurements) of 
the light absorption and scattering caused by PM2.5 under 
ambient conditions. Calculated PM2.5 light extinction uses 
the IMPROVE algorithm to calculate PM2.5 light extinction 
using measured PM2.5 mass, speciated PM2.5 mass, 
and measured relative humidity. The Policy Assessment evaluated each of 
these alternatives, finally concluding that consideration should be 
given to establishing a new calculated PM2.5 light 
extinction indicator (U.S. EPA, 2011a, p. 4-51).
    As discussed in section VI.D.1 of the proposal, the Policy 
Assessment concluded that consideration of the use of either directly 
measured PM2.5 light extinction or calculated 
PM2.5 light extinction as an indicator is justified because 
light extinction is a physically meaningful measure of the 
characteristic of ambient PM2.5 that is most relevant and 
directly related to PM-related visibility effects (U.S. EPA, 2011a, p. 
4-41). Further, as noted above, PM2.5 is the component of PM 
responsible for most of the visibility impairment in most urban areas. 
In these areas, the contribution of PM10-2.5 is a minor 
contributor to visibility impairment most of the time. The Policy 
Assessment also indicated that the available evidence demonstrated a 
strong correspondence between calculated PM2.5 light 
extinction and PM-related visibility impairment, as well as the 
significant degree of variability in visibility protection across the 
U.S. allowed by a PM2.5 mass indicator. The Policy 
Assessment recognized that while in the future it would be appropriate 
to consider a direct measurement of PM2.5 light extinction 
it was not an appropriate option in this review because a suitable 
specification of the equipment and associated performance verification 
procedures cannot be developed in the time frame for this review.
(a) PM2.5 Mass
    In terms of utilizing a PM2.5 mass indicator, the 
proposal noted that PM2.5 mass monitoring methods are in 
widespread use, including the FRM involving the collection of periodic 
(usually 1-day-in-6 or 1-day-in-3) 24-hour filter samples. However, 
these routine monitoring activities do not include measurement of the 
full water content of the ambient PM2.5 that contributes, 
often significantly, to visibility impacts. Further, the 
PM2.5 mass concentration monitors do not provide information 
on the composition of the ambient PM2.5, which plays a 
central role in the relationship between PM-related visibility 
impairment and ambient PM2.5 mass concentrations. Additional 
analyses discussed in the proposal that looked at the contribution of 
PM2.5 to total PM-related light extinction (defined in terms 
of hourly PM10 calculated light extinction) indicate that 
there is a poor correlation between hourly PM10 light 
extinction and hourly PM2.5 mass principally due to the 
impact of the water content of the particles on light extinction, which 
depends on both the composition of the PM2.5 and the ambient 
relative humidity. Both composition and especially relative humidity 
vary during a single day, as well as from day-to-day, at any site and 
time of year. Also, there are systematic regional and seasonal 
differences in the distribution of ambient humidity and 
PM2.5 composition conditions that make it impossible to 
select a PM2.5 concentration that generally would correspond 
to the same PM-related light extinction levels across all areas of the 
nation. Analyses discussed in the proposal quantify the projected 
uneven protection that would result from the use of 1-hour average 
PM2.5 mass as the indicator.
(b) Directly Measured PM2.5 Light Extinction
    PM light extinction has a nearly one-to-one relationship to light 
extinction, unlike PM2.5 mass concentration. As explained 
above, PM2.5 is the component responsible for the large 
majority of PM light extinction in most places and times. 
PM2.5 light extinction can be directly measured using 
several instrumental methods, some of which have been used for decades 
to routinely monitor the two components of PM2.5 light 
extinction (light scattering and absorption) or to jointly measure both 
as total light extinction (from which Rayleigh scattering is subtracted 
to get PM2.5 light extinction). As noted at the time of 
proposal, there are a number of advantages to direct measurements of 
light extinction for use in a secondary standard relative to estimates 
of PM2.5 light extinction calculated using PM2.5 
mass and speciation data. These include greater accuracy of direct 
measurements with shorter averaging times and overall greater 
simplicity when compared to the need for measurements of multiple 
parameters to calculate PM light extinction.
    In evaluating whether direct measurement of PM2.5 or 
PM10 light extinction is appropriate to consider in the 
context of this PM NAAQS review, the EPA solicited comment from the 
Ambient Air Monitoring and Methods

[[Page 3195]]

Subcommittee (AAMMS) of CASAC. The CASAC AAMMS recommended that 
consideration of direct measurement should be limited to 
PM2.5 light extinction, and that although instruments 
suitable for this purpose are commercially available at present, 
research is expected to produce even better instruments in the near 
term. The CASAC AAMMS advised against choosing any currently available 
commercial instrument, or even a general measurement approach, as an 
FRM because to do so could discourage development of other potentially 
superior approaches. Instead, the CASAC AAMMS recommended that the EPA 
develop performance-based approval criteria for direct measurement 
methods in order to put all approaches on a level playing field.
    At the present time, the EPA has not undertaken to develop and test 
such performance-base approval criteria. The EPA anticipates that if an 
effort were begun it would take at least several years before such 
criteria would be ready for regulatory use. Thus, the Policy Assessment 
concluded that while in the future it would be appropriate to consider 
a direct measurement of PM2.5 light extinction, or the sum 
of separate measurements of light scattering and light absorption, as 
the indicator for the secondary PM2.5 standard, this is not 
an appropriate option in this review because a suitable specification 
of the equipment or appropriate performance-based verification 
procedures cannot be developed in the time frame for this review (U.S. 
EPA, 2011a, p. 4-51, -52).
(c) Calculated PM2.5 Light Extinction
    For the reasons discussed above, the Policy Assessment concluded 
that a calculated PM2.5 light extinction indicator would be 
the preferred approach. PM2.5 light extinction can be 
calculated from PM2.5 mass, combined with speciated 
PM2.5 mass concentration data plus relative humidity data, 
as is presently routinely done on a 24-hour average basis under the 
Regional Haze Program using data from the rural IMPROVE monitoring 
network. This same calculation procedure, using a 24-hour average 
basis, could be used for a NAAQS focused on protecting against PM-
related visibility impairment primarily in urban areas. This approach 
would use the type of data that is routinely collected from the urban 
CSN \168\ in combination with monthly average relative humidity data 
based on long-term climatological means as used in the Regional Haze 
Program (U.S. EPA, 2011a, Appendix G, section G.2). The proposal 
discussed the complex approach utilized in the Visibility Assessment 
for calculating hourly PM2.5 light extinction \169\ and 
discussed various simplified approaches for calculating these hourly 
values that were analyzed in the Policy Assessment. The Policy 
Assessment concluded that each of these simplified approaches provided 
reasonably good estimates of PM2.5 light extinction and each 
would be appropriate to consider as the indicator for a distinct hourly 
or multi-hour secondary standard (U.S. EPA, 2011a, p. 4-48). The 
proposal also recognized that the Policy Assessment identified a number 
of variations on these simplified approaches that it would be 
appropriate to consider, including:
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    \168\ About 200 sites in the CSN routinely measure 24-hour 
average PM2.5 chemical components using filter-based 
samplers and chemical analysis in a laboratory, on either a 1-day-
in-3 or 1-day-in-6 schedule (U.S. EPA, 2011a, Appendix B, section 
B.1.3).
    \169\ As noted at the time of proposal, the sheer size of the 
ambient air quality, meteorological, and chemical transport modeling 
data files involved with the Visibility Assessment approach would 
make it very difficult for state agencies or any interested party to 
consistently apply such an approach on a routine basis for the 
purpose of implementing a national standard defined in terms of the 
Visibility Assessment approach.

    (1) The use of the split-component mass extinction efficiency 
approach from the revised IMPROVE algorithm\170\
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    \170\ If the revised IMPROVE algorithm were used to define the 
calculated PM2.5 mass-based indicator, it would not be 
possible to algebraically reduce the revised algorithm to a two-
factor version as described above and in Appendix F of the Policy 
Assessment for the simplified approaches. Instead, five component 
fractions would be determined from each day of speciated sampling, 
and then either applied to hourly measurements of PM2.5 
mass on the same day or averaged across a month and then applied to 
measurements of PM2.5 mass on each day of the month.
---------------------------------------------------------------------------

    (2) The use of more refined value(s) for the organic carbon 
multiplier \171\
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    \171\ An organic carbon (OC)-to-organic mass (OM) multiplier of 
1.6 was used for the assessment, which was found to produce a value 
of OM comparable to the one derived with the original, albeit more 
complex, Visibility Assessment method.
---------------------------------------------------------------------------

    (3) The use of the reconstructed 24-hour PM2.5 mass 
(i.e., the sum of the five PM2.5 components from 
speciated monitoring) as a normalization value for the hourly 
measurements from the PM2.5 instrument as a way of better 
reflecting ambient nitrate concentrations
    (4) The use of historical monthly or seasonal, or regional, 
speciation averages

    Overall, the analyses conducted for the Visibility Assessment and 
Policy Assessment indicated that the use of a calculated 
PM2.5 light extinction indicator would provide a much higher 
degree of uniformity in terms of the degree of protection from 
visibility impairment across the country than a PM2.5 mass 
indicator, because a calculated PM2.5 light extinction 
indicator would directly incorporate the effects of humidity and 
PM2.5 composition differences between various regions. 
Further, the proposal noted that the Policy Assessment concluded that 
consideration could be given to defining a calculated PM2.5 
light extinction indicator on either a 24-hour or a sub-daily basis 
(U.S. EPA, 2011a, p. 4-52). However, the Policy Assessment noted that 
approval of continuous FEM monitors has been based only on 24-hour 
average, not hourly, PM2.5 mass. In addition, there are 
mixed results of data quality assessments on a 24-hour basis for these 
monitors, as well as the near absence of performance data for sub-daily 
averaging periods. Thus, while it is possible to utilize data from 
PM2.5 continuous FEMs on a 1-hour or multi-hour (e.g., 4-
hour) basis, these factors increase the uncertainty of utilizing 
continuous methods to support 1-hour or 4-hour PM2.5 mass 
measurements as an input to the light extinction calculation. 
Therefore, as noted at the time of proposal, until issues regarding the 
comparability of 24-hour PM2.5 mass values derived from 
continuous FEMs and filter-based FRMs \172\ are resolved, there is 
reason to be cautious about relying on a calculation procedure that 
uses hourly PM2.5 mass values reported by continuous FEMs in 
combination with speciated PM2.5 mass values from 24-hour 
filter-based samplers.
---------------------------------------------------------------------------

    \172\ Filter-based FRMs are designed to adequately quantify the 
amount of PM2.5 collected over 24-hours. They cannot be 
presumed to be appropriate for quantifying average concentrations 
over 1-hour or 4-hour periods.
---------------------------------------------------------------------------

(d) CASAC Advice
    In reviewing the second draft Policy Assessment, CASAC stated that 
it ``overwhelmingly * * * would prefer the direct measurement of light 
extinction,'' recognizing it as the property of the atmosphere that 
most directly relates to visibility effects (Samet, 2010d, p. iii). 
CASAC noted that ``[I]t has the advantage of relating directly to the 
demonstrated harmful welfare effect of ambient PM on human visual 
perception.'' However, CASAC also concluded that the calculated 
PM2.5 light extinction indicator ``appears to be a 
reasonable approach for estimating hourly light extinction'' (Samet, 
2010d, p. 11). Further, based on CASAC's understanding of the time that 
would be required to develop an FRM for this indicator, CASAC agreed 
with the staff preference presented in the second draft Policy 
Assessment for a calculated PM2.5 light extinction 
indicator. CASAC noted that ``[I]ts reliance on procedures that

[[Page 3196]]

have already been implemented in the CSN and routinely collected 
continuous PM2.5 data suggest that it could be implemented 
much sooner than a directly measured indicator'' (Samet, 2010d, p. 
iii).\173\
---------------------------------------------------------------------------

    \173\ In commenting on the second draft Policy Assessment, CASAC 
did not have an opportunity to review the assessment of continuous 
PM2.5 FEMs compared to collocated FRMs (Hanley and Reff, 
2011) as presented and discussed in the final Policy Assessment 
(U.S. EPA, 2011a, p. 4-50).
---------------------------------------------------------------------------

(e) Administrator's Proposed Conclusions on Indicator
    At the time of proposal, while agreeing with CASAC that a directly 
measured PM light extinction indicator would provide the most direct 
link between PM in the ambient air and PM-related light extinction, the 
Administrator provisionally concluded that this was not an appropriate 
option in this review because a suitable specification of currently 
available equipment or performance-based verification procedures cannot 
be developed in the time frame of this review. Taking all of the above 
considerations and CASAC advice into account, the Administrator 
provisionally concluded that a new calculated PM2.5 light 
extinction indicator, similar to that used in the Regional Haze Program 
(i.e., using an IMPROVE algorithm as translated into the deciview 
scale), was the appropriate indicator to replace the current 
PM2.5 mass indicator. Such an indicator, referred to as a 
PM2.5 visibility index, would appropriately reflect the 
relationship between ambient PM and PM-related light extinction, based 
on the analyses discussed in the proposal and incorporation of factors 
based on measured PM2.5 speciation concentrations and 
relative humidity data. In addition, selection of this type of 
indicator would address, in part, the issues raised in the court's 
remand of the 2006 p.m.2.5 standards. The Administrator also 
noted that such a PM2.5 visibility index would afford a 
relatively high degree of uniformity of visual air quality protection 
in areas across the country by virtue of directly incorporating the 
effects of differences in PM2.5 composition and relative 
humidity across the country.
    Based on these above considerations, the Administrator proposed to 
set a distinct secondary standard for PM2.5 defined in terms 
of a PM2.5 visibility index (i.e., a calculated 
PM2.5 light extinction indicator, translated into the 
deciview scale) to protect against PM-related visibility impairment 
primarily in urban areas. The Administrator proposed that such an index 
be based on the original IMPROVE algorithm in conjunction with monthly 
average relative humidity data based on long-term climatological means 
as used in the Regional Haze Program. The EPA solicited comment on all 
aspects of the proposed indicator, especially:

    (1) The proposed use of a PM2.5 visibility index 
rather than a PM10 visibility index which would include 
an additional term for coarse particles;
    (2) Using the revised IMPROVE algorithm rather than the original 
IMPROVE algorithm;
    (3) The use of alternative values for the organic carbon 
multiplier in conjunction with either the original or revised 
IMPROVE algorithm;
    (4) The use of historical monthly, seasonal, or regional 
speciation averages;
    (5) Alternative approaches to determining relative humidity; and
    (6) Simplified approaches to generating hourly PM2.5 
light extinction values for purposes of calculating an hourly or 
multi-hour indicator.

iii. Averaging Times
    In this review, as discussed in section VI.D.2 of the proposal, 
consideration of appropriate averaging times for use in conjunction 
with a PM2.5 visibility index was informed by information 
related to the nature of PM visibility effects and the nature of inputs 
to the calculation of PM2.5 light extinction, as discussed 
above. The EPA considered both sub-daily (1- and 4-hour averaging 
times) and 24-hour averaging times. In considering sub-daily averaging 
times, the EPA has also considered what diurnal periods and ambient 
relative humidity conditions would be appropriate to consider in 
conjunction with such an averaging time.
    As an initial matter, the Policy Assessment considered sub-daily 
averaging times. Taking into account what is known from available 
studies concerning how quickly people experience and judge visibility 
conditions, the possibility that some fraction of the public 
experiences infrequent or short periods of exposure to ambient 
visibility conditions, and the typical rate of change of the path-
averaged PM light extinction over urban areas, the initial analyses 
conducted as part of the Visibility Assessment focused on a 1-hour 
averaging time. In its review of the first draft Policy Assessment, 
CASAC agreed that a 1-hour averaging time would be appropriate to 
consider, noting that PM effects on visibility can vary widely and 
rapidly over the course of a day and such changes are almost 
instantaneously perceptible to human observers (Samet, 2010c, p. 19). 
The Policy Assessment noted that this view related specifically to a 
standard defined in terms of a directly measured PM light extinction 
indicator, in that CASAC also noted that a 1-hour averaging time is 
well within the instrument response times of the various currently 
available and developing optical monitoring methods.
    However, CASAC also advised that if a PM2.5 mass 
indicator were to be used, it would be appropriate to consider 
``somewhat longer averaging times--2 to 4 hours--to assure a more 
stable instrumental response'' (Samet, 2010c, p. 19). In considering 
this advice, the Policy Assessment concluded that since a calculated 
PM2.5 light extinction indicator relies in part on measured 
PM2.5 mass, it would be appropriate to consider a multi-hour 
averaging time on the order of a few hours (e.g. 4-hours). A multi-hour 
averaging time might reasonably characterize the visibility effects 
experienced by the segment of the population who have access to 
visibility conditions often or continuously throughout the day. For 
this segment of the population, it may be that their perception of 
visual air quality reflects some degree of offsetting an hour with poor 
visual air quality with one or more hours of clearer visual conditions. 
Further, the Policy Assessment recognized that a multi-hour averaging 
time would have the effect of averaging away peak hourly visibility 
impairment, which can change significantly from one hour to the next 
(U.S. EPA, 2011a, p. 4-53; U.S. EPA, 2010b, Figure 3-12).
    In considering either 1-hour or multi-hour averaging times, the 
Policy Assessment recognized that no data are available with regard to 
how the duration and variation of time a person spends outdoors during 
the daytime impacts his or her judgment of the acceptability of 
different degrees of visibility impairment. As a consequence, it is not 
clear to what degree, if at all, the protection levels found to be 
acceptable in the public preference studies would change for a multi-
hour averaging time as compared to a 1-hour averaging time. Thus, the 
Policy Assessment concluded that it is appropriate to consider a 1-hour 
or multi-hour (e.g., 4-hour) averaging time as the basis for a sub-
daily standard defined in terms of a calculated PM2.5 light 
extinction indicator (U.S. EPA, 2011a, p. 4-53).
    In addition, as discussed above, some data quality uncertainties 
have been observed with regard to hourly data collected by FEMs. 
Specifically, as part of the review of data from all continuous FEM 
PM2.5 instruments operating at state/local monitoring sites, 
the Policy Assessment noted that the occurrence of questionable 
outliers in 1-

