Document ID: EPA_FRDOC_0001-25354
Agency: epa
Document Type: Proposed Rule
Title: Review of theNational Ambient Air Quality Standards for Particulate Matter
Posted Date: 2020-04-30T04:00Z

[Federal Register Volume 85, Number 84 (Thursday, April 30, 2020)]
[Proposed Rules]
[Pages 24094-24144]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2020-08143]

[[Page 24093]]

Vol. 85

Thursday,

No. 84

April 30, 2020

Part II

Environmental Protection Agency

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40 CFR Part 50

Review of the National Ambient Air Quality Standards for Particulate 
Matter; Proposed Rule

  Federal Register / Vol. 85 , No. 84 / Thursday, April 30, 2020 / 
Proposed Rules  

[[Page 24094]]

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 50

[EPA-HQ-OAR-2015-0072; FRL-10008-31-OAR]
RIN 2060-AS50

Review of the National Ambient Air Quality Standards for 
Particulate Matter

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed action.

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SUMMARY: Based on the Environmental Protection Agency's (EPA's) review 
of the air quality criteria and the national ambient air quality 
standards (NAAQS) for particulate matter (PM), the Administrator has 
reached proposed decisions on the primary and secondary PM NAAQS. With 
regard to the primary standards meant to protect against fine particle 
exposures (i.e., annual and 24-hour PM2.5 standards), the 
primary standard meant to protect against coarse particle exposures 
(i.e., 24-hour PM10 standard), and the secondary 
PM2.5 and PM10 standards, the EPA proposes to 
retain the current standards, without revision.

DATES: Comments must be received on or before June 29, 2020.
    Public Hearings: The EPA will hold one or more virtual public 
hearings on this proposed rule. These will be announced in a separate 
Federal Register notice that provides details, including specific 
dates, times, and contact information for these hearings.

ADDRESSES: You may submit comments, identified by Docket ID No. EPA-HQ-
OAR-2015-0072, by any of the following means:
     Federal eRulemaking Portal: https://www.regulations.gov 
(our preferred method). Follow the online instructions for submitting 
comments.
     Email: a-and-r-Docket@epa.gov. Include the Docket ID No. 
EPA-HQ-OAR-2015-0072 in the subject line of the message.
    Instructions: All submissions received must include the Docket ID 
No. for this document. Comments received may be posted without change 
to https://www.regulations.gov, including any personal information 
provided. For detailed instructions on sending comments, see the 
SUPPLEMENTARY INFORMATION section of this document. Out of an abundance 
of caution for members of the public and our staff, the EPA Docket 
Center and Reading Room was closed to public visitors on March 31, 
2020, to reduce the risk of transmitting COVID-19. Our Docket Center 
staff will continue to provide remote customer service via email, 
phone, and webform. We encourage the public to submit comments via 
https://www.regulations.gov or email, as there is a temporary 
suspension of mail delivery to EPA, and no hand deliveries are 
currently accepted. For further information of EPA Docket Center 
services and the current status, please visit us online at https://www.epa.gov/dockets.

FOR FURTHER INFORMATION CONTACT: Dr. Scott Jenkins, 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) 541-1167; fax: (919) 
541-5315; email: jenkins.scott@epa.gov.

SUPPLEMENTARY INFORMATION: 

General Information

    Written Comments: Submit your comments, identified by Docket ID No. 
EPA-HQ-OAR-2015-0072, at https://www.regulations.gov (our preferred 
method), or the other methods identified in the ADDRESSES section. Once 
submitted, comments cannot be edited or removed from the docket. The 
EPA may publish any comment received to its public docket. Do not 
submit electronically any information you consider to be Confidential 
Business Information (CBI) or other information whose disclosure is 
restricted by statute. Multimedia submissions (audio, video, etc.) must 
be accompanied by a written submission. The written submission is 
considered the official submission and should include discussion of all 
points you wish to make. The EPA will generally not consider 
submissions or submission content located outside of the primary 
submission (i.e., on the web, cloud, or other file sharing system). For 
additional submission methods, the full EPA public comment policy, 
information about CBI or multimedia submissions, and general guidance 
on making effective comments, please visit https://www.epa.gov/dockets/commenting-epa-dockets.
    The EPA is temporarily suspending its Docket Center and Reading 
Room for public visitors to reduce the risk of transmitting COVID-19. 
Written comments submitted by mail are temporarily suspended and no 
hand deliveries will be accepted. Our Docket Center staff will continue 
to provide remote customer service via email, phone, and webform. We 
encourage the public to submit comments via https://www.regulations.gov. For further information and updates on EPA Docket 
Center services, please visit us online at https://www.epa.gov/dockets.
    The EPA continues to carefully and continuously monitor information 
from the Centers for Disease Control and Prevention (CDC), local area 
health departments, and our Federal partners so that we can respond 
rapidly as conditions change regarding COVID-19.

Availability of Information Related to This Action

    A number of the documents that are relevant to this proposed 
decision are available through the EPA's website at https://www.epa.gov/naaqs/particulate-matter-pm-air-quality-standards. These 
documents include the Integrated Review Plan for the National Ambient 
Air Quality Standards for Particulate Matter (U.S. EPA, 2016), 
available at https://www3.epa.gov/ttn/naaqs/standards/pm/data/201612-final-integrated-review-plan.pdf, the Integrated Science Assessment for 
Particulate Matter (U.S. EPA, 2019), available at https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=347534, and the Policy 
Assessment for the Review of the National Ambient Air Quality Standards 
for Particulate Matter (U.S. EPA, 2020), available at https://www.epa.gov/naaqs/particulate-matter-pm-standards-policy-assessments-current-review-0. These and other related documents are also available 
for inspection and copying in the EPA docket identified above.

Table of Contents

    The following topics are discussed in this preamble:

Executive Summary

I. Background
    A. Legislative Requirements
    B. Related PM Control Programs
    C. Review of the Air Quality Criteria and Standards for 
Particulate Matter
    1. Reviews Completed in 1971 and 1987
    2. Review Completed in 1997
    3. Review Completed in 2006
    4. Review Completed in 2012
    5. Current Review
    D. Air Quality Information
    1. Distribution of Particle Size in Ambient Air
    2. Sources and Emissions Contributing to PM in the Ambient Air
    3. Monitoring of Ambient PM
    4. Ambient Concentrations and Trends
    a. PM2.5 mass
    b. PM2.5 components
    c. PM10
    d. PM10-2.5
    a. UFP
    5. Background PM
II. Rationale for Proposed Decisions on the Primary PM2.5 
Standards
    A. General Approach

[[Page 24095]]

    1. Approach Used in the Last Review
    a. Indicator
    b. Averaging Time
    c. Form
    d. Level
    2. Approach in the Current Review
    B. Health Effects Related to Fine Particle Exposures
    1. Nature of Effects
    a. Mortality
    b. Cardiovascular Effects
    c. Respiratory Effects
    d. Cancer
    e. Nervous System Effects
    2. Populations at Risk of PM2.5-Related Health 
Effects
    3. CASAC Advice
    C. Proposed Conclusions on the Current Primary PM2.5 
Standards
    1. Evidence- and Risk-Based Considerations in the Policy 
Assessment
    a. Evidence-Based Considerations
    b. Risk-Based Considerations
    2. CASAC Advice
    3. Administrator's Proposed Decision on the Current Primary 
PM2.5 Standards
III. Rationale for Proposed Decisions on the Primary PM10 
Standard
    A. General Approach
    1. Approach Used in the Last Review
    2. Approach in the Current Review
    B. Health Effects Related to Thoracic Coarse Particle Exposures
    1. Mortality
    a. Long-Term Exposures
    b. Short-Term Exposures
    2. Cardiovascular Effects
    a. Long-Term Exposures
    b. Short-Term Exposures
    3. Respiratory Effects--Short-Term Exposures
    4. Cancer--Long-Term Exposures
    5. Metabolic Effects--Long-Term Exposures
    6. Nervous System Effects--Long-Term Exposures
    C. Proposed Conclusions on the Current Primary PM10 
Standard
    1. Evidence-Based Considerations in the Policy Assessment
    2. CASAC Advice
    3. Administrator's Proposed Decision on the Current Primary 
PM10 Standard
IV. Rationale for Proposed Decisions on the Secondary PM Standards
    A. General Approach
    1. Approach Used in the Last Review
    a. Non-Visibility Effects
    b. Visibility Effects
    2. Approach for the Current Review
    B. PM-Related Visibility Impairment
    1. Nature of PM-Related Visibility Impairment
    2. Relationship between Ambient PM and Visibility
    3. Public Perception of Visibility Impairment
    C. Other PM-Related Welfare Effects
    1. Climate
    2. Materials
    D. Proposed Conclusions on the Current Secondary PM Standards
    1. Evidence- and Quantitative Information-Based Considerations 
in the Policy Assessment
    2. CASAC Advice
    3. Administrator's Proposed Decision on the Current Secondary PM 
Standards
V. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review and 
Executive Order 13563: Improving Regulation and Regulatory Review
    B. Executive Order 13771: Reducing Regulations and Controlling 
Regulatory Costs
    C. Paperwork Reduction Act (PRA)
    D. Regulatory Flexibility Act (RFA)
    E. Unfunded Mandates Reform Act (UMRA)
    F. Executive Order 13132: Federalism
    G. Executive Order 13175: Consultation and Coordination With 
Indian Tribal Governments
    H. Executive Order 13045: Protection of Children From 
Environmental Health and Safety Risks
    I. Executive Order 13211: Actions Concerning Regulations That 
Significantly Affect Energy Supply, Distribution or Use
    J. National Technology Transfer and Advancement Act (NTTAA)
    K. Executive Order 12898: Federal Actions to Address 
Environmental Justice in Minority Populations and Low-Income 
Populations
    L. Determination Under Section 307(d)
References

Executive Summary

    This document presents the Administrator's proposed decisions on 
the primary (health-based) and secondary (welfare-based) National 
Ambient Air Quality Standards (NAAQS) for particulate matter (PM). In 
ambient air, PM is a mixture of substances suspended as small liquid 
and/or solid particles. Particles in the atmosphere range in size from 
less than 0.01 to more than 10 micrometers ([mu]m) in diameter. 
Particulate matter and its precursors are emitted from both 
anthropogenic sources (e.g., electricity generating units, cars and 
trucks, agricultural operations) and natural sources (e.g., sea salt, 
wildland fires, biological aerosols).
    When describing PM, subscripts are used to denote particle size. 
For example, PM2.5 includes particles with diameters 
generally less than or equal to 2.5 [mu]m and PM10 includes 
particles with diameters generally less than or equal to 10 [mu]m.
    The EPA has established primary (health-based) and secondary 
(welfare-based) NAAQS for PM2.5 and PM10. This 
includes two primary PM2.5 standards, an annual average 
standard with a level of 12.0 [mu]g/m\3\ and a 24-hour standard with a 
98th percentile form and a level of 35 [mu]g/m\3\. It also includes a 
primary PM10 standard with a 24-hour averaging time, a 1-
expected exceedance form, and a level of 150 [mu]g/m\3\. Secondary PM 
standards are set equal to the primary standards, except that the level 
of the secondary annual PM2.5 standard is 15.0 [mu]g/m\3\. 
In reaching proposed decisions on these PM standards in the current 
review, the Administrator has considered the available scientific 
evidence assessed in the Integrated Science Assessment (ISA), analyses 
in the Policy Assessment (PA), and advice from the Clean Air Scientific 
Advisory Committee (CASAC).
    For the primary PM2.5 standards, the Administrator 
proposes to conclude that there are important uncertainties in the 
evidence for adverse health effects below the current standards and in 
the potential public health impacts of reducing ambient 
PM2.5 concentrations below those standards. As a result, he 
proposes to conclude that the available evidence and information do not 
call into question the adequacy of the current primary PM2.5 
standards, and he proposes to retain those standards (i.e., both the 
annual and 24-hour standards) without revision in this review.
    For the primary PM10 standard, the Administrator 
observes that, while the available health effects evidence has 
expanded, recent studies are subject to the same types of uncertainties 
that were judged important in the last review. He proposes to conclude 
that the newly available evidence does not call into question the 
adequacy of the current primary PM10 standard, and he 
proposes to retain that standard without revision in this review.
    For the secondary standards, the Administrator observes that the 
expanded evidence for non-ecological welfare effects is consistent with 
the last review \1\ and that updated quantitative analyses show results 
similar to those in the last review. Therefore, he proposes to conclude 
that the newly available evidence and updated analyses do not call into 
question the adequacy of the current secondary PM standards, and he 
proposes to retain those standards without revision in this review.
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    \1\ The welfare effects considered in this review include 
visibility impairment, climate effects, and materials effects. 
Ecological effects associated with PM, and the adequacy of 
protection provided by the secondary PM standards for those effects, 
are being addressed in the separate review of the secondary NAAQS 
for oxides of nitrogen, oxides of sulfur and PM. Information on the 
current review of these secondary NAAQS can be found at https://www.epa.gov/naaqs/nitrogen-dioxide-no2-and-sulfur-dioxide-so2-secondary-air-quality-standards.
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    These proposed decisions are consistent with the CASAC's consensus 
advice on the primary 24-hour PM2.5 standard, the primary 
PM10 standard, and the secondary standards. The CASAC did 
not reach consensus on the primary annual PM2.5 standard, 
with some committee members

[[Page 24096]]

recommending that EPA retain the current standard and other members 
recommending revision of that standard.

I. Background

A. Legislative Requirements

    Two sections of the Clean Air Act (CAA) govern the establishment 
and revision of the NAAQS. Section 108 (42 U.S.C. 7408) directs the 
Administrator to identify and list certain air pollutants and then to 
issue air quality criteria for those pollutants. The Administrator is 
to list those pollutants ``emissions of which, in his 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 he ``plans to issue air quality criteria . . . 
. '' (42 U.S.C. 7408(a)(1)). 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(a)(2)).
    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 [42 U.S.C. 7409(a)]. Section 
109(b)(1) defines primary standards as ones ``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.'' \2\ Under section 109(b)(2), a 
secondary standard 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.'' \3\
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    \2\ 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).
    \3\ Under CAA section 302(h) (42 U.S.C. 7602(h)), effects on 
welfare include, but are not limited to, ``effects on soils, water, 
crops, vegetation, manmade materials, animals, wildlife, weather, 
visibility, and climate, damage to and deterioration of property, 
and hazards to transportation, as well as effects on economic values 
and on personal comfort and well-being.''
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    In setting primary and secondary standards that are ``requisite'' 
to protect public health and welfare, respectively, as provided in 
section 109(b), the EPA's task is to establish standards that are 
neither more nor less stringent than necessary. In so doing, the EPA 
may not consider the costs of implementing the standards. See generally 
Whitman v. American Trucking Associations, 531 U.S. 457, 465-472, 475-
76 (2001). Likewise, ``[a]ttainability and technological feasibility 
are not relevant considerations in the promulgation of national ambient 
air quality standards.'' American Petroleum Institute v. Costle, 665 
F.2d 1176, 1185 (D.C. Cir. 1981); accord Murray Energy Corporation v. 
EPA, 936 F.3d 597, 623-24 (D.C. Cir. 2019).
    The requirement that primary standards provide an adequate margin 
of safety was intended to address uncertainties associated with 
inconclusive scientific and technical information available at the time 
of standard setting. It was also intended to provide a reasonable 
degree of protection against hazards that research has not yet 
identified. See Lead Industries Association v. EPA, 647 F.2d 1130, 1154 
(D.C. Cir 1980); American Petroleum Institute v. Costle, 665 F.2d at 
1186; Coalition of Battery Recyclers Ass'n v. EPA, 604 F.3d 613, 617-18 
(D.C. Cir. 2010); Mississippi v. EPA, 744 F.3d 1334, 1353 (D.C. Cir. 
2013). Both kinds of uncertainties are components of the risk 
associated with pollution at levels below those at which human health 
effects can be said to occur with reasonable scientific certainty. 
Thus, in selecting primary standards that include an adequate margin of 
safety, the Administrator is seeking not only to prevent pollution 
levels that have been demonstrated to be harmful but also to prevent 
lower pollutant levels that may pose an unacceptable risk of harm, even 
if the risk is not precisely identified as to nature or degree. The CAA 
does not require the Administrator to establish a primary NAAQS at a 
zero-risk level or at background concentration levels, see Lead 
Industries Ass'n v. EPA, 647 F.2d at 1156 n.51, Mississippi v. EPA, 744 
F.3d at 1351, but rather at a level that reduces risk sufficiently so 
as to protect public health with an adequate margin of safety.
    In addressing the requirement for an adequate margin of safety, the 
EPA considers such factors as the nature and severity of the health 
effects involved, the size of the sensitive population(s), and the kind 
and degree of uncertainties. 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 Ass'n 
v. EPA, 647 F.2d at 1161-62; Mississippi v. EPA, 744 F.3d at 1353.
    Section 109(d)(1) of the Act requires the review every five years 
of existing air quality criteria and, if appropriate, the revision of 
those criteria to reflect advances in scientific knowledge on the 
effects of the pollutant on public health and welfare. Under the same 
provision, the EPA is also to review every five years and, if 
appropriate, revise the NAAQS, based on the revised air quality 
criteria.
    Section 109(d)(2) addresses the appointment and advisory functions 
of an independent scientific review committee. Section 109(d)(2)(A) 
requires the Administrator to appoint this committee, which is to be 
composed of ``seven members including at least one member of the 
National Academy of Sciences, one physician, and one person 
representing State air pollution control agencies.'' Section 
109(d)(2)(B) provides that the 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 1980s, this independent review function has been performed by 
the Clean Air Scientific Advisory Committee (CASAC) of the EPA's 
Science Advisory Board. A number of other advisory functions are also 
identified for the committee by section 109(d)(2)(C), which reads:

