[Federal Register Volume 77, Number 126 (Friday, June 29, 2012)]
[Proposed Rules]
[Pages 38889-39055]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 2012-15017]



[[Page 38889]]

Vol. 77

Friday,

No. 126

June 29, 2012

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; Proposed 
Rule

Federal Register / Vol. 77, No. 126 / Friday, June 29, 2012 / 
Proposed Rules

[[Page 38890]]


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ENVIRONMENTAL PROTECTION AGENCY

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

[EPA-HQ-OAR-2007-0492; FRL-9682-9]
RIN 2060-AO47


National Ambient Air Quality Standards for Particulate Matter

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed 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 proposes to make revisions to the primary and secondary 
NAAQS for PM to provide requisite protection of public health and 
welfare, respectively, and to make corresponding revisions to the data 
handling conventions for PM and ambient air monitoring, reporting, and 
network design requirements. The EPA also proposes revisions to the 
prevention of significant deterioration (PSD) permitting program with 
respect to the proposed NAAQS revisions. With regard to primary 
standards for fine particles (generally referring to particles less 
than or equal to 2.5 micrometers ([mu]m) in diameter, 
PM2.5), the EPA proposes to revise the annual 
PM2.5 standard by lowering the level to within a range of 
12.0 to 13.0 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. The EPA proposes changes to the Air Quality Index (AQI) for 
PM2.5 to be consistent with the proposed primary 
PM2.5 standards. With regard to the primary standard for 
particles generally less than or equal to 10 [mu]m in diameter 
(PM10), the EPA proposes 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 proposes to revise the suite of secondary PM 
standards by adding a distinct standard for PM2.5 to address 
PM-related visibility impairment and to retain the current standards 
generally to address non-visibility welfare effects. The proposed 
distinct secondary standard would 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 dv or 28 dv.

DATES: Comments must be received on or before August 31, 2012.
    Public Hearings: The EPA intends to hold public hearings on this 
proposed rule in July 2012. These will be announced in a separate 
Federal Register notice that provides details, including specific 
dates, times, addresses, and contact information for these hearings.

ADDRESSES: Submit your comments, identified by Docket ID No. EPA-HQ-
OAR-2007-0492 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.
    Docket: 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 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:

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General Information

What should I consider as I prepare my comments for EPA?

    1. Submitting CBI. Do not submit this information to the EPA 
through www.regulations.gov or email. Clearly mark the part or all of 
the information that you claim to be CBI. For CBI information in a disk 
or CD ROM that you mail to the EPA, mark the outside of the disk or CD 
ROM as CBI and then identify electronically within the disk or CD ROM 
the specific information that is claimed as CBI. In addition to one 
complete version of the comment that includes information claimed as 
CBI, a copy of the comment that does not contain the information 
claimed as CBI must be submitted for inclusion in the public docket. 
Information so marked will not be disclosed except in accordance with 
procedures set forth in 40 CFR part 2.
    2. Tips for Preparing Your Comments. When submitting comments, 
remember to:
     Identify the rulemaking by docket number and other 
identifying information (subject heading, Federal Register date and 
page number).
     Follow directions--the agency may ask you to respond to 
specific questions or organize comments by referencing a Code of 
Federal Regulations (CFR) part or section number.
     Explain why you agree or disagree, suggest alternatives, 
and substitute language for your requested changes.
     Describe any assumptions and provide any technical 
information and/or data that you used.
     Provide specific examples to illustrate your concerns, and 
suggest alternatives.
     Explain your views as clearly as possible, avoiding the 
use of profanity or personal threats.
     Make sure to submit your comments by the comment period 
deadline identified.

Availability of Related Information

    A number of the documents that are relevant to this rulemaking are 
available through 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. 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
III. Rationale for Proposed 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. Health Effects Related to Exposure to Fine Particles
    1. Nature of Effects
    a. Health Effects Associated With Long-term PM2.5 
Exposures
    b. Health Effects Associated With Short-term PM2.5 
Exposures
    c. Summary
    2. Limitations and Uncertainties Associated With the Currently 
Available Evidence
    3. At-Risk Populations
    4. Potential PM2.5-Related Impacts on Public Health
    C. Quantitative Characterization of Health Risks
    1. Overview
    2. Summary of Design Aspects
    3. Risk Estimates and Key Observations
    D. Conclusions on the Adequacy of the Current Primary 
PM2.5 Standards
    1. Evidence-Based Considerations in the Policy Assessment
    a. Associations With Long-term PM2.5 Exposures
    b. Associations With Short-term PM2.5 Exposures
    2. Summary of Risk-Based Considerations in the Policy Assessment
    3. CASAC Advice
    4. Administrator's Proposed 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. Approach Used in the Policy Assessment
    b. Consideration of the Annual Standard in the Policy Assessment
    c. Consideration of the 24-Hour Standard in the Policy 
Assessment
    d. CASAC Advice
    e. Administrator's Proposed Conclusions on the Primary 
PM2.5 Standard Levels
    F. Administrator's Proposed Decisions on Primary 
PM2.5 Standards
IV. Rationale for Proposed 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 Policy Assessment for the 
Current Review
    B. Health Effects Related to Exposure to Thoracic Coarse 
Particles
    1. Nature of Effects
    a. Short-term PM10-2.5 Exposure and Mortality
    b. Short-term PM10-2.5 Exposure and Cardiovascular 
Effects
    c. Short-term PM10-2.5 Exposure and Respiratory 
Effects
    2. Potential Impacts of Sources and Composition on 
PM10-2.5 Toxicity
    3. Ambient PM10 Concentrations in PM10-2.5 
Study Locations
    4. At-Risk Populations
    5. Limitations and Uncertainties Associated With the Currently 
Available Evidence
    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
    a. Indicator
    b. Averaging Time
    c. Form
    d. Level
    i. Evidence-Based Considerations in the Policy Assessment
    ii. Air Quality-Based Considerations in the Policy Assessment
    iii. Integration of Evidence-Based and Air Quality-Based 
Considerations in the Policy Assessment
    D. CASAC Advice
    E. Administrator's Proposed Conclusions Concerning the Adequacy 
of the Current Primary PM10 Standard
    F. Administrator's Proposed Decision on the Primary 
PM10 Standard
V. Communication of Public Health Information

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VI. Rationale for Proposed 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. PM-Related Visibility Impairment
    1. Nature of PM-Related Visibility Impairment
    a. Relationship Between Ambient PM and Visibility
    b. Temporal Variations of Light Extinction
    c. Periods During the Day of Interest for Assessment of 
Visibility
    d. Exposure Durations of Interest
    2. Public Perception of Visibility Impairment
    C. Adequacy of the Current Standards for PM-Related Visibility 
Impairment
    1. Visibility Under Current Conditions
    2. Protection Afforded by the Current Standards
    3. CASAC Advice
    4. Administrator's Proposed Conclusions on the Adequacy of the 
Current Standards for PM-Related Visibility Impairment
    D. Consideration of Alternative Standards for Visibility 
Impairment
    1. Indicator
    a. Alternative Indicators Considered in the Policy Assessment
    i. PM2.5 Mass
    ii. Directly Measured PM2.5 Light Extinction
    iii. Calculated PM2.5 Light Extinction
    iv. Conclusions in the Policy Assessment
    b. CASAC Advice
    c. Administrator's Proposed Conclusions on Indicator
    2. Averaging Times
    a. Alternative Averaging Times
    i. Sub-Daily
    ii. 24-Hour
    iii. Conclusions in the Policy Assessment
    b. CASAC Advice
    c. Administrator's Proposed Conclusions on Averaging Time
    3. Form
    4. Level
    E. Other PM-Related Welfare Effects
    1. Climate
    2. Ecological Effects
    a. Plants
    b. Soil and Nutrient Cycling
    c. Wildlife
    d. Water
    e. Effects Associated With Ambient PM Concentrations
    f. Conclusions in the Policy Assessment
    3. Materials Damage
    4. CASAC Advice
    5. Administrator's Proposed Conclusions on Secondary Standards 
for Other PM-related Welfare Effects
    F. Administrator's Proposed Decisions on Secondary PM Standards
VII. Interpretation of the NAAQS for PM
    A. Proposed 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 Annual and 24-Hour PM2.5 
NAAQS
    5. Data Handling Procedures for the Proposed New Secondary 
PM2.5 Visibility Index NAAQS
    B. Exceptional Events
    C. Proposed Updates for Data Handling Procedures for Reporting 
the Air Quality Index
VIII. Proposed 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 CSN Methods to Support the Proposed New Secondary 
PM2.5 Visibility Index NAAQS
    B. Proposed Changes to 40 CFR Part 58 (Ambient Air Quality 
Surveillance)
    1. Proposed Terminology Changes
    2. Special Considerations for Comparability of PM2.5 
Ambient Air Monitoring Data to the NAAQS
    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. Proposed Changes to Monitoring for the National Ambient Air 
Monitoring System
    a. Background
    b. Primary PM2.5 NAAQS
    i. Proposed 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 Requirements at NCore Sites
    d. Measurements for the Proposed New PM2.5 Visibility 
Index NAAQS
    4. Proposed 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. Proposed 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
    B. Section 110(a)(2) Infrastructure SIP Requirements
    C. Implementing the Proposed Revised Primary Annual 
PM2.5 NAAQS in Nonattainment Areas
    D. Implementing the Primary and Secondary PM10 NAAQS
    E. Implementing the Proposed New PM2.5 Visibility 
Index NAAQS in Nonattainment Areas
    F. Prevention of Significant Deterioration and Nonattainment New 
Source Review Programs for the Proposed Revised Primary Annual 
PM2.5 NAAQS and the Proposed New Secondary 
PM2.5 Visibility Index NAAQS
    1. Prevention of Significant Deterioration
    a. Grandfathering Provision
    b. Recent Guidance Applicable to the Proposed Revised Primary 
Annual PM2.5 NAAQS
    c. Surrogacy Approach for the Proposed New Secondary 
PM2.5 Visibility Index NAAQS
    d. PSD Screening Provisions: Significant Emissions Rates, 
Significant Impact Levels, and Significant Modeling Concentration
    e. PSD Increments
    2. Nonattainment New Source Review
    G. Transportation Conformity Program
    H. 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
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 proposed rulemaking is being done pursuant to these 
statutory requirements. The schedule for this proposed rule is set out 
in a court order.
    In 2006, the EPA completed the 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

[[Page 38893]]

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 [mu]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. (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 EPA because EPA 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 is responding to the court's remands as part of the 
current review of the PM NAAQS.
    This review was initiated in June 2007. Between 2007 and 2011, 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 peer reviewed 
by EPA's Clean Air Scientific Advisory Committee (CASAC). This proposed 
rulemaking is the next step in the review process.
    In this rulemaking, the EPA proposes to make revisions to the suite 
of primary and secondary standards for PM to provide increased 
protection of public health and welfare. We also discuss EPA's current 
perspectives on implementation issues related to the proposed revisions 
to the PM NAAQS. The EPA proposes revisions to the Prevention of 
Significant Deterioration (PSD) permitting regulations to address the 
proposed changes in the primary and secondary PM NAAQS. The EPA also 
proposes an approach for implementing the PSD program specifically for 
the proposed secondary standard. The EPA is also proposing to update 
the Air Quality Index (AQI) for PM2.5 and to make changes in 
the data handling conventions for PM and ambient air monitoring, 
reporting, and network design requirements to correspond with the 
proposed changes to the standards.

B. Summary of Major Provisions

    With regard to the primary standards for fine particles, EPA 
proposes to revise the annual PM2.5 standard by lowering the 
level from 15.0 to within a range of 12.0 to 13.0 [mu]g/m\3\ so as to 
provide increased protection against health effects associated with 
long- and short-term exposures. The EPA proposes to retain the level 
(35 [mu]g/m\3\) and the form (98th percentile) of the 24-hour 
PM2.5 standard to provide supplemental protection against 
health effects associated with short-term exposures. This proposed 
action would provide 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 proposes to eliminate spatial averaging 
provisions as part of the form of the annual standard to avoid 
potential disproportionate impacts on at-risk populations.
    The proposed changes to the primary annual PM2.5 
standard are within the range that CASAC advised the Agency to 
consider. These changes 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 proposed changes also 
reflect 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 proposes to conclude 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 the 
proposed revisions are warranted to provide the appropriate degree of 
increased public health protection. The EPA solicits comment on all 
aspects of the proposed primary PM2.5 standards.
    With regard to the primary standard for coarse particles, EPA 
proposes to retain 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 proposes to conclude 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 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. The 
Administrator welcomes the public's views on these approaches to 
considering and accounting for the evidence and its limitations and 
uncertainties.
    With regard to the secondary PM standards, the EPA proposes to 
revise the suite of secondary PM standards by adding a distinct 
standard for PM2.5 to address PM-related visibility 
impairment. More specifically, the EPA proposes to establish a 
secondary standard 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, 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 deciviews (dv) or 28 dv. The EPA also 
proposes to rely upon the existing Chemical Speciation Network (CSN) to 
provide appropriate monitoring data for calculating PM2.5 
visibility index values.
    The proposed secondary standard is based on the long-standing 
science characterizing the contribution of PM, especially fine 
particles, to visibility impairment and on air quality analyses, with 
consideration also given to a reanalysis of public perception surveys 
regarding people's stated preferences regarding acceptable and 
unacceptable visual air quality. Based on this information, the 
Administrator proposes to conclude that the current secondary 
PM2.5 standards are not sufficiently protective of the 
public welfare with respect to visual air quality. The EPA solicits 
comment on all aspects of the proposed secondary standard.
    To address other non-visibility welfare effects including 
ecological effects, effects on materials, and climate impacts, the EPA 
proposes to retain the current suite of secondary PM standards 
generally, while proposing to revise only the form of the secondary 
annual PM2.5 standard to remove the option for spatial 
averaging consistent with this

[[Page 38894]]

proposed change to the primary annual PM2.5 standard.
    The proposed revisions to the PM NAAQS would 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 
proposed new or revised NAAQS for PM2.5. States will also 
review, modify and supplement their existing state implementation 
plans. The proposed NAAQS revisions would affect the applicable air 
permitting requirements and the transportation conformity and general 
conformity processes. This notice provides background information for 
understanding the implications of the proposed NAAQS revisions for 
these implementation processes and describes and requests comment on 
EPA's current perspectives on implementation issues. In addition, the 
EPA proposes to revise its PSD regulations to provide limited 
grandfathering from the requirements that result from the revised PM 
NAAQS for permit applications for which the public comment period has 
begun when the revised PM NAAQS take effect. The EPA also proposes to 
implement a surrogate approach that would provide a mechanism for 
permit applicants to demonstrate that they will not cause or contribute 
to a violation of the proposed secondary PM2.5 visibility 
index NAAQS. It is the EPA's intention to finalize any time-sensitive 
revisions to its PSD regulations at the same time as any new or revised 
NAAQS are finalized.
    With regard to implementation-related activities, the EPA intends 
to promulgate rules or develop guidance related to NAAQS implementation 
on a schedule that provides timely clarity to the states, tribes, and 
other parties responsible for NAAQS implementation. The EPA solicits 
comment on all implementation aspects during the public comment period 
for this notice and will consider these comments as it develops future 
rulemaking or guidance, as appropriate.
    On other topics, the EPA proposes changes to the Air Quality Index 
(AQI) for PM2.5 to be consistent with the proposed primary 
PM2.5 standards. The EPA also proposes revisions to the data 
handling procedures consistent with the proposed 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 proposes updates to several 
aspects of the monitoring regulations and specifically proposes 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.

C. Costs and Benefits

    In setting the NAAQS, the EPA may not consider the costs of 
implementing the standards. This was confirmed by the Supreme Court in 
Whitman v. American Trucking Associations, 531 U.S. 457, 465-472, 475-
76 (2001), as discussed in section II.A of this notice. 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 proposed 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 $88 million to $220 million (for 13.0 [mu]g/m\3\) and from 
$2.3 billion to $5.9 billion per year (for 12.0 [mu]g/m\3\) in 2020 and 
costs ranging from $2.9 million (for 13.0 [mu]g/m\3\) to $69 million 
(for 12.0 [mu]g/m\3\) per year.

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 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.''
---------------------------------------------------------------------------

    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

[[Page 38895]]

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 sensitive population(s) at risk, and the 
kind and degree of the uncertainties that must be addressed. The 
selection of any particular approach to providing an adequate margin of 
safety is a policy choice left specifically to the Administrator's 
judgment. 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), 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 
Clean Air Scientific Advisory Committee (CASAC).\3\
---------------------------------------------------------------------------

    \3\ Lists of CASAC members and of members of the CASAC PM Review 
Panel are available at: http://yosemite.epa.gov/sab/sabproduct.nsf/WebCASAC/CommitteesandMembership?OpenDocument.
---------------------------------------------------------------------------

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.
---------------------------------------------------------------------------

    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 [mu]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).

           Table 1--Summary of National Ambient Air Quality Standards Promulgated for PM 1971-2006 \5\
----------------------------------------------------------------------------------------------------------------
          Final rule                 Indicator        Averaging time          Level                 Form
----------------------------------------------------------------------------------------------------------------
1971--36 FR 8186 April 30,      TSP...............  24-hour...........  260 [mu]g/m\3\     Not to be exceeded
 1971.                                                                   (primary), 150     more than once per
                                                                         [mu]g/m\3\         year.
                                                                         (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
 1987.                                                                                      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.
1997--62 FR 38652, July 18,     PM2.5.............  24-hour...........  65 [mu]g/m\3\....  98th percentile,
 1997.                                                                                      averaged over 3
                                                                                            years.\6\
                                                    Annual............  15.0 [mu]g/m\3\..  Annual arithmetic
                                                                                            mean, averaged over
                                                                                            3 years.7 8
                                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 17,  PM2.5.............  24-hour...........  35 [mu]g/m\3\....  98th percentile,
 2006.                                                                                      averaged over 3
                                                                                            years.\6\
                                                    Annual............  15.0 [mu]g/m\3\..  Annual arithmetic
                                                                                            mean, averaged over
                                                                                            3 years.\7\ \9\

[[Page 38896]]

 
                                PM10..............  24-hour...........  150 [mu]g/m\3\...  Not to be exceeded
                                                                                            more than once per
                                                                                            year on average over
                                                                                            a 3-year period.
----------------------------------------------------------------------------------------------------------------

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

    \5\ When not specified, primary and secondary standards are 
identical.
    \6\ 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).
    \7\ 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).
    \8\ 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'').
    \9\ 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 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).\10\ The EPA 
established two new PM2.5 standards: an annual standard of 
15 [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 \11\; 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 \12\ 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.
---------------------------------------------------------------------------

    \10\ See 40 CFR parts 50, 53, and 58 for more information on 
reference and equivalent methods for measuring PM in ambient air.
    \11\ 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)].
    \12\ 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 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 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 (DC Cir. 
1999), rehearing granted in part and denied in part, 195 F. 3d 4 (DC 
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 EPA's decision to regulate coarse particle 
pollution, but vacated the 1997 PM10 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 PM10 standards from the CFR (69 FR 45592, July 
30, 2004) and deleted the regulatory provision [at 40 CFR section 
50.6(d)] that controlled the transition from the pre-existing 1987 
PM10 standards to the 1997 PM10 standards. The 
pre-existing 1987 PM10 standards remained in place (65 FR 
80776, December 22, 2000). The court also upheld 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 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 EPA's long-standing interpretation and DC 
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 (DC 
Cir. 1999). The full Court of Appeals denied 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 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 EPA's 
discretion, affirming EPA's approach of setting standards that are 
neither more nor less stringent than necessary. The Supreme Court 
remanded the case to the Court of Appeals for resolution of any 
remaining issues that had not been addressed in

[[Page 38897]]

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 EPA's PM2.5 
standards were reasonably supported by the administrative record and 
were not ``arbitrary and capricious.'' American Trucking Associations 
v. EPA, 283 F. 3d 355, 369-72 (DC 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, 
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 their 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 in the range of 35 to 
30 [micro]g/m\3\ with a 98th percentile form, in concert with revising 
the level of the primary annual PM2.5 standard in the range 
of 14 to 13 [micro]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 [micro]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, this proposed indicator 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 [micro]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.\13\ 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).
---------------------------------------------------------------------------

    \13\ 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 [micro]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 promulgated 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 [micro]g/m\3\, retained the level of 
the primary annual PM2.5 standard at 15 [micro]g/m\3\, and 
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 [micro]g/m\3\ with a 
one-expected exceedance form), 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 
61212-13). 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 ``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 [micro]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).\14\ 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 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 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).
---------------------------------------------------------------------------

    \14\ 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).

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

[[Page 38898]]

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 
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. 3d 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. 
3d 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).\15\ 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.
---------------------------------------------------------------------------

    \15\ 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.16 17
---------------------------------------------------------------------------

    \16\ 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, EPA 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, 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).
    \17\ 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.
---------------------------------------------------------------------------

    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 with 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). The first external review draft 
Integrated Science Assessment (U.S. EPA, 2008b; 73 FR 77686, December 
19, 2008) was reviewed by CASAC and the public 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. 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) were 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 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,

[[Page 38899]]

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, 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 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. CASAC's comments on 
this preliminary draft were considered in developing a first draft PA 
(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. The second draft 
document was reviewed by CASAC at a meeting on July 26 and 27, 2010 (75 
FR 32763, June 9, 2010). CASAC (Samet, 2010d) and public comments on 
the second draft Policy Assessment were considered by EPA staff 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 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. The court order, entered on June 
2, 2012 and amended on June 6, 2012, provides that EPA will sign, for 
publication, a notice of proposed rulemaking concerning its review of 
the PM NAAQS no later than June 14, 2012.
    The EPA is aware that a number of new scientific studies on the 
health effects of PM have been published since the mid-2009 cutoff date 
for inclusion in the Integrated Science Assessment. As in the last PM 
NAAQS review, the EPA intends to conduct a provisional review and 
assessment of any significant new studies published since the close of 
the Integrated Science Assessment, including studies that may be 
submitted during the public comment period on this proposed rule in 
order to ensure that, before making a final decision, the Administrator 
is fully aware of the new science that has developed since 2009. In 
this provisional assessment, the EPA will examine these new studies in 
light of the literature evaluated in the Integrated Science Assessment. 
This provisional assessment and a summary of the key conclusions will 
be placed in the rulemaking docket.
    Today's action presents the Administrator's proposed decisions on 
the current PM standards. Throughout this preamble there are a number 
of conclusions, findings, and determinations that are part of the 
rationales for the decisions proposed by the Administrator. They are 
referred to throughout as ``provisional'' conclusions, findings, and 
determinations to reflect that they are not intended to be final or 
conclusive but rather proposals for public comment. The EPA invites 
general, specific, and technical comments on all issues involved with 
this proposal, including all such proposed decisions and provisional 
conclusions, findings, and determinations.

C. Related Control Programs To Implement PM Standards

    States are primarily responsible for ensuring attainment and 
maintenance of ambient air quality standards once the EPA has 
established them. Under section 110 of the CAA, and related provisions, 
states are to submit, for EPA's approval, state implementation plans 
(SIPs) that provide for the attainment and maintenance of such 
standards through control programs directed to sources of the 
pollutants involved. The states, in conjunction with the EPA, also 
administer the PSD 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 for stationary sources under section 111 of the 
CAA.
    Currently, there are 55 areas in the U.S. (with a population of 
more than 100 million) that are designated as nonattainment for either 
the annual or 24-hour PM2.5 standards. Regarding the 1997 
PM2.5 standards, the EPA designated 39 nonattainment areas 
in 2005. Regarding the 2006 24-hour PM2.5 standard, the EPA 
designated 31 areas in 2009 and added one area in 2010. Sixteen areas 
are currently designated as nonattainment for both the 1997 and 2006 
PM2.5 standards. With regard to the PM10 NAAQS, 
45 areas (with a population of more than 25 million) are currently 
designated as nonattainment. Upon any revisions to the PM NAAQS, the 
EPA would work with the states to conduct a new area designation 
process. Upon designation of new nonattainment areas, certain states 
would then be required to develop SIPs to attain the standards. In 
developing their attainment plans, states would first take into account 
projected emission reductions from federal and state rules that have 
been already adopted at the time of plan submittal. A number of 
significant emission reduction programs that will lead to reductions of 
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 
the Transport Rule for electric generating units, 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 would 
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

[[Page 38900]]

regulations as appropriate. Section IX includes additional discussion 
of designation and implementation issues associated with any revised PM 
NAAQS.

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

    This section presents the rationale for the Administrator's 
proposed decision to revise the level and form of the existing primary 
annual PM2.5 standard and to retain the existing primary 24-
hour PM2.5 standard. As discussed more fully below, this 
rationale 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. This proposal also 
takes 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; and (3) public comments received 
during the development of these documents, either in connection with 
CASAC meetings or separately.
    In developing this proposal, the Administrator recognizes that the 
CAA requires her to reach a public health policy judgment as to what 
standards would be requisite 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 standards to propose, 
and subsequently 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, 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 well 
reflect different demographic and air pollution characteristics.\18\
---------------------------------------------------------------------------

    \18\ 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 proposed rule, the EPA has drawn upon an integrative 
synthesis of the entire body of evidence between 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 in taking into account potential impacts on 
at-risk populations\19\ and in considering alternative standard levels 
that provide protection with an appropriate margin of safety.
---------------------------------------------------------------------------

    \19\ In this proposal, the term ``at-risk'' is the broadly 
encompassing term used for groups with specific factors that 
increase the risk of PM-related health effects in a population. In 
the Integrated Science Assessment, as discussed in section III.B.3 
below, the term ``susceptibility'' was used broadly to recognize 
populations at greater risk.
---------------------------------------------------------------------------

    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 opportunities for public review and comment.
    Although important uncertainties remain in the qualitative and 
quantitative characterizations of health effects attributable to 
ambient fine particles, the review of this information has been 
extensive and deliberate. This intensive evaluation of the scientific 
evidence and quantitative assessments has provided an 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 potential 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 will draw 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 this proposal.
    In presenting the rationale for the proposed revisions of 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 those standards in 2006 (section 
III.A.1). The D.C. 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 history, section II.A.3 
describes EPA's general approach used in the current review for 
considering the need to retain or revise the current suite of fine 
particle standards. Section III.B summarizes the body of scientific 
evidence supporting the rationale for the proposed decisions, including 
key health endpoints associated with long- and short-term exposures to 
ambient fine particles. This overview includes a discussion of at-risk 
populations and potential PM2.5-related impacts on public 
health. Section III.C outlines the approach taken by the EPA to assess 
health risks associated with exposure to ambient PM2.5, 
including a discussion of key uncertainties and limitations associated 
with these analyses. Section III.D discusses the scientific evidence, 
air quality, risk-based information; CASAC advice; and the 
Administrator's proposed decisions related to the adequacy of the 
current standards. Section III.E discusses the scientific evidence, air 
quality, and risk-based information; CASAC advice; and the

[[Page 38901]]

Administrator's proposed decisions related to alternative standards. 
Section III.F summarizes the Administrator's proposed decisions with 
regard to the primary PM2.5 NAAQS.

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 PM 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, 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. 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. 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.
1. General Approach Used in Previous Reviews
    The general approach used to translate scientific information into 
standards used in the previous reviews focused on consideration of 
alternative standard levels that were somewhat below the long-term mean 
PM2.5 concentrations reported in epidemiological studies 
(U.S. EPA, 2011a, section 2.1.1). This approach recognized that the 
strongest evidence of PM2.5-related associations occurs at 
concentrations near 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.\20\ 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 a national annual 
standard (62 FR 38669/3).
---------------------------------------------------------------------------

    \20\ 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 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 both short- 
and long-term exposure studies, 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 [mu]g/m\3\ to protect against effects associated with short-term 
exposures. The EPA relied upon evidence from long-term exposure studies 
as the principal basis for retaining the level of the annual 
PM2.5 standard at 15 [mu]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

[[Page 38902]]

any of the litigants and, thus, 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 PM NAAQS decisions, as summarized in section III.A.1. 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 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 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, 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 is broader and more integrative than in past reviews. Our 
general approach also reflects consideration of the issues raised by 
the court in its remand of the primary annual PM2.5 
standard, 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 takes 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, EPA staff developed a final Policy 
Assessment (U.S. EPA, 2011a) which provides as broad an array of policy 
options as is supportable 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 presented in this document.
    The Policy Assessment concludes it is 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).\21\ As the 
EPA recognized in 1997, 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.
---------------------------------------------------------------------------

    \21\ By utilizing this approach, the Agency would also be 
responsive to the remand of the 2006 standard. As noted in section 
III.A.2, the DC 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 recognizes 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 recognizes 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

[[Page 38903]]

recognizes that the short-term exposure studies are 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 is 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 demonstrate 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 is no evidence suggesting that 
risks associated with long-term exposures are likely to be 
disproportionately driven by peak 24-hour concentrations.\22\ For these 
reasons, strategies that focus primarily on reducing peak days are less 
likely to achieve reductions in the PM2.5 concentrations 
that are most strongly associated with the observed health effects.
---------------------------------------------------------------------------

    \22\ In confirmation, a number of studies that have presented 
analyses excluding higher PM concentration days 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).
---------------------------------------------------------------------------

    Furthermore, a policy approach that focuses 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, 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 
discussed above.
    The Policy Assessment concludes 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, is 
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 concludes, however, 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 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 concludes it is 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 focuses 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 is 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 considers effects and estimated risks 
associated with short-term exposures. In both cases, the Policy 
Assessment places greatest weight on health effects that have 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 focuses on specific at-risk populations. 
The Policy Assessment places relatively greater weight on statistically 
significant associations that yield relatively more precise effect 
estimates and that are judged to be robust to confounding by other air 
pollutants. In the case of short-term exposure studies, the Policy 
Assessment places 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 considers a number of factors 
(U.S. EPA, 2011a, section 2.2). As an initial matter, the Policy 
Assessment considers 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 
considers evidence on at-risk populations and potential impacts on such 
populations. Further, the Policy Assessment explores the extent to 
which PM2.5-related health effects have been observed in 
areas where air quality distributions extend to lower levels 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 alternative standard levels for 
consideration (U.S. EPA, 2011a, sections 2.1.3 and 2.3.4), the Policy 
Assessment first recognizes 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).\23\ 
In the absence of any discernible thresholds, the Agency's general 
approach for identifying appropriate standard levels for consideration 
involves 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 recognizes 
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 decisions to either retain or 
revise the standards. Final decisions will necessarily also take into 
account

[[Page 38904]]

public health policy judgments as to the degree of health protection 
that is to be achieved.
---------------------------------------------------------------------------

    \23\ 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 concludes that the 
studies evaluated support the use of a no-threshold, log-linear 
model but recognizes 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 is appropriate to consider, the Policy Assessment focuses 
on identifying an annual standard that provides requisite protection 
from effects associated with both long- and short-term exposures. In so 
doing, the Policy Assessment explores 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.
    The approach that most directly addresses this issue considers 
studies that present confidence intervals around concentration-response 
relationships, and in particular, analyses that average across multiple 
concentration-response models rather than considering a single 
concentration-response model.\24\ The Policy Assessment explores the 
extent to which such analyses have 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 concludes 
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.
---------------------------------------------------------------------------

    \24\ 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 explores other approaches that 
consider different statistical metrics from epidemiological studies. 
The Policy Assessment first takes 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.\25\ This approach recognizes that the strongest evidence of 
PM2.5-related associations occurs at concentrations near the 
long-term (i.e., annual) mean. In using this approach, the Policy 
Assessment places greatest weight on those long- and short-term 
exposure studies that reported statistically significant associations 
with mortality and morbidity effects.
---------------------------------------------------------------------------

    \25\ Epidemiological studies typically report PM2.5 
concentrations averaged across the available ambient monitors. For 
multi-city studies, this metric reflects concentrations averaged 
across one or more ambient monitors within each area included in a 
given study 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 considers 
information beyond a single statistical metric of PM2.5 
concentrations (i.e., the mean) to the extent such information is 
available. In so doing, the Policy Assessment employs 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 epidemiological studies. Thus, the Policy 
Assessment considers the range of PM2.5 concentrations where 
the data analyzed in the study (i.e., air quality and population-level 
data, as discussed below) are most concentrated, specifically, the 
range of PM2.5 concentrations around the long-term mean over 
which our confidence in the associations observed in the 
epidemiological studies is greatest. The Policy Assessment then focuses 
on the lower part of this range to characterize where in the 
distributions the data become appreciably more sparse and, thus, where 
our understanding of the associations correspondingly becomes more 
uncertain. The Policy Assessment recognizes there is no one percentile 
value within a given distribution that is the most appropriate or 
``correct'' way to characterize where our confidence in the 
associations becomes appreciably lower. The Policy Assessment concludes 
that the range from the 25th to 10th percentiles is a reasonable range 
to consider as a region where we have appreciably less confidence in 
the associations observed in epidemiological studies.\26\
---------------------------------------------------------------------------

    \26\ In the PM NAAQS review completed in 2006, the Staff Paper 
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 advices are most useful to consider 
in identifying the PM2.5 concentrations most influential in 
generating the health effect estimates reported in the epidemiological 
studies.\27\ Consistent with CASAC advice, the most relevant 
information is 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, as 
discussed in section III.E.4.b below, the Policy Assessment also 
considers the number of study participants within each study area as an 
appropriate surrogate for health event data.
---------------------------------------------------------------------------

    \27\ The second draft Policy Assessment focused on the 
distributions of PM2.5 concentrations 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, EPA considered area-specific health event and area-
specific population data along with corresponding PM2.5 
concentrations to generate a cumulative distribution of the 
population data relative to long-term mean PM2.5 
concentrations to determine the most influential range (U.S. EPA, 
2011a, Figure 2-7 and associated text).
---------------------------------------------------------------------------

    The Policy Assessment recognizes that an approach considering 
analyses of confidence intervals around concentration-response 
functions is intrinsically related to an approach that considers 
different distributional statistics. Both of these approaches could be 
employed to identify the range of PM2.5 concentrations over 
which we have the most confidence in the associations reported in 
epidemiological studies.
    In applying these approaches, the Policy Assessment considers 
PM2.5 concentrations from long- and short-term 
PM2.5 exposure studies using composite monitor 
distributions.\28\ For multi-city studies, this distribution reflects 
concentrations averaged across one or more ambient monitors within

[[Page 38905]]

each area included in a given study and then averaged across study 
areas for an overall study mean PM2.5 concentration. Beyond 
considering air quality concentrations based on composite monitor 
distributions, the second draft Policy Assessment also considered 
PM2.5 concentrations based on measurements at the monitor 
within each area that records the highest concentration (i.e., maximum 
monitor) (U.S. EPA, 2010f, sections 2.1.3 and 2.3.4.1).\29\ Although 
the second draft Policy Assessment discussed whether consideration of 
alternative annual standard levels should be based on composite or 
maximum monitor distributions, the final Policy Assessment, consistent 
with CASAC advice (Samet, 2010d, p. 3), concluded that it is most 
reasonable to place more weight on an approach based on composite 
monitor distributions, which represent the PM2.5 
concentrations typically presented and used in epidemiological analyses 
and which provide a direct link between PM2.5 concentrations 
and the observed health effects reported in both long- and short-term 
exposure studies (U.S. EPA, 2011a, p. 2-13).
---------------------------------------------------------------------------

    \28\ 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, as noted above, the 
composite monitor concentration represents the average concentration 
across one or more monitors within each area included in a given 
study and then averaged across study areas for an overall study mean 
PM2.5 concentration.
    \29\ The maximum monitor distribution is relevant because it is 
generally used to determine whether a given standard is met in an 
area and the extent to which ambient PM2.5 concentrations 
need to be reduced in order to bring an area into attainment with 
the standard. However, maximum monitor distributions represent a far 
less robust metric than composite monitor distributions for 
consideration of alternative annual standard levels in part because 
they are available for only a few epidemiological studies.
---------------------------------------------------------------------------

    In reaching staff conclusions on alternative standard levels that 
are appropriate to consider, the Policy Assessment also includes a 
broader consideration of the uncertainties related to the 
concentration-response relationships from multi-city, long- and short-
term exposure studies. Most notably, these uncertainties relate to our 
currently limited understanding of the heterogeneity of relative risk 
estimates in areas across the country. This heterogeneity may be 
attributed, in part, to the potential for different components within 
the mix of ambient fine particles to differentially contribute to 
health effects observed in the studies and to exposure-related factors 
(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 further 
discussed in section III.B.2 below.
    The Policy Assessment recognizes that the level of protection 
afforded by the NAAQS relies both on the level and the form of the 
standard. The Policy Assessment concludes that a policy approach that 
uses data based on composite monitor distributions to identify 
alternative standard levels, and then compares those levels to 
concentrations at maximum monitors to determine if an area meets a 
given standard, inherently has the potential to build in some margin of 
safety (U.S. EPA, 2011a, p. 2-14).\30\ This conclusion is 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).
---------------------------------------------------------------------------

    \30\ 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. 
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 and the composite 
monitor distributions may be well below the maximum monitor 
distributions (U.S. EPA, 2011a, p. 2-14).
---------------------------------------------------------------------------

    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 informs 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 evaluates both the distributions of 24-hour PM2.5 
concentrations, with a focus on the 98th percentile concentrations to 
match the form of the current 24-hour PM2.5 standard, to the 
extent such data were available, 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 
considers air quality information based on county-level 24-hour and 
annual design values \31\ to understand the policy implications of the 
alternative standard levels supported by the underlying science. In 
particular, the Policy Assessment considers 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. Based on the evidence-based 
considerations outlined above, the Policy Assessment develops 
integrated conclusions with regard to alternative suites of standards, 
including both annual and 24-hour standards that are appropriate to 
consider in this review based on the currently available evidence and 
air quality information. In so doing, the Policy Assessment discusses 
the roles that each standard might be expected to play in the 
protection afforded by alternative suites of standards.
---------------------------------------------------------------------------

    \31\ 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 considers the quantitative risk estimates and the key observations 
presented in the Risk Assessment. This assessment includes 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 considers 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 characterizes 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 recognizes the 
uncertainties inherent in such risk estimates, and takes 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 considers 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

[[Page 38906]]

related to both long- and short-term PM2.5 exposures.
    The Policy Assessment conclusions related to the primary 
PM2.5 standards reflect an 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) potential alternative standards, if any, that are 
appropriate to consider in this review to protect against effects 
associated with both long- and short-term exposures to fine particles. 
In addressing these broad questions, the discussions included in the 
Policy Assessment were organized around a series of more specific 
questions reflecting different aspects of each overarching question 
(U.S. EPA, 2011a, chapter 2, Figure 2-1). When evaluating the health 
protection afforded by the current or any alternative suites of 
standards considered, the Policy Assessment takes 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 provides a comprehensive basis to help inform 
the judgments required of the Administrator in reaching decisions about 
the current and potential alternative primary fine particle standards 
and in responding to the remand of the 2006 primary annual 
PM2.5 standard.

B. Health Effects Related to Exposure to Fine Particles

    This section outlines key information contained 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. As was true in the last review, evidence from 
epidemiologic studies plays a key role in the Integrated Science 
Assessment's evaluation of the scientific evidence. The following 
sections discuss 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).
1. Nature of Effects
    In considering the strength of the associations between long- and 
short-term exposures to PM2.5 and health effects, the Policy 
Assessment notes that in the PM NAAQS review completed in 2006 the 
Agency 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 epidemiological 
evidence 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).\32\
---------------------------------------------------------------------------

    \32\ The term ``likely causal association'' was used in the 2004 
Criteria Document to summarize the strength of the available 
epidemiological 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 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 (U.S. EPA, 
2009a, sections 2.3.1 and 2.3.2). The currently available evidence is 
stronger in comparison to evidence available in the last review because 
of its breadth and the substantiation of previously observed health 
effects. A number of large multi-city epidemiological studies have been 
conducted throughout the U.S., including extended analyses of studies 
that were important to inform decision-making in the last review. These 
studies have reported consistent increases in morbidity and/or 
mortality related to ambient PM2.5 concentrations, with the 
strongest evidence reported for cardiovascular-related effects. In 
addition, the findings of new toxicological and controlled human 
exposure studies greatly expand and provide stronger support for a 
number of potential biologic mechanisms or pathways for cardiovascular 
and respiratory effects associated with long- and short-term PM 
exposures (U.S. EPA, 2009a, p. 2-17; chapter 5; Figures 5-4 and 5-5).
    With regard to causal inferences described in the Integrated 
Science Assessment, the Policy Assessment notes that since the last 
review, the Agency has developed a more formal framework for reaching 
causal determinations that 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 (U.S. EPA, 2011a, p. 2-18; 
U.S. EPA, 2009a, section 1.5). 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 
a 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).\33\
---------------------------------------------------------------------------

    \33\ Causal inferences, as discussed in the Integrated Science 
Assessment, 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).
---------------------------------------------------------------------------

    Using this causal framework, the Integrated Science Assessment 
concludes that the collective evidence is largely consistent with past 
studies and substantially strengthens what was known about fine 
particles in the last review to reach the conclusion that a causal 
relationship exists between both long- and short-term exposures to 
PM2.5 and mortality and cardiovascular effects including 
cardiovascular-related mortality. The Integrated Science Assessment 
also concludes that the collective evidence continues to support a 
likely causal relationship between long- and short-term 
PM2.5 exposures and respiratory effects, including 
respiratory-related mortality. Further, the Integrated Science 
Assessment concludes that the currently available evidence is 
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) (U.S. EPA, 2009a, sections 2.3.1 and 2.6; Table 2-6; U.S. 
EPA, 2011a, Table 2-1).
a. Health Effects Associated With Long-Term PM2.5 Exposures
    With regard to mortality, the Integrated Science Assessment 
concludes that newly available evidence significantly strengthens the 
link between long-term exposure to PM2.5 and mortality, 
while providing indications that the magnitude of the PM2.5-
mortality association may be larger than previously estimated (U.S. 
EPA, 2009a, sections 7.2.10, 7.2.11, and 7.6.1; Figures 7-6 and 7-7). A 
number of large U.S. cohort studies have been published since the last 
review, including extended analyses of the

[[Page 38907]]

American Cancer Society (ACS) and Harvard Six Cities studies (U.S. EPA, 
2009a, pp. 7-84 to 7-85; Figure 7-6; Krewski et al., 2009; Pope et al., 
2004; Jerrett et al., 2005; Laden et al., 2006). In addition, new long-
term PM2.5 exposure studies evaluating mortality impacts in 
additional cohorts are now available (U.S. EPA, 2009a, section 7.6). 
For example, the Women's Health Initiative (WHI) Observational Study 
reported effects of PM2.5 on cardiovascular-related 
mortality in post-menopausal women with no previous history of cardiac 
disease (Miller et al., 2007). The PM2.5 effect estimate in 
this study remained positive and statistically significant in a multi-
pollutant model that included gaseous co-pollutants as well as coarse 
particles. In addition, multiple studies observed PM2.5-
associated mortality among older adults using Medicare data (Eftim et 
al., 2008; Zeger et al., 2007, 2008). Collectively, these new studies, 
along with evidence available in the last review, provide consistent 
and stronger evidence of associations between long-term exposure to 
PM2.5 and mortality (U.S. EPA, 2009a, sections 2.3.1 and 
7.6).
    The strength of the causal relationship between long-term 
PM2.5 exposure and mortality also builds upon new studies 
providing evidence of improvement in community health following 
reductions in ambient fine particles. Pope et al. (2009) documented the 
population health benefits of reducing ambient air pollution by 
correlating past reductions in ambient PM2.5 concentrations 
with increased life expectancy. These investigators reported that 
reductions in ambient fine particles during the 1980s and 1990s account 
for as much as 15 percent of the overall improvement in life expectancy 
in 51 U.S. metropolitan areas, with the fine particle reductions 
reported to be associated with an estimated increase in mean life 
expectancy of approximately 5 to 9 months (U.S. EPA, 2009a, p. 7-95; 
Pope et al., 2009). An extended analysis of the Harvard Six Cities 
study found that as cities cleaned up their air, locations with the 
largest reductions in PM2.5 saw the largest improvements in 
reduced mortality rates, while those with the smallest decreases in 
PM2.5 concentrations saw the smallest improvements (Laden et 
al., 2006). Another extended follow-up to the Harvard Six Cities study 
investigated the delay between changes in ambient PM2.5 
concentrations and changes in mortality (Schwartz et al., 2008) and 
reported that the effects of changes in PM2.5 were seen 
within the 2 years prior to death (U.S. EPA, 2009a, p. 7-92; Figure 7-
9).
    With regard to cardiovascular effects, 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. The Integrated Science Assessment concludes that the 
strongest evidence comes from recent studies investigating 
cardiovascular-related mortality. This includes evidence from a number 
of large, multi-city U.S. long-term cohort studies including extended 
follow-up analyses of the ACS and Harvard Six Cities studies, as well 
as the WHI study (U.S. EPA, 2009a, sections 7.2.10 and 7.6.1; Krewski 
et al., 2009; Pope et al., 2004; Laden et al., 2006; Miller et al., 
2007). Pope et al. (2004) reported a positive association between 
mortality and long-term PM2.5 exposure for a number of 
specific cardiovascular diseases, including ischemic heart disease, 
dysrhythmia, heart failure, and cardiac arrest (U.S. EPA, 2009a, p. 7-
84; Figure 7-7). Krewski et al. (2009) provides further evidence for 
mortality related to ischemic heart disease associated with long-term 
PM2.5 exposure (U.S. EPA, 2009a, p. 7-84, Figure 7-7).
    With regard to cardiovascular-related morbidity associated with 
long-term PM2.5 exposures, studies were not available in the 
last review. Recent studies, however, have provided new evidence 
linking long-term exposure to PM2.5 with cardiovascular 
outcomes that has ``expanded upon the continuum of effects ranging from 
the more subtle subclinical measures to cardiopulmonary mortality'' 
(U.S. EPA, 2009a, p. 2-17). In the current review, studies are now 
available that evaluated a number of endpoints ranging from subtle 
indicators of cardiovascular health to serious clinical events 
associated with coronary heart disease and cardiovascular and 
cerebrovascular disease.\34\ The most important new evidence comes from 
the WHI study which provides evidence of nonfatal cardiovascular events 
including both coronary and cerebrovascular events (Miller et al., 
2007; U.S. EPA, 2009a, sections 7.2.9 and 7.6.1). Toxicological studies 
provide supportive evidence that the cardiovascular morbidity effects 
observed in long-term exposure epidemiological studies are biologically 
plausible and coherent with studies of cardiovascular-related mortality 
as well as with studies of cardiovascular-related effects associated 
with short-term exposures to PM2.5, as described below (U.S. 
EPA, 2009a, p. 7-19).
---------------------------------------------------------------------------

    \34\ Coronary and cerebrovascular events include myocardial 
infarction, coronary artery revascularization (e.g., bypass graft, 
angioplasty, stent, atherectomy), congestive heart failure and 
stroke.
---------------------------------------------------------------------------

    With regard to respiratory effects, the Integrated Science 
Assessment concludes that 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 associated with long-term PM2.5 exposure. The 
strongest evidence for respiratory-related effects available in this 
review is from studies that evaluated decrements in lung function 
growth, increased respiratory symptoms, and asthma development (U.S. 
EPA, 2009a, sections 2.3.1.2, 7.3.1.1, and 7.3.2.1).\35\ Specifically, 
extended analyses of the Southern California Children's Health Study 
provide evidence that clinically important deficits in lung function 
\36\ associated with long-term exposure to PM2.5 persist 
into early adulthood (U.S. EPA, 2009a, p. 7-27; Gauderman et al., 
2004). Additional analyses of the 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 24; 
McConnell et al., 2003) 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, 2009, 7-24; Islam et al., 2007). The outcomes 
observed in these 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 previously reported. Supporting these results are new 
longitudinal cohort studies conducted by other researchers in varying 
locations using different methods (U.S. EPA, 2009a, section 7.3.9.1). 
New evidence from a U.S. cohort of cystic fibrosis patients provided 
evidence of association between long-term PM2.5 exposures 
and exacerbations of respiratory symptoms

[[Page 38908]]

resulting in hospital admissions or use of home intravenous antibiotics 
(U.S. EPA, 2009a, p. 7-25; Goss et al., 2004).
---------------------------------------------------------------------------

    \35\ Supporting evidence comes from studies ``that observed 
associations between long-term exposure to PM10 and an 
increase in respiratory symptoms and reductions in lung function 
growth in areas where PM10 is dominated by 
PM2.5'' (U.S. EPA, 2009a, p. 2-12).
    \36\ 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 includes 
this interpretation for FEV1 (75 FR 35525, June 22, 
2010).
---------------------------------------------------------------------------

    Toxicological studies provide coherence and biological plausibility 
for the respiratory effects observed in epidemiological studies (U.S. 
EPA, 2009a, p. 7-42). For example, pre- and postnatal exposure to 
ambient levels of urban particles has been found to affect lung 
development in an animal model (U.S. EPA, 2009a, section 7.3.2.2; p. 7-
43). This finding is important because impaired lung development is one 
mechanism by which PM exposure may decrease lung function growth in 
children (U.S. EPA, 2009a, p. 2-12; section 7.3).
    With regard to respiratory-related mortality associated with long-
term PM2.5 exposure, the Integrated Science Assessment 
concludes that ``when deaths due to respiratory causes are separated 
from all-cause (nonaccidental) and cardiopulmonary deaths, there is 
limited and inconclusive evidence for an effect of PM2.5 on 
respiratory mortality, with one large cohort study finding a reduction 
in deaths due to respiratory causes associated with reduced 
PM2.5 concentrations, and another large cohort study finding 
no PM2.5 associations with respiratory mortality'' (U.S. 
EPA, 2009a, p. 7-41). The extended follow-up of the Harvard Six Cities 
study reported a positive but statistically non-significant association 
between long-term PM2.5 exposure and respiratory-related 
mortality (Laden et al., 2006), whereas Pope et al. (2004) found no 
association in the ACS cohort (U.S. EPA, 2009a, p. 7-84). There is 
emerging but limited evidence for an association between long-term 
PM2.5 exposure and respiratory mortality in post-neonatal 
infants where long-term exposure was considered as approximately one 
month to one year (U.S. EPA, 2009a, pp. 7-54 to 7-55). Emerging 
evidence of short- and long-term exposure to PM2.5 and 
respiratory morbidity and infant mortality provide some support for the 
weak respiratory-related mortality effects observed.
    Beyond effects considered to have causal or likely causal 
relationships with long-term PM2.5 exposure as discussed 
above, the following health outcomes are classified in the Integrated 
Science Assessment as having evidence suggestive of a causal 
relationship with long-term PM2.5 exposure: (1) Reproductive 
and developmental effects and (2) cancer, mutagenicity, and 
genotoxicity effects (U.S. EPA, 2009a, Table 2-6). With regard to 
reproductive and developmental effects, the Integrated Science 
Assessment notes, ``[e]vidence is accumulating for PM2.5-
related effects on low birth weight and infant mortality, especially 
due to respiratory causes during the post-neonatal period'' (U.S. EPA, 
2009a, p. 2-13). New evidence available in this review reports 
significant associations between exposure to PM2.5 during 
pregnancy and lower birth weight and infant mortality, with less 
consistent evidence for pre-term birth and intrauterine growth 
restriction. (U.S. EPA, 2009a, section 7.4). The Integrated Science 
Assessment further notes that ``[i]nfants and fetal development 
processes may be particularly vulnerable to PM exposure, and although 
the physical mechanisms are not fully understood, several hypotheses 
have been proposed involving direct effects on fetal health, altered 
placenta function, or indirect effects on the mother's health'' (U.S. 
EPA, 2009a, section 7.4.1). Although toxicological studies provide some 
evidence that supports an association between long-term 
PM2.5 exposure and adverse reproductive and developmental 
outcomes, there is ``little mechanistic information or biological 
plausibility for an association between long-term PM exposure and 
adverse birth outcomes (e.g., low birth weight, infant mortality)'' 
(U.S. EPA, 2009a, p. 2-13).
    With regard to cancer, mutagenic and genotoxic 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 
and sections 2.3.1.2 and 7.5; Table 7-7; Figures 7-6 and 7-7). The 
extended follow-up to the ACS study reported an association between 
long-term PM2.5 exposure and lung cancer mortality (U.S. 
EPA, 2009a, p. 7-71; Krewski et al., 2009) as did the extended follow-
up to the Harvard Six Cities study when considering the entire 25-year 
follow-up period (Laden et al., 2006). There is some evidence, 
primarily from in vitro studies, providing biological plausibility for 
the PM-lung cancer relationships observed in epidemiological studies 
(U.S. EPA, 2009a, p. 7-80), although in vivo toxicological studies of 
carcinogenicity generally reported mixed results (U.S. EPA, 2009a, 
section 7.5).
b. Health Effects Associated With Short-Term PM2.5 Exposures
    In considering effects associated with short-term PM2.5 
exposure, the body of currently available scientific evidence has 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 fine particle 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 climate and 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 (U.S. EPA 
2009a, chapter 6).
    With regard to mortality, new U.S. and Canadian multi-city and 
single-city PM2.5 exposure studies have found generally 
consistent positive associations between short-term PM2.5 
exposures and cardiovascular- and respiratory-related mortality as well 
as all-cause (non-accidental) mortality (U.S. EPA, 2009a, sections 
2.3.1.1, 6.2.11 and 6.5.2.2; Figures 6-26, 6-27, and 6-28). In an 
analysis of the National Morbidity, Mortality, and Air Pollution Study 
(NMMAPS) data, Dominici et al. (2007) reported associations between 
fine particle exposures and all-cause and cardiopulmonary-related 
mortality (U.S. EPA, 2009a, p. 6-175, Figure 6-26). In a study of 112 
U.S. cities, Zanobetti and Schwartz (2009) reported positive 
associations (in 99 percent of the cities) and frequently statistically 
significant associations (in 55 percent of the cities) between short-
term PM2.5 exposure and total (non-accidental) mortality 
(U.S. EPA, 2009a, pp. 6-176 to 6-179; Figures 6-23 and 6-24).\37\ A 
Canadian 12-city study (Burnett et al., 2004) is generally consistent 
with an earlier Canadian 8-city study (Burnett and Goldberg, 2003). 
Both studies reported a positive and statistically significant 
association between short-term PM2.5 exposure and mortality 
(U.S. EPA, 2009a, p. 6-182, Figure 2-1), although the influence of 
nitrogen dioxide (NO2) and limited PM2.5 data for 
several years during the study period somewhat diminished the findings 
reported in the 12-city study. In addition to these multi-city studies, 
evidence from available single-city studies suggests that gaseous 
copollutants do not confound the PM2.5-mortality association 
(U.S. EPA, 2009a, section 2.3.1.1). Collectively, these studies provide 
generally consistent and much stronger evidence for PM2.5-

[[Page 38909]]

associated mortality than the evidence available in the last review 
(U.S. EPA, 2011a, p. 2-24).
---------------------------------------------------------------------------

    \37\ Single-city Bayes-adjusted effect estimates for the 112 
cities analyzed in Zanobetti and Schwartz (2009) were provided by 
the study author (personal communication with Dr. Antonella 
Zanobetti, 2009; see also U.S. EPA, 2009a, Figure 6-24).
---------------------------------------------------------------------------

    With regard to cardiovascular effects, new multi-city as well as 
single-city short-term PM2.5 exposure studies conducted 
since the last review support a largely positive and frequently 
statistically significant association between short-term exposure to 
PM2.5 and cardiovascular-related morbidity and mortality, 
substantiating prior findings. For example, among a multi-city cohort 
of older adults participating in the Medicare Air Pollution Study 
(MCAPS), investigators reported evidence of a positive association 
between short-term PM2.5 exposures and hospital admissions 
related to cardiovascular outcomes (U.S. EPA, 2009a, pp. 6-57 to 58; 
Dominici et al, 2006a; Bell et al, 2008). The strongest evidence for 
cardiovascular effects has been observed predominantly for hospital 
admissions and emergency department visits for ischemic heart disease 
and congestive heart failure, and cardiovascular-related mortality 
(U.S. EPA, 2009a, Figure 2-1, p. 6-79, sections 6.2.10.3, 6.2.10.5, and 
6.2.11; Bell et al., 2008; Dominici et al., 2006a; Tolbert et al., 
2007; Zanobetti and Schwartz, 2009). In studies that evaluated the 
potential for confounding using co-pollutant models, PM2.5 
effect estimates for cardiovascular-related hospital admissions and 
emergency department visits generally remained positive, with the 
magnitude of PM2.5 effect estimates increasing in some 
models and decreasing in others (U.S. EPA, 2009a, Figure 6-5). 
Furthermore, these findings are supported by a recent study of a multi-
city cohort of women participating in the WHI study that reported a 
positive but statistically nonsignificant association between short-
term exposure to PM2.5 and electrocardiogram measures of 
myocardial ischemia (Zhang et al., 2009).
    In focusing on respiratory effects, the strongest evidence from 
short-term PM2.5 exposure studies has been observed for 
respiratory-related emergency department visits and hospital admissions 
for chronic obstructive pulmonary disease (COPD) and respiratory 
infections (U.S. EPA, 2009a, sections 2.3.1.1 and 6.3.8.3; Figures 2-1 
and 6-13; Dominici et al., 2006a). In studies that employed co-
pollutant models to evaluate the potential for confounding, 
PM2.5 effect estimates for respiratory-related hospital 
admissions and emergency department visits generally remained positive, 
with the magnitude of PM2.5 effect estimates increasing in 
some models and decreasing in others (U.S. EPA, 2009a, Figure 6-15). 
Evidence for PM2.5-related respiratory effects has also been 
observed in panel studies, which indicate associations with respiratory 
symptoms, pulmonary function, and pulmonary inflammation among 
asthmatic children (U.S. EPA, 2009a, p. 2-10). Although not 
consistently observed, some controlled human exposure studies have 
reported small decrements in various measures of pulmonary function 
following controlled exposures to PM2.5 (U.S. EPA, 2009a, p. 
2-10). Furthermore, the comparatively larger body of toxicological 
evidence since the last review is coherent with the evidence from 
epidemiological and controlled human exposure studies that examined 
short-term exposures to PM2.5 and respiratory effects (U.S. 
EPA, 2009a, section 6.3.10.1).
c. Summary
    In considering the extent to which newly available scientific 
evidence strengthens or calls into question evidence of associations 
identified in the last review between ambient fine particle exposures 
and health effects, the Policy Assessment recognizes that much progress 
has been made in assessing some key uncertainties related to our 
understanding of health effects associated with long- and short-term 
exposure to PM2.5. As briefly discussed above as well as in 
the more complete discussion of the evidence as presented and assessed 
in the Integrated Science Assessment, the Policy Assessment notes that 
the newly available information combined with information available in 
the last review provides substantially stronger confidence in a causal 
relationship between long- and short-term exposures to PM2.5 
and mortality and cardiovascular effects. In addition, the newly 
available evidence reinforces and expands the evidence supporting a 
likely causal relationship between long- and short-term exposure to 
PM2.5 and respiratory effects. The body of scientific 
evidence is somewhat expanded 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 Integrated Science Assessment concludes that 
these data provide evidence that is suggestive of a causal relationship 
for these effects. Thus, the Policy Assessment concludes there is 
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.
2. Limitations and Uncertainties Associated With the Currently 
Available Evidence
    With respect to understanding the nature and magnitude of 
PM2.5-related risks, the Policy Assessment recognizes that 
important uncertainties remain in the current review (U.S. EPA, 2011a, 
p. 2-25). Epidemiological studies evaluating health effects associated 
with long- and short-term PM2.5 exposures have reported 
heterogeneity in responses both within and between cities and 
geographic regions within the U.S. In particular, the Policy Assessment 
notes that there are challenges with interpreting differences in health 
effects observed in the eastern versus western parts of the U.S., 
including evaluating effects stratified by seasons.\38\ This 
heterogeneity may be attributed, in part, to differences in the fine 
particle composition or related to exposure measurement error.
---------------------------------------------------------------------------

    \38\ Seasonal differences in effects may be related to 
PM2.5 composition as well as regional differences in 
climate and infrastructure that may affect time spent outdoors or 
indoors, housing characteristics including air conditioning usage, 
and differences in baseline incidence rates (U.S. EPA, 2009a, p. 3-
182).
---------------------------------------------------------------------------

    In considering the relationships between PM composition and health 
effects, the ISA notes that the scientific evidence continues to evolve 
and concludes that, while many constituents of PM 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 (U.S. EPA, 2009a, p. 2-17). 
In particular, based on assessing the body of available evidence, the 
ISA notes that (1) cardiovascular effects have been linked with 
elemental carbon as well as with PM2.5 from crustal sources, 
traffic, and wood smoke/vegetative burning; (2) respiratory effects 
have been linked with secondary sulfate PM2.5 as well as 
with PM2.5 from crustal/soil/road dust and traffic sources; 
and (3) a few studies have reported associations between total 
mortality and secondary sulfate/long-range transport, traffic, and 
salt. While specific PM2.5 constituents have been linked 
with various PM2.5-related health effects in a small number 
of studies, research continues to focus on the identification of 
specific constituents or sources that may be most closely related to 
specific PM2.5-related health outcomes.

[[Page 38910]]

    Exposure measurement error is also an important source of 
uncertainty (U.S. EPA, 2009a, section 3.8.6). 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, climate, 
source characteristics, ventilation usage, personal activity patterns, 
photochemistry). As noted in the Integrated Science Assessment, 
exposure measurement error can introduce bias and increased uncertainty 
in associated health effect estimates (U.S. EPA, 2009a, p. 2-17).
    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).\39\ As noted above, 
although short-term studies of cardiovascular and respiratory hospital 
admissions and emergency department visits generally reported that 
PM2.5 effect estimates remained positive, the magnitude of 
those effect estimates increased in some models and decreased in 
others. In addition, although evidence from single-city studies 
available in the last review suggests that gaseous copollutants do not 
confound the PM2.5-related mortality association (U.S. EPA, 
2004, section 8.4.3.3), a conclusion that is supported by studies that 
examined the PM10-mortality relationship (U.S. EPA, 2009a, 
p. 6-182 and 6-201), many recent U.S. multi-city studies have not 
analyzed multipollutant models. 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.
---------------------------------------------------------------------------

    \39\ 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).
---------------------------------------------------------------------------

3. At-Risk Populations
    In identifying population groups or lifestages at greatest risk for 
health risk from a specific pollutant, the terms susceptibility, 
vulnerability, sensitivity, and at-risk are commonly employed. The 
definition for these terms sometimes varies, but in most instances 
``susceptibility'' refers to biological or intrinsic factors (e.g., 
lifestage, gender, preexisting disease/conditions) while 
``vulnerability'' refers to nonbiological or extrinsic factors (e.g., 
socioeconomic factors). However, factors included in the terms 
``susceptibility'' and ``vulnerability'' are intertwined and are 
difficult to distinguish. In the Integrated Science Assessment, the 
term ``susceptibility'' has been used broadly to recognize populations 
that have a greater likelihood of experiencing effects related to 
ambient PM exposure\40\, such that use of the term ``susceptible 
populations'' in the Integrated Science Assessment is used as a term 
that encompasses factors related both to susceptibility and 
vulnerability.\41\ In the development of a more recent Integrated 
Science Assessment, the Agency is using the term ``at-risk'' groups to 
more broadly define the populations with characteristics that increase 
the risk of pollutant-related health effects (U.S. EPA, 2011d, p. 8-1). 
Therefore, in this proposal, the term ``at-risk'' is the broadly 
encompassing term used for groups with specific factors that increase 
the risk of PM-related health effects in a population. At-risk 
populations could exhibit a greater risk of PM-related health effects 
than the general population for a number of reasons including: being 
affected by lower concentrations of PM, experiencing a larger health 
impact at a given PM concentration or being exposed to higher PM 
concentrations than the general population. Given the heterogeneity of 
individual responses to PM exposures, the severity of the health 
effects experienced by an at-risk population may be much greater than 
that experienced by the population at large.
---------------------------------------------------------------------------

    \40\ Although studies have primarily used exposures to 
PM10 or PM2.5, the available evidence suggests 
that the identified factors also increase risk from 
PM10-2.5 (U.S. EPA, 2009a, section 8.1.8).
    \41\ The term ``susceptible population'' is defined in the 
Integrated Science Assessment as ``[P]opulations that have a greater 
likelihood of experiencing health effects related to exposure to an 
air pollutant (e.g., PM) due to a variety of factors including, but 
not limited to: Genetic or developmental factors, race, gender, 
lifestage, lifestyle (e.g., smoking status and nutrition) or 
preexisting disease; as well as population-level factors that can 
increase an individual's exposure to an air pollutant (e.g., PM) 
such as socioeconomic status [SES], which encompasses reduced access 
to health care, low educational attainment, residential location, 
and other factors (U.S. EPA, 2009a, p. 8-1).
---------------------------------------------------------------------------

    As summarized below and presented in more detail in chapter 8 of 
the Integrated Science Assessment and section 2.2.1 of the Policy 
Assessment, the currently available epidemiological and controlled 
human exposure 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) (U.S. EPA, 2009a, 
section 2.4.1, Table 8-2). In addition, toxicological studies provide 
underlying support for the biological mechanisms that potentially lead 
to increased susceptibility to PM-related health effects (U.S. EPA, 
2009a, sections 2.4.1 and 8.1.8).
    Two different lifestages have been associated with increased risk 
to PM-related health effects: childhood (i.e., less than 18 years of 
age) and older adulthood (i.e., 65 years of age and older). Childhood 
represents a lifestage where susceptibility to PM exposures may be 
related to the following observations: children spend more time 
outdoors; children have greater activity levels than adults; children 
have exposures resulting in higher doses per body weight and lung 
surface area; and the developing lung is prone to damage, including 
irreversible effects, from environmental pollutants as it continues to 
develop through adolescence (U.S. EPA, 2009a, section 8.1.1.2). Older 
adults represent a lifestage where susceptibility to PM-associated 
health effects may be related to the higher prevalence of pre-existing 
cardiovascular and respiratory diseases found in this age group 
compared to younger age groups as well as the gradual decline in 
physiological processes that occur as part of the aging process (U.S. 
EPA, 2009a, section 8.1.1.1). In addition, accumulating evidence 
suggests that the developing fetus may also represent an additional 
lifestage that is at greater risk to PM exposures (U.S. EPA, 2009a, 
sections 2.3.1.2 and 7.4).
    With regard to mortality, recent epidemiological studies have 
continued to find that older adults are at greater risk of all-cause 
(non-accidental) mortality associated with short-term exposure to both 
PM2.5 and PM10, providing consistent and stronger 
evidence of effects in this at-risk population compared to the last 
review (U.S. EPA, 2009a, Figure 7-7, section 8.1.1.1, Zeger et al., 
2008). Evidence is accumulating for PM2.5-related infant 
mortality, especially due to respiratory causes during the post-
neonatal period (U.S. EPA, 2009a, sections 2.3.1.2 and 7.4).