[[Page 3197]]

hour data submitted to AQS from continuous FEM PM2.5 
instruments had been observed at some of these sites (Evangelista, 
2011). Some of these outliers were questionable simply by virtue of 
their extreme magnitude, as high as 985 [mu]g/m\3\, whereas other 
values were questionable because they were isolated to single hours 
with much lower values before and after, a pattern that is much less 
plausible than if the high concentrations were more sustained.\174\ The 
Policy Assessment noted that any current data quality problems might be 
resolved in the normal course of monitoring program evolution as 
operators become more adept at instrument operation and maintenance and 
data validation or by improving the approval criteria and testing 
requirements for continuous instruments. Regardless, the Policy 
Assessment noted that multi-hour averaging of FEM data could serve to 
reduce the effects of such outliers relative to the use of a 1-hour 
averaging time.
---------------------------------------------------------------------------

    \174\ Similarly questionable hourly data were not observed in 
the 2005 to 2007 continuous PM2.5 data used in the 
Visibility Assessment, all of which came from early-generation 
continuous instruments that had not been approved as FEMs. However, 
only 15 sites and instruments were involved in the Visibility 
Assessment analyses, versus about 180 currently operating FEM 
instruments submitting data to AQS. Therefore, there were more 
opportunities for very infrequent measurement errors to be observed 
in the larger FEM data set.
---------------------------------------------------------------------------

    The Policy Assessment noted that there are significant reasons to 
consider using PM2.5 light extinction calculated on a 24-
hour basis to reduce the various data quality concerns described above 
with respect to relying on continuous PM2.5 monitoring data. 
However, the Policy Assessment recognized that 24 hours is far longer 
than the hourly or multi-hour time periods that might reasonably 
characterize the visibility effects experienced by various segments of 
the population, including both those who do and do not have access to 
visibility conditions often or continuously throughout the day. Thus, 
the Policy Assessment concluded that the appropriateness of considering 
a 24-hour averaging time would depend upon the extent to which PM-
related light extinction calculated on a 24-hour average basis would be 
a reasonable and appropriate surrogate for PM-related light extinction 
calculated on a sub-daily basis.
    To examine this relationship, the EPA conducted comparative 
analyses of 24-hour and 4-hour averaging times in conjunction with a 
calculated PM2.5 indicator. For these analyses, 4-hour 
average PM2.5 light extinction was calculated based on using 
the Visibility Assessment approach. The 24-hour average 
PM2.5 light extinction was calculated using the original 
IMPROVE algorithm and long-term relative humidity conditions to 
calculate PM2.5 light extinction. Based on these 
analyses,\175\ which are presented and discussed in Appendix G of the 
Policy Assessment, scatter plots comparing 24-hour and 4-hour 
calculated PM2.5 light extinction were constructed for each 
of the 15 cities included in the Visibility Assessment and for all 15 
cities pooled together (U.S. EPA, 2011a, Figures G-4 and G-5). Though 
there was some scatter around the regression line for each city because 
the calculated 4-hour light extinction values included day-specific and 
hour-specific influences that are not captured by the simpler 24-hour 
approach, these analyses generally showed good correlation between 24-
hour and 4-hour average PM2.5 light extinction, as evidenced 
by reasonably high city-specific and pooled R\2\ values, generally in 
the range of over 0.6 to over 0.8.\176\ This suggested that 
PM2.5 light extinction calculated on a 24-hour basis is a 
reasonable and appropriate surrogate to PM2.5 light 
extinction calculated on a sub-daily basis.
---------------------------------------------------------------------------

    \175\ These analyses are also based on the use of a 90th 
percentile form, averaged over 3 years, as discussed below in 
section VI.D.3 and in section 4.3.3 of the Policy Assessment (U.S. 
EPA, 2011a).
    \176\ The EPA staff noted that the R\2\ value (0.44) for Houston 
was notably lower than for the other cities.
---------------------------------------------------------------------------

    Taking the above considerations and CASAC's advice into account, 
the Policy Assessment concluded that it would be appropriate to 
consider a 24-hour averaging time, in conjunction with a calculated 
PM2.5 light extinction indicator and an appropriately 
specified standard level, as discussed below. By using site-specific 
daily data on PM2.5 composition and site-specific long-term 
relative humidity conditions, this 24-hour average indicator would 
provide more consistent protection from PM2.5-related 
visibility impairment than would a secondary PM2.5 NAAQS 
based only on 24-hour or annual average PM2.5 mass. In 
particular, this approach would account for the systematic difference 
in humidity conditions between most eastern states and most western 
states. The Policy Assessment also concluded that it would also be 
appropriate to consider a multi-hour, sub-daily averaging time, for 
example a period of 4 hours, in conjunction with a calculated 
PM2.5 light extinction indicator and with further 
consideration of the data quality issues discussed above. Such an 
averaging time, to the extent that data quality issues can be 
appropriately addressed, would be more directly related to the short-
term nature of the perception of visibility impairment, short-term 
variability in PM-related visual air quality, and the short-term nature 
(hourly to multiple hours) of relevant exposure periods for segments of 
the viewing public. Such an averaging time would still result in an 
indicator that is less sensitive than a 1-hour averaging time to short-
term instrument variability with respect to PM2.5 mass 
measurement. In conjunction with consideration of a multi-hour, sub-
daily averaging time, the Policy Assessment concluded that 
consideration should be given to including daylight hours only and to 
applying a relative humidity screen of approximately 90 percent to 
remove hours in which fog or precipitation is much more likely to 
contribute to the observed visibility impairment (U.S. EPA, 2011a, p. 
4-58). Recognizing that a 1-hour averaging time would be even more 
sensitive to data quality issues, including short-term variability in 
hourly data from currently available continuous monitoring methods, the 
Policy Assessment concluded that it would not be appropriate to 
consider a 1-hour averaging time in conjunction with a calculated 
PM2.5 light extinction indicator in this review (U.S. EPA, 
2011a, p. 4-58).
    As noted above, in its review of the first draft Policy Assessment, 
CASAC concluded that PM effects on visibility can vary widely and 
rapidly over the course of a day and such changes are almost 
instantaneously perceptible to human observers (Samet, 2010c, p. 19). 
Based in part on this consideration, CASAC agreed that a 1-hour 
averaging time would be appropriate to consider in conjunction with a 
directly measured PM light extinction indicator, noting that a 1-hour 
averaging time is well within the instrument response times of the 
various currently available and developing optical monitoring methods. 
At that time, CASAC also advised that if a PM2.5 mass 
indicator were to be used, it would be appropriate to consider 
``somewhat longer averaging times--2- to 4-hours--to assure a more 
stable instrumental response'' (Samet, 2010c, p. 19). Thus, CASAC's 
advice on averaging times that would be appropriate for consideration 
was predicated in part on the capabilities of monitoring methods that 
were available for the alternative indicators discussed in the draft 
Policy Assessment. CASAC's views on a multi-hour averaging time would 
also apply to the calculated PM2.5 light extinction 
indicator since hourly PM2.5 mass measurements are also 
required for this

[[Page 3198]]

indicator when calculated on a sub-daily basis.
    It is important to note that at the time it provided advice on 
suitable averaging times, CASAC did not have the benefit of EPA's 
subsequent assessment of the data quality issues associated with the 
use of continuous FEMs as the basis for hourly PM2.5 mass 
measurements. Furthermore, since CASAC only commented on the first and 
second drafts of the Policy Assessment, neither of which included 
discussion of a calculated PM2.5 indicator based on a 24-
hour averaging time, CASAC did not have a basis to offer advice 
regarding a 24-hour averaging time. In addition, the 24-hour averaging 
time is not based on consideration of 24-hours as a relevant exposure 
period, but rather as a surrogate for a sub-daily period of 4 hours, 
which is consistent with CASAC's advice concerning an averaging time 
associated with the use of a PM2.5 mass indicator.
    Taking into account the information discussed above with regard to 
analyses and conclusions presented in the final Policy Assessment the 
Administrator recognized that hourly or sub-daily, multi-hour averaging 
times, within daylight hours and excluding hours with relative humidity 
above approximately 90 percent, are more directly related than a 24-
hour averaging time to the short-term nature of the perception of PM-
related visibility impairment and the relevant exposure periods for 
segments of the viewing public. On the other hand, she recognized that 
data quality uncertainties have recently been associated with currently 
available instruments that would be used to provide the hourly 
PM2.5 mass measurements that would be needed in conjunction 
with an averaging time shorter than 24-hours. As a result, while the 
Administrator recognized the desirability of a sub-daily averaging 
time, she had strong reservations about proposing to set a standard at 
this time in terms of a sub-daily averaging time.
    In considering the information and analyses related to 
consideration of a 24-hour averaging time, the Administrator recognized 
that the Policy Assessment concluded that PM2.5 light 
extinction calculated on a 24-hour averaging basis is a reasonable and 
appropriate surrogate for sub-daily PM2.5 light extinction 
calculated on a 4-hour average basis. In light of this finding and the 
views of CASAC based on its reviews of the first and second drafts of 
the Policy Assessment, the Administrator proposed to set a distinct 
secondary standard with a 24-hour averaging time in conjunction with a 
PM2.5 visibility index.
iv. Form
    As discussed in section VI.D.3 of the proposal, the ``form'' of a 
standard defines the air quality statistic that is to be compared to 
the level of the standard in determining whether the standard is 
achieved. The form of the current 24-hour PM2.5 NAAQS is 
such that the level of the standard is compared to the 3-year average 
of the annual 98th percentile value of the measured indicator. The 
purpose in averaging for three years is to provide stability from the 
occasional effects of inter-annual meteorological variability that can 
result in unusually high pollution levels for a particular year. The 
use of a multi-year percentile form, among other things, makes the 
standard less subject to the possibility of transient violations caused 
by statistically unusual indicator values, thereby providing more 
stability to the air quality management process that may enhance the 
practical effectiveness of efforts to implement the NAAQS. Also, a 
percentile form can be used to take into account the number of times an 
exposure might occur as part of the judgment on protectiveness in 
setting a NAAQS. For all of these reasons, the Policy Assessment 
concluded it would be appropriate to consider defining the form of a 
distinct secondary standard in terms of a 3-year average of a specified 
percentile air quality statistic (U.S. EPA, 2011a, p. 4-58).
    The urban visibility preference studies that provided results 
leading to the range of CPLs being considered in this review offer no 
information that addresses the frequency of time that visibility levels 
should be below those values. Given this lack of information, and 
recognizing that the nature of the public welfare effect is one of 
aesthetics and/or feelings of well-being, the Policy Assessment 
concluded that it would not be appropriate to consider eliminating all 
exposures above the level of the standard and that allowing some number 
of hours/days with reduced visibility can reasonably be considered 
(U.S. EPA, 2011a, p. 4-59). In the Visibility Assessment, 90th, 95th, 
and 98th percentile forms were assessed for alternative PM light 
extinction standards (U.S. EPA, 2010b, section 4.3.3). In considering 
these alternative percentiles, the Policy Assessment noted that the 
Regional Haze Program targets the 20 percent most impaired days for 
improvements in visual air quality in Federal Class I areas. If 
improvement in the 20 percent most impaired days were similarly judged 
to be appropriate for protecting visual air quality in urban areas, a 
percentile well above the 80th percentile would be appropriate to 
increase the likelihood that all days in this range would be improved 
by control strategies intended to attain the standard. A focus on 
improving the 20 percent most impaired days suggests that the 90th 
percentile, which represents the median of the distribution of the 20 
percent worst days, would be an appropriate form to consider. 
Strategies that are implemented so that 90 percent of days have visual 
air quality that is at or below the level of the standard would 
reasonably be expected to lead to improvements in visual air quality 
for the 20 percent most impaired days. Higher percentile values within 
the range assessed could have the effect of limiting the occurrence of 
days with peak PM-related light extinction in urban areas to a greater 
degree. In considering the limited information available from the 
public preference studies, the Policy Assessment found no basis to 
conclude that it would be appropriate to consider limiting the 
occurrence of days with peak PM-related light extinction in urban areas 
to a greater degree.
    Another aspect of the form discussed in the proposal for a sub-
daily averaging time was whether to include all daylight hours or only 
the maximum daily daylight hour(s). The maximum daily daylight 1-hour 
or multi-hour form would be most directly protective of the welfare of 
people who have limited, infrequent or intermittent exposure to 
visibility during the day (e.g., during commutes), but spend most of 
their time without an outdoor view. For such people a view of poor 
visibility during their morning commute may represent their perception 
of the day's visibility conditions until the next time they venture 
outside during daylight, which may be hours later or perhaps the next 
day. Other people have exposure to visibility conditions throughout the 
day. For those people, it might be more appropriate to include every 
daylight hour in assessing compliance with a standard, since it is more 
likely that each daylight hour could affect their welfare.
    The Policy Assessment did not have information regarding the 
fraction of the public that has only one or a few opportunities to 
experience visibility during the day, nor did it have information on 
the role the duration of the observed visibility conditions has on 
wellbeing effects associated with those visibility conditions. However, 
it is logical to conclude that people with limited opportunities to 
experience visibility conditions on a daily basis

[[Page 3199]]

would experience the entire impact associated with visibility based on 
their short-term exposure. The impact of visibility for those who have 
access to visibility conditions often or continuously during the day 
may be based on varying conditions throughout the day.
    In light of these considerations, the analyses conducted as part of 
the Visibility Assessment analyses included both the maximum daily hour 
and the all daylight hours forms. The Policy Assessment noted that 
there is a close correspondence between the level of protection 
afforded for all 15 urban areas by a maximum daily daylight 1-hour 
approach using the 90th percentile form and an all daylight hours 
approach combined with the 98th percentile form (U.S. EPA, 2010b, 
section 4.1.4). This suggested that reductions in visibility impairment 
required to meet either form of the standard would provide protection 
to both fractions of the public (i.e., those with limited opportunities 
and those with greater opportunities to view PM-related visibility 
conditions). CASAC generally supported consideration of both types of 
forms without expressing a preference based on its review of 
information presented in the second draft Policy Assessment (Samet, 
2010d, p. 11).
    In conjunction with a calculated PM2.5 light extinction 
indicator and alternative 24-hour or sub-daily (e.g., 4-hour) averaging 
times, based on the above considerations, and given the lack of 
information on and the high degree of uncertainty over the impact on 
public welfare of the number of days with visibility impairment over a 
year, the Policy Assessment concluded that it would be appropriate to 
give primary consideration to a 90th percentile form, averaged over 
three years (U.S. EPA, 2011a, p. 4-60). Further, in the case of a 
multi-hour, sub-daily alternative standard, the Policy Assessment 
concluded that it would be appropriate to give primary consideration to 
a form based on the maximum daily multi-hour period in conjunction with 
the 90th percentile form (U.S. EPA, 2011a, p. 4-60). This sub-daily 
form would be expected to provide appropriate protection for various 
segments of the population, including those with limited opportunities 
during a day and those with more extended opportunities over the 
daylight hours to experience PM-related visual air quality.
    Though CASAC did not provide advice as to a specific form that 
would be appropriate, it took note of the alternative forms considered 
in that document and encouraged further analyses in the final Policy 
Assessment that might help to clarify a basis for selecting from within 
the range of forms identified. In considering the available information 
and the conclusions in the final Policy Assessment in light of CASAC's 
comments, at the time of proposal the Administrator concluded that a 
90th percentile form, averaged over 3 years, is appropriate, and 
proposed such a form in conjunction with a PM2.5 visibility 
index and a 24-hour averaging time.
v. Level
    As discussed in section VI.D.4 of the proposal, in considering 
appropriate levels for a 24-hour standard defined in terms of a 
PM2.5 visibility index and an 90th percentile form, averaged 
over 3 years, the Policy Assessment took into account the evidence- and 
impact-based considerations discussed above, with a focus on the 
results of public perception and attitude surveys related to the 
acceptability of various levels of visual air quality and on the 
important limitations in the design and scope of such available 
studies. The Policy Assessment considered a variety of approaches for 
identifying appropriate levels for such a standard, including utilizing 
both adjusted and unadjusted CPLs derived from the visibility 
preference studies.
    The Policy Assessment interpreted the results from the visibility 
preferences studies conducted in four urban areas to define a range of 
low, middle, and high CPLs for a sub-daily standard (e.g., 1- to 4-hour 
averaging time) of 20, 25, and 30 dv, which are approximately 
equivalent to PM2.5 light extinction of values of 65, 110, 
and 190 Mm-1. The CASAC generally supported this approach, 
noting that the ``EPA staff's approach for translating and presenting 
the technical evidence and assessment results is logically conceived 
and clearly presented. The 20-30 deciview range of levels chosen by EPA 
staff as `Candidate Protection Levels' is adequately supported by the 
evidence presented'' (Samet, 2010d, p. 11).\177\ The Policy Assessment 
also recognized that to define a range of alternative levels that would 
be appropriate to consider for a 24-hour calculated PM2.5 
light extinction standard, it would be appropriate to consider whether 
some adjustment to these CPLs is warranted since these preference 
studies cannot be directly interpreted as applying to a 24-hour 
exposure period (as noted above and in Policy Assessment section 
4.3.1). Considerations related to such adjustments are more 
specifically discussed below.
---------------------------------------------------------------------------

    \177\ In 2009, the DC Circuit remanded the secondary 
PM2.5 standards to the EPA in part because the Agency 
failed to identify a target level of protection, even though EPA 
staff and CASAC had identified a range of target levels of 
protection that were appropriate for consideration. The court 
determined that the Agency's failure to identify a target level of 
protection as part of its final decision was contrary to the statute 
and therefore unlawful, and that it deprived EPA's decision-making 
of a reasoned basis. See 559F. 3d at 528-31; see also section VI.A.2 
above and the Policy Assessment, section 4.1.2.
---------------------------------------------------------------------------

    In considering alternative levels for a sub-daily standard based 
directly on the four preference study results, the Policy Assessment 
noted that the individual low and high CPLs are in fact generally 
reflective of the results from the Denver and Washington, DC studies 
respectively, and the middle CPL is very near to the 50th percentile 
criteria result from the Phoenix study, which was by far the best of 
the studies, providing somewhat more support for the middle CPL.
    In considering the results from the four visibility preference 
studies, the Policy Assessment recognized that currently available 
studies are limited in that they were conducted in only four areas, 
three in the U.S. and one in Canada. Further, the Policy Assessment 
recognized that available studies provide no information on how the 
duration and variation of time a person spends outdoors during the 
daytime may impact their judgment of the acceptability of different 
degrees of visibility impairment. As such, there is a relatively high 
degree of uncertainty associated with using the results of these 
studies to inform consideration of a national standard for any specific 
averaging time. Nonetheless, the Policy Assessment concluded, as did 
CASAC, that these studies are appropriate to use for this purpose (U.S. 
EPA, 2011a, p. 4-61).
    Using approaches described in section VI.C.4 of the proposal, the 
Policy Assessment explored various approaches to adjusting the CPLs 
derived from the preference studies to inform alternative levels for a 
24-hour standard. These various approaches, based on analyses of 2007-
2009 data from the 15 urban areas assessed in the Visibility 
Assessment, focused on estimating CPLs for a 24-hour standard that 
would provide generally equivalent protection as that provided by a 4-
hour standard with CPLs of 20, 25, and 30 dv. In conducting these 
analyses, staff initially expected that the values of 24-hour average 
PM2.5 light extinction and daily maximum daylight 4-hour 
average PM2.5 light extinction would differ on any given 
day, with the shorter term peak value generally being larger. This 
would mean that, in concept, the level of a 24-hour standard should 
include a