    Such committee shall also (i) advise the Administrator of areas 
in which additional knowledge is required to appraise the adequacy 
and basis of existing, new, or revised national ambient air quality 
standards, (ii) describe the research efforts necessary to provide 
the required information, (iii) advise the Administrator on the 
relative contribution to air pollution concentrations of natural as 
well as anthropogenic activity, and (iv) advise the Administrator of 
any adverse public health, welfare, social, economic, or energy 
effects which may result from various strategies for attainment and 
maintenance of such national ambient air quality standards.

    As previously noted, the Supreme Court has held that section 109(b) 
``unambiguously bars cost considerations from the NAAQS-setting 
process.'' Whitman v. Am. Trucking Associations, 531 U.S. 457, 471 
(2001). Accordingly, while some of these issues regarding which 
Congress has directed the CASAC to advise the Administrator are ones 
that are relevant to the standard setting process, others are not. 
Issues

[[Page 24097]]

that are not relevant to standard setting may be relevant to 
implementation of the NAAQS once they are established.\4\
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    \4\ Some aspects of the CASAC's advice may not be relevant to 
the EPA's process of setting primary and secondary standards that 
are requisite to protect public health and welfare. Indeed, were EPA 
to consider costs of implementation when reviewing and revising the 
standards ``it would be grounds for vacating the NAAQS.'' Whitman, 
531 U.S. at 471 n.4. At the same time, the CAA directs the CASAC to 
provide advice on ``any adverse public health, welfare, social, 
economic, or energy effects which may result from various strategies 
for attainment and maintenance'' of the NAAQS to the Administrator 
under section 109(d)(2)(C)(iv). In Whitman, the Court clarified that 
most of that advice would be relevant to implementation but not 
standard setting, as it ``enable[s] the Administrator to assist the 
States in carrying out their statutory role as primary implementers 
of the NAAQS.'' Id. at 470 (emphasis in original). However, the 
Court also noted that the CASAC's ``advice concerning certain 
aspects of `adverse public health . . . effects' from various 
attainment strategies is unquestionably pertinent'' to the NAAQS 
rulemaking record and relevant to the standard setting process. Id. 
at 470 n.2.
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B. Related PM Control Programs

    States are primarily responsible for ensuring attainment and 
maintenance of ambient air quality standards once the EPA has 
established them. Under section 110 and 171-190 of the CAA, and related 
provisions and regulations, 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 Prevention of Significant Deterioration (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.

C. Review of the Air Quality Criteria and Standards for Particulate 
Matter

1. Reviews Completed in 1971 and 1987
    The EPA first established NAAQS for PM in 1971 (36 FR 8186, April 
30, 1971), based on the original Air Quality Criteria Document (AQCD) 
(DHEW, 1969).\5\ The federal reference method (FRM) 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 [micro]m 
(referred to as total suspended particulates or TSP). The primary 
standards were set at 260 [micro]g/m\3\, 24-hour average, not to be 
exceeded more than once per year, and 75 [micro]g/m\3\, annual 
geometric mean. The secondary standards were set at 150 [micro]g/m\3\, 
24-hour average, not to be exceeded more than once per year, and 60 
[micro]g/m\3\, annual geometric mean.
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    \5\ Prior to the review initiated in 2007 (see below), the AQCD 
provided the scientific foundation (i.e., the air quality criteria) 
for the NAAQS. Beginning in that review, the Integrated Science 
Assessment (ISA) has replaced the AQCD.
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    In October 1979 (44 FR 56730, October 2, 1979), the EPA announced 
the first periodic review of the air quality criteria and NAAQS for PM. 
Revised primary and secondary standards were promulgated in 1987 (52 FR 
24634, July 1, 1987). In the 1987 decision, the EPA changed the 
indicator for particles from TSP to PM10, in order to focus 
on the subset of inhalable particles small enough to penetrate to the 
thoracic region of the respiratory tract (including the 
tracheobronchial and alveolar regions), referred to as thoracic 
particles.\6\ The level of the 24-hour standards (primary and 
secondary) was set at 150 [micro]g/m\3\, and the form was one expected 
exceedance per year, on average over three years. The level of the 
annual standards (primary and secondary) was set at 50 [micro]g/m\3\, 
and the form was annual arithmetic mean, averaged over three years.
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    \6\ PM10 refers to particles with a nominal mean 
aerodynamic diameter less than or equal to 10 [micro]m. More 
specifically, 10 [micro]m is the aerodynamic diameter for which the 
efficiency of particle collection is 50 percent.
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2. Review Completed in 1997
    In April 1994, the EPA announced its plans for the second periodic 
review of the air quality criteria and NAAQS for PM, and in 1997 the 
EPA promulgated revisions to the NAAQS (62 FR 38652, July 18, 1997). In 
the 1997 decision, the EPA determined that the fine and coarse 
fractions of PM10 should be considered separately. This 
determination was based on evidence that serious health effects were 
associated with short- and long-term exposures to fine particles in 
areas that met the existing PM10 standards. The EPA added 
new standards, using PM2.5 as the indicator for fine 
particles (with PM2.5 referring to particles with a nominal 
mean aerodynamic diameter less than or equal to 2.5 [micro]m). The new 
primary standards were as follows: (1) An annual standard with a level 
of 15.0 [micro]g/m\3\, based on the 3-year average of annual arithmetic 
mean PM2.5 concentrations from single or multiple community-
oriented monitors; \7\ and (2) a 24-hour standard with a level of 65 
[micro]g/m\3\, based on the 3-year average of the 98th percentile of 
24-hour PM2.5 concentrations at each monitor within an area. 
Also, the EPA established a new reference method for the measurement of 
PM2.5 in the ambient air and adopted rules for determining 
attainment of the new standards. To continue to address the health 
effects of the coarse fraction of PM10 (referred to as 
thoracic coarse particles or PM10-2.5; generally including 
particles with a nominal mean aerodynamic diameter greater than 2.5 
[micro]m and less than or equal to 10 [micro]m), the EPA retained the 
primary annual PM10 standard and revised the form of the 
primary 24-hour PM10 standard to be based on the 99th 
percentile of 24-hour PM10 concentrations at each monitor in 
an area. The EPA revised the secondary standards by setting them equal 
in all respects to the primary standards.
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    \7\ The 1997 annual PM2.5 standard was compared with 
measurements made at the community-oriented monitoring site 
recording the highest concentration or, if specific constraints were 
met, measurements from multiple community-oriented monitoring sites 
could be averaged (i.e., ``spatial averaging''). In the last review 
(completed in 2012) the EPA replaced the term ``community-oriented'' 
monitor with the term ``area-wide'' monitor. Area-wide monitors are 
those sited at the neighborhood scale or larger, as well as those 
monitors sited at micro- or middle-scales that are representative of 
many such locations in the same core-based statistical area (CBSA) 
(78 FR 3236, January 15, 2013).
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    Following promulgation of the 1997 PM NAAQS, petitions for review 
were filed by several parties, addressing a broad range of issues. In 
May 1999, the U.S. Court of Appeals for the District of Columbia 
Circuit (D.C. Circuit) upheld the EPA's decision to establish fine 
particle standards, holding that ``the growing empirical evidence 
demonstrating a relationship between fine particle pollution and 
adverse health effects amply justifies establishment of new fine 
particle standards.'' American Trucking Associations, Inc. v. EPA, 175 
F. 3d 1027, 1055-56 (D.C. Cir. 1999). The D.C. Circuit also found 
``ample support'' for the EPA's decision to regulate coarse particle 
pollution, but vacated the 1997 PM10 standards, concluding 
that the EPA had not provided a reasonable explanation justifying use 
of PM10 as an indicator for coarse particles. American 
Trucking Associations v. EPA, 175 F. 3d at 1054-55. Pursuant to the 
D.C. Circuit's decision, the EPA removed the vacated 1997 
PM10 standards, and the pre-existing 1987 PM10 
standards remained in place (65 FR 80776, December 22, 2000). The D.C. 
Circuit also upheld the EPA's determination not to establish more 
stringent secondary standards for fine particles to address

[[Page 24098]]

effects on visibility. American Trucking Associations v. EPA, 175 F. 3d 
at 1027.
    The D.C. Circuit also addressed more general issues related to the 
NAAQS, including issues related to the consideration of costs in 
setting NAAQS and the EPA's approach to establishing the levels of 
NAAQS. Regarding the cost issue, the court reaffirmed prior rulings 
holding that in setting NAAQS the EPA is ``not permitted to consider 
the cost of implementing those standards.'' American Trucking 
Associations v. EPA, 175 F. 3d at 1040-41. Regarding the levels of 
NAAQS, the court held that the EPA's approach to establishing the level 
of the standards in 1997 (i.e., both for PM and for the ozone NAAQS 
promulgated on the same day) effected ``an unconstitutional delegation 
of legislative authority.'' American Trucking Associations v. EPA, 175 
F. 3d at 1034-40. Although the court 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.
    The D.C. Circuit's holding on the cost and constitutional issues 
were appealed to the United States Supreme Court. In February 2001, the 
Supreme Court issued a unanimous decision upholding the EPA's position 
on both the cost and constitutional issues. Whitman v. American 
Trucking Associations, 531 U.S. 457, 464, 475-76. On the constitutional 
issue, the Court held that the statutory requirement that NAAQS be 
``requisite'' to protect public health with an adequate margin of 
safety sufficiently guided the EPA's discretion, affirming the EPA's 
approach of setting standards that are neither more nor less stringent 
than necessary.
    The Supreme Court remanded the case to the D.C. Circuit for 
resolution of any remaining issues that had not been addressed in that 
court's earlier rulings. Id. at 475-76. In a March 2002 decision, the 
D.C. Circuit rejected all remaining challenges to the standards, 
holding that the EPA's PM2.5 standards were reasonably 
supported by the administrative record and were not ``arbitrary and 
capricious.'' American Trucking Associations v. EPA, 283 F. 3d 355, 
369-72 (D.C. Cir. 2002).
3. Review Completed in 2006
    In October 1997, the EPA published its plans for the third periodic 
review of the air quality criteria and NAAQS for PM (62 FR 55201, 
October 23, 1997). After the CASAC and public review of several drafts, 
the EPA's National Center for Environmental Assessment (NCEA) finalized 
the AQCD in October 2004 (U.S. EPA, 2004). The EPA's Office of Air 
Quality Planning and Standards (OAQPS) finalized a Risk Assessment and 
Staff Paper in December 2005 (Abt Associates, 2005, U.S. EPA, 2005).\8\ 
On December 20, 2005, the EPA announced its proposed decision to revise 
the NAAQS for PM and solicited public comment on a broad range of 
options (71 FR 2620, January 17, 2006). On September 21, 2006, the EPA 
announced its final decisions to revise the primary and secondary NAAQS 
for PM to provide increased protection of public health and welfare, 
respectively (71 FR 61144, October 17, 2006). With regard to the 
primary and secondary standards for fine particles, the EPA revised the 
level of the 24-hour PM2.5 standards to 35 [micro]g/m\3\, 
retained the level of the annual PM2.5 standards at 15.0 
[micro]g/m\3\, and revised the form of the annual PM2.5 
standards by narrowing the constraints on the optional use of spatial 
averaging. With regard to the primary and secondary standards for 
PM10, the EPA retained the 24-hour standards, with levels at 
150 [micro]g/m\3\, and revoked the annual standards.\9\ The 
Administrator judged that the available evidence generally did not 
suggest a link between long-term exposure to existing ambient levels of 
coarse particles and health or welfare effects. In addition, a new 
reference method was added for the measurement of PM10-2.5 
in the ambient air in order to provide a basis for approving federal 
equivalent methods (FEMs) and to promote the gathering of scientific 
data to support future reviews of the PM NAAQS.
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    \8\ Prior to the review initiated in 2007, the Staff Paper 
presented the EPA staff's considerations and conclusions regarding 
the adequacy of existing NAAQS and, when appropriate, the potential 
alternative standards that could be supported by the evidence and 
information. More recent reviews present this information in the 
Policy Assessment.
    \9\ In the 2006 proposal, the EPA proposed to revise the 24-hour 
PM10 standard in part by establishing a new 
PM10-2.5 indicator for thoracic coarse particles (i.e., 
particles generally between 2.5 and 10 [micro]m in diameter). The 
EPA proposed to include any ambient mix of PM10-2.5 that 
was dominated by resuspended dust from high density traffic on paved 
roads and by PM from industrial sources and construction sources. 
The EPA proposed to exclude any ambient mix of PM10-2.5 
that was dominated by rural windblown dust and soils and by PM 
generated from agricultural and mining sources. In the final 
decision, the existing PM10 standard was retained, in 
part due to an ``inability . . . to effectively and precisely 
identify which ambient mixes are included in the 
[PM10-2.5] indicator and which are not'' (71 FR 61197, 
October 17, 2006).
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    Several parties filed petitions for review following promulgation 
of the revised PM NAAQS in 2006. These petitions addressed the 
following issues: (1) Selecting the level of the primary annual 
PM2.5 standard; (2) retaining PM10 as the 
indicator of a standard for thoracic coarse particles, retaining the 
level and form of the 24-hour PM10 standard, and revoking 
the PM10 annual standard; and (3) setting the secondary 
PM2.5 standards identical to the primary standards. On 
February 24, 2009, the D.C. Circuit issued its opinion in the case 
American Farm Bureau Federation v. EPA, 559 F. 3d 512 (D.C. Cir. 2009). 
The court remanded the primary annual PM2.5 NAAQS to the EPA 
because the Agency had failed to adequately explain why the standards 
provided the requisite protection from both short- and long-term 
exposures to fine particles, including protection for at-risk 
populations. Id. at 520-27. With regard to the standards for 
PM10, the court upheld the EPA's decisions to retain the 24-
hour PM10 standard to provide protection from thoracic 
coarse particle exposures and to revoke the annual PM10 
standard. Id. at 533-38. With regard to the secondary PM2.5 
standards, the court remanded the standards to the EPA because the 
Agency 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. Id. at 528-32. The EPA responded to the court's remands as 
part of the next review of the PM NAAQS, which was initiated in 2007 
(discussed below).
4. Review Completed in 2012
    In June 2007, the EPA initiated the fourth periodic review of the 
air quality criteria and the PM NAAQS by issuing a call for information 
(72 FR 35462, June 28, 2007). Based on the NAAQS review process, as 
revised in 2008 and again in 2009,\10\ the EPA held science/policy 
issue workshops on the primary and secondary PM NAAQS (72 FR 34003, 
June 20, 2007; 72 FR 34005, June 20, 2007), and prepared and released 
the planning and assessment documents that comprise the review process 
(i.e., IRP (U.S. EPA, 2008), ISA (U.S. EPA, 2009c), REA planning 
documents for health and welfare (U.S. EPA, 2009b, U.S. EPA, 2009a), a 
quantitative health risk assessment (U.S. EPA, 2010a) and an urban-
focused visibility assessment

