National Ambient Air Quality Standards for Particulate Matter

Federal RegisterJun 29, 2012

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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.

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 (μm) in diameter, PM

2.5

), the EPA proposes to revise the annual PM

2.5

standard by lowering the level to within a range of 12.0 to 13.0 micrograms per cubic meter (μg/m

), 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 PM

2.5

standard. The EPA proposes changes to the Air Quality Index (AQI) for PM

2.5

to be consistent with the proposed primary PM

2.5

standards. With regard to the primary standard for particles generally less than or equal to 10 μm in diameter (PM

), the EPA proposes to retain the current 24-hour PM

standard to continue to provide protection against effects associated with short-term exposure to thoracic coarse particles (i.e., PM

10-2.5

). With regard to the secondary PM standards, the EPA proposes to revise the suite of secondary PM standards by adding a distinct standard for PM

2.5

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 PM

2.5

visibility index, which would use speciated PM

2.5

mass concentrations and relative humidity data to calculate PM

2.5

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:

General Information

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

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.

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 PM

2.5

Standards

A. Background

1. General Approach Used in Previous Reviews

2. Remand of Primary Annual PM

2.5

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 PM

2.5

Exposures

b. Health Effects Associated With Short-term PM

2.5

Exposures

c. Summary

2. Limitations and Uncertainties Associated With the Currently Available Evidence

3. At-Risk Populations

4. Potential PM

2.5

-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 PM

2.5

Standards

1. Evidence-Based Considerations in the Policy Assessment

a. Associations With Long-term PM

2.5

Exposures

b. Associations With Short-term PM

2.5

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 PM

2.5

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 PM

2.5

Standard Levels

F. Administrator's Proposed Decisions on Primary PM

2.5

Standards

IV. Rationale for Proposed Decision on Primary PM

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 PM

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 PM

10-2.5

Exposure and Mortality

b. Short-term PM

10-2.5

Exposure and Cardiovascular Effects

c. Short-term PM

10-2.5

Exposure and Respiratory Effects

2. Potential Impacts of Sources and Composition on PM

10-2.5

Toxicity

3. Ambient PM

Concentrations in PM

10-2.5

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 PM

Standard

F. Administrator's Proposed Decision on the Primary PM

Standard

V. Communication of Public Health Information

VI. Rationale for Proposed Decisions on the Secondary PM Standards

A. Background

1. Approaches Used in Previous Reviews

2. Remand of 2006 Secondary PM

2.5

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. PM

2.5

Mass

ii. Directly Measured PM

2.5

Light Extinction

iii. Calculated PM

2.5

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 PM

2.5

1. General

2. Monitoring Considerations

3. Requirements for Data Use and Reporting for Comparison With the NAAQS for PM

2.5

4. Comparisons With the Annual and 24-Hour PM

2.5

NAAQS

5. Data Handling Procedures for the Proposed New Secondary PM

2.5

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. PM

2.5

and PM

10-2.5

Federal Equivalent Methods

2. Use of CSN Methods to Support the Proposed New Secondary PM

2.5

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 PM

2.5

Ambient Air Monitoring Data to the NAAQS

a. Revoking Use of Population-Oriented as a Condition for Comparability of PM

2.5

Monitoring Sites to the NAAQS

b. Applicability of Micro- and Middle-Scale Monitoring Sites to the Annual PM

2.5

NAAQS

3. Proposed Changes to Monitoring for the National Ambient Air Monitoring System

a. Background

b. Primary PM

2.5

NAAQS

i. Proposed Addition of a Near-Road Component to the PM

2.5

Monitoring Network

ii. Use of PM

2.5

Continuous FEMs at SLAMS

c. Revoking PM

10-2.5

Requirements at NCore Sites

d. Measurements for the Proposed New PM

2.5

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 PM

2.5

Samplers and Monitors

5. Proposed Probe and Monitoring Path Siting Criteria

a. Near-Road Component to the PM

2.5

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 PM

2.5

NAAQS in Nonattainment Areas

D. Implementing the Primary and Secondary PM

NAAQS

E. Implementing the Proposed New PM

2.5

Visibility Index NAAQS in Nonattainment Areas

F. Prevention of Significant Deterioration and Nonattainment New Source Review Programs for the Proposed Revised Primary Annual PM