[[Page 3200]]

downward adjustment compared to the level of a 4-hour standard to 
provide generally equivalent protection. As discussed more fully in 
section G.5 of Appendix G and summarized below, this initial 
expectation was not found to be the case across the range of CPLs 
considered. In fact, as shown in Tables G-7 and G-8 of Appendix G and 
in the corrected version of Table G-6 found in Frank et al. 
(2012b),\178\ in considering estimates aggregated or averaged over all 
15 cities as well as the range of city-specific estimates for the 
various approaches considered, these analyses indicated that the 
generally equivalent 24-hour levels ranged from somewhat below the 4-
hour level to just above the 4-hour level for each of the CPLs.\179\ In 
all cases, the range of city-specific estimates of generally equivalent 
24-hour levels included the 4-hour level for each of the CPLs of 20, 
25, and 30 dv. As noted in the proposal, looking more broadly at these 
results could support consideration of using the same CPL for a 24-hour 
standard as for a 4-hour standard, recognizing that there is no one 
approach that can most closely identify a generally equivalent 24-hour 
standard level in each urban area for each CPL. The use of such an 
unadjusted CPL for a 24-hour standard would place more emphasis on the 
relatively high degree of spatial and temporal variability in relative 
humidity and fine particle composition observed in urban areas across 
the country, so as to reduce the potential of setting a 24-hour 
standard level that would require more than the intended degree of 
protection in some areas.
---------------------------------------------------------------------------

    \178\ Note that the city-specific ranges shown in Table G-6, 
Appendix G of the Policy Assessment are incorrectly stated for 
Approaches C and E. Drawing from the more detailed and correct 
results for Approaches C and E presented in Tables G-7 and G-8, 
respectively, the city-specific ranges in Table G-6 for Approach C 
should be 17-21 dv for the CPL of 20 dv; 21-25 dv for the CPL of 25 
dv; and 24-30 dv for the CPL of 30 dv; the city-specific ranges in 
Table G-6 for Approach E should be 17-21 dv for the CPL of 20 dv; 
21-26 dv for the CPL of 25 dv; and 25-31 dv for the CPL of 30 dv. In 
the EPA's reanalysis comparing 4- vs. 24-hour values, Frank et al. 
(2012b) recreated Table G-6 using the correct values from Tables G-7 
and G-8.
    \179\ As discussed in more detail in Appendix G of the Policy 
Assessment, some days have higher values for 24-hour average light 
extinction than for daily maximum 4-hour daylight light extinction, 
and consequently an adjusted ``equivalent'' 24-hour CPL can be 
greater than the original 4-hour CPL. This can happen for two 
reasons. First, the use of monthly average historical RH data will 
lead to cases in which the f(RH) values used for the calculation of 
24-hour average light extinction are higher than all or some of the 
four hourly values of f(RH) used to determine daily maximum 4-hour 
daylight light extinction on the same day. Second, PM2.5 
concentrations may be greater during non-daylight periods than 
during daylight hours.
---------------------------------------------------------------------------

    In considering the appropriate level of a secondary standard 
focused on protection from PM-related urban visibility impairment based 
on either a 24-hour or a multi-hour, sub-daily (e.g., 4-hour) averaging 
time, the EPA has been mindful of the important limitations in the 
available evidence from public preference studies. These uncertainties 
and limitations are due in part to the small number of stated 
preference studies available for this review; the relatively small 
number of study participants and the extent to which the study 
participants may not be representative of the broader study area 
population in some of the studies; and the variations in the specific 
materials and methods used in each study such as scene characteristics, 
the range of VAQ levels presented to study participants, image 
presentation methods and specific wording used to frame the questions 
used in the group interviews. In addition the EPA has noted that the 
scenic vistas available on a daily basis in many urban areas across the 
country generally may not have the inherent visual interest or the 
distance between viewer and object of greatest intrinsic value as in 
the Denver and Phoenix preference studies, and that there is the 
possibility that there could be regional differences in individual 
preferences for VAQ.
    It is also important to note that as in past reviews, the EPA is 
considering a national visibility standard 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 Federal Class I areas across the country. The EPA recognizes 
that programs implemented to meet a national standard focused primarily 
on the visibility problems in urban areas can be expected to improve 
visual air quality in surrounding non-urban areas as well, as would 
programs now being developed to address the requirements of the 
Regional Haze Program established for protection of visual air quality 
in Federal Class I areas. The EPA also believes that the development of 
local programs, such as those in Denver and Phoenix, can continue to be 
an effective and appropriate approach to provide additional protection, 
beyond that afforded by a national standard, for unique scenic 
resources in and around certain urban areas that are particularly 
highly valued by people living in those areas.
    The Policy Assessment concluded that it is appropriate to give 
primary consideration to alternative standard levels toward the upper 
end of the ranges identified above for 24-hour and sub-daily standards, 
respectively (U.S. EPA, 2011a, p. 4-63). Thus, the Policy Assessment 
concluded it is appropriate to consider the following alternative 
levels: A level of 28 dv or somewhat below, down to 25 dv, for a 
standard defined in terms of a calculated PM2.5 light 
extinction indicator, a 90th percentile form, and a 24-hour averaging 
time; and a standard level of 30 dv or somewhat below, down to 25 dv, 
for a similar standard but with a 4-hour averaging time (U.S. EPA, 
2011a, p. 4-63). The Policy Assessment judged that such standards would 
provide appropriate protection against PM-related visibility impairment 
primarily in urban areas. The Policy Assessment noted that CASAC 
generally supported consideration of the 20-30 dv range as CPLs and, 
more specifically, that support for consideration of the upper part of 
the range of the CPLs derived from the public preference studies was 
expressed by some CASAC Panel members during the public meeting on the 
second draft Policy Assessment. The Policy Assessment concluded that 
such a standard would be appropriate in conjunction with the Regional 
Haze Program to achieve appropriate levels of protection against PM-
related visibility impairment in areas across the country (U.S. EPA, 
2011a, p. 4-63).
    Based on the considerations discussed above and in section VI.D.4 
of the proposal, and taking into account the advice of CASAC, at the 
time of proposal the Administrator concluded that it would be 
appropriate to establish a target level of protection--for a standard 
defined in terms of a PM2.5 visibility index; a 90th 
percentile form averaged over 3 years; and a 24-hour averaging time--
equivalent to the protection afforded by such a sub-daily (i.e., 4-
hour) standard at a level of 30 dv, which is the upper end of the range 
of CPLs identified in the Policy Assessment and generally supported by 
CASAC. More specifically, the Administrator provisionally concluded 
that a 24-hour level of either 30 dv or 28 dv could be construed as 
providing such a degree of protection, and that either level was 
supported by the available information and was generally consistent 
with the advice of CASAC. Thus, the EPA proposed two options for the 
level of a new 24-hour standard (defined in terms of a PM2.5 
visibility index and a 90th percentile form, averaged over 3 years) to 
provide appropriate protection from PM-related visibility impairment: 
Either 30 dv or 28 dv. As noted in the proposal, the option of setting 
such a 24-hour standard at a level of 30 dv would reflect recognition 
that there is considerable spatial and

[[Page 3201]]

temporal variability in the key factors that determine the value of the 
PM2.5 visibility index in any given urban area, such that 
there is a relatively high degree of uncertainty as to the most 
appropriate approach to use in selecting a 24-hour standard level that 
would be generally equivalent to a specific 4-hour standard level. 
Selecting a 24-hour standard level of 30 dv would reflect a judgment 
that such substantial degrees of variability and uncertainty should be 
reflected in a higher standard level than would be appropriate if the 
underlying information were more consistent and certain. Alternatively, 
the option of setting such a 24-hour standard at a level of 28 dv would 
reflect placing more weight on statistical analyses of aggregated data 
from across the study cities and not placing as much emphasis on the 
city-to-city variability as a basis for determining an appropriate 
degree of protection on a national scale.
    The information available for the Administrator to consider when 
setting the secondary PM standard raises a number of uncertainties. 
While CASAC supported moving forward with a new standard on the basis 
of the available information, CASAC also recognized these 
uncertainties, referencing the discussion of key uncertainties and 
areas for future research in the second draft of the Policy Assessment. 
In discussing areas of future research, CASAC stated that: ``The range 
of 50% acceptability values discussed as possible standards are based 
on just four studies (Figure 4-2), which, given the large spread in 
values, provide only limited confidence that the benchmark candidate 
protection levels cover the appropriate range of preference values. 
Studies using a range of urban scenes (including, but not limited to, 
iconic scenes--``valued scenic elements'' such as those in the 
Washington, DC study), should also be considered'' (Samet, 2010d, p. 
12). The EPA solicited comment on how the Administrator should weigh 
those uncertainties as well as any additional comments and information 
to inform her consideration of these uncertainties.
    In addition, the EPA solicited comment on a number of other issues 
related to the level of the standard, including:

    (1) Both of the proposed levels and the various approaches to 
identifying generally equivalent levels upon which the alternative 
proposed levels are based.
    (2) A broader range of levels down to 25 dv in conjunction with 
a 24-hour averaging time.
    (3) A range of alternative levels from 30 to 25 dv in 
conjunction with a sub-daily (e.g., 4-hour) averaging time.
    (4) The strengths and limitations associated with the public 
preference studies and the use of these studies to inform the 
selection of a range of levels that could be used to provide an 
appropriate degree of public welfare protection when combined with 
the other elements of the standard (i.e. indicator, form and 
averaging time).
    (5) Specific aspects of the public preference studies, including 
the extent to which the 50 percent acceptability criterion is an 
appropriate basis for establishing target protection levels in the 
context of establishing a distinct secondary NAAQS to address PM-
related visibility impairment in urban areas; how the variability 
among preference studies in the extent to which study participants 
may be representative of the broader study area population should be 
weighed in the context of considering these studies in reaching 
proposed conclusions on a distinct secondary NAAQS; and the extent 
to which the ranges of VAQ levels presented to participants in each 
of the studies may have influenced study results and on how this 
aspect of the study designs should appropriately be weighed in the 
context of considering these studies in the context of this review.
vi. Administrator's Proposed Conclusions Regarding PM Standards To 
Protect Visibility
    At the time of proposal, based on the considerations described 
above, the Administrator proposed to revise the suite of secondary PM 
standards by adding a distinct standard for PM2.5 to address 
PM-related visibility impairment, focused primarily on visibility in 
urban areas. This proposed visibility standard was to be defined in 
terms of a PM2.5 visibility index, which would use measured 
PM2.5 mass, combined with PM2.5 speciation data 
and relative humidity data, to calculate PM2.5 light 
extinction, translated into the deciview (dv) scale; a 24-hour 
averaging time; a 90th percentile form, averaged over 3 years; and a 
level of 28-30 dv.
vii. Related Technical Analysis
    At the time of proposal, the EPA conducted a two-pronged technical 
analysis of the relationships between the proposed PM2.5 
visibility index standard and the current 24-hour PM2.5 
mass-based standard (Kelly, et al., 2012a). This analysis was designed 
to provide technical information to inform key issues related to 
implementing a distinct secondary standard for visibility as proposed. 
Specifically, the EPA recognized that significant technical issues were 
likely to arise for new or modified emissions sources conducting air 
quality analyses for purposes of demonstrating that they would not 
cause or contribute to a violation of the visibility standard under the 
Prevention of Significant Deterioration (PSD) program. Such a 
demonstration for the proposed secondary PM2.5 visibility 
index standard could require each PSD applicant to predict, via air 
quality modeling, the increase in visibility impairment, in terms of 
the proposed PM2.5 visibility index, that would result from 
the proposed source's emissions in conjunction with an assessment of 
existing air quality (visibility impairment) conditions in terms of the 
proposed PM2.5 visibility index. The EPA noted that if this 
demonstration were to be attempted using the six-step procedure that 
the EPA proposed to use for calculating PM2.5 visibility 
index design values from monitored air concentrations of 
PM2.5 components, significant technical issues with the 
modeling procedures could arise.
    To address these technical issues, the EPA sought to explore 
whether sources that met the requirements pertaining to the 24-hour 
mass-based standard of 35 [micro]g/m\3\ would also meet the 
requirements pertaining to the proposed visibility index standard. As 
described in Kelly et al. (2012a), the first prong of the analysis 
addressed aspects of a PSD significant impact analysis by evaluating 
whether an individual source's impact resulting in a small increase in 
the ambient PM2.5 concentration would produce a comparably 
small increase in visibility impairment. This analysis included 
estimates of PM2.5 speciation profiles based on direct 
PM2.5 emission profiles for a broad range of source 
categories and for theoretical upper and lower bound scenarios.
    The second prong of the analysis addressed aspects of a PSD 
cumulative impact analysis by exploring the relationship between the 
three-year design values for the existing 24-hour PM2.5 
standard and coincident design values for the proposed PM2.5 
visibility index standard based on recent air quality data. This aspect 
of the analysis indicated that increases in 24-hour PM2.5 
design values generally correspond to increases in visibility index 
design values, and vice-versa. The analysis further explored the 
appropriateness of using a demonstration that a source does not cause 
or contribute to a violation of the 24-hour PM2.5 standard 
as a surrogate for a demonstration that a source does not cause or 
contribute to a violation of the proposed secondary PM2.5 
visibility index standard. This analysis was based on 2008 to 2010 air 
quality data, and compared the proposed level of 35 [mu]g/m\3\ for the 
24-hour PM2.5 standard and for illustrative purposes an 
alternative standard level of 12 [micro]g/m\3\ for the annual 
PM2.5 standard with the

[[Page 3202]]

proposed levels of 28 or 30 dv for the secondary PM2.5 
visibility index standard with a 24-hour averaging time and a 90th 
percentile form. The results indicated that all (for the 30 dv level) 
or nearly all (for the 28 dv level) areas in attainment of the 24-hour 
PM2.5 standard would also have been in attainment of the 
proposed secondary PM2.5 visibility index standard.
    Based on this technical analysis, the EPA proposed that there is 
sufficient evidence that a demonstration that a source does not cause 
or contribute to a violation of the mass-based 24-hour PM2.5 
standard serves as a suitable surrogate for demonstrating that a source 
does not cause or contribute to a violation of the proposed secondary 
24-hour PM2.5 visibility index standard under the PSD 
program. As such, the EPA proposed to conclude that many or all sources 
undergoing PSD review for PM2.5 could rely upon their 
analysis for demonstrating that they do not cause or contribute to a 
violation of the mass-based 24-hour PM2.5 standard to also 
show that they do not cause or contribute to a violation of the 
proposed secondary PM2.5 visibility index standard, if a 
distinct visibility standard were finalized.
    Although this proposed ``surrogacy policy'' was designed to address 
an implementation-related issue, the second prong of the technical 
analysis addresses the broader technical question of the relationship 
between the existing 24-hour PM2.5 standard and the proposed 
PM2.5 visibility index standard in terms of the degree of 
protection likely to be afforded by each standard. Specifically, the 
analysis indicated that depending on the level of the proposed 
PM2.5 visibility index standard, the existing 24-hour 
PM2.5 mass-based standard would be as protective or in some 
areas more protective of visibility than a distinct secondary standard 
set within the range of levels proposed. Commenters on the proposed 
PM2.5 visibility index explored the implications of this 
analysis at length, as discussed further below in section VI.C.1.f. For 
this reason, the analysis is described in some detail here.
    Kelly et al. (2012a) noted that the relationship between design 
values for the 24-hour PM2.5 standard and the proposed 
secondary visibility index standard is not obvious a priori because of 
differences in design value calculations for the standards. However, 
closer examination of this relationship indicated that increases or 
decreases in 24-hour PM2.5 design values correspond, 
respectively, to increases or decreases in visibility index values. 
Specifically, based on measurements from 102 sites with complete data 
from 2008-2010, Kelly et al. (2012a) found linear correlations between 
the 24-hour PM2.5 design values and the visibility index 
design values with r\2\ values ranging from 0.65 to 0.98 across these 
sites, with an average r\2\ value of 0.75 across all U.S. sites. 
Moreover, the data indicated that no design value existed where the 
visibility index design value exceeded 30 dv, but the 24-hour 
PM2.5 standard level of 35 [micro]g/m\3\ was attained. 
Visibility index design values for certain sites in the Industrial 
Midwest were shown to exceed 28 dv despite the fact that the 24-hour 
PM2.5 design values for these sites were below 35 [micro]g/
m\3\. This was attributed to the combination of high nitrate and 
sulfate fractions, substantial RH adjustment factors, and 
PM2.5 distribution characteristics that led to relatively 
high visibility index design values for a given 24-hour 
PM2.5 design value for counties in the Industrial 
Midwest.\180\ Kelly et al. (2012a) concluded that the ``overall, design 
values based on 2008-2010 data suggest that counties that attain 24-
hour PM2.5 NAAQS level of 35 [micro]g/m\3\ would attain the 
proposed secondary PM2.5 visibility index NAAQS level of 30 
dv and generally attain the level of 28 dv'' (pp. 17-18). In addition, 
the Kelly et al. analysis indicated that at sites that violated both 
the 24-hour PM2.5 level and the proposed visibility index 30 
dv level, the proposed level of 30 dv would likely be attained if 
PM2.5 concentrations were reduced such that the 24-hour 
PM2.5 level of 35 [micro]g/m\3\ was attained (Kelly et al., 
2012a, p.15).\181\ A key implication of this analysis, therefore, was 
that within the range of levels proposed by the EPA for a visibility 
index standard (28-30 dv), the 24-hour PM2.5 standard of 35 
[micro]g/m\3\ would be controlling in almost all (at 28 dv) or all (at 
30 dv) instances.
---------------------------------------------------------------------------