[[Page 24099]]

(U.S. EPA, 2010a), and PA (U.S. EPA, 2011)). In June 2012, the EPA 
announced its proposed decision to revise the NAAQS for PM (77 FR 
38890, June 29, 2012).
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    \10\ The history of the NAAQS review process, including 
revisions to the process, is discussed at https://www.epa.gov/naaqs/historical-information-naaqs-review-process.
---------------------------------------------------------------------------

    In December 2012, the EPA announced its final decisions to revise 
the primary NAAQS for PM to provide increased protection of public 
health (78 FR 3086, January 15, 2013). With regard to primary standards 
for PM2.5, the EPA revised the level of the annual 
PM2.5 standard \11\ to 12.0 [micro]g/m\3\ and retained the 
24-hour PM2.5 standard, with its level of 35 [micro]g/m\3\. 
For the primary PM10 standard, the EPA retained the 24-hour 
standard to continue to provide protection against effects associated 
with short-term exposure to thoracic coarse particles (i.e., 
PM10-2.5). With regard to the secondary PM standards, the 
EPA generally retained the 24-hour and annual PM2.5 
standards \12\ and the 24-hour PM10 standard to address 
visibility and non-visibility welfare effects.
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    \11\ The EPA also eliminated the option for spatial averaging.
    \12\ Consistent with the primary standard, the EPA eliminated 
the option for spatial averaging with the annual standard.
---------------------------------------------------------------------------

    As with previous reviews, petitioners challenged the EPA's final 
rule. Petitioners argued that the EPA acted unreasonably in revising 
the level and form of the annual standard and in amending the 
monitoring network provisions. On judicial review, the revised 
standards and monitoring requirements were upheld in all respects. NAM 
v. EPA, 750 F.3d 921 (D.C. Cir. 2014).
5. Current Review
    In December 2014, the EPA announced the initiation of the current 
periodic review of the air quality criteria for PM and of the 
PM2.5 and PM10 NAAQS and issued a call for 
information (79 FR 71764, December 3, 2014). On February 9 to 11, 2015, 
the EPA's NCEA and OAQPS held a public workshop to inform the planning 
for the current review of the PM NAAQS (announced in 79 FR 71764, 
December 3, 2014). Workshop participants, including a wide range of 
external experts as well as EPA staff representing a variety of areas 
of expertise (e.g., epidemiology, human and animal toxicology, risk/
exposure analysis, atmospheric science, visibility impairment, climate 
effects), were asked to highlight significant new and emerging PM 
research, and to make recommendations to the Agency regarding the 
design and scope of this review. This workshop provided for a public 
discussion of the key science and policy-relevant issues around which 
the EPA has structured the current review of the PM NAAQS and of the 
most meaningful new scientific information that would be available in 
this review to inform understanding of these issues.
    The input received at the workshop guided the EPA staff in 
developing a draft IRP, which was reviewed by the CASAC Particulate 
Matter Panel and discussed on public teleconferences held in May 2016 
(81 FR 13362, March 14, 2016) and August 2016 (81 FR 39043, June 15, 
2016). Advice from the CASAC, supplemented by the Particulate Matter 
Panel, and input from the public were considered in developing the 
final IRP (U.S. EPA, 2016). The final IRP discusses the approaches to 
be taken in developing key scientific, technical, and policy documents 
in this review and the key policy-relevant issues.
    In May 2018, the Administrator issued a memorandum describing a 
``back-to-basics'' process for reviewing the NAAQS (Pruitt, 2018). This 
memo announced the Agency's intention to conduct the current review of 
the PM NAAQS in such a manner as to ensure that any necessary revisions 
are finalized by December 2020. Following this memo, on October 10, 
2018 the Administrator additionally announced that the role of 
reviewing the key assessments developed as part of the ongoing review 
of the PM NAAQS (i.e., drafts of the ISA and PA) would be performed by 
the seven-member chartered CASAC (i.e., rather than the CASAC 
Particulate Matter Panel that reviewed the draft IRP).\13\
---------------------------------------------------------------------------

    \13\ The CASAC charter is available at: https://
yosemite.epa.gov/sab/sabproduct.nsf/WebCASAC/2019casaccharter/$File/
CASAC%202019%20Renewal%20Charter%203.21.19%20-%20final.pdf. The 
Administrator's announcement is available at: https://archive.epa.gov/epa/newsreleases/acting-administrator-wheeler-announces-science-advisors-key-clean-air-act-committee.html.
---------------------------------------------------------------------------

    The EPA released the draft ISA in October 2018 (83 FR 53471, 
October 23, 2018). The draft ISA was reviewed by the chartered CASAC at 
a public meeting held in Arlington, VA in December 2018 (83 FR 55529, 
November 6, 2018) and was discussed on a public teleconference in March 
2019 (84 FR 8523, March 8, 2019). The CASAC provided its advice on the 
draft ISA in a letter to the EPA Administrator dated April 11, 2019 
(Cox, 2019b). In that letter, the CASAC's recommendations address both 
the draft ISA's assessment of the science for PM-related effects and 
the process under which this review of the PM NAAQS is being conducted.
    Regarding the assessment of the evidence, the CASAC letter states 
that ``the Draft ISA does not provide a sufficiently comprehensive, 
systematic assessment of the available science relevant to 
understanding the health impacts of exposure to particulate matter 
(PM)'' (Cox, 2019b, p. 1 of letter). The CASAC recommended that this 
and other limitations (i.e., ``[i]nadequate evidence for altered causal 
determinations'' and the need for a ``[c]learer discussion of causality 
and causal biological mechanisms and pathways'') be remedied in a 
revised ISA (Cox, 2019b, p. 1 of letter).
    Given the Administrator's timeline for this review, as noted above 
(Pruitt, 2018), the EPA did not prepare a second draft ISA. Rather, the 
EPA has taken steps to address the CASAC's comments in the Final PM ISA 
(U.S. EPA, 2019). In particular, the final ISA includes additional text 
and a new appendix to clarify the comprehensive and systematic process 
employed by the EPA to develop the PM ISA. In addition, several 
causality determinations were re-examined and, consistent with the 
CASAC advice, the final ISA reflects a revised causality determination 
for long-term ultrafine particle (UFP) exposures and nervous system 
effects (i.e., from ``likely to be causal'' to ``suggestive of, but not 
sufficient to infer, a causal relationship'').\14\ The final ISA also 
contains additional text to clarify the evidence for biological 
pathways of particular PM-related effects and the role of that evidence 
in causality determinations.
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    \14\ Based on the CASAC's comments, the EPA also re-examined the 
causality determinations for cancer and for nervous system effects 
following long-term PM2.5 exposures. The EPA's 
consideration of these comments in the final ISA is discussed below 
in sections II.B.1.d and II.B.1.e.
---------------------------------------------------------------------------

    Among its comments on the process, the chartered CASAC recommended 
``that the EPA reappoint the previous CASAC PM panel (or appoint a 
panel with similar expertise)'' (Cox, 2019b). The Agency's response to 
this advice was provided in a letter from the Administrator to the 
CASAC chair dated July 25, 2019.\15\ In that letter, the Administrator 
announced his intention to identify a pool of non-member subject matter 
expert consultants to support the CASAC's review activities for the PM 
and ozone NAAQS. A Federal Register notice requesting the nomination of 
scientists from a broad range of disciplines ``with demonstrated 
expertise and research in the field of air pollution related to PM and 
ozone'' was published in August 2019 (84 FR 38625,

[[Page 24100]]

August 7, 2019). The Administrator selected consultants from among 
those nominated, and input from members of this pool of consultants 
informed the CASAC's review of the draft PA.
---------------------------------------------------------------------------

    \15\ Available at: https://yosemite.epa.gov/sab/sabproduct.nsf/
0/6CBCBBC3025E13B4852583D90047B352/$File/EPA-CASAC-19-
002_Response.pdf.
---------------------------------------------------------------------------

    The EPA released the draft PA in September 2019 (84 FR 47944, 
September 11, 2019). The draft PA drew from the assessment of the 
evidence in the draft ISA. It was reviewed by the chartered CASAC and 
discussed in October 2019 at a public meeting held in Cary, NC. Public 
comments were received via a separate public teleconference (84 FR 
51555, September 30, 2019). A public meeting to discuss the chartered 
CASAC letter and response to charge questions on the draft PA was held 
in Cary, NC in December 2019 (84 FR 58713, November 1, 2019), and the 
CASAC provided its advice on the draft PA, including its advice on the 
current primary and secondary PM standards, in a letter to the EPA 
Administrator dated December 16, 2019 (Cox, 2019a).
    With regard to the primary standards, the CASAC recommended 
retaining the current 24-hour PM2.5 and PM10 
standards but did not reach consensus on the adequacy of the current 
annual PM2.5 standard. With regard to the secondary 
standards, the CASAC recommended retaining the current standards. The 
CASAC's advice on the primary and secondary PM standards, and the 
Administrator's consideration of that advice in reaching proposed 
decisions, is discussed in detail in sections II.C.2 and II.C.3 
(primary PM2.5 standards), III.C.2 and III.C.3 (primary 
PM10 standards), and IV.D.2 and IV.D.3 (secondary standards) 
of this document.
    The CASAC additionally made a number of recommendations regarding 
the information and analyses presented in the draft PA. Specifically, 
the CASAC recommended that a revised PA include (1) additional 
discussion of the current CASAC and NAAQS review process; (2) 
additional characterization of PM-related emissions, monitoring and air 
quality information, including uncertainties in that information; (3) 
additional discussion and examination of uncertainties in the 
PM2.5 health evidence and the risk assessment; (4) updates 
to reflect changes in the ISA's causality determinations; and (5) 
additional discussion of the evidence for PM-related welfare effects, 
including uncertainties (Cox, 2019a, pp. 2-3 in letter). In response to 
the CASAC's comments, the final PA \16\ incorporated a number of 
changes, including the following (U.S. EPA, 2020):
---------------------------------------------------------------------------

    \16\ Given the Administrator's timeline for this review, as 
noted above (Pruitt, 2018), the EPA did not prepare a second draft 
PA. Rather, the CASAC's advice was considered in developing the 
final PA (U.S. EPA, 2020).
---------------------------------------------------------------------------

     Text was added to Chapter 1 to clarify the process 
followed for this review of the PM NAAQS, including how the process has 
evolved since the initiation of the review.
     Text and figures were added to Chapter 2 on emissions of 
PM and PM precursors, and a section discussing uncertainty in emissions 
estimates was added. A discussion of measurement uncertainty for FRM, 
FEM, CSN, and IMPROVE monitors was also added.
     Chapter 3 and Appendices B and C include a number of 
changes, including:
    [cir] An expanded characterization and discussion of the evidence 
related to exposure measurement error, the potential confounders 
examined by key studies, the shapes of concentration-response 
functions, and the results of causal inference and quasi-experimental 
studies.
    [cir] An expanded and clarified discussion of uncertainties in the 
risk assessment, and additional air quality model performance 
evaluations for each of the urban study areas included in the risk 
assessment.
    [cir] Additional detail on the procedure used to derive 
concentration-response functions used in the risk assessment.
    [cir] Changes in the text to reflect the change in the final ISA's 
causality determination from ``likely to be causal'' to ``suggestive 
of, but not sufficient to infer, a causal relationship.''
     Throughout the document (Chapters 3, 4 and 5), summaries 
of the CASAC advice on the PM standards are included, and expanded 
discussions of data gaps and areas for future research in the health 
and welfare effects evidence are presented.

D. Air Quality Information

    This section provides a summary of basic information related to PM 
ambient air quality. It summarizes information on the distribution of 
particle size in ambient air (I.D.1), sources and emissions 
contributing to PM in the ambient air (I.D.2), monitoring of ambient PM 
in the U.S. (I.D.3), ambient PM concentrations and trends in the U.S. 
(I.D.4), and background PM (I.D.5). Additional detail on PM air quality 
can be found in Chapter 2 of the Policy Assessment (U.S. EPA, 2020; 
PA).
1. Distribution of Particle Size in Ambient Air
    In ambient air, PM is a mixture of substances suspended as small 
liquid and/or solid particles (U.S. EPA, 2019, section 2.2). Particle 
size is an important consideration for PM, as distinct health and 
welfare effects have been linked with exposures to particles of 
different sizes. Particles in the atmosphere range in size from less 
than 0.01 to more than 10 [mu]m in diameter (U.S. EPA, 2019, section 
2.2). When describing PM, subscripts are used to denote the aerodynamic 
diameter \17\ of the particle size range, in [micro]m, of 50% cut 
points of sampling devices. The EPA defines PM2.5, also 
referred to as fine particles, as particles with aerodynamic diameters 
generally less than or equal to 2.5 [mu]m. The size range for 
PM10-2.5, also called coarse or thoracic coarse particles, 
includes those particles with aerodynamic diameters generally greater 
than 2.5 [mu]m and less than or equal to 10 [mu]m. PM10, 
which is comprised of both fine and coarse fractions, includes those 
particles with aerodynamic diameters generally less than or equal to 10 
[mu]m. In addition, UFP are often defined as particles with a diameter 
of less than 0.1 [mu]m based on physical size, thermal diffusivity or 
electrical mobility (U.S. EPA, 2019, section 2.2).
---------------------------------------------------------------------------

    \17\ Aerodynamic diameter is the size of a sphere of unit 
density (i.e., 1 g/cm\3\) that has the same terminal settling 
velocity as the particle of interest (U.S. EPA, 2019, section 
4.1.1).
---------------------------------------------------------------------------

    Atmospheric distributions of particle size generally exhibit 
distinct modes that roughly align with the PM size fractions defined 
above. The nucleation mode is made up of freshly generated particles, 
formed either during combustion or by atmospheric reactions of 
precursor gases. The nucleation mode is especially prominent near 
sources like heavy traffic, industrial emissions, biomass burning, or 
cooking (Vu et al., 2015). While nucleation mode particles are only a 
minor contributor to overall ambient PM mass and surface area, they are 
the main contributors to ambient particle number (U.S. EPA, 2019, 
section 2.2). By number, most nucleation mode particles fall into the 
UFP size range, though some fraction of the nucleation mode number 
distribution can extend above 0.1 [mu]m in diameter. Nucleation mode 
particles can grow rapidly through coagulation or uptake of gases by 
particle surfaces, giving rise to the accumulation mode. The 
accumulation mode is typically the predominant contributor to 
PM2.5 mass, though only a minor contributor to particle 
number (U.S. EPA, 2019, section 2.2). PM2.5 sampling methods 
measure most of the accumulation mode mass, although a small fraction 
of particles that make up the accumulation mode are greater than 2.5 
[mu]m in diameter. Coarse mode particles are formed by mechanical 
generation, and through processes like dust resuspension and sea spray 
formation

[[Page 24101]]