2.5

NAAQS and the Proposed New Secondary PM

2.5

Visibility Index NAAQS

1. Prevention of Significant Deterioration

a. Grandfathering Provision

b. Recent Guidance Applicable to the Proposed Revised Primary Annual PM

2.5

NAAQS

c. Surrogacy Approach for the Proposed New Secondary PM

2.5

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 (μm) in diameter, PM

2.5

), at that time, the EPA

revised the level of the primary 24-hour PM

2.5

standard from 65 to 35 μg/m

and retained the level of the primary annual PM

2.5

standard. (2) With regard to the primary standards for particles less than or equal to 10 μm in diameter (PM

), the EPA retained the primary 24-hour PM

standard to continue to provide protection against effects associated with short-term exposure to thoracic coarse particles (i.e., PM

10-2.5

) and revoked the primary annual PM

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 PM

2.5

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 PM

2.5

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 PM

2.5

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 PM

2.5

standard by lowering the level from 15.0 to within a range of 12.0 to 13.0 μg/m

so as to provide increased protection against health effects associated with long- and short-term exposures. The EPA proposes to retain the level (35 μg/m

) and the form (98th percentile) of the 24-hour PM

2.5

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 PM

2.5

-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 PM

2.5

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 PM

2.5

-related health effects in the last review, including extended analyses of key epidemiological studies, and evidence of health effects observed at lower ambient PM

2.5

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 PM

2.5

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 PM

2.5

standards.

With regard to the primary standard for coarse particles, EPA proposes to retain the current 24-hour PM

standard, with a level of 150 μg/m

and a one-expected exceedance form, to continue to provide protection against effects associated with short-term exposure to PM

10-2.5

, 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 PM

10-2.5

, 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 PM

10-2.5

does not need to be increased beyond that provided by the current PM

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 PM

2.5

to address PM-related visibility impairment. More specifically, the EPA proposes to establish a secondary standard defined in terms of a PM

2.5

visibility index, which would use speciated PM

2.5

mass concentrations and relative humidity data to calculate PM

2.5

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 PM

2.5

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 PM

2.5

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 PM

2.5

standard to remove the option for spatial averaging consistent with this

proposed change to the primary annual PM

2.5

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 PM

2.5

. 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 PM

2.5

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 PM

2.5

to be consistent with the proposed primary PM

2.5

standards. The EPA also proposes revisions to the data handling procedures consistent with the proposed primary and secondary standards for PM

2.5

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 PM

2.5

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

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 PM

2.5

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 μg/m

) and from $2.3 billion to $5.9 billion per year (for 12.0 μg/m

) in 2020 and costs ranging from $2.9 million (for 13.0 μg/m

) to $69 million (for 12.0 μg/m

) 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.”

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.”

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).

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

EPA,

647 F.2d 1130, 1154 (D.C. Cir 1980);

American Petroleum Institute

Costle,

665 F.2d 1176, 1186 (D.C. Cir. 1981;

American Farm Bureau Federation

EPA,

559 F. 3d 512, 533 (D.C. Cir. 2009);

Association of Battery Recyclers

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

EPA,

647 F.2d at 1156

n.51, but rather at a level that reduces risk sufficiently so as to protect public health with an adequate margin of safety.

In addressing the requirement for an adequate margin of safety, the EPA considers such factors as the nature and severity of the health effects involved, the size of 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

EPA,

647 F.2d at 1161-62;

Whitman

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

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

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).

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

and level. Table 1 provides a summary of the PM NAAQS that have been promulgated to date. These decisions are briefly discussed below.