[[Page 38911]]

    With regard to morbidity effects, currently available studies 
provide evidence that older adults have heightened responses, 
especially for cardiovascular-related effects, and children have 
heightened responses for respiratory-related effects (U.S. EPA, 2009a, 
p. 2-23). In considering respiratory-related effects in children 
associated with long-term PM exposures, the Policy Assessment 
recognizes that our understanding of effects on lung development has 
been strengthened based on newly available evidence that is consistent 
and coherent across different study designs, locations, and research 
groups (U.S. EPA, 2011a, p. 2-28). The strongest evidence comes from 
the extended follow-up for the Southern California Children's Health 
Study which includes several new studies that report positive 
associations between long-term exposure to PM2.5 and 
respiratory morbidity, particularly for such endpoints as lung function 
growth, respiratory symptoms (e.g., bronchitic symptoms), and 
respiratory disease incidence (U.S. EPA, 2009a, section 7.3; McConnell 
et al, 2003; Gauderman et al., 2004; Islam et al., 2007). These 
analyses provide evidence that PM2.5-related effects persist 
into early adulthood and are more robust and larger in magnitude than 
previously reported. With regard to respiratory effects in children 
associated with short-term exposures to PM2.5, currently 
available studies provide stronger evidence of respiratory-related 
hospitalizations with larger effect estimates observed among children. 
In addition, reductions in lung function (i.e., FEV1) and 
increases in respiratory symptoms and medication use associated with PM 
exposures have been reported among asthmatic children (U.S. EPA, 2009a, 
sections 6.3.1, 6.3.2.1, and 8.4.9).
    A number of health conditions have been found to put individuals at 
greater risk for adverse effects following exposure to PM. The 
currently available evidence confirms and strengthens evidence in the 
last review that individuals with underlying cardiovascular and 
respiratory diseases are more susceptible to PM exposures (U.S. EPA, 
2009a, section 8.1.6; U.S. EPA, 2011a, section 2.2.1). There is also 
emerging evidence that people with diabetes, who are at risk for 
cardiovascular disease, as well as obese individuals, may have 
increased susceptibility to PM exposures (U.S. EPA, 2009a, section 
8.1.6.4). As discussed in section 8.1.6 of the Integrated Science 
Assessment, this body of evidence includes findings from 
epidemiological and human clinical studies that associations with 
mortality or morbidity are greater in those with pre-existing 
conditions, and also includes evidence from toxicological studies using 
animal models of cardiopulmonary disease.
    Stronger evidence is available in this review than the last 
indicating that people from lower socioeconomic strata are an at-risk 
population relative to PM exposures (U.S. EPA, 2009a, section 8.1.7; 
U.S. EPA, 2011a, section 2.2.1). Persons with lower socioeconomic 
status (SES) \42\ have been generally found to have a higher prevalence 
of pre-existing diseases; limited access to medical treatment; and 
increased nutritional deficiencies, which can increase this 
population's risk to PM-related effects.
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    \42\ Socioeconomic status is a composite measure that usually 
consists of economic status, measured by income; social status 
measured by education; and work status measured by occupation (U.S. 
EPA, 2009a, p. 8-14).
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    Investigation of potential genetic susceptibility has provided 
evidence that individuals with specific genetic differences are more 
susceptible to PM-related effects (U.S. EPA, 2009a, pp. 8-7 to 8-9). 
More research is needed to better understand the relationship between 
genetic effects and potential susceptibility to PM-related effects 
(U.S. EPA, 2011a, p. 2-109).
    In summary, there are several at-risk populations that may be 
especially susceptible or vulnerable to PM-related effects. 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. The available evidence does not generally allow distinctions to 
be drawn between the PM indicators in terms of whether populations are 
more at-risk to a particular size fraction (i.e., PM2.5 and 
PM10-2.5).
4. Potential PM2.5-Related Impacts on Public Health
    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 as described in section III.B.3. 
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. In addition, it appears that mortality 
risks are not limited to the very frail. Taken together, these results 
suggest that exposure to ambient PM2.5 concentrations can 
have substantial public health impacts.
    With regard to at-risk populations in the United States, 
approximately 7 percent of adults (approximately 16 million adults) and 
9 percent of children (approximately 7 million children) have asthma 
(U.S. EPA 2009a, Table 8-3; CDC, 2008 \43\). In addition, approximately 
4 percent of adults have been diagnosed with chronic bronchitis and 
approximately 2 percent with emphysema (U.S. EPA, 2009a, Table 8-3). 
Approximately 11 percent of adults have been diagnosed with heart 
disease, 6 percent with coronary heart disease, 23 percent with 
hypertension, and 8 percent with diabetes (U.S. EPA, 2009a, Table 8-3). 
In addition, approximately 3 percent of the U.S. adult population has 
suffered a stroke (U.S. EPA, 2009a, Table 8-3). Therefore, large 
portions of the United States population are in groups that may be at 
increased risk to health effects associated with exposures to ambient 
PM2.5. The size of the potentially at-risk population 
suggests that exposure to ambient PM2.5 has significant 
impact on public health in the United States.
---------------------------------------------------------------------------

    \43\ For percentages, see http://www.cdc.gov/ASTHMA/nhis/06/table4-1.htm. For population estimates, see http://www.cdc.gov/ASTHMA/nhis/06/table3-1.htm.
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C. Quantitative Characterization of Health Risks

1. Overview
    In this review, the quantitative risk assessment builds on the 
approach used and lessons learned in the last review and focuses 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. The goals of this quantitative risk assessment are largely 
the same as those articulated in the risk assessment conducted for the 
last review. These goals include: (1) To provide estimates of the 
potential magnitude of premature mortality and/or selected morbidity 
effects in the population associated with recent ambient level of 
PM2.5 and with simulating just meeting the current and 
alternative suites of PM2.5 standards in 15 selected urban 
study areas, including, where data were available, consideration of 
impacts on at-risk

[[Page 38912]]

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.
    The 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 I.B.3.
2. Summary of Design Aspects
    Based on a review of the evidence presented and assessed in the 
Integrated Science Assessment and criteria for selecting specific 
health effect endpoints discussed in the Risk Assessment (U.S. EPA, 
2010a, section 3.3.1), the following broad categories of health 
endpoints were included in the quantitative risk assessment: (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 associated with short-term PM2.5 exposure. 
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 (U.S. 
EPA, 2010a, p. 3-29). The quantitative risk assessment estimates risks 
for various health effects in 15 urban study areas. The selection of 
urban study areas was based on a number of criteria including: (1) 
Consideration of urban study areas evaluated in the last PM risk 
assessment; (2) consideration of locations evaluated in key 
epidemiological studies; (3) preference for locations with relatively 
elevated annual and/or 24-hour PM2.5 monitored 
concentrations; and (4) preference for including locations from 
different regions across the country, reflecting potential differences 
in PM2.5 sources, composition, and potentially other factors 
which might impact PM2.5-related risk (U.S. EPA, 2010a, 
section 3.3.2). Based on the results of several analyses examining the 
representativeness of these 15 urban study areas in the broader 
national context, the Risk 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 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).\44\
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    \44\ 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 have 
relatively low PM2.5 concentrations (e.g., urban areas in 
Florida) and, consequently, the Risk Assessment concludes failure to 
include these areas in the set of urban study areas is unlikely to 
exclude high PM2.5-risk locations (U.S. EPA, 2010a, 
section 4.4.1).
---------------------------------------------------------------------------

    In order to estimate the incidence of a particular health effect 
associated with recent ambient conditions in a specific urban study 
area attributable to PM2.5 exposures, as well as the change 
in incidence corresponding to a given change in PM2.5 
concentrations resulting from simulating just meeting current or 
alternative PM2.5 standards, three elements are required 
(U.S. EPA, 2010a, section 3.1.1, Figures 3-2 and 3-3). These elements 
are: (1) Air quality information (including recent air quality data for 
PM2.5 from ambient monitors for the selected location, 
estimates of background PM2.5 concentrations appropriate for 
that location, and a method for adjusting the recent data to reflect 
patterns of air quality estimated to occur when the area just meets a 
given set of PM2.5 standards); (2) relative risk-based 
concentration-response functions that provide an estimate of the 
relationship between the health endpoints of interest and ambient 
PM2.5 concentrations; and (3) baseline health effects 
incidence rates and population data, which are needed to provide an 
estimate of the incidence of health effects in an area before any 
changes in PM2.5 air quality.\45\
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    \45\ Incidence rates express the occurrence of a disease or 
event (e.g., death, hospital admission) in a specific period of 
time, usually per year. Rates are expressed either as a value per 
population group (e.g., the number of cases in Philadelphia County) 
or a value per number of people (e.g., the number of cases per 
10,000 residents in Philadelphia County), and may be age- and/or 
sex-specific. Incidence rates vary among geographic areas due to 
differences in populations characteristics (e.g., age distribution) 
and factors promoting illness (e.g., smoking rates, air pollution 
concentrations). The baseline incidence rate provides an estimate of 
the incidence rate (i.e., number of cases of the health effect per 
year, usually per 10,000 or 100,000 general population) in the 
assessment location unrelated to changes in ambient PM2.5 
concentrations in that location (U.S. EPA, 2010a, section 3.4).
---------------------------------------------------------------------------

    The Risk Assessment includes 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 provide information to evaluate and 
rank the potential impacts of key sources of uncertainty on the core 
risk estimates (U.S. EPA, 2010a, sections 3.5 and 4.3, Table 4-3). In 
addition, the sensitivity analyses represent a set of reasonable 
alternatives to the core set of risk estimates that fall within an 
overall set of plausible risk estimates surrounding the core estimates 
(U.S. EPA, 2010a, section 4.3.2).
    Recent air quality was characterized for the 15 urban study areas 
based on 24-hour PM2.5 concentrations measured for 3 years 
(i.e., 2005, 2006, and 2007) as described in section 3.2.1 of the Risk 
Assessment. Different methodologies were then used to simulate 
conditions for just meeting the current or alternative PM2.5 
standards (U.S. EPA, 2010a, section 3.2.3). This included using the 
single rollback approach used in the risk assessment conducted for the 
last review which reflects a uniform regional pattern of reductions in 
ambient PM2.5 concentrations across monitors (i.e., 
proportional rollback approach). The proportional rollback approach was 
used in generating the core risk estimates (U.S. EPA, 2010a, section 
3.2.3.1). In sensitivity analyses, the Risk Assessment also applied two 
alternative rollback approaches (i.e., hybrid and locally-focused 
rollback approaches)\46\ to better characterize

[[Page 38913]]

potential variability in the way air quality in urban areas responds to 
programs put in place to meet the current or alternative 
PM2.5 standards. In considering the three rollback 
approaches collectively, the proportional and locally-focused methods 
are approaches that are more likely to represent ``bounding'' scenarios 
related to the spatial pattern of future reductions in ambient 
PM2.5 concentrations. In contrast, the hybrid approach, in 
principle, reflects a more plausible or representative rollback 
strategy since it: (1) Reflects consideration for site-specific 
information regarding larger PM2.5 sources and their 
potential impact on source-oriented monitors and (2) combines elements 
of more locally-focused and regionally-focused patterns of reductions 
(U.S. EPA, 2010a, section 3.2.3).
---------------------------------------------------------------------------

    \46\ The hybrid rollback approach involves a combination of an 
initial step of a more localized reduction in ambient 
PM2.5 concentrations at source-oriented monitors followed 
by a regional pattern of reduction across all monitors in a study 
area (U.S. EPA, 2010a, section 3.2.3.2). The locally-focused 
rollback approach involves a focused reduction of concentrations 
only at those monitors exceeding the current or alternative 24-hour 
standard levels (U.S. EPA, 2010a, section 3.2.3.3).
---------------------------------------------------------------------------

    The peak-to-mean ratio of ambient PM2.5 concentrations 
within a study area informs the type of rollback approach used to 
simulate just meeting the current or alternative suites of standards to 
determine the magnitude of the reduction in annual mean 
PM2.5 concentrations for that study area and consequently 
the degree of risk reduction.\47\ For example, study areas with 
relatively high peak-to-mean ratios are likely to have greater 
estimated risk reductions for the current suite of standards (depending 
on the combination of 24-hour and annual design values), and such 
locations can be especially sensitive to the type of rollback approach 
used, with the proportional rollback approach resulting in notably 
greater estimated risk reduction compared with the locally-focused 
rollback approach. In contrast, study areas with lower peak-to-mean 
ratios typically experience greater risk reductions when simulating 
just meeting the current or alternative annual-standard level than with 
simulating just meeting the current or alternative 24-hour standard 
level (again depending on the combination of 24-hour and annual design 
values). In addition, the type of rollback approach used will tend to 
have less of an impact on the magnitude of risk reductions for study 
areas with lower peak-to-mean ratios. Consideration of these two 
factors helps to inform an understanding of the nature and pattern of 
estimated risk reductions and risk remaining upon simulation of just 
meeting the current and alternative suites of standards across the 
urban study areas (U.S. EPA, 2010a, section 5.2.1).
---------------------------------------------------------------------------

    \47\ The peak-to-mean ratio of ambient PM2.5 
concentrations also has a direct bearing on whether the 24-hour or 
annual standard will be the generally controlling standard for a 
particular study area, with higher peak-to-mean ratios generally 
being associated with locations where the 24-hour standard is likely 
the controlling standard.
---------------------------------------------------------------------------

    The concentration-response functions used in the risk assessment 
were based on findings from epidemiological studies that have relied on 
fixed-site, population-oriented, ambient monitors as a surrogate for 
actual ambient PM2.5 exposures. The risk assessment 
addresses risks attributable to anthropogenic sources and activities 
(i.e., risk associated with concentrations above policy-relevant 
background).\48\ This approach of estimating risks in excess of 
background was judged to be more relevant to policy decisions regarding 
ambient air quality standards than risk estimates that include effects 
potentially attributable to PM2.5 concentrations that are 
not associated with North American anthropogenic emissions.
---------------------------------------------------------------------------

    \48\ Policy-relevant background estimates used in the risk 
assessment model were based on information presented in the 
Integrated Science Assessment (U.S. EPA, 2009a, section 3.7, Table 
3-23) and discussed in the Risk Assessment (U.S. EPA, 2010a, section 
3.2.2). These values were generated based on a combination of 
Community Multiscale Air Quality model (CMAQ) and Goddard Earth 
Observing System (GEOS)-Chem modeling (U.S. EPA, 2009a, section 
3.7.1.2; U.S. EPA, 2010a, section 3.2.2).
---------------------------------------------------------------------------

    In modeling risk associated with long- and short-term 
PM2.5 exposures, the Risk Assessment initially focused on 
selecting concentration-response functions from multi-city studies.\49\ 
Concentration-response functions from two single-city studies provided 
coverage for additional health effect endpoints (i.e., emergency 
department visits for cardiovascular and/or respiratory effects) 
associated with short-term PM2.5 exposures (U.S. EPA, 2010a, 
p. 3-37).
---------------------------------------------------------------------------

    \49\ As noted in section 3.3.3 of the Risk Assessment, multi-
city studies have a number of advantages over single-city studies 
including: increased statistical power providing effect estimates 
with relatively greater precision and reduced problems with 
publication bias (i.e., in which studies with statistically 
insignificant or negative results are less likely to get published 
than those with positive and/or statistically significant results).
---------------------------------------------------------------------------

    With regard to modeling risks associated with long-term 
PM2.5 exposure, concentration-response functions used in the 
risk model are all based on cohort studies, in which a cohort of 
individuals is followed over time. In the core analysis, estimated 
premature mortality risk associated with long-term PM2.5 
concentrations used concentration-response functions from the extended 
ACS study (Krewski et al., 2009). This study had a number of advantages 
including: analyses that expanded upon previous publications presenting 
evaluations of the ACS long-term cohort study and extending the follow-
up period to eighteen years; a rigorous examination of different model 
forms for estimating effect estimates; coverage for a range of 
ecological variables (e.g., social, economic, and demographic factors) 
which allowed for consideration of whether these factors confound or 
modify the relationship between PM2.5 exposure and 
mortality; and updated and expanded data sets on incidence and exposure 
(U.S. EPA, 2010a, p 2-9 and 3-38).
    As discussed in section III.B.3, persons of lower socioeconomic 
status have been identified as an at-risk population. The ACS study 
cohort does not provide representative coverage for persons of lower-
socioeconomic status and, thus, the Risk Assessment concludes that 
using the concentration-response functions from this study may result 
in risk estimates that are biased low (U.S. EPA, 2010a, p. 5-7). 
Therefore, concentration-response functions from a reanalysis of the 
Harvard Six Cities study (Krewski et al., 2000) were used in a 
sensitivity analysis to better support generalizing the results of the 
risk assessment across the broader national population.\50\
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    \50\ As noted in the last review, the ACS study population has 
persons generally representative of a higher SES (e.g., higher 
educational status) relative to the Harvard Six Cities study 
population (12 percent versus 28 percent of the cohort had less than 
a high school education, respectively) (U.S. EPA, 2004, p. 8-118). 
The Policy Assessment concludes that the Harvard Six Cities study 
cohort may provide a more representative sample of the broader 
national population than the ACS study cohort (U.S. EPA, 2011a, p. 
2-40).
---------------------------------------------------------------------------

    While being mindful that the use of concentration-response 
functions from Krewski et al. (2009) introduces potential for low bias 
in the core risk estimates, the Policy Assessment also recognizes many 
strengths of this study and reasons for its continued use for 
generating the core risk estimates, including: consideration of a large 
number of metropolitan statistical areas, inclusion of two time periods 
for the air quality data which allowed us to consider different 
exposure windows, and analysis of a wide range of concentration-
response function models. Therefore, the Risk Assessment concludes that 
concentration-response functions obtained from this study had the 
greatest overall support and were appropriate to incorporate in the 
core risk model (U.S. EPA, 2010a, p. 3-38).

[[Page 38914]]

    In the core analysis, for modeling health endpoints associated with 
long-term exposure, the Risk Assessment concluded that modeling risks 
down to policy-relevant background would require substantial 
extrapolation of the estimated concentration-response functions below 
the range of the data on which they were estimated (i.e., the lowest 
measured levels reported in the epidemiological studies were 
substantially above policy-relevant background). Therefore, the Risk 
Assessment concluded it was most appropriate in the core analysis to 
estimate risk only down to the lowest measured level to avoid 
introducing additional uncertainty into the analysis (U.S. EPA, 2010a, 
3-1 to 3-3).\51\ A sensitivity analysis comparing the impact of 
estimated risks down to policy-relevant background rather than down to 
the lowest measured level (U.S. EPA, 2010a, section 3.5.4.1) used 
annual estimates of policy-relevant background values for specific 
geographic regions (U.S. EPA, 2010a, section 3.2.2, Table 3-2).
---------------------------------------------------------------------------

    \51\ To provide consistency for the different concentration-
response functions selected from the long-term exposure studies, 
and, in particular, to avoid the choice of lowest measured levels 
unduly influencing the results of the risk assessment, the Risk 
Assessment concluded it was appropriate to select a single lowest 
measured level--5.8 [mu]g/m\3\ from the later exposure period 
evaluated in Krewski et al. (2009)--to use in estimating risks 
associated with long-term PM2.5 exposures (U.S. EPA, 
2010a, p. 3-3).
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    With regard to modeling risks associated with short-term 
PM2.5 exposure, concentration-response functions from two 
time-series studies were selected as the primary studies to support the 
core analysis. Concentration-response functions from Zanobetti and 
Schwartz (2009) were used in estimating premature non-accidental, 
cardiovascular-related, and respiratory-related mortality. 
Concentration-response functions from Bell et al. (2008) were used in 
estimating cardiovascular-related and respiratory-related hospital 
admissions. In addition, concentration-response functions from two 
single-city studies were used to estimate emergency department visits 
for cardiovascular and/or respiratory illnesses associated with short-
term PM2.5 exposure (Tolbert et al., 2007; Ito et al., 2007; 
U.S. EPA, 2010a, p. 3-37).
    For modeling health endpoints associated with short-term 
PM2.5 exposure, the Risk Assessment estimates risk down to 
policy-relevant background exclusively using quarterly values to 
represent the appropriate block of days within a simulated year (U.S. 
EPA, 2010a, section 3.2.2, Table 3-2).
    To estimate the change in incidence of a health endpoint associated 
with a given change in PM2.5 concentrations, information on 
the baseline incidence of that endpoint is needed (U.S. EPA, 2010a, 
section 3.4). In calculating a baseline incidence rate to be used with 
a concentration-response function from a given epidemiological study, 
the Risk Assessment matched the counties, age grouping, and 
International Classification of Diseases (ICD) codes used in that study 
(U.S. EPA, 2010a, section 3.4.2).
    An important component of a population health risk assessment is 
the characterization of both uncertainty and variability.\52\ The 
design of the risk assessment includes a number of elements to address 
these issues, including using guidance from the World Health 
Organization (WHO, 2008) as a framework for developing the approach 
used for characterizing uncertainty in the analyses (U.S. EPA, 2010a, 
section 3.5).
---------------------------------------------------------------------------

    \52\ 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).
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    The Risk Assessment considers key sources of variability that can 
impact the nature and magnitude of risks associated with simulating 
just meeting current and alternative standard levels across the urban 
study areas (U.S. EPA, 2010a, section 3.5.2). These sources of 
variability include those that contribute to differences in risk across 
urban study areas, but do 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 
focuses on factors that not only introduce variability into risk 
estimates across study areas, but also play 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 are likely to affect population 
risks and the degree to which they were (or were not) fully captured in 
the design of the risk assessment are discussed in section 3.5.2 of the 
Risk Assessment. These sources include: PM2.5 composition; 
intra-urban variability in ambient PM2.5 concentrations; 
variability in the patterns of reductions in PM2.5 
concentrations associated with different rollback approaches when 
simulating just meeting the current or alternative standards; co-
pollutant exposures; factors related to demographic and socioeconomic 
status; behavioral differences across urban study areas (e.g., time 
spent outdoors); baseline incidence rates; and 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 (U.S. EPA, 2010a, pp. 3-67 to 3-69).
    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 (U.S. EPA, 2010a, sections 3.5.3 
and 3.5.4). The quantitative sensitivity analyses informed our 
understanding of sources of uncertainty that may have a moderate to 
large impact on the core risk estimates including: (1) Characterizing 
intra-urban population exposure in the context of epidemiology 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 associate with long-term PM2.5 exposures; and (8) 
potential variation in effect estimates reflecting compositional 
differences in PM2.5 (U.S. EPA, 2011a, section 5.1.4). In 
addition to identifying sources of uncertainty with a moderate to large 
impact on the core risk estimates, the single and multi-element 
sensitivity analyses also produced a set of reasonable alternative risk 
estimates that allowed us to place the results of the core analysis in 
context with regard to uncertainty and potential bias (U.S. EPA, 2010a, 
section 5.1.4). The qualitative uncertainty analysis supplemented the 
quantitative sensitivity analyses by allowing coverage for sources of 
uncertainty that could not be readily included in the sensitivity 
analysis (U.S. EPA, 2010a, section 3.5.3).
    With respect to the long-term exposure-related mortality risk

[[Page 38915]]

estimates,\53\ the most important sources of uncertainty identified in 
the quantitative sensitivity analyses included: selection of 
concentration-response functions; modeling risk down to policy-relevant 
background versus lowest measured level; and the choice of rollback 
approach used to simulate just meeting current or alternative standards 
(U.S. EPA, 2011a, p. 2-39). With regard to the qualitative analysis of 
uncertainty, the following sources were identified as potentially 
having a large impact on the core risk estimates for the long-term 
exposure-related mortality: characterization of intra-urban population 
exposures; impact of historical air quality; and potential variation in 
effect estimates reflecting differences in PM2.5 composition 
(U.S. EPA, 2011a, p. 2-39).
---------------------------------------------------------------------------

    \53\ Given increased emphasis placed in this analysis on long-
term exposure-related mortality, the uncertainty analyses completed 
for this health endpoint category were more comprehensive than those 
conducted for analyses of short-term exposure-related mortality and 
morbidity. This reflects, to some extent, limitations in the 
epidemiological data available for addressing uncertainty in the 
latter categories (U.S. EPA, 2010a, section 3.5.4.2).
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    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). Two additional analyses examined potential bias 
and overall confidence in the risk estimates. The first analysis 
explored potential bias in the core risk estimates by considering a set 
of alternative reasonable risk estimates generated as part of a 
sensitivity analysis. The second analysis compared the annual mean 
PM2.5 concentrations associated with simulating just meeting 
the current and alternative suites of standards with the air quality 
distribution used in deriving the concentration-response functions 
applied in modeling mortality risk. 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).
3. Risk Estimates and Key Observations
    As discussed below, three factors figure prominently in the 
interpretation of the risk estimates associated with simulating just 
meeting the current and alternative suites of standards, including: (1) 
The importance of changes in annual mean PM2.5 
concentrations for a specific study area in estimating changes in risks 
related to both long- and short-term exposures associated with recent 
air quality conditions and air quality simulated to just meet the 
current and alternative suites of PM2.5 standards; (2) the 
ratio of peak- to-mean ambient PM2.5 concentrations in a 
study area; and (3) the spatial pattern of ambient PM2.5 
reductions that result from using different approaches to simulate just 
meeting the current standard levels (i.e., rollback approaches). The 
latter two factors are interrelated and influence the degree of risk 
reduction estimated under the current suite of standards.
    The magnitude of both long- and short-term exposure-related risk 
estimated to remain upon just meeting the current suite of standards is 
strongly associated with the simulated change in annual mean 
PM2.5 concentrations. The role of annual mean 
PM2.5 concentrations in driving long-term exposure-related 
risk estimates is intuitive given that risks are modeled using the 
annual mean air quality metric.\54\ The fact that short-term exposure-
related risk estimates are also driven by changes in long-term mean 
PM2.5 concentrations is less intuitive, since changes in 
mean 24-hour PM2.5 concentrations are used to estimate 
changes in risk for this time period.\55\ Analyses show that short-term 
exposure-related risks are 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 (U.S. EPA, 2010a, section 3.1.2.2). Consequently, the 
largest part of the estimates of short-term exposure-related risk is 
related to the changes in the portion of the distribution of short term 
PM2.5 exposures that are well represented by changes in the 
annual mean. Therefore, the Policy Assessment focuses on changes in 
annual mean PM2.5 concentrations to inform our understanding 
of patterns of both long- and short-term exposure-related risk 
estimates across the set of urban study areas evaluated in the 
quantitative risk assessment (U.S. EPA, 2011a, pp. 2-36 to 2-37).
---------------------------------------------------------------------------

    \54\ As noted in section 3.2.1 of the Risk Assessment (U.S. EPA, 
2010a), estimates of long-term exposure-related mortality are 
actually based on an annual mean PM2.5 concentration that 
is the average across monitors in a study area (i.e., based on the 
composite monitor distribution). Therefore, in considering changes 
in long-term exposure-related mortality, it is most appropriate to 
compare composite monitor estimates generated for a study area under 
each alternative suite of standards considered. The annual mean at 
the highest reporting monitor (i.e., based on the maximum monitor 
distribution) for a study area is the annual design value. The 
annual design value is used to determine the percent reduction in 
PM2.5 concentrations required to meet a particular 
standard. Both types of air quality estimates are provided in Table 
3-4 of the Risk Assessment (U.S. EPA, 2010a, pp. 3-25 to 3-27).
    \55\ Estimates of short-term PM2.5 exposure-related 
mortality and morbidity are based on composite monitor 24-hour 
PM2.5 concentrations. However, similar to the case with 
long-term exposure-related mortality, under the current rules, it is 
the 98th percentile 24-hour concentration estimated at the maximum 
monitor (the 24-hour design value) that will determine the degree of 
reduction required to meet a given 24-hour standard level (U.S. EPA, 
2011a, p. 2-37).
---------------------------------------------------------------------------

    In estimating PM2.5-related risks likely to remain upon 
simulation of just meeting the current annual and 24-hour standards in 
the 15 urban study areas, the Risk Assessment focuses on the 13 areas 
that would likely not have met the current suite of PM2.5 
standards based on recent air quality (2005 to 2007). These 13 areas 
have annual and/or 24-hour design values that are above the levels of 
the current standards (U.S. EPA, 2010a, Table 3-3).\56\ Based on the 
core risk estimates for these areas, using the proportional rollback 
approach, the Policy Assessment makes the following key observations 
regarding the magnitude of risk remaining upon simulation of just 
meeting the current suite of standards:
---------------------------------------------------------------------------

    \56\ Of the 15 urban study areas, only Dallas and Phoenix have 
both annual and 24-hour design values below the levels of the 
current standards based on 2005-2007 air quality data (U.S. EPA, 
2010a, Table 3-3).

    (1) Long-term exposure-related mortality risk estimated to 
remain upon just meeting the current standards are significant: 
Premature mortality related to ischemic heart disease attributable 
to long-term PM2.5 exposure was estimated to range from 
less than 100 to approximately 2,000 cases per year across the urban 
study areas. The variability in these estimates reflects, to a

[[Page 38916]]

great extent, differences in the size of study area populations. 
These estimates represent from 4 to 17% of all mortality related to 
ischemic heart disease in a given year for the urban study areas 
evaluated, representing a measure of risk that takes into account 
differences in population size and baseline mortality rates (U.S. 
EPA, 2011a, p. 2-43, Table 2-2). These estimates of risk for 
mortality related to ischemic heart disease associated with long-
term PM2.5 exposure would likely be in a range of 
thousands of deaths per year for the 15 urban study areas \57\ (U.S. 
EPA, 2011a, pp. 2-46 to 2-47). Based on these risk estimates for 
premature mortality related to ischemic heart disease alone, the 
Policy Assessment concludes that risks estimated to remain upon 
simulation of just meeting the current suite of standards are 
important from a public health standpoint (U.S. EPA, 2011a, p. 2-
47). The Risk Assessment also includes estimated risks for premature 
mortality related to cardiopulmonary effects and lung cancer, which 
increase the total annual incidence of mortality attributable to 
long-term PM2.5 exposure (see U.S. EPA, 2010a, section 
4.2.1).
---------------------------------------------------------------------------

    \57\ Premature mortality for all causes attributed to 
PM2.5 exposure was estimated to be in a range 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).
---------------------------------------------------------------------------

    (2) Short-term exposure-related mortality risk estimated to 
remain upon just meeting the current standards are much smaller than 
long-term exposure-related mortality risks: Cardiovascular-related 
mortality associated with short-term PM2.5 exposure was 
estimated to range from less than 10 to 500 cases per year across 
the urban study areas. These estimates represent approximately 1 to 
2 percent of total cardiovascular-related mortality in a given year 
for the urban study areas evaluated (U.S. EPA, 2011a, p. 2-43, Table 
2-3). 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 substantially lower, up 
to an order of magnitude smaller, for short-term exposure-related 
mortality (U.S. EPA, 2011a, p. 2-47).
    (3) Short-term exposure-related morbidity risk estimated to 
remain upon just meeting the current standards indicate 
hospitalizations are significantly larger for cardiovascular-related 
rather than respiratory-related events and emergency department 
visits for asthma-related events are significant: Cardiovascular-
related hospitalizations were estimated to range from approximately 
10 to 800 cases per year across the study areas, which are less than 
1 percent of total cardiovascular-related hospitalizations (U.S. 
EPA, 2011a, p. 2-43, Table 2-3). Respiratory-related hospital 
admissions attributable to short-term PM2.5 exposure were 
significantly smaller than those related to cardiovascular events 
(U.S. EPA, 2010a, Tables E-102 and E-111). Cardiovascular- and 
respiratory-related hospital admissions together ranged up to 
approximately 1,000 admissions per year across the urban study 
areas. The estimated incidence of asthma-related emergency 
department visits is several times larger than the estimates of 
cardiovascular- and respiratory-related hospital admissions (U.S. 
EPA, 2011a, p. 2-47; U.S. EPA, 2010a, Tables E-118 to E-123
    (4) Substantial variability exists in the magnitude of risk 
remaining across urban study areas: Estimated risks remaining upon 
just meeting the current suite of standards vary substantially 
across study areas, even when considering risks normalized for 
differences in population size and baseline incidence rates. This 
variability is a consequence of the substantial differences in the 
annual mean PM2.5 concentrations across study areas that 
result from simulating just meeting the current standards. This is 
important because, as discussed above, annual mean concentrations 
are highly correlated with both long- and short-term exposure-
related risk. The variability in annual mean PM2.5 
concentrations occurred primarily in those study areas in which the 
24-hour standard was the generally controlling standard. In such 
areas, the variability in estimated risks across study areas was 
largest when regional patterns of reductions in PM2.5 
concentrations were simulated, using the proportional rollback 
approach, as was done in the core analysis. Less variability was 
observed when more localized patterns of PM2.5 reductions 
were simulated using the locally-focused rollback approach, as was 
done in a sensitivity analysis. When simulations were done using the 
locally-focused rollback approach, estimated risks remaining upon 
just meeting the current suite of standards were appreciably larger 
than those estimated in the core analysis (U.S. EPA, 2011a, p. 2-46; 
U.S. EPA, 2010a, section 4.3.1.1).
    (5) Simulation of just meeting the current suite of standards 
results in annual mean PM2.5 concentrations well below 
the current standard for some study areas: In simulating just 
meeting the current suite of standards, the resulting composite 
monitor annual mean PM2.5 concentrations ranged from 
about 15 [micro]g/m\3\ (for those study areas in which the annual 
standard was controlling) down to as low as about 8 [micro]g/m\3\ 
(for those study areas in which the 24-hour standard was the 
generally controlling standard or the annual mean concentration was 
well below 15 [micro]g/m\3\ based on recent air quality) (U.S. EPA, 
2011a, p. 2-46).

    Reductions in risk associated with simulating air quality to just 
meet alternative standard levels were also estimated in this review 
(U.S. EPA, 2010a, sections 4.2.2, 5.2.2, and 5.2.3; U.S. EPA, 2011a, 
section 2.3.4.2). The estimated percent of risk reductions are depicted 
graphically in the Policy Assessment (US 2011a, Figures 2-11 and 2-12), 
showing patterns of estimated risk reductions associated with 
alternative suites of standards.\58\ These figures also depict the 
level of confidence associated with the risk estimates generated for 
simulating just meeting the current standards as well as alternative 
standard levels considered. As would be expected, patterns of 
increasing estimated risk reductions are generally observed as either 
the annual or 24-hour standard, or both, are reduced over the ranges 
considered in the Risk Assessment. A number of the key observations 
regarding the magnitude of risk remaining upon simulation of just 
meeting the alternative suites of standards are analogous to the 
observations identified above for simulation of just meeting the 
current standards (U.S. EPA, 2011a, pp. 2-97 to 2-100).
---------------------------------------------------------------------------

    \58\ Patterns of risk reduction across alternative annual 
standard levels, in terms of percent change relative to risk 
estimates upon simulating just meeting the current standards, are 
similar for all health endpoints modeled (i.e., all-cause, ischemic 
heart disease-related, and cardiopulmonary-related mortality). This 
similarity reflects the fact that the concentration-response 
functions used in the quantitative risk assessment are close to 
linear across the range of ambient PM2.5 concentrations 
evaluated. However, estimated incidence will vary by health endpoint 
(U.S. EPA, 2011a, pp. 2-93 to 2-94, footnote 70).
---------------------------------------------------------------------------

    With regard to characterizing estimates of PM2.5-related 
risk associated with simulation of alternative standards, the Policy 
Assessment recognizes that greater overall confidence is associated 
with estimates of risk reduction than for estimates of absolute risk 
remaining (U.S. EPA, 2011a, p. 2-94). Furthermore, the Policy 
Assessment recognizes 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 
(U.S. EPA, 2011a, p. 2-97). In addition, the Policy Assessment observes 
that in considering the overall confidence associated with the 
quantitative analyses, the Risk Assessment recognizes that: (1) 
Substantial variability exists in the magnitude of risk remaining 
across urban study areas and (2) in general, higher confidence is 
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.
    The variability in risk is a consequence of the substantial 
differences in the annual mean PM2.5 concentrations across 
urban study areas that result from simulating just meeting current or 
alternative standards. As PM2.5 concentrations decrease from 
the mean PM2.5 concentrations, the Risk Assessment concludes 
there is decreasing confidence in the risk estimates (U.S. EPA, 2010a, 
p. 5-16). As lower long-term mean PM2.5 concentrations are 
simulated (i.e., ambient concentrations further from

[[Page 38917]]

recent air quality conditions), the potential variability in such 
factors as the spatial pattern of ambient PM2.5 reductions 
(i.e., rollback) increases, thereby introducing greater uncertainty 
into the simulation of composite monitor annual mean PM2.5 
concentrations, and, consequently, in the risk estimates (U.S. EPA, 
2010a, Appendix J).
    Based on consideration of the composite monitor annual mean 
PM2.5 concentrations involved in estimating long-term 
exposure-related mortality, the Risk Assessment has higher confidence 
in using those concentrations that generally fall well within the range 
of ambient PM2.5 concentrations considered in fitting the 
concentration-response functions used (i.e., within one standard 
deviation of the mean PM2.5 concentration reported in 
Krewski et al. (2009) for 1999-2000) as inputs to the risk model. For 
example, with the exception of one urban study area, those areas 
estimated to have risk reductions using alternative annual standard 
levels of 13 and 14 [micro]g/m\3\ had simulated composite monitor 
annual mean concentrations ranging from approximately 10.6 to 13.3 
[micro]g/m\3\. With lower alternative annual standard levels of 12 
[micro]g/m\3\ and 10 [micro]g/m\3\, the composite monitor annual mean 
values ranged from approximately 9.0 to 11.4 [micro]g/m\3\ and 7.6 and 
8.9 [micro]g/m\3\, respectively. These concentrations are towards the 
lower end of the range of ACS data (in some cases approaching the 
lowest measured level) used in fitting the concentration-response 
functions, particularly for an annual standard level of 10 [micro]g/
m\3\, and, thus, the Policy Assessment concludes there is less 
confidence in the risk estimates associated with these levels compared 
with those for the higher alternative annual standard levels considered 
(U.S. EPA, 2011a, p. 2-99). Thus, while simulation of risks for an 
alternative annual standard level of 10 [micro]g/m\3\ suggests that 
additional risk reductions could be expected with alternative annual 
standards below 12 [micro]g/m\3\, the Policy Assessment recognizes that 
there is potentially greater uncertainty associated with these risk 
estimates compared with estimates generated for the higher alternative 
annual standard levels considered in the quantitative risk assessment, 
since these estimates required simulation of relatively greater 
reductions in ambient PM2.5 concentrations (U.S. EPA, 2011a, 
p. 2-98).
    The results of simulating alternative suites of PM2.5 
standards including a combination of alternative annual and 24-hour 
standard levels suggest 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 the alternative 24-hour standard is highly 
variable (U.S. EPA, 2010a, section 4.2.2). Thus, the Risk Assessment 
concludes more consistent reductions in estimated risk and consequently 
degrees of public health protection are estimated to result from 
simulating just meeting the alternative annual standard levels 
considered (U.S. EPA, 2010a, pp. 5-15 to 5-16). Furthermore, the Policy 
Assessment concludes that the urban study areas with the greatest 
degree of estimated reduction associated with simulating just meeting 
alternative 24-hour standard levels of 30 and 25 [micro]g/m\3\ also had 
the lowest estimated annual mean PM2.5 concentrations, and, 
therefore, there was substantially lower confidence in these risk 
estimates (U.S. EPA, 2011a, pp. 2-99 to 2-100).
    Based on the consideration of both the qualitative and quantitative 
assessments of uncertainty, the Risk Assessment concludes 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 Risk Assessment conclude 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 \59\ (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 recognizes that the currently available scientific 
information includes evidence for a broader range of health endpoints 
and at-risk populations beyond those included in the quantitative risk 
assessment, including lung function growth and respiratory symptoms in 
children and reproductive and developmental effects (U.S. EPA, 2011a, 
section 2.2.1).
---------------------------------------------------------------------------

    \59\ Most of the alternative model specifications supported by 
the currently available scientific information produced risk 
estimates that are higher (by up to a factor of 2 to 3) than the 
core risk estimates (U.S. EPA, 2011a, pp. 2-40 and 2-41).
---------------------------------------------------------------------------

    In considering the set of quantitative risk estimates and related 
uncertainties and limitations related to long- and short-term 
PM2.5 exposure discussed above together with consideration 
of the health endpoints which could not be quantified, the Policy 
Assessment concludes this information provides 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 (U.S. EPA, 2011a, 
p. 2-47). Furthermore, while the alternative 24-hour standard levels 
considered (when controlling) did result in additional estimated risk 
reductions beyond those estimated for alternative annual standards 
alone, these additional estimated reductions are highly variable, in 
part due to different rollback approaches. Conversely, the Risk 
Assessment recognizes 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).

D. Conclusions on the Adequacy of the Current Primary PM2.5 Standards

    The initial issue to be addressed in the current review of the 
primary PM2.5 standards is whether, in view of the 
additional information now available, the existing standards should be 
retained or revised. In evaluating whether it is appropriate to retain 
or revise the current suite of standards, the Administrator considered 
the scientific information from the last review and the broader body of 
evidence and information now available. The Administrator has taken 
into account both evidence- and risk-based considerations in developing 
conclusions on the adequacy of the current primary PM2.5 
standards. Evidence-based considerations (section III.D.1) 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). The risk-
based considerations (section III.D.2) 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). The advice received from CASAC is discussed in section 
III.D.3. Finally, the Administrator's proposed conclusion on the 
adequacy of the current PM2.5 primary standards is provided 
in section III.D.4.

[[Page 38918]]

1. Evidence-Based Considerations in the Policy Assessment
    In light of the health evidence described above, specifically with 
regard to factors contributing to greater susceptibility to health 
effects associated with ambient PM2.5 exposures, the Policy 
Assessment considers the extent to which the currently available 
scientific evidence reports associations between fine particle 
exposures and health effects that extend to air quality concentrations 
that are lower than had previously been observed or that have been 
observed in areas that would likely meet the current suite of 
PM2.5 standards (U.S. EPA, 2011a, section 2.2.1). As noted 
above, the Integrated Science Assessment concludes there is no evidence 
to support the existence of a discernible threshold below which effects 
would not occur (U.S. EPA, 2009a, section 2.4.3).
a. Associations With Long-term PM2.5 Exposures
    With regard to associations observed in long-term PM2.5 
exposure studies, the Policy Assessment recognizes that extended 
follow-up analyses of the ACS and Harvard Six Cities studies provide 
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 
[micro]g/m\3\ across 50 metropolitan areas for 1979-1983 (Pope et al., 
1995; Krewski et al., 2000).\60\ 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 (1979-1983) 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 with a long-term mean PM2.5 concentration of 18.0 
[micro]g/m\3\ for 1980-1985 (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 [micro]g/m\3\ 
for 1980-1988 \61\ (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).
---------------------------------------------------------------------------

    \60\ The study periods referred to in the Policy Assessment 
(U.S. EPA, 2011a) and in this proposed 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 status data that were included.
    \61\ Aggregate mean concentration provided by study author 
(personal communication from Dr. Francine Laden, 2009).
---------------------------------------------------------------------------

    New cohort studies provide 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 greater in magnitude (U.S. EPA, 2011a, 
pp. 2-32 to 2-33). The WHI study reported positive and most often 
statistically significant associations between long-term 
PM2.5 exposure and cardiovascular-related mortality, with 
much larger relative risk estimates than in the ACS and Harvard Six 
Cities studies, as well as morbidity effects at an aggregate long-term 
mean PM2.5 concentration of 12.9 [mu]g/m\3\ for 2000 (Miller 
et al., 2007).\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 2000-2002. The MCAPS 
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 [micro]g/m\3\, although 
no association was reported for the western region with an aggregate 
long-term PM2.5 median concentration of 13.1 [micro]g/m\3\ 
(U.S. EPA, 2009a, p. 7-88; Zeger et al., 2008).\63\ 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).
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    \62\ Miller et al. (2007) studied postmenopausal women without 
previous cardiovascular disease in 36 study areas from 1994 to 1998, 
with a median follow-up period of six years. The ambient 
PM2.5 monitor nearest to a study subject's residence 
(within 30 miles or 48 kilometers) was identified and used to assign 
long-term mean PM2.5 concentrations to each subject. The 
annual average concentration in the year 2000 was the primary 
exposure measure because of the substantially increased network of 
monitors in that year, as compared with previous years. Miller et 
al. (2007) reported a long-term mean PM2.5 concentration 
across study areas of 13.5 [mu]g/m\3\. This concentration was 
presented in the Integrated Science Assessment (U.S. EPA, 2009a, 
Figure 2-2, Table 7-8) and discussed in the second draft Policy 
Assessment (U.S. EPA, 2010f, Figure 2-4). In response to a request 
from the EPA for additional information on the air quality data used 
in selected epidemiological studies (Hassett-Sipple and Stanek, 
2009), study investigators provided updated air quality data for the 
study period. The updated long-term mean PM2.5 
concentration provided by the study authors was 12.9 [mu]g/m\3\ 
(personal communication from Cynthia Curl, 2009; Stanek et al., 
2010). The EPA notes that this updated long-term mean concentration 
matches the composite monitor approach annual mean calculated by 
staff for the year of air quality data (i.e., 2000) considered by 
the study investigators (Hassett-Sipple et al., 2010, Attachment A, 
p. 6). The updated air quality data for the Women's Health 
Initiative study was presented and considered in the final Policy 
Assessment (U.S. EPA, 2011a, p. 2-32). The Policy Assessment notes 
that in comparison to other long-term exposure studies, the WHI 
study was more limited in that it was based on only one year of air 
quality data (U.S. EPA, 2011a, p. 2-82).
    \63\ 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 
[micro]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).
---------------------------------------------------------------------------

    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 [mu]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\. 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).
    In also considering health effects for which the Integrated Science 
Assessment concludes evidence is suggestive of a causal relationship, 
the Policy Assessment notes a limited number of birth outcome studies 
that reported positive and statistically significant effects related to 
aggregate long-term mean PM2.5 concentrations

[[Page 38919]]

down to approximately 12 [mu]g/m\3\ (U.S. EPA, 2011a, p. 2-33).
    Collectively, the Policy Assessment concludes that currently 
available evidence provides support for associations between long-term 
PM2.5 exposure and mortality and morbidity effects that 
extend to air quality concentrations that are lower than had previously 
been observed, with aggregate long-term mean PM2.5 
concentrations extending to well below the level of the current annual 
standard. These studies evaluated a broader range of health outcomes in 
the general population and in at-risk populations than were considered 
in the last review, and include extended follow-up for prospective 
epidemiological studies that were important in the last review as well 
as additional evidence in important new cohorts.
b. Associations With Short-term PM2.5 Exposures
    In light of the mixed findings reported in single-city, short-term 
exposure studies, the Policy Assessment places 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 recognizes 
that long-term mean concentrations reported in new multi-city U.S. and 
Canadian studies provide evidence of associations between short-term 
PM2.5 exposure and mortality at similar air quality 
distributions than 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 indicate the association between short-term 
exposure to PM2.5 and total mortality and cardiovascular- 
and respiratory-related mortality is 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 notes 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 [mu]g/m\3\ (personal 
communication with Dr. Antonella Zanobetti, 2009).\64\
---------------------------------------------------------------------------

    \64\ 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 MCAPS 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).\65\ In an 
extended analysis of the MCAPS study, 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 indicate that effects are occurring in areas that would meet the 
current standards but also suggest 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).
---------------------------------------------------------------------------

    \65\ This sub-analysis was not included in the original 
publication (Dominici et al., 2006a). 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 concludes that the findings 
from short-term PM2.5 exposure studies provide 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 are 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, the Policy Assessment first concludes that 
there is 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 
addresses the question of whether the available evidence supports 
consideration of standards that are more protective than the current 
standards. In so doing, the Policy Assessment considers whether there 
is 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 provide 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 recognizes 
that important limitations and uncertainties associated with this 
expanded body of scientific evidence, noted above in section III.B.2, 
need 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 
concludes 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 concludes this evidence provides strong support 
for considering fine particle standards that would afford increased 
protection beyond that

[[Page 38920]]

afforded by the current standards (U.S. EPA, 2011a, p. 2-35).
2. Summary of Risk-Based Considerations in the Policy Assessment
    In addition to evidence-based consideration, the Policy Assessment 
also considers 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 notes 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; 
\66\ 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.
---------------------------------------------------------------------------

    \66\ 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 concludes these risks are 
important from a public health standpoint (U.S. EPA, 2011a, p. 2-47). 
This conclusion reflects consideration of both the severity and the 
magnitude of the effects. For example, the risk assessment indicates 
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 \67\ (U.S. 
EPA, 2011a, pp. 2-46 to 2-47). Moreover, additional risks are 
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 quantitative assessments of 
uncertainty completed as part of the quantitative risk assessment, the 
Risk Assessment concludes that it is 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 above provides 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).
---------------------------------------------------------------------------

    \67\ 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).
---------------------------------------------------------------------------

    In considering the risks estimated to occur upon simulating just 
meeting the current PM2.5 standards, the Policy Assessment 
concludes that these estimated risks can reasonably be judged to be 
important from a public health perspective and provide strong support 
for consideration of alternative standards that would provide increased 
protection beyond that afforded by the current PM2.5 
standards (U.S. EPA, 2011a, p. 2-48).
3. CASAC Advice
    CASAC, based on their review of drafts of the Integrated Science 
Assessment, the Risk Assessment, and the Policy Assessment, has 
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).
4. Administrator's Proposed Conclusions Concerning the Adequacy of the 
Current Primary PM2.5 Standards
    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 staff conclusions and associated 
rationales presented in the Policy Assessment, views expressed by 
CASAC, and public comments. In particular, the Administrator recognizes 
that the Integrated Science Assessment concludes 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. Moreover, the 
Administrator reflects that these effects have 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. See 
American Trucking Associations v. EPA, 283 F. 3d at 369, 376 (revision 
of level of existing standards justified when effects are observed in 
areas that meet those standards). With regard to the results of the 
quantitative risk assessment, the Administrator notes that the Risk 
Assessment concludes that the risks estimated to remain upon simulation 
of just meeting the current standards are important from a public 
health standpoint in terms of both the severity and magnitude of the 
effects.
    Based on her consideration of these conclusions, as well as 
consideration of CASAC's conclusion that the evidence and risk 
assessment clearly call into question the adequacy of the public health 
protection provided by the current PM2.5 NAAQS, the 
Administrator provisionally concludes that the current primary 
PM2.5 standards, taken together, are not requisite to 
protect public health with an adequate margin of safety and that 
revision is needed to provide increased public health protection. The 
Administrator provisionally concludes that the scientific evidence and 
information on risk provide strong support for consideration of 
alternative standards that would provide increased public health 
protection beyond that afforded by the current PM2.5 
standards.

[[Page 38921]]

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 concludes that it is 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, 2011, p. 2-50).
    The Policy Assessment also considers the expanded body of evidence 
available in this review to consider whether there is sufficient 
evidence to support a separate standard for ultrafine particles \68\ or 
whether there is 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).
---------------------------------------------------------------------------

    \68\ 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 concludes this evidence is still very 
limited and largely focused on exposure to diesel exhaust, for which 
the Integrated Science Assessment concludes it is unclear if 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 notes 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 recognizes 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 concludes 
that the currently available evidence is suggestive of a causal 
relationship between short-term exposures to ultrafine particles and 
cardiovascular and respiratory effects. Furthermore, the Integrated 
Science Assessment concludes that evidence is 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 
recognizes a national characterization of concentrations, temporal and 
spatial patterns, and trends is not possible at this time, and the 
availability of ambient ultrafine measurements to support health 
studies is 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 
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 suggest 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

[[Page 38922]]

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 concludes that this 
information is 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 concludes 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, there are 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). Data from the 
CSN provide 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).\69\ 
The CSN provides PM2.5 speciation measurements generally on 
a one-in-three or one-in-six day sampling schedule and, thus, do not 
capture data every day at most sites.\70\
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    \69\ 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).
    \70\ 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 recognizes that several new multi-city 
studies evaluating short-term exposures to fine particle constituents 
are now available. These studies continue 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 notes the most 
significant new evidence is 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., 2006). Using health data from a 
cohort of U.S. military veterans and PM2.5 measurement data 
from the CSN, Lipfert et al. (2006) 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 associated with 
a range of health endpoints, the Integrated Science Assessment reports 
that the currently available evidence suggests associations between 
cardiovascular effects and a number of specific PM2.5-
related source categories, specifically 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. These studies include a study that 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). There 
is less consistency in associations observed between sources of fine 
particles and respiratory health effects, which may be partially due to 
the fact that fewer studies have evaluated respiratory-related outcomes 
and measures. However, there is 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). Respiratory effects relating to 
the crustal/soil/road dust and traffic sources of PM have been observed 
in asthmatic children and adults (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). 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 
morality 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

[[Page 38923]]

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 concludes 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 concludes 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 concludes 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 concludes 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 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 proposes to retain 
PM2.5 as the indicator for fine particles. Further, the 
Administrator provisionally concludes 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. 
Furthermore, the Administrator also provisionally concludes that the 
currently available scientific information does 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.
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.
    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 concludes 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 notes 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 is much 
sparser than for cardiovascular effects and continues to be very 
limited. The Integrated Science Assessment concludes 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 concludes 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 provide 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 observes that there are few studies 
presently available to deduce a general

[[Page 38924]]

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 concludes that there is no 
basis to consider a seasonal averaging time separate from a 24-hour 
averaging time.
    Based on the above considerations, the Policy Assessment concludes 
that the currently available information provides strong support for 
consideration of retaining current annual and 24-hour averaging timers 
but does not provide support for considering alternative averaging 
times (U.S. EPA, 2011a, p. 2-58). In addition, CASAC considers it 
appropriate to retain the current annual and 24-hour averaging times 
for the primary PM2.5 standards (Samet, 2010c, pp. 2 to 3). 
The Administrator concurs with the staff conclusions and CASAC advice 
and proposes 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 concludes, 
consistent with conclusions reached in the Policy Assessment and by 
CASAC, that the currently available information is 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.
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, we consider whether 
currently available information supports consideration of alternative 
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 (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 area or 
were taken from a single monitor that was selected to represent 
community-wide exposures.
    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 discussed in section III.B.3, 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 is 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, would provide 
appropriate public health protection.
    In considering the potential for disproportionate impacts on at-
risk populations, the Policy Assessment recognizes an update of an air 
quality analysis conducted for the last review (U.S. EPA, 2011a, pp. 2-
59 to 60; Schmidt, 2011a, Analysis A). This analysis focuses on 
determining if 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 
considers whether persons of lower socioeconomic status 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, 
2011a, Analysis A). Recognizing the limitations of such cross-sectional 
analyses, the Policy Assessment observes 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 
concludes 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 concludes that it is 
appropriate to consider revising the form of the annual 
PM2.5 standard such that it does 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 \71\ (U.S. EPA, 2011a, p. 2-
60).
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    \71\ As discussed in section VIII.B.1 below, the EPA is 
proposing to revise several terms associated with PM2.5 
monitor placement. Specifically, the EPA is proposing to revoke the 
term ``community-oriented'' and replace it with the term ``area-
wide'' monitoring.
---------------------------------------------------------------------------

    The CASAC agreed with staff conclusions that it is ``reasonable'' 
for the EPA to eliminate the spatial averaging provisions (Samet, 
2010d, p. 2). Further, in CASAC's comments on

[[Page 38925]]

the first draft Policy Assessment, they 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 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 proposes to revise the form of the 
annual PM2.5 standard to eliminate the use of spatial 
averaging. Thus, the Administrator proposes 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\72\ 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.
---------------------------------------------------------------------------

    \72\ As discussed in section VIII.B.2.b below, the EPA proposes 
that PM2.5 monitoring sites at micro- and middle-scale 
locations be comparable to the annual standard unless the monitoring 
site has been approved by the Regional Administrator as a 
``relatively unique micro-scale, or localized hot-spot, or unique 
middle-scale site.''
---------------------------------------------------------------------------

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.\73\
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    \73\ 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).
---------------------------------------------------------------------------

    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 considers air quality data reported in 2000 to 2008 
to update our understanding of the ratio between peak-to-mean 
PM2.5 concentrations. This analysis provides evidence that 
the 98th percentile value is a more stable metric than the 99th 
percentile (U.S. EPA, 2011a, Figure 2-2, p. 2-62).
    The Agency recognizes 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.\74\ As in previous 
reviews, the EPA recognizes that a concentration-based form, compared 
to an exceedance-based form, is 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 concludes 
that a concentration-based form, when averaged over three years, 
provides an appropriate balance between limiting peak pollutant 
concentrations and providing a stable regulatory target, thus 
facilitating the development of more stable implementation programs.
---------------------------------------------------------------------------

    \74\ 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. The context for the court's discussion is 
identical to that here; whether to adopt a 98th percentile form for 
a 24-hour primary PM2.5 standard intended to provide 
supplemental protection for a generally controlling annual standard.
---------------------------------------------------------------------------

    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 proposes to retain the 
98th percentile form of the 24-hour standard. The Administrator 
provisionally concludes 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.
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 exposures 
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 [micro]g/m\3\ \75\ was challenged and on judicial 
review, the D.C. 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.
---------------------------------------------------------------------------

    \75\ Throughout this section, the annual standard level is 
denoted as an integer value for simplicity, although, as noted above 
in section II.B.1, Table 1, the standard level is defined to one 
decimal place, such that the current standard level is 15.0 
[micro]g/m\3\. Alternative standard levels discussed in this section 
are similarly defined to one decimal place.
---------------------------------------------------------------------------

a. Approach Used in the Policy Assessment
    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 uses an 
approach that integrates evidence-based and risk-based considerations, 
takes into account CASAC advice, and considers the issues raised by the 
court in remanding the primary annual PM2.5 standard. 
Following the general approach outlined in section III.A.3, for the 
reasons discussed below, the Policy Assessment concludes it is 
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 is 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.