(Whitby et al., 1972). Most coarse mode mass is captured by 
PM10-2.5 sampling, but small fractions of coarse mode mass 
can be smaller than 2.5 [mu]m or greater than 10 [mu]m in diameter 
(U.S. EPA, 2019, section 2.2).
    Most particles are found in the lower troposphere, where they can 
have residence times ranging from a few hours to weeks. Particles are 
removed from the atmosphere by wet deposition, such as when they are 
carried by rain or snow, or by dry deposition, when particles settle 
out of suspension due to gravity. Atmospheric lifetimes are generally 
longest for PM2.5, which often remains in the atmosphere for 
days to weeks (U.S. EPA, 2019, Table 2-1) before being removed by wet 
or dry deposition. In contrast, atmospheric lifetimes for UFP and 
PM10-2.5 are shorter. Within hours, UFP can undergo 
coagulation and condensation that lead to formation of larger particles 
in the accumulation mode, or can be removed from the atmosphere by 
evaporation, deposition, or reactions with other atmospheric 
components. PM10-2.5 are also generally removed from the 
atmosphere within hours, through wet or dry deposition (U.S. EPA, 2019, 
Table 2-1).
2. Sources and Emissions Contributing to PM in the Ambient Air
    PM is composed of both primary (directly emitted particles) and 
secondary particles. Primary PM is derived from direct particle 
emissions from specific PM sources while secondary PM originates from 
gas-phase chemical compounds present in the atmosphere that have 
participated in new particle formation or condensed onto existing 
particles (U.S. EPA, 2019, section 2.3). As discussed further in the 
ISA (U.S. EPA, 2019, section 2.3.2.1), secondary PM is formed in the 
atmosphere by photochemical oxidation reactions of both inorganic and 
organic gas-phase precursors. Precursor gases include sulfur dioxide 
(SO2), nitrogen oxides (NOX), and volatile 
organic compounds (VOC) (U.S. EPA, 2019, section 2.3.2.1). Ammonia also 
plays an important role in the formation of nitrate PM by neutralizing 
sulfuric acid and nitric acid. Sources and emissions of PM are 
discussed in more detail in section 2.1.1 of the PA (U.S. EPA, 2020).
    Direct emissions of PM have remained relatively unchanged in recent 
years, while emissions of some precursor gases have declined 
substantially.\18\ From 1990 to 2014, SO2 emissions have 
undergone the largest declines while NH3 emissions have 
undergone the smallest change. Declining SO2 emissions 
during this time period are primarily a result of reductions at 
stationary sources such as EGUs, with substantial reductions also from 
mobile sources (U.S. EPA, 2019, section 2.3.2.1).\19\
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    \18\ More information on these trends, including details on 
methods and explanations on the noted changes over time is available 
at https://gispub.epa.gov/neireport/2014/.
    \19\ State-specific emission trends data for 1990 to 2014 can be 
found at: https://www.epa.gov/air-emissions-inventories/air-pollutant-emissions-trends-data.
---------------------------------------------------------------------------

3. Monitoring of Ambient PM
    To promote uniform enforcement of the air quality standards set 
forth under the CAA and to achieve the degree of public health and 
welfare protection intended for the NAAQS, the EPA established PM 
Federal Reference Methods (FRMs) \20\ for both PM10 and 
PM2.5 (40 CFR appendix J and L to Part 50) and performance 
requirements for approval of Federal Equivalent Methods (FEMs) (40 CFR 
part 53). Amended following the 2006 and 2012 p.m. NAAQS reviews, the 
current PM monitoring network relies on FRMs and automated continuous 
FEMs, in part to support changes necessary for implementation of the 
revised PM standards. The requirements for measuring ambient air 
quality and reporting ambient air quality data and related information 
are the basis for 40 CFR appendices A through E to Part 58. More 
information on PM ambient monitoring networks is available in section 
2.2 of the PA (U.S. EPA, 2020).
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    \20\ FRMs provide the methodological basis for comparison to the 
NAAQS and also serve as the ``gold-standard'' for the comparison of 
other methods being reviewed for potential approval as equivalent 
methods. The EPA keeps a complete list of designated reference and 
equivalent methods available on its Ambient Monitoring Technology 
Information Center (AMTIC) website (https://www.epa.gov/amtic/air-monitoring-methods-criteria-pollutants).
---------------------------------------------------------------------------

4. Ambient Concentrations and Trends
    This section summarizes available information on recent ambient PM 
concentrations in the U.S. and on trends in PM air quality. Sections 
I.D.4.a and I.D.4.b summarize information on PM2.5 mass and 
components, respectively. Section I.D.4.c summarizes information on 
PM10. Sections I.D.4.d and I.D.4.e summarize the more 
limited information on PM10-2.5 and UFP, respectively. 
Additional detail on PM air quality and trends can be found in section 
2.3 of the PA (U.S. EPA, 2020).
a. PM2.5 Mass
    At monitoring sites in the U.S., annual PM2.5 
concentrations from 2015 to 2017 averaged 8.0 [mu]g/m\3\ (and ranged 
from 3.0 to 18.2 [mu]g/m\3\) and the 98th percentiles of 24-hour 
concentrations averaged 20.9 [mu]g/m\3\ (and ranged from 9.2 to 111 
[mu]g/m\3\) (U.S. EPA, 2020, section 2.3.2.1). The highest ambient 
PM2.5 concentrations occur in the west, particularly in 
California and the Pacific northwest (U.S. EPA, 2020, Figure 2-8). Much 
of the eastern U.S. has lower ambient concentrations, with annual 
average concentrations generally at or below 12.0 [mu]g/m\3\ and 98th 
percentiles of 24-hour concentrations generally at or below 30 [mu]g/
m\3\ (U.S. EPA, 2020, section 2.3.2).
    Recent ambient PM2.5 concentrations reflect the 
substantial reductions that have occurred across much of the U.S. (U.S. 
EPA, 2020, section 2.3.2.1). From 2000 to 2017, national annual average 
PM2.5 concentrations have declined from 13.5 [mu]g/m\3\ to 
8.0 [mu]g/m\3\, a 41% decrease (U.S. EPA, 2020, section 2.3.2.1).\21\ 
These declines have occurred at urban and rural monitoring sites, 
although urban PM2.5 concentrations remain consistently 
higher than those in rural areas (Chan et al., 2018) due to the impact 
of local sources in urban areas. Analyses at individual monitoring 
sites indicate that declines in ambient PM2.5 concentrations 
have been most consistent across the eastern U.S. and in parts of 
coastal California, where both annual average and 98th percentiles of 
24-hour concentrations have declined significantly (U.S. EPA, 2020, 
section 2.3.2.1). In contrast, trends in ambient PM2.5 
concentrations have been less consistent over much of the western U.S., 
with no significant changes since 2000 observed at some sites in the 
Pacific northwest, the northern Rockies and plains, and the southwest, 
particularly for 98th percentiles of 24-hour concentrations (U.S. EPA, 
2020, section 2.3.2.1).
---------------------------------------------------------------------------

    \21\ See https://www.epa.gov/air-trends/particulate-matter-pm25-trends and https://www.epa.gov/air-trends/particulate-matter-pm25-trends#pmnat for more information.
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    The recent deployment of PM2.5 monitors near major roads 
in large urban areas provides information on PM2.5 
concentrations near an important emissions source. Of the 25 CBSAs with 
valid design values at near-road monitoring sites,\22\ 52% measured the 
highest annual design value at the near-road site while 24% measured 
the highest 24-hour design value at the near-road site (U.S. EPA, 2020, 
section 2.3.2.2). Of the CBSAs with highest

[[Page 24102]]

annual design values at near-road sites, those design values were, on 
average, 0.7 [mu]g/m\3\ higher than at the highest measuring non-near-
road sites (range is 0.1 to 2.0 [mu]g/m\3\ higher at near-road sites). 
Although most near-road monitoring sites do not have sufficient data to 
evaluate long-term trends in near-road PM2.5 concentrations, 
analyses of the data at one near-road-like site in Elizabeth, NJ,\23\ 
show that the annual average near-road increment has generally 
decreased between 1999 and 2017 from about 2.0 [mu]g/m\3\ to about 1.3 
[mu]g/m\3\ (U.S. EPA, 2020, section 2.3.2.2).
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    \22\ A design value is considered valid if it meets the data 
handling requirements given in 40 CFR appendix N to Part 50. Several 
large CBSAs such as Chicago-Naperville-Elgin, IL-IN-WI and Houston-
The Woodlands-Sugar Land, TX had near-road sites that did not have 
valid PM2.5 design values for the 2015-2017 period.
    \23\ The Elizabeth Lab site in Elizabeth, NJ is situated 
approximately 30 meters from travel lanes of the Interchange 13 toll 
plaza of the New Jersey Turnpike and within 200 meters of travel 
lanes for Interstate 278 and the New Jersey Turnpike.
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b. PM2.5 Components
    Based on recent air quality data, the major chemical components of 
PM2.5 have distinct spatial distributions. Sulfate 
concentrations tend to be highest in the eastern U.S., while in the 
Ohio Valley, Salt Lake Valley, and California nitrate concentrations 
are highest, and relatively high concentrations of organic carbon are 
widespread across most of the continental U.S. (U.S. EPA, 2020, section 
2.3.2.3). Elemental carbon, crustal material, and sea-salt are found to 
have the highest concentrations in the northeast U.S., southwest U.S., 
and coastal areas, respectively.
    An examination of PM2.5 composition trends can provide 
insight into the factors contributing to overall reductions in ambient 
PM2.5 concentrations. The biggest change in PM2.5 
composition that has occurred in recent years is the reduction in 
sulfate concentrations due to reductions in SO2 emissions. 
Between 2000 and 2015, the nationwide annual average sulfate 
concentration decreased by 17% at urban sites and 20% at rural sites. 
This change in sulfate concentrations is most evident in the eastern 
U.S. and has resulted in organic matter or nitrate now being the 
greatest contributor to PM2.5 mass in many locations (U.S. 
EPA, 2019, Figure 2-19). The overall reduction in sulfate 
concentrations has contributed substantially to the decrease in 
national average PM2.5 concentrations as well as the decline 
in the fraction of PM10 mass accounted for by 
PM2.5 (U.S. EPA, 2019, section 2.5.1.1.6; U.S. EPA, 2020, 
section 2.3.1).
c. PM10
    At monitoring sites in the U.S., the 2015-2017 average of 2nd 
highest 24-hour PM10 concentration was 56 [mu]g/m\3\ 
(ranging from 18 to 173 [mu]g/m\3\) (U.S. EPA, 2020, section 
2.3.2.4).\24\ The highest PM10 concentrations tend to occur 
in the western U.S. Seasonal analyses indicate that ambient 
PM10 concentrations are generally higher in the summer 
months than at other times of year, though the most extreme high 
concentration events are more likely in the spring (U.S. EPA, 2019, 
Table 2-5). This is due to fact that the major PM10 emission 
sources, dust and agriculture, are more active during the warmer and 
drier periods of the year.
---------------------------------------------------------------------------

    \24\ The form of the current 24-hour PM10 standard is 
one-expected-exceedance, averaged over three years.
---------------------------------------------------------------------------

    Recent ambient PM10 concentrations reflect reductions 
that have occurred across much of the U.S. (U.S. EPA, 2020, section 
2.3.2.4). From 2000 to 2017, annual second highest 24-hour 
PM10 concentrations have declined by about 30% (U.S. EPA, 
2020, section 2.3.2.4).\25\ These PM10 concentrations have 
generally declined in the eastern U.S., while concentrations in the 
much of the midwest and western U.S. have remained unchanged or 
increased since 2000 (U.S. EPA, 2020, section 2.3.2.4). Analyses at 
individual monitoring sites indicate that annual average 
PM10 concentrations have also declined at most sites across 
the U.S., with much of the decrease in the eastern U.S. associated with 
reductions in PM2.5 concentrations.
---------------------------------------------------------------------------

    \25\ For more information, see https://www.epa.gov/air-trends/particulate-matter-pm10-trends#pmnat.
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d. PM10-2.5
    Since the last review, the availability of PM10-2.5 
ambient concentration data has greatly increased. As illustrated in the 
PA (U.S. EPA, 2020, section 2.3.2.5), annual average and 98th 
percentile PM10-2.5 concentrations exhibit less distinct 
differences between the eastern and western U.S. than for either 
PM2.5 or PM10. Additionally, compared to 
PM2.5 and PM10, changes in PM10-2.5 
concentrations have been small in magnitude and inconsistent in 
direction (U.S. EPA, 2020, section 2.3.2.5).
e. UFP
    Compared to PM2.5 mass, there is relatively little data 
on U.S. particle number concentrations, which are dominated by UFP. 
Based on measurements in two urban areas (New York City, Buffalo) and 
at a background site (Steuben County) in New York, urban particle 
number counts were several times higher than at the background site 
(U.S. EPA, 2020, section 2.3.2.6; U.S. EPA, 2019, Figure 2-18). The 
highest particle number counts in an urban area with multiple sites 
(Buffalo) were observed at a near-road location.
    Long-term trends in UFP are not routinely available at U.S. 
monitoring sites. At one site in Illinois with long-term data 
available, the annual average particle number concentration declined 
between 2000 and 2017, closely matching the reductions in annual 
PM2.5 mass over that same period (U.S. EPA, 2020, section 
2.3.2.6). In addition, a small number of published studies have 
examined UFP trends over time. While limited, these studies also 
suggest that UFP number concentrations have declined over time along 
with decreases in PM2.5 (U.S. EPA, 2020, section 2.3.2.6).
5. Background PM
    In this review, background PM is defined as all particles that are 
formed by sources or processes that cannot be influenced by actions 
within the jurisdiction of concern. U.S. background PM is defined as 
any PM formed from emissions other than U.S. anthropogenic (i.e. 
manmade) emissions. Potential sources of U.S. background PM include 
both natural sources (i.e., PM that would exist in the absence of any 
anthropogenic emissions of PM or PM precursors) and transboundary 
sources originating outside U.S. borders. Background PM is discussed in 
more detail in section 2.4 of the PA (U.S. EPA, 2020).
    At annual and national scales, estimated background PM 
concentrations in the U.S. are small compared to contributions from 
domestic anthropogenic emissions. For example, based on zero-out 
modeling in the last review of the PM NAAQS, annual background 
PM2.5 concentrations were estimated to range from 0.5-3 
[micro]g/m\3\ across the sites examined. In addition, speciated 
monitoring data from IMPROVE sites can provide some insights into how 
contributions from different PM sources, including sources of 
background PM, may have changed over time. As discussed further in the 
PA (U.S. EPA, 2020, section 2.4), such data suggests that estimates of 
background concentrations at IMPROVE monitors are around 1-3 [micro]g/
m\3\, and have not changed significantly since the last PM NAAQS 
Review.
    As discussed further in the PA (U.S. EPA, 2020, section 2.4), 
sources that contribute to natural background PM include dust from the 
wind erosion of natural surfaces, sea salt, wildland fires, primary 
biological aerosol particles such as bacteria and pollen, oxidation of 
biogenic hydrocarbons such as isoprene and terpenes to produce 
secondary

[[Page 24103]]

organic aerosols (SOA), and geogenic sources such as sulfate formed 
from volcanic production of SO2 and oceanic production of 
dimethyl-sulfide. While most of these sources release or contribute 
predominantly to fine aerosol, some sources including windblown dust, 
and sea salt also produce particles in the coarse size range (U.S. EPA, 
2019, section 2.3.3).
    The magnitude and sources of background PM can vary widely by 
region and time of year. Coastal sites may experience a consistent 
contribution of PM from sea spray aerosol, while other areas covered 
with dense vegetation may be impacted by biogenic aerosol production 
during the summertime. Sources of background PM also operate across a 
range of time scales. While some sources like biogenic aerosol vary at 
monthly to seasonal scales, many sources of background PM are episodic 
in nature. These episodic sources (e.g., large wildfires) can be 
characterized by infrequent contributions to high-concentration events 
occurring over shorter periods of time (e.g., hours to several days). 
Such episodic events are sporadic and do not necessarily occur in all 
years. While these exceptional episodes can lead to exceedances of the 
24-hour PM2.5 standard (35 [micro]g/m\3\) in some cases 
(Schweizer et al., 2017), such events are routinely screened for and 
usually identifiable in the monitoring data. As described further in 
the PA (U.S. EPA, 2020, section 2.4), contributions to background PM in 
the U.S. result mainly from sources within North America. Contributions 
from intercontinental events have also been documented (e.g., transport 
from dust storms occurring in deserts in North Africa and Asia), but 
these events are less frequent and represent a relatively small 
fraction of background PM in most places.