    \180\ Kelly et al. (2012a) also noted that ``Regional reductions 
in sulfate PM2.5 due to emission controls planned as part 
of national rules as well as emission reductions associated with 
potential annual standard violations are expected to improve 
visibility in this region'' (p. 17).
    \181\ The analysis also showed that attaining the 24-hour 
PM2.5 standard level of 35 [micro]g/m\3\ would result in 
achieving a lower PM2.5 visibility index level in certain 
areas of the country, largely western areas, than would be achieved 
in other areas of the country. This is due to differences in the 
composition of ambient PM2.5 and the lower relative 
humidity in those areas.
---------------------------------------------------------------------------

2. Other (Non-Visibility) PM-related Welfare Effects
    In the 2006 review, the EPA concluded that there was insufficient 
information to consider a distinct secondary standard based on PM-
related impacts to ecosystems, materials damage and soiling, and 
climatic and radiative processes (71 FR 61144, October 17, 2006). 
Specifically, there was a lack of evidence linking various non-
visibility welfare effects to specific levels of ambient PM. In that 
review, to provide a level of protection for these welfare-related 
effects, the secondary standards were set equal to the revised primary 
standards to directionally improve the level of protection afforded 
vegetation, ecosystems, and materials (71 FR 61210, October 17, 2006).
    This section briefly outlines key conclusions discussed more fully 
in section VI.E of the proposal regarding the non-visibility welfare 
effects of PM. These conclusions relate to the climate, ecological 
(including effects on plants, soil and nutrient cycling, wildlife and 
water) and materials damage effects of PM. For all of these effects, 
the Policy Assessment concluded that there is insufficient information 
at this time to revise the current suite of secondary standards. It is 
important to note that the Policy Assessment explicitly excluded 
discussion of the effects associated with deposited particulate matter 
components of NOX and SOx and their 
transformation products which are addressed fully in the joint review 
of the secondary NO2 and SO2 NAAQS.
a. Evidence of Other Welfare Effects Related to PM
    With regard to the role of PM in climate, the proposal noted that 
there is considerable ongoing research focused on understanding aerosol 
contributions to changes in global mean temperature and precipitation 
patterns. The Integrated Science Assessment concluded ``that a causal 
relationship exists between PM and effects on climate, including both 
direct effects on radiative forcing and indirect effects that involve 
cloud feedbacks that influence precipitation formation and cloud 
lifetimes'' (U.S. EPA, 2009a, section 9.3.10). These effects are 
discussed in more detail in section VI.E.1 of the proposal, which 
provides information on the major aerosol components of interest for 
climate processes, including black carbon (BC), organic carbon (OC), 
sulfates, nitrates, and mineral dusts, and the nature, magnitude, and 
direction (e.g., cooling vs. warming) of various aerosol impacts on 
climate.\182\ The Policy Assessment concluded that aerosols alter 
climate processes directly through radiative forcing and by indirect 
effects on cloud brightness, changes in precipitation, and

[[Page 3203]]

possible changes in cloud lifetimes (U.S. EPA, 2011a, p. 5-10). 
Further, the Policy Assessment noted that the major aerosol components 
that contribute to climate processes (i.e. BC, OC, sulfate, nitrate and 
mineral dusts) vary in their reflectivity, forcing efficiencies and 
even in the direction of climate forcing, though there is an overall 
net climate cooling associated with aerosols in the global atmosphere 
(U.S. EPA, 2009a, section 9.2.10). The Policy Assessment concluded that 
the current mass-based PM2.5 and PM10 secondary 
standards were not an appropriate or effective means of focusing 
protection against PM-associated climate effects due to these 
differences in components (U.S. EPA, 2011a, p. 5-11). In addition, in 
light of the significant uncertainties in current scientific 
information and the lack of sufficient data, the Policy Assessment 
concluded it is not currently feasible to conduct a quantitative 
analysis for the purpose of informing revisions of the current 
secondary PM standards based on climate (U.S. EPA, 2011a, p. 5-11). 
Overall the Policy Assessment concluded that there is insufficient 
information at this time to base a national ambient standard on climate 
impacts associated with current ambient concentrations of PM or its 
constituents (U.S. EPA, 2011a, p. 5-11, -12).\183\
---------------------------------------------------------------------------

    \182\ Atmospheric PM is referred to as aerosols in the remainder 
of this section to be consistent with the Integrated Science 
Assessment.
    \183\ This conclusion would apply for both the secondary 
(welfare-based) and the primary (health-based) standards.
---------------------------------------------------------------------------

    With regard to ecological effects, the proposal noted that several 
ecosystem components (e.g., plants, soils and nutrient cycling, 
wildlife and water) are impacted by PM air pollution, which may alter 
the services provided by affected ecosystems. Ecological effects 
include both direct effects due to deposition (e.g., wet, dry or 
occult) to vegetation surfaces and indirect effects occurring via 
deposition to ecosystem soils or surface waters where the deposited 
constituents of PM then interact with biological organisms. Some of the 
ecological effects considered in this review include direct effects to 
metabolic processes of plant foliage; contribution to total metal 
loading resulting in alteration of soil biogeochemistry and 
microbiology, and plant and animal growth and reproduction; and 
contribution to total organics loading resulting in bioaccumulation and 
biomagnification across trophic levels. Section VI.E.2 of the proposal 
summarizes key findings related to:

    (1) Impacts on plants and the ecosystem services they provide 
due to deposition of PM to vegetative surfaces, which alters the 
radiation received by the plant, and uptake of deposited PM 
components by plants from soil or foliage, which can lead to stress 
and decreased photosynthesis;
    (2) Impacts on ecosystem support services such as nutrient 
cycling, products such as crops and the regulation of flooding and 
water quality;
    (3) Impacts on wildlife, especially due to biomagnification of 
heavy metals (especially Hg) up the food chain and bioconcentration 
of POPs and PBDEs; and
    (4) Impacts of deposited PM, especially metals and organics, on 
the ecosystem services provided by water bodies, including primary 
production, provision of fresh water, regulation of climate and 
floods, recreational fishing and water purification.

    The proposal noted that the Integrated Science Assessment had 
concluded that ecological evidence is sufficient to conclude that a 
causal relationship is likely to exist between deposition of PM and a 
variety of effects on individual organisms and ecosystems (U.S. EPA, 
2009a, sections 2.5.3 and 9.4.7), and also noted that vegetation and 
other ecosystem components are affected more by particulate chemistry 
than size fraction. However, the proposal also pointed to the 
Integrated Science Assessment conclusion that it is generally difficult 
to characterize the nature and magnitude of effects and to quantify 
relationships between ambient concentrations of PM and ecosystem 
response due to significant data gaps and uncertainties as well as 
considerable variability that exists in the components of PM and their 
various ecological effects. There are few studies that link ambient PM 
concentrations to observed effect. Most direct ecosystem effects 
associated with particulate pollution occur in severely polluted areas 
near industrial point sources (quarries, cement kilns, metal smelting) 
(U.S. EPA, 2009a, sections 9.4.3 and 9.4.5.7).
    Based on the evidence available at this time, the proposal noted 
the following key conclusions in the Policy Assessment:

    (1) A number of significant environmental effects that either 
have already occurred or are currently occurring are linked to 
deposition of chemical constituents found in ambient PM.
    (2) Ecosystem services can be adversely impacted by PM in the 
environment, including supporting, provisioning, regulating and 
cultural services.
    (3) The lack of sufficient information to relate specific 
ambient concentrations of particulate metals and organics to a 
degree of impairment of a specific ecological endpoint hinders the 
identification of a range of appropriate indicators, levels, forms 
and averaging times of a distinct secondary standard to protect 
against associated effects.
    (4) Data from regionally-based ecological studies can be used to 
establish probable local, regional and/or global sources of 
deposited PM components and their concurrent effects on ecological 
receptors.

    The proposal noted that the Policy Assessment had concluded that 
the currently available information is insufficient for purposes of 
assessing the adequacy of the protection for ecosystems afforded by the 
current suite of PM secondary standards or establishing a distinct 
national standard for ambient PM based on ecosystem effects of 
particulates not addressed in the NOX/SOX 
secondary review (e.g., metals, organics) (U.S. EPA, 2011a, p. 5-24). 
Furthermore, the Policy Assessment had concluded that in the absence of 
information providing a basis for specific standards in terms of 
particle composition, the observations continue to support retaining an 
appropriate degree of control on both fine and coarse particles to help 
address effects to ecosystems and ecosystem components associated with 
PM (U.S. EPA, 2011a, p. 5-24).
    With regard to materials damage, the proposal discussed effects 
associated with deposition of PM, including both physical damage 
(materials damage effects) and impaired aesthetic qualities (soiling 
effects). As with the other categories of welfare effects discussed 
above, the Integrated Science Assessment concluded that evidence is 
sufficient to support a causal relationship between PM and effects on 
materials (U.S. EPA, 2009a, sections 2.5.4 and 9.5.4). The deposition 
of PM can physically affect materials, adding to the effects of natural 
weathering processes, by potentially promoting or accelerating the 
corrosion of metals, by degrading paints and by deteriorating building 
materials such as stone, concrete and marble (U.S. EPA, 2009a, section 
9.5). In addition, the deposition of ambient PM can reduce the 
aesthetic appeal of buildings and objects through soiling. The Policy 
Assessment made the following observations:

    (1) Materials damage and soiling that occur through natural 
weathering processes are enhanced by exposure to atmospheric 
pollutants, most notably sulfur dioxide and particulate sulfates.
    (2) While ambient particles play a role in the corrosion of 
metals and in the weathering of materials, no quantitative 
relationships between ambient particle concentrations and rates of 
damage have been established.
    (3) While soiling associated with fine and course particles can 
result in increased cleaning frequency and repainting of surfaces, 
no quantitative relationships between particle characteristics and 
the frequency of cleaning or repainting have been established.
    (4) Limited new data on the role of microbial colonizers in 
biodeterioration

[[Page 3204]]

processes and contributions of black crust to soiling are not 
sufficient for quantitative analysis.
    (5) While several studies in the PM Integrated Science 
Assessment and NOX/SOX Integrated Science 
Assessment suggest that particles can promote corrosion of metals 
there remains insufficient evidence to relate corrosive effects to 
specific particulate levels or to establish a quantitative 
relationship between ambient PM and metal degradation. With respect 
to damage to calcareous stone, numerous studies suggest that wet or 
dry deposition of particles and dry deposition of gypsum particles 
can enhance natural weathering processes.

    The Policy Assessment concluded that none of the new evidence in 
this review called into question the adequacy of the current standards 
for protecting against material damage effects, that such effects could 
play no quantitative role in determining whether revisions to the 
secondary PM NAAQS are appropriate at this time, and that observations 
continue to support retaining an appropriate degree of control on both 
fine and coarse particles to help address materials damage and soiling 
associated with PM (U.S. EPA, 2011a, p. 5-29).
b. CASAC Advice
    In advising the EPA regarding the non-visibility welfare effects, 
CASAC stated that it ``concurs with the Policy Assessment's conclusions 
that while these effects are important, and should be the focus of 
future research efforts, there is not currently a strong technical 
basis to support revisions of the current standards to protect against 
these other welfare effects'' (Samet, 2010c). More specifically, with 
regard to climate impacts, CASAC concluded that while there is 
insufficient information on which to base a national standard, the 
causal relationship is established and the risk of impacts is high, so 
further research on a regional basis is urgently needed (Samet, 2010c, 
p. 5). CASAC also noted that reducing certain aerosol components could 
lead to increased radiative forcing and regional climate warming while 
having a beneficial effect on PM-related visibility. As a consequence, 
CASAC noted that a secondary standard directed toward reducing PM-
related visibility impairment has the potential to be accompanied by 
regional warming if light scattering aerosols are preferentially 
targeted.
    With regard to ecological effects, CASAC concluded that the 
published literature is insufficient to support a national standard for 
PM effects on ecosystem services (Samet, 2010c, p.23). CASAC noted that 
the best-established effects are related to particles containing 
nitrogen and sulfur, which are being considered in the EPA's ongoing 
review of the secondary NAAQS for NOX/SOX. With 
regard to PM-related effects on materials, CASAC concluded that the 
published literature, including literature published since the last 
review, is insufficient either to call into question the current level 
of the standard or to support any specific national standard for PM 
effects on materials (Samet, 2010c, p.23). Nonetheless, with regard to 
both types of effects, CASAC noted the importance of maintaining an 
appropriate degree of control of both fine and coarse particles to 
address such effects, even in the current absence of sufficient 
information to develop a standard.
c. Summary of Proposed Decisions Regarding Other Welfare Effects
    Based on the above considerations and the advice of CASAC, at the 
time of proposal the Administrator provisionally concluded that it 
would not be appropriate to establish any distinct secondary PM 
standards to address other non-visibility PM-related welfare effects, 
including ecological effects, effects on materials, and climate 
impacts. Nonetheless, the Administrator concurred with the conclusions 
of the Policy Assessment and CASAC advice that it is important to 
maintain an appropriate degree of control of both fine and coarse 
particles to address such effects. Noting that there is an absence of 
information that would support any different standards, the 
Administrator proposed generally to retain the current suite of 
secondary PM standards \184\ to address non-visibility welfare effects. 
Specifically, the Administrator proposed to retain all aspects of the 
current secondary 24-hour PM2.5 and PM10 
standards. With regard to the secondary annual PM2.5 
standard, the Administrator proposed to retain the level of the current 
standard and to revise the form of the standard by removing the option 
for spatial averaging consistent with this change to the primary annual 
PM2.5 standard.
---------------------------------------------------------------------------

    \184\ As summarized in section VI.A and Table 1 above, the 
current suite of secondary PM standards includes annual and 24-hour 
PM2.5 standards and a 24-hour PM10 standard.
---------------------------------------------------------------------------

C. Public Comments on Proposed Decisions Regarding Secondary PM 
Standards

    The EPA received a large number of comments on its proposed 
decisions with regard to secondary PM standards, with the large 
majority of those comments focusing on the proposal to set a distinct 
standard to protect against visibility impairment, discussed below in 
section VI.C.1. Very few commenters addressed the proposal to retain 
the existing secondary standards for non-visibility welfare effects, 
discussed below in section VI.C.2. As discussed in section VI.D. below, 
the Administrator has decided to retain the current suite of secondary 
PM standards generally, while revising only the form of the secondary 
annual PM2.5 standard to remove the option for spatial 
averaging consistent with this change to the primary annual 
PM2.5 standard. The Administrator has also decided, contrary 
to what was proposed, not to establish a distinct secondary standard to 
address PM-related visibility impairment. This section discusses EPA's 
responses to the comments EPA received on its proposal, and the 
rationale behind the Administrator's final decisions is discussed in 
section VI.D. below.
1. Comments on Proposed Secondary Standard for Visibility Protection
a. Overview of Comments
    Among those commenting on the proposal to set a distinct secondary 
PM2.5 visibility index standard, a large majority of 
commenters, including more than 25 state and local agencies; regional 
organizations such as NACAA, NESCAUM, and WESTAR; and industry 
commenters, such as ACC, API, BP, EPRI, NCBA, NEDA-CAP, NMA, NSSGA, and 
UARG, opposed setting a distinct secondary standard for visibility at 
this time. Many commenters in this group expressed the view that such a 
standard was not needed, primarily on the basis that adequate 
protection was provided by the existing 24-hour secondary 
PM2.5 standard. Some of these commenters also expressed 
legal concerns with the nature of the proposed standard. Other 
commenters in this group supported a distinct secondary standard for 
visibility in concept, but expressed the view that it was premature to 
set such a standard pending collection of additional visibility 
preference study data and the resolution of a number of key technical 
issues. Support for setting such a distinct secondary standard for 
visibility at this time came from a second group of commenters, 
including the Department of the Interior (National Park Service), 
several states, the Mid-Atlantic/Northeast Visibility Union (MANE-VU), 
the National Tribal Air Association (NTAA), environmental organizations 
such as the Appalachian Mountain Club, National Parks Conservation 
Association, Earthjustice (AMC, et al.) and the League of Women

[[Page 3205]]

Voters of Texas. These commenters argued that the existing secondary 
standards are not sufficiently protective of visual air quality, and 
that a distinct secondary standard similar to the proposed visibility 
index standard is both necessary and appropriate to ensure adequate 
protection of visibility.
    Commenters in both groups expressed concerns about various aspects 
of the proposed distinct secondary standard, including the indicator, 
averaging time, level, and form. In addition, a large number of 
commenters, including commenters from both groups, expressed concern 
and/or confusion over the relationship between the Regional Haze 
Program and the proposed distinct secondary standard for visibility, 
raising issues such as analytical differences in methods between the 
programs, monitoring issues, and other implementation challenges.
    A discussion of the significant comments outlined above, including 
EPA's responses to the comments, is presented here, with more detailed 
discussion in the Response to Comments document. Comments relating to 
the specific elements of the proposed standard--indicator, averaging 
time, form and level--are discussed in sections VI.C.1.b-e, 
respectively. Comments related to the need for a distinct secondary 
standard at this time are discussed in section VI.C.f. Legal issues 
raised by commenters opposed to setting a secondary standard based on 
the proposed visibility index are discussed in section VI.C.g. Finally, 
comments related to the relationship between a distinct secondary 
standard and the Regional Haze Program are discussed in section 
VI.C.h.\185\ While the EPA concludes in section VI.D below to retain 
the current suite of secondary PM2.5 standards, the 
appropriateness of the protection that would be provided by the 
proposed PM2.5 visibility index standard, and the 
relationship between this degree of protection and that provided by the 
current secondary 24-hour secondary PM2.5 standard, are key 
elements in the Administrator's decision, and are discussed below.
---------------------------------------------------------------------------

    \185\ Comments pertaining to implementation issues, which the 
Administrator may not consider in making decisions about setting 
national ambient air quality standards, are discussed in the 
Response to Comments document, as are comments regarding monitoring 
issues related to the proposed distinct visibility index standard.
---------------------------------------------------------------------------

b. Indicator
    Numerous commenters, both those supporting a distinct secondary 
standard and those opposed to setting such a standard, expressed views 
on the suitability of utilizing a PM2.5 calculated light 
extinction indicator for the standard as proposed. While these groups 
of commenters differed in terms of their views on the appropriateness 
of using calculated PM2.5 light extinction as the basis for 
the indicator rather than relying on direct measurements of 
PM2.5 light extinction, commenters from both groups 
expressed concern over specific elements of the proposed method of 
calculating PM2.5 light extinction. In particular, 
commenters expressed differing views on which IMPROVE algorithm should 
be utilized; whether it is appropriate to exclude coarse particles from 
the indicator; and whether the proposed protocols for incorporating 
data on relative humidity and PM2.5 species are 
appropriate.\186\
---------------------------------------------------------------------------