[[Page 38926]]

    Beyond looking directly at the relevant epidemiologic evidence, the 
Policy Assessment considers the extent to which specific alternative 
PM2.5 standard levels are 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.3 above, patterns of increasing estimated risk reductions are 
generally observed as either the annual or 24-hour standard, or both, 
are 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 
observes, as discussed in section III.A.3, that analyses conducted for 
this and previous reviews demonstrate 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.3, the Risk Assessment observes 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 notes that while the 
results of simulating alternative suites of PM2.5 standards 
including different combinations of alternative annual and 24-hour 
standard levels suggest 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 is 
highly variable, in part due to the choice of rollback approached used 
(U.S. EPA, 2010a, p. 5-17).
    Therefore, the Policy Assessment concludes, consistent with CASAC 
advice (Samet 2010c, p. 1), that it is appropriate to set a ``generally 
controlling'' annual standard that will lower a wide range of ambient 
24-hour concentrations. The Policy Assessment concludes 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 believes 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.
    The Policy Assessment recognizes 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 concludes it is 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.
    Based on the above considerations, the approach used in the Policy 
Assessment to identify alternative standard levels that are appropriate 
for consideration focuses on translating information from 
epidemiological studies into the basis for staff conclusions on levels. 
This approach is broader and more integrative than the general approach 
used by the EPA in previous reviews (see summary in section III.A.3 
above; U.S. EPA, 2011a, sections 2.1.3 and 2.3.4.1) and reflects the 
more extensive and stronger body of scientific evidence now available 
on health effects related to long- and short-term PM2.5 
exposures, a more comprehensive quantitative risk assessment, and more 
extensive PM2.5 air quality data. In considering the 
currently available information, the Policy Assessment focuses on 
identifying levels for an annual standard and a 24-hour standard that, 
in combination, provide protection against health effects associated 
with both long- and short-term PM2.5 exposures. The Policy 
Assessment also considers the extent to which various combinations of 
annual and 24-hour standards reflect setting a generally controlling 
annual standard with a 24-hour standard providing supplemental 
protection (U.S. EPA, 2011a, sections 2.1.3, 2.3.4.1).
    As discussed in the Policy Assessment, EPA staff recognizes that 
there is no single factor or criterion that comprises the ``correct'' 
approach for reaching conclusions on alternative standard levels for 
consideration, but rather there are various approaches that are 
reasonable to consider (U.S. EPA, 2011a, section 2.3.4.1). In reaching 
conclusions in the Policy Assessment on the ranges of standard levels 
that are appropriate to consider, staff considered the relative weight 
to place on different evidence. The Policy Assessment initially focuses 
on long- and short-term PM2.5 exposure studies conducted in 
the U.S. and Canada and places the greatest weight on health outcomes 
judged in the Integrated Science Assessment as having evidence to 
support a causal or likely causal relationship. The Policy Assessment 
also considers the evidence for a broader range of health outcomes 
judged in the Integrated Science Assessment to have evidence suggestive 
of a causal relationship, specifically studies that focus on effects in 
susceptible populations, to evaluate whether this evidence provides 
support for considering lower alternative standard levels.
    Several factors were taken into account in placing relative weight 
on the body of available epidemiological studies, for example, study 
characteristics, including study design (e.g., time period of air 
quality monitoring, control for potential confounders); strength of the 
study (in terms of statistical significance and precision of results); 
and availability of population-level and air quality distribution data. 
As noted above in section III.A.3, the Policy Assessment places 
greatest weight on information from multi-city epidemiological studies 
to inform staff conclusions regarding alternative annual standard 
levels. These studies have a number of advantages compared to single-
city studies \76\ that include providing representation of ambient 
PM2.5 concentrations and potential health impacts across a 
range of diverse locations providing spatial coverage for different 
regions across the country, reflecting differences in PM2.5 
sources, composition, and potentially other exposure-related factors 
which might impact PM2.5-related risks; lack of

[[Page 38927]]

`publication bias' (U.S. EPA, 2004, p. 8-30); and consideration of 
larger study populations that afford the possibility of generalizing to 
the broader national population and provide higher statistical power 
than single-city studies to detect potentially statistically 
significant associations with relatively more precise effect estimates.
---------------------------------------------------------------------------

    \76\ As discussed in section III.B.1 above, the Policy 
Assessment recognizes that single-city studies provide ancillary 
evidence to multi-city studies in support of calling into question 
the adequacy of the current suite of standards. However, in light of 
the mixed findings reported in single-city short-term 
PM2.5 exposure studies, and the likelihood that these 
results are influenced by localized events and not representative of 
air quality across the country, the Policy Assessment places 
comparatively greater weight on the results from multi-city studies 
in considering alternative annual and 24-hour standard levels (U.S. 
EPA, 2011a, p. 2-64).
---------------------------------------------------------------------------

    In reaching conclusions in the Policy Assessment regarding 
alternative 24-hour standard levels that are appropriate to consider, 
staff also considers relevant information from single-city short-term 
PM2.5 exposure studies. Although, as discussed above, multi-
city studies have greater power to detect associations and provide 
broader geographic coverage in comparison to single-city studies, the 
extent to which effects reported in multi-city short-term 
PM2.5 exposure studies are associated with the specific 
short-term air quality in any particular location is unclear, 
especially when considering short-term concentrations at the upper end 
of the air quality distribution (i.e., at the 98th percentile value) 
for a given study area. In contrast, single-city studies are more 
limited in terms of power and geographic coverage but the link between 
reported health effects and the air quality in a given study area is 
more straightforward. Therefore, the Policy Assessment considers the 
results of both multi-city and single-city short-term exposure studies 
to inform staff conclusions regarding alternative levels that are 
appropriate to consider for a 24-hour standard that is intended to 
provide supplemental protection in areas where the annual standard may 
not offer appropriate protection against the effects of all short-term 
exposures (U.S. EPA, 2011a, pp. 2-62 to 2-65).
b. Consideration of the Annual Standard 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 are 
appropriate to consider focuses on characterizing the range of 
PM2.5 concentrations over which we have the most confidence 
in the associations reported in the epidemiological studies, and 
conversely where our confidence in the association becomes appreciably 
lower. The most direct approach to address this issue, consistent with 
CASAC advice (Samet, 2010c, p.10), is 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 three long-
term PM2.5 exposure studies reporting confidence intervals 
around concentration-response functions (i.e., Schwartz et al., 2008; 
Pope et al., 2002; Miller et al., 2007; U.S. EPA, 2011a, pp. 2-65 to 2-
70 and Figure 2-3).\77\ In its assessment of these studies, the Policy 
Assessment places greater weight on analyses that averaged across 
multiple concentration-response models since this approach represents a 
more robust examination of the underlying concentration-response 
relationship than analyses considering a single concentration-response 
model. Although these analyses of long-term exposure to 
PM2.5 provide information on the lack of any discernible 
population threshold, only Schwartz et al. (2008) conducted a multi-
model analysis to characterize confidence intervals around the 
estimated concentration-response relationship that can help inform at 
what PM2.5 concentrations we have appreciably less 
confidence in the nature of the underlying concentration-response 
relationship. Although analyses of confidence intervals associated with 
concentration-response relationships can help inform consideration of 
alternative standard levels, the Policy Assessment concludes that the 
single relevant analysis now available is too limited to serve as the 
principal basis for identifying alternative standard levels in this 
review (U.S. EPA, 2011a, p. 2-70).
---------------------------------------------------------------------------

    \77\ The EPA carefully analyzed the published evidence, but was 
unable to identify any short-term PM2.5 exposure studies 
that characterized confidence intervals around concentration-
response relationships. Nor did CASAC or public comments on this 
issue, as addressed in their comments on the second draft Policy 
Assessment, identify any additional analyses.
---------------------------------------------------------------------------

    The Policy Assessment explores other approaches that considered 
different statistical metrics to identify ranges of long-term mean 
PM2.5 concentrations that were most influential in 
generating health effect estimates in long- and short-term 
epidemiological studies, placing 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 recognizes 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 exposure studies. The long-
term mean concentrations represent air quality data typically used in 
epidemiological analyses and provide a direct link between 
PM2.5 concentrations and the observed health effects. 
Further, these data are available for all long- and short-term exposure 
studies analyzed and, therefore, represent the data set available for 
the broadest set of epidemiological studies.
    However, consistent with CASAC's comments on the second draft 
Policy Assessment \78\ (Samet, 2010d, p. 2), in preparing the final 
Policy Assessment, 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 range of PM2.5 concentrations down through 
which staff continued to have confidence in the associations observed 
in epidemiological studies and below which there is a comparative lack 
of data such that the staff's confidence in the relationship was 
appreciably less. This would also be the range of PM2.5 
concentrations which has the most influence on generating the health 
effect estimates reported in epidemiological studies. As discussed in 
section III.A.3 above, the Policy Assessment recognizes there is no one 
percentile value within a given distribution that is the most 
appropriate or ``correct'' way to characterize where our confidence in 
the associations becomes appreciably lower. The Policy Assessment 
concludes that focusing on concentrations within the lower quartile of 
a distribution, such as the range from the 25th to the 10th percentile, 
is reasonable to consider as a region within which we begin to have 
appreciably less confidence in the associations observed in 
epidemiological studies.\79\ In staff's

[[Page 38928]]

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).
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    \78\ 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).
    \79\ 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).
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    As outlined in section III.A.3 above, the Policy Assessment 
recognizes that there are 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 is 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 an 
appropriate surrogate for health event data.
    In applying this approach, the Policy Assessment focuses on 
identifying the broader range 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, 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). For the three studies for which both 
health event and study population data were available, EPA staff 
analyzed the reliability of using study population data as a surrogate 
for health event data. Based on these analyses, 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).
    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\.\80\ 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). By following the general approach used in previous PM 
NAAQS reviews, one could consider identifying alternative standard 
levels that are somewhat below the long-term mean PM2.5 
concentrations reported in these epidemiological studies.
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    \80\ 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).
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    With regard to consideration of additional information from 
epidemiological studies which is relevant to the second approach, EPA 
has compiled a summary of the range of PM2.5 concentrations 
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 \81\ (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.
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    \81\ Health event data (e.g., number of deaths, 
hospitalizations) occurring in a study population were obtained 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.
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    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. As noted above in section 
III.A.3 and discussed in the Policy Assessment, a policy approach that 
uses data based on composite monitor distributions to identify 
alternative standard levels, and then compares those levels to 
concentrations at appropriate maximum monitors to determine if an area 
meets a given standard, inherently has the potential to build in some 
margin of safety (U.S. EPA, 2011a, p. 2-14). In analyses conducted by 
EPA staff based on selected long- and short-term exposure studies, the 
Policy Assessment notes that the differences between the maximum and 
composite distributions were greater for studies with fewer years of 
air quality data (i.e., 1 to 3 years) and smaller numbers of study 
areas (i.e., 36 to 51 study areas). The differences in the maximum and 
composite monitor distribution were much smaller (i.e., generally 
within five percent) for studies with more years of air quality data 
(i.e., up to 6 years) and larger numbers of study areas (i.e., 112 to 
204 study areas) (Hassett-Sipple et al., 2010; U.S. EPA, 2010f, section 
2.3.4.1). Therefore, any margin of safety that may be provided by a 
policy approach that uses data based on composite monitor distributions 
to identify alternative standard levels, and then compares those levels 
to concentrations at appropriate maximum monitors to determine if an 
area meets a given standard, will vary depending upon the number of 
monitors and air quality distributions within a given area. See also, 
section III.A.3 above.
    Figure 4 summarizes statistical metrics for those studies included 
in Figures 1, 2, and 3 that provide evidence of statistically 
significant PM2.5-related effects, which are relevant to the 
two approaches for translating epidemiological evidence into standard 
levels discussed above. The top of Figure 4 includes information for 
long-term exposure studies evaluating health outcomes classified as 
having evidence of a casual or likely casual 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 casual or likely casual 
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.
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    In looking first at the long-term mean PM2.5 
concentrations reported in the multi-city long-term exposure studies, 
as summarized at the top of Figure 4, the Policy Assessment observes 
positive and often statistically significant associations at long-term 
mean PM2.5 concentrations ranging from 16.4 to 12.9 [mu]g/
m\3\ \82\ (Laden et al., 2006; Lipfert et al., 2006; Krewski et al., 
2009; Goss et al., 2004; Miller et al.; 2007; Zeger et al., 2008; Eftim 
et al., 2008; Dockery et al., 1996; McConnell et al., 2003). In 
considering the one long-term PM2.5 exposure study for which 
health event data are available (Krewski et al., 2009), the Policy 
Assessment observes that the long-term mean PM2.5 
concentrations corresponding with study areas contributing to the 25th 
and 10th percentiles of the distribution of mortality data are 12.0 
[mu]g/m\3\ and 10.2 [mu]g/m\3\, respectively (Figure 4; U.S. EPA, 
2011a, Figure 2-7; Rajan et al., 2011, Table 1). As identified above, 
although less directly relevant than event data, the number of 
participants within each study area can be used as a surrogate for 
health event data in relation to the distribution of PM2.5 
concentrations. The long-term mean PM2.5 concentrations 
corresponding with study areas contributing to the 25th and 10th 
percentiles of the distribution of study participants for Miller et al. 
(2007) were 11.2 [mu]g/m\3\ and 9.7 [mu]g/m\3\, respectively (Figure 4; 
U.S. EPA, 2011a, Figure 2-7; Rajan et al., 2011, Table 1).
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    \82\ As discussed in section III.D.1.a above, the lowest long-
term mean PM2.5 concentration reported in the long-term 
exposure studies was based on updated air quality data for Miller et 
al. (2007). As noted in the Policy Assessment, these air quality 
data were based on only one year of ambient measurements (2000) and 
in comparison to other long-term exposure studies that considered 
multiple years of air quality data, were much more limited (U.S. 
EPA, 2011a, pp. 2-81 to 2-82).
---------------------------------------------------------------------------

    In then considering information from multi-city, short-term 
exposure studies reporting positive and statistically significant 
associations with these same broad health effect categories, as 
summarized in the middle of Figure 4, the Policy Assessment observes 
positive and statistically significant associations at long-term mean 
PM2.5 concentrations in a similar range of 15.6 to 12.8 
[mu]g/m\3\ (Franklin et al., 2007, 2008; Klemm and Mason, 2003; Burnett 
and Goldberg, 2003; Zanobetti and Schwartz, 2009; Burnett et al., 2004; 
Bell et al., 2008; Dominici et al., 2006a; see Figure 3). In 
considering the two multi-city, short-term PM2.5 exposure 
studies for which health event data are available, the Policy 
Assessment observes that the long-term mean PM2.5 
concentrations corresponding with study areas contributing to the 25th 
and 10th percentiles of the distribution of deaths and cardiovascular-
related hospitalizations are 12.5 [mu]g/m\3\ and 10.3 [mu]g/m\3\, 
respectively, for Zanobetti and Schwartz (2009), and 11.5 [mu]g/m\3\ 
and 9.8 [mu]g/m\3\, respectively, for Bell et al. (2008) (Figure 4; 
U.S. EPA, 2011a, Figure 2-7; Rajan et al., 2011, Table 1).
    Taking into consideration additional studies of specific at-risk 
populations (i.e., children), the Policy Assessment expands its 
evaluation of the long-term exposure studies to include a broader range 
of health outcomes judged in the Integrated Science Assessment to have 
evidence suggestive of a causal relationship. This evidence was taken 
into account to evaluate whether it provides support for considering 
lower alternative levels than if weight were only placed on studies for 
which health effects have been judged in the Integrated Science 
Assessment to have evidence supporting a causal or likely causal 
relationship. The Policy Assessment makes note of a limited number of 
studies that provide emerging evidence for PM2.5-related low 
birth weight and infant mortality, especially related to respiratory 
causes during the post-neonatal period. This more limited body of 
evidence, as summarized at the bottom of Figure 4, indicates positive 
and often statistically significant effects associated with long-term 
PM2.5 mean concentrations in the range of 14.9 to 11.9 
[mu]g/m\3\ (Woodruff et al., 2008; Liu et al., 2007; Bell et al., 2007; 
see Figure 2). As illustrated in Figure 2, although Parker and Woodruff 
(2008) did not observe an association between quarterly estimates of 
exposure to PM2.5 and low birth weight in a multi-city U.S. 
study, other U.S. and Canadian studies did report positive and 
statistically significant associations between PM2.5 and low 
birth weight at lower ambient concentrations (Bell et al., 2007; Liu et 
al., 2007).\83\ There remain significant limitations (e.g., identifying 
the etiologically relevant time period) in the evaluation of evidence 
on the relationship between PM2.5 exposures and birth 
outcomes (U.S. EPA, 2009a, pp. 7-48 and 7-56) which should be taken 
into consideration in reaching judgments about how to weigh these 
studies of potential impacts on specific susceptible populations in 
considering alternative standard levels that provide protection with an 
appropriate margin of safety.
---------------------------------------------------------------------------

    \83\ As noted in section 7.4 of the Integrated Science 
Assessment, Parker et al. (2005) reported that over a 9-month 
exposure period (mean PM2.5 concentration of 15.4 [mu]g/
m\3\) a significant decrease in birth weight was associated with 
infants in the highest quartile of PM2.5 exposure as 
compared to infants exposed in the lowest quartile.
---------------------------------------------------------------------------

    With respect to carcinogenicity, mutagenicity, and genotoxicity 
(evidence suggestive of a causal relationship), the strongest evidence 
currently available is from long-term prospective cohort studies that 
report positive associations between PM2.5 and lung cancer 
mortality. At this time, 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, in 
selecting alternative standard levels appropriate to consider, the 
Policy Assessment noted that, in providing protection against mortality 
and cardiovascular and respiratory effects it is reasonable to 
anticipate that protection will also be provided for carcinogenicity, 
mutagenicity, and genotoxicity effects (U.S. EPA, 2011a, p. 2-78).
    In summarizing the currently available evidence and air quality 
information within the context of identifying potential alternative 
annual standard levels for consideration, the Policy Assessment first 
notes that the Integrated Science Assessment concludes there is no 
evidence of a discernible population threshold below which effects 
would not occur. Thus, health effects may occur over the full range of 
concentrations observed in the epidemiological studies. In the absence 
of any discernible thresholds, the general approach used in the Policy 
Assessment for identifying alternative standard levels that would 
provide appropriate protection against effects observed in 
epidemiological studies has focused on the central question of 
identifying the range of PM2.5 concentrations below the 
long-term mean concentrations where we continue to have confidence in 
the associations observed in epidemiological studies.
    In considering the evidence, the Policy Assessment recognizes 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 the weight placed on different aspects of the evidence and 
inherent uncertainties, considerations of different alternative 
standard levels could be supported.

[[Page 38935]]

    Given the currently available evidence and considering the various 
approaches discussed above, the Policy Assessment concludes it is 
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, a standard level of 12 [mu]g/m\3\, at 
the upper end of this range, is somewhat below the long-term mean 
PM2.5 concentrations reported in all the 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, 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 corresponds with 
approximately the 25th percentile of the available distributions of 
health events data in the studies for which population-level 
information was available.\84\ In addition, a level of 12 [mu]g/m\3\ 
would reflect some consideration of studies that provide more limited 
evidence of reproductive and developmental effects, which are 
suggestive of a causal relationship, in that it is about at the same 
level as the lowest long-term mean PM2.5 concentrations 
reported in such studies (see Figure 4).
---------------------------------------------------------------------------

    \84\ As outlined in section III.A.3, the Policy Assessment 
considers the 25th percentile to be the start of the range of 
PM2.5 concentrations below the mean within which the data 
become appreciably more sparse and, thus, where our confidence in 
the associations observed in epidemiological studies begins to 
become appreciably less.
---------------------------------------------------------------------------

    Alternatively, an annual standard level of 11 [mu]g/m\3\, at the 
lower end of this range, is 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 is within the 
range of PM2.5 concentrations corresponding to the 25th and 
10th percentiles of all the available distributions of such data.\85\ 
In addition, a level of 11 [mu]g/m\3\ is 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 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 concludes that the 
available evidence does not provide a sufficient basis to consider 
alternative annual standard levels below 11 [mu]g/m\3\ (U.S. EPA, 
2011a, p. 2-81).
---------------------------------------------------------------------------

    \85\ As discussed in section III.A.3, the Policy Assessment 
identifies the range from the 25th to the 10th percentiles as a 
reasonable range to consider, in that it is a range where we have 
appreciably less confidence in the associations observed in 
epidemiological studies (U.S. EPA, 2011a, p. 2-12).
---------------------------------------------------------------------------

    The Policy Assessment also considers the extent to which the 
available evidence provides a basis for considering alternative annual 
standard levels above 12 [mu]g/m\3\. As discussed below, the Policy 
Assessment concludes that it could be reasonable to consider a standard 
level up to 13 [mu]g/m\3\ based on a policy approach that tends to 
weigh 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 is placed on 
long-term exposure studies as a basis for an annual standard level. A 
level of 13 [mu]g/m\3\ is 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\ is the WHI 
study (Miller et al., 2007). As noted in section III.D.1.a above, the 
Policy Assessment observes that in comparison to other long-term 
exposure studies, the WHI study was more limited in that it was based 
on only one year of air quality data (U.S. EPA, 2011a, pp. 2-81 to 2-
82). Thus, to the extent that less weight is placed on the WHI 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, the Policy Assessment notes that a level of 13 [mu]g/m\3\ is 
below the long-term mean PM2.5 concentrations reported in 
most such studies, but is above the long-term means of 12.8 and 12.9 
[mu]g/m\3\ reported in Burnett et al. (2004) and Bell et al. (2008), 
respectively. In considering these studies, the Policy Assessment finds 
no basis to conclude that these two studies are 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 concludes 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, 
such that taken together the suite of PM2.5 standards 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 notes 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.2 above, 
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 is too uncertain to support 
consideration of any lower level.
    Beyond evidence-based considerations, the Policy Assessment also 
considered the extent to which quantitative risk assessment supports 
consideration of these alternative standard levels or provides 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 reductions estimated for 
standard levels of 13 and 12 [mu]g/m\3\, noting 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

[[Page 38936]]

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 concludes 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 concludes 
that the currently available evidence most strongly supports 
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 
concludes 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 would also reflect placing weight on information from 
studies that help 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 is appreciably less at 
lower concentrations.
c. Consideration of the 24-Hour Standard in the Policy Assessment
    As recognized in section III.A.3 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 concludes that it is 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 
considers 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 takes 
into account information from multi-city studies as well as single-city 
studies. The Policy Assessment considers 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 considers 
information on the long-term mean PM2.5 concentrations from 
these studies.
    In addition to considering the epidemiological evidence, the Policy 
Assessment also considers 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 help 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 recognizes that to the extent these 
studies were conducted in areas that likely did not meet one or both of 
the current standards, such studies do 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 places 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, 
the Policy Assessment observes 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 Policy Assessment notes 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 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). To the extent that consideration is given to revising 
the level of the annual standard, as discussed above in section 
III.E.4.b, the Policy Assessment recognizes 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 work in conjunction with an annual standard to 
provide appropriate public health protection, the Policy Assessment 
notes that to the extent that the level of the annual standard is 
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 effects observed in these multi-city studies (U.S. EPA, 2011a, 
p. 2-84).
    In summary, the Policy Assessment concludes that the multi-city, 
short-term exposure studies generally provide support for retaining the 
24-hour standard level at 35 [mu]g/m\3\ 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 concludes that 
the multi-city studies provide 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 in conjunction with an 
overall long-term mean

[[Page 38937]]

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 are appropriate to consider, the Policy Assessment 
also takes into account relevant information from single-city studies 
that evaluated effects associated with short-term PM2.5 
exposures. The Policy Assessment recognizes that these studies may 
provide additional insights regarding impacts on susceptible 
populations and/or on areas with isolated peak concentrations. 
Although, as discussed in section III.E.4.a above, multi-city studies 
have advantages over single-city studies in terms of statistical power 
to detect associations and broader geographic coverage as well as other 
factors such as less likelihood of publication bias, reflecting 
differences in PM2.5 sources, composition, and potentially 
other factors that could impact PM2.5-related effects, 
multi-city studies often present overall effect estimates rather than 
single-city effect estimates. Since short-term air quality can vary 
considerably across cities, the extent to which effects reported in 
multi-city studies are associated with short-term air quality in any 
particular location is uncertain, especially when considering short-
term concentrations at the upper end of the distribution of daily 
PM2.5 concentrations (i.e., at the 98th percentile value). 
In contrast, single-city studies are more limited in terms of power and 
geographic coverage but the link between reported health effects and 
the air quality in a given study area is more straightforward to 
establish. Therefore, the Policy Assessment also considers evidence 
from single-city, short-term exposure studies to inform staff 
conclusions regarding alternative levels that are appropriate to 
consider for a 24-hour standard that is intended to provide 
supplemental protection in areas where the annual standard may not 
provide an adequate margin of safety against the effects of all short-
term PM2.5 exposures.
    As discussed above for the multi-city studies, the Policy 
Assessment takes into account both the 24-hour PM2.5 
concentrations in the single-city studies, focusing on the 98th 
percentile air quality values, as well as the long-term mean 
PM2.5 concentrations. The Policy Assessment considers 
single-city studies conducted in areas that would likely have met the 
current suite of PM2.5 standards as most useful for 
informing staff conclusions related to the level of the 24-hour 
standard (U.S. EPA, 2011a, Figure 2-9). The Policy Assessment notes 
that additional single-city studies summarized in that Figure 2-9 were 
conducted in areas that would likely have met one but not both of the 
current PM2.5 standards. To the extent changes in air 
quality designed to just meet the current suite of PM2.5 
standards are undertaken, one could reasonably anticipate additional 
public health protection will occur in these study areas. Therefore, 
the Policy Assessment concludes that these studies are not helpful to 
inform staff conclusions regarding alternative standard levels that are 
appropriate to consider (U.S. EPA, 2011a, p. 2-87).
    With regard to single-city studies that were conducted in areas 
that would likely have met both the current 24-hour and annual 
standards, the Policy Assessment first considers studies that reported 
positive and statistically significant associations. In considering 
this group of studies, the Policy Assessment notes Mar et al. (2003) 
reported a positive and statistically significant association for 
premature mortality in Phoenix with a long-term mean concentration of 
13.5 [mu]g/m\3\ in conjunction with a 98th percentile value of 32.2 
[mu]g/m\3\ (U.S. EPA, 2011a, Figure 2-9). To the extent that 
consideration is given to revising the level of the annual standard, 
within a range of 13 to 11 [mu]g/m\3\, as discussed above, additional 
protection would be provided for the effects observed in this study 
(U.S. EPA, 2011a, p. 2-87).
    Four additional studies reported positive and statistically 
significant associations with 98th percentile values within a range of 
31.2 to 25.8 [mu]g/m\3\ and long-term mean concentrations within a 
range of 12.1 to 8.5 [mu]g/m\3\ (Delfino et al., 1997; Peters et al., 
2001; Stieb et al., 2000; and Mar et al., 2004; U.S. EPA, 2011a, Figure 
2-9). Delfino et al. (1997) reported statistically significant 
associations between PM2.5 and respiratory emergency 
department visits for older adults (greater than 64 years old) but not 
young children (less than 2 years old), in one part of the study period 
(summer 1993) but not the other (summer 1992). Peters et al. (2001) 
reported a positive and statistically significant association between 
short-term exposure to PM2.5 (2-hour and 24-hour averaging 
times) and onset of acute myocardial infarction in Boston. Stieb et al. 
(2000) reported positive and statistically significant associations 
with cardiovascular- and respiratory-related emergency department 
visits in Saint John, Canada, in single pollutant models but not in 
multi-pollutant models (U.S. EPA, 2004, pp. 8-154 and 8-252 to 8-253). 
Mar et al. (2004) reported a positive and statistically significant 
association for short-term PM2.5 exposures in relation to 
respiratory symptoms among children but not adults in Spokane, however, 
this study had very limited statistical power because of the small 
number of children and adults evaluated.
    The Policy Assessment also considers short-term single-city 
PM2.5 exposure studies that reported positive but 
nonstatistically significant associations for cardiovascular and 
respiratory endpoints in areas that would likely have met both the 
current 24-hour and annual standards. The 98th percentile values 
reported in these studies ranged from 31.6 to 17.2 [mu]g/m\3\ and the 
long-term mean concentrations ranged from 13.0 to 7.0 [mu]g/m\3\ (U.S. 
EPA, 2011a, Figure 2-9). These studies included consideration of 
cardiovascular-related mortality effects in Phoenix (Wilson et al., 
2007), asthma medication use in children in Denver (Rabinovitch et al., 
2006), hospital admissions for hemorrhagic and ischemic stroke in 
Edmonton, Canada (Villeneuve et al., 2006), and hospital admissions for 
ischemic stroke/transient ischemic attack in Nueces County, TX 
(Lisabeth et al., 2008).
    Lastly, the Policy Assessment considers single-city studies 
conducted in areas that would likely have met both the current 24-hour 
and annual standards that reported null findings. The 98th percentile 
values reported in these studies ranged from 29.6 to 24.0 [mu]g/m\3\ 
and the long-term mean concentrations ranged from 10.8 to 8.5 [mu]g/
m\3\ (U.S. EPA, 2011a, Figure 2-9). These studies reported no 
associations with short-term PM2.5 exposures and 
cardiovascular-related hospital admissions and respiratory-related 
emergency department visits (Slaughter et al., 2005) and 
cardiovascular-related emergency department visits (Schreuder et al., 
2006) in Spokane; asthma exacerbation in children in Denver 
(Rabinovitch et al., 2004); and hospital admissions for transient 
ischemic attack in Edmonton, Canada (Villeneuve et al., 2006).
    Viewing the evidence as a whole, the Policy Assessment observes a 
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. 
Many of these studies had significant limitations (e.g., limited 
statistical power, limited exposure data) or equivocal results (i.e., 
mixed results within the same study area) as briefly identified above 
and discussed in more detail in the Policy Assessment (U.S. EPA, 2011a, 
p. 2-88). Other studies reported positive but not statistically

[[Page 38938]]

significant results or null associations also in areas that would 
likely have met the current suite of PM2.5 standards. 
Overall, the entire body of results from these single-city studies is 
mixed, particularly as 24-hour 98th percentile concentrations go below 
35 [mu]g/m\3\.
    Although a number of single-city studies report 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 greater and, thus, the 
Policy Assessment concludes there is less confidence in using these 
studies as a basis for setting the level of a standard. Therefore, the 
Policy Assessment concludes that the multi-city short-term exposure 
studies provide the strongest evidence to inform decisions on the level 
of the 24-hour standard, and the single-city studies do 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 takes into account air quality information based on county-
level 24-hour and annual design values to understand the implications 
of the alternative standard levels supported by the currently available 
scientific evidence, as discussed in section III.E.4.b above. As 
discussed in section III.A.3 above, the Policy Assessment concludes 
that a policy goal which includes 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, is 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. Therefore, the Policy 
Assessment considers the extent to which different combinations of 
alternative annual and 24-hour standard levels based on the evidence 
would support this policy goal (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 examines 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 concludes 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 represent 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 
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 concludes 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 would recognize 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).
    The Policy Assessment also recognizes an alternative approach to 
considering the evidence that provides some support for revising the 
level below 35 [mu]g/m\3\, perhaps as low as 30 [mu]g/m\3\ (U.S. EPA, 
2011a, p. 2-92). This alternative 24-hour standard level would be more 
compatible with an alternative annual standard of 13 [mu]g/m\3\ based 
on placing greater weight on one multi-city short-term exposure study 
(Bell et al., 2008) that reported positive and statistically 
significant effects at a 98th percentile value less than 35 [mu]g/m\3\ 
(i.e., 34.2 [mu]g/m\3\) in conjunction with a long-term mean 
concentration less than 13 [mu]g/m\3\ (i.e., 12.9 [mu]g/m\3\).
    Beyond evidence-based considerations, the Policy Assessment also 
considered the extent to which the quantitative risk assessment 
supports consideration of retaining the current 24-hour standard level 
or provides 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 provides 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 
does 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 concludes that while it is 
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 supports 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).
d. 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

[[Page 38939]]

``[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 and 24-hour 
standards that are appropriate to consider, including consideration of 
the combination of these standards supported by the available 
information (Samet, 2010d, p. ii). Specifically, 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 (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, EPA staff conducted additional analyses 
and modified their conclusions regarding alternative standard levels 
that are appropriate to consider. The staff conclusions in the final 
Policy Assessment (U.S. EPA, 2011a, section 2.3.4.4) differ 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, the EPA analyzed additional 
population-level data obtained from several study investigators. In 
commenting on draft staff conclusions in the second draft Policy 
Assessment, CASAC did not have an opportunity to review the staff 
analyses of distributional statistics to identify the broader range of 
PM2.5 concentrations that were most influential in 
generating health effect estimates in epidemiological studies (Rajan et 
al., 2011). In addition, CASAC was not aware of the revised long-term 
mean PM2.5 concentration in the WHI study as discussed in 
section III.D.1.a above or the staff's inclusion of that value in its 
evaluation of the evidence (i.e., in Figures 1 and 4 above and related 
discussion). The WHI study is the only long-term cohort study that 
provides information regarding effects classified as having evidence of 
a causal or likely causal relationship associated with a long-term 
PM2.5 concentration below 13 [mu]g/m\3\. Furthermore, CASAC 
did not have an opportunity to review the staff's revised rationale for 
the combinations of alternative standards suggested in the final Policy 
Assessment.
e. Administrator's Proposed Conclusions on the Primary PM2.5 
Standard Levels
    In reaching her conclusions regarding appropriate alternative 
standard levels to consider, 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 and the advice of CASAC. As an initial matter, the 
Administrator agrees with the 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. This is consistent with the approach taken in the 
review completed in 1997, in contrast to considering each standard 
separately, as was done in the review completed in 2006. Furthermore, 
based on the evidence and quantitative risk assessment, the 
Administrator provisionally concludes 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 concludes 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.
    In reaching decisions on alternative standard levels to propose, 
the Administrator judges that it is most appropriate to examine where 
the evidence of associations observed in the epidemiological studies is 
strongest and, conversely, where she has 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 recognizes 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 takes note that these 
studies, along with evidence available in the last review, provide 
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 recognizes 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 
recognizes new U.S. multi-city studies that greatly expand and 
reinforce our understanding of mortality and morbidity effects

[[Page 38940]]

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).
    The newly available scientific evidence builds 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 
recognizes that the weight of evidence, as evaluated in the Integrated 
Science Assessment, is strongest for health endpoints classified as 
having evidence of a causal relationship. These relationships include 
those between long- and short-term PM2.5 exposures and 
mortality and cardiovascular effects. She recognizes that the weight of 
evidence is also strong for health endpoints classified as having 
evidence of a likely causal relationship, which include those between 
long- and short-term PM2.5 exposures and respiratory 
effects. In addition, the Administrator makes 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.
    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 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 
recognizes, in taking note of CASAC advice and the distributional 
statistics analysis discussed in section III.E.4.b above and in the 
Policy Assessment, that there is significantly greater confidence in 
observed associations over certain parts of the air quality 
distributions in the studies, and conversely, that there is 
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.d above, 
the Administrator generally agrees that it is appropriate to consider a 
level for an annual standard that is 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 is strongest at and around the long-term average where 
the data in the study are most concentrated, she understands that this 
approach does not provide a bright line for reaching decisions about 
appropriate standard levels. The Administrator notes that long-term 
mean PM2.5 concentrations are available for each study 
considered and, therefore, represent the most robust data set to inform 
her decisions on appropriate annual standard levels. She also notes 
that the overall study mean PM2.5 concentrations are 
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 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 understands that it is appropriate to consider the weight 
of evidence for the health endpoints evaluated in such studies in 
giving weight to this information.
    Based on the information summarized in Figure 4 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 observes 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). She observes 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 observes 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).\86\
---------------------------------------------------------------------------

    \86\ 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 considers 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 understands 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 within 
which the data become appreciably more sparse and, thus, where her 
confidence in the associations observed in epidemiological studies 
would become appreciably less. As discussed in section III.E.4.b above 
and summarized in Figure 4, the Administrator takes note of additional 
population-level data that are available for four studies (Krewski et 
al., 2009; Miller et al., 2007; Bell et al., 2008; Zanobetti and 
Schwartz, 2009), each of which report 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 observes that these 
values range from somewhat above to somewhat below 12 [mu]g/m\3\ 
(Figure 4). The Administrator recognizes that these four studies 
represent some of the strongest evidence available within the overall 
body of scientific evidence and notes 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). However, 
the Administrator also recognizes that additional population-level data 
are available for only these four studies and, therefore, she believes 
that these studies comprise a more limited data set than one based on 
long-term mean PM2.5 concentrations for which data are 
available for all studies considered, as discussed above. In 
considering this information, the Administrator notes that CASAC 
advised that information about the long-term PM2.5 
concentrations that were most influential in generating the health 
effect estimates in epidemiological

[[Page 38941]]

studies can help to inform selection of an appropriate annual standard 
level.
    The Administrator recognizes, as summarized in section III.B.2 
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 fixed monitor sites, and the nature, 
magnitude, and confidence in 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. The Administrator 
also recognizes 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.
    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 standard levels are appropriate to propose, the 
Administrator recognizes, as did EPA staff in the Policy Assessment, 
that there is no single factor or criterion that comprises the 
``correct'' approach to weighing the various types of available 
evidence and information, but rather there are various approaches that 
are appropriate to consider. The Administrator further recognizes 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 leads the Administrator to consider various approaches 
to weighing the evidence so as to identify appropriate standard levels 
to propose. In so doing, the Administrator encourages 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 notes 
the advice of CASAC that the currently available scientific 
information, including epidemiological and toxicological evidence as 
well as risk and air quality information, provides 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 recognizes that the Policy 
Assessment concludes 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 
concludes that the evidence most strongly supports 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 
observes that CASAC did not express support for any specific levels or 
combinations of standards within in these ranges, although she 
recognizes that CASAC did not have an opportunity to review additional 
information and analyses presented in the final Policy Assessment 
prepared in response to CASAC's recommendations on the second draft 
Policy Assessment. Nor did CASAC have an opportunity to review the EPA 
staff's revised rationale for the combinations of alternative standards 
presented in the final document.
    In considering the extent to which the currently available evidence 
and information provide support for specific standard levels within the 
ranges identified by CASAC and the Policy Assessment as appropriate for 
consideration, the Administrator initially considers standard levels 
within the range of 13 to 11 [mu]g/m\3\ for the annual standard. In so 
doing, the Administrator first considers 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 and discussed more broadly 
above. She notes 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 notes that the evidence of an 
association in any such study is strongest at and around the long-term 
average where the data in the study are most concentrated, and that 
long-term mean PM2.5 concentrations are available for each 
study considered and, therefore, represent the most robust data set to 
inform her decisions on appropriate annual standard levels. Further, 
she is mindful that this approach does not provide a bright line for 
reaching decisions about appropriate standard levels.
    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 notes 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 
notes 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 considers additional 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 notes 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

[[Page 38942]]

these studies. The Administrator also notes 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 
notes that there is 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 becomes appreciably 
less.
    In considering mean PM2.5 concentrations and 
distributional analyses from the various sets of epidemiological 
studies noted above, the Administrator is 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 uses 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 is set, concentrations at appropriate maximum 
monitors within an area are generally used to determine if an area 
meets a given standard.
    The Administrator also recognizes 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 remain in the 
evidence and information being considered in this review. The 
Administrator notes that the extent to which these uncertainties 
influence 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 concludes that 
it is 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 concludes 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 concludes 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 recognizes that 
setting an annual standard level at the lower end of this range would 
reflect an approach that places 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 place 
more emphasis on the remaining uncertainties in the evidence to avoid 
potentially overestimating public health improvements, and would 
generally support a view that the uncertainties remaining in the 
evidence are too great to warrant setting a lower annual standard 
level.
    While the Administrator recognizes that CASAC advised, and the 
Policy Assessment concluded, that the available scientific information 
provides support for considering a range that extended down to 11 
[mu]g/m\3\, she concludes that proposing such an extended range would 
reflect a public health policy approach that places more weight on 
relatively limited evidence and more uncertain information and analyses 
than she considers appropriate at this time. Nonetheless, the 
Administrator solicits 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 recognizes 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 recognizes 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 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, 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, the 
Administrator has 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 agrees 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 recognizes that to the extent these studies 
were conducted in areas that likely did not meet one or both of the 
current standards, such studies do 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 
anticipates that additional public health protection will occur. 
Therefore, the Administrator has 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 has 
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, 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. 
The Administrator views the single-city, short-term exposure studies as 
a much

[[Page 38943]]

more limited data set providing mixed results and, therefore, she has 
less confidence in using these 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 
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 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 notes that, to the extent air quality distributions are 
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 
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 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 
recognizes 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 notes that the study-wide mean concentrations are 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 
believes it would be reasonable to conclude that revision to the 24-
hour standard would not be warranted in conjunction with an annual 
standard within this range. Based on the above considerations related 
to the epidemiological evidence, the Administrator provisionally 
concludes 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 in the proposed range of 12 to 13 [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 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 has considered whether this suite of 
standards would meet a public health policy goal which includes 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 concludes that this 
approach is 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 
recognizes that there is 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 observes 
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 
proposes 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 recognizes 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 is 
solicited, must be clearly responsive to the issues raised by the D.C. 
Circuit's remand of the 2006 primary annual PM2.5 standard. 
Furthermore, the final suite of standards will reflect the 
Administrator's 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.
    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 solicits 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 solicits 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 
plays in considering the appropriate annual and 24-hour levels, the 
Administrator solicits public comment on issues related to translating 
epidemiological evidence into standards, including approaches for 
addressing the uncertainties and

[[Page 38944]]

limitations associated with this evidence.

F. Administrator's Proposed 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 proposes to revise the current primary PM2.5 
standards. Specifically, the Administrator proposes to revise: (1) The 
level of the primary annual PM2.5 standard to a level within 
the range of 12.0 to 13.0 [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 allowance 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 
proposes to retain the level and form of the primary 24-hour 
PM2.5 standard to provide supplemental protection for areas 
with high peak PM2.5 concentrations. The Administrator 
provisionally concludes that such a revised suite of standards, 
including a revised annual standard together with the current 24-hour 
standard, could provide requisite protection against health effects 
potentially associated with long- and short-term PM2.5 
exposures. The Administrator is not proposing any revisions to the 
current PM2.5 indicator and the annual and 24-hour averaging 
times for the primary PM2.5 standards. Data handling 
conventions are specified in proposed revisions to appendix N, as 
discussed in section VII below. The Administrator solicits comment on 
all aspects of this proposed decision.

IV. Rationale for Proposed Decision on Primary PM10 Standard

    This section presents the rationale for the Administrator's 
proposed decision to retain the current 24-hour PM10 
standard to continue to provide public health protection against short-
term exposures to thoracic coarse particles, that is inhalable 
particles which can penetrate into the trachea, bronchi, and deep lungs 
and which are in the size range of 2.5 to 10 [mu]m 
(PM10-2.5). As discussed more fully below, this rationale 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 
thoracic coarse particles in the ambient air. This proposal also takes 
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 health evidence presented in 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 and Policy Assessment at public meetings, 
in separate written comments, and in CASAC's letters to the 
Administrator; and (3) public comments received during the development 
of these documents, either in connection with CASAC meetings or 
separately. The EPA notes that the final decision for retaining or 
revising the current primary PM10 standard is a public 
health policy judgment made by the Administrator. The Administrator's 
final decision will draw upon scientific information and analyses 
related to health effects; judgments about uncertainties that are 
inherent in the scientific evidence and analyses; CASAC advice; and 
comments received in response to this proposal.
    In presenting the rationale for the proposed decision to retain the 
current primary PM10 standard, this section begins with 
background information on EPA's past reviews of the PM NAAQS and the 
general approach taken to review the current PM10 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), and the Administrator's proposed 
conclusions regarding the adequacy of the current primary 
PM10 standard (section IV.E). Section IV.F summarizes the 
Administrator's proposed decision with regard to the primary 
PM10 NAAQS.

A. Background

    The following sections discuss previous reviews of the PM NAAQS 
(section IV.A.1), the litigation of the 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). 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). Thus, the 
EPA concluded that a PM10 standard could provide requisite 
protection against effects associated with particles in the size range 
of 2.5 to 10 [mu]m.\87\ 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, it was judged 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 newly adopted PM2.5 
standards.
---------------------------------------------------------------------------

    \87\ 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).

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[[Page 38945]]

    In May 1998, a three-judge panel of the U.S. Court of Appeals for 
the District of Columbia Circuit found ``ample support'' for 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 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.\88\ 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).
---------------------------------------------------------------------------

    \88\ 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 decision that it was requisite to provide 
protection from exposure to all thoracic coarse PM, regardless of its 
origin, rejecting 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 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 
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 to 96, October 17, 2006). 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, 
potentially increasing the toxicity of thoracic coarse particles in 
urban areas (Id.). Given the evidence that the existing PM10 
standard afforded requisite protection with an adequate margin of 
safety, the Agency retained the level and form of the 24-hour 
PM10 standard.\89\
---------------------------------------------------------------------------

    \89\ 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.
---------------------------------------------------------------------------

    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.
    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

[[Page 38946]]

NAAQS (71 FR 61202/3, October 17, 2006).
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 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 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 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 accorded 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 EPA's decision to revoke the annual 
PM10 standard. Specifically, the court stated the following:

    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 Policy Assessment for 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 basic elements of the NAAQS: Indicator, averaging time, form, and 
level.
    In contrast to previous reviews, where PM10 studies 
conducted in locations where PM10 is comprised predominantly 
of PM10-2.5 were considered (U.S. EPA, 2005, pp. 5-49 to 5-
50), the focus in the current review is on PM10-2.5 studies. 
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). In light of the 
considerable uncertainty in the extent to which PM10 effect 
estimates reflect associations with PM10-2.5 versus 
PM2.5, together with the availability in this review of a 
number of studies that evaluated associations with PM10-2.5 
and the fact that the Integrated Science Assessment weight of evidence 
conclusions for thoracic coarse particles were based on studies of 
PM10-2.5, the EPA focuses in this review on studies that 
have specifically evaluated PM10-2.5.
    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, section 2.4.3). 
As a result, any approach to 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.
    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

[[Page 38947]]

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).

B. Health Effects Related to Exposure to Thoracic Coarse Particles

    The following sections discuss available information on the health 
effects associated with exposures to PM10-2.5, including the 
nature of such health effects (section IV.B.1), the impacts of sources 
and composition on particle toxicity (section IV.B.2), ambient 
PM10 concentrations in PM10-2.5 study locations 
(section IV.B.3), at-risk populations (section IV.B.4), and limitations 
and uncertainties (section IV.B.5).
1. Nature of Effects
    Since the conclusion of the last review, the Agency has developed a 
more formal framework for reaching causal inferences from the body of 
scientific evidence. As discussed above in section III.B.1, this 
framework uses a five-level hierarchy that classifies the overall 
weight of evidence using the following categorizations: Causal 
relationship, likely to be a 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, section 1.5). 
Applying this framework to thoracic coarse particles, the Integrated 
Science Assessment concludes 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).\90\ In 
contrast, the Integrated Science Assessment concludes that available 
evidence is ``inadequate'' to infer a causal relationship between long-
term PM10-2.5 exposures and various health effects (U.S. 
EPA, 2009a, sections 7.2 to 7.6). Similar to the judgment made in the 
2004 AQCD regarding long-term exposures (U.S. EPA, 2004, p. 9-79), the 
Integrated Science Assessment states, ``To date, a sufficient amount of 
evidence does not exist in order to draw conclusions regarding the 
health effects and outcomes associated with long-term exposure to 
PM10-2.5'' (U.S. EPA, 2009a, section 2.3.4). Given these 
weight of evidence conclusions in the Integrated Science Assessment, 
EPA's consideration of the scientific evidence for PM10-2.5 
focuses on effects that have been linked with short-term exposures. The 
evidence supporting a link between short-term thoracic coarse particle 
exposures and adverse health effects is discussed in detail in the 
Integrated Science Assessment (U.S. EPA, 2009a, Chapter 6) and is 
summarized briefly below for mortality (section IV.B.1.a), 
cardiovascular effects (section IV.B.1.b), and respiratory effects 
(section IV.B.1.c).
---------------------------------------------------------------------------

    \90\ The Integrated Science Assessment discusses the framework 
for causality determinations (U.S. EPA, 2009a, section 1.5). 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).
---------------------------------------------------------------------------

a. Short-Term PM10-2.5 Exposure and Mortality
    The Integrated Science Assessment assesses a number of multi-city 
and single-city epidemiological studies that have evaluated 
associations between mortality and short-term PM10-2.5 
concentrations (U.S. EPA, 2009a, Figure 6-30 presents 
PM10-2.5 mortality studies assessed in the last review and 
the current review). Different studies have used different approaches 
to estimate ambient PM10-2.5. Some studies have used the 
difference between PM10 and PM2.5 mass, either 
measured at co-located monitors (e.g., Lipfert et al., 2000; Mar et 
al., 2003; Ostro et al., 2003; Sheppard et al., 2003; Wilson et al., 
2007) or as the difference in county-wide average concentrations 
(Zanobetti and Schwartz, 2009), while other studies have measured 
PM10-2.5 directly with dichotomous samplers (e.g., Burnett 
and Goldberg, 2003; Fairley et al., 2003; Burnett et al., 2004; Klemm 
et al., 2004). Despite differences in the approaches used to estimate 
ambient PM10-2.5 concentrations, the majority of multi- and 
single-city studies have reported positive associations between 
PM10-2.5 and mortality, though most of these associations 
were not statistically significant (U.S. EPA, 2009a, Figure 6-30).
    One important PM10-2.5 study conducted since the last 
review of the PM NAAQS is the U.S. multi-city study by Zanobetti and 
Schwartz (2009), which reported positive and statistically significant 
associations with PM10-2.5 for all-cause, cardiovascular-
related, and respiratory-related mortality (U.S. EPA, 2009a, section 
6.5.2.3). In this study, effect estimates for all-cause and 
respiratory-related mortality remained statistically significant in co-
pollutant models that included PM2.5, while the effect 
estimate for cardiovascular-related mortality remained positive but not 
statistically significant. Several other multi-city studies have 
reported positive, but not statistically significant, 
PM10-2.5 effect estimates for mortality (U.S. EPA, 2009a, 
Figure 6-30).
    When risk estimates in the study by Zanobetti and Schwartz (2009) 
were evaluated by climatic region (U.S. EPA, 2009a, Figure 6-28), a mix 
of positive and negative PM10-2.5 effect estimates were 
reported in the regions that typically have the highest ambient 
PM10-2.5 concentrations (i.e., regions corresponding to the 
western and southwestern U.S.). Regional effect estimates from western 
regions of the United States were generally not statistically 
significant. Positive and statistically significant effect estimates 
were more often reported in regions that typically have lower 
PM10-2.5 concentrations (i.e., regions generally 
corresponding to the eastern half of the U.S.) (Schmidt and Jenkins, 
2010 for PM10-2.5 concentrations). In addition, single-city 
empirical Bayes-adjusted effect estimates (calculated using the methods 
discussed in Le Tertre et al., 2005) for the 47 cities evaluated by 
Zanobetti and Schwartz (2009) were generally positive, though typically 
not statistically significant (U.S. EPA, 2009a, Figure 6-29).
    Of the available single-city PM10-2.5 mortality studies, 
most reported positive, but not statistically significant, 
PM10-2.5 effect estimates (U.S. EPA, 2009a, Figure 6-30). Of 
the three studies that did report statistically significant effect 
estimates (Mar et al., 2003; Ostro et al., 2003; Wilson et al., 2007), 
Ostro et al. (2003) reported that PM10-2.5 effect estimates 
remained statistically significant in co-pollutant models that included 
either ozone or NO2. The single-city studies by Mar et al. 
(2003) and Wilson et al. (2007) did not utilize co-pollutant models.
b. Short-Term PM10-2.5 Exposure and Cardiovascular Effects
    The Integrated Science Assessment assesses a number of studies that 
have evaluated the link between short-term ambient concentrations of 
thoracic coarse particles and cardiovascular effects. Single- and 
multi-city epidemiological studies generally report positive 
associations between short-term PM10-2.5 concentrations and 
hospital admissions or emergency department visits for cardiovascular 
causes (U.S. EPA, 2009a, sections 2.3.3 and 6.2.12.2). However, as is 
the case for the mortality studies, most of these positive associations 
are not statistically significant. In addition, most 
PM10-2.5 effect estimates remained positive, but not 
statistically significant, in co-pollutant models that included either

[[Page 38948]]

gaseous or particulate co-pollutants (U.S. EPA, 2009a, Figure 6-5).
    An important cardiovascular morbidity study published since the 
last review of the PM NAAQS is the U.S. multi-city study by Peng et al. 
(2008). This study evaluates hospital admissions and emergency 
department visits for cardiovascular disease in Medicare patients 
(MCAPS, Peng et al., 2008). The authors report a positive and 
statistically significant association between 24-hour 
PM10-2.5 concentrations and cardiovascular disease 
hospitalizations in a single pollutant model using air quality data for 
108 U.S. counties with co-located PM10 and PM2.5 
monitors. The magnitude of this effect estimate was larger in counties 
with higher degrees of urbanization and larger in the eastern U.S. than 
the western U.S., though this regional difference was not statistically 
significant (Peng et al., 2008). The PM10-2.5 effect 
estimate was reduced only slightly in a co-pollutant model that 
included PM2.5, but it was no longer statistically 
significant (U.S. EPA, 2009a, sections 2.3.3, 6.2.10.9).
    In addition to this U.S. multi-city study, positive associations 
reported for short-term PM10-2.5 exposures and 
cardiovascular-related morbidity reached statistical significance in a 
multi-city study in France (Host et al., 2007) and single-city studies 
in Detroit (Ito, 2003) and Toronto (Burnett et al., 1999) (U.S. EPA, 
2009a, Figures 6-2 and 6-3). In contrast, associations were positive 
but not statistically significant in single-city studies conducted in 
Atlanta (Metzger et al., 2004; Tolbert et al., 2007) and Boston (Peters 
et al., 2001) (and for some endpoints in Detroit (Ito, 2003)) (U.S. 
EPA, 2009a, Figures 6-1 to 6-3, and 6-5).
    The plausibility of the positive associations reported for 
PM10-2.5 and cardiovascular-related hospital admissions and 
emergency department visits receives some measure of support from a 
small number of controlled human exposure studies that have reported 
alterations in heart rate variability following short-term exposure to 
PM10-2.5 (Gong et al., 2004; Graff et al., 2009); by short-
term PM10-2.5 epidemiological studies reporting positive 
associations with cardiovascular-related mortality; by a small number 
of recent epidemiological studies that have examined dust storm events 
and reported increases in cardiovascular-related emergency department 
visits and hospital admissions (see below); and by associations with 
other cardiovascular effects including heart rhythm disturbances and 
changes in heart rate variability (U.S. EPA, 2009a, sections 2.3.3 and 
6.2.12.2). The few toxicological studies that examined the effect of 
PM10-2.5 on cardiovascular health effects used intratracheal 
instillation and, as a result, provide only limited evidence on the 
biological plausibility of PM10-2.5 induced cardiovascular 
effects (U.S. EPA, 2009a, sections 2.3.3 and 6.2.12.2).
c. Short-Term PM10-2.5 Exposure and Respiratory Effects
    The Integrated Science Assessment also assesses a number of studies 
that have evaluated the link between short-term ambient concentrations 
of thoracic coarse particles and respiratory effects. This includes 
recent studies conducted in the U.S., Canada, and France (U.S. EPA, 
2009a, section 6.3.8), including the U.S. multi-city study of Medicare 
patients by Peng et al. (2008). As discussed above, Peng estimated 
PM10-2.5 concentrations as the difference between 
PM10 and PM2.5 concentrations measured by co-
located monitors. The authors reported a positive, but not 
statistically significant, PM10-2.5 effect estimate for 
respiratory-related hospital admissions. Single-city studies have 
reported positive, and in some cases statistically significant, 
PM10-2.5 effect estimates for respiratory-related hospital 
admissions and emergency department visits (U.S. EPA, 2009a, Figures 6-
10 to 6-15). Some of these PM10-2.5 respiratory morbidity 
studies have reported positive and statistically significant 
PM10-2.5 effect estimates in co-pollutant models that 
included gaseous pollutants while others reported that 
PM10-2.5 effect estimates remain positive, but not 
statistically significant, in such co-pollutant models (U.S. EPA, 
2009a, Figure 6-15).
    A limited number of epidemiological studies have focused on 
specific respiratory morbidity outcomes and reported both positive and 
negative, but generally not statistically significant, associations 
between PM10-2.5 and lower respiratory symptoms, wheeze, and 
medication use (U.S. EPA, 2009a, sections 2.3.3.1 and 6.3.1.1; Figures 
6-7 to 6-9). Although controlled human exposure studies have not 
observed an effect on lung function or respiratory symptoms in healthy 
or asthmatic adults in response to short-term exposure to 
PM10-2.5, healthy volunteers have exhibited increases in 
markers of pulmonary inflammation.\91\ Toxicological studies using 
inhalation exposures are still lacking, but pulmonary injury and 
inflammation has been reported in animals after intratracheal 
instillation exposure (U.S. EPA, 2009a, section 6.3.5.3) and, in some 
cases, PM10-2.5 was found to be more potent than 
PM2.5.
---------------------------------------------------------------------------

    \91\ PM10-2.5 controlled human exposure studies have 
not been conducted in children.
---------------------------------------------------------------------------

2. Potential Impacts of Sources and Composition on PM10-2.5 
Toxicity
    In the absence of a systematic national effort to characterize 
PM10-2.5 components, relatively little information (e.g., 
compared to fine particles) is available in the current review to 
inform consideration of the potential for composition to impact 
PM10-2.5 toxicity. Given this, the Integrated Science 
Assessment concludes that currently available evidence is insufficient 
to draw distinctions in 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 is associated with 
adverse health effects (U.S. EPA, 2009a, section 2.4.4).
    As discussed above, positive associations between short-term 
PM10-2.5 concentrations and mortality and morbidity have 
been reported in a number of urban locations in the U.S., Canada, and 
Europe. While little is known about how PM10-2.5 composition 
varies across these locations or about how that variation could affect 
particle toxicity (U.S. EPA, 2009a, sections 2.3.3, 2.3.4, 2.4.4), a 
number of trace elements (e.g., chromium, cobalt, nickel, copper, zinc, 
arsenic, selenium, and lead) have been detected in PM10-2.5 
from urban locations (U.S. EPA, 2004, section 3.2.4).
    An indication of the sources of some of these trace elements (e.g., 
metals such as lead, copper, and zinc) in ambient PM10-2.5 
samples has been obtained by examining urban runoff (U.S. EPA, 2004, 
section 3.2.4). Wind-abrasion on building siding and roofs (coatings 
such as lead paint and building material such as brick, metal, and wood 
siding); brake wear (brake pads contain significant quantities of 
copper and zinc); tire wear (zinc is used as a filler in tire 
production); and burning engine oil could all produce particles 
containing metals (U.S. EPA, 2004, section 3.2.4). Once deposited on 
the ground, these elements can be resuspended with other material as 
PM10-2.5. In addition, resuspended crustal particles may 
become contaminated with trace elements and other components from 
previously deposited fine PM (e.g., metals from smelters or steel 
mills, PAHs from automobile exhaust, pesticides from agricultural 
lands) (U.S. EPA, 2004, section 8.5, p. 8-344).
    In considering the potential for PM10-2.5 composition to 
impact toxicity,

[[Page 38949]]

it is useful to consider studies conducted in locations where 
PM10-2.5 composition is expected to be very different from 
that in typical urban locations. Specifically, a small number of 
studies have examined the health impacts of dust storm events (U.S. 
EPA, 2009a, sections 6.2.10.1 and 6.5.2.3). Although these studies do 
not link specific particle constituents to health effects, they do 
provide some information on the toxicity of particles of non-urban 
crustal origin. Several of these studies have reported positive and 
statistically significant associations between dust storm events and 
morbidity or mortality, including the following:

    (1) Middleton et al. (2008) reported that dust storms in Cyprus 
were associated with a statistically significant increase in risk of 
hospitalization for all causes and a non-significant increase in 
hospitalizations for cardiovascular disease.
    (2) Chan et al. (2008) studied the effects of Asian dust storms 
on cardiovascular-related hospital admissions in Taipei, Taiwan and 
reported a statistically significant increase associated with 39 
Asian dust events. Evaluating the same data, Bell et al. (2008) also 
reported positive and statistically significant associations between 
hospitalization for ischemic heart disease and PM10-2.5.
    (3) Perez et al. (2008) tested the hypothesis that outbreaks of 
Saharan dust exacerbate the effects of PM10-2.5 on daily 
mortality in Spain. During Saharan dust days, the 
PM10-2.5 effect estimate was larger than on non-dust days 
and it became statistically significant, whereas it was not 
statistically significant on non-dust days.

In addition, a study in Coachella Valley by Ostro et al. (2003) 
reported statistically significant associations in a location where 
thoracic coarse particles are expected to be largely due to windblown 
dust.
    In contrast to the studies noted above, some dust storm studies 
have reported associations that were not statistically significant. 
Specifically, Bennett et al. (2006) reported on a dust storm in the 
Gobi desert that transported PM across the Pacific Ocean, reaching 
western North America in the spring of 1998. The authors reported no 
excess risk of cardiovascular-related or respiratory-related hospital 
admissions associated with the dust storm in the population of British 
Columbia's Lower Fraser Valley (Bennett et al., 2006). In addition, 
Yang et al. (2009) reported that hospitalizations for congestive heart 
failure were elevated during or immediately following 54 Asian dust 
storm events, though effect estimates were not statistically 
significant.
3. Ambient PM10 Concentrations in PM10-2.5 Study 
Locations
    As discussed above, a 24-hour PM10 standard is in place 
to protect public health against exposures to PM10-2.5. 
Given this, the EPA considers ambient PM10 concentrations in 
locations where PM10-2.5 health studies have been conducted 
(U.S. EPA, 2011a, section 3.2.1). Specifically, the Agency considers 
study locations for which ambient PM10 data are available 
for comparison to the current standard,\92\ including study locations 
evaluated in single-city U.S. studies, in Bayes-adjusted single-city 
analyses of the U.S. locations assessed by Zanobetti and Schwartz 
(2009), in single-city studies conducted outside the U.S., and in 
recent U.S. multi-city studies (Peng et al., 2008; Zanobetti and 
Schwartz, 2009).
---------------------------------------------------------------------------

    \92\ As discussed in more detail in the Policy Assessment (U.S. 
EPA, 2011a), these analyses are based on comparison of the one-
expected-exceedance concentration-equivalent design values in study 
locations to the level of the current standard. The one-expected-
exceedance concentration-equivalent design value is used as a 
surrogate concentration for comparison to the standard level in 
order to gain insight into whether a particular area would likely 
have met or violated the current PM10 standard. 
Therefore, locations with one-expected-exceedance concentration-
equivalent design values below the level of the current 
PM10 standard (i.e., 150 [mu]g/m\3\) would likely meet 
that standard (U.S. EPA, 2011a, section 3.2.1).
---------------------------------------------------------------------------

    In considering 24-hour PM10 concentrations in locations 
of specific PM10-2.5 epidemiological studies, the EPA has 
focused primarily on U.S. study locations where single-city analyses 
have been conducted (U.S. EPA, 2011a, sections 3.2.1 and 3.3.4). While 
multi-city studies are particularly important when drawing conclusions 
about health effect associations,\93\ it can be difficult to use these 
studies to link air quality in a given location to health effects in 
that same location. Multi-city studies often present overall effect 
estimates rather than single-city effect estimates, while short-term 
air quality can vary considerably across cities. Therefore, the extent 
to which effects reported in multi-city studies are associated with the 
short-term air quality in any particular location is uncertain, 
especially when considering short-term concentrations at the upper end 
of the distribution of daily concentrations for pollutants with 
relatively heterogeneous spatial distributions such as 
PM10-2.5 and PM10 (U.S. EPA, 2009a, section 
2.1.1.2). In contrast, single-city studies are more limited in terms of 
power and geographic coverage but the link between reported health 
effects and the short-term air quality in a given city is more 
straightforward to establish. As a result, in considering 24-hour 
PM10 concentrations in locations of epidemiological studies, 
the EPA has focused primarily on single-city studies and single-city 
analyses of the locations evaluated in the multi-city study by 
Zanobetti and Schwartz (2009) (U.S. EPA, 2011a, sections 3.2.1 and 
3.3.4).
---------------------------------------------------------------------------

    \93\ Multi-city studies assess PM10-2.5-associated 
health effects among large study populations and provide enhanced 
power to detect PM10-2.5-associated health effects. In 
addition, multi-city studies often provide spatial coverage for 
different regions across the country, reflecting differences in 
PM10-2.5 sources, composition, and potentially other 
factors that could impact PM10-2.5-related effects. These 
factors make multi-city studies particularly important when drawing 
conclusions about health effect associations.
---------------------------------------------------------------------------

    Of the 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).\94\ 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). Amongst 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, PM10-2.5 
effect estimates were both positive and negative, with most not 
statistically significant (U.S. EPA, 2011a, Figure 3-3).
---------------------------------------------------------------------------

    \94\ See a previous footnote above and the Policy Assessment 
(U.S. EPA, 2011a, section 3.2.1) for an explanation of how 
PM10 air quality in study locations was compared to the 
current PM10 standard.
---------------------------------------------------------------------------

    As discussed above, PM10-2.5 effect estimates for 
mortality were generally positive but not statistically significant in 
Bayes-adjusted single-city analyses in the locations evaluated by 
Zanobetti and Schwartz (U.S. EPA, 2009a, Figure 6-30). These effect 
estimates were generally similar in magnitude and precision, 
particularly for cardiovascular-related mortality, across a wide range 
of estimated PM10-2.5 concentrations (U.S. EPA, 2009a, 
Figure 6-29). In most of the cities evaluated (37 of the 45 for which 
appropriate PM10 air quality data were available for 
comparison to the current standard, as described in Schmidt and Jenkins 
(2010) and Jenkins (2011), PM10 concentrations were below 
those that would have been allowed by the current PM10 
standard (U.S. EPA, 2011a, section 3.2.1). Of these 37 cities that 
would likely have met the current PM10 standard during

[[Page 38950]]

the study period, positive and statistically significant 
PM10-2.5 effect estimates were reported in three locations 
(Chicago, Pittsburgh, Birmingham). Of the eight cities likely to have 
violated the current PM10 standard during the study period, 
PM10-2.5 effect estimates were positive and statistically 
significant in three (Detroit, St. Louis, Salt Lake City).
    In considering PM10-2.5 epidemiological studies 
conducted in Canada and elsewhere outside the U.S., the EPA notes that 
PM10 air quality information beyond that published by the 
study authors is generally not available. The available PM10 
concentration data for these study areas is typically not appropriate 
for comparison to the current PM10 standard (i.e., 
concentrations are averaged across monitors, rather than from the 
highest monitor in the study area, and/or concentrations are reported 
as means or medians). However, in a small number of cases it is 
possible to draw conclusions based on available air quality information 
about whether a study area would likely have met or violated the 
current PM10 standard.
    For example, Lin et al. (2002) reported positive and statistically 
significant associations between PM10-2.5 and asthma 
hospital admissions in children in Toronto (U.S. EPA, 2009a; Figures 6-
12 and 6-15). The authors reported a maximum PM10 
concentration measured at a single monitor in the study area of 116 
[mu]g/m\3\, indicating that the PM10 air quality in Toronto 
during this study would have been allowed by the current 24-hour 
PM10 standard.\95\
---------------------------------------------------------------------------

    \95\ This is the case because the maximum monitored 24-hour 
PM10 concentration (116 [mu]g/m\3\) was below the level 
of the current PM10 standard (150 [mu]g/m\3\).
---------------------------------------------------------------------------

    In contrast Middleton et al. (2008), who reported that dust storms 
in Cyprus were associated with a statistically significant increase in 
risk of hospitalization for all causes and a non-significant increase 
in hospitalizations for cardiovascular diseases, reported a maximum 24-
hour PM10 concentration of 1,371 [mu]g/m\3\. Thus, the dust 
storm-associated increases in hospitalizations reported in this study 
occurred in an area with PM10 concentrations that were 
likely well above those allowed by the current standard. Other dust 
storm studies did not report maximum 24-hour PM10 
concentrations from individual monitors, though the studies by Chan et 
al. (2008) and Bell et al. (2008), which reported positive and 
statistically significant associations between dust storm metrics and 
cardiovascular-related hospital admissions, reported that 24-hour 
PM10 concentrations, averaged across monitors, exceeded 200 
[mu]g/m\3\. It is likely that peak concentrations measured at 
individual monitors in these studies were much higher and, therefore, 
24-hour PM10 concentrations in these study areas were likely 
above those allowed by the current standard.
    In addition to the single-city studies discussed above, multi-city 
averages of PM10 one-expected-exceedance concentration-
equivalent design values \96\ for recent U.S. multi-city studies were 
110 [mu]g/m\3\, for the locations evaluated by Zanobetti and Schwartz 
(2009), and 100 [mu]g/m\3\, for the locations evaluated by Peng et al. 
(2008) (U.S. EPA, 2011a, section 3.2.1). As discussed above, the extent 
to which multi-city PM10-2.5 effect estimates are associated 
with the air quality in any particular location is uncertain.
---------------------------------------------------------------------------

    \96\ The one-expected-exceedance concentration-equivalent design 
value is used as a surrogate concentration for comparison to the 
standard level in order to gain insight into whether a particular 
area would likely have met or violated the current PM10 
standard. Therefore, locations with one-expected-exceedance 
concentration-equivalent design values below the level of the 
current PM10 standard (i.e., 150 [mu]g/m\3\) would likely 
meet that standard (U.S. EPA, 2011a, section 3.2.1).
---------------------------------------------------------------------------

4. At-Risk Populations
    Specific groups within the general population are likely at 
increased risk for suffering adverse effects following 
PM10-2.5 exposures. As discussed in section III.B.3 above, 
in this proposal, the term ``at-risk'' is the all encompassing term 
used for groups with specific factors that increase the risk of PM-
related health effects in a population.
    Although studies have primarily used exposures to PM10 
or PM2.5 to investigate potential at-risk populations, the 
available evidence suggests that the identified factors also increase 
risk from PM10-2.5 \97\ (U.S. EPA, 2009a, section 8.1.8). As 
discussed in section III.B.3 above, at-risk populations include those 
with preexisting heart and lung diseases (e.g., asthma), 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 (U.S. EPA, 2009a, section 2.4.1 and Table 8-2).
---------------------------------------------------------------------------

    \97\ Although the Integrated Science Assessment notes that in 
PM10-2.5 studies of respiratory-related hospital 
admissions and emergency department visits, ``the strongest 
relationships were observed among children'' (U.S. EPA, 2009a, 
section 2.3.3.1). As discussed above (section III.B.3), children may 
be more at increased risk for effects associated with ambient PM 
exposures because, compared to adults, children typically spend more 
time outdoors and at higher activity levels; they have exposures 
that result in higher doses per body weight and lung surface area; 
and there is the potential for irreversible effects on the 
developing lung (U.S. EPA, 2009a, section 8.1.1.2).
---------------------------------------------------------------------------

    Given the range of at-risk groups, the population potentially 
affected by PM10-2.5 is large. In the United States, 
approximately 7 percent of adults (approximately 16 million adults) and 
9 percent of children (approximately 7 million children) have asthma 
(U.S. EPA, 2009a, Table 8-3; CDC, 2008 \98\). In addition, 
approximately 4 percent of adults have been diagnosed with chronic 
bronchitis and approximately 2 percent with emphysema (U.S. EPA, 2009a, 
Table 8-3). Approximately 11 percent of adults have been diagnosed with 
heart disease, 6 percent with coronary heart disease, 23 percent with 
hypertension, and 8 percent with diabetes (U.S. EPA, 2009a, Table 8-3). 
In addition, approximately 3 percent of the U.S. adult population has 
suffered a stroke (U.S. EPA, 2009a, Table 8-3). Therefore, although 
exposures to ambient PM10-2.5 have not been well 
characterized on a national scale, the size of the potentially at-risk 
population suggests that ambient PM10-2.5 could have a 
significant impact on public health in the United States.
---------------------------------------------------------------------------

    \98\ For percentages, see http://www.cdc.gov/ASTHMA/nhis/06/table4-1.htm. For population estimates, see http://www.cdc.gov/ASTHMA/nhis/06/table3-1.htm.
---------------------------------------------------------------------------

5. Limitations and Uncertainties Associated With the Currently 
Available Evidence
    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, are discussed below.
    The Integrated Science Assessment concludes that an important 
uncertainty in interpreting PM10-2.5 epidemiological studies 
is the potential for confounding by co-occurring pollutants, 
particularly PM2.5. This issue 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

[[Page 38951]]

experimental evidence (i.e., controlled human exposure and animal 
toxicology studies) available to support the plausibility of 
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.
    Another important uncertainty is related to exposure error. The 
Integrated Science Assessment concludes that ``there is greater spatial 
variability in PM10-2.5 concentrations than PM2.5 
concentrations, resulting in increased exposure error for the larger 
size fraction'' (U.S. EPA, 2009a, p. 2-8) and that 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).
    In addition, there is uncertainty in the air quality estimates used 
in PM10-2.5 epidemiological studies (U.S. EPA, 2009a, 
sections 2.3.3, 2.3.4) and, therefore, in the ambient 
PM10-2.5 concentrations that are associated with mortality 
and morbidity. 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.\99\ Given these limitations in routine monitoring, 
epidemiological studies have employed different approaches for 
estimating PM10-2.5 concentrations. For example, several of 
the studies discussed above, including the multi-city study by Peng et 
al. (2008), estimated PM10-2.5 by taking the difference 
between mass measured at co-located PM10 and 
PM2.5 monitors while the study by Zanobetti and Schwartz 
(2009) used the difference between county-wide average PM10 
and PM2.5 concentrations. In addition, a small number of 
studies have directly measured PM10-2.5 concentrations with 
dichotomous samplers (e.g., Burnett et al., 2004; Villeneuve et al., 
2003; Klemm et al., 2004). 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 with a 
dichotomous sampler or by calculating the difference using co-located 
samplers (U.S. EPA, 2009a, section 6.5.2.3).\100\ 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 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).
---------------------------------------------------------------------------

    \99\ The EPA has required PM10-2.5 mass monitoring, 
as part of the NCore network, beginning January 1, 2011 at 
approximately 80 stations. The NCore network is a multi-pollutant 
network that includes measurements of particles, gases, and 
meteorology (71 FR 61236, October 17, 2006). NCore monitoring 
stations are located away from direct emissions sources that could 
substantially impact the detection of area-wide concentrations. The 
network is comprised of stations in both urban and rural areas. 
Urban NCore stations are generally to be located at an urban or 
neighborhood scale to provide exposure concentrations that are 
expected to be representative of the metropolitan area. Rural NCore 
stations are to be located, to the maximum extent practicable, at a 
regional or larger scale away from any large local emission source, 
so that they represent ambient concentrations over an extensive area 
(U.S. EPA, 2011a, Appendix B, section B.4).
    \100\ In addition, several sources of uncertainty can be 
specifically associated with PM10-2.5 concentrations that 
are estimated based on co-located monitors. For example, the 
potential for differences among operational flow rates and 
temperatures for PM10 and PM2.5 monitors add 
to the potential for exposure misclassification. As discussed in 
Appendix B of the Policy Assessment (U.S. EPA, 2011a, sections B.2 
and B.3), PM10 data are often reported at standard 
temperature and pressure (STP) while PM2.5 is reported at 
local conditions (LC). In these cases, the PM10 data 
should be adjusted to LC when estimating PM10-2.5 
concentrations. In many of the epidemiological studies that 
estimated PM10-2.5 concentrations based on co-located 
monitors, it is not made explicitly clear whether this adjustment 
was made, adding to the overall uncertainty in the 
PM10-2.5 concentrations that are associated with health 
effects.
---------------------------------------------------------------------------

    Another uncertainty results from the relative lack of 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). As discussed above, a few recent studies have evaluated 
associations between health effects and particles of non-urban, crustal 
origin by evaluating the health impacts of dust storm events. Though 
these studies provide some information on the health effects of ambient 
particles that likely differ in composition from the particles of urban 
origin that are typically studied, 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).
    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 (U.S. EPA, 2009b, Appendix H). The 
decision not to conduct a PM10-2.5 risk assessment for the 
current review was based on consideration of several key uncertainties, 
including the following:

    (1) Concerns that monitoring data that would be used in a 
PM10-2.5 risk assessment (i.e., for the period 2005 to 
2007) would not match ambient monitoring data used in the underlying 
epidemiological studies providing concentration-response functions.
    (2) Uncertainty in the prediction of ambient levels under 
current and alternative standard levels.
    (3) Concerns that locations used in the risk assessment may not 
be representative of areas experiencing the most significant 24-hour 
peak PM10-2.5 concentrations (and consequently, may not 
capture locations with the highest risk).
    (4) Concerns about the relatively small (i.e., compared to 
PM2.5) health effects database that supplies the 
concentration-response relationships.