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

    This section provides the rationale supporting the Administrator's 
proposed decisions on the primary PM2.5 standards. Section 
II.A describes the Agency's approach to reaching decisions on the 
primary PM2.5 standards in the last review and summarizes 
the general approach to reaching proposed decisions in this review. 
Section II.B summarizes the scientific evidence for PM2.5-
related health effects. Section II.C presents the Administrator's 
proposed conclusions regarding the adequacy of the current primary 
PM2.5 standards and his proposed decision to retain those 
standards in this review.\26\
---------------------------------------------------------------------------

    \26\ Sections III and IV provide the rationales supporting the 
Administrator's proposed decisions on the primary PM10 
standard and secondary standards, respectively.
---------------------------------------------------------------------------

A. General Approach

1. Approach Used in the Last Review
    The last review of the primary PM NAAQS was completed in 2012 (78 
FR 3086, January 15, 2013). As noted above (section 1.3), in the last 
review the EPA lowered the level of the primary annual PM2.5 
standard from 15.0 to 12.0 [mu]g/m\3\,\27\ and retained the existing 
24-hour PM2.5 standard with its level of 35 [mu]g/m\3\. The 
2012 decision to strengthen the suite of primary PM2.5 
standards was based on the prior Administrator's consideration of the 
extensive body of scientific evidence assessed in the 2009 ISA (U.S. 
EPA, 2009c); the quantitative risk analyses presented in the 2010 
health risk assessment (U.S. EPA, 2010a); the advice and 
recommendations of the CASAC (Samet, 2009; Samet, 2010c; Samet, 2010b); 
and public comments on the proposed rule (78 FR 3086, January 15, 2013; 
U.S. EPA, 2012a). She particularly noted the ``strong and generally 
robust body of evidence of serious health effects associated with both 
long- and short-term exposures to PM2.5'' (78 FR 3120, 
January 15, 2013). This included epidemiologic studies reporting health 
effect associations based on long-term average PM2.5 
concentrations ranging from about 15.0 [mu]g/m\3\ or above (i.e., at or 
above the level of the then-existing annual standard) to concentrations 
``significantly below the level of the annual standard'' (78 FR 3120, 
January 15, 2013). Based on her ``confidence in the association between 
exposure to PM2.5 and serious public health effects, 
combined with evidence of such an association in areas that would meet 
the current standards'' (78 FR 3120, January 15, 2013), the prior 
Administrator concluded that revision of the suite of primary 
PM2.5 standards was necessary in order to provide increased 
public health protection.
---------------------------------------------------------------------------

    \27\ The Agency also eliminated spatial averaging provisions as 
part of the form of the annual standard.
---------------------------------------------------------------------------

    The prior Administrator next considered what specific revisions to 
the existing primary PM2.5 standards were appropriate, given 
the available evidence and quantitative risk information. She 
considered both the annual and 24-hour PM2.5 standards, 
focusing on the basic elements of those standards (i.e., indicator, 
averaging time, form, and level). These considerations, and the prior 
Administrator's conclusions, are summarized in sections II.A.1.a to 
II.A.1.d below.
a. Indicator
    In the last review, the EPA considered issues related to the 
appropriate indicator for fine particles, with a focus on evaluating 
support for the existing PM2.5 mass-based indicator and for 
potential alternative indicators based on the UFP fraction or on fine 
particle composition (78 FR 3121, January 15, 2013).\28\ With regard to 
PM2.5 mass, as in the 1997 and 2006 reviews, the health 
studies available during the last review continued to link adverse 
health outcomes (e.g., premature mortality, hospital admissions, 
emergency department visits) with long- and short-term exposures to 
fine particles indexed largely by PM2.5 mass (78 FR 3121, 
January 15, 2013). With regard to the ultrafine fraction of ambient PM, 
the 2011 PA noted the limited body of health evidence assessed in the 
2009 ISA (summarized in U.S. EPA, 2009c, section 2.3.5 and Table 2-6) 
and the limited monitoring information available to characterize 
ambient concentrations of UFP (U.S. EPA, 2011, section 1.3.2). With 
regard to PM composition, the 2009 ISA concluded that ``the evidence is 
not yet sufficient to allow differentiation of those constituents or 
sources that are more closely related to specific health outcomes'' 
(U.S. EPA, 2009c, pp. 2-26 and 6-212; 78 FR 3123, January 15, 2013). 
The 2011 PA further noted that ``many different constituents of the 
fine particle mixture as well as groups of components associated with 
specific source categories of fine particles are linked to adverse 
health effects'' (U.S. EPA, 2011, p. 2-55; 78 FR 3123, January 15, 
2013). Consistent with the considerations and conclusions in the 2011 
PA, the CASAC advised that it was appropriate to consider retaining 
PM2.5 as the indicator for fine particles. In light of the 
evidence and the CASAC's advice, the prior Administrator concluded that 
it was ``appropriate to retain PM2.5 as the indicator for 
fine particles'' (78 FR 3123, January 15, 2013).
---------------------------------------------------------------------------

    \28\ In the last review, the ISA defined UFP as generally 
including particles with a mobility diameter less than or equal to 
0.1 [micro]m. Mobility diameter is defined as the diameter of a 
particle having the same diffusivity or electrical mobility in air 
as the particle of interest, and is often used to characterize 
particles of 0.5 [micro]m or smaller (U.S. EPA, 2009c, pp. 3-2 to 3-
3).

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[[Page 24104]]

b. Averaging Time
    In 1997, the EPA set an annual PM2.5 standard to provide 
protection from health effects associated with long- and short-term 
exposures to PM2.5, and a 24-hour standard to supplement the 
protection afforded by the annual standard (62 FR 38667 to 38668, July 
18, 1997). In the 2006 review, the EPA retained both annual and 24-hour 
averaging times (71 FR 61164, October 17, 2006). In the last review, 
the EPA again considered issues related to the appropriate averaging 
times for PM2.5 standards, with a focus on evaluating 
support for the existing annual and 24-hour averaging times and for 
potential alternative averaging times based on sub-daily or seasonal 
metrics.
    Based on the evidence assessed in the ISA, the 2011 PA noted that 
the overwhelming majority of studies that had been conducted since the 
2006 review continued to utilize annual (or multi-year) or 24-hour PM 
averaging periods (U.S. EPA, 2011, section 2.3.2). Given this, and 
limitations in the data for alternatives, the 2011 PA reached the 
overall conclusions that the available information provided strong 
support for considering retaining the current annual and 24-hour 
averaging times (U.S. EPA, 2011, p. 2-58). The CASAC agreed that these 
conclusions were reasonable (Samet, 2010a, p. 13). The prior 
Administrator concurred with the PA conclusions and with the CASAC's 
advice. Specifically, she judged that it was ``appropriate to retain 
the current annual and 24-hour averaging times for the primary 
PM2.5 standards to protect against health effects associated 
with long- and short-term exposure periods'' (78 FR 3124, January 15, 
2013).
c. Form
    In 1997, the EPA established the form of the annual 
PM2.5 standard as an annual arithmetic mean, averaged over 3 
years, from single or multiple community-oriented monitors.\29\ That 
is, the level of the annual standard was to be compared to measurements 
made at each community-oriented monitoring site or, if specific 
criteria were met, measurements from multiple community-oriented 
monitoring sites could be averaged together (i.e., spatial averaging) 
\30\ (62 FR 38671 to 38672, July 18, 1997). In the 1997 review, the EPA 
also established the form of the 24-hour PM2.5 standard as 
the 98th percentile of 24-hour concentrations at each monitor within an 
area (i.e., no spatial averaging), averaged over three years (62 FR at 
38671 to 38674, July 18, 1997). In the 2006 review, the EPA retained 
these standard forms but tightened the criteria for using spatial 
averaging with the annual standard (71 FR 61167, October 17, 2006).\31\
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    \29\ In the last review, the EPA replaced the term ``community-
oriented'' monitor with the term ``area-wide'' monitor (U.S. EPA, 
2020, section 1.3). Area-wide monitors are those sited at the 
neighborhood scale or larger, as well as those monitors sited at 
micro- or middle scales that are representative of many such 
locations in the same core-based statistical area (CBSA; 78 FR 3236, 
January 15, 2013). CBSAs are required to have at least one area-wide 
monitor sited in the area of expected maximum PM2.5 
concentration.
    \30\ The original criteria for spatial averaging included: (1) 
The annual mean concentration at each site shall be within 20% of 
the spatially averaged annual mean, and (2) the daily values for 
each monitoring site pair shall yield a correlation coefficient of 
at least 0.6 for each calendar quarter (62 FR 38671 to 38672, July 
18, 1997).
    \31\ Specifically, the Administrator revised spatial averaging 
criteria such that ``(1) [t]he annual mean concentration at each 
site shall be within 10 percent of the spatially averaged annual 
mean, and (2) the daily values for each monitoring site pair shall 
yield a correlation coefficient of at least 0.9 for each calendar 
quarter (71 FR 61167, October 17, 2006).
---------------------------------------------------------------------------

    In the last review, the EPA's consideration of the form of the 
annual PM2.5 standard again included a focus on the issue of 
spatial averaging. An analysis of air quality and population 
demographic information indicated that the highest PM2.5 
concentrations in a given area tended to be measured at monitors in 
locations where the surrounding populations were more likely to live 
below the poverty line and to include larger percentages of racial and 
ethnic minorities (U.S. EPA, 2011, p. 2-60). Based on this analysis, 
the 2011 PA concluded that spatial averaging could result in 
disproportionate impacts in at-risk populations, including minority 
populations and populations with lower socioeconomic status (SES). 
Therefore, the PA concluded that it was appropriate to consider 
revising the form of the annual PM2.5 standard such that it 
did not allow for the use of spatial averaging across monitors (U.S. 
EPA, 2011, p. 2-60). The CASAC agreed with the PA conclusions that it 
was ``reasonable'' for the EPA to eliminate the spatial averaging 
provisions (Samet, 2010c, p. 2).
    The prior Administrator concluded that public health would not be 
protected with an adequate margin of safety in all locations, as 
required by law, if disproportionately higher PM2.5 
concentrations in low income and minority communities were averaged 
together with lower concentrations measured at other sites in a large 
urban area. Therefore, she concluded that the form of the annual 
PM2.5 standard should be revised to eliminate spatial 
averaging provisions (78 FR 3124, January 15, 2013).
    In the last review, the EPA also considered the form of the 24-hour 
PM2.5 standard. The Agency recognized that the existing 98th 
percentile form for the 24-hour standard was originally selected to 
provide a balance between limiting the occurrence of peak 24-hour 
PM2.5 concentrations and identifying a stable target for 
risk management programs.\32\ Updated air quality analyses in the last 
review provided additional support for the increased stability of the 
98th percentile PM2.5 concentration, compared to the 99th 
percentile (U.S. EPA, 2011, Figure 2-2, p. 2-62). Consistent with the 
PA conclusions based on this analysis, the prior Administrator 
concluded that it was appropriate to retain the 98th percentile form 
for the 24-hour PM2.5 standard (78 FR 3127, January 15, 
2013).
---------------------------------------------------------------------------

    \32\ See ATA III, 283 F.3d at 374-76 which concludes that it is 
legitimate for the EPA to consider overall stability of the standard 
and its resulting promotion of overall effectiveness of NAAQS 
control programs in setting a standard that is requisite to protect 
the public health.
---------------------------------------------------------------------------

d. Level
    The EPA's approach to considering alternative levels of the 
PM2.5 standards in the last review was based on evaluating 
the public health protection afforded by the annual and 24-hour 
standards, taken together, against mortality and morbidity effects 
associated with long-term or short-term PM2.5 exposures. 
This approach recognized that it is appropriate to consider the 
protection provided by attaining the air quality needed to meet the 
suite of standards, and that there is no bright line clearly directing 
the choice of levels. Rather, the choice of what is appropriate is a 
public health policy judgment entrusted to the Administrator. See 
Mississippi, 744 F.3d at 1358, Lead Industries Ass'n, 647 F.2d at 1147.
    In selecting the levels of the annual and 24-hour PM2.5 
standard, the prior Administrator placed the greatest emphasis on 
health endpoints for which the evidence was strongest, based on the 
assessment of the evidence in the ISA and on the ISA's causality 
determinations (U.S. EPA, 2009c, section 2.3.1). She particularly noted 
that the evidence was sufficient to conclude a causal relationship 
exists between PM2.5 exposures and mortality and 
cardiovascular effects (i.e., for both long- and short-term exposures) 
and that the evidence was sufficient to conclude a causal relationship 
is ``likely'' to exist between PM2.5 exposures and 
respiratory effects (i.e., for both long-

[[Page 24105]]

and short-term exposures). She also noted additional, but more limited, 
evidence for a broader range of health endpoints, including evidence 
``suggestive of a causal relationship'' between long-term exposures and 
developmental and reproductive effects as well as carcinogenic effects 
(78 FR 3158, January 15, 2013).
    To inform her decisions on an appropriate level for the annual 
standard, the prior Administrator considered the degree to which 
epidemiologic studies indicate confidence in the reported health effect 
associations over distributions of ambient PM2.5 
concentrations. She noted that a level of 12.0 [micro]g/m\3\ was below 
the long-term mean PM2.5 concentrations reported in key 
epidemiologic studies that provided evidence of an array of serious 
health effects (78 FR 3161, January 15, 2013). She further noted that 
12.0 [micro]g/m\3\ generally corresponded to the lower portions (i.e., 
about the 25th percentile) of distributions of health events in the 
limited number of epidemiologic studies for which population-level 
information was available. A level of 12.0 [micro]g/m\3\ also reflected 
placing some weight on studies of reproductive and developmental 
effects, for which the evidence was more uncertain (78 FR 3161-3162, 
January 15, 2013).\33\
---------------------------------------------------------------------------

    \33\ With respect to cancer, mutagenic, and genotoxic effects, 
the Administrator observed that the PM2.5 concentrations 
reported in studies evaluating these effects generally included 
ambient concentrations that are equal to or greater than ambient 
concentrations observed in studies that reported mortality and 
cardiovascular and respiratory effects (U.S. EPA, 2009c, section 
7.5). Therefore, the Administrator concluded that, in selecting a 
standard level that provides protection from mortality and 
cardiovascular and respiratory effects, it is reasonable to 
anticipate that protection will also be provided for carcinogenic 
effects (78 FR 3161-3162, January 15, 2013).
---------------------------------------------------------------------------