    \186\ Some commenters expressed concern about the omission of 
other contributors to visibility impairment from the visibility 
index, as discussed in the Response to Comments document.
---------------------------------------------------------------------------

i. Comments on Calculated vs. Directly Measured Light Extinction
    The majority of commenters in both groups noted the uncertainties 
associated with relying on a calculated light extinction indicator and 
stated a preference for utilizing direct light extinction measurements. 
However, recognizing the limitations on applying direct measurements at 
present, commenters supporting the proposal to set a distinct standard 
argued that relying on ``calculated light extinction is a reasonable 
first approach'' (DOI, p. 2). These commenters pointed to the advice of 
CASAC, which had acknowledged that it was not possible for the EPA to 
develop an FRM for direct measurement of light extinction within the 
time frame of this review and had concluded that relying on a 
calculated PM2.5 light extinction indicator represented a 
reasonable approach that could be implemented sooner than a directly 
measured indicator. These commenters generally supported the proposal 
to adopt a calculated PM2.5 light extinction indicator, at 
least as an interim approach.
    Commenters opposed to setting a distinct standard generally argued 
that it was inappropriate to rely on a calculated light extinction 
indicator rather than direct measurements. Some of these commenters 
argued that the proposed calculated light extinction indictor is ill 
suited for a bright line standard because the method uses average 
humidity and a reconstructed visibility measurement calculated from 
PM2.5 speciation filter analysis, rather than measuring what 
is actually observed by individuals. A number of commenters advocated 
postponing setting a distinct standard until an approach based on 
direct light extinction measurements can be adopted. Many of these 
commenters stated that relying on direct light extinction measurements 
would enable a standard to be based on a shorter averaging time, either 
1-hour or sub-daily (4 to 6 hours), consistent with the more 
instantaneous nature of perceptions of visual air quality and the 
advice of CASAC in this review.
    The EPA generally agrees with commenters that an indicator based on 
directly measured light extinction would provide the most direct link 
between PM in the ambient air and PM-related light extinction. However, 
as noted at the time of proposal and in accordance with the advice of 
CASAC, the EPA has concluded that this is not an appropriate option in 
this review because a suitable specification of currently available 
equipment or performance-based verification procedures could not be 
developed in the time frame of this review. Moreover, CASAC concluded 
that relying on a calculated PM2.5 light extinction 
indicator based on PM2.5 chemical speciation and relative 
humidity data represented a reasonable approach. The inputs that are 
necessary include measurements that are available through existing 
monitoring networks and approved protocols. Thus, the EPA remains 
confident that the available evidence demonstrates that a strong 
correspondence exists between calculated PM2.5 light 
extinction and PM-related visibility impairment. Furthermore, CASAC 
agreed, noting that the proposed calculated PM2.5 light 
extinction indicator based on the original IMPROVE algorithm ``appears 
to be a reasonable approach for estimating hourly light extinction'' 
(Samet, 2010d, p. 11) and ``its reliance on procedures that have 
already been implemented in the CSN and routinely collected continuous 
PM2.5 data suggest that it could be implemented much sooner 
than a directly measured indicator'' (Samet, 2010d, p. iii). Thus it 
would not be appropriate to postpone setting a distinct secondary 
standard until an approach based on direct light extinction 
measurements could be adopted.
ii. Comments on Specific Aspects of Calculated Light Extinction 
Indicator
    Some commenters, even those supporting the adoption of a calculated 
light extinction indicator, also expressed concern over specific 
aspects of the proposed indicator. First, a

[[Page 3206]]

number of commenters expressed concern over the proposal to use the 
original IMPROVE algorithm as the basis for the calculated light 
extinction indicator. These commenters noted that the original IMPROVE 
algorithm has been shown to have consistent biases at both low and high 
levels of light extinction. In particular, these commenters expressed 
concern with the algorithm's bias at higher levels of light extinction, 
which they pointed out were the conditions that might be encountered on 
hazier days in urban areas.
    Some commenters supported use of the revised IMPROVE algorithm. 
These commenters noted that the revised equation has been through a 
peer review which confirmed that it is based on the best science and 
corrects the biases inherent in the original algorithm. Commenters also 
noted that this revised algorithm has been widely incorporated into 
Regional Haze plans, and urged the EPA to use this same equation in the 
visibility index for the sake of consistency: ``EPA approved this 
approach for regional haze and does not dispute its greater accuracy. 
Therefore, a national secondary ambient air quality standard based on 
criteria that accurately reflect the latest scientific knowledge 
logically should not revert to the original IMPROVE algorithm'' 
(Oklahoma DEQ, p. 2). Other commenters noted that both the original and 
the revised IMPROVE algorithms were designed in support of the Regional 
Haze Program which is focused on largely rural Class I areas, and that 
neither algorithm is necessarily suitable for urban areas. Noting that 
the EPA has not thoroughly evaluated the applicability of either 
IMPROVE algorithm in urban areas, these commenters urged additional 
research to evaluate the suitability of either algorithm (or an 
alternative approach) in urban areas.
    Second, a number of commenters argued that exclusion of coarse PM 
from the calculated light extinction indicator was inappropriate. These 
commenters noted that coarse particulate matter is an important 
contributor to visibility impairment in many areas, particularly in the 
western U.S., and that the levels of ``acceptable'' visual air quality 
derived from the visibility preference studies reflected total light 
extinction due to the full mix of particles (including coarse PM) in 
ambient air. A few commenters noted that due to the exclusion of coarse 
particles, a ``deciview'' calculated for purposes of the proposed 
PM2.5 visibility index is inconsistent with the unit as 
conventionally defined under the Regional Haze Program. Other 
commenters, however, supported the proposal to exclude coarse PM from 
the calculated light extinction indicator, noting the important role 
that PM2.5 plays in urban visibility and arguing it would be 
more difficult to control the contribution of coarse particle sources 
such as wind-blown dust to urban visibility impairment.
    Third, some commenters questioned why the EPA was proposing to rely 
on monthly average relative humidity (f(RH)) values when hourly 
humidity data are widely available, particularly in urban areas. One 
commenter argued that the EPA's proposed approach involves ``guessing 
relative humidity'' rather than relying on accurate, readily available 
measurements (Oklahoma DEQ, p. 1). The commenter stated that since 
relative humidity is highly variable and weather dependent, the 
proposed approach ``effectively undermines the capacity of the 
prescribed monitoring regime to identify periods when PM2.5 
adversely affects visibility.'' Other commenters supported this view, 
noting that relative humidity can vary substantially even within a 24-
hour period, and that light extinction can be very sensitive to these 
changes. These commenters recommended that hourly or daily humidity 
measurements should be utilized in place of the proposed monthly 
average f(RH) values.
    Some commenters also recommended that the EPA should utilize a 90 
percent relative humidity screen rather than 95 percent cap for 
purposes of eliminating periods in which visibility impairment is due 
to rain or fog. These commenters claimed that under a 95 percent cap, 
both the average f(RH) values and the PM2.5 visibility index 
values could be inflated in locations frequently affected by fog and/or 
precipitation. These commenters preferred the approach of excluding 
hours with relative humidity above 90 percent on the grounds that this 
approach would eliminate foggy/rainy hours irrespective of the 
frequency of occurrence.
    The EPA does not agree with commenters who advocated using the 
revised IMPROVE algorithm. Both the original and the revised IMPROVE 
algorithms have been evaluated by comparing the calculated estimates of 
light extinction with coincident optical measurements. As discussed 
above in section VI.B.1.a.i, the revised algorithm was developed to 
address observed biases in the predictions using the original algorithm 
under very low and very high light extinction conditions, with further 
modifications and additions to better account for differences in 
particle composition and aging in remote areas.\187\ However, the EPA 
does not believe that these same modifications and additions would 
necessarily be appropriate for calculating light extinction in urban 
areas. Instead, the EPA considers the original algorithm to be suitable 
for purposes of calculating urban light-extinction, although some 
adjustments may be appropriate for urban environments as well. The 
reasons why the original algorithm is suited to urban environments are 
discussed further below, along with adjustments that the EPA believes 
are likely appropriate based on the current (limited) state of 
knowledge.
---------------------------------------------------------------------------

    \187\ Specifically, the revised IMPROVE algorithm incorporates 
additional terms to account for particles representing the different 
dry extinction and water uptake (f(RH)) from two size modes of 
sulfate, nitrate and organic mass, as well as adding a term for 
hygroscopic sea salt. There are also adjustments for the calculation 
of OM as 1.8*OC compared to 1.4*OC in the original algorithm to 
better account for the more aged PM organic components found in 
remote areas.
---------------------------------------------------------------------------

    First, the EPA considers that the multiplier of 1.8 used to convert 
OC to OM in the revised IMPROVE algorithm is too high for urban 
environments. The EPA is aware that there has been considerable debate 
within the research community about the appropriate multiplier to use 
to best represent urban environments. As discussed in Appendix F of the 
Policy Assessment (U.S. EPA, 2011a), the EPA used the SANDWICH mass 
closure approach (Frank, 2006) in the Urban Focused Visibility 
Assessment (U.S. EPA, 2010b) for purposes of calculating maximum 
daylight hourly PM2.5 light extinction and evaluated which 
multiplier would produce 24-hour results most similar to the SANDWICH 
approach using 24-hour PM2.5 organic carbon derived from the 
new Chemical Speciation Network (CSN) carbon monitoring protocol 
established in 2007.\188\ Analyses presented in Appendix F of the 
Policy Assessment indicate that a multiplier of 1.6 is most appropriate 
for purposes of comparing the hourly PM2.5 light extinction 
with calculated 24-hour extinction (see Appendix F, section F.6 for a 
full explanation). The EPA also considers this higher multiplier to be 
a better approach for urban CSN monitoring sites where the new 
measurements of organic carbon tend to be lower than those produced by 
the older NIOSH-type monitoring protocol

[[Page 3207]]

(Malm, 2011). A multiplier of 1.6 is now used to calculate OM from OC 
measurements at CSN sites.
---------------------------------------------------------------------------

    \188\ Starting in 2007, the CSN adopted the IMPROVE monitoring 
protocol for the measurement of organic and elemental carbon using 
the IMPROVE analytical method and an IMPROVE-like sampler. The 
transition was completed in 2009. (See ``Modification of Carbon 
Procedures in the Speciation Network,'' http://www.epa.gov/ttn/amtic/files/ambient/pm25/spec/faqcarbon.pdf.)
---------------------------------------------------------------------------

    At the time of proposal, the EPA proposed to use the original 
IMPROVE algorithm with its 1.4 multiplier for converting OC to OM, but 
requested comment on whether this value was appropriate. Comments 
received by the Agency generally indicate that the OC-to-OM multiplier 
of 1.4 used in the original IMPROVE algorithm is too low for urban 
areas. Based on the analyses presented in Appendix F of the Policy 
Assessment, the EPA agrees with these commenters. However, the EPA also 
believes that it would be inappropriate to use a multiplier as high as 
1.8 to convert OC to OM in urban areas. As noted by commenters, the 
organic mass contribution to visibility impairment can be large, and 
generally OM is significantly larger in urban areas compared to 
surrounding rural areas.\189\ Because a large portion of the organic 
component of urban PM results from nearby emissions sources, the total 
OM mass is generally closer to the measured OC from which it is 
derived. This means it is appropriate to use a smaller multiplier to 
convert OC to OM in urban areas as compared to the value of 1.8 used in 
the revised algorithm, which is tailored to remote areas. The CASAC 
noted that urban OM includes fresh emissions and the EPA concluded in 
the Visibility Assessment that ``the original version is considered 
more representative of urban situations when emissions are still fresh 
rather than aged as at remote IMPROVE sites'' (U.S. EPA, 2010b, p. 3-
19). Although the revised algorithm represents the best science of 
estimating extinction in remote areas with its aged aerosol, the 
commenters did not address how the EPA should modify the revised 
algorithm to best represent the more complex and different urban 
aerosol, particularly for OM. In light of all of these considerations, 
in particular the analyses the EPA conducted for Appendix F of the 
Policy Assessment and the fact that the monitoring method for organic 
carbon has recently changed in the CSN network, the EPA judges that a 
multiplier of 1.6 for urban areas would be most appropriate for 
purposes of calculating PM2.5 light extinction in urban 
areas.\190\ In formulating this judgment, the EPA recognizes that 
neither the original nor the revised IMPROVE algorithm has been tested 
for suitability in urban areas and that additional research is 
necessary to reduce the uncertainties about the most appropriate value 
for the OC to OM multiplier in urban environments. With regard to other 
changes between the original and revised IMPROVE algorithms, the EPA 
also does not believe that it would be appropriate to include a term 
for hygroscopic sea salt for urban light extinction, or to 
differentiate between different size modes of sulfate, nitrate, and 
organic mass as empirically defined by the revised IMPROVE algorithm. 
Unlike in some remote coastal locations, sea salt is not major 
contributor to light extinction in urban areas. Moreover, urban sources 
of salt include sanding of roads during the winter and those re-
entrained particles are mostly in the coarse size range.
---------------------------------------------------------------------------

    \189\ The difference between higher PM2.5 mass in 
urban areas compared to surrounding regions, known as the urban 
excess, is largely attributed to organic mass (U.S. EPA, 2004b).
    \190\ The implications of this shift to a 1.6 multiplier for OC 
in urban areas for decisions about averaging time, level, and need 
for a distinct secondary standard are discussed further below in 
sections VI.C.1.c, VI.C.1.e, and VI.C.1.f, respectively.
---------------------------------------------------------------------------

    Like in remote areas, small and large size modes of sulfate, 
nitrate and organic mass would exist in the urban environment. However, 
the apportionment of the total fine particle concentration of each of 
the three PM2.5 components into the concentrations of the 
small and large size fractions would likely need a different approach 
than that used for remote areas. This is because of the closer 
proximity of urban sources to their emissions. This is a particular 
concern not only for organic mass, which as explained previously has a 
large contribution from nearby urban emission sources, but also for 
PM2.5 nitrate whose concentrations are also higher in urban 
areas compared to the surrounding regions. Thus, a higher portion of 
the total urban concentration may be in the small mode compared to 
remote areas and thus a different apportionment algorithm would be 
needed.
    Finally, the EPA does not consider it necessary to employ site-
specific Rayleigh light scattering terms in place of a universal 
Rayleigh light scattering value for purposes of calculating light 
extinction in urban areas for purposes of calculating the 90th 
percentile values. The site-specific Rayleigh value is most important 
to accurately estimate extinction on the best visibility days which is 
an essential metric for the regional haze program.
    For all of these reasons, the EPA considers the original IMPROVE 
algorithm better suited to the task of calculating urban light 
extinction than the revised IMPROVE algorithm. However, the EPA does 
consider it appropriate to make certain adjustments to the original 
algorithm for purposes of calculating urban light extinction. As 
discussed above, the EPA believes it is appropriate to use a 1.6 
multiplier to convert OC to OM in urban areas. In addition, the EPA 
believes it is appropriate to exclude the term for coarse particles 
from the equation. The EPA does not agree with commenters who suggested 
that coarse particles should be included in the calculated light 
extinction indicator. As noted in the proposal, PM2.5 is the 
component of PM responsible for most of the visibility impairment in 
most urban areas. Currently available data suggest that 
PM10-2.5 is a minor contributor to visibility impairment 
most of the time, although at some locations (U.S. EPA, 2010b, Figure 
3-13 for Phoenix) PM10-2.5 can be a major contributor to 
urban visibility effects. While it is reasonable to assume that other 
urban areas in the desert southwestern region of the country may have 
conditions similar to the conditions shown for Phoenix, in fact few 
urban areas conduct continuous PM10-2.5 monitoring. This 
significantly increases the difficulty of assessing the role of coarse 
particles in urban visibility impairment. For example, among the 15 
urban areas assessed in this review, only four areas had collocated 
continuous PM10 data allowing calculation of hourly 
PM10-2.5 data for 2005 to 2007. In addition, 
PM10-2.5 is generally less homogenous in urban areas than 
PM2.5 in that coarse particle concentrations exhibit greater 
temporal variability and a steeper gradient across urban areas than 
fine particles (U.S. EPA, 2009a, p. 3-72). This makes it more 
challenging to select sites that would adequately represent urban 
visibility conditions. Thus, while it would be possible to include a 
PM10-2.5 light extinction term in a calculated light 
extinction indicator, as was done in the Visibility Assessment, there 
is insufficient information available at this time to assess the impact 
and effectiveness of such a refinement in providing public welfare 
protection in areas across the country (U.S. EPA, 2011a, pp. 4-41 to 4-
42). Therefore, the EPA concludes that it is not appropriate to set a 
standard based on a calculated light extinction indicator that includes 
coarse particles at this time, and the calculated indicator should be 
based on PM2.5 light extinction.
    With regard to the suggestion by some commenters that the 
calculated light extinction indicator should be calculated using hourly 
humidity data,