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 μm (referred to as total suspended particles or TSP). The primary standards (measured by the indicator TSP) were 260 μg/m

, 24-hour average, not to be exceeded more than once per year, and 75 μg/m

, annual geometric mean. The secondary standard was 150 μg/m

, 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 PM

, the latter including particles with an aerodynamic diameter less than or equal to a nominal 10 μm, 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 PM

standard of 150 μg/m

with no more than one expected exceedance per year; and (2) replacing the annual TSP standard with a PM

standard of 50 μg/m

, annual arithmetic mean. The secondary standard was revised by replacing it with 24-hour and annual PM

standards identical in all respects to the primary standards. The revisions also included a new reference method for the measurement of PM

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

EPA,

902 F. 2d 962 (D.C. Cir. 1990).

Table 1—Summary of National Ambient Air Quality Standards Promulgated for PM 1971-2006

Final rule

Indicator

Averaging time

Level

Form

1971—36 FR 8186 April 30, 1971

TSP

24-hour

260 μg/m

(primary), 150 μg/m

(secondary)

Not to be exceeded more than once per year.

Annual

75 μg/m

(primary)

Annual average.

1987—52 FR 24634, July 1, 1987

24-hour

150 μg/m

Not to be exceeded more than once per year on average over a 3-year period.

Annual

50 μg/m

Annual arithmetic mean, averaged over 3 years.

1997—62 FR 38652, July 18, 1997

2.5

24-hour

65 μg/m

98th percentile, averaged over 3 years.

Annual

15.0 μg/m

Annual arithmetic mean, averaged over 3 years.

7 8

24-hour

150 μg/m

Initially promulgated 99th percentile, averaged over 3 years; when 1997 standards for PM

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 μg/m

Annual arithmetic mean, averaged over 3 years.

2006—71 FR 61144, October 17, 2006

2.5

24-hour

35 μg/m

98th percentile, averaged over 3 years.

Annual

15.0 μg/m

Annual arithmetic mean, averaged over 3 years.

24-hour

150 μg/m

Not to be exceeded more than once per year on average over a 3-year period.

When not specified, primary and secondary standards are identical.

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 μg/m

would round to 35 μg/m

and thus meet the 24-hour standard and a 3-year average of 35.50 μg/m

would round to 36 and, hence, violate the 24-hour standard (40 CFR part 50, appendix N).

The level of the annual standard is defined to one decimal place (i.e., 15.0 μg/m

) as determined by rounding. For example, a 3-year average annual mean of 15.04 μg/m

would round to 15.0 μg/m

and, thus, meet the annual standard and a 3-year average of 15.05 μg/m

would round to 15.1 μg/m

and, hence, violate the annual standard (40 CFR part 50, appendix N).

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”).

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 (PM

), the fine and coarse fractions of PM

should be considered separately. New standards were added, using PM

2.5

as the indicator for fine particles. The PM

standards were retained for the purpose of regulating the coarse fraction of PM

(referred to as thoracic coarse particles or PM

10-2.5

The EPA established two new PM

2.5

standards: an annual standard of 15 μg/m

, based on the 3-year average of annual arithmetic mean PM

2.5

concentrations from single or multiple monitors sited to represent community-wide air quality

; and a 24-hour standard of 65 μg/m

, based on the 3-year average of the 98th percentile of 24-hour PM

2.5

concentrations at each population-oriented monitor

within an area. Also, the EPA established a new reference method for the measurement of PM

2.5

in the ambient air and rules for determining attainment of the new standards. To continue to address thoracic coarse particles, the annual PM

standard was retained, while the form, but not the level, of the 24-hour PM

standard was revised to be based on the 99th percentile of 24-hour PM

concentrations at each monitor in an area. The EPA revised the secondary standards by making them identical in all respects to the primary standards.

See 40 CFR parts 50, 53, and 58 for more information on reference and equivalent methods for measuring PM in ambient air.

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 PM

2.5

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)].

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

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

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 PM

standards, concluding, in part, that PM

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 PM

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 PM

standards to the 1997 PM

standards. The pre-existing 1987 PM

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

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

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

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 PM

2.5

standards were reasonably supported by the administrative record and were not “arbitrary and capricious.”