    Given the uncertainties remaining in the scientific information, 
the prior Administrator judged that an annual standard level below 12.0 
[micro]g/m\3\ was not supported. She specifically noted uncertainties 
related to understanding the relative toxicity of the different 
components in the fine particle mixture, the role of PM2.5 
in the complex ambient mixture, exposure measurement errors in 
epidemiologic studies, and the nature and magnitude of estimated risks 
at relatively low ambient PM2.5 concentrations. Furthermore, 
she noted that epidemiologic studies had reported heterogeneity in 
responses both within and between cities and in geographic regions 
across the U.S. She recognized that this heterogeneity may be 
attributed, in part, to differences in fine particle composition in 
different regions and cities. With regard to evidence for reproductive 
and developmental effects, the prior Administrator recognized that 
there were a number of limitations associated with this body of 
evidence, including the following: The limited number of studies 
evaluating such effects; uncertainties related to identifying the 
relevant exposure time periods of concern; and limited toxicological 
evidence providing little information on the mode of action(s) or 
biological plausibility for an association between long-term 
PM2.5 exposures and adverse birth outcomes. On balance, she 
found that the available evidence, interpreted in light of these 
remaining uncertainties, did not justify an annual standard level set 
below 12.0 [micro]g/m\3\ as being ``requisite'' to protect public 
health with an adequate margin of safety (i.e., a standard with a lower 
level would have been more stringent than necessary).
    In conjunction with a revised annual standard with a level of 12.0 
[micro]g/m\3\, the prior Administrator concluded that the evidence 
supported retaining the 35 [micro]g/m\3\ level of the 24-hour 
PM2.5 standard. She noted that the existing 24-hour 
standard, with its 35 [micro]g/m\3\ level and 98th percentile form, 
would provide supplemental protection, particularly for areas with high 
peak-to-mean ratios possibly associated with strong seasonal sources 
and for areas with PM2.5-related effects that may be 
associated with shorter than daily exposure periods (78 FR 3163, 
January 15, 2013). Thus, she concluded that the available evidence and 
information, interpreted in light of remaining uncertainties, supported 
an annual standard with a level of 12.0 [micro]g/m\3\ combined with a 
24-hour standard with a level of 35 [micro]g/m\3\.
2. Approach in the Current Review
    The EPA's approach to reaching proposed decisions on the primary 
PM2.5 standards in the current review builds on the 
decisions made in the last review. Consistent with that review, the 
approach focuses on evaluating the public health protection afforded by 
the annual and 24-hour standards, taken together, against mortality and 
morbidity associated with long-term or short-term PM2.5 
exposures. As discussed in the PA (U.S. EPA, 2020, section 3.1.2), in 
adopting this approach the EPA recognizes that changes in 
PM2.5 air quality designed to meet an annual standard would 
likely result not only in lower annual average PM2.5 
concentrations, but also in fewer and lower short-term peak 
PM2.5 concentrations. Additionally, changes designed to meet 
a 24-hour standard, with a 98th percentile form, would result not only 
in fewer and lower peak 24-hour PM2.5 concentrations, but 
also in lower annual average PM2.5 concentrations. Thus, the 
EPA's approach recognizes that it is appropriate to consider the 
protection provided by attaining the air quality needed to meet the 
suite of standards.
    This approach to reviewing the primary PM2.5 standards 
is based most fundamentally on considering the available scientific 
evidence and technical information as assessed and discussed in the ISA 
(U.S. EPA, 2019) and PA (U.S. EPA, 2020), including the uncertainties 
inherent in that evidence and information, and on consideration of 
advice received from the CASAC in this review (Cox, 2019a). The EPA 
emphasizes the health outcomes for which the ISA determines that the 
evidence supports either a ``causal'' or a ``likely to be causal'' 
relationship with PM2.5 exposures (U.S. EPA, 2019). This 
approach focuses proposed decisions on the health outcomes for which 
the evidence is strongest. Such a focus, which is supported by the 
CASAC (Cox, 2019a, p. 12 of consensus responses), recognizes that 
standards set based on evidence supporting ``causal'' and ``likely to 
be causal'' health outcomes will also provide some measure of 
protection against the broader range of PM2.5-associated 
outcomes, including those for which the evidence is less certain.
    As in past reviews, the EPA's approach recognizes that there is no 
bright line clearly directing the choice of standards. Rather, the 
choice of what is appropriate is a public health policy judgment 
entrusted to the Administrator. Specifically, the CAA requires primary 
standards that, in the judgment of the Administrator, are requisite to 
protect public health with an adequate margin of safety. In setting 
primary standards that are ``requisite'' to protect public health, the 
EPA's task is to establish standards that are neither more nor less 
stringent than necessary for this purpose. Thus, as discussed above 
(I.A), the CAA does not require that primary standards be set at a 
zero-risk level, but rather at a level that, in the judgment of the 
Administrator, limits risk sufficiently so as to protect public health 
with an adequate margin of safety. As in previous reviews, this 
judgment includes consideration of the strengths and limitations of the 
scientific and technical information, and the appropriate inferences to 
be drawn from that information.

[[Page 24106]]

B. Health Effects Related to Fine Particle Exposures

    This section draws from the EPA's synthesis and assessment of the 
scientific evidence presented in the ISA (U.S. EPA, 2019) and the 
summary of that evidence in the PA (U.S. EPA, 2020, section 3.2.1). The 
ISA uses a weight-of-evidence framework for characterizing the strength 
of the available scientific evidence for health effects attributable to 
PM exposures (U.S. EPA, 2015, Preamble, Section 5). As in the last 
review (U.S. EPA, 2009c), the ISA for this review has adopted a five-
level hierarchy to classify the overall weight-of-evidence into one of 
the following categories: Causal relationship; a likely to be causal 
relationship; suggestive of, but not sufficient to infer, a causal 
relationship; \34\ inadequate to infer the presence or absence of a 
causal relationship; and not likely to be a causal relationship (U.S. 
EPA, 2015, Preamble Table II). In using the weight-of-evidence approach 
to inform judgments about the likelihood that various health effects 
are caused by PM exposures, evidence is evaluated for major outcome 
categories or groups of related outcomes (e.g., respiratory effects), 
integrating evidence from across disciplines, including epidemiologic, 
controlled human exposure, and animal toxicological studies and 
evaluating the coherence of evidence across a spectrum of related 
endpoints as well as biological plausibility of the effects observed 
(U.S. EPA, 2015, Preamble, Section 5.c.). Based on application of this 
approach, the EPA believes that the final ISA ``accurately reflects 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 [PM] in the ambient air, in varying 
quantities'' as required by the CAA (42 U.S.C. 7408(a)(2)).
---------------------------------------------------------------------------

    \34\ As noted in the 2019 p.m. ISA (U.S. EPA, 2019, p. ES-15), 
this causality determination language has been updated since the 
last review.
---------------------------------------------------------------------------

    In this review of the NAAQS, the EPA considers the full body of 
health evidence, placing the greatest emphasis on the health effects 
for which the evidence has been judged in the ISA to demonstrate a 
``causal'' or a ``likely to be causal'' relationship with PM exposures. 
The ISA defines these causality determinations as follows (U.S. EPA, 
2019, p. p-20):
     Causal relationship: The pollutant has been shown to 
result in health effects at relevant exposures based on studies 
encompassing multiple lines of evidence and chance, confounding, and 
other biases can be ruled out with reasonable confidence.
     Likely to be a causal relationship: There are studies in 
which results are not explained by chance, confounding, or other 
biases, but uncertainties remain in the health effects evidence 
overall. For example, the influence of co-occurring pollutants is 
difficult to address, or evidence across scientific disciplines may be 
limited or inconsistent.
    The sections below briefly summarize the health effects evidence 
determined in the ISA to support either a ``causal'' or a ``likely to 
be causal'' relationship with fine particle exposures (II.B.1), the 
populations potentially at increased risk for PM-related effects 
(II.B.2), and the CASAC's advice on the draft ISA (II.B.3). Additional 
detail on these topics can be found in the ISA (U.S. EPA, 2019) and in 
the PA (U.S. EPA, 2020, section 3.2).
1. Nature of Effects
    Drawing from the assessment of the evidence in the ISA (U.S. EPA, 
2019), and the summaries of that assessment in the PA (U.S. EPA, 2020), 
the sections below summarize the evidence for relationships between 
long- or short-term PM2.5 exposures and mortality 
(II.B.1.a), cardiovascular effects (II.B.1.b), respiratory effects 
(II.B.1.c), cancer (II.B.1.d), and nervous system effects (II.B.1.e). 
For these outcomes, the ISA concludes that the evidence supports either 
a ``causal'' or a ``likely to be causal'' relationship with 
PM2.5 exposures.
a. Mortality
i. Long-term PM2.5 exposures
    In the last review, the 2009 PM ISA reported that the evidence was 
``sufficient to conclude that the relationship between long-term 
PM2.5 exposures and mortality is causal'' (U.S. EPA, 2009c, 
p. 7-96). The strongest evidence supporting this conclusion was 
provided by epidemiologic studies, particularly those examining two 
seminal cohorts, the American Cancer Society (ACS) cohort and the 
Harvard Six Cities cohort. Analyses of the Harvard Six Cities cohort 
included demonstrations that reductions in ambient PM2.5 
concentrations are associated with reduced mortality risk (Laden et 
al., 2006) and with increases in life expectancy (Pope et al., 2009). 
Further support was provided by other cohort studies conducted in North 
America and Europe that also reported positive associations between 
long-term PM2.5 exposures and risk of mortality (U.S. EPA, 
2009c).
    Recent cohort studies, which have become available since the 2009 
ISA, continue to provide consistent evidence of positive associations 
between long-term PM2.5 exposures and mortality. These 
studies add support for associations with total and non-accidental 
mortality,\35\ as well as with specific causes of death, including 
cardiovascular disease and respiratory disease (U.S. EPA, 2019, section 
11.2.2). Many of these recent studies have extended the follow-up 
periods originally evaluated in the ACS and Harvard Six Cities cohort 
studies and continue to observe positive associations between long-term 
PM2.5 exposures and mortality (U.S. EPA, 2019, section 
11.2.2.1; Figures 11-18 and 11-19). Adding to recent evaluations of the 
ACS and Six Cities cohorts, studies conducted with other cohorts also 
show consistent, positive associations between long-term 
PM2.5 exposure and mortality across various demographic 
groups (e.g., age, sex, occupation), spatial and temporal extents, 
exposure assessment metrics, and statistical techniques (U.S. EPA, 
2019, sections 11.2.2.1, 11.2.5). This includes some of the largest 
cohort studies conducted to date, with analyses of the U.S. Medicare 
cohort that include nearly 61 million enrollees (Di et al., 2017b) and 
studies that control for a range of individual and ecological 
covariates.
---------------------------------------------------------------------------

    \35\ The majority of these studies examined non-accidental 
mortality outcomes, though some Medicare studies lack cause-specific 
death information and, therefore, examine total mortality.
---------------------------------------------------------------------------

    A recent series of retrospective studies has additionally tested 
the hypothesis that past reductions in ambient PM2.5 
concentrations have been associated with increased life expectancy or a 
decreased mortality rate (U.S. EPA, 2019, section 11.2.2.5). Pope et 
al. (2009) conducted a cross-sectional analysis using air quality data 
from 51 metropolitan areas across the U.S., beginning in the 1970s 
through the early 2000s, and found that a 10 [micro]g/m\3\ decrease in 
long-term PM2.5 concentration was associated with a 0.61-
year increase in life expectancy. In a subsequent analysis, the authors 
extended the period of analysis to include 2000 to 2007 (Correia et 
al., 2013), a time period with lower ambient PM2.5 
concentrations. In this follow-up study, a decrease in long-term 
PM2.5 concentration continued to be associated with an 
increase in life expectancy, though the magnitude of the increase was 
smaller than during the earlier time period (i.e., a 10 [micro]g/m\3\ 
decrease in long-term PM2.5

[[Page 24107]]

concentration was associated with a 0.35-year increase in life 
expectancy). Additional studies conducted in the U.S. or Europe 
similarly report that reductions in ambient PM2.5 are 
associated with improvements in longevity (U.S. EPA, 2019, section 
11.2.2.5).
    The 2019 ISA specifically evaluates the degree to which recent 
studies that examine the relationship between long-term 
PM2.5 exposure and mortality have addressed key policy-
relevant issues and/or previously identified data gaps in the 
scientific evidence. For example, based on its assessment of the 
evidence, the ISA concludes that positive associations between long-
term PM2.5 exposures and mortality are robust across 
analyses examining a variety of study designs (e.g., U.S. EPA, 2019, 
section 11.2.2.4), approaches to estimating PM2.5 exposures 
(U.S. EPA, 2019, section 11.2.5.1), approaches to controlling for 
confounders (U.S. EPA, 2019, sections 11.2.3 and 11.2.5), geographic 
regions and populations, and temporal periods (U.S. EPA, 2019, sections 
11.2.2.5 and 11.2.5.3). Recent evidence further demonstrates that 
associations with mortality remain robust in copollutant analyses (U.S. 
EPA, 2019, section 11.2.3), and that associations persist in analyses 
restricted to long-term exposures below 12 [mu]g/m\3\ (Di et al., 
2017b) or 10 [mu]g/m\3\ (Shi et al., 2016).
    An additional important consideration in characterizing the public 
health impacts associated with PM2.5 exposure is whether 
concentration-response relationships are linear across the range of 
concentrations or if nonlinear relationships exist along any part of 
this range. Several recent studies examine this issue, and continue to 
provide evidence of linear, no-threshold relationships between long-
term PM2.5 exposures and all-cause and cause-specific 
mortality (U.S. EPA, 2019, section 11.2.4). However, interpreting the 
shapes of these relationships, particularly at PM2.5 
concentrations near the lower end of the air quality distribution, can 
be complicated by relatively low data density in the lower 
concentration range, the possible influence of exposure measurement 
error, and variability among individuals with respect to air pollution 
health effects. These sources of variability and uncertainty tend to 
smooth and ``linearize'' population-level concentration-response 
functions, and thus could obscure the existence of a threshold or 
nonlinear relationship (U.S. EPA, 2015, Preamble section 6.c).
    The biological plausibility of PM2.5-attributable 
mortality is supported by the coherence of effects across scientific 
disciplines (i.e., animal toxicological, controlled human exposure 
studies, and epidemiologic), including in recent studies evaluating the 
morbidity effects that are the largest contributors to total 
(nonaccidental) mortality. The ISA outlines the available evidence for 
plausible pathways by which inhalation exposure to PM2.5 
could progress from initial events (e.g., respiratory tract 
inflammation, autonomic nervous system modulation) to endpoints 
relevant to population outcomes, particularly those related to 
cardiovascular diseases such as ischemic heart disease, stroke and 
atherosclerosis (U.S. EPA, 2019, section 6.2.1), and to metabolic 
disease and diabetes (U.S. EPA, 2019, section 7.2.1). The ISA notes 
``more limited evidence from respiratory morbidity'' (U.S. EPA, 2019, 
p. 11-101) to support the biological plausibility of mortality due to 
long-term PM2.5 exposures (U.S. EPA, 2019, section 11.2.1).
    Taken together, recent studies reaffirm and further strengthen the 
body of evidence from the 2009 ISA for the relationship between long-
term PM2.5 exposure and mortality. Recent epidemiologic 
studies consistently report positive associations with mortality across 
different geographic locations, populations, and analytic approaches. 
Such studies reduce key uncertainties identified in the last review, 
including those related to potential copollutant confounding, and 
provide additional information on the shape of the concentration-
response curve. Recent experimental and epidemiologic evidence for 
cardiovascular effects, and respiratory effects to a more limited 
degree, supports the plausibility of mortality due to long-term 
PM2.5 exposures. The 2019 ISA concludes that, 
``collectively, this body of evidence is sufficient to conclude that a 
causal relationship exists between long-term PM2.5 exposure 
and total mortality'' (U.S. EPA, 2019, section 11.2.7; p. 11-102).
ii. Short-term PM2.5 exposures
    The 2009 PM ISA concluded that ``a causal relationship exists 
between short-term exposure to PM2.5 and mortality'' (U.S. 
EPA, 2009c). This conclusion was based on the evaluation of both multi- 
and single-city epidemiologic studies that consistently reported 
positive associations between short-term PM2.5 exposure and 
non-accidental mortality. These associations were strongest, in terms 
of magnitude and precision, primarily at lags of 0 to 1 days. 
Examination of the potential confounding effects of gaseous 
copollutants was limited, though evidence from single-city studies 
indicated that gaseous copollutants have minimal effect on the 
PM2.5-mortality relationship (i.e., associations remain 
robust to inclusion of other pollutants in copollutant models). The 
evaluation of cause-specific mortality found that effect estimates were 
larger in magnitude, but also had larger confidence intervals, for 
respiratory mortality compared to cardiovascular mortality. Although 
the largest mortality risk estimates were for respiratory mortality, 
the interpretation of the results was complicated by the limited 
coherence from studies of respiratory morbidity. However, the evidence 
from studies of cardiovascular morbidity provided both coherence and 
biological plausibility for the relationship between short-term 
PM2.5 exposure and cardiovascular mortality.
    Recent multicity studies evaluated since the 2009 ISA continue to 
provide evidence of primarily positive associations between daily 
PM2.5 exposures and mortality, with percent increases in 
total mortality ranging from 0.19% (Lippmann et al., 2013) to 2.80% 
(Kloog et al., 2013) \36\ at lags of 0 to 1 days in single-pollutant 
models. Whereas most studies rely on assigning exposures using data 
from ambient monitors, associations are also reported in recent studies 
that employ hybrid modeling approaches using additional 
PM2.5 data (i.e., from satellites, land use information, and 
modeling, in addition to monitors), allowing for the inclusion of more 
rural locations in analyses (Kloog et al., 2013, Shi et al., 2016, Lee 
et al., 2015).
---------------------------------------------------------------------------