[[Page 3208]]

the EPA disagrees that concurrent humidity measurements should be used. 
The use of longer-term averages for each monitoring site adequately 
captures the seasonal variability of relative humidity and its effects 
of visibility impairment, and this approach focuses more on the 
underlying aerosol contributions to visibility impairment and less on 
the day-to-day variations in humidity. This provides a more stable 
indicator for comparison to the NAAQS and one that is more directly 
related to the underlying emissions that contribute to visibility 
impairment.
    With regard to the comments advocating the use of a 90 percent 
humidity screen as opposed to a 95 percent humidity cap, the EPA 
believes that relying on monthly average relative humidity values based 
on 10 years of climatological data appropriately reduces the effect of 
fog and precipitation. Although the approach of using a 95 percent 
humidity cap, as in the Regional Haze Program, includes some hours with 
relative humidity between 90-95 percent, the general approach of using 
a longer-term average for each monitoring site effectively eliminates 
the effect of very high humidity conditions on visibility at those 
locations.
    Therefore, taking all of the above considerations and CASAC advice 
into account, the EPA continues to conclude that a calculated 
PM2.5 light extinction indicator, similar to that used in 
the Regional Haze Program (i.e., using an IMPROVE algorithm as 
translated into the deciview scale), would be the most appropriate 
indicator to replace the current PM2.5 mass indicator for a 
distinct secondary standard. Moreover, the EPA continues to conclude 
that this calculated indicator should based on the original IMPROVE 
algorithm, adjusted to use a 1.6 OC multiplier and exclude the term for 
coarse particles, in conjunction with monthly average relative humidity 
data (i.e., f(RH) values) based on long-term climatological means as 
used in the Regional Haze Program. A PM2.5 visibility index 
defined in this way would appropriately reflect the relationship 
between ambient PM and PM-related light extinction, based on the 
analyses discussed in the proposal and reflecting the aerosol and 
relative humidity contributions to visibility impairment by 
incorporation of factors based on measured PM2.5 speciation 
concentrations and climatological average relative humidity data. In 
addition, this type of indicator would address, in part, the issues 
raised in the court's remand of the 2006 PM2.5 standards. 
Such a PM2.5 visibility index would afford a relatively high 
degree of uniformity of visual air quality protection in areas across 
the country by virtue of directly incorporating the effects of 
differences in PM2.5 composition and relative humidity 
across the country.
c. Averaging Time
    Few commenters specifically addressed the issue of averaging time. 
Those who did generally expressed the view that an hourly or sub-daily 
averaging time would be the most appropriate approach, as supported by 
CASAC and the EPA's own analyses in this review. These comments were 
generally consistent with the emphasis among all commenters on the 
desirability of adopting a directly measured light extinction indicator 
that could be measured on an hourly or sub-daily time scale. Some 
commenters noted that a standard based on a 4-6 hour averaging time 
would better capture peak daily light extinction while allowing stable 
signal quality; others urged EPA to adopt a 1-hour averaging time in 
conjunction with direct measurements. Commenters pointed to significant 
limitations associated with using a 24-hour averaging time, including 
the uncertainties in translating hourly or sub-daily visibility index 
values into 24-hour equivalent values. Some commenters criticized the 
analysis presented in the Policy Assessment comparing the 24-hour 
calculated light extinction values to the maximum daylight 4-hour 
calculated light extinction values. These commenters stated that the 
scatter plots and regressions presented in the Policy Assessment 
indicate there is considerable variation in the 24-hour vs. 4-hour 
relationship, and interpreted this to mean that 24-hour light 
extinction values are a poor surrogate for 4-hour values. For example, 
several industry commenters cited an analysis which noted that the 
correlation coefficient between the 24-hour and 4-hour values was as 
low as r\2\ = 0.42 in Houston, and stated that the EPA was being overly 
``optimistic'' in concluding that city-specific and pooled r\2\ values 
in the range of 0.6 to 0.8 showed good correlation (UARG, Attachment 2, 
p. 27).
    In addition, some commenters expressed concern over potential bias 
and greater uncertainty introduced by the inclusion of nighttime hours, 
noting that because relative humidity tends to be higher at night, 
inclusion of these hours could cause areas to ``record NAAQS 
exceedances that have no corresponding visibility impairment value'' 
(UARG, p. 36). Commenters also emphasized the poor fit of a 24-hour 
averaging time with the near instantaneous judgments about visibility 
impairment reflected in the visibility preference studies. Commenters 
also noted that there is greater hourly variation in PM concentrations 
and resulting visibility conditions in urban areas than in Class I 
areas; thus, while the Regional Haze Program uses 24-hour IMPROVE data, 
the commenters stated that a shorter averaging time is needed for an 
urban-focused PM2.5 visibility standard. Some commenters 
objected to a 24-hour averaging time as unsupported by the record in 
this review: ``Because the science the Administrator relies on for the 
other elements of the proposed visibility standard is tied to short-
term exposures to visibility impairment, the EPA has no basis for 
promulgating a standard that uses a 24-hour averaging time'' (API, p. 
43). These commenters claimed that while the EPA may not have the 
information or infrastructure in place to allow the Agency to set a 
standard based on a 1-hour or other sub-daily averaging time, this does 
not justify moving to a 24-hour averaging time.
    Among commenters supporting the proposed distinct secondary 
standard for visibility, many commenters recognized the limitations on 
monitoring methods and currently available data that led to the EPA's 
proposal to adopt a standard based on a 24-hour averaging time. Most of 
these commenters acknowledged that the lack of reliable hourly 
speciation data means that a 24-hour averaging time is the only 
workable approach for a standard based on calculated light extinction. 
Commenters advocating a distinct secondary standard for visibility 
therefore generally supported the proposal to adopt a 24-hour averaging 
time, at least as an interim approach until a directly measured light 
extinction indicator could be adopted in the future. This approach was 
also supported by a few industry commenters who noted that since a 
visibility index standard would be based on data from the IMPROVE and 
CSN monitors, which operate on a 24-hour basis with 1-in-3 (or 1-in-6) 
day sampling, ``it is imperative that EPA retain a 24-hour averaging 
time if a secondary visibility standard is promulgated'' (API, 
Attachment 2, p. 9).
    In response to comments supporting a 1-hour or sub-daily (4- to 6- 
hour) averaging time in conjunction with a direct light extinction 
measurements, the EPA notes that, as discussed above in the response to 
comments on indicator, the Agency has concluded

[[Page 3209]]

that a directly measured light extinction indicator is not an 
appropriate option in this review, independent of the decision on 
averaging time. Having reached the conclusion that a calculated 
PM2.5 light extinction indicator would be most appropriate, 
the EPA has next considered what averaging time would be most desirable 
for such an indicator. As noted in the proposal, the EPA has recognized 
that hourly or sub-daily (4- to 6-hour) averaging times, within 
daylight hours and excluding hours with high relative humidity, are 
more directly related than a 24-hour averaging time to the short-term 
nature of the perception of PM-related visibility impairment and the 
relevant exposure periods for segments of the viewing public. Thus, the 
Agency agrees with commenters' general point that, as a starting 
premise, a sub-daily averaging time would generally be preferable.
    However, as noted at the time of proposal and discussed above in 
section VI.B.1.c, important data quality uncertainties have recently 
been identified in association with currently available instruments 
that would be used to provide the hourly PM2.5 mass 
measurements that would be needed in conjunction with an averaging time 
shorter than 24 hours. As a result, at this time the Agency has strong 
technical reservations about a secondary standard that would be defined 
in terms of a sub-daily averaging time. The data quality issues which 
have been identified, including short-term variability in hourly data 
from currently available continuous monitoring methods, effectively 
preclude adoption of a 1-hour averaging time in this review, given the 
sensitivity of a 1-hour averaging time to these data quality 
limitations. Even with regard to multi-hour averaging times, the EPA 
continues to conclude that the data quality concerns preclude adoption 
of a sub-daily averaging time.
    Moreover, analyses conducted for the Policy Assessment indicate 
that PM2.5 light extinction calculated on a 24-hour average 
basis would be a reasonable and appropriate surrogate for 
PM2.5 light extinction calculated on a 4-hour basis. The 
scatter plots comparing 24-hour and 4-hour calculated PM2.5 
light extinction in the Policy Assessment (U.S. EPA, 2011a, Figures G-4 
and G-5) do show some scatter around the regression line for each city. 
This was to be expected, since the calculated 4-hour light extinction 
includes day-specific and hour-specific influences that are not 
captured by the simpler 24-hour approach. Overall, however, in the 
EPA's view, both the city-specific and pooled 15-city 24-hour vs. 4-
hour comparisons show strong correlation between the two averaging 
times. Moreover, the 90th percentile design values calculated for 4-
hour vs. 24-hour light extinction are much more closely correlated than 
are the values for individual days in particular urban areas calculated 
using these two approaches. Thus, while the EPA agrees with commenters 
who pointed out the relatively low correlation between 4- and 24-hour 
values in cities such as Houston, the Agency points out that the 
correlations of 90th percentile values are much higher, particularly 
when one considers the average values across urban areas. In general, 
the 90th percentile values line up better and demonstrate closer to a 
one-to-one relationship.
    The EPA has conducted a reanalysis (Frank et al., 2012b) of the 
relationships between estimated 24-hour and 4-hour visibility 
impairment based on the variety of metrics discussed in Appendix G of 
the Policy Assessment that further supports this finding. The 
reanalysis more appropriately considered the uncertainty of the 
calculated 4-hour values. It also considered the effect of changing the 
OC to OM multiplier used in urban areas with the new CSN monitoring 
protocol from 1.4 to 1.6. The revised analysis shows that the 24-hour 
values are generally closer to the 4-hour values than originally 
estimated.
    Since conclusions in the proposal about the relationship between 4-
hour and 24-hour values were drawn not just on the basis of the city-
specific results but also on the more robust 90th percentile values, 
the EPA disagrees with commenters who state that the Agency was overly 
optimistic in considering 24-hour values an appropriate surrogate for 
4-hour values. Also, it is appropriate to focus on the 90th percentile 
design value comparison since the design values would determine 
attainment status and the degree of improvement in air quality that 
could be expected in areas instituting controls to meet the NAAQS. 
Therefore the EPA does not agree with commenters who state that a 24-
hour averaging time cannot serve as an appropriate surrogate for sub-
daily periods of visibility impairment. On the contrary, the EPA 
continues to conclude, on the basis of this analysis, that 
PM2.5 light extinction calculated on a 24-hour basis is a 
reasonable and appropriate surrogate for sub-daily PM2.5 
light extinction calculated on a 4-hour basis.
    The EPA recognizes that the effect of adopting a 24-hour averaging 
time may be to smooth out some of the hour-by-hour variability in 
visibility index values. (Indeed, this is true if we compare a 4-hour 
averaging time to a 1-hour averaging time as well.) Hour-specific 
influences which would be evident if an hourly or sub-daily averaging 
time were to be used will be masked to some extent when those hours are 
averaged together with other hours. This means, in part, that a 24-hour 
averaging time may effectively reduce peak values by means of averaging 
them together with other hours, which may have lower values. However, 
given the well documented variability in hourly visibility conditions, 
especially in urban areas, as noted by commenters, it is reasonable to 
assume that in some cases peak hours may be significantly influenced by 
atypical conditions, making it appropriate to adopt an averaging time 
that is sufficiently long to ensure that hour-specific influences are 
balanced against more typical conditions. Perhaps even more important 
is the concern that many peak hourly measurements may be significantly 
influenced by atypical instrument performance; this reinforces the 
conclusion that it is appropriate to adopt a longer averaging time, to 
ensure that hour-specific uncertainties are balanced against more 
robust measurements.
    Thus, in agreement with commenters who supported a daily averaging 
time, the EPA concludes that a 24-hour averaging time would be 
appropriate for a distinct secondary standard based on a calculated 
PM2.5 light extinction indicator.
d. Form
    The EPA received very few comments with regard to the proposal to 
adopt a 90th percentile form, averaged over 3-years, in conjunction 
with a PM2.5 visibility index and a 24-hour averaging time. 
One commenter stated that it was inappropriate to use a 90th percentile 
form, noting that this would result in the exclusion of a minimum of 36 
days of data annually. The commenter expressed particular concern that 
this proposed approach, in combination with a 24-hour standard based on 
an unadjusted CPL, would not capture the worst visibility impairment 
and that this would undermine ``the intent of setting a meaningful 
secondary visibility standard'' (AMC, et al., p. 2). Another commenter 
argued that the EPA had provided no scientific basis for why the 90th 
percentile form was suitable, and claimed that the Agency was making 
``a somewhat arbitrary judgment that people's welfare would be affected 
only if adverse urban visibility were to occur

[[Page 3210]]

more than 10 percent of the time'' (API, Attachment 2, p. 4).
    On other hand, a few commenters who appeared to generally support 
the proposal to use a 90th percentile form advocated averaging the 90th 
percentile values over longer time periods, arguing that averaging over 
only 3 years would not provide a stable assessment of visual air 
quality in the West because this time period is insufficient to 
properly account for western drought and fire cycles. These commenters 
pointed to the approach in the Regional Haze Program of averaging 
visibility impairment over 5 years, and noted that even within this 
longer time period data can be significantly influenced by high 
emissions during significant fire years.
    The EPA disagrees with all of these comments. With regard to the 
comment opposing the 90th percentile form as inappropriately excluding 
the worst visibility days, the EPA notes that there is a significant 
lack of information on, and a high degree of uncertainty regarding, the 
impact on public welfare of the number of days with visibility 
impairment over the course of a year. For example, the visibility 
preference studies used to derive the range of CPLs considered in this 
review offered no information regarding the frequency of time that 
visibility levels should be below those values. Based on this 
limitation, the EPA concluded in the Policy Assessment that it would 
not be appropriate to consider eliminating all exposures above the 
level of the standard and that it was reasonable to consider allowing 
some number of days with reduced visibility. Recognizing that the 
Regional Haze Program focuses attention on the 20 percent worst 
visibility days (i.e., those at or above the 80th percentile of 
visibility impairment), the EPA continues to believe, as noted in the 
proposal, that a percentile well above the 80th percentile would be 
appropriate to increase the likelihood that all days in this range 
would be improved by control strategies intended to help areas attain 
the standard. Focusing on the 90th percentile, which represents the 
median of the distribution of the 20 percent worst visibility days, 
could be reasonably expected to lead to improvements in visual air 
quality on the 20 percent most impaired days. Thus, the EPA has made a 
reasoned judgment based on a full consideration of the upper end of the 
distribution of visibility impairment conditions and continues to 
conclude that it is appropriate to focus on the 90th percentile of 
visibility impairment values.
    With regard to comments requesting the EPA adopt a longer multi-
year averaging period for the 90th percentile values, the EPA disagrees 
that it would be appropriate to average the 90th percentile values over 
periods longer than 3 years. The EPA recognizes that a multi-year 
percentile form offers greater stability to the air quality management 
process by reducing the possibility that statistically unusual 
indicator values will lead to transient violations of the standard. 
Utilizing a 3-year average form provides stability from the occasional 
effects of inter-annual meteorological variability that can result in 
unusually high pollution levels for a particular year. The Agency has 
adopted this approach in other NAAQS, including the current secondary 
24-hour PM2.5 NAAQS, which has a 98th percentile form 
averaged over 3 years. However, adopting a multi-year averaging period 
longer than 3 years would increase the number of days with visibility 
impairment above the target level of protection and would therefore 
reduce the protectiveness of the standard. Based on this the EPA does 
not believe it would be appropriate to average 90th percentile values 
over a period as long as five years. Therefore, the EPA continues to 
conclude that a 90th percentile form, averaged over 3 years, would be 
appropriate, in conjunction with a calculated PM2.5 light 
extinction indicator and a 24-hour averaging time.
e. Level
    With regard to level, commenters focused on two main themes. First, 
a large number of commenters addressed the information available from 
the public preference studies with regard to the acceptability of 
various levels of visual air quality. These comments, which are 
discussed in subsection VI.C.1.e.i below, address the EPA's use of 
visibility preference studies as the basis for the selection of a range 
of appropriate levels for the Administrator to consider. Many 
commenters challenged the use of these studies as the basis for setting 
a distinct secondary standard, arguing that limitations in these 
studies rendered them an unsuitable and insufficient basis on which to 
establish such a standard. Second, commenters expressed different views 
as to what level(s) of a distinct secondary standard would be 
appropriate, if the EPA were to set such a standard. These comments 
reflected consideration of the results of the public preference studies 
as well as analyses conducted in the Visibility Assessment and the 
Policy Assessment, as discussed in the proposal. Comments addressing 
the appropriateness of specific levels are discussed in subsection 
VI.C.1.e.ii below.
i. Comments on Visibility Preference Studies
    A majority of commenters expressed the view that the existing 
preference studies provide an insufficient basis for selection by the 
Administrator of an appropriate level of public welfare visibility 
protection for a national standard. These commenters highlighted a 
number of limitations and uncertainties (enumerated below) associated 
with these studies as support for this view. In contrast, other 
commenters felt that despite certain limitations, these studies do 
provide a sufficient basis on which the Administrator can select an 
appropriate level of a standard to provide national public welfare 
visibility protection. The remainder of this section organizes and 
discusses these comments under four broad topic areas, including: (a) 
Limitations and uncertainties associated with the visibility preference 
studies; (b) preference study methods and design; (c) use of preference 
study results for determining adversity; (d) the appropriateness of 
using regionally varying preference study results to select a single 
level for a national standard.
(a) Preference Study Limitations and Uncertainties
    A large and diverse number of limitations and uncertainties 
associated with the visibility preference studies have been identified 
and discussed in the public comments. Many of these same limitations 
and uncertainties were also identified and discussed by the EPA in the 
various documents developed throughout this review. The most important 
and fundamental limitations and uncertainties will be discussed here in 
the preamble, while more specific, unique or detailed comments will be 
addressed in the Response to Comments document.
    The primary or most frequent limitation cited by many commenters 
relates to the small number of preference studies that are available in 
this review. In particular, some commenters note that these preference 
studies cover just four locations, only three of which occur in the 
U.S., that the two studies conducted in Washington, DC were pilot 
studies, not full preference studies, and/or that three of the 
preference studies were conducted in the West, while only one was 
conducted in the East, providing only limited geographic coverage. 
Typically, these same commenters also pointed out that taken together, 
these

[[Page 3211]]

limited studies only included a total of 852 participants, which they 
claimed was too small a sample size and unrepresentative nationally. 
These commenters thus concluded that there is insufficient information, 
both geographically and demographically, upon which to select a 
national level of a visibility index for purposes of visibility 
protection.
    In contrast, several commenters stated support for using the 
preference studies, concluding they provide an adequate basis, in spite 
of their limited nature. In particular, AMC et al. state:

    We believe that these studies provide sufficient results to 
inform setting a national visibility standard. While the number of 
studies is small, they do incorporate spatial variation and, in the 
case of Denver and Phoenix, varied populations* * *. EPA should have 
confidence, rather than uncertainty, in the fact that these studies 
used different methods and respondents and yield a range of 20-24 
dv, with one outlier of 29. (AMC, et al., pp. 6-7)