    When considered together, the limitations outlined above resulted 
in the conclusion that a quantitative PM10-2.5 risk 
assessment would not significantly enhance the review of the NAAQS for 
coarse-fraction PM. Specifically, these limitations would likely result 
in sufficient uncertainty in the resulting risk estimates to 
significantly limit their utility to inform policy-related questions, 
including the assessment of whether the current standard is protective 
of public health and characterization of the degree of additional 
public health protection potentially afforded by alternative standards. 
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.

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

    The following sections discuss EPA's consideration of whether to 
revise the current PM10 standard, as well as our 
consideration of potential alternative

[[Page 38952]]

standards, drawing from such considerations in the Policy Assessment 
(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 (section IV.C.2.a), averaging 
time (section IV.C.2.b), form (section IV.C.2.c), and level (section 
IV.C.2.d).
1. Consideration of the Current Standard in the Policy Assessment
    As discussed above, a 24-hour PM10 standard is in place 
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 EPA considers 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 EPA notes 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 notes 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 3.2.1):

    (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 
in 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 characterize.
    (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 concludes 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 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. Although such an approach to considering the 
adequacy of the current standard would recognize the positive, and in 
some cases statistically significant, associations between 
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 notes that, when considering the 
uncertainties and limitations in the PM10-2.5 health 
evidence and air quality information, the EPA judged that it would not 
be 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 notes an alternative approach to 
considering the evidence and its uncertainties would place emphasis on 
the following:

    (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 
concludes 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 concludes 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

[[Page 38953]]

evidence, air quality information, and the uncertainties and 
limitations associated with that evidence and information.
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. These considerations are discussed below in terms 
of indicator, averaging time, form, and level.
a. Indicator
    As noted above, PM10 includes both PM10-2.5 
and PM2.5, with the relative contribution of each to 
PM10 mass varying across locations and over time. In the 
most recent review completed in 2006, the EPA concluded that the 
PM10 indicator remained appropriate in large part because a 
PM10 standard would provide some measure of protection 
against exposures to all PM10-2.5 regardless of source or 
location, while also targeting protection to urban areas, where the 
evidence of effects from exposure to coarse PM is the strongest (71 FR 
at 61196, October 17, 2006). As noted above, the court explicitly 
endorsed this reasoning. 559 F. 3d at 535-36.
    In considering the indicator in the current review, the Policy 
Assessment evaluated the extent to which PM10 is comprised 
of PM10-2.5 across locations and over time. Based on the air 
quality analyses in the Integrated Science Assessment (U.S. EPA, 2009a, 
section 3.5.1.1) and Schmidt and Jenkins (2010), and based on the 
concentration estimates of Zanobetti and Schwartz (2009), the Policy 
Assessment notes that PM10-2.5 typically makes up a larger 
portion of PM10 mass in the western United States, with the 
southwest region having the highest ratios of PM10-2.5 to 
PM10. In addition, the ratios of PM10-2.5 to 
PM10 across the U.S. tended to be higher on days with 
relatively high PM10 concentrations than on days with more 
typical PM10 concentrations (i.e., comparing days with 
concentrations at or above the 95th percentile to all days) (U.S. EPA, 
2011a, section 3.3.1, Figure 3-4). Given this, the Policy Assessment 
concludes that high daily PM10 concentrations are driven, at 
least in part, by elevated PM10-2.5 mass and that a 
PM10 standard focusing on the upper end of the distribution 
of daily PM10 concentrations could effectively control 
ambient PM10-2.5 concentrations (U.S. EPA, 2011a, p. 3-28).
    The Policy Assessment also considered the appropriateness of a 
PM10 standard, given that such a standard allows lower 
PM10-2.5 concentrations in areas with higher fine particle 
concentrations (urban areas) than areas with lower fine particle 
concentrations (rural areas) (U.S. EPA, 2011a, section 3.3.1). In 
considering this issue, the Policy Assessment notes that most of the 
evidence for positive associations between PM10-2.5 and 
morbidity and mortality, particularly evidence for these associations 
at relatively low concentrations of PM10-2.5, comes from a 
number of studies conducted in locations where the PM10-2.5 
is expected to be largely of urban origin (U.S. EPA, 2009a, Chapter 6). 
Although some studies have reported positive associations between 
relatively high concentrations of particles of non-urban origin (i.e., 
crustal material from windblown dust in non-urban areas, see above) and 
mortality and morbidity, the Policy Assessment notes that the extent to 
which these associations would remain at the lower particle 
concentrations more typical of U.S. and Canadian urban study locations 
remains uncertain.\101\
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    \101\ Other than the dust storm studies, we note that the study 
in Coachella Valley by Ostro et al. (2003) reported statistically 
significant associations in a location where thoracic coarse 
particles are expected to be largely due to windblown dust. 
Specifically, we note the CASAC conclusion in the last review that 
``studies from Ostro et al. showed significant adverse health 
effects, primarily involving exposures to coarse-mode particles 
arising from crustal sources'' (Henderson, 2005b). In considering 
this study, we also note the relatively high PM10 
concentrations in the study area (U.S. EPA, 2011a, Figure 3-2), 
which would not have met the current PM10 standard.
---------------------------------------------------------------------------

    Given these considerations, and given the increased potential for 
coarse particles in urban areas to become contaminated by toxic 
components of fine particles from urban/industrial sources (U.S. EPA, 
2004 at 8-344; 71 FR 61196, October 17, 2006), the Policy Assessment 
concludes that it is reasonable to consider an indicator that targets 
control to areas with the types of ambient mixes generally present in 
urban areas. The Policy Assessment notes that such an indicator would 
focus control on areas with ambient mixes known with greater certainty 
to be associated with adverse health effects and, therefore, would 
provide public health benefits with the greatest degree of certainty. 
Therefore, as in the last review, the Policy Assessment reaches the 
conclusion that a PM10 indicator would appropriately target 
protection to those locations where the evidence is strongest for 
associations between adverse health effects and exposures to thoracic 
coarse particles (U.S. EPA, 2011a, p. 3-29).
    In contrast, the Policy Assessment notes that a PM10-2.5 
indicator, for a standard set at a single unvarying level, would not 
achieve this targeting, given that allowable thoracic coarse particle 
concentrations would be the same regardless of the location or the 
likely sources of PM. Therefore, given the currently available 
evidence, one possible result of using a PM10-2.5 indicator 
would be a standard that is overprotective in rural areas and/or 
underprotective in urban areas (Id.).
    Given all of the above considerations, the Policy Assessment 
concludes that the available evidence supports consideration in the 
current review of a PM10 indicator for a standard that 
protects against exposures to thoracic coarse particles. The Policy 
Assessment further concludes that consideration of alternative 
indicators (e.g., PM10-2.5) in future reviews is desirable 
and could be informed by additional research (U.S. EPA, 2011a, section 
3.5).
b. Averaging Time
    Based primarily on epidemiological studies that reported positive 
associations between short-term (24-hour) PM10-2.5 
concentrations and mortality and morbidity, the Administrator concluded 
in the last review that the available evidence supported a 24-hour 
averaging time for a standard intended to protect against exposures to 
thoracic coarse particles. In contrast, given the relative lack of 
studies supporting a link between long-term exposures to thoracic 
coarse particles and morbidity or mortality (U.S. EPA, 2004, Chapter 
9), the Administrator further concluded that an annual coarse particle 
standard was not warranted at that time (71 FR 61198-61199, October 17, 
2006).
    In the current review, the Policy Assessment notes the conclusions 
from the Integrated Science Assessment regarding the weight of evidence 
for short-term and long-term PM10-2.5 exposures as well as 
the studies on which those conclusions are based. Specifically, as 
discussed above, the Integrated Science Assessment concludes 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). This 
conclusion is based largely on epidemiological studies which have 
primarily evaluated associations between 24-hour PM10-2.5 
concentrations and morbidity and

[[Page 38954]]

mortality (e.g., U.S. EPA, 2009a, Figure 2-3), though a small number of 
controlled human exposure studies have reported effects following 
shorter exposures (i.e., 2-hours) to PM10-2.5 (U.S. EPA, 
2009a, sections 6.2.1.2 and 6.3.3.2). In contrast, with respect to 
long-term exposures, the Integrated Science Assessment concludes that 
available evidence is inadequate to infer a causal relationship with 
all health outcomes evaluated (U.S. EPA, 2009a, section 2.3). 
Specifically, the Integrated Science Assessment states, ``To date, a 
sufficient amount of evidence does not exist in order to draw 
conclusions regarding the health effects and outcomes associated with 
long-term exposure to PM10-2.5'' (U.S. EPA, 2009a, section 
2.3.4).
    In considering these weight-of-evidence determinations, the Policy 
Assessment concludes that, at a minimum, they suggest the importance of 
maintaining a standard that protects against short-term exposures to 
thoracic coarse particles. Given that the majority of the evidence 
supporting the link between short-term PM10-2.5 and 
morbidity and mortality is based on 24-hour average thoracic coarse 
particle concentrations, the Policy Assessment concludes that the 
evidence available in this review continues to support consideration of 
a 24-hour averaging time for a PM10 standard meant to 
protect against effects associated with short-term exposures to 
PM10-2.5 (U.S. EPA, 2011a, p. 3-31).
    The Policy Assessment further concludes that the available evidence 
does not support consideration of an annual thoracic coarse particle 
standard at this time. In reaching this conclusion, the Policy 
Assessment also notes that, to the extent a short-term standard 
requires areas to reduce their 24-hour ambient particle concentrations, 
long-term concentrations would also be expected to decrease (Id.). 
Therefore, a 24-hour standard meant to protect against short-term 
exposures to thoracic coarse particles would also be expected to 
provide some protection against potential effects associated with long-
term exposures to ambient concentrations.
c. Form
    The ``form'' of a standard defines the air quality statistic that 
is to be compared to the level of the standard in determining whether 
an area attains that standard. As discussed above, in the last review 
the Administrator retained the one-expected exceedance form of the 
primary 24-hour PM10 standard. This decision was linked to 
the overall conclusion that ``the level of protection from coarse 
particles provided by the current 24-hour PM10 standard 
remains requisite to protect public health with an adequate margin of 
safety'' (71 FR 61202, October 17, 2006). Because revising either the 
level or the form of the standard would have altered the protection 
provided, the Administrator concluded that such changes ``would not be 
appropriate based on the scientific evidence available at this time'' 
(71 FR 61202). Therefore, the decision in the last review to retain the 
one-expected-exceedance form was part of the broader decision that the 
existing 24-hour standard provided requisite public health protection.
    In the current review, the Policy Assessment considers the form of 
the standard within the context of the overall decision on whether, and 
if so how, to revise the current 24-hour PM10 standard. 
Given the conclusions above regarding the appropriate indicator and 
averaging time for consideration for potential alternative standards, 
the Policy Assessment considers potential alternative forms for a 24-
hour PM10 standard.
    Although the selection of a specific form must be made within the 
context of decisions on the other elements of the standard, the Policy 
Assessment notes that the EPA generally favors concentration-based 
forms for short-term standards. In 1997, the EPA established a 98th 
percentile form for the 24-hour PM2.5 standard and, in 2010, 
the EPA established a 98th percentile form for the primary 1-hour 
NO2 standard (62 FR 38671, July 18, 1997; 75 FR 6474, 
February 9, 2010) and a 99th percentile form for the primary 1-hour 
SO2 standard (75 FR 35541, June 22, 2010).\102\ In making 
these decisions, the EPA noted that, compared to an exceedance-based 
form, a concentration-based form is 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. In addition, when averaged 
over three years, these concentration-based forms were judged to 
provide an appropriate balance between limiting peak pollutant 
concentrations and providing a stable regulatory target, facilitating 
the development of stable implementation programs.
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    \102\ As noted above (section IV.A.1.a), in the 1997 review the 
EPA revised the form of the 24-hour PM10 standard to the 
99th percentile. However, the D.C. Circuit Court vacated the revised 
rule, based on EPA's retention of the PM10 indicator, and 
the 1987 standards remained in place (including the one-expected-
exceedance form for the 24-hour standard).
---------------------------------------------------------------------------

    These considerations are also relevant in the current review of the 
24-hour PM10 standard. Specifically, the Policy Assessment 
concludes that it is appropriate to consider concentration-based forms 
that would provide a balance between limiting peak pollutant 
concentrations and providing a stable regulatory target. To accomplish 
this, it would be appropriate to consider forms from the upper end of 
the annual distribution of 24-hour PM10 concentrations.\103\ 
However, given the potential for local sources to have important 
impacts on monitored PM10 concentrations (U.S. EPA, 2009a, 
section 2.1.1.2), the Policy Assessment also notes that it would be 
appropriate to consider forms that, when averaged over three years, 
would be expected to promote the stability of local implementation 
programs.\104\ In considering these issues in the most recent review of 
the primary NO2 NAAQS, the Policy Assessment notes that a 
98th percentile form was adopted, rather than a 99th percentile form, 
due to the potential for ``instability in the higher percentile 
concentrations'' near local sources (75 FR 6493, February 9, 
2010).105 106
---------------------------------------------------------------------------

    \103\ With regard to this conclusion, the Policy Assessment also 
notes that PM10-2.5 is likely to make a larger 
contribution to PM10 mass on days with relatively high 
PM10 concentrations than on days with more typical 
PM10 concentrations (see above).
    \104\ As noted in section III.E.3.b above, stability of 
implementation programs has been held to be a legitimate 
consideration in determining a NAAQS (American Trucking Associations 
v. EPA, 283 F. 3d at 374 to 75).
    \105\ See also, ATA III, 283 F. 3d at 374-75 (upholding 98th 
percentile form since ``otherwise States would have to design their 
pollution control programs around single high exposure events that 
may be due to unusual meteorological conditions alone, rendering the 
programs less stable--and hence, we assume, less effective--than 
programs designed to address longer-term average conditions.''). In 
contrast, in the recently completed review of the primary 
SO2 NAAQS, a 99th percentile form was adopted. However, 
in the case of SO2, the standard was intended to limit 5-
minute exposures and a 99th percentile form was markedly more 
effective at doing so than a 98th percentile form (75 FR 35540 to 
41, June 22, 2010).
    \106\ Similar considerations are noted in section III.E.3.b 
above, with regard to the form of the primary 24-hour 
PM2.5 standard.
---------------------------------------------------------------------------

    In considering the potential appropriateness of a 98th percentile 
form in the current review, the Policy Assessment notes that, compared 
to the current PM10 standard, attainment status for a 
PM10 standard with a 98th percentile form would be based on 
a more stable air quality statistic and would be expected to be less 
influenced by relatively rare events that can cause elevations in 
PM10 concentrations over short periods of time (Schmidt, 
2011b).

[[Page 38955]]

Specifically, the Policy Assessment notes that in areas that monitor 
PM10 every six days, every three days, or every day the 
PM10 concentrations that are comparable to the current 
standard level are, respectively, the highest, 2nd highest, or 4th 
highest 24-hour PM10 concentrations measured during a three 
year period. In contrast, for the same monitoring frequencies, the 
PM10 concentrations that would be comparable to the level of 
a standard with a 98th percentile form would be the three-year average 
of the 2nd highest, 3rd highest, or 7th/8th highest 24-hour 
PM10 concentrations measured during a single year (U.S. EPA, 
2011a, p. 3-33).
    In further considering this issue the Policy Assessment notes that, 
compared to the current one-expected-exceedance form, a concentration-
based form specified as a percentile of the annual distribution of 
PM10 concentrations (e.g., such as a 98th percentile form) 
would be expected to better compensate for missing data and less-than-
daily monitoring. This is a particularly important consideration in the 
case of PM10 because, depending largely on ambient 
concentrations, the frequency of PM10 monitoring differs 
across locations (i.e., either daily, 1 in 2 days, 1 in 3 days, or 1 in 
6 days) (U.S. EPA, 2011a, section 1.3 and Appendix B). As discussed in 
earlier reviews of the PM NAAQS (e.g., 62 FR 38671, July 18, 1997), an 
area's attainment status for a standard with a 98th percentile form 
would be based directly on monitoring data rather than on a calculated 
value adjusted for missing data or less-than-every-day monitoring, as 
is the case with the current one-expected-exceedance form.
    In light of all of the above considerations, the Policy Assessment 
concludes that, 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 
(U.S. EPA, 2011a, p. 3-34).\107\
---------------------------------------------------------------------------

    \107\ As noted above, local sources can have important impacts 
on monitored PM10 concentrations. In the recent review of 
the NO2 primary NAAQS, where this was also an important 
consideration, a 98th percentile form was adopted, rather than a 
99th percentile form, due to the potential for ``instability in the 
higher percentile concentrations'' near local sources (75 FR 6493, 
February 9, 2010). A similar conclusion in the current review led 
the Policy Assessment to focus on the 98th percentile rather than 
the 99th percentile, in considering potential alternative forms for 
a PM10 standard.
---------------------------------------------------------------------------

    In their review of the second draft Policy Assessment, CASAC noted 
that such a change in form ``will lead to changes in levels of 
stringency across the country'' and recommended that this issue be 
explored further (Samet, 2010d). In considering this issue, the Policy 
Assessment acknowledges that, given differences in PM10 air 
quality distributions across locations (U.S. EPA, 2009a, Table 3-10), a 
revised standard with a 98th percentile form would likely target public 
health protection to some different locations than does the current 
standard with its one-expected-exceedance form (U.S. EPA, 2011a, p. 3-
34). The final Policy Assessment notes that a further consideration 
with regard to the appropriateness of revising the form of the current 
PM10 standard is the extent to which, when compared with the 
current standard, a revised standard with a 98th percentile form would 
be expected to target public health protection to areas where we have 
more confidence that ambient PM10-2.5 is associated with 
adverse health effects (Id., p. 3-34 to 3-35).
    In giving initial consideration to this issue, the Policy 
Assessment used recent PM10 air quality concentrations 
(i.e., from 2007-2009) to identify counties that would meet, and 
counties that would violate, the current PM10 standard as 
well as potential alternative standards with 98th percentile forms 
(Schmidt, 2011b).108 109 In some cases, counties that would 
violate the current standard do so because of a small number of 
``outlier'' days (e.g., as few as one such day in three years) with 
PM10 concentrations well-above more typical concentrations 
(Schmidt, 2011b). Mean and 98th percentile PM10 and 
PM10-2.5 concentrations were higher in counties that would 
have violated a revised standard with a 98th percentile form but met 
the current standard \110\ than in counties that violated the current 
standard, but would have met a revised standard with a 98th percentile 
form (Schmidt, 2011b). This analysis suggests that, to the extent a 
revised PM10 standard with a 98th percentile form could 
target public health protection to different areas than the current 
standard, those areas preferentially targeted by a revised standard 
generally have higher ambient concentrations of thoracic coarse 
particles. The issue of targeting public health protection is 
considered further in section 3.3.4 of the Policy Assessment (U.S. EPA, 
2011a) and below, within the context of considering specific potential 
alternative standard levels for a 24-hour PM10 standard with 
a 98th percentile form.
---------------------------------------------------------------------------

    \108\ Section 3.3.4 of the Policy Assessment (U.S. EPA, 2011a) 
discusses potential alternative standard levels that would be 
appropriate to consider in conjunction with a revised standard with 
a 98th percentile form.
    \109\ The memo by Schmidt (2011b) identifies specific counties 
that are expected to meet, and counties that are not likely to meet 
the current standard and potential alternative standards with 98th 
percentile forms.
    \110\ This analysis considered a revised PM10 
standard with a 98th percentile form and a level from the middle of 
the range discussed in section 3.3.4 of the Policy Assessment (i.e., 
75 [micro]g/m\3\) (U.S. EPA, 2011a).
---------------------------------------------------------------------------

d. Level
    As noted above, the Policy Assessment concluded that, to the extent 
it is judged in the current review that the 24-hour PM10 
standard does not provide adequate public health protection against 
exposures to thoracic coarse particles, potential alternative standards 
could be considered. The Policy Assessment considers potential 
alternative levels for a 24-hour PM10 standard with a 98th 
percentile form. To inform consideration of this issue, the Policy 
Assessment considers the available scientific evidence and air quality 
information (U.S. EPA, 2011a, section 3.3.4).
i. Evidence-Based Considerations in the Policy Assessment
    As discussed above, in considering the evidence as it relates to 
potential alternative standard levels, the Policy Assessment first 
considers the relative weight to place on specific epidemiological 
studies, including the weight to place on the uncertainties associated 
with those studies. The Policy Assessment considers several factors in 
placing weight on specific epidemiological studies including the extent 
to which studies report statistically significant associations with 
PM10-2.5 and the extent to which the reported associations 
are robust to co-pollutant confounding, in particular confounding by 
PM2.5. In addition, the Policy Assessment considers the 
extent to which associations with PM10-2.5 can be linked to 
the air quality in a specific location. With regard to this, as noted 
above, the Policy Assessment places the greatest weight on information 
from single-city analyses.
    In considering PM air quality in study locations, the Policy 
Assessment also notes that the available evidence does not support the 
existence of thresholds, or lowest-observed-effects levels, in terms of 
24-hour average concentrations (U.S. EPA, 2009a, section 2.4.3).\111\ 
In the absence of an apparent threshold, for purposes of identifying a 
range of

[[Page 38956]]

standard levels potentially supported by the health evidence, the 
Policy Assessment focuses on the range of PM10 
concentrations that have been measured in locations where U.S. 
epidemiological studies have reported associations with 
PM10-2.5 (U.S. EPA, 2009a, Figures 6-1 to 6-30 for studies).
---------------------------------------------------------------------------

    \111\ Most studies that have evaluated the potential for 
thresholds have focused on PM10 or PM2.5. 
However, there is no scientific basis for drawing different 
conclusions for PM10-2.5.
---------------------------------------------------------------------------

    In single-city mortality studies, as well as the single-city 
analyses of the locations evaluated by Zanobetti and Schwartz (2009), 
positive and statistically significant PM10-2.5 effect 
estimates were reported in some locations with 98th percentile 
PM10 concentrations ranging from 200 [mu]g/m\3\ to 91 [mu]g/
m\3\ (U.S. EPA, 2011a, section 3.3.4). Lower PM10 
concentrations were present in locations where positive, but not 
statistically significant, effect estimates were reported and when 
averaged across locations evaluated in the multi-city study by 
Zanobetti and Schwartz (2009) (U.S. EPA, 2011a, section 3.3.4).
    Among U.S. morbidity studies, Ito (2003) reported a positive and 
statistically significant PM10-2.5 effect estimate for 
hospital admissions for ischemic heart disease in Detroit, where the 
98th percentile PM10 concentration (102 [mu]g/m\3\) was also 
within this range (U.S. EPA, 2011a, section 3.3.4 and Figure 3-6). 
PM10-2.5 effect estimates in this study remained positive, 
and in some cases statistically significant, in co-pollutant models 
with gaseous pollutants (U.S. EPA, 2009a, Figures 6-5 and 6-15). Lower 
PM10 concentrations were present in locations where 
positive, but not statistically significant, effect estimates were 
reported and when averaged across locations evaluated in the multi-city 
study by Peng et al. (2008) (U.S. EPA, 2011a, section 3.3.4).
ii. Air Quality-based Considerations in the Policy Assessment
    In addition to the evidence-based considerations described above, 
the Policy Assessment estimated the level of a 24-hour PM10 
standard with a 98th percentile form that would approximate the degree 
of protection, on average across the country, provided by the current 
24-hour PM10 standard with its one-expected-exceedance form. 
The initial approach to estimating this ``generally equivalent'' 98th 
percentile PM10 concentration was to use EPA's Air Quality 
System (AQS)\112\ as the basis for evaluating correlations between 98th 
percentile PM10 concentrations and one-expected-exceedance 
concentration equivalent design values (Schmidt and Jenkins, 
2010).\113\ Based on these correlations, using monitoring data from 
1988 to 2008, a 98th percentile PM10 concentration of 87 
[mu]g/m\3\ is, on average, generally equivalent to the current standard 
level (U.S. EPA, 2011a, Figure 3-7). However, given the variability in 
the distributions of PM10 concentrations across locations 
(U.S. EPA, 2009a, Table 3-10; Schmidt and Jenkins, 2010), the range of 
equivalent concentrations varies considerably (95 percent confidence 
interval ranges from 63 to 111 [mu]g/m\3\) (Schmidt and Jenkins, 2010). 
As a consequence, the Policy Assessment notes that in some locations a 
98th percentile standard with a level of 87 [mu]g/m\3\ would likely be 
more protective than the current standard while in other locations it 
would likely be less protective than the current standard.\114\
---------------------------------------------------------------------------

    \112\ See http://www.epa.gov/ttn/airs/airsaqs/.
    \113\ As discussed above, the one-expected-exceedance 
concentration-equivalent design value is used as a surrogate 
concentration for comparison to the standard level in order to gain 
insight into whether a particular area would likely have met or 
violated the current PM10 standard. Therefore, locations 
with one-expected-exceedance concentration-equivalent design values 
below the level of the current PM10 standard (i.e., 150 
[mu]g/m\3\) would likely meet that standard (U.S. EPA, 2011a, 
section 3.2.1).
    \114\ The ``generally equivalent'' concentration also differs 
depending on the years of monitoring data used. For example, when 
this analysis was restricted to only the most recent years available 
(i.e., 2007 to 2009), the ``generally equivalent'' 98th percentile 
PM10 concentration was 78 [mu]g/m\3\. Given the temporal 
variability in the relationship between the current standard level 
and 98th percentile PM10 concentrations, and the 
potential for the ``generally equivalent'' 98th percentile 
concentration to vary year-to-year, staff concluded that it remains 
appropriate to consider the correlation analyses that use the 
broader range of available monitoring years (i.e., 1998-2008), as 
these analyses are likely to be more robust than analyses based on a 
shorter period of time.
---------------------------------------------------------------------------

    The Policy Assessment also evaluates regional differences in the 
relationship between 98th percentile PM10 concentrations and 
one-expected-exceedance concentration equivalent design values (U.S. 
EPA, 2011a, Figure 3-8), based on air quality data from 1988 to 2008. 
The 98th percentile PM10 concentrations that are, on 
average, generally equivalent to the current standard level ranged from 
just below 87 [mu]g/m\3\ in the Southeast, Southwest, upper Midwest, 
and outlying areas (i.e., generally equivalent 98th percentile 
PM10 concentrations ranged from 82 to 85 [mu]g/m\3\ in these 
regions) to just above 87 [mu]g/m\3\ in the Northeast, industrial 
Midwest, and southern California (i.e., generally equivalent 98th 
percentile PM10 concentrations ranged from 88 to 93 [mu]g/
m\3\ in these regions) (Schmidt, 2011b). However, within each of these 
regions there is considerable variability in the ``generally 
equivalent'' 98th percentile PM10 concentration across 
monitoring sites (U.S. EPA, 2011a, Figure 3-8).
    To provide a broader perspective on the relationship between the 
current standard and potential alternative standards with 98th 
percentile forms, the Policy Assessment also compares the size of the 
populations living in counties with PM10 one-expected-
exceedance concentration-equivalent design values greater than the 
current standard level to the size of the populations living in 
counties with 98th percentile PM10 concentrations above 
different potential alternative standard levels (based on air quality 
data from 2007 to 2009 \115\). Such comparisons can be considered as 
surrogates for comparisons of the breadth of public health protection 
provided by the current and potential alternative standards. Based on 
these comparisons, a 98th percentile PM10 standard with a 
level between 75 and 80 [mu]g/m\3\ would be most closely equivalent to 
the current standard. That is, compared to the number of people living 
in counties that would violate the current PM10 standard, a 
similar number live in counties that would violate a revised 24-hour 
PM10 standard with a 98th percentile form and a level 
between 75 and 80 [mu]g/m\3\ (U.S. EPA, 2011a, Table 3-2). However, 
there is considerably more variability across regions in the potential 
alternative standard that, based on this analysis, would be generally 
equivalent to the current PM10 standard (U.S. EPA, 2011a, 
section 3.3.4).
---------------------------------------------------------------------------

    \115\ These analyses are based on three years of air quality 
data in order to simulate the requirements for determining whether 
areas attain or violate the current PM10 standard, which 
requires consideration of 3 years of air quality data.
---------------------------------------------------------------------------

    Given the variability in the relationship between the current 
standard and potential alternative standards with 98th percentile 
forms, the Policy Assessment concludes that no single potential 
alternative standard level, for a revised standard with a 98th 
percentile form, would provide public health protection equivalent to 
that provided by the current standard, consistently over time and 
across locations.
    One consequence of this variability, as noted above in the 
discussion of the form of the standard, would be that a 24-hour 
PM10 standard with a 98th percentile form and a revised 
level would likely target public health protection to some different 
locations than does the current standard. Therefore, in further 
considering the appropriateness of revising the form and level of the 
current PM10 standard, the

[[Page 38957]]

Policy Assessment considered the extent to which, when compared with 
the current standard, a revised PM10 standard would be 
expected to target public health protection to areas where we have more 
confidence that PM10-2.5 is associated with adverse health 
effects. To address this question, the Policy Assessment considered the 
potential impact of revising the form and level of the PM10 
standard in locations where health studies have reported associations 
with PM10-2.5.
    The Policy Assessment initially considers U.S. study locations that 
would likely have met the current PM10 standard during the 
study period and where positive and statistically significant 
associations with PM10-2.5 were reported. Only Birmingham, 
Chicago, Pittsburgh, and Detroit \116\ met these criteria. During study 
periods, none of these areas would likely have met a 98th percentile 
24-hour PM10 standard with a level at or below 87 [mu]g/m\3\ 
(U.S. EPA, 2011a, section 3.3.4 and Table 3-3).
---------------------------------------------------------------------------

    \116\ Positive and statistically significant PM10-2.5 
effect estimates for Birmingham, Chicago, and Pittsburgh are 
reported in the Integrated Science Assessment (U.S. EPA, 2009a, 
Figure 6-29; from cities evaluated by Zanobetti and Schwartz, 2009). 
Effect estimates for Detroit are reported by Ito et al. (2003).
---------------------------------------------------------------------------

    The Policy Assessment also considered U.S. locations where health 
studies have reported positive associations (both statistically 
significant and non-significant) between PM10-2.5 and 
mortality or morbidity. Such positive associations were reported in 47 
locations that would likely have met the current PM10 
standard during the study period.\117\ Of these 47 locations, 13 would 
likely not have met a 98th percentile 24-hour PM10 standard 
with a level at 87 [mu]g/m\3\, 20 would likely not have met a 98th 
percentile 24-hour PM10 standard with a level of 75 [mu]g/
m\3\, and 31 would likely not have met a 98th percentile 24-hour 
PM10 standard with a level of 65 [mu]g/m\3\ (U.S. EPA, 
2011a, section 3.3.4).
---------------------------------------------------------------------------

    \117\ Philadelphia (Lipfert et al., 2000), Detroit (Ito et al., 
2003), Santa Clara (CA) (Fairley et al., 2003), Seattle (Sheppard et 
al., 2003), Atlanta (Klemm et al., 2004), Spokane (Slaughter et al., 
2005), Bronx and Manhattan (NYS DOH, 2006), and 39 of the cities 
evaluated by Zanobetti and Schwartz (2009) (U.S. EPA, 2009a, Figure 
6-29).
---------------------------------------------------------------------------

    In addition to the above analyses, the Policy Assessment also 
considered locations where health studies reported positive 
associations with PM10-2.5 and where ambient PM10 
concentrations were likely to have exceeded those allowed under the 
current PM10 standard during the study period. Nine 
locations met these criteria.\118\ Of these locations, all would also 
likely have exceeded a 98th percentile PM10 standard with a 
level at or below 87 [mu]g/m\3\ (U.S. EPA, 2011a, section 3.3.4).
---------------------------------------------------------------------------

    \118\ Pittsburgh (Chock et al., 2000), Coachella Valley (CA) 
(Ostro et al., 2003), Phoenix (Mar et al., 2003; Wilson et al., 
2007), and 6 of the cities evaluated by Zanobetti and Schwartz 
(2009) (U.S. EPA, 2009a, Figure 6-29).
---------------------------------------------------------------------------

    Therefore, among U.S. study locations where PM10-2.5-
associated health effects have been reported, some areas met the 
current standard but would likely have violated a 98th percentile 
PM10 standard with a level at or below 87 [mu]g/m\3\. In 
contrast, the locations that violated the current standard would also 
likely have violated a 98th percentile PM10 standard with a 
level at or below 87 [mu]g/m\3\. Given this, the Policy Assessment 
concludes that, compared to the current PM10 standard, a 24-
hour PM10 standard with a 98th percentile form could 
potentially better target public health protection to locations where 
we have more confidence that ambient PM10-2.5 concentrations 
are associated with mortality and/or morbidity (U.S. EPA, 2011a, pp. 3-
45 to 3-46).
iii. Integration of Evidence-Based and Air Quality-Based Considerations 
in the Policy Assessment
    In considering the integration of the evidence and air quality 
information within the context of identifying potential alternative 
standard levels for consideration, the Policy Assessment first notes 
the following:

    (1) Analyses of air quality correlations suggest that a 98th 
percentile 24-hour PM10 concentration as high as 87 
[mu]g/m\3\ could be considered generally equivalent to the current 
PM10 standard, over time and across the country.
    (2) A 98th percentile 24-hour PM10 standard with a 
level at or below 87 [mu]g/m\3\ would be expected to maintain 
PM10 and PM10-2.5 concentrations below those 
present in U.S. locations where single-city studies have reported 
PM10-2.5 effect estimates that are positive and 
statistically significant (lowest concentration in such a location 
was 91 [mu]g/m\3\). Although some single-city studies have reported 
positive PM10-2.5 effect estimates in locations with 98th 
percentile PM10 concentrations below 87 [mu]g/m\3\, these 
effect estimates were not statistically significant.
    (3) Multi-city average 98th percentile PM10 
concentrations were below 87 [mu]g/m\3\ for recent U.S. multi-city 
studies, which have reported positive and statistically significant 
PM10-2.5 effect estimates. However, the extent to which 
effects reported in multi-city studies are associated with the 
short-term air quality in any particular location is highly 
uncertain.
    (4) Epidemiological studies have reported positive, and in a few 
instances statistically significant, associations with 
PM10-2.5 in some locations likely to have met the current 
PM10 standard but not a PM10 standard with a 
98th percentile form and a level at or below 87 [mu]g/m.\3\
    To the extent the above considerations are emphasized, the Policy 
Assessment notes 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. 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. Given this, when compared to the current 
standard, a 24-hour PM10 standard with a 98th percentile 
form and a level at or below 85 [mu]g/m\3\ could have the effect of 
focusing public health protection on locations where there is more 
confidence that PM10-2.5 is associated with mortality and/or 
morbidity.
    Given the above, the Policy Assessment concludes that a 98th 
percentile standard with a level as high as 85 [mu]g/m\3\ could be 
considered to the extent that more weight is placed on the 
appropriateness of focusing public health protection in areas where 
positive and statistically significant associations with 
PM10-2.5 have been reported, and to the extent less weight 
is placed on PM10-2.5 effect estimates that are not 
statistically significant and/or that reflect estimates across multiple 
cities. The Policy Assessment notes that it could be judged appropriate 
to place less weight on PM10-2.5 effect estimates that are 
not statistically significant given the relatively large amount of 
uncertainty that is associated with the broader body of 
PM10-2.5 health evidence, including uncertainty in the 
extent to which health effects evaluated in epidemiological studies 
result from exposures to PM10-2.5 itself, rather than one or 
more co-occurring pollutants. This uncertainty, as well as other 
uncertainties discussed above, are reflected in the Integrated Science 
Assessment conclusions that the evidence is ``suggestive'' of a causal 
relationship (i.e., rather than ``causal'' or ``likely causal'') 
between short-term PM10-2.5 and mortality, respiratory 
effects, and cardiovascular effects. In addition, the Policy Assessment 
concludes that it could be appropriate to place less weight on 98th 
percentile PM10 concentrations averaged across multiple 
cities, given the uncertainty in

[[Page 38958]]

linking multi-city effect estimates with the air quality in any 
particular location.
    However, the Policy Assessment also notes that, overall across the 
U.S., based on recent air quality information (i.e., 2007-2009), fewer 
people live in counties with 98th percentile 24-hour PM10 
concentrations above 85 [mu]g/m\3\ than in counties likely to exceed 
the current PM10 standard (U.S. EPA, 2011a, Table 3-2 and p. 
3-48). These results could be interpreted to suggest that a 98th 
percentile standard with a level of 85 [mu]g/m\3\ would decrease 
overall public health protection compared to the current standard. 
Based on this 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. To the extent these 
population counts are emphasized in comparing the public health 
protection provided by the current and potential alternative standards, 
and to the extent it is judged appropriate to set a revised standard 
that provides at least the level of public health protection that is 
provided by the current standard based on such population counts, the 
Policy Assessment concludes that it would be appropriate to consider 
standard levels in the range of approximately 75 to 80 [mu]g/m\3\ 
(Id.).
    The Policy Assessment concludes that alternative approaches to 
considering the evidence could also lead to consideration of standard 
levels below 75 [mu]g/m\3\. 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.5 above), 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 multi-
city averages of 98th percentile PM10 concentrations in the 
locations evaluated by Zanobetti and Schwartz (2009) and Peng et al. 
(2008) were 77 and 68 [mu]g/m\3\, respectively. Both of these multi-
city studies reported positive and statistically significant 
PM10-2.5 effect estimates that remained positive in co-
pollutant models that included PM2.5, though only Zanobetti 
and Schwartz (2009) reported PM10-2.5 effect estimates that 
remained statistically significant in such co-pollutant models. Despite 
uncertainties in the extent to which effects reported in these multi-
city studies are associated with the short-term air quality in any 
particular location, emphasis could be placed on these multi-city 
associations. The Policy Assessment concludes 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\ (U.S. EPA, 2011a, p. 3-48). A standard level of 
65 [mu]g/m\3\ would be expected to provide a substantial margin of 
safety against health effects that have been associated with 
PM10-2.5 and, as discussed above, could better focus 
(compared to the current standard) public health protection on areas 
where health studies have reported associations with 
PM10-2.5.
    In considering potential alternative standard levels below 65 
[mu]g/m\3\, the Policy Assessment notes that, as discussed above, 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\. Specifically, the Policy Assessment notes the following (Id., p. 
3-49):

    (1) Epidemiological studies, either single-city or multi-city, 
have not reported positive and statistically significant 
PM10-2.5 effect estimates in locations with 98th 
percentile PM10 concentrations (multi-city average 98th 
percentile concentrations in the case of multi-city studies) at or 
below 65 [mu]g/m\3\.
    (2) Although some single-city morbidity studies have reported 
positive, but not statistically significant, associations with 
PM10-2.5 in locations with 98th percentile 
PM10 concentrations below 65 [mu]g/m\3\, the results of 
U.S. morbidity studies were generally less consistent than those of 
mortality studies, with some PM10-2.5 effect estimates 
being positive while others were negative (i.e., negative effect 
estimates were reported in several studies conducted in Atlanta, 
where the 98th percentile PM10 concentrations ranged from 
67 [mu]g/m\3\ to 71 [mu]g/m\3\).
    (3) Although Bayes-adjusted single-city PM10-2.5 
effect estimates were positive, but not statistically significant, 
in some locations with PM10 concentrations below 65 
[mu]g/m\3\, these effect estimates were based on the difference 
between community-wide PM10 and PM2.5 
concentrations. As discussed above, it is not clear how these 
estimates of PM10-2.5 concentrations compare to those 
more typically used in other studies to calculate 
PM10-2.5 effect estimates. At present, few corroborating 
studies are available that use other approaches (i.e., co-located 
monitors, dichotomous samplers) to estimate/measure 
PM10-2.5 in locations with 98th percentile 
PM10 concentrations below 65 [mu]g/m\3\.

    In light of these 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 concludes that while it could be judged 
appropriate to consider standard levels as low as 65 [mu]g/m\3\, it is 
not appropriate, based on the currently available body of evidence, to 
consider standard levels below 65 [mu]g/m\3\.

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).\119\ 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). 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 current

[[Page 38959]]

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.
---------------------------------------------------------------------------

    \119\ With regard to limitations and uncertainties in the 
evidence, CASAC endorsed the ISA 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 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).

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 notes 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 above, the 
Administrator provisionally concludes that the available evidence 
continues 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 PM10-2.5. In drawing 
conclusions as to whether the current PM10 standard is 
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 has considered:

    (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, in the last review the EPA 
concluded 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 an adequate margin of safety (71 FR 61197, October 17, 2006). 
In that review, the EPA concluded that 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). In litigation of the decisions from the last review, the D.C. 
Circuit affirmed the conclusion that it was appropriate 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 current review, the Administrator 
judges that the expanded body of scientific evidence 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 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). While 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).
    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

[[Page 38960]]

Integrated Science Assessment's conclusions regarding the links between 
adverse health effects and PM sources and composition, the 
Administrator provisionally 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.
    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 notes 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 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 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 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 provisionally concludes that it remains 
appropriate to maintain a standard that 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 provisionally 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.120 121
---------------------------------------------------------------------------

    \120\ 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 D.C. 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.
    \121\ 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 initial 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). 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 judges 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 notes that the Policy Assessment discusses 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 in detail (section IV.C.1), lead 
to different

[[Page 38961]]

conclusions regarding the appropriateness of the degree of public 
health protection provided by the current PM10 standard. The 
Administrator further notes 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;
    (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 used 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 notes that when the available 
PM10-2.5 scientific evidence and its associated 
uncertainties are considered, the Integrated Science Assessment 
concludes 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 
notes that she is proposing 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). The suggestive judgment for 
PM10-2.5 reflects the greater degree of uncertainty 
associated with this body of evidence, as discussed above in detail 
(sections IV.B.5 and IV.C.1) 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.
    (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 provisional 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.
    With regard to this provisional 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 the single-city analyses for the 
locations evaluated in the multi-city study by Zanobetti and Schwartz 
(2009), the Administrator notes that associations in most of these 
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 above, 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 with a 
dichotomous sampler or by calculating the difference using co-located 
samplers (U.S. EPA,

[[Page 38962]]

2009a, section 6.5.2.3).\122\ For these reasons, the Administrator 
notes that there is considerable uncertainty in interpreting the 
associations in these single-city analyses.
---------------------------------------------------------------------------

    \122\ As noted in section IV.B.5 above and in the Policy 
Assessment (U.S. EPA, 2011a, p. 3-16), there are also important 
uncertainties in estimates of ambient PM10-2.5 
concentrations based on the difference between PM10 mass 
and PM2.5 mass, as measured at co-located monitors.
---------------------------------------------------------------------------

    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 provisional conclusions:

    (1) Given 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; given 
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 given 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; 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.\123\
---------------------------------------------------------------------------

    \123\ 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 D.C. 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 provisional 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) Given 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, the degree of public health protection provided by the 
current standard is not greater than warranted.\124\
---------------------------------------------------------------------------

    \124\ 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 at 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 provisional conclusions, the Administrator notes 
that the Policy Assessment also discusses 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).\125\ 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.
---------------------------------------------------------------------------

    \125\ 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

[[Page 38963]]

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 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 reaches 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 proposes to retain the current 
PM10 standard in order to protect against health effects 
associated with short-term exposures to PM10-2.5.
    The Administrator recognizes that her proposed conclusions and 
decision to retain the current PM10 standard differ 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 welcomes 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.

F. Administrator's Proposed Decision on the Primary PM10 Standard

    For the reasons discussed above, and taking into account the 
information and assessments presented in the Integrated Science 
Assessment and the Policy Assessment and the advice and recommendations 
of CASAC, the Administrator proposes to retain the current primary 
PM10 standard. The Administrator solicits comment on all 
aspects of this 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.

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.\126\ The 
current AQI has been in use since its inception in 1999.\127\ 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 dioxide, PM and sulfur dioxide. The AQI is 
also recognized internationally as a proven tool to effectively 
communicate air quality information to the public. In fact, many 
countries have created similar indices based on the AQI.
---------------------------------------------------------------------------

    \126\ See http://www.airnow.gov/.
    \127\ 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). Decisions about the pollutant concentrations at 
which to set the various AQI breakpoints that delineate the various AQI 
categories for each pollutant specific sub-index within the AQI draw 
directly from the underlying health information that supports the NAAQS 
review.
    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 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 and can 
encourage the public to take actions that will reduce air pollution on 
days when levels are projected to be at levels of concern to local 
communities. Thus, these programs have significantly broadened the ways 
in which state and local agencies can meet the nationally uniform AQI 
reporting requirements, and are contributing to state and local efforts 
to provide community health protection and to attain or maintain 
compliance with the NAAQS. The EPA and state and local agencies 
recognize that these programs are interrelated with AQI reporting and 
with the information on the effects of air pollution on public health 
that is generated through the periodic review, and revision when 
appropriate, of the NAAQS.
    In recognition of the proposed change to the primary annual 
PM2.5 standard summarized in section III.F above, the EPA 
proposes a conforming change to the PM2.5 sub-index of the 
AQI to be

[[Page 38964]]

consistent with the proposed change to the annual standard. The health 
effects information that supports the proposed decisions on the 
PM2.5 standards, as discussed in section III.B above, is 
also the basis for the proposed decisions on the AQI discussed below in 
this section. The EPA intends to finalize conforming changes to the AQI 
in conjunction with the Agency's final decisions on the primary annual 
and 24-hour PM2.5 standards, if revisions to such standards 
are promulgated.
    With respect to an AQI value of 50, as discussed above, the 
historical approach is to set it at the same level of the annual 
standard, if there is one. This is consistent with the current AQI sub-
index for PM2.5, in which the current AQI value of 50 is set 
at 15 [mu]g/m\3\, consistent with the level of the current primary 
annual PM2.5 standard. The EPA sees no basis for deviating 
from this approach in this review. Thus, the EPA proposes to set an AQI 
value of 50 within a range of 12 to 13 [mu]g/m\3\, 24-hour average, 
consistent with the proposed annual PM2.5 standard level 
(section III.F). The final AQI value of 50 will be set at the level of 
the annual PM2.5 standard that is promulgated.
    With respect to an AQI value of 100, which is the basis for 
advisories to individuals in sensitive groups, there are 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 the 
other sub-indices as noted above, is to set an AQI value of 100 at the 
same level as the short-term standard. The EPA recognizes 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. 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 proposes to use this 
approach to set the AQI value of 100 at 35 [mu]g/m\3\, 24-hour average, 
consistent with the proposal to retain the current 24-hour 
PM2.5 standard (section III.F). If the 24-hour standard is 
set at a different level, the EPA proposes to set an AQI value of 100 
at the level of the 24-hour PM2.5 standard that is 
promulgated.
    An alternative approach is 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,\128\ 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 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. The EPA is 
soliciting 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 recognizes that some state and local air quality agencies have 
expressed support for this alternative approach. Using this alternative 
approach could result 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 
above. The EPA encourages state and local air quality agencies that use 
the AQI to comment on both the approach and the level at which to set 
an AQI value of 100 together with any supporting rationale.
---------------------------------------------------------------------------

    \128\ 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 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 believe that the health effects evidence indicates that the level of 
55 [mu]g/m\3\, 24-hour average, is appropriate to use \129\ in 
conjunction with an AQI value of 100 set at the proposed level of 35 
[mu]g/m\3\. Thus, if the EPA sets an AQI value of 100 at the 
PM2.5 level of 35 [mu]g/m\3\, 24-hour average, the Agency 
proposes to set an AQI value of 150 at the PM2.5 level of 55 
[mu]g/m\3\, 24-hour average. If, however, the EPA decides to set an AQI 
value of 100 at a lower level, then the EPA would adjust an AQI value 
of 150 proportionally. 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, EPA concludes the use of 
a proportional adjustment would be appropriate.
---------------------------------------------------------------------------

    \129\ 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.
---------------------------------------------------------------------------

    With respect to an AQI value of 500, a review of the history of the 
AQI value of 500 for PM10 and of the AQI value of 500 for 
PM2.5 is useful background. The current AQI value of 500 for 
PM10 was set in 1987 at the level of 600 [mu]g/m\3\, 24-hour 
average, on the basis of increased mortality associated with historical 
wintertime pollution episodes in London (52 FR 24687 to 24688, July 1, 
1987). Particle concentrations during these episodes, measured by the 
British Smoke method, were in the range of 500 to 1000 [mu]g/m\3\. In 
the 1987 rulemaking that established the upper bound index value for 
PM10, the EPA cited a generally held opinion that the 
British Smoke method measures PM with a cutpoint of approximately 4.5 
microns (52 FR 24688, July 1, 1987). In establishing this value for 
PM10, the EPA assumed that concentrations of 
PM10, which includes both coarse and fine particles, during 
episodes of concern, would be about 100 [mu]g/m\3\ higher than the PM 
concentration measured in terms of British Smoke (52 FR 24688, July 1, 
1987). The upper bound index value of 600 [mu]g/m\3\ was developed by 
selecting the lower end of the range of harmful concentrations during 
the historical wintertime pollution episodes in London (500 [mu]g/m\3\) 
and adding a margin of 100 [mu]g/m\3\ to account for this measurement 
difference. The current PM2.5 concentration corresponding to 
an AQI value of 500 set in the 1999 rulemaking is 500 [mu]g/m\3\, 24-
hour average.\130\ Because there were few PM2.5 monitoring 
data available at that time, the decision was based on the stated 
assumption that PM concentrations measured by the British Smoke method 
were approximately equivalent to PM2.5 concentrations. In 
considering whether it is appropriate to retain or revise the AQI value 
of 500 for PM2.5, the EPA notes that the 1999 rulemaking was 
based on an assumption of approximate equivalence between the British 
Smoke

[[Page 38965]]

method and the current PM2.5 method. This assumption is not 
entirely consistent with the view cited in 1987 that the British Smoke 
method has a size cutpoint of 4.5 microns (52 FR 24688, July 1, 1987), 
such that it would be reasonable to expect based on considering size 
cutpoint alone that a level of 500 [mu]g/m\3\ based on the British 
Smoke method would generally be equivalent to a somewhat lower level 
based on the current PM2.5 method. Nonetheless, more recent 
comparisons between British Smoke and PM2.5 measurement 
methods (Heal, et al., 2005; Chaloulakou, et al., 2005) suggest that on 
average British Smoke can be less than or more than PM2.5, 
but generally represents a larger fraction in the seasons and locations 
when PM2.5 predominantly results from directly emitted 
carbonaceous particles such as from combustion sources. More generally, 
the EPA recognizes that extremely high PM concentrations that would 
most likely be associated with combustion sources (e.g., coal burning 
in historic the London event, wildfires in contemporary U.S. 
environments) are typically dominated by fine particles, such that 
there may be very little difference between these measurement methods 
at such high levels.
---------------------------------------------------------------------------

    \130\ We note that a level of 350 [mu]g/m\3\ is recommended for 
an AQI value of 500 in the more recent EPA guidance (Harnett, 2009, 
Attachment B).
---------------------------------------------------------------------------

    Further, in considering the body of more recent health effects 
evidence available in this review, the EPA concludes that there is 
little information about more recent air pollution episodes similar to 
the wintertime pollution episodes in London and associated impacts on 
community health upon which to base a decision. Thus, the EPA concludes 
that it remains appropriate to use the historical wintertime pollution 
episodes in London as the basis for setting an AQI value of 500 for 
PM2.5 as described above because it is still the best 
available directly relevant information. Nonetheless, the EPA takes 
note of a limited number of more recent studies cited in the Integrated 
Science Assessment that evaluated wood smoke health impacts which found 
effects such as cardiovascular morbidity and mortality as well as 
respiratory effects, albeit at much lower levels (U.S. EPA, 2009a, 
sections 6.2 and 6.6). These more recent health studies may provide 
some support for considering a lower PM2.5 level for an AQI 
value of 500.
    Based on the above considerations, the EPA concludes that it is 
appropriate to propose to retain the current level of 500 [mu]g/m\3\, 
24-hour average, for the AQI value of 500. The EPA solicits comment on 
alternative approaches to setting a level for the AQI value of 500 and 
on alternative levels that commenters believe may be appropriate as 
well as supporting information and rationales for such alternative 
levels. The EPA also solicits 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.
    For the intermediate breakpoints in the AQI between the values of 
150 and 500, the EPA proposes PM2.5 concentrations that 
generally reflect a linear relationship between increasing index values 
and increasing PM2.5 values. 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 suggest 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 proposed breakpoints for the 
PM2.5 sub-index.\131\ 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.
---------------------------------------------------------------------------

    \131\ As discussed in section VII.C below, the EPA is also 
proposing to update 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--Proposed Breakpoints for PM2.5 Sub-Index
------------------------------------------------------------------------
                                                   Proposed breakpoints
          AQI category            Index values     ([mu]g/m\3\, 24-hour
                                                         average)
------------------------------------------------------------------------
Good...........................            0-50          0.0-(12.0-13.0)
Moderate.......................          51-100         (12.1-13.1)-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 proposing to retain the 500 level for the AQI as described 
above, we note that the EPA is not proposing to establish 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 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.

VI. Rationale for Proposed Decisions on the Secondary PM Standards

    This section presents the rationale for the Administrator's 
proposed decisions to revise the current suite of secondary PM 
standards by adding a distinct standard for PM2.5 to address 
PM-related

[[Page 38966]]

visibility impairment while retaining the current secondary 
PM2.5 and PM10 standards to address the other 
welfare effects considered in this review. In particular, this section 
presents background information on EPA's previous and current reviews 
of the secondary PM standards (section VI.A), information on visibility 
impairment (section VI.B), conclusions on the adequacy of the current 
secondary PM2.5 standards to protect against PM-related 
visibility impairment (section VI.C), conclusions on alternative 
standards to protect against PM-related visibility impairment (section 
VI.D), conclusions on secondary PM standards to address other PM-
related welfare effects (section VI.E), and a summary of the 
Administrator's proposed decisions on the secondary PM standards 
(section VI.F).

A. Background

    The current suite of secondary PM standards is identical to the 
current suite of primary PM standards, 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 builds upon and broadens the approaches used in previous PM 
NAAQS reviews. The following discussion focuses particularly on the 
current 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 current approach 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, utilize different ways to 
consider the underlying body of scientific evidence. They also reflect 
an evolution in EPA's understanding of the nature of the effect on 
public welfare from visibility impairment, from an approach focusing 
only on Federal Class I area visibility impacts to a more multifaceted 
approach that also considers PM-related impacts on non-Federal Class I 
area visibility, such as in urban areas. This evolution has occurred in 
conjunction with the expansion of available PM data and information 
from associated studies of public perception, valuation, and personal 
comfort and well-being.
    In 1997, the EPA revised the identical primary and secondary 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 regional differences. These 
differences included several factors that influence visibility such as 
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 these considerations, the EPA assessed potential 
visibility improvements in urban areas and on a regional scale that 
would result from attainment of the new primary standards for 
PM2.5. The agency concluded that the spatially averaged form 
of the annual PM2.5 standard was well suited to the 
protection of visibility, which involves effects of PM2.5 
throughout an extended viewing distance across an urban area. Based on 
air quality data available at that time, many urban areas in the 
Northeast, Midwest, and Southeast, as well as Los Angeles, were 
expected to see perceptible improvement in visibility if the annual 
PM2.5 primary standard were attained. The EPA also concluded 
that attainment of the 24-hour PM2.5 standard in some areas 
would be expected to reduce, to some degree, the number and intensity 
of ``bad visibility'' days, resulting in improvement in the 20 percent 
of days having the greatest impairment over the course of a year.
    Having concluded that attainment of the annual and 24-hour 
PM2.5 primary standards would lead to visibility 
improvements in many eastern and some western urban areas, the EPA also 
considered whether these standards could provide potential improvements 
to visibility on a regional scale. Based on information available at 
the time, the EPA concluded that attainment of secondary 
PM2.5 standards set identical to the primary 
PM2.5 standards would be expected to result in 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 then considered the potential effectiveness of a regional 
haze program, required by sections 169A and 169B of the CAA \132\ 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 a nationally applicable secondary NAAQS. The 
regional haze visibility goal is more protective than a secondary NAAQS 
since the goal addresses any anthropogenic impairment rather than just 
impairment at levels determined to be adverse to 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

[[Page 38967]]

emissions of pollutants that impair visibility in many of these areas.
---------------------------------------------------------------------------

    \132\ 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, 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, in conjunction with a regional haze program 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, 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 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

[[Page 38968]]

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 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 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 EPA's decision lacked a reasoned basis. 
First, they asserted that EPA never determined what level of visibility 
was ``requisite to protect the public welfare.'' They argued that EPA 
unreasonably

[[Page 38969]]

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 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 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 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 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 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 broadens 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 is taking 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 are appropriate for consideration in this 
review. As in past reviews, the EPA is also considering 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 
recognizes that such an approach is the most appropriate and effective 
means of addressing the public welfare effects associated with 
visibility impairment in areas across the country.
    The Policy Assessment draws from the qualitative evaluation of all 
studies discussed in the Integrated Science Assessment (U.S. EPA, 
2009a). Specifically, the Policy Assessment considers the extensive new 
air quality and source apportionment information available from the 
regional planning organizations, long-standing evidence of PM effects 
on visibility, and public preference studies 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 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. On these bases, the Policy 
Assessment again focuses assessments on visibility conditions in urban 
areas.
    The conclusions in the Policy Assessment reflect 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 takes into account the four basic elements of the NAAQS: 
indicator, averaging time, level, and form.