American Trucking Associations

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 PM

2.5

standard in the range of 35 to 30 µg/m

with a 98th percentile form, in concert with revising the level of the primary annual PM

2.5

standard in the range of 14 to 13 µg/m

(Henderson, 2005a, p.7). For thoracic coarse particles, the Panel had reservations in recommending a primary 24-hour PM

10-2.5

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 PM

2.5

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 PM

2.5

standard to 35 µg/m

; (2) revising the form, but not the level, of the primary annual PM

2.5

standard by tightening the constraints on the use of spatial averaging; (3) replacing the primary 24-hour PM

standard with a 24-hour standard defined in terms of a new indicator, PM

10-2.5

, this proposed indicator was qualified so as to include any ambient mix of PM

10-2.5

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 µg/m

based on the 3-year average of the 98th percentile of 24-hour PM

10-2.5

concentrations; (4) revoking the primary annual PM

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.

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 PM

2.5

standards as well as the standards for thoracic coarse particles (Henderson, 2006a).

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 PM

2.5

standard to address visibility impairment, considering alternative standard levels within a range of 20 to 30 µg/m

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 PM

2.5

standard to 35 µg/m

, retained the level of the primary annual PM

2.5

standard at 15 µg/m

, and revised the form of the primary annual PM

2.5

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 PM

standard (such that the standard remained at a level of 150 µg/m

with a one-expected exceedance form), and revoked the annual PM

standard. The EPA also established a new Federal Reference Method (FRM) for the measurement of PM

10-2.5

in the ambient air (71 FR 61212-13). Although the standards for thoracic coarse particles were not defined in terms of a PM

10-2.5

indicator, the EPA adopted a new FRM for PM

10-2.5

to facilitate consistent research on PM

10-2.5

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 PM

2.5

annual standard, CASAC expressed serious concerns regarding the decision to retain the level of the standard at 15 µg/m

. Specifically, CASAC stated, “It is the CASAC's consensus scientific opinion that the decision to retain without change the annual PM

2.5

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).

With regard to EPA's final decision to retain the 24-hour PM

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 PM

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 PM

2.5

standards to be identical in all respects to the revised primary PM

2.5

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).

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

2. Litigation Related to the 2006 PM Standards

Several parties filed petitions for review following promulgation of the revised PM NAAQS in 2006. These petitions addressed the following issues: (1) Selecting the level of the primary annual PM

2.5

standard; (2) retaining PM

as the indicator of a standard for thoracic coarse particles, retaining the level and form of the 24-hour PM

standard, and revoking the PM

annual standard; and (3) setting the secondary PM

2.5

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

EPA,

559 F. 3d 512 (D.C. Cir. 2009). The court remanded the primary annual PM

2.5

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

EPA,

559 F. 3d 512, 520-27 (D.C. Cir. 2009). With regard to the standards for PM

, the court upheld EPA's decisions to retain the 24-hour PM

standard to provide protection from thoracic coarse particle exposures and to revoke the annual PM

standard.

American Farm Bureau Federation

EPA,

559 F. 3d at 533-38. With regard to the secondary PM

2.5

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

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).

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.

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

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).

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 PM

2.5

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,

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

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 PM

2.5

standards. Regarding the 1997 PM

2.5

standards, the EPA designated 39 nonattainment areas in 2005. Regarding the 2006 24-hour PM

2.5

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 PM

2.5

standards. With regard to the PM

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

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 PM

2.5

Standards

This section presents the rationale for the Administrator's proposed decision to revise the level and form of the existing primary annual PM

2.5

standard and to retain the existing primary 24-hour PM

2.5

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 PM

2.5

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 PM

2.5

standards, the EPA has placed greater weight on U.S. and Canadian studies using PM

2.5

measurements, since studies conducted in other countries may well reflect different demographic and air pollution characteristics.

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

likely causal relationship

with long- or short-term PM

2.5

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 PM

2.5

exposures in taking into account potential impacts on at-risk populations

and in considering alternative standard levels that provide protection with an appropriate margin of safety.

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 PM

2.5

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 PM

2.5

standards, this section begins with a summary of the approaches used in setting the initial primary PM

2.5

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 PM

2.5

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 PM

2.5

-related impacts on public health. Section III.C outlines the approach taken by the EPA to assess health risks associated with exposure to ambient PM

2.5

, 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

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

2.5

NAAQS.