    Regarding the first group of commenters, the EPA notes that it is 
well aware of the limited nature of the information, which it has 
described in great detail in the Integrated Science Assessment, 
Visibility Assessment, and Policy Assessment, as well as in section 
VI.B.2 of the proposed rule (77 FR 38973). The EPA further notes, 
however, that limited information does not preclude the Administrator 
from making judgments based on the best available science, taking into 
account the existing uncertainties and limitations associated with that 
available science. Thus, in reaching judgments based on the science, 
the Administrator appropriately weighs the associated uncertainties. 
The CASAC supported this view and concluded that the available 
information provided a sufficient basis on which the Administrator 
could form a judgment about requisite PM-related public welfare 
visibility protection. Specifically, CASAC stated ``[t]he 20-30 
deciview range of levels chosen by EPA staff as `Candidate Protection 
Levels' is adequately supported by the evidence presented'' (Samet, 
2010b, p. iii). As discussed in the proposed rule (77 FR 38990), the 
Administrator recognized and explicitly took into account the 
uncertainties and limitations in the science in determining an 
appropriate degree of protection when she proposed a level at the upper 
end of the recommended range. As discussed below, the Administrator 
continues to be mindful of these uncertainties and limitations in 
reaching her final determination regarding what constitutes an 
appropriate degree of protection with respect to PM-related visibility 
impairment.
    With respect to the comments of AMC et al., the EPA agrees that 
these studies provide a sufficient basis to inform the Administrator's 
judgments regarding an appropriate level of protection from PM-related 
visibility impairment, but she recognizes that these studies, which are 
the only studies before her, are a limited source of information. 
However, the EPA does not agree that the Washington, DC, results 
represent an outlier, and thus the EPA believes these results are 
appropriately included in the range identified for the Administrator to 
consider.
    Some commenters made the point that the EPA relied on much of this 
same evidence to reach the conclusion in 2006 that the information was 
too limited to allow selection of a national standard. For example, API 
stated:

    [T]he bulk of the VAQ preference studies were available during 
the previous PM NAAQS review and were considered by the Agency in 
its establishment of the 2006 p.m. secondary NAAQS * * *. The 
Proposed Rule does not mention this fact and does not explain why 
many of these same studies now compel EPA to propose this new 
secondary NAAQS * * *. The Proposed Rule notes in passing that, 
since the last review of the PM NAAQS, `limited information that has 
become available regarding the characterization of public 
preferences in urban areas has provided some new perspectives on the 
usefulness of this information in informing the selection of target 
levels of urban visibility protection.' 77 Fed. Reg. at 38969/2. It 
is a serious oversight that the Proposed Rule makes no attempt to 
explain what that information is or how it affects the 
interpretation of the VAQ preference studies. This `limited 
information' is an apparent reference to information provided by Dr. 
Anne Smith. (API, p. 37)

    The EPA disagrees with these commenters. First, the EPA disagrees 
that it failed to distinguish between studies that were available in 
the previous review and the current review. The discussion in section 
VI.A.1 of the proposal specifically identifies the studies from Denver, 
Phoenix and British Columbia (77 FR 38967/2) as being considered in the 
last review. The EPA further disagrees with the implication that it is 
being circumspect about identifying the ``limited information that has 
become available regarding the characterization of public preferences 
in urban areas.'' Beginning in section VI.A.3 of the proposed rule (77 
FR 38969), the EPA was clear about what information, both preexisting 
and new, it relied upon in this review to inform its views and provide 
the basis for its proposal. In section VI.B.2, the EPA elaborates on 
the specific information, tools, methods and data which are considered 
in relation to the public preference studies, including the new 
information available since the last review.
    As noted above and in the proposal, in addition to the substantial 
PM urban air quality information and analyses new to this review, there 
are three other sources of information that have specifically 
``provided some new perspectives on the usefulness of'' the preference 
studies ``in informing the selection of target levels of urban 
visibility protection'' (77 FR 38969). They include: (1) Results from 
additional urban visibility preference study experiments conducted for 
Washington, DC by Smith and Howell (2009) which added to the preference 
data for that location and shed light on the role of location in 
preference responses; (2) a review and reanalysis (Stratus Consulting, 
2009) of the urban visibility public preference studies from the four 
urban areas, including the newly available Smith and Howell (2009) 
experiments which examined the similarities and differences between the 
studies and evaluated the potential significance of those differences 
on the study results; and (3) additional analyses, including most 
importantly a logit analysis (Deck and Lawson, 2010, as discussed in 
Chapter 2 and Appendix J of the Visibility Assessment), which was 
requested and reviewed by CASAC, which showed that each city's 
responses represented unique and statistically different curves. Taken 
together, these sources contributed to the EPA's current knowledge and 
understanding of each survey study's results, the appropriateness of 
comparing each study's results to the others, and the key uncertainties 
relevant to data interpretation. In addition, in the last review the 
decision to not adopt a distinct secondary standard was remanded as 
contrary to law and failing to provide a reasoned explanation for the 
decision. As such it is not appropriate for purposes of comparison with 
the Administrator's judgment and reasoning in this review.
(b) Preference Study Methods and Design
    In addition to the limitations and uncertainties noted above, many 
comments also asserted the methodologies used in the preference studies 
are fundamentally flawed. Many commenters cited some of the same issues 
that have already been identified by the EPA as sources of uncertainty 
and potential factors in producing the statistically different study 
results (see section VI.B.1.b above). As noted above,

[[Page 3212]]

the EPA is well aware of the issues raised regarding the adequacy of 
the preference studies to serve as a basis for a secondary NAAQS (see 
77 FR 38975) and solicited comment on how these uncertainties should be 
considered (see 77 FR 38990). Most of these same commenters also 
pointed to an assessment of the preference studies methodology provided 
by Smith and Howell (2009) as the basis for their views, as indicated 
by the following comments:

    Smith and Howell (2009) show that VAQ preference study outcomes 
are malleable and depend entirely on the design of the study. 
Accordingly, such studies do not identify any meaningful threshold 
of acceptable visibility conditions. Despite Smith and Howell's 
conclusions, EPA continues to assert that the VAQ preference studies 
can be used to identify minimally acceptable visibility conditions 
even though the Agency has never provided any valid scientific basis 
for discounting the Smith and Howell (2009) results. (API, p. 38)
    Well-controlled preference studies discussed by Anne Smith of 
Charles River Associates at the March 2010 CASAC meeting 
demonstrated that the judgment of panel members was affected by the 
order in which photographs were presented and tendency to identify 
the middle of the range of visibility degredation as a threshold of 
acceptability. This points to a potential flaw in these studies and 
that artifacts caused by these tendencies may have influenced study 
results. Dismissing these inherent flaws in the existing preference 
studies and then using these studies to set a secondary NAAQS is 
arbitrary and capricious. (API, Attachment 2, p. 12)
    EPA also fails to acknowledge that the only study conducted 
since the last review rebuts the validity of the VAQ preference 
studies previously conducted. (UARG, Attachment 2, p. 28)

    As is explained in a more detailed discussion in the Response to 
Comments document, the EPA disagrees that the study conducted by Smith 
and Howell (2009) supports the conclusion that the preference study 
methodologies were fundamentally flawed; however, the EPA notes that 
their experiments do identify areas where additional research would be 
useful to further inform our limited understanding of public 
preferences in urban areas. The EPA views the Smith and Howell 
experiments as increasing the EPA's knowledge and understanding of the 
findings of the 2001 Washington, DC focus group pilot study (Abt, 2001) 
in several important ways, although this information still remains 
limited overall. Specifically, the Smith and Howell results suggest: 
(1) The 2001 results, while based on a small sample size of 9, were 
consistent with results from a larger sample of the general Washington, 
DC population; (2) an individual's preferences for visibility in one 
location may not depend on whether they live in that location; and (3) 
presentation method (i.e., changing from slide projection to computer 
monitor) did not appear to affect the reported preferences.
(c) Preference Study Results and Adversity
    A number of comments were received regarding the EPA's use of 
preference study results to make the determination that adverse 
PM2.5-related visibility effects on the public welfare are 
occurring. In this context, several commenters questioned whether the 
EPA had made the case that unacceptable levels of visual air quality 
based on preference study results alone can be equated with an adverse 
public welfare effect. These commenters suggested that unless 
preference study information is linked to personal comfort and well-
being or other associated welfare effects, it cannot form the basis of 
a determination of adversity. For example, Kennecott Utah Copper LLC 
stated that:

    Thus, EPA seemingly was building the foundation for a 
determination of what constitutes an adverse effect on visibility in 
the context of public welfare. However * * * EPA subsequently veered 
toward an oversimplified focus on public acceptance of visibility 
conditions * * *. EPA's discussion of visibility in the Policy 
Assessment and its proposed rule in the Federal Register focuses 
entirely on ``acceptable'' and ``unacceptable'' visual air quality 
and make no mention of an ``adverse effect'' in the context of 
visibility. EPA's reliance on only 3 urban preference studies 
represents a paucity of data and a wholesale abandonment of any 
effort to seek a scientifically measurable adverse effect. 
(Kennecott Utah Copper LLC, p. 26)

    In response, the EPA first notes that the definition of effects on 
welfare included in section 302(h) of the CAA identifies both 
visibility and the broader category of effects on personal comfort and 
well-being as effects on welfare. In setting a secondary standard to 
address visibility impairment, the EPA considers the effect on the 
public from impairment of visibility as a separate and distinct welfare 
effect in its own right. The EPA is not required to translate this into 
terms of personal comfort and well-being, as visibility impairment is 
designated explicitly by Congress as an effect on welfare. While there 
may be a large degree of overlap among these different welfare effects, 
the EPA properly focuses on evaluating all of the information before 
the Agency on the effect visibility impairment has on the public, 
whether or not this impairment would also be categorized as having an 
adverse effect on personal comfort and well-being. It is in the context 
of all of this information that the EPA makes the judgment as to the 
appropriate degree of protection from known and anticipated adverse 
effects on the public from visibility impairment. The EPA recognizes 
that there is uncertainty about the degree of adversity to the public 
welfare associated with PM-related visibility impairment. However a 
secondary standard is designed to provide protection from ``known or 
anticipated'' adverse effects, and a bright line determination of 
adversity is not required in judging the requisite degree of protection 
under section 109(b)(2). Furthermore, the EPA disagrees that it has 
abandoned its consideration of visibility-related impacts on the 
welfare effect of personal comfort and well-being, as is made clear in 
the following quote:

    Research has demonstrated that people are emotionally affected 
by low visual air quality, that perception of pollution is 
correlated with stress, annoyance, and symptoms of depression, and 
that visual air quality is deeply intertwined with a ``sense of 
place,'' affecting people's sense of the desirability of a 
neighborhood (U.S. EPA, 2009a, section 9.2.4). Though it is not 
known to what extent these emotional effects are linked to different 
periods of exposure to poor visual air quality, providing additional 
protection against short-term exposures to levels of visual air 
quality considered unacceptable by subjects in the context of the 
preference studies would be expected to provide some degree of 
protection against the risk of loss in the public's ``sense of well-
being.'' (77 FR 38973/1, emphasis added)

    The approach taken to address such qualitative, but policy-
relevant, information in this review is the same as in other NAAQS 
reviews. The review is initiated with a comprehensive assessment of all 
possible public health and welfare effects associated with PM in the 
Integrated Science Assessment. Then policy-relevant effects for which 
there is sufficient quantitative information to allow a determination 
of the change in risks associated with incremental changes in air 
quality are assessed (in this review, in the Visibility Assessment) and 
used to provide a quantitative basis to inform the selection of an 
appropriate range of levels for further consideration in the Policy 
Assessment. In the Policy Assessment, the EPA considers all important 
policy-relevant evidence and information, both quantitative and 
qualitative, in making recommendations regarding the range of policy 
options appropriate for the Administrator to consider. It is in the 
context of all of this information that the Administrator

[[Page 3213]]

makes her final judgment as to the appropriate degree of protection 
from known and anticipated adverse effects on the public from 
visibility impairment.
    Another issue raised in the comments regarding adversity is the 
EPA's decision to use the 50 percent acceptability criterion from the 
public preference studies in determining candidate protection levels of 
visibility impairment for the selection of a national level of 
visibility protection. For example, AMC et al. recommended ``a 75% 
acceptability criterion as a target that is in line with protecting the 
broader public from the negative effects of visibility impairment'' 
(AMC, et al., p. 9).
    In the Visibility Assessment, the EPA noted that the use of the 50 
percent acceptance level for urban visibility was first presented in 
Ely et al. (1991) (U.S. EPA, 2010b, p. 2-5). Ely discussed the use of 
the 50 percent acceptability criterion as a reasonable basis for 
setting an urban visibility standard.

    The standard was determined based on a 50% acceptability 
criterion, that is, the standard was set at the level of extinction 
that would divide the slides into two groups: those judged 
acceptable and those judged unacceptable by a majority of the people 
in the study. The criterion is politically reasonable because it 
defines the point where a majority of the study participants begin 
to judge slides as representing unacceptable visibility. It is also 
consistent with psychological scaling theory which indicates that a 
``true score'' exceeds a standard when more than 50% of the 
``observed scores'' exceed that standard. (Ely et al., 1991, p. 11)

    As Ely described, the 50 percent acceptability criterion and the 
preference study conducted by Ely were used as the basis for setting 
the level of the Denver Visibility Standard in 1990. That same 
criterion was judged appropriate and selected for use in the Phoenix 
preference study (BBC research, 2003) and as the basis for setting the 
level of the Phoenix Visibility Standard in 2003. Most recently, the 50 
percent acceptability criterion has been recommended by the British 
Columbia Visibility Coordinating Committee as the basis for the 
visibility standard currently under consideration by British Columbia, 
Canada. Furthermore, CASAC supported this approach, while recognizing 
the uncertainty associated with this issue. Specifically, CASAC agreed 
that ``the 50th percentile for the acceptability criteria is logical, 
given the noted similarities in methodologies employed in the 4 study 
areas. * * * In terms of choosing a specific percentile from the 
preference studies, we note that there may not be a ``preferred'' one, 
but in assessing preference studies to propose a PM secondary NAAQS, 
the 50th percentile is sufficient, as it is the basis for existing 
visibility indexes used in the Denver/Colorado Front Range and Phoenix 
metropolitan areas'' (Samet, 2009c, pp. 8-9). Therefore, after 
considering the information that served as the original basis for its 
selection as described in Ely et al., 1991, and given its acceptance 
and use in existing visibility programs, the EPA continues to conclude, 
consistent with the advice of CASAC, that it is reasonable to use the 
50 percent acceptability criterion in determining target levels of 
protection from visibility impairment.
    (d) Appropriateness of using regionally varying preference study 
results to select a single level for a national standard.
    A number of commenters raised concerns regarding the bases for and 
implications of the differences observed in the preference study 
results, concluding that these results were due to regionally varying 
factors and thus could not be used to set a national standard. For 
example, some commenters asserted that because the confidence intervals 
around the four 50 percent acceptability levels do not overlap at all, 
and because there are variations in preference study designs and 
inherent differences in the visual setting among cities and panels, the 
four preference curves and their associated 50 percent dv values are 
city-specific and statistically different. The commenters concluded, 
therefore, that it was inappropriate to aggregate the 50th percentile 
dv values from multiple studies and that they should instead be 
evaluated individually.
    Other commenters expressed the related view that the preference 
study results cannot be used to set a national standard for visibility 
impairment because the results show that visibility preferences vary 
regionally. For example, API stated that:

    The `one-size-fits-all' approach * * * is not viable because it 
does not account for regional and city-specific factors that have 
been made evident in the disparity of preference study data * * *. 
It is well known, for example, that the level of light extinction to 
which people in different areas of the country are accustomed, as 
well as the urban setting, are the primary factors that affect a 
person's visual perception of an urban vista. Thus, the degree to 
which extinction threshold can be related to human welfare is 
inevitably regionally-dependent. (API, Attachment 2, p. 4)

Some of these commenters argued that because acceptable visual air 
quality is regionally dependent, it would be more appropriate to 
develop distinct visibility standards at the state or local level. 
Others pointed out that areas which lack ``important visibility 
vistas'' might not need standards at all, since flat areas without 
significant terrain have a limited maximum visual range (NEDA/CAP, p. 
3).
    Other commenters stated that due to regionally varying factors, 
such as relative humidity, it is not possible to select a single level 
for a national standard to protect visibility across the United States. 
In particular, these commenters pointed to differences between Eastern 
and Western areas, arguing that a single national standard could not 
offer the appropriate degree of protection in locations with distinct 
characteristics. For example:

    [T]he proposed method falls short because it is not temporally 
or geographically representative enough to have any meaning * * *. 
The uncertainty evidenced in these studies and the non-uniformity 
between the western and eastern vistas makes it impossible at this 
time to set an acceptable light extinction value that would 
appropriately address visibility concerns in non-Class I areas. (New 
York DOH/DEC, pp. 5-6)

    The EPA agrees that the preference curves and the 50 percent dv 
levels are separate and distinct data points representing four 
different VAQ preference curves for four unique urban scenes. However, 
the EPA does not consider the fact that the four curves are distinct as 
a weakness of the approach or a reason that the results cannot be 
compared. In addition, the EPA does not agree that the study results 
necessarily support a conclusion that preferences are regionally 
dependent. In particular, the EPA notes that the results of Smith and 
Howell (2009) which show that participants in Houston and Washington, 
DC did not have significantly different views on acceptable air quality 
in Washington, DC, provide limited support for the conclusion that 
people's preferences differ less because of where they live and more 
because of the scene they are viewing.
    On the other hand, the existing literature indicates that people's 
preferences for VAQ depend in large part on the characteristics and 
sensitivity of the scene being viewed. The EPA understands there is a 
wide variety or range of urban scenes within the United States. These 
sensitive urban scenes include those with natural vistas such as the 
Colorado Rocky Mountains as well as those with iconic man-made urban 
structures like the Washington Monument. The EPA believes that the 
scenes presented in the four urban areas