    \36\ As detailed in the Preface to the ISA, risk estimates are 
for a 10 [micro]g/m\3\ increase in 24-hour avg PM2.5 
concentrations, unless otherwise noted (U.S. EPA, 2019).
---------------------------------------------------------------------------

    Some recent studies have expanded the examination of potential 
confounders (e.g., U.S. EPA, 2019, section 11.1.5.1), including 
copollutants. Associations between short-term PM2.5 
exposures and mortality remain positive and relatively unchanged in 
copollutant models with both gaseous pollutants and PM10-2.5 
(U.S. EPA, 2019, Section 11.1.4). Additionally, the low (r < 0.4) to 
moderate correlations (r = 0.4-0.7) between PM2.5 and 
gaseous pollutants and PM10-2.5 increase the confidence in 
PM2.5 having an independent effect on mortality (U.S. EPA, 
2019, section 11.1.4).
    The generally positive associations reported with mortality are 
supported

[[Page 24108]]

by a small group of studies employing causal inference or quasi-
experimental statistical approaches (U.S. EPA, 2019, section 11.1.2.1). 
For example, a recent study examines whether a specific regulatory 
action in Tokyo, Japan (i.e., a diesel emission control ordinance) 
resulted in a subsequent reduction in daily mortality (Yorifuji et al., 
2016). The authors report a reduction in mortality in Tokyo due to the 
ordinance, compared to Osaka, which did not have a similar diesel 
emission control ordinance in place.
    The positive associations for total mortality reported across the 
majority of studies evaluated are further supported by analyses 
reporting generally consistent, positive associations with both 
cardiovascular and respiratory mortality (U.S. EPA, 2019, section 
11.1.3). For both cardiovascular and respiratory mortality, there has 
been only limited assessment of potential copollutant confounding, 
though initial evidence indicates that associations remain positive and 
relatively unchanged in models with gaseous pollutants and 
PM10-2.5. This evidence further supports the copollutant 
analyses conducted for total mortality. The evidence for ischemic 
events and heart failure, as detailed in the assessment of 
cardiovascular morbidity (U.S. EPA, 2019, Chapter 6), provides 
biological plausibility for PM2.5-related cardiovascular 
mortality, which comprises the largest percentage of total mortality 
(i.e., ~33%) (U.S. National Institutes of Health, 2013). Although there 
is evidence for exacerbations of chronic obstructive pulmonary disease 
(COPD) and asthma, the collective body of evidence, particularly from 
controlled human exposure studies of respiratory effects, provides only 
limited support for the biological plausibility of PM2.5-
related respiratory mortality (U.S. EPA, 2019, Chapter 5).
    In the 2009 ISA, one of the main uncertainties identified was the 
regional and city-to-city heterogeneity in PM2.5-mortality 
associations. Recent studies examine both city-specific as well as 
regional characteristics to identify the underlying contextual factors 
that could contribute to this heterogeneity (U.S. EPA, 2019, section 
11.1.6.3). Analyses focusing on effect modification of the 
PM2.5-mortality relationship by PM2.5 components, 
regional patterns in PM2.5 components and city-specific 
differences in composition and sources indicate some differences in the 
PM2.5 composition and sources across cities and regions, but 
these differences do not fully explain the observed heterogeneity. 
Additional studies find that factors related to potential exposure 
differences, such as housing stock and commuting, as well as city-
specific factors (e.g., land-use, port volume, and traffic 
information), may explain some of the observed heterogeneity (U.S. EPA, 
2019, section 11.1.6.3). Collectively, recent studies indicate that the 
heterogeneity in PM2.5-mortality risk estimates cannot be 
attributed to one factor, but instead a combination of factors 
including, but not limited to, PM composition and sources as well as 
community characteristics that could influence exposures (U.S. EPA, 
2019, section 11.1.12).
    A number of recent studies conducted systematic evaluations of the 
lag structure of associations for the PM2.5-mortality 
relationship by examining either a series of single-day or multiday 
lags and these studies continue to support an immediate effect (i.e., 
lag 0 to 1 days) of short-term PM2.5 exposures on mortality 
(U.S. EPA, 2019, section 11.1.8.1). Recent studies also conducted 
analyses comparing the traditional 24-hour average exposure metric with 
a sub-daily metric (i.e., 1-hour max). These initial studies provide 
evidence of a similar pattern of associations for both the 24-hour 
average and 1-hour max metric, with the association larger in magnitude 
for the 24-hour average metric.
    Recent multicity studies indicate that positive and statistically 
significant associations with mortality persist in analyses restricted 
to short-term PM2.5 exposures below 35 [mu]g/m\3\ (Lee et 
al., 2015),\37\ below 30 [mu]g/m\3\ (Shi et al., 2016), and below 25 
[mu]g/m\3\ (Di et al., 2017a). Additional studies examine the shape of 
the concentration-response relationship and whether a threshold exists 
specifically for PM2.5 (U.S. EPA, 2019, section 11.1.10). 
These studies have used various statistical approaches and consistently 
found linear relationships with no evidence of a threshold. Recent 
analyses provide initial evidence indicating that PM2.5-
mortality associations persist and may be stronger (i.e., a steeper 
slope) at lower concentrations (e.g., Di et al., 2017a; Figure 11-12 in 
U.S. EPA, 2019). However, given the limited data available at the lower 
end of the distribution of ambient PM2.5 concentrations, the 
shape of the concentration-response curve remains uncertain at these 
low concentrations and, to date, studies have not conducted extensive 
analyses exploring alternatives to linearity when examining the shape 
of the PM2.5-mortality concentration-response relationship.
---------------------------------------------------------------------------

    \37\ Lee et al. (2015) also report that positive and 
statistically significant associations between short-term 
PM2.5 exposures and mortality persist in analyses 
restricted to areas with long-term concentrations below 12 [mu]g/
m\3\.
---------------------------------------------------------------------------

    Overall, recent epidemiologic studies build upon and extend the 
conclusions of the 2009 ISA for the relationship between short-term 
PM2.5 exposures and total mortality. Supporting evidence for 
PM2.5-related cardiovascular morbidity, and more limited 
evidence from respiratory morbidity, provides biological plausibility 
for mortality due to short-term PM2.5 exposures. The 
primarily positive associations observed across studies conducted in 
diverse geographic locations is further supported by the results from 
co-pollutant analyses indicating robust associations, along with 
evidence from analyses of the concentration-response relationship. The 
2019 ISA states that, collectively, ``this body of evidence is 
sufficient to conclude that a causal relationship exists between short-
term PM2.5 exposure and total mortality'' (U.S. EPA, 2019, 
pp. 11-58).
b. Cardiovascular Effects
i. Long-Term PM2.5 Exposures
    The scientific evidence reviewed in the 2009 PM ISA was 
``sufficient to infer a causal relationship between long-term 
PM2.5 exposure and cardiovascular effects'' (U.S. EPA, 
2009c). The strongest line of evidence comprised findings from several 
large epidemiologic studies of U.S. cohorts that consistently showed 
positive associations between long-term PM2.5 exposure and 
cardiovascular mortality (Pope et al., 2004, Krewski et al., 2009, 
Miller et al., 2007, Laden et al., 2006). Studies of long-term 
PM2.5 exposure and cardiovascular morbidity were limited in 
number. Biological plausibility and coherence with the epidemiologic 
findings were provided by studies using genetic mouse models of 
atherosclerosis demonstrating enhanced atherosclerotic plaque 
development and inflammation, as well as changes in measures of 
impaired heart function, following 4- to 6-month exposures to 
PM2.5 concentrated ambient particles (CAPs), and by a 
limited number of studies reporting CAPs-induced effects on coagulation 
factors, vascular reactivity, and worsening of experimentally induced 
hypertension in mice (U.S. EPA, 2009c).
    Studies conducted since the last review continue to support the 
relationship between long-term exposure to PM2.5 and 
cardiovascular effects. As discussed above, results from recent U.S. 
and Canadian cohort studies consistently report positive associations 
between long-term PM2.5 exposure and cardiovascular 
mortality (U.S. EPA, 2019, Figure 6-19) in evaluations

[[Page 24109]]

conducted at varying spatial scales and employing a variety of exposure 
assessment and statistical methods (U.S. EPA, 2019, section 6.2.10). 
Positive associations between long-term PM2.5 exposures and 
cardiovascular mortality are generally robust in copollutant models 
adjusted for ozone, NO2, PM10-2.5, or 
SO2. In addition, most of the results from analyses 
examining the shape of the concentration-response relationship for 
cardiovascular mortality support a linear relationship with long-term 
PM2.5 exposures and do not identify a threshold below which 
effects do not occur (U.S. EPA, 2019, section 6.2.16; Table 6-52).\38\
---------------------------------------------------------------------------

    \38\ As noted above for mortality, uncertainty in the shape of 
the concentration-response relationship increases near the upper and 
lower ends of the concentration distribution where the data are 
limited.
---------------------------------------------------------------------------

    The body of literature examining the relationship between long-term 
PM2.5 exposure and cardiovascular morbidity has greatly 
expanded since the 2009 PM ISA, with positive associations reported in 
several cohorts (U.S. EPA, 2019, section 6.2). Though results for 
cardiovascular morbidity are less consistent than those for 
cardiovascular mortality (U.S. EPA, 2019, section 6.2), recent studies 
provide some evidence for associations between long-term 
PM2.5 exposures and the progression of cardiovascular 
disease. Positive associations with cardiovascular morbidity (e.g., 
coronary heart disease, stroke) and atherosclerosis progression (e.g., 
coronary artery calcification) are observed in several epidemiologic 
studies (U.S. EPA, 2019, sections 6.2.2. to 6.2.9). Associations in 
such studies are supported by toxicological evidence for increased 
plaque progression in mice following long-term exposure to 
PM2.5 collected from multiple locations across the U.S. 
(U.S. EPA, 2019, section 6.2.4.2). A small number of epidemiologic 
studies also report positive associations between long-term 
PM2.5 exposure and heart failure, changes in blood pressure, 
and hypertension (U.S. EPA, 2019, sections 6.2.5 and 6.2.7). 
Associations with heart failure are supported by animal toxicological 
studies demonstrating decreased cardiac contractility and function, and 
increased coronary artery wall thickness following long-term 
PM2.5 exposure (U.S. EPA, 2019, section 6.2.5.2). Similarly, 
a limited number of animal toxicological studies demonstrating a 
relationship between long-term exposure to PM2.5 and 
consistent increases in blood pressure in rats and mice are coherent 
with epidemiologic studies reporting positive associations between 
long-term exposure to PM2.5 and hypertension.
    Longitudinal epidemiologic analyses also report positive 
associations with markers of systemic inflammation (U.S. EPA, 2019, 
section 6.2.11), coagulation (U.S. EPA, 2019, section 6.2.12), and 
endothelial dysfunction (U.S. EPA, 2019, section 6.2.13). These results 
are coherent with animal toxicological studies generally reporting 
increased markers of systemic inflammation, oxidative stress, and 
endothelial dysfunction (U.S. EPA, 2019, section 6.2.12.2 and 6.2.14).
    In summary, the 2019 ISA concludes that there is consistent 
evidence from multiple epidemiologic studies illustrating that long-
term exposure to PM2.5 is associated with mortality from 
cardiovascular causes. Associations with CHD, stroke and 
atherosclerosis progression were observed in several additional 
epidemiologic studies providing coherence with the mortality findings. 
Results from copollutant models generally support an independent effect 
of PM2.5 exposure on mortality. Additional evidence of the 
independent effect of PM2.5 on the cardiovascular system is 
provided by experimental studies in animals, which support the 
biological plausibility of pathways by which long-term exposure to 
PM2.5 could potentially result in outcomes such as CHD, 
stroke, CHF and cardiovascular mortality. The combination of 
epidemiologic and experimental evidence results in the ISA conclusion 
that ``a causal relationship exists between long-term exposure to 
PM2.5 and cardiovascular effects'' (U.S. EPA, 2019, p. 6-
222).
ii. Short-Term PM2.5 Exposures
    The 2009 PM ISA concluded that ``a causal relationship exists 
between short-term exposure to PM2.5 and cardiovascular 
effects'' (U.S. EPA, 2009c). The strongest evidence in the 2009 PM ISA 
was from epidemiologic studies of emergency department visits and 
hospital admissions for ischemic heart disease (IHD) and heart failure 
(HF), with supporting evidence from epidemiologic studies of 
cardiovascular mortality (U.S. EPA, 2009c). Animal toxicological 
studies provided coherence and biological plausibility for the positive 
associations reported with myocardial ischemia, emergency department 
visits, and hospital admissions. These included studies reporting 
reduced myocardial blood flow during ischemia and studies indicating 
altered vascular reactivity. In addition, effects of PM2.5 
exposure on a potential indicator of ischemia (i.e., ST segment 
depression on an electrocardiogram) were reported in both animal 
toxicological and epidemiologic panel studies.\39\ Key uncertainties 
from the last review resulted from inconsistent results across 
disciplines with respect to the relationship between short-term 
exposure to PM2.5 and changes in blood pressure, blood 
coagulation markers, and markers of systemic inflammation. In addition, 
while the 2009 PM ISA identified a growing body of evidence from 
controlled human exposure and animal toxicological studies, 
uncertainties remained with respect to biological plausibility.
---------------------------------------------------------------------------

    \39\ Some animal studies included in the 2009 PM ISA examined 
exposures to mixtures, such as motor vehicle exhaust or woodsmoke. 
In these studies, it was unclear if the resulting cardiovascular 
effects could be attributed specifically to the particulate 
components of the mixture.
---------------------------------------------------------------------------

    A large body of recent evidence confirms and extends the evidence 
from the 2009 ISA supporting the relationship between short-term 
PM2.5 exposure and cardiovascular effects. This includes 
generally positive associations observed in multicity epidemiologic 
studies of emergency department visits and hospital admissions for IHD, 
HF, and combined cardiovascular-related endpoints. In particular, 
nationwide studies of older adults (65 years and older) using Medicare 
records report positive associations between PM2.5 exposures 
and hospital admissions for HF (U.S. EPA, 2019, section 6.1.3.1). 
Additional multicity studies conducted in the northeast U.S. report 
positive associations between short-term PM2.5 exposures and 
emergency department visits or hospital admissions for IHD (U.S. EPA, 
2019, section 6.1.2.1) while studies conducted in the U.S. and Canada 
reported positive associations between short-term PM2.5 
exposures and emergency department visits for HF. Epidemiologic studies 
conducted in single cities contribute some support, though associations 
reported in single-city studies are less consistently positive than in 
multicity studies, and include a number of studies reporting null 
associations (U.S. EPA, 2019, sections 6.1.2 and 6.1.3). When 
considered as a whole; however, the recent body of IHD and HF 
epidemiologic evidence supports the evidence from previous ISAs 
reporting mainly positive associations between short-term 
PM2.5 concentrations and emergency department visits and 
hospital admissions.
    In addition, a number of more recent controlled human exposure, 
animal toxicological, and epidemiologic panel studies provide evidence 
that PM2.5