A. Background

There are currently two primary PM

2.5

standards providing public health protection from effects associated with fine particle exposures. The annual standard is set at a level of 15.0 μg/m

, based on the 3-year average of annual arithmetic mean PM

2.5

concentrations from single or multiple monitors sited to represent community-wide air quality. The 24-hour standard is set at a level of 35 μg/m

, based on the 3-year average of the 98th percentile of 24-hour PM

2.5

concentrations at each population-oriented monitor within an area.

The past and current approaches for reviewing the primary PM

2.5

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 PM

2.5

standards are complicated by the recognition that no population threshold, below which it can be concluded with confidence that PM

2.5

-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 PM

2.5

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 PM

2.5

concentrations reported in epidemiological studies (U.S. EPA, 2011a, section 2.1.1). This approach recognized that the strongest evidence of PM

2.5

-related associations occurs at concentrations near the long-term (i.e., annual) mean.

In setting primary PM

2.5

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 PM

2.5

concentrations in studies providing evidence of associations with short-term PM

2.5

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 PM

2.5

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 PM

standards (62 FR 38665/1, July 18, 1997).

The EPA considered the epidemiological evidence and data on air quality relationships to set an annual PM

2.5

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 PM

2.5

In conjunction with the annual standard, the EPA also established a 24-hour PM

2.5

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).

In so doing, the EPA noted that because an annual standard would focus control programs on annual average PM

2.5

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 PM

2.5

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 PM

2.5

standard from 65 to 35 μg/m

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 PM

2.5

standard at 15 μg/m

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 PM

2.5

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 PM

2.5

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 PM

2.5

annual standard. The primary 24-hour PM

2.5

standard was not challenged by

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 PM

2.5

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

EPA,

559 F. 3d 512 (D.C. Cir. 2009). With respect to human health protection from short-term PM

2.5

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 PM

2.5

standard by itself would provide the protection needed from short-term exposures and remanded the primary annual PM

2.5

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 PM

2.5

.”

American Farm Bureau Federation

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 PM

2.5

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 PM

2.5

exposure (Gauderman et al., 2000) and a multi-city study (24-Cities Study) evaluating decreased lung function in children associated with long-term PM

2.5

exposures (Raizenne et al., 1996), did not warrant a more stringent annual PM

2.5

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 PM

2.5

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 PM

2.5

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 PM

2.5

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 PM

2.5

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 PM

2.5

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 PM

2.5

-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).

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.

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 PM

2.5

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 PM

2.5

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 PM

2.5

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 PM

2.5

air quality designed to meet an annual standard would likely result not only in lower annual average PM

2.5

concentrations but also in fewer and lower peak 24-hour PM

2.5

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 PM

2.5

concentrations but also in lower annual average PM

2.5

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

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 PM

2.5

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

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.

For these reasons, strategies that focus primarily on reducing peak days are less likely to achieve reductions in the PM

2.5

concentrations that are most strongly associated with the observed health effects.

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 PM

2.5

concentrations, as opposed to focusing on control of peak 24-hour PM

2.5

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 PM

2.5

exposures cannot be expected to offer an adequate margin of safety against the effects of all short-term PM

2.5

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 PM

2.5

-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 PM

2.5

-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 PM

2.5

-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 PM

2.5

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 PM

2.5

-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 PM

2.5

exposure studies (U.S. EPA, 2011a, section 2.4.3).

In the absence of any discernible thresholds, the Agency's general approach for identifying appropriate standard levels for consideration involves characterizing the range of PM

2.5

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

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

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 PM

exposures and premature mortality associated with long-term PM

2.5

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 PM

2.5

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 PM

2.5

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.

The Policy Assessment explores the extent to which such analyses have been published for studies of health effects associated with long- or short-term PM

2.5

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.

This is distinct from confidence intervals around concentration-response relationships that are related to the magnitude of effect estimates generated at specific PM

2.5

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 PM

2.5

concentrations reported in epidemiological studies.