B. PM-Related Visibility Impairment

    As discussed below, the rationale for the Administrator's proposed 
decision regarding secondary PM standards to protect against visibility 
impairment focuses on those considerations most influential in the 
Administrator's proposed decisions, including consideration of: (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 outlines key information contained in the Integrated 
Science Assessment, the Visibility Assessment and the Policy Assessment 
on: (1) 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 and (2) public 
perceptions and attitudes about visibility impairment and the impacts 
of visibility impairment on public welfare.
1. Nature of PM-Related Visibility Impairment
    New research conducted by regional planning organizations in 
support of the Regional Haze Rule, as 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

[[Page 38970]]

contributions to that effect in rural and remote locations. Additional 
by-products of this research include 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. 
Information from both of these sources of PM data, while useful, has 
not however changed the fundamental and long understood science 
characterizing the contribution of PM, especially fine particles, to 
visibility impairment. This science, briefly summarized below, provides 
the basis for the Integrated Science Assessment designation of the 
relationship between PM and visibility impairment as causal.
a. 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. 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 is 
quantified by a light extinction coefficient with units of 1/distance, 
which is often expressed in the technical literature as 1/(1 million 
meters) or inverse megameters (abbreviated Mm-1). When PM is 
present in the air, its contribution to light extinction typically 
greatly exceeds that of gases.
    The amount of light extinction contributed by PM depends on the 
particle size distribution and composition, as well as its particle 
concentration. If details of the ambient particle size distribution and 
composition (including the mixing of components) are known, Mie theory 
can be used to accurately calculate PM light extinction (U.S. EPA, 
2009a, chapter 9). However, routine monitoring rarely includes 
measurements of particle size and composition information with 
sufficient detail for such calculations. To make estimation of light 
extinction more practical, visibility scientists have developed a much 
simpler algorithm, known as the IMPROVE algorithm,\133\ to estimate 
light extinction using routinely monitored fine particle 
(PM2.5) speciation and coarse particle mass 
(PM10-2.5) data. In addition, relative humidity information 
is needed to estimate the contribution by liquid water that is in 
solution with hygroscopic PM components (U.S. EPA, 2009a, section 
9.2.2.2; U.S. EPA, 2010b, chapter 3). 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 
light extinction conditions.\134\ 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.
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    \133\ 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).
    \134\ 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. 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. They have been evaluated by comparing the algorithm 
estimates of light extinction with coincident optical measurements. 
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. The dry extinction efficiency for 
particulate organic mass is larger than those for particulate sulfate 
and nitrate principally because the density of the dry inorganic 
compounds is higher than that assumed for the PM organic mass 
components.
    For the first two terms, ``sulfate'' is defined in terms of 
ammonium sulfate and ``nitrate'' is defined in terms of ammonium 
nitrate. Since IMPROVE does not include ammonium ion monitoring, the 
assumption is made that all sulfate is fully neutralized ammonium 
sulfate and all nitrate is assumed to be ammonium nitrate.\135\ Though 
often reasonable, neither assumption is always true (see U.S. EPA, 
2009a, section 9.2.3.1). In the eastern U.S. during the summer there is 
insufficient ammonia in the atmosphere to neutralize the sulfate fully. 
Fine particle nitrates can include sodium or calcium nitrate, which are 
the fine particle fraction of generally much coarser particles due to 
nitric acid interactions with sea salt at near-coastal areas (sodium 
nitrate) or nitric acid interactions with calcium carbonate in crustal 
aerosol (calcium nitrate). Despite the simplicity of the 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.
---------------------------------------------------------------------------

    \135\ To calculate ammonium sulfate, multiply the CSN 
measurement of the sulfate ion by 1.375. To calculate ammonium 
nitrate, multiply the CSN measurement of the nitrate ion by 1.29 
(Lowenthal and Kumar, 2006).
---------------------------------------------------------------------------

    The f(RH) term 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

[[Page 38971]]

approximately 85 percent where their total light extinction efficiency 
exceeds the 10 m\2\/g associated with elemental carbon (EC). Based on 
this algorithm, particulate sulfate and nitrate are estimated to have 
comparable light extinction efficiencies (i.e., the same dry extinction 
efficiency and f(RH) water growth terms), so on a per unit mass 
concentration basis at any specific relative humidity they are treated 
as equally effective contributors to visibility effects.
    As noted above, 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. 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 high relative humidity 
conditions.\136\
---------------------------------------------------------------------------

    \136\ The IMPROVE algorithm does not explicitly separate the 
light-scattering and light-absorbing effects of elemental carbon.
---------------------------------------------------------------------------

    With regard to the fifth and sixth terms, the fine soil component 
is based on measurement of five elements: Aluminum (Al), silicon (Si), 
calcium (Ca), iron (Fe), and titanium (Ti).\137\ 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.
---------------------------------------------------------------------------

    \137\ Consistent with calculations used in the IMPROVE network 
and the Regional Haze Program, the fine soil component is calculated 
using the following formula:
    Fine Soil = 2.20 x [Al] + 2.49 x [Si] + 1.63 x [Ca] + 2.42 x 
[Fe] + 1.94 x [Ti].
---------------------------------------------------------------------------

    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) has been 
developed. It employs a more complex split-component mass extinction 
efficiency to correct biases believed to be related to particle size 
distributions, a sea salt term that can be important for remote coastal 
areas, a different multiplier for organic carbon for purposes of 
estimating organic carbonaceous material,\138\ and site-specific 
Rayleigh light scattering terms in place of a universal Rayleigh light 
scattering value. These features of the revised IMPROVE algorithm are 
described in section 9.2.3.1 of the Integrated Science Assessment, 
which also presents a comparison of the estimates produced by the two 
algorithms for rural areas. Compared to the original algorithm, the 
revised IMPROVE algorithm can yield higher estimates of current light 
extinction levels in urban areas on days with relatively poor 
visibility (Pitchford, 2010). This difference is primarily attributable 
to the split-component mass extinction efficiency treatment in the 
revised algorithm rather than to the inclusion of a sea salt term or 
the use of site-specific Rayleigh scattering values.
---------------------------------------------------------------------------

    \138\ The revised IMPROVE algorithm uses a multiplier of 1.8 
instead of 1.4 as used in the original algorithm for the mean ratio 
of organic mass to organic carbon.
---------------------------------------------------------------------------

    As mentioned above, particles are not the only contributor to 
ambient visibility conditions. Light scattering by gases also occurs in 
ambient air. Under pristine atmospheric conditions, naturally occurring 
gases such as elemental nitrogen and oxygen cause what is known as 
Rayleigh scattering. Rayleigh scattering depends on the density of air, 
which is a function primarily of the elevation above sea level, and can 
be treated as a site-dependent constant. The Rayleigh scattering 
contribution to light extinction is only significant under pristine 
conditions. The only other commonly occurring atmospheric gas to 
appreciably absorb light in the visible spectrum is nitrogen dioxide. 
Nitrogen dioxide forms in the atmosphere from nitrogen oxide emissions 
associated with combustion processes. These combustion processes also 
emit PM at levels that generally contribute much higher light 
extinction than the nitrogen dioxide (i.e., nitrogen dioxide absorption 
is generally less than approximately 5 percent of the light extinction, 
except where emission controls remove most of the PM prior to releasing 
the remaining gases to the atmosphere). The final term in the IMPROVE 
algorithm of 10 Mm-1 is known as the Rayleigh scattering 
term and accounts for light scattering by the natural gases in 
unpolluted air. The remainder of this section focuses on the 
contribution of PM, which is typically much greater than that of gases, 
to ambient light extinction, unless otherwise specified.
    In the following discussions, visual air quality is characterized 
in terms of both light extinction, as discussed above, and an 
alternative scale for characterizing visibility--the deciview scale--
that is defined directly in terms of light extinction (expressed in 
units of Mm-1) by the following equation: \139\

    \139\ 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 a 
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 and 
regulatory 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.
b. 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). 
Generally, low mixing heights at night and during the early morning 
hours tend to trap locally produced emissions, which are diluted as the 
mixing height increases due to heating during the day. Low temperatures 
and high relative humidity at night are conducive to the presence of 
ammonium nitrate particles and water growth by hygroscopic particles 
compared with the generally higher temperatures and lower relative 
humidity later in the day. These combine to make early morning the most 
likely time for peak urban light extinction. Superimposed on such 
systematic time-of-day variations are the effects of synoptic 
meteorology (i.e., those associated with changing weather) and 
regional-scale air quality that can generate peak light extinction 
impacts any time of day. The net effects of the systematic urban- and 
larger-scale variations are that peak daytime PM light extinction 
levels can occur any time of day, although in many areas they most 
often occur 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

[[Page 38972]]

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.
c. Periods During the Day of Interest for Assessment of Visibility
    Visibility is typically associated with daytime periods because 
people are outside more during the day than at night and there are more 
viewable scenes at a distance during the day than at night. The Policy 
Assessment recognizes, however, that physically PM light extinction 
behaves the same at night as during the day, 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. These 
effects produce the visual result of a reduction in the number of 
visible stars and the disappearance of diffuse or subtle phenomena such 
as the Milky Way. The extinction of starlight is a secondary and minor 
effect also caused by increased PM scattering and absorption.
    However, there are significant and important differences between 
daytime and nighttime visual environments with regard to how light 
extinction per se relates to visual air quality (or visibility) and 
public welfare. First, daytime visibility has dominated the attention 
of those who have studied the visibility effects of air pollution, 
particularly in urban areas. As a result, little research has been 
conducted on nighttime visibility and the state of the science is not 
comparable to that associated with daytime visibility impairment. As 
noted in the Policy Assessment, no urban-focused preference or 
valuation studies providing information on public preferences for 
nighttime visual air quality have been identified (U.S. EPA, 2011a, p. 
4-17). Second, in addition to air pollution, nighttime visibility is 
affected by the addition of light into the sight path from numerous 
sources, including anthropogenic light sources in urban environments 
such as artificial outdoor lighting, which varies dramatically across 
space, and natural sources including the moon, planets, and stars. 
Light sources and ambient light conditions are typically five to seven 
orders of magnitude dimmer at night than in sunlight. Moonlight, like 
sunlight, introduces light throughout an observer's sight path at a 
constant angle. On the other hand, dim starlight emanates from all over 
the celestial hemisphere while artificial lights are concentrated in 
cities and illuminate the atmosphere from below. These different light 
sources will yield variable changes in visibility as compared to what 
has been established for the daytime scenario, in which a single 
source, the sun, is by far the brightest source of light. Third, the 
human psychophysical response (e.g., how the human eye sees and 
processes visual stimuli) at night is expected to differ (U.S. EPA, 
2009a, section 9.2.2).
    Given the above, 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). Thus, the 
Policy Assessment focuses its assessments of PM visibility impacts in 
urban areas on daylight hours. For simplicity, and because perceptions 
and welfare effects from light extinction-related visual effects during 
the minutes of actual sunrise and sunset have not been explored, 
daylight hours are defined as those hours entirely after the local 
sunrise time and before the local sunset time.
    In so doing, the Policy Assessment notes that the 24-hour averaging 
time used in the Regional Haze Program includes nighttime conditions 
(U.S. EPA, 2011a, p. 4-18). It also notes, however, that the goal of 
the Regional Haze Program is to address any manmade impairment of 
visibility without regard to distinctions between daylight and 
nighttime conditions. Moreover, because of the lack of strong diurnal 
patterns in most Federal Class I areas, both nighttime and daylight 
visibility are strongly correlated with 24-hour average visibility 
conditions, so a 24-hour averaging period is suitable for driving both 
daylight and nighttime visibility towards their natural conditions. 
Also, the focus on 24-hour average visibility allows the Regional Haze 
Program to make use of more practically obtained ambient speciated PM 
measurements of adequate accuracy than if a shorter averaging period 
were used, which is an important consideration especially given the 
remoteness of many Federal Class I area monitoring sites and given the 
low PM concentrations that must be measured accurately in such areas.
    In addition, when natural conditions such as fog and rain cause 
poor visibility, it can be reasonably assumed that the light extinction 
properties of the air that are attributable to air pollution are not 
important from a public welfare perspective. Thus, it is appropriate to 
give special treatment to such periods when considering whether current 
PM2.5 standards adequately protect public welfare from PM-
related visibility impairment. In evaluating alternative sub-daily 
standards, the Policy Assessment addresses this issue by screening out 
hours with particularly high relative humidity. As discussed further 
below, the Policy Assessment uses a relative humidity screen of 90 
percent on the basis that it serves as a reasonable surrogate for 
excluding hours affected by fog and rain (U.S. EPA, 2011a, p. 4-18).
d. Exposure Durations of Interest
    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 
has 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. A momentary glance at an image of a scene (i.e., less than 
a minute) is enough for study participants to judge the acceptability 
or unacceptability of the viewed visual air quality conditions. 
Moreover, individual participants in general consistently judge the 
acceptability of same-scene images that differed only with respect to 
light extinction levels when these images were presented repeatedly for 
such short periods. That is, individuals generally did not say that a 
higher-light extinction image was acceptable while saying a lower-light 
extinction, same-scene image was unacceptable, even though they could 
not compare images side-to-side. However, the Policy Assessment does 
not have information about what assumptions, if any, the participants 
may have made about the duration of exposure in determining the 
acceptability of the images and EPA staff 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.
    In the absence of such studies, the Policy Assessment considers 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

[[Page 38973]]

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. In such circumstances, the viewer may consciously evaluate how 
the visual air quality at that time either enhances or diminishes the 
experience or view. However, the public also 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). These 
scenes, whether iconic or generic, may not be consciously viewed for 
their scenic value and may not even be noticed for periods comparable 
to what would be the case during purposeful visits to scenic visits, 
but their visual air quality may still affect a person's sense of 
wellbeing. 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 wellbeing.''
    Some people have mostly intermittent opportunities on a daily basis 
(e.g., during morning and/or afternoon commutes) to experience ambient 
visibility conditions because they spend much of their time indoors 
without access to windows. For such people a view of poor visual air 
quality during their morning commute may provide their perception of 
the day's visibility conditions until the next time they venture 
outside during daylight hours later or perhaps the next day. Other 
people have exposure to visibility conditions throughout the day, 
conditions that may differ from hour to hour. A day with multiple hours 
of visibility impairment would likely be judged as having a greater 
impact on their wellbeing than a day with just one such hour followed 
by clearer conditions.
    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 logical to conclude 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 
judges 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. 
This approach would recognize that at least some but not all of the 
population of an area will actually be exposed to this worst hour and 
that some of the people who are exposed to this worst hour may not have 
an opportunity to observe clearer conditions in other hours if they 
were to occur. Moreover, because visibility conditions and people's 
daily activities on work/school days both tend to follow the same 
diurnal pattern day after day, those who are exposed only to the worst 
hour will tend to have this experience day after day.
    For another group of observers, those who have access to visibility 
conditions often or continuously throughout the day, the impact of the 
day's visibility conditions on their welfare may be based on the 
varying visibility conditions they observe 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. Based on these considerations, the Policy Assessment judges 
that it would also be appropriate to use a maximum multi-hour daylight 
period for evaluating the adequacy of the current suite of secondary 
standards (U.S. EPA, 2011a, p. 4-20).
    The above discussion is based on what people see, which is 
determined by the extinction of light along the paths between observers 
and the various objects they view. A related but separate issue is what 
measurement period is relevant, if what will be measured is the light 
extinction property or the PM concentration of the local air at a fixed 
site. Light extinction conditions at a fixed site can change quickly 
(i.e., in less than a minute). Sub-hourly variations in light 
extinction determined at any point in the atmosphere are likely the 
result of small-scale spatial pollution features (i.e., high 
concentration plumes that have just been generated in the immediate 
vicinity due to local sources or that have been transported by the wind 
across that point). These small-scale pockets of air causing short 
periods of higher light extinction at the fixed site likely do not 
determine the visual effect for scenes with longer sight paths. In 
contrast, atmospheric sight path-averaged light extinction which is 
pertinent to visibility impacts generally changes more slowly (i.e., 
tens of minutes generally), because a larger air mass must be affected 
by a broader set of emission sources or the larger air mass must be 
replaced by a cleaner or dirtier air mass due to the wind operating 
over time. At typical wind speeds found in U.S. cities, an hour 
corresponds to a few tens of kilometers of air flowing past a point, 
which is similar to sight path lengths of interest in urban areas. 
Based on the above considerations, the Policy Assessment concludes 
hourly average light extinction would generally be reasonably 
representative of the net visibility effect of the spatial pattern of 
light extinction levels, especially along site paths that generally 
align with the wind direction (U.S. EPA, 2011a, p. 4-21).
2. Public Perception of Visibility Impairment
    As noted in the Integrated Science Assessment, 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 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.'' 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

[[Page 38974]]

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 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.\140\ The public perception studies are 
described in detail below.
---------------------------------------------------------------------------

    \140\ 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.\141\ 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.
---------------------------------------------------------------------------

    \141\ 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. Urban 
visibility preference studies for four urban areas were reviewed in the 
Visibility Assessment (U.S. EPA, 2010b, chapter 2) 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 includes 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 answered while 
viewing a set of images of various urban visual air quality conditions.
    The approaches used in the four studies are similar and are 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

[[Page 38975]]

extinction than do images representing higher levels.
    Given the similarities in the approaches used, it is 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, variations 
in the specific materials and methods used in each study introduce 
uncertainties that should also be considered when interpreting the 
results of these comparisons. Key differences between the studies 
include: (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 the criterion as a useful index for 
comparison between studies. The results of each individual 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.\142\ 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.\143\
---------------------------------------------------------------------------

    \142\ Only 47 percent of the British Columbia participants rated 
a 19.2 dv photograph as acceptable.
    \143\ 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.
    \144\ 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] TP29JN12.004

    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. 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

[[Page 38976]]

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 4-1 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.
    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 by the Policy Assessment in a 
range from 20 dv to 30 dv (75 to 200 Mm-1) \145\ 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). 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 least noisy preference results and the most 
representative selection of participants. Based on the currently 
available information, the Policy Assessment concludes 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).
---------------------------------------------------------------------------

    \145\ 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 
considers 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 reflect 
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% 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,\146\ 
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 continues to be used in 
this 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 notes that the term CPL is meant only 
to indicate target levels of visibility within a range that EPA staff 
feels is appropriate for consideration that could, in conjunction with 
other elements of the standard, including indicator, averaging time, 
and form, provide an appropriate degree of visibility protection.
---------------------------------------------------------------------------

    \146\ 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 considers whether 
they represent a true regional distinction in preferences for urban 
visibility conditions between western and eastern U.S. There is 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.\147\ 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.
---------------------------------------------------------------------------

    \147\ 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 notes 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 and many areas in the eastern 
U.S., and possibly some areas in the western U.S. as well. 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. Based on the 
currently available information, the Policy Assessment concludes that 
the high end of the CPL range (30 dv) is an appropriate level to 
consider (U.S. EPA, 2011a, p. 4-27).
    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 concludes 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

[[Page 38977]]

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 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 concludes 
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 supports this set of CPLs for 
consideration by the EPA in this review. CASAC notes 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. CASAC concludes that this range of levels is 
``adequately supported by the evidence presented'' (Samet, 2010d, p. 
iii).

C. Adequacy of the Current Standards for PM-Related Visibility 
Impairment

    As noted above, visibility impairment occurs during periods with 
fog or precipitation irrespective of the presence or absence of PM. 
While it is a popular notion that areas with many foggy or rainy days 
are ``dreary'' places to live compared to areas with more sunny days 
per year, the Policy Assessment has no basis for taking into account 
how the occurrence of such days might modify the effect of pollution-
induced hazy days on public welfare. It is logical that periods with 
naturally impaired visibility due to fog or precipitation should not be 
treated as having PM-impaired visibility. Moreover, depending on the 
specific indicator, averaging time, and measurement approach used for 
the NAAQS, foggy conditions might result in measured or calculated 
indicator values that are higher than the light extinction actually 
caused by PM.\148\ Therefore, in order to avoid precipitation and fog 
confounding estimates of PM visibility impairment, and as advised by 
CASAC as part of its comments on the first draft Visibility Assessment, 
the assessment of visibility conditions was restricted to daylight 
hours with relative humidity less than or equal to 90 percent when 
evaluating sub-daily alternative standards (U.S. EPA, 2010b, section 
3.3.5, Appendix G).
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    \148\ One example of an indicator and measurement approach for 
which indicator values could be higher than true PM light extinction 
as a result of fog would be a light extinction indicator measured in 
part by an unheated nephelometer, which is an optical instrument for 
measuring PM light scattering from an air sample as it flows through 
a measurement chamber. Raindrops would be removed by the initial 
size-selective inlet device, although some particles associated with 
fog may be small enough that they might pass through the inlet and 
enter the measurement chamber of the instrument. This would result 
in a reported scattering coefficient that does not correspond to 
true PM light extinction. Direct measurement of light extinction 
using an open-path instrument would be even more affected by both 
fog and precipitation.
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    The EPA recognizes that not all periods with relative humidity 
above 90 percent have fog or precipitation. Removing those hours from 
consideration for a secondary PM standard would involve a tradeoff 
between the benefits of not including many of the hours with 
meteorological causes of visibility impacts and the loss of public 
welfare protection of not including some hours with high relative 
humidity without fog or precipitation, where the growth of hygroscopic 
PM into large solution droplets results in enhanced PM visibility 
impacts. For the 15 urban areas included in the assessment for which 
meteorological data were obtained to allow an examination of the co-
occurrence of high relative humidity and fog or precipitation, a 90 
percent relative humidity cutoff criterion is effective in that on 
average less than 6 percent of the daylight hours are removed from 
consideration, yet those hours have 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. Based on these findings, the Policy Assessment concludes that 
it is appropriate that a sub-daily standard intended to protect against 
PM-related visibility impairment would be defined in such a way as to 
exclude hours with relative humidity greater than approximately 90 
percent, regardless of measured values of light extinction or PM (U.S. 
EPA, 2011a, p. 4-29).
1. Visibility Under Current Conditions
    Recent visibility conditions have been characterized in the Policy 
Assessment in terms of PM-related light extinction \149\ levels for the 
15 urban areas \150\ that were selected for analysis in the Visibility 
Assessment. Hourly average PM-related light extinction was analyzed in 
terms of both PM10 and PM2.5 light extinction. 
These recent visibility conditions were then compared to the CPLs 
identified above. From Figure 4-3 and Table 4-2 in the Policy 
Assessment (U.S. EPA, 2010b, Figure 3-8 and Table 3-7, respectively) it 
can be seen that among these 14 urban areas, those in the East and in 
California tend to have a higher frequency of visibility conditions 
estimated to be above the high CPL compared with those in the western 
U.S. Both the figure and table are based on data from the 2005 to 2007 
time period and exclude hours with relative humidity greater than 90 
percent. These displays indicate that all 14 urban areas have daily 
maximum hourly PM10 light extinction values that are 
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 are estimated to exceed the high CPL from about 
20 percent to over 60 percent of the days. It is also noted that all 14 
of the urban areas are estimated to exceed the low CPL from about 40 
percent to over 90 percent of the days.
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    \149\ 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% 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% cutpoint of 2.5 [mu]m diameter.
    \150\ 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. 
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, is not representative of the urban area and resulted in 
unrealistically large PM10-2.5 values. The EPA staff 
considers these comments credible and has set aside the St. Louis 
assessment results for PM10 light extinction. Thus, 
results and statements in this Policy Assessment regarding 
PM10 light extinction apply to only the other 14 areas. 
However, results regarding PM2.5 light extinction in most 
cases apply 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 repeats 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), as described in Policy Assessment Appendix F. 
Figure 4-4 and Table 4-3 in the Policy Assessment present the same type 
of information as do Figure 4-3 and Table

[[Page 38978]]

4-2, respectively. While the estimates of the percentage of daily 
maximum hourly PM2.5 light extinction values exceeding the 
CPLs are somewhat lower than for PM10 light extinction, the 
patterns of these estimates across the study areas are similar. More 
specifically, except for the two Texas and the non-California western 
urban areas, all of the other urban areas are estimated to exceed the 
high CPL from about 10 percent up to about 50 percent of the days based 
on PM2.5 light extinction, while all 15 areas are estimated 
to exceed the low CPL from over 10 percent to over 90 percent of the 
days.
2. Protection Afforded by the Current Standards
    The Policy Assessment also conducted analyses to assess the 
likelihood that PM-related visibility impairment would exceed the 
various CPLs for a scenario based on simulating just meeting the 
current suite of PM2.5 secondary standards: 15 [mu]g/m\3\ 
annual average PM2.5 concentration and 35 [mu]g/m\3\ 24-hour 
average PM2.5 concentration with a 98th percentile form, 
averaged over three years. As described in the Visibility Assessment, 
the steps needed to model meeting the current NAAQS involve explicit 
consideration of changes in PM2.5 components. First, the 
Policy Assessment applied proportional rollback to all the 
PM2.5 monitoring sites in each study area, taking into 
account policy-relevant background PM2.5 mass, to ``just 
meet'' the current NAAQS scenario for the area as a whole, not just at 
the visibility assessment study site. The quantitative health risk 
assessment document (U.S. EPA, 2010a) describes this air quality roll-
back procedure in detail. The degree of rollback (i.e., the percentage 
reduction in non-policy-relevant background PM2.5 mass) is 
controlled by the highest annual or 24-hour design value, which in most 
study areas is from a site other than the site used in this visibility 
assessment.\151\ The relevant result from this analysis is the 
percentage reduction in non-policy-relevant background PM2.5 
mass needed to ``just meet'' the current NAAQS, for each study area. 
These percentage reductions are shown in Table 4-4 of the Visibility 
Assessment. It was noted that Phoenix and Salt Lake City meet the 
current PM2.5 NAAQS under current conditions and require no 
reduction. PM2.5 levels in these two cities were not 
``rolled up.'' Second, for each day and hour for each PM2.5 
component, the Policy Assessment subtracted the policy-relevant 
background concentration from the current conditions concentration to 
determine the non- policy-relevant background portion of the current 
conditions concentration. Third, the Policy Assessment applied the same 
percentage reduction from the first step to the non- policy-relevant 
background portion of each of the five PM2.5 components and 
added back the policy-relevant background portion of the component. 
Finally, the Policy Assessment applied the original IMPROVE algorithm, 
using the reduced PM2.5 component concentrations, the 
current conditions PM10-2.5 concentration for the day and 
hour, and relative humidity for the day and hour to calculate the 
PM10 light extinction.
---------------------------------------------------------------------------

    \151\ The selection of the site used to assess visibility was 
driven by the need for several types of PM data, and for most study 
areas the site with the highest annual or 24-hour design value did 
not have the needed types of data.
---------------------------------------------------------------------------

    In these analyses, the Policy Assessment has 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 display 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 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 show 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 are estimated 
to exceed the high CPL from about 8 percent up to over 30 percent of 
the days and the middle CPL from about 30 percent up to about 70 
percent of the days, while all areas except Phoenix are estimated to 
exceed the low CPL from 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 are estimated to exceed the 
high CPL from about 4 percent up to over 15 percent of the days and the 
middle CPL from about 15 percent up to about 45 percent of the days, 
while all areas except Phoenix are estimated to exceed the low CPL from 
over 10 percent to about 75 percent of the days. A similar set of 
figures and tables have been developed in terms of PM10 
light extinction (U.S. EPA, 2011a, Figures 4-5 and 4-6, Tables 4-4 and 
4-5).
    Taking into account the above considerations, the Policy Assessment 
concludes that the available information in this review, as described 
above and in the Visibility Assessment and Integrated Science 
Assessment, clearly calls 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 
supports consideration of alternative standards to provide appropriate 
protection (U.S. EPA, 2011a, p. 4-39).
    This conclusion is based in part on the large percentage of days, 
in many urban areas, 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 is simulated to just meet the current 
PM2.5 standards, greater than 10 percent of the days are 
estimated to 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 exceed the highest CPL ranges from over 10 percent 
to over 30 percent. Similarly, when the middle CPL of 25 dv is 
considered, greater than 30 percent up to approximately 70 percent of 
the days are estimated to exceed 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 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 exceeding the middle 
CPL in terms of PM2.5 light extinction. For the lowest CPL 
of 20 dv, the percentages of days estimated to exceed that CPL are even 
higher for all cases considered. Based on all of the above, the Policy 
Assessment concludes 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 exceeds 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 concludes that use of the current 
indicator of PM2.5 mass, in conjunction

[[Page 38979]]

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 confounded by 
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 notes 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, as 
discussed above in section 4.1.2.
    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 concludes 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).
3. CASAC Advice
    Based on its review of the second draft Policy Assessment, CASAC 
concludes 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 notes 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 states, 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).
4. Administrator's Proposed Conclusions on the Adequacy of Current 
Standards for PM-Related Visibility Impairment
    In considering whether the current suite of secondary 
PM2.5 standards is requisite to protect the public welfare 
against PM-related visibility impairment primarily in urban areas, the 
Administrator has taken 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 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. As an initial matter, the Administrator 
recognizes the clear causal relationship between PM in the ambient air 
and impairment of visibility. She takes note of the evidence from the 
visibility preference studies, and the rationale for determining a 
range of candidate protection levels based on those studies. She notes 
the relatively large number of days 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 perceptions of the acceptability of 
varying degree of visibility impairment and the information on the 
number of days estimated to exceed the CPLs, the Administrator 
concludes that such information provides 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 concludes 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 recognizes 
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. She recognizes that 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 plays a 
central role in determining the relationship between the mix of PM in 
the ambient air and impairment of visibility. The Administrator notes 
that such considerations were reflected in CASAC's advice to set a 
distinct secondary standard that would more directly reflect the 
relationship between ambient PM and visibility impairment. The 
Administrator also notes that such considerations were reflected in the 
court's remand of the current secondary PM2.5 standards. 
Based on the above considerations, the Administrator provisionally 
concludes that the current secondary PM2.5 standards, taken 
together, are neither sufficiently protective nor are they suitably 
structured to provide an appropriate degree of public welfare 
protection from PM-related visibility impairment, primarily in urban 
areas. Thus, the Administrator has considered alternative standards by 
looking at each of the elements of the standards--indicator, averaging 
time, form, and level--as discussed below.

D. Consideration of Alternative Standards for Visibility Impairment

1. Indicator
a. Alternative Indicators Considered in the Policy Assessment
    As described below, the Policy Assessment considers three 
indicators: The current PM2.5 mass indicator and two 
alternative indicators, including directly measured PM2.5 
light extinction and calculated PM2.5 light extinction (U.S. 
EPA, 2011a, section 4.3.1.1).\152\ 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 speciated PM2.5 mass and measured relative 
humidity.\153\
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    \152\ In the second draft Policy Assessment, the calculated 
PM2.5 light extinction indicator was referred to as 
speciated PM2.5 mass calculated light extinction.
    \153\ In 2009, the D.C. Circuit remanded the secondary 
PM2.5 standards to the Agency in part because the EPA did 
not address the problem that a PM2.5 mass-based standard 
using a daily averaging time would be confounded by regional 
differences in relative humidity, although EPA had acknowledged this 
problem. The EPA notes that the light extinction indicators 
considered in the Policy Assessment explicitly took into account 
differences in relative humidity in areas across the country (U.S. 
EPA, 2011a, section 4.3.1).
---------------------------------------------------------------------------

    The Policy Assessment concludes 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 characteristic that 
is most relevant and directly related to PM-related visibility effects 
(U.S. EPA, 2011a,

[[Page 38980]]

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, 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. Few urban areas conduct continuous PM10-2.5 
monitoring. 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 the absence of PM10-2.5 air quality information 
from a much larger number of urban areas across the country, it is not 
possible at this time to know in how many urban areas 
PM10-2.5 is a major contributor to urban visibility effects, 
though 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. PM10-2.5 is generally less 
homogenous in urban areas than PM2.5, making it more 
challenging to select sites that would adequately represent urban 
visibility conditions. 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).
    The basis for considering each of these three indicators is 
discussed below. The discussion also addresses monitoring data 
requirements for directly measured PM2.5 light extinction 
and for calculated PM2.5 light extinction. The following 
discussion also takes into consideration different averaging times 
since the combination of indicator and averaging time is relevant to 
understanding the monitoring data requirements. Consideration of 
alternative averaging times is addressed more specifically in section 
VI.D.2 on averaging time.
i. PM2.5 Mass
    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. Blank and loaded filters 
are weighed to determine 24-hour PM2.5 mass. Continuous 
PM2.5 monitoring produces hourly average mass concentrations 
and is conducted at about 900 locations. About 180 of these locations 
employ newer model continuous instruments that have been approved by 
EPA as FEMs, although the Policy Assessment notes that FEM approval has 
been based only on 24-hour average, not hourly, PM2.5 mass. 
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.\154\ 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.\155\
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    \154\ FRM filters are stabilized in a laboratory at fixed 
temperature and relative humidity levels, which alters whatever 
water content was present on the filter when removed from the 
sampler. FEM instruments are designed to meet performance criteria 
compared to FRM measurements, and accordingly typically manage 
temperature and/or humidity at the point of measurement to levels 
that are not the same as ambient conditions.
    \155\ As discussed below, 24-hour average PM2.5 
chemical component mass is measured at about 200 CSN sites.
---------------------------------------------------------------------------

    The overall performance of 1-hour average PM2.5 mass as 
a predictor of PM-related visibility impairment as indicated by 
PM10 calculated light extinction can be seen in scatter 
plots shown in Figure 4-9 of the Policy Assessment for two illustrative 
urban areas, Pittsburgh and Philadelphia (Similar plots for all 14 
urban areas that have estimates of PM10 light extinction are 
in Appendix D, Figure D-2 of U.S. EPA, 2010b). These illustrative 
examples demonstrate the large variations in hourly PM10 
light extinction corresponding to any specific level of hourly 
PM2.5 mass concentration as well as differences in the 
statistical average relationships (depicted as the best fit lines) 
between cities. This poor correlation between hourly PM10 
light extinction and hourly PM2.5 mass is not due to any 
great extent to the contribution of PM10-2.5 to light 
extinction, but rather is 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. This 
contributes to the noisiness of the data on the relationship 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.
    As part of the Visibility Assessment, an assessment was conducted 
that estimated PM10 light extinction levels that may prevail 
if areas were simulated to just meet a range of alternative secondary 
standards based on hourly PM2.5 mass as the indicator. 
Appendix E of the Policy Assessment contains the results of this 
rollback-based assessment. This assessment quantifies the projected 
uneven protection, noted qualitatively above, that would result from 
the use of 1-hour average PM2.5 mass as the indicator.
ii. Directly Measured PM2.5 Light Extinction
    PM light extinction is the major contributor to light extinction, 
which is the property of the atmosphere that is most directly related 
to visibility effects. It differs from light extinction by the nearly 
constant contributions for Rayleigh (or clean air) light scattering and 
the minor contributions by NO2 light absorption. The net 
result is that 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. Direct 
measurement of PM2.5 light extinction can be accomplished 
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). 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.
    As part of the Visibility Assessment, an assessment was conducted 
that estimated PM10 light extinction levels that may prevail 
in 14 urban study areas if the areas were simulated to just meet a 
secondary standard based on directly measured hourly PM10 
light extinction as the indicator (U.S. EPA, 2010b,

[[Page 38981]]

section 4.3).\156\ As would be expected, this assessment indicated that 
a secondary standard based on a directly measured PM10 light 
extinction indicator would provide the same percentage of days having 
values above the level of the standard in each of the areas, with the 
percentage being dependent on the statistical form of the standard. The 
Policy Assessment considers this assessment reasonably informative for 
a directly measured PM2.5 light extinction indicator as 
well, because in most of the assessment study areas PM10 
light extinction is dominated by PM2.5 light extinction.
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    \156\ This assessment was conducted prior to staff's decision to 
focus on PM2.5 light extinction indicators in the Policy 
Assessment.
---------------------------------------------------------------------------

    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 produced a White Paper on 
Particulate Matter (PM) Light Extinction Measurements (U.S. EPA, 
2010g), and solicited comment on the White Paper from the Ambient Air 
Monitoring and Methods Subcommittee (AAMMS) of CASAC. In its review of 
the White Paper (Russell and Samet, 2010a), the CASAC AAMMS made the 
recommendation that consideration of direct measurement should be 
limited to PM2.5 light extinction as this can be 
accomplished by a number of commercially available instruments and 
because PM2.5 is generally responsible for most of the PM 
visibility impairment in urban areas. The CASAC AAMMS indicated that it 
is technically more challenging at this time to accurately measure the 
PM10-2.5 component of light extinction.
    The CASAC AAMMS also commented on the capabilities of currently 
available instruments, and expressed optimism regarding the near-term 
development of even better instruments for such measurement than are 
now commercially available. 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 EPA develop performance-based approval 
criteria for direct measurement methods in order to put all approaches 
on a level playing field. Such criteria would necessarily include 
procedures and pass/fail requirements for demonstrating that the 
performance criteria have been met. For example, instruments might be 
required to demonstrate their performance in a wind tunnel, where the 
concentration of PM2.5 components, and thus of 
PM2.5 light extinction, could be controlled to known values. 
It might also be possible to devise approval testing procedures based 
on operation in ambient air, although knowing the true light extinction 
level (without in effect treating some particular instrument as if it 
were the FRM) would be more challenging. 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.
iii. Calculated PM2.5 Light Extinction
    As discussed above in section VI.B.1 above, PM2.5 light 
extinction can be calculated from 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 also be 
used for a NAAQS focused on protecting against PM-related visibility 
impairment primarily in urban areas. This could use the type of data 
that is routinely collected from the urban CSN \157\ in combination 
with climatological relative humidity data as used in the Regional Haze 
Program (U.S. EPA, 2011a, Appendix G, section G.2). This calculation 
procedure, using the original IMPROVE light extinction equation 
presented above in section VI.B.1 on a 24-hour basis (or the revised 
IMPROVE equation), does not require PM2.5 mass concentration 
measurements.
---------------------------------------------------------------------------

    \157\ 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).
---------------------------------------------------------------------------

    Alternatively, a conceptually similar approach could be applied in 
urban areas on an hourly or multi-hour basis. Applying this conceptual 
approach on a sub-daily basis would involve translating 24-hour 
speciation data into hourly estimates of species concentrations, and 
using 24-hour average species concentrations in conjunction with hourly 
PM2.5 mass concentrations. This translation can be made 
using more or less complex alternative approaches, as discussed below.
    The approach used to generate hourly PM10 light 
extinction for the Visibility Assessment was a relatively more complex 
method for implementing such a conceptual approach. It involved the use 
of the original IMPROVE algorithm \158\ with estimates of hourly PM 
2.5 components derived from day-specific 24-hour and hourly 
measurements of PM 2.5 mass, 24-hour measurements of PM 
2.5 composition, hourly measurements of PM 2.5 
mass and (for some but not all study sites) hourly PM10-2.5 
mass, along with hourly relative humidity information (U.S. EPA, 2010b, 
section 3.3). The Visibility Assessment approach also involved the use 
of output from a chemical transport modeling run to provide initial 
estimates of diurnal profiles for PM2.5 components at 
particular sites. The Visibility Assessment approach entailed numerous 
and complex data processing steps to generate hourly PM2.5 
composition information from these less time-resolved data, including 
application of a mass-closure approach, referred to as the SANDWICH 
approach \159\ (Frank, 2006), to adjust for nitrate retention 
differences between FRM and CSN filters, which is a required step for 
consistency with the IMPROVE algorithm and for estimating organic 
carbonaceous material via mass balance.\160\ The EPA staff employed 
complex custom software to do these data processing steps.
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    \158\ The original IMPROVE algorithm was selected for the 
described analysis in the Visibility Assessment because of its 
simplicity relative to the revised algorithm.
    \159\ Sulfate, adjusted nitrate, derived water, inferred 
carbonaceous mass (SANDWICH) approach.
    \160\ Daily temperature data were also used as part of the 
SANDWICH method.
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    While the complexity of the approach used in the Visibility 
Assessment was reasonable for assessment purposes at 15 urban areas, 
the Policy Assessment recognizes that a relatively more simple approach 
would be more straightforward and have greater transparency, and thus 
should be considered for purposes of a national standard.\161\ 
Therefore, the Policy Assessment evaluated the degree to which simpler 
approaches would correlate with the results of the highly complex 
method used in the Visibility Assessment. This evaluation of two 
specific simpler approaches (described briefly below and in more detail 
in U.S. EPA, 2011a, Appendix F, especially Table F-1) demonstrated that 
the PM2.5 portions of the PM10 light extinction

[[Page 38982]]

values developed for the Visibility Assessment can be well approximated 
using the same IMPROVE algorithm applied to hourly PM2.5 
composition values that were much more simply generated than with the 
method used in the Visibility Assessment.
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    \161\ 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.
---------------------------------------------------------------------------

    The simplified approaches examined were aimed at calculating hourly 
PM2.5 light extinction using the original IMPROVE algorithm 
(see section VI.B.1.a. above) excluding the Rayleigh term for light 
scattering by atmospheric gases and the term for 
PM10-2.5.\162\ These approaches, including a description of 
the sources of the data and steps required to determine calculated 
PM2.5 light extinction for these simplified approaches, are 
described in more detail in the Policy Assessment (U.S. EPA, 2011a, pp. 
4-46 to 48, Appendix F, Table F-2). Also, Table F-1 of Appendix F of 
the Policy Assessment compares and contrasts each of these approaches 
with the Visibility Assessment approach and with each other.
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    \162\ The original IMPROVE algorithm was the basis for the 
approaches considered in the Policy Assessment to maintain 
comparability to the estimates developed in the Visibility 
Assessment. This allowed the effects of other simplifications 
relative to the Visibility Assessment approach to be better 
discerned.
---------------------------------------------------------------------------

    The hourly PM2.5 light extinction values generated by 
using either simplified approach are comparable to those developed for 
use in the Visibility Assessment as indicated by the regression 
statistics for scatter plots of the paired data (i.e., the slopes of 
the regression equation and the R\2\ values are near 1 as shown in U.S. 
EPA, 2011a, Appendix F, Tables F-3 and F-4). Appendix F notes that both 
approaches underestimate PM2.5 light extinction on some days 
in a few study areas, which the Policy Assessment attributes to the 
occurrence of very high nitrate concentrations and the failure of the 
FRM-correlated/adjusted FEM instrument to report the entire nitrate 
mass. Nevertheless, the Policy Assessment concludes that each of these 
simplified approaches provides reasonably good estimates of 
PM2.5 light extinction and each is appropriate to consider 
as the indicator for a distinct hourly or multi-hour secondary standard 
(U.S. EPA, 2011a, p. 4-48).
    In addition, the Policy Assessment notes that there are variations 
of these simplified approaches that may also be appropriate to 
consider. For example, some variations that may improve the correlation 
with actual ambient light extinction in certain areas of the country 
include the use of the split-component mass extinction efficiency 
approach from the revised IMPROVE algorithm,\163\ the use of more 
refined value(s) for the organic carbon multiplier (see U.S. EPA, 
2011a, Appendix F),\164\ and 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. Other variations may 
serve to simplify the calculation of PM2.5 light extinction 
values, such as those suggested by CASAC for consideration, including 
the use of historical monthly or seasonal speciation averages as well 
as speciation estimates on a regional basis (Samet, 2010d, p. 11). Some 
of these variations would also be appropriate to consider in 
conjunction with a 24-hour average calculated PM2.5 light 
extinction indicator, including the use of the revised IMPROVE 
algorithm, the use of an alternative value for the organic carbon 
multiplier (e.g., 1.6), and the use of historical monthly or seasonal, 
or regional, speciation averages.
---------------------------------------------------------------------------

    \163\ 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.
    \164\ 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.
---------------------------------------------------------------------------

    As mentioned above, as part of the Visibility Assessment, an 
assessment was conducted of PM10 light extinction levels 
that would prevail if areas met a standard based on directly measured 
hourly PM10 light extinction as the indicator. This 
assessment indicated that a standard based on a directly measured 
PM10 light extinction indicator would provide the same 
percentage of days having indicator values above the level of the 
standard across areas, with the percentage being dependent on the 
statistical form of the standard. This assessment was based on the more 
complex Visibility Assessment approach to estimating PM10 
light extinction, rather than the simpler approaches for estimating 
PM2.5 light extinction. Nevertheless, the generally close 
correspondence between design values for PM2.5 light 
extinction developed consistent with the Visibility Assessment approach 
and design values based on the simplified approaches (U.S. EPA, 2011a, 
Appendix F, Figure F-5) suggest that the findings regarding the 
protection offered by alternative PM10 light extinction 
standards using directly measured light extinction would also hold 
quite well for standards based on the simplified indicators.\165\ Thus, 
the Policy Assessment concludes that the use of a calculated 
PM2.5 light extinction indicator would provide a much higher 
degree of uniformity in terms of the visibility levels across the 
country than is possible using PM2.5 mass as the indicator 
(U.S. EPA, 2011a, p. 4-49). This is due to the fact that the 
PM2.5 mass indicator does not account for the effects of 
humidity and PM2.5 composition differences between various 
regions, while a calculated PM2.5 light extinction indicator 
directly incorporates those effects.
---------------------------------------------------------------------------

    \165\ The degree of emission reduction needed to meet a standard 
is tightly tied to the degree to which the design value exceeds the 
level of the standard.
---------------------------------------------------------------------------

    The inputs that would be necessary to use either simplified 
approach to calculate a sub-daily PM2.5 light extinction 
indicator (e.g., 1- or 4-hour averaging time) include PM2.5 
chemical speciation, relative humidity, and hourly PM2.5 
mass measurements. In defining a standard in terms of calculated light 
extinction, the criteria for allowable protocols for these calculations 
would need to be specified. It would be appropriate to base these 
criteria on the protocols utilized in the IMPROVE \166\ and CSN 
networks, as well as sampling and analysis protocols for ambient 
relative humidity sensors, and approved FEM mass monitors for 
PM2.5. Any approach to approving methods for use in 
calculating a light extinction indicator should take advantage of the 
existing inventory of monitoring and analysis methods.
---------------------------------------------------------------------------

    \166\ Several monitoring agencies utilize IMPROVE in urban areas 
to meet their chemical speciation monitoring needs. These sites are 
known as IMPROVE-protocol stations.
---------------------------------------------------------------------------

    The CSN measurements have a strong history of being reviewed by 
CASAC technical committees, both during their initial deployment about 
ten years ago (Mauderly 1999a,b) and during the more recent transition 
to carbon sampling that is consistent with the IMPROVE protocols 
(Henderson, 2005c). Because the methods for the CSN are well documented 
in a nationally implemented Quality Assurance Project Plan (QAPP) and 
accompanying standard operating procedures (SOPs), are validated 
through independent performance testing, and are used to meet multiple 
data objectives (e.g., source apportionment, trends, and as an input to 
health studies), consideration

[[Page 38983]]

should be given to an approach that utilizes the existing methods as 
the basis for criteria for allowable sampling and analysis protocols 
for purposes of a calculated light extinction indicator. Such an 
approach of basing criteria on the current CSN and IMPROVE methods 
provides a nationally consistent way to provide the chemical species 
data used in the light extinction calculation, while preserving the 
opportunity for improved methods for measuring the chemical species. 
For relative humidity, in conjunction with either hourly, multi-hour, 
or 24-hour average calculated PM2.5 light extinction, 
consideration should be given to simply using criteria based on 
available relative humidity sensors such as already utilized by the 
National Oceanic and Atmospheric Administration (NOAA) at routine 
weather stations. These relative humidity sensors are already widely 
used by a number of monitoring agencies and can be easily compared to 
other relative humidity measurements.\167\ Finally, the simplified 
approaches for a sub-daily averaging period depend on having values of 
hourly PM2.5 mass, as discussed below.
---------------------------------------------------------------------------

    \167\ For the purposes of using relative humidity measurements 
to derive multi-hour or 24-hour average PM2.5 calculated 
light extinction, the non-linear f(RH) enhancement factor should be 
developed separately for each hour and then averaged over the 
desired multi-hour period. This averaging approach is consistent 
with derivation of climatological f(RH) factors used by the IMPROVE 
program and for the Regional Haze rule.
---------------------------------------------------------------------------

    Since 2008, EPA has approved several PM2.5 continuous 
mass monitoring methods as FEMs.\168\ These methods have several 
advantages over filter-based FRMs, such as producing hourly data and 
the ability to report air quality information in near real-time. 
However, initial assessments of the data quality as operated by state 
and local monitoring agencies have had mixed results. A recent 
assessment of continuous FEMs and collocated FRMs conducted by EPA 
staff (Hanley and Reff, 2011) found some sites and continuous FEM 
instruments to have an acceptable degree of comparability of 24-hour 
average PM2.5 mass values derived from continuous FEMs and 
filter-based FRMs, while others had poor data quality that would not 
meet current data quality objectives. The EPA is working closely with 
the monitoring committee of the National Association of Clean Air 
Agencies (NACAA), instrument manufacturers, and monitoring agencies to 
document and communicate best practices on these methods to improve 
quality and consistency of resulting data. It should be noted that 
performance testing submitted to EPA for purposes of designating the 
PM2.5 continuous methods as FEMs, and the recent assessment 
of collocated FRMs and continuous FEMs, are both based on 24-hour 
sample periods. Therefore, the EPA does not have similar performance 
data for continuous PM2.5 FEMs for 1-hour or 4-hour 
averaging periods, nor is there an accepted practice to generate 
performance standards for these time periods.\169\ Until issues 
regarding the comparability of 24-hour PM2.5 mass values 
derived from continuous FEMs and filter-based FRMs 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 
and speciated PM2.5 mass values from 24-hour filter-based 
samplers. Section 4.3.2.1 of the Policy Assessment discusses another 
reason for such caution, based on a preliminary assessment of hourly 
data from continuous FEMs (U.S. EPA, 2011a, pp. 4-52 to 4-54).
---------------------------------------------------------------------------

    \168\ The EPA maintains a list of designated Reference and 
Equivalent Methods on its Web site at: http://www.epa.gov/ttn/amtic/files/ambient/criteria/reference-equivalent-methods-list.pdf.
    \169\ 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.
---------------------------------------------------------------------------

    This section has addressed the types of measurements that would be 
necessary to support a calculated PM2.5 light extinction 
indicator for either 24-hour or sub-daily (e.g., 1-hour and 4-hour) 
averaging periods. Considerations related specifically to each of these 
alternative averaging times, in conjunction with a standard defined in 
terms of a calculated PM2.5 light extinction indicator, are 
discussed further in section 4.3.2 of the Policy Assessment.
iv. Conclusions in the Policy Assessment
    Taking the above considerations and CASAC's advice into account, 
the Policy Assessment concludes that consideration should be given to 
establishing a new calculated PM2.5 light extinction 
indicator (U.S. EPA, 2011a, p. 4-51). This conclusion takes into 
consideration the available evidence that demonstrates 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. While a secondary standard that uses a 
PM2.5 mass indicator could be set to provide additional 
protection from PM2.5-related visibility impairment, the 
Policy Assessment concludes that the advantages of using a calculated 
PM2.5 light extinction indicator make it the preferred 
choice (U.S. EPA, 2011a, p. 4-51). In addition, the Policy Assessment 
recognizes 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, it concludes 
that 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).
    Further, the Policy Assessment concludes 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). In either case, it would be appropriate to base criteria for 
allowable monitoring and analysis protocols to obtain PM2.5 
speciation measurements on the protocols utilized in the IMPROVE and 
CSN networks. Further, in the case of a calculated PM2.5 
light extinction indicator defined on a sub-daily basis, it would be 
appropriate to consider using the simplified approaches described, or 
some variations on these approaches. In reaching this conclusion, as 
discussed above, the Policy Assessment notes that while it is possible 
to utilize data from PM2.5 continuous FEMs on a 1-hour or 
multi-hour (e.g., 4-hour) basis, the mixed results of data quality 
assessments on a 24-hour basis, as well as the near absence of 
performance data for sub-daily averaging periods, increases 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.
b. CASAC Advice
    Based on its review of 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 concludes 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

[[Page 38984]]

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 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).\170\
---------------------------------------------------------------------------

    \170\ 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).
---------------------------------------------------------------------------

c. Administrator's Proposed Conclusions on Indicator
    In reaching a proposed conclusion on the appropriate indicator for 
a standard intended to protect against PM-related visibility 
impairment, as an initial matter, the Administrator concurs 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. However, she also recognizes that while instruments 
currently exist that can directly measure PM2.5 light 
extinction, they are not an appropriate option in this review because a 
suitable specification of the equipment or performance-based 
verification procedures cannot be developed in the time frame of this 
review.
    Taking the above considerations and CASAC advice into account, the 
Administrator provisionally concludes 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), is an appropriate indicator to replace the current 
PM2.5 mass indicator. Such an indicator, referred to as a 
PM2.5 visibility index, appropriately reflects the 
relationship between ambient PM and PM-related light extinction, based 
on the analyses discussed above and incorporation of factors based on 
measured PM2.5 speciation concentrations and relative 
humidity data. In addition, this addresses, in part, the issues raised 
in the court's remand of the 2006 PM2.5 standards. The 
Administrator also notes 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 the above considerations, the Administrator proposes 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 proposes that such an index 
be based on the original IMPROVE algorithm in conjunction with 
climatological relative humidity data as used in the Regional Haze 
Program. A more detailed discussion of the steps involved in the 
calculation of PM2.5 visibility index values is presented in 
section VII.A.5 below.
    The Administrator solicits comment on all aspects of the proposed 
indicator. In particular, the Administrator solicits comment on the 
proposed use of a PM2.5 visibility index rather than a 
PM10 visibility index which would include an additional term 
for coarse particles. The Administrator also solicits comment on 
alternatively using the revised IMPROVE algorithm rather than the 
original IMPROVE algorithm the use of alternative values for the 
organic carbon multiplier in conjunction with either the original or 
revised IMPROVE algorithm; the use of historical monthly, seasonal, or 
regional speciation averages; and on alternative approaches to 
determining relative humidity, as discussed above. Further, in 
conjunction with an hourly or multi-hour indicator, comment is 
solicited on variations on the simplified approaches discussed above 
and on other approaches that may be appropriate to consider for such an 
indicator.
2. Averaging Times
a. Alternative Averaging Times
    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, as discussed above in 
section VI.B.1 and in section 4.2.1 of the Policy Assessment, and the 
nature of inputs to the calculation of PM2.5 light 
extinction, as discussed above in section VI.D.1 and in section 4.3.1 
of the Policy Assessment. Based on this information, the Policy 
Assessment considered both sub-daily (1- and 4-hour averaging times) 
and 24-hour averaging times, as discussed below. In considering sub-
daily averaging times, the Policy Assessment also addressed what 
diurnal periods and ambient relative humidity conditions would be 
appropriate to consider in conjunction with such an averaging time.
i. Sub-daily
    As an initial matter, in considering sub-daily averaging times, the 
Policy Assessment took 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. While perception of 
change in visibility can occur in less than a minute, meaningful 
changes to path-averaged light extinction occur more slowly. As 
discussed above and in section 4.2.1 of the Policy Assessment, one hour 
is a short enough averaging period to result in indicator values that 
are close to the maximum one- or few-minute visibility impact that an 
observer could be exposed to within the hour. Further, a 1-hour 
averaging time could reasonably characterize the visibility effects 
experienced by the segment of the population that experiences 
infrequent short-term exposures during peak visibility impairment 
periods in each area/site. Based on the above considerations, the 
initial analyses conducted in the Policy Assessment as part of the 
Visibility Assessment to support consideration of alternative standards 
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 notes 
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 concludes 
that since a calculated PM2.5 light extinction indicator 
relies in part on measured PM2.5 mass, as discussed above 
and in section 4.3.1 of the Policy Assessment, it is also appropriate 
to consider a multi-hour averaging time in

[[Page 38985]]

conjunction with such an indicator (U.S. EPA, 2011a, p. 4-53).
    Thus, the Policy Assessment has considered multi-hour averaging 
times, on the order of a few hours as illustrated by a 4-hour averaging 
time. Such averaging times 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 recognizes 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 recognizes 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 concludes 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).
    Additionally, as part of the review of data from all continuous FEM 
PM2.5 instruments operating at state/local monitoring sites, 
as discussed above, the Policy Assessment notes that the occurrence of 
questionable outliers in 1-hour data submitted to AQS from continuous 
FEM PM2.5 instruments has been observed at some of these 
sites (Evangelista, 2011). Some of these outliers are questionable 
simply by virtue of their extreme magnitude, as high as 985 [micro]g/
m\3\, whereas other values are questionable because they are 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.\171\ The nature and frequency of questionable 1-hour FEM 
data collected in the past two years are being investigated. At this 
time, the Policy Assessment notes 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 notes 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.
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    \171\ 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.
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    In considering an appropriate diurnal period for use in conjunction 
with a sub-daily averaging time, the Policy Assessment recognizes that 
nighttime visibility impacts, described in the Integrated Science 
Assessment (U.S. EPA, 2009a, section 9.2.2) are significantly different 
from daytime impacts and are not sufficiently well understood to be 
included at this time. As a result, consistent with CASAC advice 
(Samet, 2010c, p. 4), the Policy Assessment concludes that it would be 
appropriate to define a sub-daily standard in terms of only daylight 
hours at this time (U.S. EPA, 2011a, p. 4-54). In the Visibility 
Assessment, daylight hours were defined to be those morning hours 
having no minutes prior to local sunrise and afternoon hours having no 
minutes after local sunset. This definition ensures the exclusion of 
periods of time where the sun is not the primary outdoor source of 
light to illuminate scenic features.
    In considering the well-known interaction of PM with ambient 
relative humidity conditions, the Policy Assessment recognizes that PM 
is not generally the primary source of visibility impairment during 
periods with fog or precipitation. In order to reduce the probability 
that hours with a high degree of visibility impairment caused by fog or 
precipitation are unintentionally used for purposes of determining 
compliance with a standard, the Policy Assessment determined that a 
relative humidity screen that excludes daylight hours with average 
relative humidity above approximately 90 percent is appropriate (U.S. 
EPA, 2001, pp. 4-54 to 4-55; see also U.S. EPA, 2010b, section 3.3.5, 
Appendix G). For example, for the 15 urban areas \172\ included in the 
Visibility Assessment, a 90 percent relative humidity cutoff criterion 
proved effective in that on average less than 6 percent of the daylight 
hours were removed from consideration, yet those same hours had on 
average 10 times the likelihood of rain, 6 times the likelihood of 
snow/sleet, and 34 times the likelihood of fog compared with hours with 
90 percent or lower relative humidity. However, not all periods with 
relative humidity above 90 percent have fog or precipitation. The 
Policy Assessment recognizes that removing those hours from 
consideration involves a tradeoff between the benefits of avoiding many 
of the hours with meteorological causes of visibility impacts and not 
counting some hours without fog or precipitation in which high humidity 
levels (e.g., greater than 90 percent) lead to the growth of 
hygroscopic PM to large solution droplets resulting in larger PM 
visibility impacts.
---------------------------------------------------------------------------

    \172\ The 90 percent relative humidity cap assessment was 
conducted as part of the Visibility Assessment on all 15 of the 
urban areas, including St. Louis.
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ii. 24-Hour
    As discussed in section 4.3.1 of the Policy Assessment and below, 
there are significant reasons to consider using PM2.5 light 
extinction calculated on a 24-hour basis to reduce the various data 
quality concerns over relying on continuous PM2.5 monitoring 
data. However, the Policy Assessment recognizes 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, as 
discussed above and in section 4.3.2.1 of the Policy Assessment. Thus, 
consideration of a 24-hour averaging time depends 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, as discussed below in this 
section. Further, since a 24-hour averaging time combines daytime and 
nighttime periods, the Policy Assessment recognizes that the public 
preference studies do not directly provide a basis for identifying an 
appropriate level of protection, in terms of 24-hour average light 
extinction, based on judgments of acceptable daytime visual air quality 
obtained in

[[Page 38986]]

those studies. Thus, consideration of a 24-hour averaging time also 
depends upon developing an approach to translate the candidate levels 
of protection derived from the public preference studies, which the 
Policy Assessment has interpreted on an hourly or multi-hour basis, to 
a candidate level of protection defined in terms of a 24-hour average 
calculated light extinction, as discussed in section.VI.D.4 below.
    To determine whether 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, the 
Policy Assessment performed comparative analyses of 24-hour and 4-hour 
averaging times in conjunction with a calculated PM2.5 
indicator.\173\ These analyses are presented and discussed in Appendix 
G, section G.4 of the Policy Assessment. 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 calculations used the original 
IMPROVE algorithm and long-term (1988 to 1997) average relative 
humidity conditions, to calculate monthly average values of the 
relative humidity term in the IMPROVE algorithm, consistent with the 
approach used for the Regional Haze Program. Similar to the approach 
used to assess a sub-daily visibility index discussed in section 
VI.2.a.i above, these 1988-1997 humidity data are similarly screened to 
remove the effect of high hourly relative humidity. In this case, any 
relative humidity value great than 95 percent was treated as 95 
percent. Because 10-years of hourly data were used to produce a single 
humidity term for each month, the EPA believes that the resulting 
monthly average of the humidity term is sufficient and appropriate to 
reduce the effects of fog or precipitation. Based on these analyses, 
scatter plots comparing 24-hour and 4-hour calculated PM2.5 
light extinction are shown 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). It can be seen, as expected, that there is 
some scatter around the regression line for each city, because the 
calculated 4-hour light extinction includes day-specific and hour-
specific influences that are not captured by the simpler 24-hour 
approach. The Policy Assessment notes that this scatter could be 
reduced by the use of same-day hourly relative humidity data to 
calculate a 24-hour average value of the relative humidity term in the 
IMPROVE algorithm. In the Policy Assessment, scatter plots are also 
shown for the annual 90th percentile values, based on data from 2007 to 
2009, for 4-hour and 24-hour calculated PM2.5 light 
extinction across all 15 cities (U.S. EPA, 2011a, Figure G-7) and for 
the 3-year design values across all 15 cities (U.S. EPA, 2011a, Figure 
G-8). These analyses 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.\174\
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    \173\ 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).
    \174\ The EPA staff note that the R\2\ value (0.44) for Houston 
was notably lower than for the other cities.
---------------------------------------------------------------------------

iii. Conclusions in the Policy Assessment
    Taking the above considerations and CASAC's advice into account, 
the Policy Assessment concludes that it is appropriate to consider in 
this review a 24-hour averaging time, in conjunction with a calculated 
PM2.5 light extinction indicator and an appropriately 
specified standard level (U.S. EPA, 2011a, p. 4-57). This conclusion 
reflects the judgment 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 average 
basis. This conclusion is also predicated on consideration of a 24-hour 
average standard level, as discussed below and in section 4.3.4 of the 
Policy Assessment, that is appropriately translated from the CPLs 
derived from the public preference studies, which the Policy Assessment 
has interpreted as providing information on the acceptability of 
daytime visual air quality over an hourly or multi-hour exposure 
period.
    A 24-hour average calculated PM2.5 light extinction 
indicator would avoid data quality uncertainties that have recently 
been associated with currently available instruments for measurement of 
hourly PM2.5 mass. The particular 24-hour indicator 
considered by the Policy Assessment uses the original IMPROVE algorithm 
and long-term relative humidity conditions to calculate 
PM2.5 light extinction. 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.
    Further, the Policy Assessment concludes 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 that have been raised by the 
recent EPA study of continuous FEMs (U.S. EPA, 2011a, p. 4-58). 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 concludes 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 concludes 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).
b. CASAC Advice
    As noted above, in its review of the first draft Policy Assessment, 
CASAC concludes 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

[[Page 38987]]

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 indicator when calculated on a sub-daily basis.
    In considering this advice, the Policy Assessment first notes that 
CASAC did not have the benefit of EPA's recent assessment of the data 
quality issues associated with the use of continuous FEMs as the basis 
for hourly PM2.5 mass measurements. The Policy Assessment 
also notes that since earlier drafts of this Policy Assessment did not 
include 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.
c. Administrator's Proposed Conclusions on Averaging Time
    In reaching a proposed conclusion on the appropriate averaging time 
for a standard intended to protect against PM-related visibility 
impairment, the Administrator has taken into account the information 
discussed above with regard to analyses and conclusions presented in 
the final Policy Assessment as well as the views of CASAC based on its 
reviews of the first and second drafts of the Policy Assessment. As an 
initial matter, the Administrator recognizes 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 recognizes 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 recognizes the desirability of a sub-
daily averaging time, she has 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 recognizes 
that the Policy Assessment concludes 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 (U.S. EPA, 2011a, p. 4-57). In 
light of this finding, the Administrator proposes to set a distinct 
secondary standard with a 24-hour averaging time in conjunction with a 
PM2.5 visibility index.
    Further, in light of the desirability of a sub-daily averaging 
time, the Administrator solicits comment on a sub-daily (e.g., 4-hour) 
averaging time and related data quality issues associated with 
currently available monitoring instrumentation. In so doing, the 
Administrator notes that CASAC's advice on averaging times was 
predicated in part on the capabilities of available monitoring 
instrumentation as CASAC understood them when it provided its advice.
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 
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 concludes it is 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 
concludes 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 notes 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 finds no basis to 
conclude that it would be appropriate to consider limiting the 
occurrence of days with peak PM-

[[Page 38988]]

related light extinction in urban areas to a greater degree.
    Another aspect of the form that was considered in the Visibility 
Assessment for a sub-daily (i.e., 1-hour) averaging time is whether to 
include all daylight hours or only the maximum daily daylight hour. 
This consideration would also be relevant for a multi-hour (e.g., 4-
hour) averaging time, although such an analysis was not included in the 
Visibility Assessment. The maximum daily daylight 1-hour or multi-hour 
form is 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 does not have information regarding the 
fraction of the public that has only one or a few opportunities to 
experience visibility during the day, nor does 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 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 Visibility Assessment 
analyses included both the maximum daily hour and the all daylight 
hours forms. The Policy Assessment observed a close correspondence 
between the level of protection afforded for all 15 urban areas in the 
assessment by the maximum daily daylight 1-hour approach using the 90th 
percentile form and the all daylight hours approach combined with the 
98th percentile form (U.S. EPA, 2010b, section 4.1.4). On this basis, 
the Policy Assessment notes that the 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). The Policy Assessment also notes that 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 concludes that it is 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 concludes that it 
is 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.
    Based on its review of the second draft Policy Assessment, CASAC 
did not provide advice as to a specific form that would be appropriate, 
but 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, the Administrator provisionally concludes that a 90th 
percentile form, averaged over 3 years, is appropriate, and proposes 
such a form in conjunction with a PM2.5 visibility index and 
a 24-hour averaging time.
4. Level
    In considering alternative levels for a new standard that would 
provide requisite protection against PM-related visibility impairment 
primarily in urban areas, the Policy Assessment has taken into account 
the evidence- and impact-based considerations discussed above and in 
section 4.2.1 of the Policy Assessment, 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 this information in the context of a standard 
defined in terms of a calculated PM2.5 light extinction 
indicator, discussed above and in the Policy Assessment section 4.3.1; 
with alternative averaging times of 24-hours or multi-hour, sub-daily 
periods (e.g., 4-hours), discussed above and in Policy Assessment 
section 4.3.2; and a 90th percentile-based form, discussed above and in 
section 4.3.3 of the Policy Assessment.
    As part of the Policy Assessment's assessment of the adequacy of 
the current standards, summarized in section VI.B. above and in Policy 
Assessment section 4.2.1, it 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 Policy Assessment notes that 
CASAC agreed that this was an appropriate range of levels to consider 
for such a standard (Samet, 2010d, p. 11).\175\ The Policy Assessment 
also recognizes 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 is 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.
---------------------------------------------------------------------------

    \175\ In 2009, the D.C. 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.
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    As an initial matter, in considering alternative levels for a sub-
daily standard based directly on the four preference study results, the 
Policy Assessment notes that the individual

[[Page 38989]]

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. As discussed above and in section 4.2.1 of the Policy 
Assessment, the Phoenix study was by far the best of the studies, 
providing somewhat more support for the middle CPL. In considering the 
results from these studies, the Policy Assessment recognizes that the 
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 recognizes 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 concludes, as did 
CASAC, that these studies are appropriate to use for this purpose (U.S. 
EPA, 2011a, p. 4-61).
    In considering potential alternative levels for a 24-hour standard, 
the Policy Assessment explores various approaches to adjusting the CPLs 
derived directly from the preference studies, as presented and 
discussed in Appendix G of the Policy Assessment, especially section G-
5. 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 so doing, staff recognized that there are 
multiple approaches for estimating generally equivalent levels on a 
city-specific or national basis, and that the inherent spatial and 
temporal variability in relative humidity and fine particle composition 
across cities leads to a set of alternative estimates of levels that 
may be construed as being generally equivalent on a national basis.
    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 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 Table G-6 of Appendix G,\176\ 
in considering estimates aggregated or averaged over all 15 cities as 
well as the range of city-specific estimates for the various approaches 
considered, 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.\177\
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    \176\ 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.
    \177\ 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.
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    Some of the approaches used in these analyses focused on comparing 
24-hour and 4-hour light extinction values in each of the 15 urban 
areas, whereas other approaches focused on comparisons based on using 
aggregated data across the urban areas. Two of these approaches, which 
used regressions of city-specific annual 90th percentile light 
extinction values or 3-year light extinction design values, gave nearly 
identical results and were considered by staff to be most appropriate 
for further consideration. These approaches (shown in U.S. EPA, 2011a, 
Appendix G, Figures G-7 and G-8, referred to as Approaches A and B) 
were preferred by staff based on the high R\2\ values of the 
regressions and because the regressions were determined by data from 
days with PM2.5 light extinction conditions in the range of 
20 to 40 dv. This contrasted with the other approaches that were 
influenced by PM2.5 light extinction conditions well below 
this range. Based on these analyses (presented in Appendix G of the 
Policy Assessment), the Policy Assessment notes that the single 
approach thought by staff to be more appropriate for further 
consideration (referred to as Approach B in Appendix G) yielded 
adjusted 24-hour CPLs of 21, 25, and 28 dv as being levels that are 
generally equivalent in an aggregate or central tendency sense to 4-
hour CPLs of 20, 25, and 30 dv.\178\
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    \178\ To provide some perspective in considering these results 
(U.S. EPA, 2011a, Appendix G, Table G-6), the Policy Assessment 
notes that 1 dv is about the amount that persons can distinguish 
when viewing scenic vistas, and that a difference of 1 dv is 
equivalent to about a 10 percent difference in light extinction 
expressed in Mm-1.
---------------------------------------------------------------------------

    Two of the approaches yielded not only estimates of generally 
equivalent levels on an aggregated basis but also city-specific 
estimates (referred to as Approaches C and E in Appendix G) that showed 
greater variability than the aggregated estimates. 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 
shown in Tables G-7 and G-8, Appendix G of the Policy Assessment, for 
Approaches C and E, respectively). 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.
    In more broadly considering alternative standard levels that would 
be appropriate for a nationally applicable secondary standard focused 
on protection from PM-related urban visibility impairment based on 
either a 24-hour or multi-hour, sub-daily (e.g., 4-hour) averaging 
time, the Policy Assessment was mindful of the important limitations in 
the available evidence from public preference studies. While the Policy 
Assessment concluded, consistent with CASAC advice, that it is 
appropriate to consider a distinct secondary PM2.5 standard 
to address PM-related visibility impairment focused primarily in urban 
areas based on the evidence from public preference studies, it also 
recognized that there are a number of uncertainties and limitations 
associated with the preference studies that have served as a basis for 
selecting an appropriate range of levels to consider, as discussed 
above

[[Page 38990]]

in section VI.B.2. 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 Policy Assessment was mindful 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, and that there is the possibility that there could 
be regional differences in individual preferences for VAQ.
    Given the uncertainties and limitations noted above, the EPA 
broadly solicits comment on the strengths and limitations associated 
with these 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). In particular, the EPA solicits comment on the following 
specific aspects of the public preference studies and on how these 
studies should appropriately be considered in this review. Recognizing 
that all of these studies evaluated a 50 percent acceptability 
criterion as the basis for reaching judgments in the context of each 
study, the EPA requests comment on the extent to which this 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. Recognizing that these 
studies vary in the extent to which the study participants may be 
representative of the broader study area population, the EPA requests 
comment on how this aspect of the study designs should appropriately be 
weighed in the context of considering these studies in reaching 
proposed conclusions on a distinct secondary NAAQS. The EPA also 
solicits comment on 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.
    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.
    Based on the above considerations, the Policy Assessment concludes 
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 concludes 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 judges 
that such standards would provide appropriate protection against PM-
related visibility impairment primarily in urban areas. The Policy 
Assessment notes 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 concludes 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 above considerations, taking into account the 
conclusions in the Policy Assessment and the extent to which those 
conclusions reflected consideration of CASAC advice during the 
development of the Policy Assessment, as an initial matter, the 
Administrator provisionally concludes that it is 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 concludes 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 is supported by the 
available information and is generally consistent with the advice of 
CASAC. The option of setting such a 24-hour standard 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. 
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.
    In light of these provisional conclusions, the Administrator 
proposes to set 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 based on one of two options. One option is to set 
the level of such a standard at 30 dv and the other option is to set 
the level at 28 dv. In so doing, the

[[Page 38991]]

Administrator solicits comment on each of these levels and on the 
various approaches to identifying generally equivalent levels discussed 
above upon which the alternative proposed levels are based. Recognizing 
that there was some support for consideration of a broader range of 
levels, the Administrator also solicits comment on a range of levels 
down to 25 dv in conjunction with a 24-hour averaging time. Further, 
having solicited comment on a sub-daily (e.g., 4-hour) averaging time, 
the Administrator also solicits comment on a range of alternative 
levels from 30 to 25 dv in conjunction with such a sub-daily averaging 
time.
    Finally, as we have indicated, 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). We invite comment on how the Administrator should weigh 
those uncertainties as well as any additional comments and information 
to inform her consideration of these uncertainties.