[[Page 3214]]

include important types of sensitive valued urban scenes and therefore, 
when considered together, can inform the selection of a level of 
acceptable urban VAQ at the national scale, taking into account the 
variation across the country evidenced in the studies. This is 
discussed further in the next section, below.
    The EPA does agree with commenters that there are regionally 
varying factors that are important to take into account when setting a 
national standard for visibility protection. Section VI.A above 
regarding the history of the secondary PM NAAQS review discusses the 
evolution of the EPA's understanding regarding the regional differences 
in PM concentrations, relative humidity and other factors. As a result, 
the current review has gone to great lengths to address these factors, 
leading to the EPA's proposal to use the IMPROVE algorithm to calculate 
light extinction in order to take into account the varying effects of 
relative humidity and speciated PM. While this approach does not result 
in a uniform level of ambient PM2.5, it does ensure a 
nationally uniform level of visibility protection. The EPA refers the 
reader to other sections of the final rule, including sections 
VI.B.1.a, VI.B.1.c, VI.C.1.b and VI.C.1.f, and the Response to Comments 
document for a more detailed response as to how it is taking these 
variables into account.
ii. Specific Comments on Level
    The EPA received relatively few comments endorsing a specific level 
for a distinct secondary standard for visibility. In general, 
commenters who opposed setting a distinct secondary standard at this 
time did not address the question of what level would be appropriate if 
the EPA were to set a distinct secondary standard for visibility; 
similarly, commenters who supported adopting a distinct secondary 
standard at this time generally did not recommend a specific level. 
However, a few commenters did provide comments in support of a specific 
level or range of levels, with some commenters advocating standards at 
the upper end of the range of proposed levels (i.e., 30 dv), while 
others supported levels below the lower end of the proposed range 
(i.e., below 28 dv).
    As discussed above, a large number of commenters argued that the 
currently available data are insufficient to determine what constitutes 
a standard that would be neither more nor less protective than 
necessary and that no standard should be set at this time. These 
commenters pointed to the limitations and uncertainties in the 
preference studies discussed above as the basis for this claim. These 
commenters pointed to significant variation in the results of the 
preference studies in support of their arguments that the studies 
should not be used to derive a level for a distinct secondary standard 
for visibility. For example, one consultant cited by several industry 
commenters argued that the proposed level of 28 or 30 dv did not 
reflect the substantial difference in visibility preferences between 
the East and the West reflected in the preference studies (UARG, 
Attachment 2, p. 11), and that it did not reflect the full range of 
preferences (i.e., potential 50 percent acceptability levels) likely to 
exist nationwide (UARG, Attachment 2, p. 19). This commenter further 
objected to the EPA's proposal for a level of 28 or 30 dv on the 
grounds that the EPA had inaccurately adjusted 4-hour values into 24-
hour values. Based on his analysis, the consultant concluded that ``a 
range of adjusted values from 28 to 32 dv is needed'' to account for 
the majority of the spread between the 4-hour vs. 24-hour equivalent 
values at the upper end of the distribution of values.
    A number of commenters questioned whether the proposed range of 
levels was appropriate. One industry commenter claimed that the EPA had 
not explicitly justified why a standard within the proposed range was 
requisite, stating that ``EPA makes no attempt to explain how the 
proposed level of the standard is neither lower nor higher than 
necessary to protect public welfare'' (NSSGA, p. 15). Arizona DEQ noted 
that since the proposed calculated light extinction indicator excluded 
coarse particles and Rayleigh scattering, the proposed levels of 28 or 
30 dv were inconsistent with the visibility preference studies, which 
considered total light extinction. Noting these perceived problems with 
the proposed range of levels, a few commenters noted that if the EPA 
were to set a distinct secondary standard, the level should be set no 
lower than 30 dv, ``to account for inconsistent value judgments, a 
great deal of spatial and temporal variability, and a very high level 
of uncertainty'' (Texas CEQ, p. 7).
    In contrast, some commenters supporting the EPA's proposal for a 
distinct secondary standard for visibility stated that the proposed 
range of levels from 28-30 dv was insufficiently protective based on a 
24-hour averaging time, and recommended a lower level for the 
visibility index standard. These commenters expressed the view that the 
proposed levels of 28 or 30 dv represented neither adequate surrogates 
for equivalent 4-hour values, as the EPA claimed, nor sufficiently 
protective levels based on recent air quality data. Several commenters 
stated that the EPA's own analyses suggested that a standard set at a 
level of 28 or 30 dv was insufficiently protective based on a 24-hour 
averaging time. One commenter emphasized that the Policy Assessment had 
indicated a level between 25-28 dv was appropriate for a standard 
calculated on a 24-hour average, and encouraged the EPA to adopt a 
standard level of 25 dv. Several environmental groups provided comments 
stating that a 24-hour average would underestimate a 4-hour value by 
13-42 percent and certain areas of the country--particularly the 
Northeast--would be affected disproportionately. These commenters 
suggested that a 24-hour PM2.5 visibility index standard 
should be set at a level of 18.6-20 dv. The Department of the Interior 
pointed to recent air quality data indicating that visibility on the 
20% worst days in several large metropolitan areas, including 
Birmingham, Fresno, New York City, Phoenix, and Washington, DC was 
below 29 dv. While noting that these calculations were based on IMPROVE 
calculations which include contributions from coarse PM mass, DOI 
expressed the view that the proposed level of 28 to 30 dv would not 
provide adequate visibility protection compared to the current 24-hour 
PM2.5 standard of 35 [micro]g/m\3\ and recommended that the 
standard be set at a level of 25 dv consistent with the results of the 
Phoenix visibility preference study.
    In contrast, the states of Arizona and Colorado submitted comments 
arguing that the visibility preference studies conducted in Phoenix and 
Denver, respectively, were designed to address a specific local problem 
and that the results of these studies were not an appropriate basis for 
selecting the level of a national standard. For example, Arizona DEQ 
noted:

    The cited studies were conducted considering total light 
extinction; including extinction resulting from particulate matter 
and Rayleigh scattering. Visibility impairment due to coarse 
particulate matter can be an important contributor in Arizona, 
specifically in the Phoenix area where ongoing measurements have 
been made. Therefore, ADEQ believes that the proposed levels of the 
secondary visibility standard are inconsistent with applicable urban 
studies. (Arizona DEQ, p. 2)

Similarly, the Colorado Department of Public Health and the Environment 
noted that the Denver visibility standard was designed to address 
``brown clouds'', i.e., strong inversions that occur in the Denver 
metropolitan area, and that this standard ``is based on a

[[Page 3215]]

specific view of Denver'' associated with particular sight paths and 
direct measurement methods. The commenter stated that this standard 
``is applicable only to this location,'' and that these limitations 
make it potentially unsuitable for application as ``a national 
secondary standard, particularly a proposed standard that does not use 
a direct measurement method'' (Colorado DPHE, p. 2).
    While acknowledging the uncertainties and limitations associated 
with the visibility preference studies as discussed above, the EPA 
continues to conclude, as did CASAC, that the preference studies are 
appropriate to use as the basis for selecting a target level of 
protection from visibility impairment. However, the EPA agrees with 
commenters who emphasize the high degree of variability in visibility 
conditions and the potential variability in visibility preferences 
across different parts of the country. In light of the associated 
uncertainty, as noted in the proposal, the Administrator judged it 
appropriate to establish a target level of protection equivalent to the 
upper end of the range of Candidate Protection Levels (CPLs) identified 
in the Policy Assessment and generally supported by CASAC. Thus, the 
EPA proposed to set a 24-hour visibility index standard that would 
provide protection equivalent to the protection afforded by a 4-hour 
standard set at a level of 30 dv. In light of the comments received on 
the proposal, in particular comments emphasizing the uncertainty and 
variability in the results of the public preference studies, the EPA 
continues to conclude that this approach is warranted, and that it is 
appropriate to set a target level of protection equivalent to the 
protection that would be afforded by a 4-hour, 30 dv visibility index 
standard.
    Moreover, the EPA disagrees with commenters who argued that the 
EPA's approach for translating 4-hour CPLs into equivalent 24-hour 
values was inappropriate. In adjusting 4-hour values for purposes of 
defining an appropriate level for a 24-hour standard, the EPA noted at 
the time of proposal that there were multiple approaches for estimating 
generally equivalent levels on a city-specific or national basis. While 
expressing the view that it was appropriate to consider the two 
approaches with the highest r\2\ values (Approaches A and B in Appendix 
G of the Policy Assessment),\191\ which used regressions of 90th 
percentile light extinction values, the EPA determined it would also be 
appropriate to consider the city-specific estimates resulting from 
Approaches C and E which showed greater variability than the aggregated 
estimates. Approaches C and E generated a range of city-specific 
estimates of generally equivalent 24-hour levels that encompassed the 
range of levels considered appropriate for 4-hour CPLs, including the 
CPL of 30 dv at the upper end of that range. This information provided 
support for using the same CPL for a 24-hour standard as for a 4-hour 
standard, since no single approach could generate an equivalent 24-hour 
standard level in each urban area for each CPL. The EPA continues to 
conclude, as it did at the time of proposal, that using an unadjusted 
4-hour CPL for purposes of establishing a target level of protection 
for a 24-hour standard is appropriate because this approach places more 
emphasis on the relatively high degree of spatial and temporal 
variability in relative humidity and fine particle composition observed 
in urban areas across the country, consistent with EPA's reanalysis 
discussed below.
---------------------------------------------------------------------------

    \191\ In particular, EPA staff expressed a preference for 
Approach B in the Policy Assessment. However, in light of the 
additional information provided by the other approaches explored in 
Appendix G of the Policy Assessment and the reanalysis in Frank, et 
al. (2012b), the EPA judges it more appropriate to consider the 
range of values resulting from all five analytical approaches for 
purposes of informing decisions about the equivalent level of a 24-
hour standard.
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    The EPA has conducted a reanalysis (Frank et al., 2012b) of the 
relationships between estimated 24-hour and 4-hour visibility 
impairment based on the variety of metrics discussed in Appendix G of 
the Policy Assessment. The reanalysis has more appropriately considered 
the uncertainty of the calculated 4-hour values. The revised analysis 
shows that the 24-hour equivalent level is generally closer to the 4-
hour value at the upper end of the range of CPLs than originally 
estimated, as can be seen in the results for Approaches B, C, and 
D.\192\ For example, the reanalysis indicates that Approach B yields an 
adjusted 24-hour CPL of 29 dv\193\ as generally being equivalent to a 
4-hour CPL of 30 dv, while Approach C yields a 24-hour equivalent CPL 
of 29 dv averaged across cities and a range of city-specific values 
from 25-36 dv.194 195 Not only are the 90th percentile and 
pooled average values closer to the 4-hour CPL of 30 dv, the range of 
city-specific results shows a wider spread that clearly encompasses the 
unadjusted 4-hour value of 30 dv near the midpoint of the city-specific 
range. This provides support for concluding that the EPA's approach to 
translating of 4-hour CPLs into equivalent 24-hour values was 
appropriate, and that it is appropriate to use unadjusted 4-hour values 
for purposes of selecting a level for a standard based on a 24-hour 
averaging time.\196\
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    \192\ Approach E as presented in the Policy Assessment is based 
on the median values for each city; these results are not affected 
by the regression analyses. Therefore, Approach E was not included 
in the reanalysis, and the results remain unchanged from those 
reported in the corrected Table G-6 as reported in Frank, et al., 
2012b.
    \193\ In Appendix G of the Policy Assessment, a 24-hour adjusted 
CPL of 28 dv was estimated to be equivalent to a 4-hour value of 30 
dv under Approach B (annual 90th percentile values regression).
    \194\ In Appendix G of the Policy Assessment, under Approach C 
(all-days city-specific regression), a 24-hour adjusted CPL of 27 dv 
was estimated to be equivalent to a 4-hour CPL of 30 dv when 
averaged across cities, while city-specific values were estimated to 
range from 24-30 dv.
    \195\ In the reanalysis, Approach D (all days pooled regression) 
generated results of 28 dv for the 24-hour CPL equivalent to a 4-
hour value of 30 dv as compared to a value of 27 dv in the original 
analysis described in Appendix G.
    \196\ The analysis in Appendix G of the Policy Assessment used 
the 4-hour light extinction value treated as the independent (x-
axis) variable in an ordinary least squares regression. The EPA now 
concludes that this regression approach was not the most appropriate 
approach because that variable has error and in fact may be more 
uncertain than the calculated 24-hour extinction values. The Frank 
et al. (2012b) reanalysis uses an orthogonal regression instead of 
ordinary least squares regression and results in slopes closer to 
the 1:1 line for all the results, particularly for Dallas, TX. 
Furthermore, consistent with the EPA's conclusion that a higher 
multiplier for converting OC to OM would be appropriate (see section 
VI.C.1.b.ii above), the reanalysis substitutes a 1.6 multiplier for 
converting OC to OM in the calculation of 24-hour values instead of 
the value of 1.4 that was used in calculating 24-hour values for 
Appendix G. The higher multiplier is more consistent with the 
SANDWICH approach used to calculate the 4-hour values found in 
Appendix G. See Frank et al. (2012b) for a more detailed 
explanation.
---------------------------------------------------------------------------

    Moreover, the EPA disagrees with commenters who argue that the 
currently available evidence is sufficient to justify establishing a 
target level of protection at 25 dv or below. The EPA recognizes that 
25 dv represents the middle of the range of 50 percent acceptability 
levels from the 4 cities studied, and represents the 50 percent 
acceptability level from the Phoenix study, which the Agency has 
acknowledged as the best of the four studies in terms of having the 
least noise in the preference study results and the most representative 
selection of participants. The EPA also notes the caveats discussed in 
the proposal regarding whether it would be appropriate to interpret 
results from the western studies as generally representative of a 
broader range of scenic vistas in urban areas across the country. The 
Policy Assessment noted significant differences in the

[[Page 3216]]

characteristics of the urban scenes used in each study, with western 
urban visibility preference study scenes including mountains in the 
background and objects at greater distances, while scenes in the 
eastern study did not. Since objects at a greater distance have a 
greater sensitivity to perceived visibility changes as light extinction 
changes compared to otherwise similar scenes with objects at a shorter 
range, this likely explains part of the difference between the results 
of the eastern study and results of the western studies. In the 
proposal, the EPA noted that the scenic vistas available on a daily 
basis in many urban areas across the country generally do not have the 
inherent visual interest or the distance between viewer and object of 
greatest intrinsic value as in the Denver and Phoenix preference 
studies. Also, the Agency takes note of the caution expressed by 
Colorado and Arizona about using the results of the Denver and Phoenix 
preference studies, which were aimed at addressing specific local 
visibility problems, to inform the choice of level for a national 
standard. Therefore, the Agency considers it reasonable to conclude, 
especially in light of the significant uncertainties, that it is 
appropriate to place less weight on the western preference results and 
that the high CPL value (30 dv) that is based on the eastern preference 
results is likely to be more representative of urban areas that do not 
have associated mountains or other valued objects visible in the 
distant background. These areas would include the middle of the country 
and many areas in the eastern U.S., as well as some western areas. As a 
result, the EPA concludes that it is more appropriate to establish a 
target level of protection at the upper end of the range of 24-hour 
CPLs considered, recognizing that no one level will be ``correct'' for 
every urban area in the country.
    In considering the upper end of this range, the EPA must identify a 
target level of protection that is considered requisite to protect 
public welfare from a national perspective, recognizing that the same 
target level would apply in all locations. Making this judgment 
requires a balancing of the risks to the public welfare and the 
substantial uncertainties surrounding appropriate levels of visibility 
protection. As acknowledged in the proposal, the EPA recognizes that 
setting a target level of protection for a 24-hour standard at 30 dv 
would reflect a judgment that the current substantial degrees of 
variability and uncertainty inherent in the public preference studies 
should be reflected in a higher target protection level than would be 
appropriate if the underlying information were more consistent and 
certain. Also, a 24-hour visibility index at a level of 30 dv would 
reflect recognition that there is considerable spatial and temporal 
variability in the key factors that determine the value of the 
PM2.5 visibility index in any given urban area, such that 
there is a relatively high degree of uncertainty as to the most 
appropriate approach to use in selecting a 24-hour standard level that 
would be generally equivalent to a specific 4-hour standard level. In 
light of these uncertainties, the EPA continues to believe that it is 
appropriate to establish a target level of protection for visual air 
quality of 30 dv, averaged over 24-hours, with a form as discussed 
above.
    In reaching this conclusion, the EPA notes that any national 
ambient air quality standard for visibility would be designed to work 
in conjunction with the Regional Haze Program as a means of achieving 
appropriate levels of protection against PM-related visibility 
impairment in all areas of the country, including urban, non-urban, and 
Federal Class I areas. While the Regional Haze Program is focused on 
improving visibility in Federal Class I areas and a secondary 
visibility index NAAQS would focus on protecting visual air quality 
principally in urban areas, both programs could be expected to provide 
benefits in surrounding areas. In addition, the development of local 
programs, such as those in Denver and Phoenix, can continue to be an 
effective and appropriate approach to provide additional protection, 
beyond that afforded by a national standard, for unique scenic 
resources in and around certain urban areas that are particularly 
highly valued by people living in those areas. With regard to comments 
from the Department of Interior noting that many large metropolitan 
areas have 24-hour IMPROVE values below 30 dv on the worst 20 percent 
of days already, the EPA notes that the purpose of establishing NAAQS 
is to ensure adequate protection of public welfare everywhere, not to 
mandate continuous improvements in areas that may already be relatively 
clean. In fact, the evidence from the IMPROVE program that many urban 
areas have total 24-hour PM-related light extinction below 29 dv on the 
20 percent worst visibility days suggests that many areas have 
relatively good visual air quality already.
f. Need for a Distinct Secondary Standard To Protect Visibility
    Numerous commenters questioned whether a distinct secondary 
standard for visibility is necessary in light of the analysis described 
in section VI.B.1.c.vii above (Kelly et al., 2012a) which indicated 
that a 24-hour mass-based PM2.5 standard of 35 [mu]g/m\3\ 
would protect against visibility impacts exceeding the range of levels 
considered in the proposal (28-30 dv). While this analysis was 
conducted in support of proposed implementation requirements for a 
distinct secondary standard (specifically, the modeling demonstrations 
that would be required under the PSD program), the second prong of the 
analysis showed that within the range of levels proposed by the EPA for 
the visibility index NAAQS (28-30 dv), the 24-hour PM2.5 
standard of 35 [mu]g/m\3\ would generally be controlling. Kelly et al. 
(2012a) concluded that ``overall, design values based on 2008-2010 data 
suggest that counties that attain 24-hour PM2.5 NAAQS level 
of 35 [mu]g/m\3\ would attain the proposed secondary PM2.5 
visibility index NAAQS level of 30 dv and generally attain the level of 
28 dv'' (pp. 17-18).
    Citing this conclusion, many state and local agencies and industry 
commenters argued that a visibility index standard in the range 
proposed (28-30 dv) would provide no additional protection beyond that 
afforded by the existing secondary 24-hour PM2.5 NAAQS, and 
therefore no distinct visibility standard was necessary. These 
commenters advocated retaining the current 24-hour PM2.5 
mass-based standard to protect against visibility effects. ``Since the 
24-hour PM2.5 standard already protects the welfare the 24-
hour PM2.5 visibility standard is designed to protect, the 
new standard is duplicative and unnecessary'' (South Dakota DENR, p. 
2). Furthermore, a number of state commenters objected to the 
additional resource burden associated with implementing a standard 
which had, in their view, no practical effect: ``If the 24-hour 
PM2.5 mass standard has the same effect as the visibility 
standard, crafting complex regulations to implement another standard 
seems redundant'' (South Carolina DHEC, p. 3). Other states agreed: ``A 
PM2.5-related Visibility Index appears redundant since the 
benefits achieved from the current primary and secondary annual and 24-
hour PM2.5 standards already provide reductions that would 
improve visibility. Establishing a new PM2.5 secondary 
standard