This approach recognizes that the strongest evidence of PM

2.5

-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.

Epidemiological studies typically report PM

2.5

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 PM

2.5

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 PM

2.5

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 PM

2.5

concentrations that had the most influence on the calculation of relative risk estimates in epidemiological studies. Thus, the Policy Assessment considers the range of PM

2.5

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 PM

2.5

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.

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 PM

2.5

concentrations most influential in generating the health effect estimates reported in the epidemiological studies.

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 PM

2.5

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.

The second draft Policy Assessment focused on the distributions of PM

2.5

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 PM

2.5

concentrations to generate a cumulative distribution of the population data relative to long-term mean PM

2.5

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 PM

2.5

concentrations over which we have the most confidence in the associations reported in epidemiological studies.

In applying these approaches, the Policy Assessment considers PM

2.5

concentrations from long- and short-term PM

2.5

exposure studies using composite monitor distributions.

For multi-city studies, this distribution 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 PM

2.5

concentration. Beyond considering air quality concentrations based on composite monitor distributions, the second draft Policy Assessment also considered PM

2.5

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).

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 PM

2.5

concentrations typically presented and used in epidemiological analyses and which provide a direct link between PM

2.5

concentrations and the observed health effects reported in both long- and short-term exposure studies (U.S. EPA, 2011a, p. 2-13).

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 PM

2.5

concentration.

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 PM

2.5

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).

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).

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 PM

2.5

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 PM

2.5

concentrations, with a focus on the 98th percentile concentrations to match the form of the current 24-hour PM

2.5

standard, to the extent such data were available, as well as the long-term mean PM

2.5

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

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.

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 PM

2.5

exposures (U.S. EPA, 2010a). As part of the risk-based considerations, the Policy Assessment considers estimates of the magnitude of PM

2.5

-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

related to both long- and short-term PM

2.5

exposures.

The Policy Assessment conclusions related to the primary PM

2.5

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 PM

2.5

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 PM

2.5

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 PM

2.5

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 PM

2.5

, 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 PM

2.5

-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 PM

2.5

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 PM

2.5

exposures with cardiovascular-related and lung cancer mortality and respiratory-related morbidity and for linking short-term PM

2.5

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 PM

2.5

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).

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 PM

2.5

. 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 PM

2.5

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).

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 PM

2.5

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 PM

2.5

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 PM

2.5

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 PM

2.5

Exposures

With regard to mortality, the Integrated Science Assessment concludes that newly available evidence significantly strengthens the link between long-term exposure to PM

2.5

and mortality, while providing indications that the magnitude of the PM

2.5

-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

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 PM

2.5

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 PM

2.5

on cardiovascular-related mortality in post-menopausal women with no previous history of cardiac disease (Miller et al., 2007). The PM

2.5

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 PM

2.5

-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 PM

2.5

and mortality (U.S. EPA, 2009a, sections 2.3.1 and 7.6).

The strength of the causal relationship between long-term PM

2.5

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 PM

2.5

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 PM

2.5

saw the largest improvements in reduced mortality rates, while those with the smallest decreases in PM

2.5

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 PM

2.5

concentrations and changes in mortality (Schwartz et al., 2008) and reported that the effects of changes in PM

2.5

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 PM

2.5

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 PM

2.5

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 PM

2.5

exposure (U.S. EPA, 2009a, p. 7-84, Figure 7-7).

With regard to cardiovascular-related morbidity associated with long-term PM

2.5

exposures, studies were not available in the last review. Recent studies, however, have provided new evidence linking long-term exposure to PM

2.5

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.

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 PM

2.5

, as described below (U.S. EPA, 2009a, p. 7-19).

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 PM

2.5

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).

Specifically, extended analyses of the Southern California Children's Health Study provide evidence that clinically important deficits in lung function

associated with long-term exposure to PM

2.5

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 PM

2.5

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 PM

2.5

effect estimates increasing in magnitude in some models and decreasing in others, and a strong modifying effect of PM

2.5

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 PM

2.5

exposures and exacerbations of respiratory symptoms

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

Supporting evidence comes from studies “that observed associations between long-term exposure to PM

and an increase in respiratory symptoms and reductions in lung function growth in areas where PM

is dominated by PM

2.5

” (U.S. EPA, 2009a, p. 2-12).