E. Other PM-Related Welfare Effects

    In the 2006 review, the Administrator 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. To provide 
a level of protection for 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).
    In that review, the 2004 AQCD concluded that regardless of size 
fraction, particles containing nitrates and sulfates have the greatest 
potential for widespread environmental significance (U.S. EPA, 2004, 
sections 4.2.2 and 4.2.3.1). Considerable supporting evidence was 
available that indicated a significant role of oxides of nitrogen and 
sulfur, and their transformation products in acidification and nutrient 
enrichment of terrestrial and aquatic ecosystems (71 FR 61209, October 
17, 2006). The recognition of these ecological effects, coupled with 
other considerations detailed below, led EPA to initiate a joint review 
of the secondary NO2 and SO2 NAAQS that is 
considering the gaseous and particulate species of oxides of nitrogen 
and sulfur with respect to the ecosystem-related welfare effects that 
result from the deposition of these pollutants and transformation 
products.
    This section presents the Policy Assessment's conclusions with 
regard to the current suite of secondary PM standards to protect 
against non-visibility PM-related welfare effects. Specifically, the 
Policy Assessment has assessed the relevant information related to 
effects of atmospheric PM on the environment, including effects on 
climate, ecological effects, and materials. Non-visibility welfare-
based effects of oxides of nitrogen and sulfur are divided between two 
NAAQS reviews; (1) PM NAAQS review and, (2) the joint secondary NAAQS 
review for oxides of nitrogen (NOX) and oxides of sulfur 
(SOX).\179\ The scope of each document and the compounds of 
nitrogen and sulfur considered in each review are summarized in this 
section and in Table 5-1 of the Policy Assessment.
---------------------------------------------------------------------------

    \179\ For the purposes of this discussion, NOX and 
SOX refers to all oxides of nitrogen and all oxides of 
sulfur, respectively.
---------------------------------------------------------------------------

    In reviewing the current suite of secondary PM standards, the 
Policy Assessment considers all PM-related effects that are not being 
covered in the ongoing NOX/SOX review, including 
visibility impairment (U.S. EPA, 2011a, chapter 4), climate forcing 
effects (U.S. EPA, 2011a, section 5.2), ecological effects (U.S. EPA, 
2011a, section 5.3), and materials damage (U.S. EPA, 2011a, section 
5.4). By excluding the effects associated with deposited particulate 
matter components of NOX and SOX and their 
transformation products which are addressed fully in the 
NOX/SOX secondary review, the discussion of 
ecological effects of PM has been narrowed to focus on effects 
associated with the deposition of metals and, to a lesser extent, 
organics (U.S. EPA, 2011a, section 5.3). With regard to the materials 
section, because the NOX/SOX review is not 
considering materials, the discussion includes particles and gases that 
are associated with the presence of ambient NOX and 
SOX, as well as reduced forms of nitrogen such as ammonia 
and ammonium ions for completeness.
    In contrast, the proposed rulemaking for the joint NOX/
SOX secondary review (76 FR 46084, August 1, 2011) focuses 
on the welfare effects associated with exposures from deposited 
particulate and gaseous forms of oxides of nitrogen and sulfur and 
related nitrogen- and sulfur-containing compounds and transformation 
products on ecosystem receptors, including effects of acidifying 
deposition associated with particulate nitrogen and sulfur. In 
addition, the NOX/SOX secondary review includes 
evidence related to direct ecological effects of gas-phase 
NOX and SOX.
1. Climate
    Information and conclusions about what is currently known about the 
role of PM in climate is summarized in Chapter 9 of the Integrated 
Science Assessment (U.S. EPA, 2009a). The Integrated Science Assessment 
concludes ``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). The Policy Assessment summarizes and synthesizes the policy-
relevant science in the Integrated Science Assessment for the purpose 
of helping to inform consideration of climate aspects in the review of 
the secondary PM NAAQS (U.S. EPA, 2011a, section 5.2). This discussion 
is summarized below.
    Atmospheric PM (referred to as aerosols \180\ in the remainder of 
this section to be consistent with the Integrated Science Assessment) 
affects multiple aspects of climate. These include absorbing and 
scattering of incoming solar radiation, alterations in terrestrial 
radiation, effects on the hydrological cycle, and changes in cloud 
properties (U.S. EPA, 2009a, section 9.3.1). Major aerosol components 
that contribute to climate processes include black carbon (BC),

[[Page 38992]]

organic carbon (OC), sulfates, nitrates, and mineral dusts. There is a 
considerable ongoing research effort focused on understanding aerosol 
contributions to changes in global mean temperature and precipitation 
patterns. The Climate Change Research Initiative identified research on 
atmospheric concentrations and effects of aerosols as a high research 
priority (National Research Council, 2001) and the IPCC 2007 Summary 
for Policymakers states that anthropogenic contributions to aerosols 
remain the dominant uncertainty in radiative forcing (IPCC, 2007). The 
current state of the science of climate alterations attributable to PM 
is in flux as a result of continually updated information.
---------------------------------------------------------------------------

    \180\ In the sections of the Integrated Science Assessment 
included from IPCC AR4 and CCSP SAP2.3 (U.S. EPA, 2009a, section 
9.3), the term ``aerosols'' is more frequently used than ``PM'' and 
that word is retained in the Policy Assessment (U.S. EPA, 2011a, 
section 5.2) and in this section of the preamble.
---------------------------------------------------------------------------

    Global climate change has increasingly been the focus of intense 
international research endeavors. As discussed in chapter 5 of the 
Policy Assessment, major efforts are underway to understand the 
complexities inherent in atmospheric aerosol interactions and to 
decrease uncertainties associated with climate estimations.
    Aerosols have direct and indirect effects on climate processes. The 
direct effects of aerosols on climate result mainly from particles 
scattering light away from Earth into space, directly altering the 
radiative balance of the Earth-atmosphere system. This reflection of 
solar radiation back to space decreases the transmission of visible 
radiation to the surface of the Earth and results in a decrease in the 
heating rate of the surface and the lower atmosphere. At the same time, 
absorption of either incoming solar radiation or outgoing terrestrial 
radiation by particles, primarily BC, results in an increased heating 
rate in the lower atmosphere. Global estimates of aerosol direct 
radiative forcing (RF) were recently summarized using a combined model-
based estimate (Forster et al., 2007). The overall, model-derived 
aerosol direct RF was estimated in the IPCC AR4 as -0.5 (-0.9 to -0.1) 
watts per square meter (W/m\2\), with an overall level of scientific 
understanding of this effect as ``medium low'' (Forster et al., 2007), 
indicating a net cooling effect in contrast to greenhouse gases (GHGs) 
which have a warming effect.
    The contribution of individual aerosol components to total aerosol 
direct radiative forcing is more uncertain than the global average 
(U.S. EPA, 2009a, section 9.3.6.6). The direct effect of radiative 
scattering by atmospheric particles exerts an overall net cooling of 
the atmosphere, while particle absorption of solar radiation leads to 
warming. For example, the presence of OC and sulfates decrease warming 
from sunlight by scattering shortwave radiation back into space. Such a 
perturbation of incoming radiation by anthropogenic aerosols is 
designated as aerosol climate forcing, which is distinguished from the 
aerosol radiative effect of the total aerosol (natural plus 
anthropogenic). The aerosol climate forcing and radiative effect are 
characterized by large spatial and temporal heterogeneities due to the 
wide variety of aerosol sources, the spatial non-uniformity and 
intermittency of these sources, the short atmospheric lifetime of 
aerosols (relative to that of the greenhouse gases), and processing 
(chemical and microphysical) that occurs in the atmosphere. For 
example, OC can be warming (positive forcer) when deposited on or 
suspended over a highly reflective surface such as snow or ice but, on 
a global average, is a negative forcer in the atmosphere.
    More information has also become available on indirect effects of 
aerosols. Particles in the atmosphere indirectly affect both cloud 
albedo (reflectivity) and cloud lifetime by modifying the cloud amount, 
and microphysical and radiative properties (U.S. EPA, 2009a, section 
9.3.6.4). The RF due to these indirect effects (cloud albedo effect) of 
aerosols is estimated in the IPCC AR4 to be -0.7 ( -1.8 to -0.3) W/m\2\ 
with the level of scientific understanding of this effect as ``low'' 
(Forster et al., 2007). Aerosols act as cloud condensation nuclei (CCN) 
for cloud formation. Increased particulates in the atmosphere available 
as CCN with no change in moisture content of the clouds have resulted 
in an increase in the number and decrease in the size of cloud droplets 
in certain clouds that can increase the albedo of the clouds (the 
Twomey effect). Smaller particles slow the onset of precipitation and 
prolong cloud lifetime. This effect, coupled with changes in cloud 
albedo, increases the reflection of solar radiation back into space. 
The altitude of the clouds also affects cloud radiative forcing. Low 
clouds reflect incoming sunlight back to space but do not effectively 
trap outgoing radiation, thus cooling the planet, while higher 
elevation clouds reflect some sunlight but more effectively can trap 
outgoing radiation and act to warm the planet (U.S. EPA, 2009a, section 
9.3.3.5).
    The total negative RF due to direct and indirect effects of 
aerosols computed from the top of the atmosphere, on a global average, 
is estimated at -1.3 (-2.2 to -0.5) W/m\2\ in contrast to the positive 
RF of +2.9 (+3.2 to +2.6) W/m\2\ for anthropogenic GHGs (IPCC 2007, p. 
200).
    The understanding of the magnitude of aerosol effects on climate 
has increased substantially in the last decade. Data on the atmospheric 
transport and deposition of aerosols indicate a significant role for PM 
components in multiple aspects of climate. Aerosols can impact 
glaciers, snowpack, regional water supplies, precipitation, and climate 
patterns (U.S. EPA, 2009a, section 9.3.9). Aerosols deposited on ice or 
snow can lead to melting and subsequent decrease of surface albedo 
(U.S. EPA, 2009a, section 9.3.9.2). Aerosols are potentially important 
agents of climate warming in the Arctic and other locations (U.S. EPA, 
2009a, section 9.3.9). Carbonaceous aerosols emitted from intermittent 
fires can occur at large enough scales to affect hemispheric aerosol 
concentrations. In addition to incidental fires, routine biomass 
burning, usually associated with agriculture in eastern Europe, has 
also been shown to contribute to hemispheric concentrations of 
carbonaceous aerosols and is therefore recognized as having a 
significant impact on PM2.5 concentrations and climate 
forcing (U.S. EPA, 2009a, section 9.3.7).
    A series of studies available since the last review examines the 
role of aerosols on local and regional scale climate processes (U.S. 
EPA, 2009a, section 9.3.9.3). Studies on the South Coast Air Basin 
(SCAB) in California indicate aerosols may reduce near-surface wind 
speeds, which, in turn reduce evaporation rates and increase cloud 
lifetimes. The overall impact can be a reduction in local precipitation 
(Jacobson and Kaufmann, 2006). Conditions in the SCAB impact 
ecologically sensitive areas including the Sierra Nevadas. 
Precipitation suppression due to aerosols in California (Givati and 
Rosenfield, 2004) and other similar studies in Utah and Colorado found 
that mountain precipitation decreased by 15 to 30 percent downwind of 
pollution sources. Evidence of regional-scale impacts of aerosols on 
meteorological conditions in other regions of the U.S. is lacking.
    Advances in the understanding of aerosol components and how they 
contribute to climate change have enabled refined global forcing 
estimates of individual PM constituents. The global mean radiative 
effect from individual components of aerosols was estimated for the 
first time in the IPCC AR4 where they were reported to be (all in W/
m\2\ units): -0.4 (+0.2) for sulfate, -0.05 (+0.05) for fossil fuel-
derived OC, +0.2 (+0.15) for fossil fuel derived BC, +0.03 (+0.12) for 
biomass burning, -0.1

[[Page 38993]]

(+0.1) for nitrates, and -0.1 (+0.2) for mineral dust (U.S. EPA, 2009a, 
section 9.3.10). Sulfate and fossil fuel-derived OC cause negative 
forcing whereas BC causes positive forcing because of its highly 
absorbing nature (U.S. EPA, 2009a, 9.3.6.3). Although BC comprises only 
a small fraction of anthropogenic aerosol mass load and aerosol optical 
depth (AOD), its forcing efficiency (with respect to either AOD or 
mass) is an order of magnitude stronger than sulfate and particulate 
organic matter (POM), so its positive shortwave forcing largely offsets 
the negative direct forcing from sulfate and POM (IPCC, 2007; U.S. EPA, 
2009a, 9.3.6.3). Global loadings for nitrates and anthropogenic dust 
remain very difficult to estimate, making the radiative forcing 
estimates for these constituents particularly uncertain (U.S. EPA, 
2009a, section 9.3.7).
    Improved estimates of anthropogenic emissions of some aerosols, 
especially BC and OC, have promoted the development of improved global 
emissions inventories and source-specific emissions factors useful in 
climate modeling (Bond et al. 2004). Recent data suggests that BC is 
one of the largest individual warming agents after carbon dioxide 
(CO2) and perhaps methane (CH4) (Jacobson 2000; 
Sato et al., 2003; Bond and Sun 2005). There are several studies 
modeling BC effects on climate and/or considering emission reduction 
measures on anthropogenic warming detailed in section 9.3.9 of the 
Integrated Science Assessment. In the U.S., most of the warming 
aerosols are emitted by biomass burning and internal engine combustion 
and much of the cooling aerosols are formed in the atmosphere by 
oxidation of SO2 or volatile organic compounds (VOCs) (U.S. 
EPA, 2009a, section 3.3). Fires release large amounts of BC, 
CO2, CH4 and OC (U.S. EPA, 2009a, section 9.3.7).
    Based on the above newly available scientific information on 
climate-aerosol relationships, the Policy Assessment concludes that 
aerosols alter climate processes directly through radiative forcing and 
by indirect effects on cloud brightness, changes in precipitation, and 
possible changes in cloud lifetimes (U.S. EPA, 2011a, p. 5-10). 
Further, the Policy Assessment notes 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). In light of this information, the 
Policy Assessment considered the appropriateness of the current 
secondary standards defined in terms of PM2.5 and 
PM10 indicators, for providing protection against potential 
climate effects of aerosols. The current standards that are defined in 
terms of aggregate size mass cannot be expected to appropriately target 
controls on components of fine and coarse particles that are related to 
climate forcing effects. Thus, the Policy Assessment concludes that the 
current mass-based PM2.5 and PM10 secondary 
standards are 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).
    Further, in light of the uncertainties associated with the spatial 
and temporal heterogeneity of PM components that contribute to climate 
forcing and the uncertainties associated with measurement of aerosol 
components, the inadequate consideration of aerosol impacts in climate 
modeling and the insufficient data on local and regional microclimate 
variations and the heterogeneity of cloud formations, the Policy 
Assessment concludes 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). Based on these considerations, the Policy Assessment concludes 
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).\181\
---------------------------------------------------------------------------

    \181\ This conclusion would apply for both the secondary 
(welfare-based) and the primary (health-based) standards.
---------------------------------------------------------------------------

2. Ecological Effects
    Information on what is currently known about ecological effects of 
PM is summarized in Chapter 9 of the Integrated Science Assessment 
(U.S. EPA, 2009a). Four main categories of ecological effects are 
identified in the Integrated Science Assessment: Direct effects, 
effects of PM-altered radiative flux, indirect effects of trace metals, 
and indirect effects of organics. Exposure to PM for direct effects 
occurs via deposition (e.g., wet, dry or occult) to vegetation 
surfaces, while indirect effects occur via deposition to ecosystem 
soils or surface waters where the deposited constituents of PM then 
interact with biological organisms. Both fine and coarse-mode particles 
may affect plants and other organisms; however, PM size classes do not 
necessarily relate to ecological effects (U.S. EPA, 1996). More often, 
the chemical constituents drive the ecosystem response to PM (Grantz et 
al., 2003). The trace metal constituents of PM considered in the 
ecological effects section of the Integrated Science Assessment are 
cadmium (Cd), copper (Cu), chromium (Cr), mercury (Hg), nickel (Ni) and 
zinc (Zn). Ecological effects of lead (Pb) in particulate form are 
covered in the Air Quality Criteria Document for Lead (U.S. EPA, 2006). 
The organics included in the ecological effects section of the PM 
Integrated Science Assessment are persistent organic pollutants (POPs), 
polyaromatic hydrocarbons (PAHs), and polybromiated diphenyl ethers 
(PBDEs).
    Ecological effects of PM 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.
    The Integrated Science Assessment states that overall, 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 based on information from the 
previous review and limited new findings in this review (U.S. EPA, 
2009a, sections 2.5.3 and 9.4.7). However the Integrated Science 
Assessment also finds, in many cases, it is 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.
    Ecological effects of PM must then be evaluated to determine if 
they are known or anticipated to have an adverse impact on public 
welfare. Characterizing a known or anticipated adverse effect to public 
welfare is an important component of developing any secondary NAAQS. 
The most recent secondary NAAQS reviews have assessed changes in 
ecosystem structure or processes using a weight-of-evidence approach 
that uses both quantitative and qualitative data. A paradigm useful in 
evaluating ecological adversity is the concept of ecosystem services. 
Ecosystem services consist of the varied and numerous ways that 
ecosystems are important to human welfare. Ecosystems provide many 
goods and services that are of vital importance for the functioning of 
the biosphere and

[[Page 38994]]

provide the basis for the delivery of tangible benefits to human 
society. An EPA initiative to consider how ecosystem structure and 
function can be interpreted through an ecosystem services approach has 
resulted in the inclusion of ecosystem services in the NOX/
SOX Risk and Exposure Assessment (U.S. EPA, 2009h). The 
Millennium Ecosystem Assessment (MEA) defines these to include 
supporting, provisioning, regulating, and cultural services (Hassan et 
al., 2005).
    An important consideration in evaluating biologically adverse 
effects of PM and linkages to ecosystem services is that many of the 
MEA categories overlap and any one pollutant may impact multiple 
services. For example, deposited PM may alter the composition of soil-
associated microbial communities, which may affect supporting services 
such as nutrient cycling. Changes in available soil nutrients could 
result in alterations to provisioning services such as timber yield and 
regulating services such as climate regulation. If enough information 
is available, these alterations can be quantified based upon economic 
approaches for estimating the value of ecosystem services. Valuation 
may be important from a policy perspective because it can be used to 
compare the benefits of altering versus maintaining an ecosystem. 
Knowledge about the relationships linking ambient concentrations and 
ecosystem services can be used to inform a policy judgment on a known 
or anticipated adverse public welfare effect.
    The Policy Assessment seeks to build upon and focus this body of 
science using the concept of ecosystem services to qualitatively 
evaluate linkages between biologically adverse effects and particulate 
deposition. This approach is similar to that taken in the 
NOX/SOX Risk and Exposure Assessment in which the 
relationship between air quality indicators, deposition of nitrogen and 
sulfur, ecologically relevant indicators, and effects on sensitive 
receptors are linked to changes in ecosystem structure and services 
(U.S. EPA, 2009h). This approach considers the benefits received from 
the resources and processes that are supplied by ecosystems. Several 
ecosystem components (e.g., plants, soils, water, and wildlife) are 
impacted by PM air pollution, which may alter the services provided by 
the ecosystems in question. Key scientific evidence regarding PM 
effects on plants, soil and nutrient cycling, wildlife, and water 
available since the last review is summarized below to evaluate how 
this information has improved understanding of ecosystem responses to 
PM.
a. Plants
    As primary producers, plants play a pivotal role in energy flow 
through ecosystems. Ecosystem services derived from plants include all 
of the categories (supporting, provisioning, regulating, and cultural) 
identified in the MEA (Hassan et al., 2005). Vegetation supports other 
ecosystem processes by cycling nutrients through food webs and serving 
as a source of organic material for soil formation and enrichment. 
Trees and plants provide food, wood, fiber, and fuel for human 
consumption. Flora help to regulate climate by sequestering 
CO2, and control flooding by stabilizing soils and cycling 
water via uptake and evapotranspiration. Plants are significant in 
aesthetic, spiritual, and recreational aspects of human interactions.
    Particulate matter can adversely impact plants and ecosystem 
services provided by plants by deposition to vegetative surfaces (U.S. 
EPA, 2009a, section 9.4.3). Particulates deposited on the surfaces of 
leaves and needles can block light, altering the radiation received by 
the plant. PM deposition can obstruct stomata limiting gas exchange, 
damage leaf cuticles, and increase plant temperatures. This level of PM 
accumulation is typically observed near sources of heavy deposition 
such as smelters and mining operations (U.S. EPA, 2009a, section 
9.4.3). Plants growing on roadsides exhibit impact damage from near-
road PM deposition, having higher levels of organics and heavy metals, 
and accumulate salt from road de-icing during winter months (U.S. EPA, 
2009a, sections 9.4.3.1 and 9.4.5.7).
    In addition to damage to plant surfaces, deposited PM can be taken 
up by plants from soil or foliage. The ability of vegetation to take up 
heavy metals and organics is dependent upon the amount, solubility, and 
chemical composition of the deposited PM. Uptake of PM by plants from 
soils and vegetative surfaces can disrupt photosynthesis, alter 
pigments and mineral content, reduce plant vigor, decrease frost 
hardiness, and impair root development. The Integrated Science 
Assessment indicates that there are little or no effects on foliar 
processes at ambient levels of PM (U.S. EPA, 2009a, sections 9.4.3 and 
9.4.7). However, damage due to atmospheric pollution can occur near 
individual point sources or under conditions where plants are subjected 
to multiple stressors.
    Although all heavy metals can be directly toxic at sufficiently 
high concentrations, only Cu, Ni, and Zn have been documented as being 
frequently toxic to plants (U.S. EPA, 2004), while toxicity due to Cd, 
Co, and Pb has been observed less frequently (Smith, 1990; U.S. EPA, 
2009a, section 9.4.5.3). In general, plant growth is negatively 
correlated with trace metal and heavy metal concentration in soils and 
plant tissue (Audet and Charest, 2007). Trace metals, particularly 
heavy metals, can influence forest growth. Growth suppression of foliar 
microflora has been shown to result from iron (Fe), aluminum (Al), and 
Zn. These three metals can also inhibit fungal spore formation, as can 
Cd, Cr, magnesium (Mg), and Ni (see Smith, 1990). Metals cause stress 
and decreased photosynthesis (Kucera et al., 2008) and disrupt numerous 
enzymes and metabolic pathways (Strydom et al., 2006). Excessive 
concentrations of metals result in phytotoxicity.
    New information since the last review provides additional evidence 
of plant uptake of organics (U.S. EPA, 2009a, section 9.4.6). An area 
of active study is the impact of PAHs on provisioning ecosystem 
services due to the potential for human and other animal exposure via 
food consumption (U.S. EPA, 2009a, section 9.4.6 page 9-190). The 
uptake of PAHs depends on the plant species, site of deposition, 
physical and chemical properties of the organic compound, and 
prevailing environmental conditions. It has been established that most 
bioaccumulation of PAHs by plants occurs via leaf uptake, and to a 
lesser extent, through roots. Differences between species in uptake of 
PAHs confound attempts to quantify impacts to ecosystem provisioning 
services.
    Plants as ecosystem regulators can serve as passive monitors of 
pollution (U.S. EPA, 2009a, section 9.4.2.3). Lichens and mosses are 
sensitive to pollutants associated with PM and have been used with 
limited success to show spatial and temporal patterns of atmospheric 
deposition of metals (U.S. EPA, 2009a, section 9.4.2.3). A limitation 
to employing mosses and lichens to detect for the presence of air 
pollutants is the difference in uptake efficiencies of metals between 
species. Thus, quantification of ecological effects is not possible due 
to the variability of species responses (U.S. EPA, 2009a, section 
9.4.2.3).
    A potentially important regulating ecosystem service of plants is 
their capacity to sequester contaminants (U.S. EPA, 2009a, section 
9.4.5.3). Ongoing research on the application of plants to 
environmental remediation efforts are

[[Page 38995]]

yielding some success in removing heavy metals and organics from 
contaminated sites (phytoremediation) with tolerant plants such as the 
willow tree (Salix spp.) and members of the family Brassicaceae (U.S. 
EPA, 2009a, section 9.4.5.3). Tree canopies can be used in urban 
locations to capture particulates and improve air quality (Freer-Smith 
et al., 2004). Plant foliage is a sink for Hg and other metals and this 
regulating ecosystem service may be impacted by atmospheric deposition 
of trace metals.
    An ecological endpoint (phytochelatin concentration) associated 
with presence of metals in the environment has been correlated with the 
ecological effect of tree mortality (Grantz et al., 2003). Metal stress 
may be contributing to tree injury and forest decline in the 
Northeastern U.S. where red spruce populations are declining with 
increasing elevation. Quantitative assessment of PM damage to forests 
potentially could be conducted by overlaying PM sampling data and 
elevated phytochelatin levels. However, limited data on phytochelatin 
levels in other species currently hinders use of this peptide as a 
general biomarker for PM.
    The presence of PM in the atmosphere affects ambient radiation as 
discussed in the Integrated Science Assessment which can impact the 
amount of sunlight received by plants (U.S. EPA, 2009a, section 9.4.4). 
Atmospheric PM can change the radiation reaching leaf surfaces through 
attenuation and by converting direct radiation to diffuse radiation. 
Diffuse radiation is more uniformly distributed in a tree canopy, 
allowing radiation to reach lower leaves. The net effect of PM on 
photosynthesis depends on the reduction of photosynthetically active 
radiation (PAR) and the increase in the diffuse fraction of PAR. 
Decreases in crop yields (provisioning ecosystem service) have been 
attributed to regional scale air pollution, however, global models 
suggest that the diffuse light fraction of PAR can increase growth 
(U.S. EPA, 2009a, section 9.4.4).
b. Soil and Nutrient Cycling
    Many of the major indirect plant responses to PM deposition are 
chiefly soil-mediated and depend on the chemical composition of 
individual components of deposited PM. Major ecosystem services 
impacted by PM deposition to soils include support services such as 
nutrient cycling, products such as crops and regulating flooding and 
water quality. Upon entering the soil environment, PM pollutants can 
alter ecological processes of energy flow and nutrient cycling, inhibit 
nutrient uptake to plants, change microbial community structure and, 
affect biodiversity. Accumulation of heavy metals in soils depends on 
factors such as local soil characteristics, geologic origin of parent 
soils, and metal bioavailability. It can be difficult to assess the 
extent to which observed heavy metal concentrations in soil are of 
anthropogenic origin (U.S. EPA, 2009a, section 9.4.5.1). Trace element 
concentrations are higher in some soils that are remote from air 
pollution sources due to parent material and local geomorphology.
    Heavy metals such as Zn, Cu, and Cd and some pesticides can 
interfere with microorganisms that are responsible for decomposition of 
soil litter, an important regulating ecosystem service that serves as a 
source of soil nutrients (U.S. EPA, 2009a, sections 9.4.5.1 and 
9.4.5.2). Surface litter decomposition is reduced in soils having high 
metal concentrations. Soil communities have associated bacteria, fungi, 
and invertebrates that are essential to soil nutrient cycling 
processes. Changes to the relative species abundance and community 
composition can be quantified to measure impacts of deposited PM to 
soil biota. A mutualistic relationship exists in the rhizophere (plant 
root zone) between plant roots, fungi, and microbes. Fungi in 
association with plant roots form mycorrhizae that are essential for 
nutrient uptake by plants. The role of mychorrizal fungi in plant 
uptake of metals from soils and effects of deposited PM on soil 
microbes is discussed in section 9.4.5.2 of the Integrated Science 
Assessment.
c. Wildlife
    Animals play a significant role in ecosystem function including 
nutrient cycling and crop production (supporting ecosystem service), 
and as a source of food (provisioning ecosystem service). Cultural 
ecosystem services provided by wildlife include bird and animal 
watching, hunting, and fishing. Impacts on these services are dependent 
upon the bioavailability of deposited metals and organics and their 
respective toxicities to ecosystem receptors. Pathways of PM exposure 
to fauna include ingestion, absorption and trophic transfer. 
Bioindicator species (known as sentinel organisms) can provide evidence 
of contamination due to atmospheric pollutants. Use of sentinel species 
can be of particular value because chemical constituents of deposited 
PM are difficult to characterize and have varying bioavailability (U.S. 
EPA, 2009a, section 9.4.5.5). Snails readily bioaccumulate contaminants 
such as PAHs and trace metals. These organisms have been deployed as 
biomonitors for urban pollution and have quantifiable biomarkers of 
exposure including growth inhibition, impairment of reproduction, 
peroxidomal proliferation, and induction of metal detoxifying proteins 
(metallothioneins) (Gomet-de Vaufleury, 2002; Regoli, et. al, 2006). 
Earthworms have also been used as sensitive indicators of soil metal 
contamination.
    Evidence of deposited PM effects on animals is limited (U.S. EPA, 
2009a, section 9.4.5.5). Trophic transfer of pollutants of atmospheric 
origin has been demonstrated in limited studies. PM may also be 
transferred between aquatic and terrestrial compartments. There is 
limited evidence for biomagnifications of heavy metals up the food 
chain except for Hg which is well known to move readily through 
environmental compartments (U.S. EPA, 2009a, section 9.4.5.6). 
Bioconcentration of POPs and PBDEs in the Arctic and deep-water oceanic 
food webs indicates the global transport of particle-associated 
organics (U.S. EPA, 2009a, section 9.4.6). Salmon migrations are 
contributing to metal accumulation in inland aquatic systems, 
potentially impacting the provisioning and cultural ecosystem service 
of fishing (U.S. EPA, 2009a, section 9.4.6). Stable isotope analysis 
can be applied to establish linkages between PM exposure and impacts to 
food webs however, the use of this evaluation tool is limited for this 
ecological endpoint due to the complexity of most trophic interactions 
(U.S. EPA, 2009a, section 9.4.5.6). Foraging cattle have been used to 
assess atmospheric deposition and subsequent bioaccumulation of Hg and 
trace metals and their impacts on provisioning services (U.S. EPA, 
2009a, section 9.4.2.3).
d. Water
    New limited information on impacts of deposited PM on receiving 
water bodies indicate that the ecosystem services of primary 
production, provision of fresh water, regulation of climate and floods, 
recreational fishing and water purification are adversely impacted by 
atmospheric inputs of metals and organics (U.S. EPA, 2009a, sections 
9.4.2.3 and 9.4.5.4). Deposition of PM to surfaces in urban settings 
increases the metal and organic component of storm water runoff (U.S. 
EPA, 2009a, sections 9.4.2.3). This atmospherically-associated 
pollutant burden can then be toxic to aquatic biota.

[[Page 38996]]

    Atmospheric deposition can be the primary source of some organics 
and metals to watersheds. The contribution of atmospherically deposited 
PAHs to aquatic food webs was demonstrated in high elevation mountain 
lakes with no other anthropogenic contaminant sources (U.S. EPA, 2009a, 
section 9.4.6). Metals associated with PM deposition limit 
phytoplankton growth, impacting aquatic trophic structure. Long-range 
atmospheric transport of 47 pesticides and degradation products to the 
snowpack in seven national parks in the Western U.S. was recently 
quantified indicating PM-associated contaminant inputs to receiving 
waters during spring snowmelt (Hageman et al., 2006).
    The recently completed Western Airborne Contaminants Assessment 
Project (WACAP) is the most comprehensive database on contaminant 
transport and PM depositional effects on sensitive ecosystems in the 
U.S. In this project, the transport, fate, and ecological impacts of 
anthropogenic contaminants from atmospheric sources were assessed from 
2002 to 2007 in seven ecosystem components (air, snow, water, sediment, 
lichen, conifer needles and fish) in eight core national parks (Landers 
et al., 2008). The goals of the study were to identify where the 
pollutants were accumulating, identify ecological indicators for those 
pollutants causing ecological harm, and to determine the source of the 
air masses most likely to have transported the contaminants to the 
parks (U.S. EPA, 2009a, section 9.4.6). The study concluded that 
bioaccumulation of semi-volatile organic compounds was observed 
throughout park ecosystems (Landers et al., 2008). Findings from this 
study included the observation of an elevational gradient in PM 
deposition with greater accumulation at higher altitude areas of the 
parks. Furthermore, specific ecological indicators were identified in 
the WACAP that can be useful in assessing contamination on larger 
spatial scales.
    In the WACAP study, bioaccumulation and biomagnification of 
airborne contaminants were demonstrated on a regional scale in remote 
ecosystems in the Western United States. Contaminants were shown to 
accumulate geographically based on proximity to individual sources or 
source areas, primarily agriculture and industry (Landers et al., 
2008). Although this assessment focuses on chemical species that are 
components of PM, it does not specifically assess the effects of 
particulates versus gas-phase forms; therefore, in most cases it is 
difficult to apply the results to this assessment based on particulate 
concentration and size fraction (U.S. EPA, 2009a, section 9.4.6). There 
is a need for ecological modeling of PM components in different 
environmental compartments to further elucidate links between PM and 
ecological indicators.
    Europe and other countries are using the critical load approach to 
assess pollutant effects at the level of the ecosystem. This type of 
assessment requires site-specific data and information on individual 
species responses to PM. In respect to trace metals and organics, there 
are insufficient data for the vast majority of U.S. ecosystems to 
calculate critical loads. However, a methodology is being presented in 
the NOX/SOX Secondary Risk and Exposure 
Assessment (U.S. EPA, 2010h) to calculate atmospheric concentrations 
from deposition that may be applicable to other environmental 
contaminants.
e. Effects Associated With Ambient PM Concentrations
    As reviewed above, there is considerable data on impacts of PM on 
ecological receptors, but few studies that link ambient PM 
concentrations to observed effect. This is due, in part, to the nature, 
deposition, transport and fate of PM in ecosystems. PM is not a single 
pollutant, but a heterogeneous mixture of particles differing in size, 
origin and chemical composition (U.S. EPA, 2009a, section 9.4.1). The 
heterogeneity of PM exists not only within individual particles or 
samples from individual sites, but to even a greater extent, between 
samples from different sites. Since vegetation and other ecosystem 
components are affected more by particulate chemistry than size 
fraction, exposure to a given mass concentration of airborne PM may 
lead to widely differing plant or ecosystem responses, depending on the 
particular mix of deposited particles. Many of the PM components 
bioaccumulate over time in organisms or plants making correlations to 
ambient concentrations of PM difficult.
    Bioindicator organisms demonstrated biological effects including 
growth inhibition, metallothionein induction and reproductive 
impairment when exposed to complex mixtures of ambient air pollutants 
(U.S. EPA, 2009a, section 9.4.5.5). Other studies quantify uptake of 
metals and organics by plants or animals. However, due to the 
difficulty in correlating individual PM components to a specific 
physiological response, these studies are limited. Furthermore, there 
may be differences in uptake between species such as differing 
responses to metal uptake observed in mosses and lichens (U.S. EPA, 
2009a, section 9.4.2.3). PM may also biomagnify across trophic levels 
confounding efforts to link atmospheric concentrations to physiological 
endpoints (U.S. EPA, 2009a, section 9.4.5.6).
    Evidence of PM effects that are linked to a specific ecological 
endpoint can be observed when ambient levels are exceeded. 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). 
Extensive research on biota near point sources provide some of the best 
evidence of ecosystem function impacts and demonstrates that deposited 
PM has the potential to alter species composition over long time 
scales. The Integrated Science Assessment indicates at 4 km distance, 
species composition of vegetation, insects, birds, and soil microbiota 
changed, and within 1 km only the most resistant organisms were 
surviving (U.S. EPA, 2009a, section 9.4.5.7).
f. Conclusions in the Policy Assessment
    Based on the above discussions, the Policy Assessment made the 
following observations:

    (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.

    Taking into consideration the responses to specific questions 
regarding the adequacy of the current secondary PM standards for 
ecological effects, the Policy Assessment concludes that the available 
information is insufficient to assess the adequacy of the protection 
for ecosystems afforded by the current suite of PM secondary standards 
(U.S. EPA, 2011a, p. 5-24). Ecosystem effects linked to PM are 
difficult to determine because the changes may not be observed until

[[Page 38997]]

pollutant deposition has occurred for many decades. Because the high 
levels necessary to cause injury occur only near a few limited point 
sources and/or on a very local scale, protection against these effects 
alone may not provide sufficient basis for considering a separate 
secondary NAAQS based on the ecological effects of particulate metals 
and organics. Data on ecological responses clearly linked with 
atmospheric PM is not abundant enough to perform a quantitative 
analysis although the WACAP study may represent an opportunity for 
quantification at a regional scale. The Policy Assessment further 
concludes that available evidence is not sufficient for 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).
    The Policy Assessment considered the appropriateness of continuing 
to use the PM2.5 and PM10 size fractions as the 
indicators for protection of ecological effects of PM. The chemical 
constitution of individual particles can be strongly correlated with 
size, and the relationship between particle size and particle 
composition can be quite complex, making it difficult in most cases to 
use particle size as a surrogate for chemistry. At this time it remains 
to be determined as to what extent PM secondary standards focused on a 
given size fraction would result in reductions of the ecologically 
relevant constituents of PM for any given area. Nonetheless, in the 
absence of information that provides a basis for specific standards in 
terms of particle composition, the Policy Assessment concludes that 
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).
3. Materials Damage
    Welfare effects on materials associated with deposition of PM 
include both physical damage (materials damage effects) and impaired 
aesthetic qualities (soiling effects). Because the effects of PM are 
exacerbated by the presence of acidic gases and can be additive or 
synergistic due to the complex mixture of pollutants in the air and 
surface characteristics of the material, this discussion will also 
include those particles and gases that are associated with the presence 
of ambient oxides of nitrogen and oxides of sulfur, as well as reduced 
forms of nitrogen (such as ammonia and ammonium ions) for completeness. 
Building upon the information presented in the last PM Staff Paper 
(U.S. EPA, 2005), and including the limited new information presented 
in Chapter 9 of the PM Integrated Science Assessment (U.S. EPA, 2009a) 
and Annex E. Effects of NOY, NHX, and 
SOX on Structures and Materials of the Integrated Science 
Assessment for Oxides of Nitrogen and Sulfur-Ecological Criteria (U.S. 
EPA, 2008c) the following sections consider the policy-relevant aspects 
of physical damage and aesthetic soiling effects of PM on materials 
including metal and stone.
    The Integrated Science Assessment concludes 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). Particles contribute to these physical effects because of their 
electrolytic, hygroscopic and acidic properties, and their ability to 
sorb corrosive gases (principally sulfur dioxide). In addition, the 
deposition of ambient PM can reduce the aesthetic appeal of buildings 
and objects through soiling. Particles consisting primarily of 
carbonaceous compounds cause soiling of commonly used building 
materials and culturally important items such as statues and works of 
art. Soiling is the deposition of particles on surfaces by impingement, 
and the accumulation of particles on the surface of an exposed material 
that results in degradation of its appearance (U.S. EPA, 2009a, section 
9.5). Soiling can be remedied by cleaning or washing, and depending on 
the soiled material, repainting.
    The majority of available new studies on materials effects of PM 
are from outside the U.S., however, they provide limited new data for 
consideration of the secondary standard.
    Metal and stone are also susceptible to damage by ambient PM. 
Considerable research has been conducted on the effects of air 
pollutants on metal surfaces due to the economic importance of these 
materials, especially steel, Zn, Al, and Cu. Chapter 9 of the PM 
Integrated Science Assessment and Annex E of the NOX/
SOX Integrated Science Assessment summarize the results of a 
number of studies on the corrosion of metals (U.S. EPA, 2009a; U.S. 
EPA, 2008c). Moisture is the single greatest factor promoting metal 
corrosion, however, deposited PM can have additive, antagonistic or 
synergistic effects. In general, sulfur dioxide is more corrosive than 
oxides of nitrogen although mixtures of oxides of nitrogen, sulfur 
dioxide and other particulate matter corrode some metals at a faster 
rate than either pollutant alone (U.S. EPA, 2008c, Annex E.5.2). 
Information from both the PM Integrated Science Assessment and 
NOX/SOX Integrated Science Assessment suggest 
that the extent of damage to metals due to ambient PM is variable and 
dependent upon the type of metal, prevailing environmental conditions, 
rate of natural weathering and presence or absence of other pollutants.
    The PM Integrated Science Assessment and NOX/
SOX Integrated Science Assessment summarize the results of a 
number of studies on PM and stone surfaces. While it is clear from the 
available information that gaseous air pollutants, in particular sulfur 
dioxide, will promote the deterioration of some types of stones under 
specific conditions, carbonaceous particles (non-carbonate carbon) and 
particles containing metal oxides may help to promote the decay 
process. Studies on metal and stone summarized in the Integrated 
Science Assessment do not show an association between particle size, 
chemical composition and frequency of repair.
    A limited number of new studies available on materials damage 
effects of PM since the last review consider the relationship between 
pollutants and biodeterioration of structures associated with microbial 
communities that colonize monuments and buildings (U.S. EPA, 2009a, 
section 9.5). Presence of air pollutants may synergistically enhance 
microbial deterioration processes. The role of heterotrophic bacteria, 
fungi and cyanobacteria in biodeterioration varied by local 
meterological conditions and pollutant components.
    Particulate matter deposition onto surfaces such as metal, glass, 
stone and paint can lead to soiling. Soiling results when PM 
accumulates on an object and alters the optical characteristics 
(appearance). The reflectivity of a surface may be changed or presence 
of particulates may alter light transmission. These effects can impact 
the aesthetic value of a structure or result in reversible or 
irreversible damage to statues, artwork and architecturally or 
culturally significant buildings. Due to soiling of building surfaces 
by PM, the frequency and duration of cleaning may be increased. Soiling 
affects the aesthetic appeal of painted surfaces. In addition to 
natural

[[Page 38998]]

factors, exposure to PM may give painted surfaces a dirty appearance. 
Pigments in works of art can be degraded or discolored by atmospheric 
pollutants, especially sulfates (U.S. EPA, 2008c, Annex E-15).
    Formation of black crusts due to carbonaceous compounds and buildup 
of microbial biofilms results in discoloration of surfaces. Black crust 
includes a carbonate component derived from building material and OC 
and EC. In limited new studies quantifying the organic carbon and 
elemental contribution to soiling by black crust, organic carbon 
predominated over elemental carbon at almost all locations (Bonazza et 
al., 2005). Limited new studies suggest that traffic is the major 
source of carbon associated with black crust formation (Putaud et al., 
2004) and that soiling of structures in Oxford, UK showed a 
relationship with traffic and nitrogen dioxide concentrations (Viles 
and Gorbushina, 2003). These findings attempt to link atmospheric 
concentrations of PM to observed damage. However, no data on rates of 
damage are available and all studies were conducted outside of the U.S.
    Based on the above discussion, the Policy Assessment makes 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 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.

    Revisiting the overarching policy question as to whether the 
available scientific evidence supports or calls into question the 
adequacy of the protection for materials afforded by the current suite 
of secondary PM standards, the Policy Assessment concludes that no new 
evidence in this review calls into question the adequacy of the 
protection for materials afforded by the current standard (U.S. EPA, 
2011a, p. 5-29). PM effects on materials can play no quantitative role 
in considering whether any revisions of the secondary PM NAAQS are 
appropriate at this time. Nonetheless, in the absence of information 
that provides a basis for establishing a different level of control, 
the Policy Assessment concludes 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).
4. CASAC Advice
    Regarding the other 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 concludes 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 notes 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 
notes 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 concludes that the 
published literature is insufficient to support a national standard for 
PM effects on ecosystem services (Samet, 2010c, p.23). CASAC notes 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 concludes 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 notes 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.
5. Administrator's Proposed Conclusions on Secondary Standards for 
Other PM-related Welfare Effects
    Based on the above considerations and the advice of CASAC, the 
Administrator provisionally concludes that it is not appropriate to 
establish any distinct secondary PM standards to address other non-
visibility PM-related welfare effects. Nonetheless, the Administrator 
concurs 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, including 
ecological effects, effects on materials, and climate impacts. In the 
absence of information that would support any different standards, the 
Administrator proposes generally to retain the current suite of 
secondary PM standards\182\ to address non-visibility welfare effects. 
These secondary standards were set identical to the primary PM 
standards in the last review. More specifically, the Administrator 
proposes to retain all aspects of the current 24-hour PM2.5 
and PM10 standards. With regard to the secondary annual 
PM2.5 standard, the Administrator proposes to retain the 
level of the current standard and to revise the form of the standard by 
removing the option for spatial averaging for the reasons discussed 
below in section VII.A. 2. In so doing, she notes that no areas in the 
country are currently using the option for spatial averaging to 
demonstrate attainment with the secondary annual PM2.5 
standard.
---------------------------------------------------------------------------

    \182\ 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.
---------------------------------------------------------------------------

F. Administrator's Proposed Decisions on Secondary PM Standards

    With regard to the secondary PM standards, the Administrator 
proposes 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 
distinct secondary standard is defined

[[Page 38999]]

in terms of a calculated PM2.5 light extinction indicator, 
translated into the deciview scale, which is referred to as a 
PM2.5 visibility index; a 24-hour averaging time; a 90th 
percentile form, averaged over 3 years; and a level set at one of two 
options--either 30 dv or 28 dv. The Administrator solicits comment on a 
range of levels for such a standard down to 25 dv, as well as on 
alternative standards to address PM-related visibility impairment, 
including a sub-daily averaging time (e.g., 4 hours) and associated 
alternative levels in the range of 30 to 25 dv. To address other non-
visibility welfare effects, the Administrator proposes to revise the 
form of the secondary annual PM2.5 standard to remove the 
option for spatial averaging and to retain all other elements of the 
current suite of secondary PM standards.

VII. Interpretation of the NAAQS for PM

    With regard to the NAAQS for PM2.5, this section 
discusses EPA's proposed revisions to the data handling procedures in 
40 CFR part 50, appendix N, for the proposed primary and secondary 
annual and 24-hour standards for PM2.5 (referred to as 
PM2.5 standards) and for the proposed distinct secondary 
standard for PM2.5 to address PM-related visibility 
impairment (referred to as the PM2.5 visibility index 
standard).\183\ Appendix N describes the computations necessary for 
determining when these standards are met and also addresses which 
measurement data are appropriate for comparison to the proposed 
standards, as well as data reporting protocols, data completeness 
criteria, and rounding conventions.
---------------------------------------------------------------------------

    \183\ With regard to the PM10 NAAQS, as summarized in 
sections IV.F and VI.F, the EPA is proposing to retain the current 
primary and secondary PM10 standards. Data handling 
procedures for these PM10 standards would remain as 
presented in 40 CFR part 50, appendix K.
---------------------------------------------------------------------------

    As discussed in sections III and VI above, the EPA is proposing to: 
(1) Revise the form and level of the primary annual PM2.5 
standard, and retain the current primary 24-hour PM2.5 
standard (section III.F); (2) retain the current secondary 24-hour 
PM2.5 standard, and revise the form and retain the level of 
the secondary annual PM2.5 standard for non-visibility-
related welfare protection (section VI.F); and (3) establish a distinct 
secondary PM2.5 visibility index standard (section VI.F). 
The EPA proposes to revise appendix N to conform to the proposed 
revisions to the standards. The Agency also proposes to make additional 
changes in the appendix N data handling provisions to codify existing 
practices currently included in guidance documents or implemented as 
EPA standard operating procedures as well as to provide greater clarity 
and consistency in the application of these provisions. The proposed 
revisions to appendix N are discussed in section VII.A below.
    Section 1(b) of appendix N refers to special considerations that 
may be given to data resulting from exceptional events. An exceptional 
event is defined in 40 CFR 50.1 as an event that affects air quality, 
is not reasonably controllable or preventable, is an event caused by 
human activity that is unlikely to recur at a particular location or a 
natural event, and is determined by the Administrator in accordance 
with 40 CFR 50.14 to be an exceptional event. Air quality data that are 
determined to have been affected by an exceptional event under the 
procedural steps, substantive criteria, and schedule specified in 
section 50.14 may be excluded from consideration when EPA makes a 
determination that an area is meeting or violating the associated 
NAAQS. Proposed revisions to the schedule specified in section 50.14 
for data flagging and submission of demonstrations for exceptional 
events data considered for initial area designations under the proposed 
revised primary and secondary PM standards are discussed in section 
VII.B below.
    Several proposed updates and clarifications to the data handling 
provisions associated with AQI reporting in 40 CFR part 58, Appendix G 
are discussed in section VII.C below. These modifications reflect the 
proposed changes to the AQI sub-index for PM2.5 as discussed 
in section V above and harmonize reporting procedures for AQI sub-
indices for other criteria pollutants.

A. Proposed Amendments to Appendix N: Interpretation of the NAAQS for 
PM2.5

    As discussed below, the proposed revisions to appendix N 
corresponding to proposed changes in the standards addressed in 
sections III and VI above are: (1) Modification of the level of the 
primary annual PM2.5 standard (sections VII.A.1 and 
VII.A.4); (2) modification of the form of the primary and secondary 
annual PM2.5 standards to remove the option for spatial 
averaging (sections VII.A.2 and VII.A.4); and (3) addition of data 
handling procedures that detail how to make comparisons to the proposed 
secondary standard for PM2.5 that addresses PM-related 
visibility impairment (section VII.A.5), as well as to summarize 
associated changes proposed in other sections of appendix N to 
accommodate this proposed standard (sections VII.A.1, VII.A.2, and 
VII.A.3). In addition to these three proposed appendix modifications 
that are discussed in depth in sections III and VI above, the EPA also 
proposes additional revisions to appendix N in order to: (1) Better 
align appendix N language and requirements with proposed changes in the 
PM2.5 ambient monitoring and reporting requirements as 
discussed in section VIII below; (2) enhance consistency with recently 
codified changes in data handling procedures for other criteria 
pollutants; (3) codify existing practices currently included in 
guidance documents or implemented as EPA standard operating procedures; 
and (4) provide enhanced clarity and consistency in the articulation 
and application of appendix N provisions. Key elements of the proposed 
revisions to appendix N are summarized in sections VII.A.1 through 
VII.A.5 below, where each of these preamble sections corresponds to the 
similarly numbered section in appendix N.
1. General
    The EPA proposes to modify section 1.0 of appendix N to provide 
additional clarity regarding the scope and interpretation of the NAAQS 
for PM2.5. This section would reference the proposed 
revisions to the primary annual PM2.5 standard and the 
proposed revision to the form of the secondary annual PM2.5 
standard (40 CFR 50.18) and the proposed addition of a distinct 
secondary PM2.5 visibility index standard (40 CFR 50.19). As 
summarized in section VI.F, the proposed secondary standard is defined 
in terms of a calculated PM2.5 light extinction indicator, 
which would use 24-hour average speciated PM2.5 mass 
concentration data, along with associated relative humidity 
information, to calculate light extinction, which would then be 
translated to the deciview scale (referred to as a PM2.5 
visibility index); a 24-hour averaging time; a 90th percentile form 
averaged over 3 years; and a level of either 30 dv or 28 dv. The result 
(i.e., the PM2.5 visibility index design value) would be 
compared to the level of the standard. As noted earlier, the NAAQS 
indicator and proposed data handling procedures are similar to those of 
the Regional Haze Program. The EPA proposes to add to section 1.0 of 
appendix N, a reference to section 2.9 of appendix C to 40 CFR part 58 
which identifies the acceptable methods for the speciated 
PM2.5 mass concentration data. With regard to the appendix N 
term definitions which are delineated in this initial section, the EPA 
proposes to

[[Page 39000]]

add, modify, or eliminate term definitions, as appropriate, in 
accordance with the proposed data handling rule revisions such as the 
addition of terms associated with the proposed secondary 
PM2.5 visibility index standard and the modification of 
terms that referenced spatial averaging. Additional term definitions 
are also being added to reference otherwise unchanged appendix N logic 
in an effort to streamline the appendix text, enhance clarity and thus 
improve readability and understanding.
2. Monitoring Considerations
    The EPA proposes revisions to section 2.0 of appendix N consistent 
with the proposed modification of the form of the primary annual 
PM2.5 standard to remove the option for spatial averaging. 
As described in more detail in section III.E.3.a above, the EPA is 
proposing to remove this option as part of the form of the primary 
annual PM2.5 standard. This proposed change is based on an 
analysis that indicates 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 susceptible populations (Schmidt 2011a, Analysis A).
    With respect to the form of the secondary annual PM2.5 
standard, while, as discussed in section VI.E.5 above, the EPA is 
proposing to retain the current secondary annual PM2.5 
standard to provide protection for non-visibility welfare effects, the 
EPA believes it would be reasonable and appropriate to align the data 
handling procedures for the primary and secondary annual 
PM2.5 standards. Therefore, the EPA proposes to remove the 
option for spatial averaging for the secondary annual PM2.5 
standard consistent with the proposed change in the form of the primary 
annual PM2.5 standard. The EPA notes that no areas in the 
country are currently using the option for spatial averaging to 
demonstrate attainment with the secondary annual PM2.5 
standard.
    Consistent with the proposed change to revise the forms of the 
primary and secondary annual PM2.5 standards, the levels of 
the standards would be compared to measurements from each appropriate 
(i.e., ``eligible'') monitoring site in an area operated in accordance 
with the network technical requirements specified in 40 CFR 58.11, the 
operating schedule described in 40 CFR 58.12, and the special 
considerations for data comparisons to the NAAQS specified in 40 CFR 
58.30, with no allowance for spatial averaging. Thus, for an area with 
multiple eligible monitoring sites, the site with the highest design 
value would determine the attainment status for that area. As a result 
of this proposed change, the EPA proposes to remove all references to 
the spatial averaging option throughout appendix N.
3. Requirements for Data Use and Reporting for Comparisons With the 
NAAQS for PM2.5
    The EPA proposes to make changes to section 3.0 of appendix N to 
correspond with the proposed secondary PM2.5 visibility 
index standard, to improve consistency with procedures used for other 
NAAQS, and to improve consistency with current standard operating 
procedures. Specifically, the EPA proposes revisions to this section 
regarding: (1) Requirements for reporting monitored aggregated 
PM2.5 and speciated PM2.5 mass concentration 
data; (2) clarification of monitoring data appropriate to compare to 
the PM2.5 and PM2.5 visibility index NAAQS; (3) 
clarification of procedures for using hourly concentrations to 
calculate 24-hour concentrations; and (4) clarification of procedures 
for combining monitoring data from collocated instruments into a single 
``combined site'' record. Further, the EPA proposes to codify, in this 
same section, modifications to the PM2.5 data handling 
provisions to make them consistent with recent changes made for other 
criteria pollutants. For example, data for which the certification 
deadline has passed, and the monitoring agency has not requested 
certification of the data, can nevertheless be used to determine 
compliance with the PM2.5 NAAQS and the PM2.5 
visibility index NAAQS when EPA judges the data to be complete and 
accurate.
    With regard to the criteria for reporting PM2.5 
concentrations, section 3.0 of appendix N specifies that 
PM2.5 mass concentrations used for NAAQS comparisons shall 
be reported in units of [micro]g/m\3\ with the values truncated (not 
rounded) to one digit to the right of the decimal point (i.e., 
truncated to one decimal place). Since, to date, appendix N has dealt 
only with PM2.5 mass concentrations, intrinsically these 
requirements have dealt only with that particular set of data.
    With regard to the proposed secondary PM2.5 visibility 
index standard, the EPA already has a requirement in 40 CFR 58.16 to 
report speciated PM2.5 mass concentration data. This 
includes the nine required speciated PM2.5 mass 
concentration inputs (i.e., sulfate, nitrate, OC (and related 
PM2.5 OC which is reported OC with an adjustment for the 
organic carbon artifact present on a filter), EC, Al, Si, Ca, Fe, and 
Ti) used to calculate PM2.5 visibility index values as 
described in section VII.A.5 below. Specifically, the EPA proposes to 
require that all nine parameters be used in the appendix N procedures 
in units of [micro]g/m\3\ with the values rounded to four decimal 
places (or three significant digits if the value is 0.1 [micro]g/m\3\ 
or larger). These rounding conventions are consistent with the AQS 
reporting protocols used in the CSN program, discussed in section 
VIII.A.2 below, which is proposed to be a major source of ambient data 
used in calculating PM2.5 visibility index design values to 
compare to the level of proposed secondary NAAQS.
    Monitoring sites eligible for comparison to the NAAQS for 
PM2.5 include those following the network technical 
requirements specified in 40 CFR 58.11 as well as following the 
eligibility criteria specified in 40 CFR 58.30.\184\ However, as 
discussed in section VIII.A.1 below, an analysis of the quality of data 
from two different methods used by FEMs has indicated that some sites 
with continuous PM2.5 FEMs have an acceptable degree of 
comparability with collocated FRMs, while other FEMs have less 
acceptable data comparability that would not meet the performance 
criteria originally used to approve the FEMs (Hanley and Reff, 2011). 
Therefore, as explained in more detail in section VIII.B.3.b.ii below, 
the EPA is proposing to allow monitoring agencies to identify 
PM2.5 FEMs that are not providing data of sufficient 
comparability to the FRM and, with EPA approval, to exclude the use of 
these data in making comparisons to the NAAQS for 
PM2.5.\185\
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    \184\ As discussed in more detail in section VIII.B.2.b below, 
the EPA is proposing to change the current presumption in 40 CFR 
58.30 that micro- and middle-scale monitoring sites are ``unique'' 
and are comparable only to the 24-hour PM2.5 standards, 
unless approved by the Regional Administrator to collectively 
identify a larger region of localized high ambient PM2.5 
concentrations. Today's proposal, if finalized, would change this 
presumption, such that micro- and middle-scale monitoring sites 
would not be presumed to be unique and, therefore, would be 
comparable to the annual PM2.5 standards as well as the 
24-hour PM2.5 standards, unless the Regional 
Administrator determines that the micro- or middle-scale site is 
unique.
    \185\ The EPA also allows use of alternative methods where 
explicitly stated in the monitoring methodology requirements 
(appendix C of 40 CFR part 58), such as PM2.5 Approved 
Regional Methods (ARMs) which can be used to determine compliance 
with the NAAQS. Monitoring agencies identifying ARMs that are not 
providing data of sufficient quality would also be allowed to 
exclude these data in making comparisons to the PM2.5 and 
PM2.5 visibility index NAAQS. Currently, there are no 
designated ARMs for PM2.5.