Clinical significance was defined as an FEV

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 (SO

) also includes this interpretation for FEV

(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 PM

2.5

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 PM

2.5

on respiratory mortality, with one large cohort study finding a reduction in deaths due to respiratory causes associated with reduced PM

2.5

concentrations, and another large cohort study finding no PM

2.5

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 PM

2.5

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 PM

2.5

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 PM

2.5

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 PM

2.5

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 PM

2.5

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 PM

2.5

-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 PM

2.5

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 PM

2.5

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 PM

2.5

and lung cancer mortality, but studies have generally not reported associations between PM

2.5

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 PM

2.5

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 PM

2.5

Exposures

In considering effects associated with short-term PM

2.5

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 PM

2.5

-associated health effects. These studies provide more precise estimates of the magnitude of effects associated with short-term PM

2.5

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 PM

2.5

exposure studies have found generally consistent positive associations between short-term PM

2.5

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 PM

2.5

exposure and total (non-accidental) mortality (U.S. EPA, 2009a, pp. 6-176 to 6-179; Figures 6-23 and 6-24).

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 PM

2.5

exposure and mortality (U.S. EPA, 2009a, p. 6-182, Figure 2-1), although the influence of nitrogen dioxide (NO

) and limited PM

2.5

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 PM

2.5

-mortality association (U.S. EPA, 2009a, section 2.3.1.1). Collectively, these studies provide generally consistent and much stronger evidence for PM

2.5

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

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 PM

2.5

exposure studies conducted since the last review support a largely positive and frequently statistically significant association between short-term exposure to PM

2.5

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 PM

2.5

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, PM

2.5

effect estimates for cardiovascular-related hospital admissions and emergency department visits generally remained positive, with the magnitude of PM

2.5

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 PM

2.5

and electrocardiogram measures of myocardial ischemia (Zhang et al., 2009).

In focusing on respiratory effects, the strongest evidence from short-term PM

2.5

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, PM

2.5

effect estimates for respiratory-related hospital admissions and emergency department visits generally remained positive, with the magnitude of PM

2.5

effect estimates increasing in some models and decreasing in others (U.S. EPA, 2009a, Figure 6-15). Evidence for PM

2.5

-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 PM

2.5

(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 PM

2.5

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 PM

2.5

. 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 PM

2.5

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 PM

2.5

and respiratory effects. The body of scientific evidence is somewhat expanded but is still limited with respect to associations between long-term PM

2.5

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 PM

2.5

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 PM

2.5

standards.

2. Limitations and Uncertainties Associated With the Currently Available Evidence

With respect to understanding the nature and magnitude of PM

2.5

-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 PM

2.5

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.

This heterogeneity may be attributed, in part, to differences in the fine particle composition or related to exposure measurement error.

Seasonal differences in effects may be related to PM

2.5

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 PM

2.5

from crustal sources, traffic, and wood smoke/vegetative burning; (2) respiratory effects have been linked with secondary sulfate PM

2.5

as well as with PM

2.5

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 PM

2.5

constituents have been linked with various PM

2.5

-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 PM

2.5

-related health outcomes.

Exposure measurement error is also an important source of uncertainty (U.S. EPA, 2009a, section 3.8.6). Variability in the associations observed across PM

2.5

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 PM

2.5

, 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 PM

2.5

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).

As noted above, although short-term studies of cardiovascular and respiratory hospital admissions and emergency department visits generally reported that PM

2.5

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 PM

2.5

-related mortality association (U.S. EPA, 2004, section 8.4.3.3), a conclusion that is supported by studies that examined the PM

-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.

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

, 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.

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.

Although studies have primarily used exposures to PM

or PM

2.5

, the available evidence suggests that the identified factors also increase risk from PM

10-2.5

(U.S. EPA, 2009a, section 8.1.8).

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 ad

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