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[[Page 39001]]

    With regard to data handling procedures for using hourly mass 
concentrations to calculate 24-hour average mass concentrations, 
current procedures are specific for handling aggregated 
PM2.5 mass concentrations and are not currently relevant for 
handling the speciated PM2.5 mass concentrations that would 
be used for calculating PM2.5 visibility index design values 
for the proposed secondary standard. In considering data handling 
procedures for hourly speciated PM2.5 mass concentrations, 
the EPA notes that the vast majority of speciation data collected 
across the country are from filter-based sampling methods which 
typically operate on a 24-hour sampling period. There are several 
monitoring sites reporting hourly speciation data, but even in these 
cases the methods employed only provide for a small number of 
speciation parameters (e.g., EC, OC, sulfate) to be reported. However, 
in anticipation that such continuous methods might be more widely 
implemented for the speciated PM2.5 mass components in the 
future, the EPA proposes to add clarifying language to section 3.0(a) 
to indicate that the data handling procedures for using hourly 
concentration data to calculate 24-hour average concentration data 
would be applicable to both aggregated PM2.5 mass 
concentrations and speciated PM2.5 mass concentrations.
    With respect to the procedures for combining monitored data from 
collocated instruments into a single ``combined site'' data record, the 
EPA proposes to revise the current methodology in situations where an 
FRM monitor operating on a non-daily schedule is collocated with a 
continuous FEM monitor (that has acceptable comparability with an FRM). 
The EPA is not proposing to change the procedures for calculating a 
combined site record \186\ but rather the subsequent evaluation of 
whether the specific measurements are considered ``creditable'' or 
``extra'' samples. Samples in the combined site record are deemed 
``creditable'' or ``extra'' according to the required sampling 
frequency for a specific monitoring site (i.e., ``site-level sampling 
frequency'') which, by default, is defined to be the same as the 
sampling frequency required of the primary monitor. Samples in the 
combined site data record that correspond to scheduled days according 
to the site-level sampling frequency are deemed ``creditable'' and, 
thus, are considered for determining whether or not a specific 
monitoring site meets data completeness requirements. These samples 
also determine which daily value in the ranked list of daily values for 
a year represents the annual 98th percentile concentration. Samples 
that are not deemed ``creditable'' are classified as ``extra'' samples. 
These samples do not count towards data completeness requirements and 
do not affect which daily values represent the annual 98th percentile 
concentration; ``extra'' samples, however, are candidates for selection 
as the 98th percentile.
---------------------------------------------------------------------------

    \186\ Data for a combined site record originates by default from 
the designated ``primary'' monitor at the site location and is then 
augmented with data from collocated FRM or FEM monitors whenever 
valid data are not generated by the primary monitor.
---------------------------------------------------------------------------

    Before the introduction of continuous PM2.5 FEMs, when 
two or more samplers were collocated at the same site, monitoring 
agencies typically identified the sampler that operated on the more 
frequent sampling schedule as the ``primary'' monitor for developing a 
single site record. However, due to concerns regarding the 
comparability of continuous PM2.5 FEMs to FRMs operated in 
some monitoring agency networks, and as briefly discussed above and in 
more detail in section VIII.A.1 below, many monitoring agencies have 
kept the FRM as the ``primary'' monitor while continuing to evaluate 
the continuous FEM monitor. In cases where the FRM either does not have 
a scheduled measurement or has a measurement that is invalidated and 
the continuous FEM data are available for use, and the continuous FEM 
data are not identified as not to be used (i.e., a special purpose 
monitor (SPM) in its first 24 months of operation) the FEM data will be 
substituted into the site record. In cases where the continuous FEM 
measurements are reported on the FRM ``off'' days, these data are 
technically considered ``extra'' samples.
    In light of this practice, the EPA modified standing operating 
procedures and now proposes a conforming revision to section 3.0(e) 
whereby collocated FEM samples reported on the FRM ``off'' days would 
be considered ``scheduled'' and ``creditable.'' Thus, collocated FEM 
samples would count towards data capture rates (actually, increasing 
both the numerator and the denominator in the capture rate equation), 
and also would count towards identifying annual 98th percentile 
concentrations. Further, consistent with current practices, if data 
from a collocated FEM are missing on an FRM ``off'' day (and no 
unscheduled FRM data are reported that day), the EPA proposes not to 
identify these as ``scheduled'' samples. Thus, reported data generated 
from the collocated continuous FEMs can only help increase data capture 
rates. The EPA specifically solicits comment on whether ``non-primary'' 
(i.e., collocated) FEM data should be combined with the primary data as 
part of the comparison to the NAAQS for PM2.5.
    The EPA proposes to utilize the same general procedures for 
combining speciated PM2.5 mass concentration data from 
collocated monitors into a single ``combined site'' record as those 
specified for the PM2.5 mass measurements.
4. Comparisons With the Annual and 24-Hour PM2.5 NAAQS
    Section 4.0 of appendix N specifies the procedures for comparing 
monitored data to the annual and 24-hour PM2.5 standards. 
The EPA proposes revisions to section 4.0 of appendix N to: (1) Provide 
consistency with the proposed primary and secondary annual 
PM2.5 standards; (2) expand the data completeness 
assessments to be consistent with current guidance and standard 
operating procedures; and (3) simplify the procedure for calculating 
annual 98th percentile concentrations when using an approved seasonal 
sampling schedule.
    Consistent with the proposed decisions to revise the level of the 
primary annual PM2.5 standard (section III.F) and to retain 
the current level of the secondary annual PM2.5 standard 
(section VI.F), the EPA proposes to modify section 4.1(a) of appendix N 
to separately list the levels of the primary and secondary annual 
PM2.5 standards. Additionally, consistent with the proposed 
decision to remove the option for spatial averaging for the primary 
annual PM2.5 standard (section III.F) as well as for the 
secondary annual PM2.5 standard (section VII.A.2), the EPA 
proposes to amend section 4.4 of appendix N to remove equations and 
associated instructions that relate to spatial averaging.
    With regard to assessments of data completeness, the EPA proposes 
to include two additional data substitution tests \187\ (making a total 
of three data substitution tests) for validating annual and 24-hour 
PM2.5 design values otherwise deemed incomplete (via the 75 
percent and 11 creditable sample minimum quarterly data completeness 
checks). Data substitution tests are diagnostic in nature; that is; 
they are only used in an illustrative manner to

[[Page 39002]]

show that the NAAQS status based on incomplete data is reasonable. If 
an ``incomplete'' design value using substituted data passes the 
diagnostic test, this ``incomplete'' design value (without the data 
substitutions) is then considered the true actual ``complete'' design 
value. If an incomplete design value does not pass any stipulated data 
substitution test, then the original design value is still considered 
incomplete.
---------------------------------------------------------------------------

    \187\ Data substitution tests are supplemental data completeness 
assessments that use estimates of 24-hour average concentrations to 
fill in for missing data (i.e., ``data substitution'').
---------------------------------------------------------------------------

    Currently, section 4.1(c) specifies one data substitution test for 
validating an otherwise incomplete design value. This diagnostic test 
is only applicable to the primary and secondary annual PM2.5 
standard and only applies in instances of a violation. The EPA proposes 
to modify the data completeness requirements by adding two additional 
data substitution tests for handling incomplete data sets in order to 
make the data handling procedures for PM2.5 more consistent 
with the procedures used for other NAAQS pollutants and to codify 
existing practices currently included in guidance documents (U.S. EPA, 
1999) and implemented as EPA standard operating procedures. The 
proposed additional data substitution tests would be applicable for 
making comparisons to the primary and secondary annual and 24-hour 
PM2.5 standards. One of these tests uses collocated 
PM10 data to fill in ``slightly incomplete'' \188\ data 
records, and the other uses quarter-specific maximum values to fill in 
``slightly incomplete'' data records.
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    \188\ ``Slightly incomplete'' is defined as less than 75 percent 
but greater than or equal to 50 percent data capture.
---------------------------------------------------------------------------

    With regard to identifying annual 98th percentile concentrations 
for comparison to the primary and secondary 24-hour PM2.5 
standards, the EPA proposes to simplify the procedures used with an 
approved seasonal sampling schedule. Specifically, the EPA proposes to 
eliminate the use of a special formula for calculating annual 98th 
percentile concentrations with a seasonal sampling schedule and 
proposes to use only one method for calculating annual 98th percentile 
concentrations at all sites.
    Currently, with an approved seasonal sampling schedule, a site 
typically samples as required during periods of the year when the 
highest concentrations are expected to occur, but less frequently 
during periods of the year when lower concentrations are expected to 
occur. This type of sampling schedule generally leads to an 
``unbalanced'' data record; that is, a data record with proportionally 
more ambient measurements (with respect to the total number of days in 
the sampling period) in the ``high'' season and proportionally fewer 
ambient measurements in the ``low'' season.
    In the last review, the EPA revised section 4.5 of appendix N to 
include a special formula for computing annual 98th percentile values 
when a site operates on an approved seasonal sampling schedule. This 
special formula accounted for an unbalanced data record and was 
consistent with guidance documentation (U.S. EPA, 1999), and, where 
appropriate, with official OAQPS design value calculations (71 FR 
61211, October 17, 2006). In cases where there is a balanced \189\ (or 
near-balanced) data record, the special formula yields the same result 
as the regular procedure for calculating annual 98th percentile 
concentrations.
---------------------------------------------------------------------------

    \189\ A balanced data record has the same proportion of ambient 
measurements (with respect to the total number of days in the 
sampling period) in the ``high'' season as in the ``low'' season.
---------------------------------------------------------------------------

    To qualify for a seasonal sampling schedule, monitoring agencies 
are required to collocate a continuous PM2.5 instrument with 
the seasonal sampling FRM. Since the last review, there has been 
considerable deployment of continuous PM2.5 FEM monitors. In 
situations where a PM2.5 FRM monitor operating on a non-
daily periodic schedule (such as a 1-day-in-3 or a 1-day-in-6 schedule) 
is collocated with a continuous PM2.5 FEM monitor, data are 
combined based on procedures stated in section 3.0 of appendix N as 
modified as discussed in section VII.A.3 above. The end result of 
combining collocated FRM and FEM data is effectively an ``every day'' 
site-based sampling frequency, resulting in a balanced data record. In 
such a case, if a site used a seasonal sampling schedule regime for the 
FRM monitor, these data would be balanced by the ``every day'' FEM data 
and there would be no need for the special formula for calculating 
annual 98th percentile concentrations on the combined site data.
    The EPA notes that currently there are very few PM2.5 
FRM monitors that actually operate on an approved seasonal sampling 
schedule (only 15 sites out of approximately 1,000 total sites in 2010) 
and that almost half of these sites have a collocated PM2.5 
FEM monitor. For the most recent 3-year period (2008-2010), the annual 
98th percentile concentrations calculated with the special formula at 
these 15 sites were approximately five percent lower than if the 
regular procedure was used. The EPA also notes that, in the last 
review, the Agency modified the monitoring requirements for areas with 
an FRM operating on a non-daily schedule such that, if the design 
values were within five percent of the 24-hour PM2.5 NAAQS, 
such areas are required to increase the frequency of sampling to every 
day (40 CFR 58.12(d)(1); 71 FR 61165, October 17, 2006; 71 FR 61249, 
October 17, 2006). Thus, the EPA proposes to simplify the data handling 
procedures for sites operating on a seasonal sampling schedule by 
eliminating the special formula and all references to it based on: (1) 
The small difference between 98th percentile concentrations calculated 
using the special formula versus the regular procedure and the small 
number of sites currently using the special formula; (2) the EPA 
requirements for every day sampling in areas with design values that 
are within five percent of the 24-hour PM2.5 NAAQS; and (3) 
the EPA requirement that FRMs operating on an approved seasonal 
sampling schedule be collocated with a continuous PM2.5 
instrument (and if that instrument were an FEM, the resulting combined 
site record would tend to be balanced over the year and thus the 
special formula would be superfluous). Thus, the EPA proposes to use 
only one method for calculating annual 98th percentile concentrations 
for all sites, that being the ``regular'' table look-up method 
specified in section 4.5(a)(1) of appendix N. The EPA solicits comment 
on the proposal to eliminate the special formula for sites operating on 
a seasonal sampling schedule.
5. Data Handling Procedures for the Proposed Secondary PM2.5 
Visibility Index NAAQS
    As summarized in section VI.F above, the EPA is proposing to 
establish a distinct secondary standard for PM2.5 to address 
PM-related visibility impairment. The EPA is proposing to define this 
standard in terms of a PM2.5 visibility index (section 
VI.D.1.c), which would use 24-hour average speciated PM2.5 
mass concentration and historic monthly average relative humidity data 
to calculate PM2.5 light extinction, translated into the 
deciview scale, similar to the Regional Haze Program.
    The EPA proposes to add a new section 5.0 to appendix N to detail 
the data handling procedures for calculating PM2.5 
visibility index design values and comparing these design values to the 
level of the proposed PM2.5 visibility index NAAQS. These 
proposed procedures are drawn from and are generally consistent with 
the original approach used in the Regional Haze Program [U.S. EPA, 
2003] and discussed

[[Page 39003]]

in the Policy Assessment (U.S. EPA, 2011a, chapter 4, Appendix G).
    As discussed in section VI.B.1.a above, visibility impairment is 
caused by the scattering and absorption of light by suspended particles 
and gases in the atmosphere. The combined effect of light scattering 
and absorption by both particles and gases is characterized as light 
extinction. The amount of light extinction contributed by PM depends on 
the particle size distribution and composition, as well as the 
concentrations of speciated components of ambient PM. To make 
estimation of light extinction more practical, visibility scientists 
have developed simple algorithms, referred to as the IMPROVE algorithms 
to relate speciated PM2.5 concentrations to light 
extinction. 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.
    The EPA proposes to define the PM2.5 visibility index 
using a PM2.5 light extinction indicator calculated on a 24-
hour basis using the original IMPROVE algorithm without the terms for 
coarse mass and Rayleigh scatter. As discussed in section VI.D.1.c 
above, using such an index appropriately reflects the relationship 
between ambient PM and PM-related light extinction. When converting 
PM2.5 light extinction values in Mm-\1\ to the 
deciview scale, the Rayleigh scattering term must be included to avoid 
the possibility of negative values.
    Consistent with the analyses and terminology used in the Policy 
Assessment (U.S. EPA, 2011a, chapter 4, Appendix G), PM2.5 
light extinction (PM2.5 bext) is defined as
[GRAPHIC] [TIFF OMITTED] TP29JN12.017

The above formula is implemented using 24-hr speciated PM2.5 
concentration data together with monthly climatological relative 
humidity factors as outlined below. The six steps involved in the 
calculation of the PM2.5 visibility index values are as 
follows:

    (1) As discussed in Section VI.B.1.a above, ``sulfate'' is 
defined as ammonium sulfate and ``nitrate'' is defined as ammonium 
nitrate. Multiply 24-hour average speciation measurements of sulfate 
and nitrate ions by factors 1.375 and 1.29, respectively, to convert 
the reported ion concentrations into sulfate and nitrate ammonium 
concentrations (appendix N, equations 5a and 5b).
    (2) Convert artifact adjusted measured OC, which is termed 
``PM2.5 OC'', into an estimate of organic mass (OM). The 
PM2.5 OC is derived by subtracting the sampler-dependent 
OC measurement artifact from the measured OC.\190\ The 
PM2.5 OC is then multiplied by 1.4 to account for the 
additional mass of hydrogen, oxygen and other elements associated 
with the carbon in measured OC (appendix N, equation 5c).
---------------------------------------------------------------------------

    \190\ In the IMPROVE program, artifact adjusted OC (i.e., 
PM2.5 OC) is simply reported as OC. That is the value 
used to produce OM for haze calculations. For the CSN measurements, 
the OC artifact needed to convert measured OC into PM2.5 
OC is estimated from sampler-specific network-wide field blanks 
(Frank, 2012).
---------------------------------------------------------------------------

    (3) Calculate fine soil/crustal PM2.5 (FS) component 
based on measurements of five soil derived elements (i.e., Al, Si, 
Ca, Fe, and Ti) together with multipliers to account for their 
normal oxides \191\ (appendix N, equation 5d).
---------------------------------------------------------------------------

    \191\ Fine Soil = 2.2[Al] + 2.49[Si] + 1.63[Ca] + 2.42[Fe] + 
1.94[Ti]
---------------------------------------------------------------------------

    (4) Determine a representative long-term monthly average of 
hourly relative humidity hygroscopic growth factors, referred to as 
f(RH) values, at the speciation monitoring site, for each month of 
the year. There will be 12 such values for any monitoring site. The 
EPA proposes that the f(RH) values be selected using historical 
data. A spatial interpolation of historical relative humidity data 
is available which presents a gridded field of f(RH) values across 
the U.S. at a resolution of 0.25 degrees (SAIC, 2001). As discussed 
in section VI.D.2.a.ii above, these monthly average values were 
developed to support the Regional Haze Program and are based on 
considering any hour with relative humidity greater than 95 percent 
as 95 percent. Because 10 years of hourly data were used to produce 
a single humidity term for each month, the EPA believes that the 
resulting monthly average of the humidity term is sufficient and 
appropriate to reduce the effects of fog or precipitation. The EPA 
proposes that the 10-year climatological data base be used to 
specify the f(RH) value associated with the grid-point closest in 
distance to the speciation monitoring site.\192\
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    \192\ To facilitate the use of relative humidity data, the EPA 
would make this ten-year climatological data base publically 
available on its Web site.
---------------------------------------------------------------------------

    (5) Apply the original IMPROVE algorithm without the terms for 
coarse mass and Rayleigh scatter (appendix N, equation 6) to 
calculate a daily average PM2.5 light extinction 
(PM2.5 bext, in units of Mm-\1\).
    (6) To translate PM2.5 light extinction to the 
deciview scale for making comparisons to the level of the proposed 
secondary PM2.5 visibility index standard, the following 
equation, which includes the term for Rayleigh scattering term, is 
used:
[GRAPHIC] [TIFF OMITTED] TP29JN12.018

The EPA solicits comment on all aspects of the calculation of the 
PM2.5 visibility index, PM2.5 bext.
    As discussed in section VI.D.3 above, the EPA is proposing a 90th 
percentile form, averaged over 3 years, for the proposed secondary 
PM2.5 visibility index standard. Thus, 3 years of valid 24-
hr speciated PM2.5 mass concentration data would be required 
to calculate PM2.5 visibility index design values. The 
proposed new section 5.0 for appendix N addresses data completeness 
requirements for speciated PM2.5 mass concentrations 
(section 5.0(b)), specifically that PM2.5 visibility index 
values be present for at least 11 creditable days of each quarter, for 
each of the three consecutive years. The 11 sample minimum is 
consistent with criteria specified for the current and proposed primary 
and secondary annual PM2.5 standards (i.e., 40 CFR part 50, 
appendix N 4.1(b)) and, furthermore, has been used extensively for 
various PM characterization exercises (e.g., U.S. EPA, 2009a; U.S. EPA, 
2011a). In addition, the proposed new section 5.0 outlines procedures 
for identifying annual 90th percentile PM2.5 visibility 
index values (section 5.0(d)(3)) similar to procedures used to identify 
annual 98th percentile values for the primary

[[Page 39004]]

and secondary 24-hour PM2.5 standards. In situations where a 
year does not contain the minimum 11 creditable samples in each 
quarter, the EPA proposes (in section 5.0) to still consider the 
identified 90th percentile index value to be valid if it, or a 3-year 
average of 90th percentile index values (i.e., a visibility impairment 
design value) including it, exceeds the level of the NAAQS. The EPA is 
not proposing any data substitution tests for PM2.5 
visibility index design values like those codified and proposed for the 
aggregated PM2.5 mass standard design values; however, the 
EPA solicits comment on the inclusion of such data substitution tests.
    With regard to rounding conventions, the EPA proposes that all 
decimal digits be retained in the intermediate steps of the calculation 
of the PM2.5 light extinction indicator and that the 
PM2.5 visibility index values be rounded to the nearest 
tenth deciview. Furthermore, the EPA proposes to round the 3-year 
average 90th percentile PM2.5 visibility index design values 
to the nearest 1 dv for comparison to the level of the proposed 
secondary standard.
    Consistent with current procedures for PM and the other criteria 
pollutants, the EPA plans to calculate design values for the proposed 
secondary PM2.5 visibility index NAAQS using the procedures 
described above. The EPA plans to post these design values on its Web 
site.\193\
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    \193\ Design values calculated by the EPA are computed and 
published annually by EPA's OAQPS and reviewed in conjunction with 
the EPA Regional Offices. These values are available at: http://www.epa.gov/airtrends/values.html.
---------------------------------------------------------------------------

B. Exceptional Events

    States \194\ are responsible for identifying air quality data that 
they believe warrant special consideration, including data affected by 
exceptional events. States identify such data by flagging (making a 
notation in a designated field in the electronic data record) specific 
values in the AQS database. States must flag the data and submit 
supporting documentation showing that the data have been affected by 
exceptional events if they wish the EPA to consider excluding the data 
in regulatory decisions, including determining whether or not an area 
is attaining the proposed revised PM NAAQS.
---------------------------------------------------------------------------

    \194\ References to ``state'' are meant to include state, local 
and tribal agencies responsible for implementing the Exceptional 
Events Rule.
---------------------------------------------------------------------------

    All states and areas of Indian country that include areas that 
could exceed the proposed PM NAAQS and could therefore be designated as 
nonattainment for the proposed PM NAAQS have the potential to be 
affected by this rulemaking. Therefore, this action would apply to all 
states; to local air quality agencies to which a state has delegated 
relevant responsibilities for air quality management including air 
quality monitoring and data analysis; and to tribal air quality 
agencies where appropriate.
    The ``Treatment of Data Influenced by Exceptional Events; Final 
Rule'' (72 FR 13560, March 22, 2007), known as the Exceptional Events 
Rule and codified at 40 CFR 50.14, contains generic deadlines for a 
state to submit to EPA specified information about exceptional events 
and associated air pollutant concentration data. A state must initially 
notify the EPA that data have been affected by an event by July 1 of 
the calendar year following the year in which the event occurred. This 
is done by flagging the data in AQS and providing an initial event 
description. The state must also, after notice and opportunity for 
public comment, submit a demonstration to justify any claim within 
three years after the quarter in which the data were collected. 
However, if a regulatory decision based on the data (for example, a 
designation action) is anticipated, the schedule to flag data in AQS 
and submit complete documentation to EPA for review may be shortened 
and all information must be submitted to the EPA no later than one year 
before the decision is to be made.
    These generic deadlines in the Exceptional Events Rule are suitable 
after initial designations have been made under a NAAQS or when an area 
is to be redesignated, either from attainment to nonattainment or from 
nonattainment to attainment, and the redesignation status may depend on 
the excluded data. However, these same generic deadlines may need to be 
adjusted to accommodate the initial area designation process and 
schedule under a newly revised NAAQS. Until the level and form of the 
NAAQS have been promulgated, a state does not know whether the criteria 
for excluding data (which are tied to the level and form of the NAAQS) 
were met for a given event. In some cases, the generic deadlines, 
especially the deadlines for flagging some relevant data, may have 
already passed by the time the new or revised NAAQS is promulgated. In 
addition, it may not be feasible for information on some exceptional 
events that may affect final designations decisions to be collected and 
submitted to EPA at least one year in advance of the final designation 
decision. This scheduling constraint could have the unintended 
consequence of the EPA designating an area nonattainment because of 
uncontrollable natural or other qualified exceptional events.
    The Exceptional Events Rule at section 50.14(c)(2)(vi) indicates 
``when EPA sets a NAAQS for a new pollutant or revises the NAAQS for an 
existing pollutant, it may revise or set a new schedule for flagging 
exceptional event data, providing initial data descriptions and 
providing detailed data documentation in AQS for the initial 
designations of areas for those NAAQS.''
    The EPA intends to promulgate the revised PM NAAQS in December 
2012. State Governors (and tribes, if they choose) should submit 
designations recommendations by December 2013, based on air quality 
data from the years 2010 to 2012 or 2011 to 2013, if there are 
sufficient data for these years. Initial designations under the revised 
NAAQS would be made by December 2014 based on air quality data from the 
years 2011 to 2013. (See section IX.A for a more detailed discussion of 
the designation schedule.) Assuming this schedule, all events to be 
considered during the designations process would need to be flagged and 
fully documented by states one year prior to designations, or by 
December 2013, under the existing generic deadline in the Exceptional 
Events Rule. Without revision to 40 CFR 50.14, a state would not be 
able to flag and submit documentation regarding events that occurred in 
December 2013 by one year before designations are made in December 
2014. The EPA believes this is not an appropriate restriction, and 
therefore is proposing revisions to 40 CFR 50.14.
    The EPA proposes revisions to 40 CFR 50.14 only to change 
submission dates for information supporting claimed exceptional events 
affecting PM data for initial area designations under the proposed new 
and revised PM NAAQS. The proposed rule language at the end of this 
notice shows the changes that would apply assuming promulgation of the 
new and revised PM NAAQS in December 2012 and initial area designations 
by December 2014. For air quality data collected in 2010 or 2011, the 
EPA proposes extending to July 1, 2013 the otherwise applicable generic 
deadlines of July 1, 2011 and July 1, 2012, respectively, for flagging 
data and providing an initial description of an event (40 CFR 
50.14(c)(2)(iii)). The EPA proposes to retain the existing generic 
deadline in the Exceptional Events Rule of July 1, 2013 for flagging 
data and providing an initial description of events occurring in 2012. 
Similarly, the EPA proposes to revise to December 12, 2013 the deadline 
for submitting

[[Page 39005]]

documentation to justify PM-related exceptional events occurring in 
2010 through 2012. The EPA believes these revisions/extensions will 
provide adequate time for states to review the impact of exceptional 
events from 2010 through 2012 on any revised standards, to notify the 
EPA by flagging the relevant data and providing an initial description 
in AQS, and to submit documentation to support claims for exceptional 
events.
    If a state intends the EPA to consider in the PM designations 
decisions whether PM data collected during 2013 have been affected by 
exceptional events, the EPA proposes that these data must be flagged by 
the generic Exceptional Event Rule deadline of July 1, 2014. The EPA 
proposes to revise to August 1, 2014 the deadline for submitting 
documentation to justify PM-related exceptional events occurring in 
2013. The EPA believes that these deadlines provide states with 
adequate time to review and identify potential exceptional events that 
occur in calendar year 2013.
    Therefore, using the authority provided in CAA section 319(b)(2) 
and in the Exceptional Events Rule at 40 CFR 50.14 (c)(2)(vi), the EPA 
proposes to modify the schedule for data flagging and submission of 
demonstrations for exceptional events data considered for initial area 
designations under the proposed PM primary and secondary NAAQS as 
presented in Table 3. If the promulgation date for a revised PM NAAQS 
occurs on a different date than in December 2012, the EPA will revise 
the final PM exceptional event flagging and documentation submission 
deadlines accordingly, consistent with the logic of this proposal, to 
provide states with reasonably adequate opportunity to review, 
identify, and document exceptional events that may affect an area 
designation under a revised NAAQS. The EPA invites comment on these 
proposed changes, shown in Table 3, to the exceptional event data 
flagging and documentation submission deadlines for the proposed 
revised PM NAAQS.

  Table 3--Revised Schedule for Exceptional Event Flagging and Documentation Submission for Data To Be Used in
                                 Initial Area Designations for the 2012 PM NAAQS
----------------------------------------------------------------------------------------------------------------
                                   Air quality data
  NAAQS pollutant/standard/     collected for  calendar    Event flagging & initial     Detailed  documentation
 (level)/ promulgation date              year                description deadline         submission deadline
----------------------------------------------------------------------------------------------------------------
PM2.5/24-Hour Standard        2010 to 2011..............  July 1, 2013..............  December 12, 2013.
 (final level and             2012......................  \a\ July 1, 2013..........  December 12, 2013.
 promulgation date TBD).      2013......................  \a\ July 1, 2014..........  August 1, 2014.
PM2.5/Annual Standard (final  2010 to 2011..............  July 1, 2013..............  December 12, 2013.
 level and promulgation date  2012......................  \a\ July 1, 2013..........  December 12, 2013.
 TBD).                        2013......................  \a\ July 1, 2014..........  August 1, 2014.
Secondary PM (final level     2010 to 2011..............  July 1, 2013..............  December 12, 2013.
 and promulgation date TBD).  2012......................  \a\ July 1, 2013..........  December 12, 2013.
                              2013......................  \a\ July 1, 2014..........  August 1, 2014.
----------------------------------------------------------------------------------------------------------------
\a\ This date is the same as the general schedule in 40 CFR 50.14. Note: The table of revised deadlines only
  applies to data the EPA will use to establish the final initial area designations for revised NAAQS. The
  general schedule applies for all other purposes, most notably, for data used by the EPA for redesignations to
  attainment. TBD = to be determined.

C. Proposed Updates for Data Handling Procedures for Reporting the Air 
Quality Index

    The EPA is proposing to update appendix G of 40 CFR part 58 to 
clarify units, breakpoint precision, and truncation methods for AQI 
sub-indices. These changes are intended to harmonize the AQI reporting 
requirements with data handling provisions expressed elsewhere in 40 
CFR part 50. Currently, the breakpoints for NO2 and 
SO2 in Table 2 of appendix G of 40 CFR part 58 are expressed 
in parts per million (ppm). The EPA proposes to change the sub-indices 
for NO2 and SO2 to be based on parts per billion 
(ppb) rather than ppm to be consistent with the units used for defining 
the current levels of the primary NO2 and SO2 
NAAQS (75 FR 6474, February 9, 2010; 75 FR 35520, June 22, 2010). In 
addition, in modifying the sub-index for NO2 to express the 
breakpoints in units of ppb, the EPA proposes to clarify the 
breakpoints for NO2 in the Very Unhealthy and Hazardous 
ranges to include four rather than three significant digits to increase 
precision. Finally, the EPA proposes to modify appendix G to explicitly 
identify truncation methods for using ambient measured concentrations 
in AQI calculations.

VIII. Proposed Amendments to Ambient Monitoring and Reporting 
Requirements

    The EPA proposes changes to the ambient air monitoring, reporting, 
and network design requirements associated with the PM NAAQS. Ambient 
PM monitoring data are used to meet a variety of monitoring objectives 
including determining whether an area is in violation of the PM NAAQS. 
Ambient PM monitoring data are collected by state, local, and tribal 
monitoring agencies (``monitoring agencies'') in accordance with the 
monitoring requirements contained in 40 CFR parts 50, 53, and 58. This 
section discusses the monitoring changes that the EPA is proposing to 
support the proposed PM NAAQS summarized in sections III.F, IV.F, and 
VI.F above.

A. Issues Related to 40 CFR Part 53 (Reference and Equivalent Methods)

    To be used in a determination of compliance with the PM NAAQS, PM 
data are typically collected using samplers or monitors employing an 
FRM or FEM. The EPA also allows use of alternative methods where 
explicitly stated in the monitoring methodology requirements (appendix 
C of 40 CFR part 58), such as PM2.5 ARMs which can be used 
to determine compliance with the NAAQS. The EPA prescribes testing and 
approval criteria for FRM and FEM methods in 40 CFR part 53.
1. PM2.5 and PM10-2.5 Federal Equivalent Methods
    In 2006, the EPA finalized new testing and performance criteria for 
Class II and Class III FEMs (71 FR 61281 to 61289, October 17, 2006). 
Class II methods are equivalent methods for PM2.5 or 
PM10-2.5

[[Page 39006]]

that utilize a PM2.5 sampler or PM10-2.5 sampler 
in which integrated PM2.5 samples or PM10-2.5 
samples are obtained from the atmosphere by filtration and are then 
subjected to a filter conditioning process followed by gravimetric mass 
determination. Class II equivalent methods are different from Class I 
equivalent methods because of substantial deviations from the design 
specifications of the sampler specified for reference methods in 
appendix L or appendix O (as applicable) of 40 CFR part 50. Class III 
refers to those methods for PM2.5 or PM10-2.5 
that are employed to provide PM2.5 or PM10-2.5 
ambient air measurements representative of one-hour or less integrated 
PM2.5 or PM10-2.5 concentrations, as well as 24-
hour measurements determined as, or equivalent to, the mean of 24 one-
hour consecutive measurements. These new testing and performance 
criteria were developed by the EPA and reviewed through consultation 
with the CASAC AAMMS \195\ and then through proposal (71 FR 2710 to 
2808, January 17, 2006) and final rulemaking in 2006 (71 FR 61236 to 
61328, October 17, 2006). The performance criteria were designed to 
ensure enough stringency in testing that subsequently deployed monitors 
would provide data of expected quality (i.e., they would meet the data 
quality objectives), but not so stringent that instrument manufacturers 
would be discouraged from testing their instrument and seeking approval 
as a Class II or III equivalent method. At the time of this proposal, 
the EPA has approved two PM10-2.5 Class II manual methods, 
one Class III PM10-2.5 continuous method, and six Class III 
PM2.5 continuous methods.\196\
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    \195\ The EPA consulted with the CASAC AAMMS on several PM 
monitoring topics in a public meeting on September 21 and 22, 2005. 
Materials from this meeting can be found on EPA's Web site at: 
http://www.epa.gov/ttn/amtic/casacinf.html.
    \196\ A list of designated Reference and Equivalent methods is 
available on EPA's Web site at: http://www.epa.gov/ttn/amtic/criteria.html.
---------------------------------------------------------------------------

    While the EPA has approved these PM2.5 Class III 
continuous FEMs, only two of those methods are deployed on a wide-
enough basis across the country to support initial analyses of data 
quality and comparability to collocated FRM samplers. The Policy 
Assessment discusses an analysis of the quality of data from these two 
FEMs (U.S. EPA, 2011a, p. 4-50). This initial analysis found that some 
sites with continuous PM2.5 FEMs have an acceptable degree 
of comparability with collocated FRMs, while others had less acceptable 
data comparability that would not meet the performance criteria used to 
approve the FEMs.
    The EPA continues to believe that an effective PM2.5 
monitoring strategy includes the use of both filter-based FRM samplers 
and well-performing continuous PM2.5 monitors. Well-
performing continuous PM2.5 monitors would include both non-
approved continuous PM2.5 monitors and approved Class III 
continuous FEMs that meet the performance criteria described in table 
C-4 of 40 CFR part 53 when comparing to a collocated FRM operated by 
the monitoring agency. The use of Class III continuous FEMs at SLAMS is 
described in more detail in section VIII.B.3.b.ii below. Monitoring 
agencies are encouraged to evaluate the quality of data being generated 
by FEMs and, where appropriate, reduce the use of manual, filter-based 
samplers to improve operational efficiency and lower overall operating 
costs. To encourage such a strategy, the EPA is working with numerous 
stakeholders including the monitoring committee of NACAA, instrument 
manufacturers, and monitoring agencies to support national data 
analyses of continuous PM2.5 FEM performance, and where such 
performance does not meet data quality objectives, to develop and 
institute a program of best practices to improve the quality and 
consistency of resulting data.
    The EPA believes that progress is being made to implement well 
performing PM2.5 continuous FEMs across the nation. As noted 
earlier, the first few steps involved the EPA developing and approving 
the testing and performance criteria which were finalized in 2006, 
followed by instrument companies performing field testing and 
submitting applications to the EPA, and EPA review and approval, as 
appropriate, of Class III FEMs. In the current step, monitoring 
agencies are testing and assessing the data comparability from 
continuous PM2.5 FEMs. While some agencies are achieving 
acceptable data comparability and others are not, the EPA wants to 
ensure that all monitoring agencies have the appropriate information to 
maximize data quality from their PM2.5 continuous FEMs 
before considering any changes to regulatory testing requirements 
intended to demonstrate equivalency of candidate Class III FEMs. Since 
we are still early in the process of learning the data comparability 
between approved PM2.5 continuous methods and collocated 
FRMs (assessments across the country are only available for two of the 
six methods), and some of the agencies operating those methods are 
achieving acceptable data comparability, the EPA does not believe it is 
appropriate at this time to propose any modifications to either the 
performance or testing criteria in 40 CFR part 53 used to approve 
PM2.5 continuous FEMs.
    While EPA is not proposing any changes to the performance or 
testing criteria in 40 CFR part 53 used to approve PM2.5 
continuous FEMs, the EPA proposes an administrative change to part 
53.9--``Conditions of designations.'' This section describes a number 
of conditions that must be met by a manufacturer as a condition of 
maintaining designation of an FRM or FEM. Subsection (c) of this 
section reads, ``Any analyzer, PM10 sampler, 
PM2.5 sampler, or PM10-2.5 sampler offered for 
sale as part of a FRM or FEM shall function within the limits of the 
performance specifications referred to in 40 CFR 53.20(a), 53.30(a), 
53.50, or 53.60, as applicable, for at least 1 year after delivery and 
acceptance when maintained and operated in accordance with the manual 
referred to in 40 CFR 53.4(b)(3).'' The EPA's intent in this 
requirement is to ensure that methods work within performance criteria, 
which includes methods for PM2.5 and PM10-2.5; 
however, there is no specific reference to performance criteria for 
Class II and III PM2.5 and PM10-2.5 methods. 
Therefore, the EPA proposes to link the performance criteria referred 
to in 40 CFR part 53.35 associated with Class II and III 
PM2.5 and PM10-2.5 methods with this requirement 
for maintaining designation of approved FEMs. The specific performance 
criteria identified in 40 CFR 53.35 for PM2.5 and 
PM10-2.5 methods are available in table C-4 to subpart C of 
40 CFR part 53.
2. Use of CSN Methods To Support the Proposed New Secondary 
PM2.5 Visibility Index NAAQS
    The EPA, monitoring agencies, and external scientists and policy 
makers use PM2.5 data from the CSN to support several 
important monitoring objectives such as: Development of modeling tools 
and the application of source apportionment modeling for control 
strategy development to implement the NAAQS; health effects and 
exposure research studies; assessment of the effectiveness of emission 
reductions strategies through the characterization of air quality; and 
development of SIPs. The initial CSN began with a pilot of 13 sites in 
2000 and grew rapidly over the next two years. Since 2006, the size of 
the CSN has remained relatively stable at approximately 200 stations.
    The methods employed in the CSN are well documented and uniformly 
implemented across the country. However, between May 2007 and

[[Page 39007]]

October 2009, the CSN transitioned to a new method of sampling and 
analyses for carbon that is consistent with the IMPROVE network 
methodology.\197\ The CSN measurements have a strong history of being 
reviewed by CASAC technical committees, both during their initial 
deployment about ten years ago, and during the more recent transition 
to carbon sampling that is consistent with the IMPROVE protocols 
(Henderson, 2005c). The CSN network is described in the Policy 
Assessment (U.S. EPA, 2011a, Appendix B, section B.1.3).
---------------------------------------------------------------------------

    \197\ In the IMPROVE program, artifact adjusted OC (i.e., 
PM2.5 OC) is simply reported as OC. That is the value 
used to produce OM for haze calculations. For the CSN measurements, 
the OC artifact needed to convert measured OC into PM2.5 
OC is estimated from sampler-specific network-wide field blanks 
(Frank, 2012).
---------------------------------------------------------------------------

    As noted in section VI.D.1.c above, the proposed new secondary 
standard for PM2.5 to address PM-related visibility 
impairment is defined in terms of a PM2.5 visibility index, 
which would use PM2.5 speciation measurement data. The EPA 
proposes that measurements using either the CSN or IMPROVE methods 
\198\ be eligible for use to calculate PM2.5 visibility 
index values. The EPA believes this proposed approach is appropriate 
because the methods for CSN and IMPROVE are well documented \199\ in 
nationally implemented Quality Assurance Project Plans (QAPPs) and 
accompanying Standard Operating Procedures (SOPs) are validated through 
independent performance testing, and because numerous state, local, and 
tribal agencies are already experienced in the use of these methods.
---------------------------------------------------------------------------

    \198\ Appendix C to 40 CFR part 58--Ambient Air Quality 
Monitoring Methodology is where EPA specifies the criteria pollutant 
monitoring methods which must be used at SLAMS and NCore, which are 
a subset of SLAMS.
    \199\ CSN documents are available at: http://www.epa.gov/ttn/amtic/speciepg.html; IMPROVE documents are available at: http://vista.cira.colostate.edu/improve/Data/QA_QC/qa_qc_Branch.htm).
---------------------------------------------------------------------------

    With reference to CSN methods, the EPA is specifically not 
proposing to include testing or performance criteria for approval of 
CSN measurements as FRMs. The EPA believes that the proposed framework 
of using the current, well-documented set of CSN and IMPROVE methods 
provides a nationally consistent way to provide the chemical species 
data used in calculating PM2.5 visibility index values, 
while preserving the flexibility for timely improvements to methods for 
measuring chemical species. Monitoring programs wishing to establish 
methods for chemical speciation in support of the proposed 
PM2.5 visibility index would do so by following the methods 
and SOP's publically available on both the IMPROVE or the EPA (for CSN) 
Web sites.\200\ The EPA solicits comment on this approach to include 
the CSN and IMPROVE measurements by reference and not require that such 
methods be approved as FRMs.
---------------------------------------------------------------------------

    \200\ SOP's for the CSN program are available in Docket number 
EPA-HQ-OAR-2007-0492 and on EPA's Web site at: http://www.epa.gov/ttn/amtic/specsop.html. SOP's for the IMPROVE program are available 
in Docket number EPA-HQ-OAR-2007-0492 and on the IMPROVE Web site 
at: http://vista.cira.colostate.edu/improve/publications/IMPROVE_SOPs.htm.
---------------------------------------------------------------------------

    As discussed in section VII.A.5 above, the calculation of the 
PM2.5 visibility index values would use historic monthly 
average relative humidity data based on a ten-year climatological data 
base. This data base would be based on measurements of relative 
humidity reported through NOAA at routine weather stations and not 
relative humidity measurements specific to the SLAMS stations.

B. Proposed Changes to 40 CFR Part 58 (Ambient Air Quality 
Surveillance)

1. Proposed Terminology Changes
    The EPA proposes to revise several terms associated with 
PM2.5 monitor placement to ensure consistency with other 
NAAQS and to conform with long-standing practices in siting of 
equipment by monitoring agencies.
    The EPA proposes to revoke the term ``community-oriented'' and 
replace it with the term ``area-wide.'' The term ``community-
oriented,'' while used within the description of the design criteria 
for PM2.5, is not defined and has not been used in the 
design criteria for other NAAQS pollutants. Appendix D to 40 CFR part 
58 presents a functional usage of the term where sites at the 
neighborhood and urban scale area are considered to be ``community-
oriented.'' In addition, population-oriented, micro-or middle-scale 
PM2.5 monitoring may also be considered ``community-
oriented'' when determined by the Regional Administrator to represent 
many such locations throughout a metropolitan area. The EPA proposes to 
replace this functional usage of ``community-oriented'' with the term 
``area-wide'' in the text of the PM2.5 network design 
criteria and to define it in 40 CFR 58.1 to provide a more consistent 
usage of this concept throughout appendix D of 40 CFR part 58. The EPA 
proposes that the terminology would read--``Area-wide means all 
monitors sited at neighborhood, urban, and regional scales, as well as 
those monitors sited at either micro- or middle-scale that are 
representative of many such locations in the same CBSA.''
    The EPA proposes to revoke the term ``Community Monitoring Zone'' 
(CMZ) and references to it in 40 CFR part 58. Community monitoring zone 
is currently defined as ``an optional averaging area with established, 
well defined boundaries, such as county or census block, within an MPA 
that has relatively uniform concentrations of annual PM2.5 
as defined by appendix N of 40 CFR part 50 of this chapter. Two or more 
community oriented state and local air monitoring stations (SLAMS) 
monitors within a CMZ that meet certain requirements as set forth in 
appendix N of 40 CFR part 50 may be averaged for making comparisons to 
the annual PM2.5 NAAQS.'' The EPA proposes to revoke this 
term and references to it since, as discussed in section VII.A.2 above, 
the EPA is proposing to eliminate all references to the spatial 
averaging option throughout appendix N.
2. Special Considerations for Comparability of PM2.5 Ambient 
Air Monitoring Data to the NAAQS
    In general, ambient monitors must meet a basic set of requirements 
before the resulting data can be used for comparison to the NAAQS; 
these requirements include the presence and implementation of an 
approved quality assurance project plan, the use of methods that are 
reference, equivalent, or other approved method as described in 
appendix C to 40 CFR part 58, and compliance with the probe and siting 
path criteria as described in appendix E to 40 CFR part 58. While these 
40 CFR part 58 requirements apply to a monitor that provides data for 
comparison to the NAAQS, only in the PM2.5 monitoring 
requirements are additional restrictions prescribed within the 
monitoring rules.\201\ These additional restrictions provide that sites 
must be ``population-oriented'' for comparison to either the 24-hour or 
annual NAAQS, and specifically for comparison to the annual NAAQS, 
sites must additionally be sited to represent area-wide locations. 
There is a related provision that provides for comparing sites at 
smaller scales to the annual NAAQS when the (micro- or middle-scale) 
site collectively identifies a larger region of localized high ambient 
PM2.5 concentration.
---------------------------------------------------------------------------

    \201\ These are referenced in 40 CFR 58.30 (Special 
considerations for data comparisons to the NAAQS).
---------------------------------------------------------------------------

    The inclusion of these provisions in the PM2.5 
monitoring requirements since the 1997 promulgation of the 
PM2.5

[[Page 39008]]

NAAQS and associated monitoring requirements has resulted in 
substantial ambiguity when the EPA and state, local, and tribal 
agencies consider the design of PM2.5 monitoring networks as 
NAAQS are revised as well as how unmonitored locations should be 
treated in modeling exercises.\202\ Accordingly, the EPA proposes to 
revise these particular PM2.5 requirements for consistency 
with long-standing practices in all other NAAQS pollutant monitoring 
networks, and to ensure interpretation of the monitoring rules does not 
cause ambiguity in considering treatment of unmonitored areas. Each of 
these topics and our proposal to revoke or modify the requirements is 
described below.
---------------------------------------------------------------------------

    \202\ Modeling can be associated with either PSD or 
transportation conformity as discussed in sections IX.F and IX.G, 
respectively, below.
---------------------------------------------------------------------------

a. Revoking Use of Population-Oriented as a Condition for Comparability 
of PM2.5 Monitoring Sites to the NAAQS
    The EPA proposes to revoke the requirement that PM2.5 
monitoring sites be ``population-oriented'' for comparison to the 
NAAQS. This requirement is inconsistent with our definition of ambient 
air which the NAAQS employ. The EPA's definition of ambient air is 
specified in 40 CFR 50.1--``Ambient air means that portion of the 
atmosphere, external to buildings, to which the general public has 
access.'' The EPA's definition of ``population-oriented'' is provided 
in 40 CFR 58.1--``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.'' The EPA's intention in proposing to revoke the 
requirement that PM2.5 monitoring sites be ``population-
oriented'' for comparison to the NAAQS is to ensure that the monitoring 
rules do not create an ambiguity in the use of data by having a 
different definition from the definition of ambient air in 40 CFR 50.1 
itself. Also, EPA's proposal to revoke this term in no way changes the 
requirements in the PM2.5 network design criteria, which 
will continue to focus on sites representing ``area-wide'' locations; 
thus continuing to represent locations with population exposure. While 
the use of the term ``population-oriented'' has little effect on how 
data from existing sites are treated (as explained below there are no 
remaining sites designated as not being ``population-oriented''), the 
inclusion of this requirement in the monitoring rules creates 
substantial ambiguity in how to treat potential locations of exposure 
such as in applying modeling across an area. By reverting to the long-
standing definition of ambient air, the EPA will be able to more 
clearly define how to treat potential exposure receptors, regardless of 
whether monitoring exists or not.
    In reviewing the impact that this proposed change might have on the 
nation's PM2.5 monitoring network, the EPA notes that there 
are no remaining sites operating affirmatively as ``non population-
oriented.'' The last known non population-oriented site at Sun Metro in 
El Paso Texas (AQS ID: 48-141-0053), was shut down in October 2010 and 
is in the process of being moved to a nearby neighborhood. While a 
monitoring agency could still set up a new site in any area, including 
one in an area that does not meet the definition of population-
oriented, which the EPA is proposing to revoke, there are other 
monitoring options that may provide more useful information and still 
result in data that are not comparable to the NAAQS; for instance, 
using a chemical speciation network sampler that provides chemical 
species information or continuous PM2.5 monitor that 
provides high time-resolution data, but is not approved as an FEM. Even 
if a monitoring agency wanted to use an FRM, an agency could still 
operate a monitor for up to 24 months as an SPM without any risk of 
data being used for comparison to the NAAQS.
b. Applicability of Micro- and Middle-scale Monitoring Sites to the 
Annual PM2.5 NAAQS
    The EPA is clarifying language used to determine when 
PM2.5 monitoring sites at micro- and middle-scale locations 
are comparable to the annual NAAQS. EPA's intent in clarifying this 
language is to provide consistency and predictability in the 
interpretation of the monitoring regulations to minimize the burden on 
state monitoring programs as they plan and implement their monitoring 
programs. The EPA's current rules, as specified in 40 CFR 58.30, state 
that ``PM2.5 data that are representative, not of area-wide 
but rather, of relatively unique population-oriented micro-scale, or 
localized hot spot, or unique population-oriented middle-scale impact 
sites are only eligible for comparison to the 24-hour PM2.5 
NAAQS. For example, if the PM2.5 monitoring site is adjacent 
to a unique dominating local PM2.5 source or can be shown to 
have average 24-hour concentrations representative of a smaller than 
neighborhood spatial scale, then data from a monitor at the site would 
only be eligible for comparison to the 24-hour PM2.5 
NAAQS.'' The EPA is clarifying language to explicitly state that 
measuring PM2.5 in micro- and middle-scale environments near 
emissions of mobile sources, such as a highway, does not constitute 
being impacted by a ``unique'' source. Mobile sources are rather 
ubiquitous and, as such, there are many locations throughout an urban 
area where elevated exposures could occur. Therefore, any potential 
location for a PM2.5 monitoring site, even micro- and 
middle-scale sites near roadways would be eligible for comparison to 
the annual NAAQS. The EPA's existing definition of middle-scale for 
PM2.5, as specified in appendix D to 40 CFR part 58, already 
states, ``(2) Middle scale--People moving through downtown areas, or 
living near major roadways, encounter particle concentrations that 
would be adequately characterized by this spatial scale. Thus, 
measurements of this type would be appropriate for the evaluation of 
possible short-term exposure public health effects of particulate 
matter pollution. In many situations, monitoring sites that are 
representative of micro- or middle-scale impacts are not unique and are 
representative of many similar situations. This can occur along traffic 
corridors or other locations in a residential district. In this case, 
one location is representative of a number of small scale sites and is 
appropriate for evaluation of long-term or chronic effects. This scale 
also includes the characteristic concentrations for other areas with 
dimensions of a few hundred meters such as the parking lot and feeder 
streets associated with shopping centers, stadia, and office 
buildings.'' With the reference to ``traffic corridors'' and related 
text, the EPA emphasizes that this type of location, which is referred 
to as near-road, should not be considered ``unique.''
    EPA and monitoring agencies already have a process for approving 
PM2.5 monitoring sites as described in the Annual Monitoring 
Network Plan due to the applicable EPA Regional Office by July 1 of 
each year (described in 40 CFR 58.10). This existing process provides 
for identification of sites that are suitable and sites that are not 
suitable for comparison against the annual PM2.5 NAAQS 
(Sec.  58.10(b)(7)). This clarifying language will provide consistency 
between the PM2.5 design criteria described in appendix D to 
40 CFR part 58 and the example provided in the special considerations 
for data comparisons to the NAAQS network design (Sec.  58.30). This 
clarifying

[[Page 39009]]

language will help to ensure a more consistent identification and 
approval of sites, and therefore a reduction in burden to monitoring 
agencies and EPA as annual monitoring network plans are prepared, 
reviewed, public comments are considered, plans are approved and 
implemented, and data are ultimately used.
3. Proposed Changes to Monitoring for the National Ambient Air 
Monitoring System
a. Background
    As described in appendix D to 40 CFR part 58, the ambient air 
monitoring networks must be designed to meet three basic monitoring 
objectives: (a) Provide air pollution data to the general public in a 
timely manner. Data can be presented to the public in a number of 
attractive ways including through air quality maps, newspapers, 
Internet sites, and as part of weather forecasts and public advisories. 
(b) Support compliance with ambient air quality standards and emissions 
strategy development. Data from FRM, FEM, and ARM monitors for NAAQS 
pollutants will be used for comparing an area's air pollution levels 
against the NAAQS. Data from monitors of various types can be used in 
the development of attainment and maintenance plans. SLAMS, and 
especially National Core Monitoring Network (NCore) \203\ station data, 
will be used to evaluate the regional air quality models used in 
developing emission strategies and to track trends in air pollution 
abatement control measures' impact on improving air quality. In 
monitoring locations near major air pollution sources, source-oriented 
monitoring data can provide insight into how well industrial sources 
are controlling their pollutant emissions. (c) Support for air 
pollution research studies. Air pollution data from the NCore network 
can be used to supplement data collected by researchers working on 
health effects assessments and atmospheric processes or for monitoring 
methods development work.
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    \203\ NCore is a multi-pollutant network that integrates several 
advanced measurements for particles, gases and meteorology (U.S. 
EPA, 2011a, Appendix B, section B.4). Measurements required at NCore 
include PM2.5 mass and speciation, PM10-2.5 
mass, ozone, CO, SO2, NO, NOy, and basic 
meteorology.
---------------------------------------------------------------------------

    To support the air quality management work indicated in the three 
basic air monitoring objectives, a network must be designed with a 
variety of types of monitoring sites. Monitoring sites must be capable 
of informing managers about many things including the peak air 
pollution levels, typical levels in populated areas, air pollution 
transported into and outside of a city or region, and air pollution 
levels near specific sources. To summarize some of these sites, here is 
a listing of six general site types: (a) Sites located to determine the 
highest concentrations expected to occur in the area covered by the 
network; (b) sites located to measure typical concentrations in areas 
of high population density; (c) sites located to determine the impact 
of significant sources or source categories on air quality; (d) sites 
located to determine general background concentration levels; and (e) 
sites located to determine the extent of regional pollutant transport 
among populated areas; and in support of secondary standards.
b. Primary PM2.5 NAAQS
    In this section, the EPA proposes to add a near-road component to 
the PM2.5 network design criteria and to clarify the use of 
approved PM2.5 continuous FEMs at SLAMS.
i. Proposed Addition of a Near-road Component to the PM2.5 
Monitoring Network
    The EPA believes that there are gradients in near-roadway 
PM2.5 that are most likely to be associated with heavily 
travelled roads, particularly those with significant heavy-duty diesel 
activity, with the largest numbers of impacted populations in the 
largest CBSAs in the country (Ntziachristos et al., 2007; Ross et al., 
2007; Yanosky et al., 2008; Zwack et al., 2011). To better understand 
the potential health impacts of these exposures, the EPA proposes to 
add a near-road component to the compliance network design for 
PM2.5 monitoring. The EPA believes that by adding a modest 
number of PM2.5 monitoring sites that are leveraged with 
measurements of other pollutants in the near-road environment, a number 
of key monitoring objectives will be supported, including collection of 
NAAQS comparable data in the near-road environment, support for long-
term health studies investigating adverse effects on people, providing 
a better understanding of pollutant gradients impacting neighborhoods 
that parallel major roads, availability of data to validate performance 
of models simulating near-road dispersion, characterization of areas 
with potentially elevated concentrations and/or poor air quality, 
implementation of a multi-pollutant paradigm as stated in the 
NO2 NAAQS proposed rule (74 FR 34442, July 15, 2009), and 
monitoring goals consistent with existing objectives noted in the 
specific design criteria for PM2.5 described in appendix D, 
4.7.1(b) to 40 CFR part 58.
    The monitoring methods that are appropriate for this purpose are an 
FRM, FEM, or ARM. The EPA recognizes that there are limitations in the 
ability of some of these PM methods to accurately measure 
PM2.5 mass due to the incomplete retention of semi-volatile 
material on the sampling medium (U.S. EPA, 2009a, section 3.4.1.1). 
This limitation is relevant to the near-road environment as well as to 
other environments where PM is expected to have semi-volatile 
components. The EPA also recognizes that continuous PM2.5 
FEMs, which provide mass concentration data on an hourly basis, are 
better suited to accomplish the goals of near-road monitoring as they 
will complement the time resolution of the other air quality 
measurements and traffic data collected at the same sites. In this 
regard, particular PM2.5 FEMs are better suited for near-
road monitoring than FRMs. However, filter-based FRMs do offer some 
advantages which may be highly desirable for near-road monitoring, such 
as readily available filters for later chemical analysis such as for 
elemental composition by x-ray fluorescence and BC by transmissometry. 
As a result of these tradeoffs, monitoring agencies are encouraged to 
select one or more PM2.5 methods for deployment at near-road 
monitoring stations that best meet their agencies monitoring objectives 
while ensuring that at least one of those methods is appropriate for 
comparison to the NAAQS (i.e., a FRM, FEM, or ARM). EPA believes that 
by allowing State monitoring agencies to choose the FRM, FEM, or ARM 
method(s) that best fits their needs, whether filter-based or 
continuous, that the data will still be able to meet the objectives 
cited above while ensuring maximum flexibility for the States in the 
operation of their network.
    Additionally, the EPA recognizes that the near-road sites would 
provide a valuable platform for evaluating emerging monitoring 
technologies and for measuring other pollutants besides 
PM2.5 mass to enhance knowledge of exposure in the near road 
environment and to support the characterization and comparison of 
specific method readings in an emission-rich environment. Further, in 
its response to the EPA on a ``Review of the ``Near-road Guidance 
Document--Outline'' and ``Near-road Monitoring Pilot Study Objectives 
and Approach'' (U.S. EPA, 2010i), the CASAC AAMMS cited several other 
measurements that may be useful or potentially linked to health and 
welfare effects such as BC, ultrafine particles,

[[Page 39010]]

and particle size distribution (Russell and Samet, 2010b, pp. xi and 
xii). The EPA agrees with these recommendations and encourages 
monitoring agencies to include these measurements, and others cited in 
the Subcommittee letter, where possible, in addition to the 
PM2.5 mass measurement. The EPA also encourages monitoring 
agencies to explore partnerships with instrument manufacturers and 
researchers to use the sites to evaluate the performance of emerging 
PM2.5 methods in the near-road environment, especially 
potential or current FEMs able to provide temporally resolved data and 
capture the semi-volatile components of PM2.5. Such emerging 
PM2.5 methods could be operated as SPMs to provide 
comparisons to the EPA approved methods supporting compliance to 
advance the understanding of instrument performance in the near-road 
environment. Monitoring agencies are also encouraged to partner with 
instrument manufacturers and researchers to operate monitors able to 
measure other PM properties relevant for the near-road environment 
(e.g., ultrafine particles, BC) to provide additional information about 
exposure to PM in this environment. The EPA is interested in supporting 
monitoring agencies willing to operate and report the data from these 
supplemental monitors. EPA notes that the implementation of additional 
measurements, while encouraged, is completely voluntary to ensure 
maximum flexibility for state monitoring programs. The EPA solicits 
comment on the best way to support such research efforts.
    The EPA believes that requiring a modest network of near-road 
compliance PM2.5 monitors is necessary to provide 
characterization of concentrations in near-road environments. These 
long-term monitors will supplement shorter-term networks operated by 
researchers to support the tracking of long-term trends of near-road 
PM2.5 mass concentrations and other pollutants in near-road 
environments. Therefore, the EPA proposes to require near-roadway 
monitoring of PM2.5 at one location within each CBSA with a 
population of one million persons or greater. The EPA believes that 
this network will be adequate to support the NAAQS since the largest 
CBSAs are likely to have greater numbers of exposed populations, a 
higher likelihood of elevated near-road PM2.5 
concentrations, and a wide range of diverse situations with regard to 
traffic volumes, traffic patterns, roadway designs, terrain/topography, 
meteorology, climate, surrounding land use and population 
characteristics. Given the latest population data available, this 
proposed requirement would result in approximately 52 required near-
road PM2.5 monitors across the country. An indirect benefit 
of this network design is that monitoring agencies in these largest 
CBSAs are more likely to have redundant monitors that could be 
relocated to the near-road environment, reducing costs for equipment 
and ongoing operation.\204\ While only a single PM2.5 
monitor is required within each of the CBSAs, agencies may elect to add 
additional PM2.5 monitoring sites in near-road environments.
---------------------------------------------------------------------------

    \204\ EPA Regional Administrator approval would be required 
prior to the discontinuation of SLAMS monitors, based on the 
criteria described in paragraph 58.14(c) to 40 CFR part 58.
---------------------------------------------------------------------------

    While the EPA recognizes that the location of maximum concentration 
of PM2.5 from roadway sources might differ from the maximum 
location of NO2 or other pollutants, the EPA proposes to 
require that near-road PM2.5 monitors be collocated with the 
planned NO2 monitors. The NO2 network design 
considers multiple factors that are also relevant for PM2.5 
concentrations (e.g., average annual daily traffic and fleet mix by 
road segment) and significant thought and review has gone into its 
design, including pilot studies at two locations, and the development 
of a technical assistance document in conjunction with the affected 
monitoring agencies and the CASAC AAMMS (Russell and Samet, 2010b) to 
support deployment. Further, this collocation will allow multiple 
pollutants to be tracked in the near-road environment. Therefore, while 
there may be limitations to collocating the proposed 52 near-road 
PM2.5 monitors with the NO2 stations that will 
also host CO monitors, on balance, EPA believes this is the most 
efficient and beneficial approach for deployment of this component of 
the network. ThU.S. EPA is seeking to maximize the utility of the 
network while also reducing the burden on monitoring agencies that have 
already put significant effort into designing their near-road stations 
for NO2 and CO.
    The EPA notes that the 52 proposed near-road monitors represent a 
small number of the total approximate 900 operating PM2.5 
monitoring stations across the country. The EPA could consider 
proposing more near-road sites; however, the addition of sites in lower 
population CBSAs is not expected to lead to much if any difference in 
characterization of air quality since the bump in PM2.5 
concentration associated with near-road environments in lower 
population CBSAs, which typically have corresponding less travelled 
roads, is expected to be very small. The EPA could also consider 
proposing multiple sites in larger CBSAs; however, State monitoring 
programs are already working towards representative near-road 
monitoring stations and there is a synergistic value in ensuring these 
measurements are collocated with multiple measurements to serve the 
monitoring objectives noted above. Since EPA has already finalized 
requirement of CO monitoring at near-road stations in CBSAs with a 
population of 1 million or more at sites that are collocated with 
NO2, there would be less value in requiring any more than 52 
PM2.5 monitors as any more stations will not have CO for use 
in multi-pollutant monitoring objectives (e.g., health studies and 
model evaluation). Also, EPA wants to ensure there is minimal 
disruption to the existing network and moving more than the proposed 52 
PM2.5 monitors may lead to losing some valuable existing 
PM2.5 stations. Therefore, EPA believes the 52 proposed near 
road monitoring stations represent the least burdensome, but most 
useful number of near-road monitoring stations to meet the monitoring 
objectives cited above for deployment across the country.
    Ideally, near-road sites would be located at the elevation and 
distance from the road where maximum concentration of PM2.5 
occurs in this environment, and within reasonable proximity to an area-
wide PM2.5 compliance monitoring site at which a similar PM 
monitor is used (i.e., for comparison purposes). Although the EPA is 
not proposing that the near-road PM2.5 monitors be located 
within a specific distance of area-wide sites, monitoring agencies are 
encouraged to consider that a near-road site selected in accordance 
with monitoring requirements and also located in proximity to a robust 
area-wide site, such as an NCore station, would provide useful 
information in characterizing the near-road contribution to multiple 
pollutants, including PM2.5.
    The timeline to implement the proposed near-road PM2.5 
monitors should be as minimally disruptive to on-going operations of 
monitoring agency programs as possible, while still meeting the need to 
collect for near-road PM2.5 data in a timely fashion. Since 
the near-road PM2.5 monitors are proposed to be collocated 
with the emerging near-road NO2 network that is scheduled to 
be operational by January 1, 2013, the EPA believes it is appropriate 
to wait

[[Page 39011]]

until after the near-road NO2 network is established before 
implementing the near-road PM2.5 monitors. Therefore, the 
EPA proposes that each PM2.5 monitor planned for collocation 
with a near-road NO2 monitoring site be implemented no later 
than January 1, 2015. The EPA believes this proposed deadline provides 
an appropriate amount of time for monitoring agencies to select 
existing PM2.5 monitors suitable for relocation, receive EPA 
approval, and physically relocate the PM2.5 monitor to the 
near-road NO2 site. Based on this proposed timeline, 
complete data sets (i.e., 3-years representing 2015-2017), from 
PM2.5 monitors in the near-road environment would be 
available to calculate site-level design values in 2018.
    In summary, the EPA proposes to specifically include a near-road 
component in the PM2.5 network design criteria for CBSA's of 
1 million persons or greater, with at least one PM2.5 
monitor collocated with a near-road NO2 and CO monitors by 
January 1, 2015. EPA believes that the 52 proposed PM2.5 
monitors to be collocated with NO2 and CO monitors in the 
near-road environment represent the minimal number of sites needed to 
characterize PM2.5 in representative near road environments 
of large population CBSA's. EPA believes that a number of 
PM2.5 monitors can be moved from single pollutant locations 
to multi-pollutant locations in the near-road environment, thus 
encouraging efficiencies in operation by monitoring agencies and 
reducing the burden of continuing to support some of the existing 
single pollutant PM2.5 stations. The EPA solicits comment on 
this approach, especially the proposed network design requirements; any 
alternative strategies that would provide comparable long-term 
characterization of PM2.5 in area-wide locations of maximum 
concentration in the absence of a specific near-road compliance 
requirement for monitoring of PM2.5; priorities for the 
collection of supplemental data at a small subset of near-road 
monitoring sites to enhance knowledge of particle exposure (e.g., non-
compliance SPMs); and the interest of monitoring agencies (or other 
parties) in the collection of supplemental (e.g., non-compliance) 
measurements relevant for the near-road environment.
ii. Use of PM2.5 Continuous FEMs at SLAMS
    The EPA proposes that each agency specify their intention to use or 
not use data from continuous PM2.5 FEMs that are eligible 
for comparison to the NAAQS as part of their annual monitoring network 
plan due to the applicable EPA Region Office by July 1 each year. The 
proposal also provides that the EPA Regional Administrator would be 
responsible for approving annual monitoring network plans where 
agencies have provided a recommendation that certain PM2.5 
FEMs be considered ineligible for comparison to the NAAQS.
    In 2006, the EPA finalized new performance criteria for approval of 
continuous PM2.5 monitors as either Class III FEMs or ARMs. 
The EPA has already approved six PM2.5 continuous FEMs and 
there are nearly 200 of these monitors already operating in State, 
local, and Tribal networks. Monitoring agencies have been deploying and 
field-testing these units over the last couple of years and the EPA 
recently compiled an assessment of the FEM data in relationship to 
collocated FRMs (Hanley and Reff, 2011; U.S. EPA, 2011a, pp. 4-50 to 4-
51). As described in section VI.D.1.a.iii above, the EPA found that 
some sites with continuous PM2.5 FEMs have an acceptable 
degree of comparability with collocated FRMs, while others had poor 
data comparability that would not meet the performance criteria used to 
approve the FEMs (71 FR 61285-61286, Table C-4, October 17, 2006). The 
EPA is encouraging use of the FEM data from those sites with acceptable 
data comparability including for purposes of comparison to the NAAQS. 
For sites with unacceptable data comparability, the EPA is working 
closely with the monitoring committee of the NACAA, instrument 
manufacturers, and monitoring agencies to document best practices on 
these methods to improve the comparability and consistency of resulting 
data wherever possible. The EPA believes that the performance of many 
of these continuous PM2.5 FEMs at locations with poor data 
comparability can be improved to a point where the acceptance criteria 
noted above can be met.
    Given the varying data comparability of continuous PM2.5 
FEMs noted above, we believe that a need exists for flexibility in the 
approaches for how such data are utilized, particularly for the 
objective of determining NAAQS compliance. Accordingly, we propose that 
monitoring agencies address the use of data from PM2.5 
continuous FEMs in their annual monitoring network plans due to the 
applicable EPA Regional Office by July 1 of each year for any cases 
where the agency believes that the data generated by PM2.5 
continuous FEMs in their network should not to be compared to the 
NAAQS. The annual network plans would include assessments such as 
comparisons of continuous FEMs to collocated FRMs, and analyses of 
whether the resulting statistical performance would meet the 
established approval criteria. Based on these quantitative analyses, 
monitoring agencies would have the option of requesting that data from 
continuous FEMs be excluded from NAAQS comparison; however, these data 
could still be utilized for other objectives such as AQI reporting.
    The issue exists of whether such data use provisions should be 
prospective only (i.e., future NAAQS comparability excluded based on an 
analysis of recent past performance) or a combination of retrospective 
and prospective (i.e., the implications of unacceptable FEM performance 
impacting usage of previously collected data as well as future data). 
The EPA believes that in most cases, monitoring agencies should be 
restricted to addressing prospective data issues to provide stability 
and predictability in the long-term PM2.5 data sets used for 
supporting attainment decisions. However in the first year after this 
proposed option would become effective, we believe it is appropriate to 
provide monitoring agencies with a one-time opportunity to review 
already reported continuous PM2.5 FEM data and request that 
data with unacceptable performance be restricted (retrospectively) from 
NAAQS comparability. Accordingly, in the first year after this rule 
becomes effective, we propose that monitoring agencies have the option 
of requesting in their annual monitoring network plans that a portion 
or all of the existing continuous PM2.5 FEM data, as 
applicable, as well as future data, be restricted from NAAQS 
comparability for the period of time that the plan covers.\205\ Annual 
monitoring network plans in subsequent years would only need to cover 
new data for the period of time that the plan covers.
---------------------------------------------------------------------------

    \205\ Data from any PM2.5 monitor being used to meet 
minimum monitoring requirements could not be restricted from NAAQS 
comparability.
---------------------------------------------------------------------------

    As noted above, in cases where an agency is operating a 
PM2.5 continuous FEM that is not meeting the expected 
performance criteria used to approve the FEMs (71 FR 61285 to 61286, 
Table C-4, October 17, 2006) when compared to their collocated FRMs, an 
agency can recommend that the data not be used for comparison to the 
NAAQS. However, all required SLAMS would still be required to have an 
operating FRM (or other well performing FEM, as evidenced by a prior 
collocation with an FRM) to ensure a data record is available for 
comparison to the NAAQS. In cases where a PM2.5 continuous 
FEM was not

[[Page 39012]]

meeting the expected performance criteria, and the Regional 
Administrator has approved that the FEM data will not be considered 
eligible for comparison to the NAAQS, the data would still be required 
to be loaded to AQS; however, these data would be stored separately 
from data used for comparison to the NAAQS.
    The goal of proposing to allow monitoring agencies the opportunity 
to recommend not having data from PM2.5 continuous FEMs as 
comparable to the NAAQS is to ensure that only high quality data (i.e., 
data from FRMs which are already well established and new continuous 
FEMs that meet the performance criteria used to approve FEMs when 
compared to collocated FRMs operated in each agencies network) are used 
when comparing data to the PM2.5 NAAQS. Under the current 
monitoring regulations, a monitoring agency can identify a 
PM2.5 continuous FEM as an SPM, which allows the method to 
be operated for up to 24 months without its data being used in 
comparison to the NAAQS. While 24 months should be sufficient time to 
operate the method across all seasons, assess the data quality, and in 
some cases resolve operational issues with the instrument, it may still 
leave some agencies with methods whose data are not sufficiently 
comparable to data from their FRMs. In these cases there may be a 
disincentive to continue operating the PM2.5 continuous FEM, 
especially in networks where the monitoring data is near the level of 
the NAAQS. With the proposed provision where a monitoring agency can 
recommend not having data from PM2.5 continuous FEMs as 
comparable to the NAAQS, a monitoring agency can continue to operate 
their PM2.5 continuous FEM to support other monitoring 
objectives (e.g., diurnal characterization of PM2.5, AQI 
forecasting and reporting), while working through options for improved 
data comparability.
    The EPA believes that an assessment of FEM performance should 
include several elements based on the original performance criteria. 
The Agency also believes that certain modifications to the performance 
criteria are appropriate in recognition of the differences between how 
monitoring agencies operate routine monitors versus how instrument 
manufacturers conduct required FRM and FEM testing protocols. The 
details below summarize these issues. The EPA proposes to use the 
performance criteria used to approve the FEMs (71 FR 61285 to 61286, 
Table C-4, October 17, 2006) for those agencies that recommend not 
having data from PM2.5 continuous FEMs as comparable to the 
NAAQS. To accommodate how routine monitoring networks operate, the EPA 
proposes that agencies seeking to demonstrate insufficient data 
comparability in an assessment base the analysis mainly on collocated 
data from FRMs and continuous FEMs at monitoring stations in their 
network. The EPA does not believe it is practical to utilize the 
requirement in table C-4 of 40 CFR part 53 for having multiple FRMs and 
FEMs at each site since such arrangements are not typically found in 
monitoring agency networks. Accordingly, the requirement for assessing 
intra-method replicate precision would be inapplicable. Another 
consideration is the range of 24-hour data concentrations, for 
instance, the performance criteria in table C-4 of 40 CFR part 53, 
provides for an acceptable concentration range of 3 to 200 [mu]g/m\3\. 
However, the EPA notes that during an evaluation of data quality from 
two FEMs (U.S. EPA, 2011a, p. 4-50), the Agency found that including 
low concentration data were helpful for understanding whether an 
intercept or slope was driving a potential bias in an instrument. 
Therefore, the EPA proposes that agencies may include low concentration 
data (i.e., below 3 [mu]g/m\3\) for purposes of evaluating the data 
comparability of continuous FEMs. With regard to the minimum number of 
samples needed for the assessment, the EPA notes that a minimum of 23 
sample pairs are specified for each season in table C-4 of 40 CFR part 
53. Having 23 sample pairs per season should be easily obtainable 
within one year for sites with a FRM operating on at least a 1 in 3-day 
sample frequency and we propose that this requirement be applicable to 
the assessments being discussed here. For sites on a one in 6-day 
sampling frequency, two years of data may be necessary to meet this 
requirement. The EPA recognizes that it would be best to assess the 
data based on the most recently available information; however, having 
data across all seasons in multiple years will provide a more robust 
data set for use in the data comparability assessment; therefore, the 
EPA proposes that data quality assessments be permitted to utilize up 
to the last three years of data for purposes of recommending not having 
data from PM2.5 continuous FEMs as comparable to the NAAQS.
    The EPA recognizes that only a portion of continuous 
PM2.5 FEMs will be collocated with FRMs, and it would be 
impractical to restrict the applicability of data comparability 
assessments to only those sites that had collocated FRM and FEM 
monitors. In these cases, the monitoring agency will be permitted to 
group the sites that are not collocated with an FRM with another 
similar site that is collocated with an FRM for purposes of 
recommending that the data are not eligible for use in comparison to 
the NAAQS. Monitoring agencies may recommend having PM2.5 
continuous FEM data eligible for comparison to the NAAQS from locations 
where the method has been demonstrated to provide acceptable data 
comparability, while also recommending not having it eligible in other 
types of areas where the method has not been demonstrated to meet data 
comparability criteria. For example, a rural site may be more closely 
associated with aged particles where volatilization issues are 
minimized resulting in acceptable data comparability between filter-
based and continuous methods, while a highly populated urban site with 
fresh emissions may result in higher readings on the PM2.5 
continuous FEM that would not meet the expected performance criteria as 
compared to a collocated FRM. In all cases where a monitoring agency 
chose to group sites for purposes of identifying a subset of 
PM2.5 continuous FEMs that would not be comparable to the 
NAAQS, the assessment submitted with the annual monitoring network plan 
would have to provide sufficient detail to support the identification 
of which combinations of method and sites would, and would not, be 
comparable to the NAAQS, as well as the rationale and quantitative 
basis for the grouping and recommendation.
    The EPA solicits comment on all aspects of this proposed approach 
of allowing monitoring agencies to recommend that PM2.5 
continuous FEM data should not be compared to the NAAQS, when 
demonstrated to not meet the performance criteria used to approve FEMs 
based on data in their own network, and as appropriate, approved by the 
EPA Regional Administrators as ineligible for comparison to the NAAQS.
    c. Revoking PM10-2.5 Speciation Requirements at NCore 
Sites
    The EPA issued revisions to the Ambient Air Monitoring Regulations 
(40 CFR parts 53 and 58) on October 17, 2006 (71 FR 61236). In the 2006 
final rule, the EPA required that PM10-2.5 speciation be 
conducted at NCore multi-pollutant monitoring stations by January 1, 
2011. PM10-2.5 speciation at NCore was intended to support 
further research in the understanding of the chemical composition and 
sources of PM10, PM10-2.5 and PM2.5 at 
a variety of urban and non-urban NCore locations.

[[Page 39013]]

Subsequent to the completion of the 2006 final monitoring rule, several 
technical issues were raised concerning the readiness of 
PM10-2.5 speciation monitoring methodologies to support such 
a nation-wide deployment strategy. Based on these issues and as 
explained in detail below, the EPA proposes to revoke the requirement 
for PM10-2.5 speciation monitoring as part of the current 
suite of NCore monitoring requirements. The requirement to monitor for 
PM10-2.5 mass (total) at all NCore multi-pollutant sites 
remains. Monitoring was commenced on January 1, 2011 as part of the 
nationwide startup of the NCore network (U.S. EPA, 2011a, p. 1-15).
    As part of the process to further define appropriate techniques for 
PM10-2.5 speciation monitoring, a public consultation with 
the CASAC AAMMS on monitoring issues related to PM10-2.5 
speciation was held in February 2009 (74 FR 4196, January 23, 2009). At 
that time, the subcommittee noted the lack of consensus on appropriate 
sampling and analytical methods for PM10-2.5 speciation and 
expressed concern that the Agency's 2006 commitment to launch the 
PM10-2.5 monitoring network without sufficient time to 
analyze the data from a planned pilot project was premature (Russell, 
2009). Based on the noted lack of consensus on PM10-2.5 
speciation monitoring techniques, the Agency did plan and implement a 
small pilot monitoring project to evaluate the available monitoring and 
analytical technologies and supplement the PM10-2.5 
speciation measurements that have mostly been done as part of other 
research efforts. The EPA expects that this field study will address 
the issues needed to develop a more robust, long-term 
PM10-2.5 speciation monitoring plan.
    The EPA pilot monitoring project will be completed in 2011, with 
plans to analyze the data and prepare a final report on findings and 
recommendations in 2012. At that time, the EPA will consider what 
PM10-2.5 speciation sampling techniques, analytical 
methodologies, and network design strategies would be most appropriate 
as part of a potential nation-wide monitoring deployment. Such a 
deployment could be based on the NCore multi-pollutant framework, or 
some other strategy that targets such measurements in areas with higher 
levels of coarse particles. This latter type of strategy would be 
consistent with CASAC AAMMS members written comments that not all NCore 
sites would be adequate for PM10-2.5 speciation and that 
more flexibility in PM10-2.5 speciation network design would 
allow for a geographically diverse network to support health studies 
and research (Russell, 2009).
    The EPA may consider reintroducing some PM10-2.5 
speciation monitoring requirements in a subsequent monitoring 
rulemaking or as part of a future review of the PM NAAQS. Until that 
time, the EPA believes it is appropriate to propose to revoke the 
current set of PM10-2.5 speciation monitoring requirements. 
The EPA solicits comment on this proposed revision to monitoring 
requirements.
d. Measurements for the Proposed New PM2.5 Visibility Index 
NAAQS
    The EPA proposes requirements for sampling of PM2.5 
chemical speciation in states with large CBSAs. The CSN has been 
operating for approximately 10 years and as described earlier in this 
proposal already supports a number of important monitoring objectives. 
Since the CSN network is already well established in states with large 
CBSAs, the EPA believes that using the data from these existing sites 
as an input for calculating PM2.5 visibility index values 
will help ensure that the network can continue to support existing 
objectives, while also supporting the proposed new secondary standard.
    To ensure the CSN network can support its existing network 
objectives while also supporting the proposed new secondary 
PM2.5 visibility index standard (section VI.F), the EPA 
proposes that each state with a CBSA over 1 million have measurements 
based on the methods in CSN (or IMPROVE), as discussed in section 
VII.A.5 above, in at least one of its CBSAs. For states with urban or 
suburban NCore Stations, their existing CSN measurements at all NCore 
sites would be appropriate to meet this proposed requirement. For 
states with multiple high population CBSAs, the EPA proposes that each 
CBSA with a population over 2.5 million people be required to have CSN 
measurements. The EPA does not believe it would be appropriate to 
require multiple cities in the same state to have CSN measurements for 
purposes of supporting the proposed new secondary PM2.5 
visibility index standard when these cities have relatively smaller 
populations (i.e., less than 2.5 million people) as the chemical 
species data may be similar across cities in the same state. The 
exception to this will be the most highly populated states and cities, 
which are either already covered by requirements for multiple NCore 
stations or the proposed population threshold of 2.5 million people. 
For example, the following high population states are already required 
to have multiple NCore stations: California, Florida, Illinois, 
Michigan, New York, North Carolina, Ohio, Pennsylvania, and Texas. The 
EPA also proposes that states be allowed to request alternative CBSAs 
to locate their CSN measurements, when the alternative location is 
better suited to support providing data for multiple monitoring 
objectives, including for the proposed new secondary PM2.5 
visibility index standard. For example, in some cases a large CBSA with 
a marine influence may have relatively cleaner air than a smaller 
inland CBSA in the same state with a lower population. In these cases, 
states may request an alternative location for their CSN measurements. 
The EPA solicits comment of these proposed requirements and on 
alternative requirements for CSN measurements to support the proposed 
new secondary PM2.5 visibility index standard.
    The EPA proposes that the network design criteria for CSN 
measurements focus on area-wide locations that are generally 
representative of long distances throughout a CBSA. For most CBSAs, 
this will mean that the existing inventory of CSN measurements can be 
used where the location of the sampling equipment is at an NCore 
station or other station(s) sited at the neighborhood or urban scale of 
representation. The EPA points out that while the existing 
PM2.5 network design criteria established to support the 
primary PM2.5 NAAQS focuses on the area-wide locations of 
expected maximum concentration, there would not necessarily be the same 
focus for the proposed new secondary PM2.5 visibility index 
standard. One reason for this difference is that for urban visibility, 
we are interested in the impact of visibility degradation over as 
representative a location as possible as the impact of the aerosol is a 
function of an entire site path and not just one monitoring location 
within a CBSA. Also, the EPA is interested in leveraging as much of the 
existing inventory of CSN and IMPROVE measurements operating in CBSAs 
where they can support the proposed new secondary PM2.5 
visibility index standard.
    The EPA considered the issue of siting measurements to support a 
new secondary standard to address PM-related visibility impairment 
during a consultation with the CASAC AAMMS (75 FR 4069, January 26, 
2010). In its letter to the EPA, the CASAC AAMMS stated that ``the 
Subcommittee strongly favored collocation of extinction measurements 
with PM mass, PM speciation, and precursor gas measurements, 
identifying continuous

[[Page 39014]]

PM mass and speciation measurements as being of particular value. NCore 
multi-pollutant monitoring sites were identified as worth considering 
even though these would not necessarily capture maximum concentrations 
and visibility impairment in an urban area'' (Russell and Samet, 2010a, 
p. 18). The EPA notes that the Subcommittee also identified that 
``[t]here was general support for making public communication an 
important consideration in network design, for example by selecting a 
monitoring site that can be associated with a vista that is recognized 
by a significant fraction of the local population'' (Russell and Samet, 
2010a, p. 18). While the EPA agrees that siting associated with a 
recognizable vista would be a useful consideration for establishing new 
sites, the EPA does not believe it would be appropriate to include such 
a requirement for cities with existing sites as this may disrupt the 
use of data to meet other important monitoring objectives. The EPA also 
notes existing long-standing public communication tools such as the 
``Haze-Cam'' network are already well suited for public communications 
of important vistas.\206\ In addition to collocation with several 
important measurements at NCore as cited by the Subcommittee, the EPA 
is also encouraging monitoring agencies to add other important 
measurements such as commercially available technologies for light 
absorption and light scattering; however, the EPA does not believe 
these technologies should be specified by regulation.
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    \206\ See http://www.hazecam.net/.
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    Since EPA's proposal to require CSN (or IMPROVE) sampling is 
consistent with a network that is largely already in place, there is no 
expectation new sites will be needed. However, from time to time there 
is a disruption of sampling due to loss of a sites lease agreement or 
other circumstances. Therefore, for any state that does not have a 
minimally required CSN (or IMPROVE) set of measurements in place, the 
EPA proposes that these measurements be in place and sampling by 
January 1, 2015.
4. Proposed Revisions to the Quality Assurance Requirements for SLAMS, 
SPMs, and PSD
a. Quality Assurance Weight of Evidence
    The EPA believes that the process by which monitoring organizations 
and the EPA use the appendix A of 40 CFR part 58 regarding quality 
assurance requirements in regulatory decision making needs to be 
articulated. Prior interpretations of appendix A have led to 
disqualification of data for noncompliance with a particular appendix A 
requirement. The proposed language described below, provides the 
interpretation the EPA would use moving forward.
    The appendix A to 40 CFR part 58 requirements represent a portion 
of the quality control activities that are implemented by monitoring 
organizations to control data quality. The EPA believes that while it 
is essential to require a minimum set of checks and procedures in 
appendix A to support the successful implementation of a quality 
system, the success or failure of any one check or series of checks 
does not preclude the EPA from determining that data are of acceptable 
quality to be used for regulatory decision-making purposes. The EPA 
proposes to use a weight-of-evidence approach for determining whether 
the quality of data is appropriate for regulatory decision-making 
purposes. Furthermore, the suitability of data for any regulatory 
purpose also relies, in part, on several other quality-related 
requirements found elsewhere in 40 CFR part 58. These requirements 
include air monitoring methodology (appendix C), network design 
criteria (appendix D) and network design plans for SLAMS, probe siting 
criteria (appendix E), the reporting of data to AQS, data completeness, 
and data certification by the reporting organization. This weight of 
evidence approach recognizes that all measurement systems have 
uncertainty and there are numerous factors that can affect data quality 
at a particular monitoring site. The specific appendix A criteria are 
designed to provide a quantification of this uncertainty, support a 
framework for assessing such uncertainty against known data quality 
goals and to support corrective actions when necessary to control 
uncertainty back to acceptable levels. Accordingly, the EPA proposes 
additional wording in appendix A to clarify the role that appendix A 
generated data quality indicators have in the overall quality system 
that supports ambient air monitoring activities.
b. Quality Assurance Requirements for the Chemical Speciation Network
    The EPA proposes to include requirements for flow rate 
verifications and flow rate audits for the PM2.5 CSN. These 
audits are currently being performed so, although they will be 
considered a new requirement, they are not new implementation 
activities. In addition, the CSN already includes six collocated sites 
which the EPA proposes to include in the 40 CFR part 58 appendix A 
requirements. The EPA proposes that PSD sites would not be required to 
collocate a second set of instruments for speciated PM2.5 
mass monitoring.
    The EPA performed an assessment of measurement uncertainty from the 
collocated CSN and IMPROVE stations using the proposed visibility index 
(Papp, 2012) and concluded that the current data quality goals for the 
PM2.5 mass can be achieved for the proposed calculated light 
extinction indicator.
c. Waivers for Maximum Allowable Separation of Collocated 
PM2.5 Samplers and Monitors
    The EPA proposes to allow waivers for the maximum allowable 
distance associated with collocated PM2.5 samplers and 
monitors. As described in section VIII.A.1 of this proposal, the EPA 
has already approved six Class III PM2.5 continuous FEMs. 
Several of these approved FEMs are required to be installed in a 
shelter with sufficient control of heating and air conditioning to 
ensure stable operation of the instrument. In many cases monitoring 
agencies are installing these approved continuous FEMs in shelters 
where they already have gas analyzers operating. Some agencies operate 
filter-based samplers (e.g., PM2.5 FRMs) on top of their 
shelter, while others operate platforms next to their shelter. In 
either case, ensuring PM2.5 continuous FEMs and 
PM2.5 FRMs meet collocation requirements (i.e., 1 to 4 
meters for PM2.5 samplers with flow rates of less than 200 
liters/minute) can be challenging, since in some cases multiple 
instruments, some installed in the shelter and some installed on a 
platform, are being sited at the same station.
    The EPA believes that maintaining the current requirement of 1 to 4 
meters for PM2.5 samplers with flow rates of less than 200 
liters/minute is useful since it ensures consistency with long-standing 
practices of collocation and ensures that any air drawn through 
collocated samplers is well within the operational precision of the 
instruments. However, the EPA also believes that instruments spaced 
farther apart could also be within the operational precision of the 
instruments, especially at sites located at larger scales of 
representation (e.g., neighborhood scale and larger). The EPA already 
defines a collocated scale in its document ``Guidance for Network 
Design and Optimum Site Exposure for PM2.5 and 
PM10 (U.S. EPA, 1997). In this document, the EPA defines a 
collocated scale as 1 to 10 meters. The EPA believes that almost all 
agencies would

[[Page 39015]]

be able to site collocated PM samplers and monitors within 10 meters. 
Therefore, the EPA proposes to allow waivers, when approved by the EPA 
Regional Administrator, for collocation of PM2.5 samplers 
and monitors of up to 10 meters so long as the site is at a 
neighborhood scale or larger. The EPA solicits comment on this proposed 
change to allow waivers of the maximum allowable distance for 
collocated PM2.5 samplers and monitors.
5. Proposed Probe and Monitoring Path Siting Criteria
a. Near-Road Component to the PM2.5 Monitoring Network
    The EPA proposes that the probe and siting criteria for the near-
road component to the PM2.5 monitoring network design follow 
the same probe and siting criteria as the NO2 near-road 
monitoring sites. These requirements would provide that the monitoring 
probe be sited ``* * * as near as practicable to the outside nearest 
edge of the traffic lanes of the target road segments; but shall not be 
located at a distance greater than 50 meters, in the horizontal, from 
the outside nearest edge of the traffic lanes of the target road 
segment'' (section 6.4 of appendix E to 40 CFR part 58). The EPA 
solicits comment on this proposed probe and siting criteria for the 
proposed near-road component to the PM2.5 monitoring network 
design.
b. CSN Network
    The EPA proposes to extend the existing probe and monitoring path 
siting criteria described in appendix E to 40 CFR part 58 for 
PM2.5 FRMs and FEMs to the CSN measurements. The EPA 
believes that monitoring agencies are already following the probe and 
siting criteria for PM2.5 when conducting CSN measurements; 
that is, at neighborhood, urban, and regional scale sites the probe 
height must be 2 to 15 meters above ground level. All other aspects of 
the existing PM2.5 probe and siting criteria would also 
apply including minimum distances from horizontal supporting structures 
(i.e., greater than 2 meters) and minimum distance to the drip-line of 
a tree (i.e., greater than 10 meters). The IMPROVE program SOP 
(IMPROVE, 1996) on site selection already provides for meeting probe 
and siting criteria described in Appendix E. The EPA solicits comment 
on extending the existing probe and siting criteria for PM to the 
speciation measurements used to support the proposed new secondary 
PM2.5 visibility index standard.
c. Reinsertion of Table E-1 to Appendix E
    The EPA is proposing to reinsert table E-1 to appendix E of 40 CFR 
part 58. This table presents the minimum separation distance between 
roadways and probes or monitoring paths for monitoring neighborhood and 
urban scale ozone (O3) and oxides of nitrogen (NO, 
NO2, NOX, NOY). This table was 
inadvertently removed during a previous CFR revision process. The EPA 
is utilizing this proposed rule to reinsert this table, unchanged from 
its prior iteration, back into the CFR.
6. Additional Ambient Air Monitoring Topics
a. Annual Monitoring Network Plan and Periodic Assessment
    In October of 2006, the EPA finalized new requirements for each 
state, or where applicable, local agency to perform and submit to their 
EPA Regional Offices an Assessment of the Air Quality Surveillance 
System (40 CFR 58.10). This assessment is required every five years. 
The first required five-year assessments were submitted to EPA Regional 
Offices on or before July 1, 2010. The assessments are intended to 
provide a comprehensive look at each monitoring agencies ambient air 
monitoring network to ensure that the network is meeting the minimum 
monitoring objectives defined in appendix D to 40 CFR part 58, whether 
new sites are needed, whether existing sites are no longer needed and 
can be terminated, and whether new technologies are appropriate for 
incorporation into the ambient air monitoring network.\207\
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    \207\ The EPA provides a link to these assessments on EPA's Web 
site at: http://www.epa.gov/ttn/amtic/plans.html. A detailed 
description of the requirements for the assessments is described in 
40 CFR 58.10.
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    Since each state has completed their first required five-year 
assessment, and several monitoring rule requirements have either been 
added or changed since this requirement was added in 2006, the EPA 
thinks it is appropriate to review this requirement and solicit comment 
on any possible changes the EPA should consider that may improve the 
usefulness of the assessments. Specifically, the EPA solicits comment 
on ways to either streamline or add additional criteria for future 
assessments. Even if no changes to the requirements are recommended by 
any commenters, the EPA is especially interested in learning from 
monitoring agencies that may have ideas on how to improve future 
assessments. Such ideas may not necessarily have to be incorporated 
into regulation, but could be referred to in our guidance on network 
assessments (U.S. EPA, 2007b).
    The EPA proposes to remove references to ``community monitoring 
zones'' and ``spatial averaging'' in the annual monitoring network 
plans due to EPA Regional Offices by July 1 of each year. The Agency 
proposes to remove these references since, as discussed in section 
VII.A.2 above, the EPA is proposing to remove all references to the 
spatial averaging option throughout 40 CFR part 50 appendix N. 
Consistent with these changes, the EPA also proposes to remove 
references to community monitoring zones under the annual monitoring 
network plans described in 40 CFR 58.10.
b. Operating Schedules
    The EPA generally requires PM2.5 SLAMS to operate on at 
least a 1-day-in-3 sampling schedule, unless a reduced sampling 
frequency is approved such as might be the case with a site that has a 
collocated continuous operating PM2.5 monitor.\208\ However, 
in the 2006 monitoring rule amendments, the EPA finalized a new 
requirement for the operating schedule of PM2.5 SLAMS sites 
(40 CFR 58.12). The new requirement stated that sites with a design 
value within plus or minus five percent of the 24-hour PM2.5 
NAAQS must have an FRM or FEM operating on a daily sampling schedule. 
This requirement was included to minimize any statistical error 
associated with the form of the 24-hour PM2.5 NAAQS (i.e., 
the 98th percentile). In section III.F, the Administrator is proposing 
to revise the level of the primary annual PM2.5 NAAQS. 
Accordingly, she is now considering whether this proposed change should 
result in any changes to sampling frequency requirements.
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    \208\ All NCore stations must operate on at least a one-in-three 
day sample frequency for filter-based PM sampling.
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    The EPA had previously considered how sample frequency affects the 
Data Quality Objectives in a consultation with the CASAC AAMMS in 
September of 2005 (70 FR 51353 to 51354, August 30, 2005). As a result 
of that consultation, the EPA proposed (71 FR 2710 to 2808, January 17, 
2006) and finalized (71 FR 61236 to 61328, October 17, 2006) changes to 
the sample frequency requirements as part of the monitoring rule 
changes in 2006. In that work, the EPA demonstrated that having a 
higher sample count is generally more useful to minimize uncertainty 
for a percentile standard than an annual average. Given the proposed 
strengthening of the primary annual

[[Page 39016]]

PM2.5 NAAQS and the known burden of performing daily 
sampling using the filter-based samplers that are still a mainstay in 
monitoring agency networks, the issue of needing daily sampling for 
sites that have design values close to the level of the 24-hour 
PM2.5 standard should be reconsidered if the site already 
has a design value above the level of the primary annual 
PM2.5 NAAQS.
    In a related issue, since the EPA finalized the requirement for 
daily sampling at sites within 5 percent of the 24-hour 
PM2.5 NAAQS in 2006, there has been confusion over the 
procedures for adjusting sample frequencies, where necessary, to 
account for variations in year-to-year design values. Therefore, the 
EPA proposes to revise this requirement in the following ways: (1) The 
EPA proposes that monitors would only be required to operate on a daily 
schedule if their 24-hour design values are within five percent of the 
24-hour PM2.5 NAAQS and the site has a design value that is 
not above the level of the annual PM2.5 NAAQS. (2) The EPA 
proposes that review of data for purposes of determining applicability 
of this requirement at a minimum be included in each agency's annual 
monitoring network plan described in 40 CFR 58.10 based on the three 
most recent years of ambient data that were certified as of the May 1 
deadline. However, monitoring agencies may request changes to sample 
frequency at any time of the year by submitting such a request to their 
applicable EPA Regional Office. Changes in sampling frequency are 
expected to take place by January 1 of the following year. Increased 
sampling is expected to be conducted for at least three years, unless a 
reduction in sampling frequency has been approved in a subsequent 
annual monitoring network plan or otherwise approved by the Regional 
Administrator. The EPA solicits comment on these proposed changes to 
the required operating schedule for PM2.5 SLAMS.
    c. Data Reporting and Certification for CSN and IMPROVE Data
    The EPA solicits comment on minor changes to reporting and 
certification of data associated with CSN and IMPROVE data. The 
chemical analyses of filters associated with CSN measurements results 
in reporting of data that are usually within three months of the sample 
collection. This fits within the existing reporting requirements for 
most ambient air measurements that data be reported within 90 days past 
the end of the previous quarterly reporting period (40 CFR 58.15). 
However, some agencies also use IMPROVE or their own internal 
laboratory for processing of chemical analyses. IMPROVE is known to 
validate and report its data on a schedule that is approximately 12 to 
18 months after sample collection. At least one state laboratory 
continues to provide chemical analysis of filters associated with sites 
that are not NCore (Note: All NCore stations use either IMPROVE or the 
CSN National Laboratory contractor for their speciation laboratory 
analysis). Therefore, the EPA solicits comment on including the 
existing reporting requirements when reporting CSN measurements. In 
addition, the EPA also solicits comment on a longer reporting and 
certification \209\ schedule specifically for CSN and IMPROVE that 
appropriately balances having sufficient time to analyze, validate, and 
report data with the need to have the data in sufficient time to use in 
assessments including calculating the proposed PM2.5 
visibility index values discussed in section VII.A.5 above. Since 2010, 
the EPA has required states to certify their data by May 1 of each 
year. Since in some cases chemical speciation data may not be fully 
validated and submitted to EPA by May 1 of a given year, the EPA 
solicits comment on having data certification of these speciation 
measurements take place by May 1 of the following year. For example, if 
the fourth quarter chemical speciation data were not fully available to 
certify by May 1 of the following year, it would be certified another 
12 months after that. The EPA solicits comment on the reporting and 
certification schedules for chemical speciation data.
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    \209\ Data certification requirements are described in 40 CFR 
58.15.
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d. Requirements for Archiving Filters
    The EPA proposes to extend the requirement for archival of 
PM2.5, PM10, and PM10-2.5 filters from 
manual low-volume samplers (samplers with a flow rate of less than 200 
liters/minute) at SLAMS from one year after data collection to five 
years after data collection. The archive of low-volume PM filters is an 
important tool for on-going research and development of emission 
control strategies and for use in health and epidemiology research. 
During a workshop on Ambient Air Quality Monitoring and Health Research 
in 2008, retaining filters for laboratory analysis was identified as a 
key recommendation to provide daily measurements of metals and elements 
(U.S. EPA, 2008d, pp. 17 to 21). The EPA's current requirement of one-
year is not sufficiently long for retrospective analysis of important 
episodes and for use in long-term epidemiology research. Since first 
requiring filter archival of low-volume PM filters in 1997, the EPA has 
always recommended longer filters archives and most agencies are 
already doing so. However, a small number of agencies have reported 
discarding older filters, despite the minimal cost of storing these 
filters. Since cold storage of a large number of filters may be cost 
prohibitive and of little benefit in retaining key aerosol species in 
the x-ray fluorescence (XRF) analyses, the EPA proposes to minimize the 
costs of retaining filters by only requiring cold storage during the 
first year after sample collection. Therefore, the EPA solicits comment 
on this proposal to extend the filter archival requirement from one to 
five years, but only require cold storage during the first year.

IX. Clean Air Act Implementation Requirements for the PM NAAQS

    The proposed revisions to the primary annual PM2.5 NAAQS 
and the proposed secondary PM2.5 visibility index NAAQS 
discussed in sections III.F and VI.F above, if finalized, would trigger 
a process under which states \210\ will make recommendations to the 
Administrator regarding area designations, and the EPA will take final 
action on these designations. States will also be required to review, 
modify, and supplement their existing implementation plans. The 
proposed PM NAAQS revisions would also affect the applicable air 
permitting requirements and the transportation conformity and general 
conformity processes. This section provides background information for 
understanding the possible implications of the proposed NAAQS changes, 
and describes the EPA's plans for providing states necessary guidance 
or rules in a timely manner to clarify how they are affected and to 
assist their implementation efforts. This section also describes 
existing EPA interpretations of CAA requirements and other EPA guidance 
relevant to implementation of new or revised NAAQS. Relevant CAA 
provisions that provide potential flexibility with regard to meeting 
implementation timelines are also discussed.
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    \210\ This and all subsequent references to ``state'' are meant 
to include state, local and tribal agencies responsible for the 
implementation of a PM2.5 control program.
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    This section also contains a discussion of several requirements of 
the stationary source construction permit programs under the CAA that 
may be affected by the proposed revisions of the PM NAAQS. These are

[[Page 39017]]

the PSD and Nonattainment New Source Review (NNSR) programs. To 
facilitate implementation of the PSD requirements, which would be the 
first of the implementation requirements to become applicable upon the 
effective date of the final NAAQS rule, the EPA proposes as part of 
this rulemaking to add a grandfathering provision to its regulations 
that would apply to certain PSD permit applications that are pending on 
the effective date of the revised PM NAAQS. If the proposed NAAQS 
revisions are finalized, this rule could be finalized at the same time 
as the revised NAAQS. This section also discusses other possible 
actions under consideration to facilitate implementation of the PSD and 
NNSR programs (see section IX.F).
    The EPA intends to propose additional appropriate regulations or 
issue guidance related to the implementation requirements for the 
revised PM NAAQS at a later date or dates. These may include additional 
revisions to both the PSD and NNSR regulations, as well as the 
promulgation of rules or development of guidance related to NAAQS 
implementation. These actions will be taken on a schedule that provides 
timely assistance to responsible states. Accordingly, in this section, 
the EPA solicits comment on several issues that the Agency anticipates 
will need to be addressed in future guidance or regulatory actions. 
Because these issues are not relevant to the establishment of the 
NAAQS, the EPA does not expect to respond, nor is the Agency required 
to respond, to these comments in the final action on this proposal, but 
the EPA expects these comments will be helpful as future guidance and 
regulations are developed.

A. Designation of Areas

    After the EPA establishes or revises a NAAQS, the CAA requires the 
EPA and the states to take steps to ensure that the new or revised 
NAAQS is met. The first step, known as the initial area designations, 
involves identifying areas of the country that either meet or do not 
meet the new or revised NAAQS along with the nearby areas contributing 
to violations.
    Section 107(d)(1) of the CAA states that, ``By such date as the 
Administrator may reasonably require, but not later than 1 year after 
promulgation of a new or revised national ambient air quality standard 
for any pollutant under section 109, the Governor of each state shall * 
* * submit to the Administrator a list of all areas (or portions 
thereof) in the State'' that designates those areas as nonattainment, 
attainment, or unclassifiable.\211\ Section 107(d)(1)(B)(i) further 
provides, ``Upon promulgation or revision of a NAAQS, the Administrator 
shall promulgate the designations of all areas (or portions thereof) * 
* * as expeditiously as practicable, but in no case later than 2 years 
from the date of promulgation. Such period may be extended for up to 
one year in the event the Administrator has insufficient information to 
promulgate the designations.'' The term ``promulgation'' has been 
interpreted by the courts with respect to the NAAQS to be signature and 
widespread dissemination of a rule. By no later than 120 days prior to 
promulgating designations, the EPA is required to notify states of any 
intended modifications to their boundaries as the EPA may deem 
necessary. States then have an opportunity to comment on the EPA's 
tentative decision. Whether or not a state provides a recommendation, 
the EPA must timely promulgate the designation that it deems 
appropriate. While section 107 of the CAA specifically addresses 
states, the EPA intends to follow the same process for tribes to the 
extent practicable, pursuant to section 301(d) of the CAA regarding 
tribal authority, and the Tribal Authority Rule (63 FR 7254; February 
12, 1998). To provide clarity and consistency in doing so, the EPA 
issued a 2011 guidance memorandum on working with tribes during the 
designations process (Page, 2011).
---------------------------------------------------------------------------

    \211\ While the CAA says ``designating'' with respect to the 
Governor's letter, in the full context of the CAA section it is 
clear that the Governor actually makes a recommendation to which the 
EPA must respond via a specified process if the EPA does not accept 
it.
---------------------------------------------------------------------------

    Monitoring data are currently available from numerous existing 
PM2.5 mass and PM2.5 speciation sites to 
determine compliance with the proposed revised primary annual 
PM2.5 NAAQS and with the proposed PM2.5 
visibility index NAAQS. As discussed in sections III and VI above, the 
EPA is proposing to: (1) Revise the form and level of the primary 
annual PM2.5 standard and retain the current primary 24-hour 
PM2.5 standard (section III.F); (2) retain the current 
secondary 24-hour PM2.5 standard and revise the form and 
retain the level of the secondary annual PM2.5 standard for 
non-visibility-related welfare protection (section VI.F); and (3) 
establish a distinct secondary PM2.5 visibility index 
standard (section VI.F). The EPA's examination of air quality 
monitoring data current at the time of this proposal indicates that, 
for the proposed levels for primary standards and the secondary 
PM2.5 visibility index standard, it is likely that the vast 
majority of monitors violating this secondary standard would overlap 
with monitors violating the primary standards. Since the same types of 
emissions sources contribute to concentrations affecting attainment 
status for both the proposed primary and secondary NAAQS, the EPA 
expects that the nonattainment area boundaries in locations with such 
overlap would be identical. The EPA will, consistent with previous area 
designations, use area-specific factor analysis \212\ to support area 
boundary decisions for both the primary and secondary standards. The 
EPA intends to more fully address issues affecting area designations in 
designations guidance that will be issued around the same time as any 
revised PM2.5 NAAQS are finalized. The EPA solicits comment 
related to establishing nonattainment area boundaries for the proposed 
revised primary annual PM2.5 NAAQS and the proposed 
secondary PM2.5 visibility index NAAQS, including any 
relevant technical information that should be considered by the EPA, 
and any input on the extent to which different considerations may be 
relevant to establishing boundaries for a secondary PM2.5 
NAAQS.
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    \212\ The EPA has used area-specific factor analyses to support 
boundary determinations by evaluating factors such as air quality 
data, emissions data, population density and degree of urbanization, 
traffic and commuting patterns, meteorology, and geography/
topography.
---------------------------------------------------------------------------

    For the reasons stated above, upon promulgation of the revised 
NAAQS, the EPA currently intends to move forward on the same schedule 
with the initial area designations for both the revised primary annual 
PM2.5 standard and the secondary PM2.5 visibility 
index standard. The EPA notes that promulgating initial area 
designations for these standards on the same schedule will provide 
early regulatory certainty for states. The EPA intends to promulgate 
the revised PM NAAQS in December 2012 and complete initial designations 
for both the revised primary annual PM2.5 NAAQS and the 
secondary PM2.5 visibility index NAAQS by December 2014 
using available air quality data from the current PM2.5 and 
speciation monitoring networks. These designations would follow the 
standard 2-year process described previously and would be based on 3 
consecutive years of certified air quality monitoring data from the 
years 2010 to 2012, or 2011 to 2013. (Note, as discussed in sections 
IV.F and VI.F above, the EPA is proposing to retain the current primary 
24-hour PM10 standard and to revise the form of the 
secondary annual PM2.5 standard to

[[Page 39018]]

remove the option for spatial averaging and to retain all other 
elements of the current suite of secondary PM standards to address non-
visibility welfare effects. A new round of mandatory designations for 
these standards would occur only if these standards change.\213\)
---------------------------------------------------------------------------

    \213\ As discussed in section in VII.A.2 above, the EPA is 
proposing to remove the option for spatial averaging from the form 
of the secondary annual PM2.5 NAAQS consistent with the 
proposed change in the form of the primary annual PM2.5 
standard. The EPA does not consider this change to trigger a new 
round of non-discretionary designations for this standard.
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    In today's action, as discussed in section VIII.B.3.b.i above, the 
EPA is proposing to add requirements for establishing near-road 
PM2.5 monitors in certain cities. If these requirements are 
finalized, the EPA anticipates that it will take up to 3 years to 
establish new monitoring sites for PM2.5 mass, plus an 
additional 3 years of monitoring thereafter to determine compliance 
with the mass-based primary and secondary PM2.5 NAAQS based 
on these new monitors. This means that a complete set of air quality 
data for use in designations from any near-road monitoring sites would 
not be available until 2018. Also, as discussed in section VIII.B.3.d 
above, the EPA is proposing that each state with a CBSA over 1 million 
in population would need to have a CSN (or IMPROVE) monitoring site in 
at least one of its CBSAs to collect speciated PM2.5 data to 
support implementation of the proposed secondary standard to address 
visibility impairment. This proposal may require the addition of new 
monitors, or the relocation of existing monitors, in some CBSAs. The 
EPA is also proposing in today's action to extend the data 
certification period for speciation measurements by 12 months. Thus, 
even if EPA were to consider taking an additional year to complete the 
designations process (i.e., in December 2015 instead of in December 
2014), data from new PM2.5 near-road monitoring sites would 
not be available prior to the extended CAA designation deadline; and 
data from certain CSN (or IMPROVE) monitors also may not be available 
prior to the extended CAA designation deadline. For these reasons, the 
EPA does not currently intend to delay designations based on 
unavailability of data for either the revised primary or distinct 
secondary standards in order to be able to include data from these new 
monitors. Initial area designations would not take into account 
monitoring data from any newly established near-road monitoring sites, 
nor from newly established speciation monitoring sites.
    The EPA recognizes that the number of PM2.5 speciation 
monitoring sites available to support the state Governors' designation 
recommendations and EPA's decisions for the proposed secondary 
PM2.5 visibility index NAAQS will be much smaller than the 
number of PM2.5 FRM/FEM/ARM sites available to support 
designation recommendations and decisions for the revised annual 
primary PM2.5 NAAQS. Therefore, it may well be that more 
areas of the nation are designated unclassifiable (or unclassifiable/
attainment) for the proposed PM2.5 visibility index NAAQS 
than for the proposed revised primary annual PM2.5 NAAQS, if 
finalized. At this time the EPA does not believe that taking an 
additional year to complete designations for the secondary 
PM2.5 visibility index NAAQS would change this outlook. 
However, the EPA intends to remain flexible with regard to the 
designation schedule for the proposed revised PM2.5 NAAQS 
and will reassess the potential need for an extended schedule upon 
issuance of the final NAAQS rule and thereafter.
    In summary, the EPA intends to provide designation guidance to the 
states at the time of the promulgation of revised NAAQS or very shortly 
thereafter, to assist them in formulating these recommendations. In 
accordance with section 107(d)(4) of the CAA, the EPA currently 
believes that state Governors (and tribes, if they choose) should 
submit their initial designation recommendations for both the revised 
primary annual PM2.5 NAAQS and the distinct secondary 
PM2.5 visibility index NAAQS to the EPA no later than 1 year 
following promulgation of any revised NAAQS (e.g., in December 2013 
assuming promulgation of the revised PM NAAQS in December 2012). If the 
Administrator intends to modify any state area recommendation, the EPA 
would notify the appropriate state Governor no later than 120 days 
prior to making final designation decisions. A state that believes the 
Administrator's modification is inappropriate would have an opportunity 
to demonstrate to EPA why it believes its original recommendation (or a 
revised recommendation) is more appropriate before designations are 
promulgated. The Administrator would take any additional input from the 
state into account in making final designation decisions.
    As previously stated, the EPA plans to issue guidance regarding 
designations for the revised PM2.5 NAAQS at or very shortly 
after the time of their final promulgation. The EPA invites preliminary 
comment on all aspects of the designation process at this time, which 
the Agency will consider in developing that guidance.

B. Section 110(a)(2) Infrastructure SIP Requirements

    The CAA directs states to address basic SIP requirements to 
implement, maintain, and enforce the standards. States are to develop 
and maintain an air quality management infrastructure that includes 
enforceable emission limitations, a permitting program, an ambient 
monitoring program, an enforcement program, air quality modeling 
capabilities, and adequate personnel, resources, and legal authority. 
Under CAA sections 110(a)(1) and 110(a)(2), states are to submit these 
SIPs within 3 years after promulgation of a new or revised primary 
standard. While the CAA allows the EPA to set a shorter time for 
submission of these SIPs, the EPA does not currently intend to do so. 
Section 110(b) of the CAA provides that the EPA may extend the deadline 
for the ``infrastructure'' SIP submission for a new secondary standard 
by up to 18 months beyond the initial 3 years. If both the revised 
primary annual PM2.5 NAAQS and the distinct secondary 
PM2.5 visibility index NAAQS are finalized, the EPA 
currently believes it would be more efficient for states and the EPA if 
each affected state submits a single section 110 infrastructure SIP 
that addresses both standards at the same time (i.e., within 3 years of 
promulgation of any revisions to the NAAQS for PM), because the EPA 
does not at present discern any need for there to be any substantive 
difference in the infrastructure SIPs for the two standards. However, 
the EPA also recognizes that states may prefer the flexibility to 
submit the secondary NAAQS infrastructure SIP at a later date. The EPA 
solicits comment on these infrastructure SIP submittal timing 
considerations. The EPA intends to provide guidance regarding the 
required date(s) for submission of infrastructure SIPs at the same time 
as or very shortly after promulgation of the revised NAAQS.
    Section 110(a)(2) of the CAA includes the following paragraphs 
describing specific requirements of infrastructure SIPs: (A) Emission 
limits and other control measures, (B) Ambient air quality monitoring/
data system, (C) Programs for enforcement of control measures and for 
construction or modification of stationary sources, (D)(i) Interstate 
pollution transport and (D)(ii) Interstate and international pollution 
abatement, (E) Adequate resources and authority, conflict of interest, 
and oversight of local governments and

[[Page 39019]]

regional agencies, (F) Stationary source monitoring and reporting, (G) 
Emergency episodes, (H) SIP revisions, (I) Plan revisions for 
nonattainment areas, (J) Consultation with government officials, public 
notification, PSD and visibility protection, (K) Air quality modeling 
and submission of modeling data, (L) Permitting fees, and (M) 
Consultation and participation by affected local entities.
    The EPA interprets the CAA such that for two of the section 
110(a)(2) elements, both of which pertain to nonattainment area 
requirements in part D, title I of the CAA, the required submittal date 
should not be governed by the 3-year submission deadline of section 
110(a)(1). Therefore, for the reasons explained below, the following 
section 110(a)(2) elements are considered by EPA to be outside the 
scope of infrastructure SIP actions: (1) Section 110(a)(2)(C) to the 
extent it refers to permit programs (known as ``nonattainment new 
source review'') under part D; and (2) section 110(a)(2)(I) (plan 
revisions for nonattainment areas) in its entirety. The EPA does not 
expect infrastructure SIP submittals to include regulations or emission 
limits developed specifically for attaining the relevant standard in 
areas designated nonattainment for the proposed revised 
PM2.5 NAAQS. Infrastructure SIPs for any final revised 
PM2.5 NAAQS will be due before PM2.5 SIPs are due 
to demonstrate attainment with the same NAAQS. (New emissions 
limitations and other control measures to attain a revised 
PM2.5 NAAQS will be due 3 years from the effective date of 
nonattainment area designation as required under CAA section 172(c) and 
will be reviewed and acted upon through a separate process.) For this 
reason, the EPA does not expect infrastructure SIP submissions to 
identify new nonattainment area emissions controls.
    It is the responsibility of each state to review its air quality 
management program's infrastructure SIP provisions in light of each 
revised NAAQS. Most states have revised and updated their 
infrastructure SIPs in recent years to address requirements associated 
with revised NAAQS. It may be the case that for a number of 
infrastructure elements, the state may believe it has adequate state 
regulations already adopted and approved into the SIP to address a 
particular requirement with respect to the revised PM NAAQS. For such 
portions of the state's infrastructure SIP submittal, the state may 
provide a ``certification'' specifying that certain existing provisions 
in the SIP are adequate. Although the term ``certification'' does not 
appear in the CAA as a type of infrastructure SIP submittal, the EPA 
sometimes uses the term in the context of infrastructure SIPs, by 
policy and convention, to refer to a state's minimal SIP submittal 
(e.g., in the form of a letter to the EPA from the state Governor or 
her/his designee).
    If a state determines that its existing SIP-approved provisions are 
adequate in light of the revised PM NAAQS with respect to a given 
infrastructure SIP element (or sub-element), then the state may make a 
``certification'' that the existing SIP contains provisions that 
address those requirements of the specific section 110(a)(2) 
infrastructure elements. In the case of a certification, the submittal 
does not have to include a copy of the relevant provision (e.g., rule 
or statute) itself. Rather, the submittal may provide citations to the 
SIP-approved state statutes, regulations, or non-regulatory measures, 
as appropriate, which meet the relevant CAA requirement. Like any other 
SIP submittal, such certification can be made only after the state has 
provided reasonable notice and opportunity for public hearing. This 
``reasonable notice and opportunity for public hearing'' requirement 
for infrastructure SIP submittals appears at section 110(a), and it 
comports with the more general SIP requirement at section 110(l) of the 
CAA. Under the EPA's regulations at 40 CFR part 51, if a public hearing 
is held, an infrastructure SIP submittal must include a certification 
by the state that the public hearing was held in accordance with the 
EPA's procedural requirements for public hearings. See 40 CFR part 51, 
appendix V, paragraph 2.1(g), and 40 CFR 51.102.
    In consultation with its EPA Regional Office, a state should follow 
applicable EPA regulations governing infrastructure SIP submittals in 
40 CFR part 51--e.g., subpart I (Review of New Sources and 
Modifications), subpart J (Ambient Air Quality Surveillance), subpart K 
(Source Surveillance), subpart L (Legal Authority), subpart M 
(Intergovernmental Consultation), subpart O (Miscellaneous Plan Content 
Requirements), subpart P (Protection of Visibility), and subpart Q 
(Reports). For the EPA's general criteria for infrastructure SIP 
submittals, refer to 40 CFR part 51, appendix V, Criteria for 
Determining the Completeness of Plan Submissions. A recent EPA guidance 
memorandum identifies a number of alternatives that are available to 
states to reduce the administrative burden, cost, and time required to 
complete the CAA-required steps that are part of submitting 
infrastructure and other SIP revisions to EPA (McCabe, 2011). The EPA 
also notes that many of the infrastructure SIP provisions are not 
NAAQS-specific, and therefore are likely to have been approved as part 
of SIP actions associated with other recently promulgated NAAQS (e.g., 
2006 PM2.5 and 2008 lead NAAQS).
    The EPA intends to issue a separate guidance document on section 
110 infrastructure SIP requirements for any revised PM NAAQS. The 
target date for issuing such guidance would be no later than 1 year 
after the revised PM NAAQS are finalized (2 years before state 
submittals are due). The EPA invites preliminary comment on all aspects 
of infrastructure SIPs at this time, which the Agency will consider in 
developing future guidance.

C. Implementing the Proposed Revised Primary Annual PM2.5 NAAQS in 
Nonattainment Areas

    Part D of the CAA describes the various program requirements that 
apply to nonattainment areas for different NAAQS. Section 172 (found in 
subpart 1 of part D) includes the general SIP requirements that govern 
the PM2.5 program. Under section 172, states are required to 
submit SIPs within 3 years of the effective date of area designations 
by the EPA. These plans need to show how the nonattainment area will 
attain the primary PM2.5 standards ``as expeditiously as 
practicable,'' but presumptively no later than within 5 years from the 
effective date of designations. However, in certain cases, the EPA can 
approve attainment dates up to 10 years from the effective date of 
designations, as appropriate, considering the severity of the air 
quality concentrations in the area, and the availability and 
feasibility of emission control measures per section 172(a)(2)(C).
    Section 172(a)(1) of the CAA authorizes the EPA to establish 
classification categories for areas designated nonattainment for the 
primary or secondary PM NAAQS, but does not require the EPA to do so. 
The implementation program for the 1997 and 2006 primary and secondary 
PM2.5 standards did not include a tiered classification 
system. This provided a relatively simple implementation structure and 
flexibility for states to implement control programs tailored to the 
specific nature of the problem and source mix in each area. For this 
same reason, the EPA also does not intend to establish classifications 
for nonattainment areas for the proposed revised primary annual 
PM2.5 standard (or for a revised primary 24-hour standard if 
one is promulgated). However, the EPA solicits comment on

[[Page 39020]]

whether a classification system would be appropriate and how a 
classification system could be designed.
    In April 2007, the EPA issued a detailed PM2.5 
implementation rule (72 FR 20586; April 25, 2007) to provide guidance 
to states regarding development of SIPs to attain the 1997 
PM2.5 NAAQS. The EPA believes that the overall framework and 
policy approach of the implementation rule for the 1997 
PM2.5 NAAQS provides effective and appropriate guidance on 
the general approach for states to follow in planning for attainment of 
the revised primary annual PM2.5 standard. The EPA intends 
to develop and propose a revised implementation rule that will address 
any new implementation requirements as a result of the proposed revised 
primary annual PM2.5 NAAQS and the proposed revised 
monitoring regulations. The EPA intends to propose this implementation 
rule within 1 year after the revised PM NAAQS are promulgated, and 
finalize the implementation rule by no later than the time the area 
designations process is finalized (approximately 1 year later). The EPA 
believes that for many issues, regulatory text similar to that of the 
existing implementation rule for the 1997 PM2.5 NAAQS can be 
included in this new implementation rule. In the implementation rule 
for the 1997 PM2.5 NAAQS, there are a few specific 
references to the 1997 annual PM2.5 NAAQS or associated 
implementation dates; in a proposed implementation rule for any revised 
PM2.5 NAAQS, such references would be updated as 
appropriate. In addition, the EPA expects to consider options for 
potentially updating certain policies in the existing implementation 
rule based on new information or implementation experience. The EPA 
solicits preliminary comment on the implementation issues that the 
Agency should consider for updating.
    Under the approach outlined in the implementation rule for the 1997 
PM2.5 NAAQS, the state begins the development of an 
attainment demonstration with the evaluation of the air quality 
improvements the nonattainment area can expect in the future due to 
``on the books'' existing federal, state, and local emission reduction 
measures. The state then must conduct a further assessment of emission 
sources in the nonattainment area, and the additional reasonably 
available control measures (RACM) and reasonably available control 
technology (RACT) that can be implemented by these sources, in 
determining how soon the area can attain the standard. (Under the 
current implementation rule, the sources for consideration would be 
those emitting SO2, direct PM2.5, and 
presumptively NOX. Sources of the other PM2.5 
precursors, VOC and ammonia, presumptively do not need to be evaluated 
for control measures unless demonstrated by the state or the EPA as 
significant contributors to PM2.5 concentrations in the 
relevant nonattainment area.) Under section 172 of the CAA as 
interpreted by the EPA, attainment demonstrations must include a RACM 
analysis showing that no additional reasonably available measures could 
be adopted and implemented such that the SIP could specify an 
attainment date that is 1 or more years earlier.
    The evaluation of these potential emission reductions and 
associated air quality improvement is commonly performed with 
sophisticated air quality modeling tools. Given that fine particle 
concentrations are affected both by regionally-transported pollutants 
(e.g., SO2 and NOX emissions from power plants) 
and emissions of direct PM2.5 from local sources in the 
nonattainment area (e.g., steel mills, rail yards, and highway mobile 
sources), the EPA recommends the use of regional grid-based models 
(such as CMAQ and CAMx) in combination with source-oriented dispersion 
models (such as AERMOD) to develop PM2.5 attainment 
strategies for the revised annual primary NAAQS. Although the EPA 
projects significant improvements in PM2.5 concentrations 
regionally from a number of recently promulgated rules such as the 
Cross State Air Pollution Rule (76 FR 48208, August 8, 2011) and the 
Mercury and Air Toxics Standards rule (77 FR 9304, February 16, 2012) 
that will result in SO2 and NOX reductions from 
many geographically dispersed sources, local reductions of direct 
PM2.5 emissions also result in important health benefits. On 
a per ton basis, reductions of direct PM2.5 emissions are 
more effective in reducing PM2.5 concentrations than 
reductions of precursor emissions. Therefore, reductions of direct 
PM2.5 emissions should play a key role in attainment 
planning as well.
    Each nonattainment area needs to ensure that it will make 
``reasonable further progress'' (RFP) in accordance with section 
172(c)(2) of the CAA from the time of SIP submittal to its attainment 
date. Under the approach outlined in the implementation rule for the 
1997 PM2.5 NAAQS, for an area that can demonstrate it will 
attain the standard within the presumptive 5-year period from 
designation, its attainment demonstration will be considered to meet 
the RFP requirement. The EPA believes it is appropriate to apply this 
same approach for the revised annual primary PM2.5 standard. 
The EPA believes there should be no additional RFP requirements for 
such an area because the SIP and attainment demonstration would be due 
3 years after designations and its attainment date will be only 2 years 
after that date. An area that cannot demonstrate attainment within the 
presumptive 5-year period would be required to provide a separate RFP 
plan showing that the area will achieve emission reductions by certain 
interim milestone dates which provide for ``generally linear'' progress 
over the course of the implementation period. All PM2.5 
attainment plans must also include contingency measures which would 
apply without significant delay in the event the area fails to attain 
by its attainment date.
    The EPA expects that the same general approach for determining 
attainment of the 1997 PM2.5 primary standard by the 
attainment deadline would be followed for determining attainment with 
any primary PM2.5 standard. Attainment would be evaluated 
based on the 3 most recent years of certified, complete, and quality-
assured air quality data in the nonattainment area. The EPA also would 
expect to include similar flexibility provisions for an area to be able 
to obtain two 1-year attainment date extensions under certain 
circumstances. In the 1997 PM2.5 NAAQS implementation rule, 
an area whose design value based on the most recent 3 years of data 
exceeds the standard could receive a 1-year attainment date extension 
if the air quality concentration for the third year alone does not 
exceed the level of the standard. Similarly, an area that has received 
a 1-year extension could receive a second 1-year extension if the 
average of the area's air quality concentration in the ``extension 
year'' and the previous year does not exceed the level of the standard.
    The EPA notes that in other sections of today's proposal, the EPA 
describes new requirements for deploying near-road monitors and 
clarifies certain existing monitoring provisions. As discussed in the 
designations section, the EPA would not expect that data from any new 
near-road PM2.5 monitors would be available in time to 
consider during the initial area designations process, and therefore 
such monitoring data would not be the basis for designating a new 
nonattainment area at the time of initial designations. The EPA plans 
to address any potential implications of the proposed monitoring

[[Page 39021]]

changes on attainment planning and development of attainment 
demonstrations by states in the future implementation rule. The EPA 
requests comment on any specific attainment planning considerations for 
future SIPs that may be associated with today's proposed changes to 
monitoring provisions.
    With regard to implementation of the pre-existing standards for 
PM2.5, the EPA's current opinion is that the changes in the 
monitoring regulations, if finalized, should not result in any new 
requirements with respect to attainment plans or maintenance plans for 
the 1997 or the 2006 PM2.5 NAAQS during some specified 
transition period.\214\ For example, if the proposed PM NAAQS revisions 
and revised monitoring regulations are finalized in December 2012, many 
states will have recently submitted, or will be close to submitting 
their implementation plans to attain the 2006 24-hour PM2.5 
NAAQS (also due in December 2012). In addition, state and EPA actions 
are still under way with regard to adopting and approving certain 
attainment plans and maintenance plans for nonattainment areas under 
the 1997 PM2.5 standards. The EPA does not believe it would 
be reasonable for requirements applicable to such attainment plans and 
maintenance plans to change beginning immediately upon any revision of 
the monitoring regulations. It could be very burdensome on state air 
quality programs to revise SIPs that have already been submitted to EPA 
or that have been under development for some time and are about to be 
submitted. The EPA believes that a more reasonable approach would be to 
provide for a transition period before the revised monitoring network 
and data comparability provisions would affect implementation plan and 
maintenance plan requirements. The EPA believes it would be important 
for the transition period to provide enough time for the EPA to 
complete action on attainment and maintenance SIPs for the 1997 or 2006 
PM2.5 NAAQS that were initiated and completed (or that are 
close to completion) by states before finalization of the proposed 
changes to the monitoring regulations. The EPA believes that if a SIP 
for the 1997 or 2006 PM2.5 NAAQS has been approved during 
the transition period, the state would not be under an obligation to 
revise it unless the EPA has made a SIP call. The EPA invites 
preliminary comment on this transition period concept, and on an 
appropriate date by which the transition period should be concluded.
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    \214\ For example, it may be possible that a new near-road 
monitoring site has collected 3 years of data and shown a violation 
before final EPA action has been taken on an attainment plan or 
maintenance plan for the 1997 or 2006 NAAQS.
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D. Implementing the Primary and Secondary PM10 NAAQS

    As summarized in sections IV.F and VI.F above, the EPA is proposing 
to retain the current primary and secondary 24-hour PM10 
standards to protect against the health effects associated with short-
term exposures to thoracic coarse particles and against welfare 
effects. If this approach is finalized, the EPA would retain the 
existing implementation strategy for meeting the CAA requirements for 
PM10. States and emission sources would continue to follow 
the existing guidance and regulations for implementing the current 
standards.

E. Implementing the Proposed Secondary PM2.5 Visibility Index NAAQS in 
Nonattainment Areas

    In past actions, the EPA has set the secondary PM standards 
identical to the primary PM standards. In this action, as summarized in 
section VI.F above, the EPA is proposing a distinct secondary 
PM2.5 visibility index NAAQS. In addition, as also 
summarized in section VI.F above, the EPA is proposing to retain the 
current annual and 24-hour secondary PM2.5 standards to 
provide protection against non-visibility welfare effects. Although the 
proposed secondary PM2.5 visibility index NAAQS would differ 
from the primary PM2.5 NAAQS (and existing secondary 
PM2.5 NAAQS) with respect to indicator/index, statistical 
form, and level, attainment of this standard would, like the 
PM2.5 mass-based standards, depend on ambient measurements 
(i.e., specifically speciated PM2.5 mass concentrations). 
The EPA expects that implementation of emission reduction measures that 
will help to achieve the mass-based 1997 and 2006 primary and secondary 
PM2.5 standards and the proposed revised primary annual 
PM2.5 standard will also provide important improvements in 
visibility and substantial progress toward meeting the proposed 
secondary PM2.5 visibility index standard because these 
emission reduction measures will address the same sources and 
pollutants which also contribute to PM-related vi