[[Page 24110]]

exposure could plausibly result in IHD or HF through pathways that 
include endothelial dysfunction, arterial thrombosis, and arrhythmia 
(U.S. EPA, 2019, section 6.1.1). The most consistent evidence from 
recent controlled human exposure studies is for endothelial 
dysfunction, as measured by changes in brachial artery diameter or flow 
mediated dilation. All but one of the available controlled human 
exposure studies examining the potential for endothelial dysfunction 
report an effect of PM2.5 exposure on measures of blood flow 
(U.S. EPA, 2019, section 6.1.13.2). These studies report variable 
results regarding the timing of the effect and the mechanism by which 
reduced blood flow occurs (i.e., availability vs sensitivity to nitric 
oxide). Some controlled human exposure studies using PM2.5 
CAPs report evidence for small increases in blood pressure (U.S. EPA, 
2019, section 6.1.6.3). In addition, although not entirely consistent, 
there is also some evidence across controlled human exposure studies 
for conduction abnormalities/arrhythmia (U.S. EPA, 2019, section 
6.1.4.3), changes in heart rate variability (HRV) (U.S. EPA, 2019, 
section 6.1.10.2), changes in hemostasis that could promote clot 
formation (U.S. EPA, 2019, section 6.1.12.2), and increases in 
inflammatory cells and markers (U.S. EPA, 2019, section 6.1.11.2). 
Thus, when taken as a whole, controlled human exposure studies are 
coherent with epidemiologic studies in that they provide evidence that 
short-term exposures to PM2.5 may result in the types of 
cardiovascular endpoints that could lead to emergency department visits 
and hospital admissions in some people.
    Animal toxicological studies published since the 2009 ISA also 
support a relationship between short-term PM2.5 exposure and 
cardiovascular effects. A recent study demonstrating decreased cardiac 
contractility and left ventricular pressure in mice is coherent with 
the results of epidemiologic studies that report associations between 
short-term PM2.5 exposure and heart failure (U.S. EPA, 2019, 
section 6.1.3.3). In addition, and as with controlled human exposure 
studies, there is generally consistent evidence in animal toxicological 
studies for indicators of endothelial dysfunction (U.S. EPA, 2019, 
section 6.1.13.3). Studies in animals also provide evidence for changes 
in a number of other cardiovascular endpoints following short-term 
PM2.5 exposure. Although not entirely consistent, these 
studies provide some evidence of conduction abnormalities and 
arrhythmia (U.S. EPA, 2019, section 6.1.4.4), changes in HRV (U.S. EPA, 
2019, section 6.1.10.3), changes in blood pressure (U.S. EPA, 2019, 
section 6.1.6.4), and evidence for systemic inflammation and oxidative 
stress (U.S. EPA, 2019, section 6.1.11.3).
    In summary, recent evidence supports the conclusions reported in 
the 2009 ISA indicating relationships between short-term 
PM2.5 exposures and hospital admissions and ED visits for 
IHD and HF, along with cardiovascular mortality. Epidemiologic studies 
reporting robust associations in copollutant models are supported by 
direct evidence from controlled human exposure and animal toxicological 
studies reporting independent effects of PM2.5 exposures on 
endothelial dysfunction as well as endpoints indicating impaired 
cardiac function, increased risk of arrhythmia, changes in HRV, 
increases in BP, and increases in indicators of systemic inflammation, 
oxidative stress, and coagulation (U.S. EPA, 2019, section 6.1.16). 
Epidemiologic panel studies, although not entirely consistent, provide 
some evidence that PM2.5 exposures are associated with 
cardiovascular effects, including increased risk of arrhythmia, 
decreases in HRV, increases in BP, and ST segment depression. Overall, 
the results from epidemiologic panel, controlled human exposure, and 
animal toxicological studies (in particular those related to 
endothelial dysfunction, impaired cardiac function, ST segment 
depression, thrombosis, conduction abnormalities, and changes in blood 
pressure) provide coherence and biological plausibility for the 
consistent results from epidemiologic studies reporting positive 
associations between short-term PM2.5 exposures and IHD and 
HF, and ultimately cardiovascular mortality. The 2019 ISA concludes 
that, overall, ``there continues to be sufficient evidence to conclude 
that a causal relationship exists between short-term PM2.5 
exposure and cardiovascular effects'' (U.S. EPA, 2019, p. 6-138).
c. Respiratory Effects
i. Long-Term PM2.5 Exposures
    The 2009 PM ISA concluded that ``a causal relationship is likely to 
exist between long-term PM2.5 exposure and respiratory 
effects'' (U.S. EPA, 2009c). This conclusion was based mainly on 
epidemiologic evidence demonstrating associations between long-term 
PM2.5 exposure and changes in lung function or lung function 
growth in children. Biological plausibility was provided by a single 
animal toxicological study examining pre- and post-natal exposure to 
PM2.5 CAPs, which found impaired lung development. 
Epidemiologic evidence for associations between long-term 
PM2.5 exposure and other respiratory outcomes, such as the 
development of asthma, allergic disease, and COPD; respiratory 
infection; and the severity of disease was limited, both in the number 
of studies available and the consistency of the results. Experimental 
evidence for other outcomes was also limited, with one animal 
toxicological study reporting that long-term exposure to 
PM2.5 CAPs results in morphological changes in the nasal 
airways of healthy animals. Other animal studies examined exposure to 
mixtures, such as motor vehicle exhaust and woodsmoke, and effects were 
not attributed specifically to the particulate components of the 
mixture.
    Recent cohort studies provide additional support for the 
relationship between long-term PM2.5 exposure and decrements 
in lung function growth (as a measure of lung development), indicating 
a robust and consistent association across study locations, exposure 
assessment methods, and time periods (U.S. EPA, 2019, section 5.2.13). 
This relationship is further supported by a recent retrospective study 
that reports an association between declining PM2.5 
concentrations and improvements in lung function growth in children 
(U.S. EPA, 2019, section 5.2.11). Epidemiologic studies also examine 
asthma development in children (U.S. EPA, 2019, section 5.2.3), with 
recent prospective cohort studies reporting generally positive 
associations, though several are imprecise (i.e., they report wide 
confidence intervals). Supporting evidence is provided by studies 
reporting associations with asthma prevalence in children, with 
childhood wheeze, and with exhaled nitric oxide, a marker of pulmonary 
inflammation (U.S. EPA, 2019, section 5.2.13). A recent animal 
toxicological study showing the development of an allergic phenotype 
and an increase in a marker of airway responsiveness supports the 
biological plausibility of the development of allergic asthma (U.S. 
EPA, 2019, section 5.2.13). Other epidemiologic studies report a 
PM2.5-related acceleration of lung function decline in 
adults, while improvement in lung function was observed with declining 
PM2.5 concentrations (U.S. EPA, 2019, section 5.2.11). A 
recent longitudinal study found declining PM2.5 
concentrations are also associated with an improvement in chronic 
bronchitis symptoms in children, strengthening evidence reported in the 
2009 ISA for a relationship between

[[Page 24111]]

increased chronic bronchitis symptoms and long-term PM2.5 
exposure (U.S. EPA, 2019, section 5.2.11). A common uncertainty across 
the epidemiologic evidence is the lack of examination of copollutants 
to assess the potential for confounding. While there is some evidence 
that associations remain robust in models with gaseous pollutants, a 
number of these studies examining copollutant confounding were 
conducted in Asia, and thus have limited generalizability due to high 
annual pollutant concentrations.
    When taken together, the 2019 ISA concludes that the 
``epidemiologic evidence strongly supports a relationship with 
decrements in lung function growth in children'' (U.S. EPA, 2019, p. 1-
34). Additional epidemiologic evidence ``supports a relationship with 
asthma development in children, increased bronchitic symptoms in 
children with asthma, acceleration of lung function decline in adults, 
and respiratory mortality, including cause-specific respiratory 
mortality for COPD and respiratory infection'' (U.S. EPA, 2019, p. 1-
34). In support of the biological plausibility of such associations 
reported in epidemiologic studies of respiratory health effects, animal 
toxicological studies continue to provide direct evidence that long-
term exposure to PM2.5 results in a variety of respiratory 
effects. Recent animal studies show pulmonary oxidative stress, 
inflammation, and morphologic changes in the upper (nasal) and lower 
airways. Other results show that changes are consistent with the 
development of allergy and asthma, and with impaired lung development. 
Overall, the ISA concludes that ``the collective evidence is sufficient 
to conclude a likely to be causal relationship between long-term 
PM2.5 exposure and respiratory effects'' (U.S. EPA, 2019, p. 
5-220).
ii. Short-Term PM2.5 Exposures
    The 2009 PM ISA (U.S. EPA, 2009c) concluded that a ``causal 
relationship is likely to exist'' between short-term PM2.5 
exposure and respiratory effects. This conclusion was based mainly on 
the epidemiologic evidence demonstrating positive associations with 
various respiratory effects. Specifically, the 2009 ISA described 
epidemiologic evidence as consistently showing PM2.5-
associated increases in hospital admissions and emergency department 
visits for COPD and respiratory infection among adults or people of all 
ages, as well as increases in respiratory mortality. These results were 
supported by studies reporting associations with increased respiratory 
symptoms and decreases in lung function in children with asthma, though 
the available epidemiologic evidence was inconsistent for hospital 
admissions or emergency department visits for asthma. Studies examining 
copollutant models showed that PM2.5 associations with 
respiratory effects were robust to inclusion of CO or SO2 in 
the model, but often were attenuated (though still positive) with 
inclusion of O3 or NO2. In addition to the 
copollutant models, evidence supporting an independent effect of 
PM2.5 exposure on the respiratory system was provided by 
animal toxicological studies of PM2.5 CAPs demonstrating 
changes in some pulmonary function parameters, as well as inflammation, 
oxidative stress, injury, enhanced allergic responses, and reduced host 
defenses. Many of these effects have been implicated in the 
pathophysiology for asthma exacerbation, COPD exacerbation, or 
respiratory infection. In the few controlled human exposure studies 
conducted in individuals with asthma or COPD, PM2.5 exposure 
mostly had no effect on respiratory symptoms, lung function, or 
pulmonary inflammation. Available studies in healthy people also did 
not clearly find respiratory effects following short-term 
PM2.5 exposures.
    Recent epidemiologic studies provide evidence for a relationship 
between short-term PM2.5 exposure and several respiratory-
related endpoints, including asthma exacerbation (U.S. EPA, 2019, 
section 5.1.2.1), COPD exacerbation (U.S. EPA, 2019, section 5.1.4.1), 
and combined respiratory-related diseases (U.S. EPA, 2019, section 
5.1.6), particularly from studies examining emergency department visits 
and hospital admissions. The generally positive associations between 
short-term PM2.5 exposure and asthma and COPD emergency 
department visits and hospital admissions are supported by 
epidemiologic studies demonstrating associations with other 
respiratory-related effects such as symptoms and medication use that 
are indicative of asthma and COPD exacerbations (U.S. EPA, 2019, 
sections 5.1.2.2 and 5.4.1.2). The collective body of epidemiologic 
evidence for asthma exacerbation is more consistent in children than in 
adults. Additionally, epidemiologic studies examining the relationship 
between short-term PM2.5 exposure and respiratory mortality 
provide evidence of consistent positive associations, demonstrating a 
continuum of effects (U.S. EPA, 2019, section 5.1.9).
    Building on the studies evaluated in the 2009 ISA, recent 
epidemiologic studies expand the assessment of potential copollutant 
confounding. There is some evidence that PM2.5 associations 
with asthma exacerbation, combined respiratory-related diseases, and 
respiratory mortality remain relatively unchanged in copollutant models 
with gaseous pollutants (i.e., O3, NO2, 
SO2, with more limited evidence for CO) and other particle 
sizes (i.e., PM10-2.5) (U.S. EPA, 2019, section 5.1.10.1).
    Insight into whether there is an independent effect of 
PM2.5 on respiratory health is provided by findings from 
animal toxicological studies. Specifically, short-term exposure to 
PM2.5 has been shown to enhance asthma-related responses in 
an animal model of allergic airways disease and lung injury and 
inflammation in an animal model of COPD (U.S. EPA, 2019, sections 
5.1.2.4.4 and 5.1.4.4.3). The experimental evidence provides biological 
plausibility for some respiratory-related endpoints, including limited 
evidence of altered host defense and greater susceptibility to 
bacterial infection as well as consistent evidence of respiratory 
irritant effects. Animal toxicological evidence for other respiratory 
effects is inconsistent and controlled human exposure studies provide 
limited evidence of respiratory effects (U.S. EPA, 2019, section 
5.1.12).
    The 2019 ISA concludes that ``[t]he strongest evidence of an effect 
of short-term PM2.5 exposure on respiratory effects is 
provided by epidemiologic studies of asthma and COPD exacerbation. 
While animal toxicological studies provide biological plausibility for 
these findings, some uncertainty remains with respect to the 
independence of PM2.5 effects'' (U.S. EPA, 2019, p. 5-155). 
When taken together, the ISA concludes that this evidence ``is 
sufficient to conclude a likely to be causal relationship between 
short-term PM2.5 exposure and respiratory effects'' (U.S. 
EPA, 2019, p. 5-155).
d. Cancer
    The 2009 ISA concluded that the overall body of evidence was 
``suggestive of a causal relationship between relevant PM2.5 
exposures and cancer'' (U.S. EPA, 2009c). This conclusion was based 
primarily on positive associations observed in a limited number of 
epidemiologic studies of lung cancer mortality. The few epidemiologic 
studies that had evaluated PM2.5 exposure and lung cancer 
incidence or cancers of other organs and systems generally did not show 
evidence of an association. Toxicological studies did not focus on 
exposures to specific PM size fractions,

[[Page 24112]]

but rather investigated the effects of exposures to total ambient PM, 
or other source-based PM such as wood smoke. Collectively, results of 
in vitro studies were consistent with the larger body of evidence 
demonstrating that ambient PM and PM from specific combustion sources 
are mutagenic and genotoxic. However, animal inhalation studies found 
little evidence of tumor formation in response to chronic exposures. A 
small number of studies provided preliminary evidence that PM exposure 
can lead to changes in methylation of DNA, which may contribute to 
biological events related to cancer.
    Since the 2009 ISA, additional cohort studies provide evidence that 
long-term PM2.5 exposure is positively associated with lung 
cancer mortality and with lung cancer incidence, and provide initial 
evidence for an association with reduced cancer survival (U.S. EPA, 
2019, section 10.2.5). Reanalyses of the ACS cohort using different 
years of PM2.5 data and follow-up, along with various 
exposure assignment approaches, provide consistent evidence of positive 
associations between long-term PM2.5 exposure and lung 
cancer mortality (U.S. EPA, 2019, Figure 10-3). Additional support for 
positive associations with lung cancer mortality is provided by recent 
epidemiologic studies using individual-level data to control for 
smoking status, by studies of people who have never smoked (though such 
studies generally report wide confidence intervals due to the small 
number of lung cancer mortality cases within this population), and in 
analyses of cohorts that relied upon proxy measures to account for 
smoking status (U.S. EPA, 2019, section 10.2.5.1.1). Although studies 
that evaluate lung cancer incidence, including studies of people who 
have never smoked, are limited in number, recent studies generally 
report positive associations with long-term PM2.5 exposures 
(U.S. EPA, 2019, section 10.2.5.1.2). A subset of the studies focusing 
on lung cancer incidence also examined histological subtype, providing 
some evidence of positive associations for adenocarcinomas, the 
predominate subtype of lung cancer observed in people who have never 
smoked (U.S. EPA, 2019, section 10.2.5.1.2). Associations between long-
term PM2.5 exposure and lung cancer incidence were found to 
remain relatively unchanged, though in some cases confidence intervals 
widened, in analyses that attempted to reduce exposure measurement 
error by accounting for length of time at residential address or by 
examining different exposure assignment approaches (U.S. EPA, 2019, 
section 10.2.5.1.2).
    The 2019 ISA evaluates the degree to which recent epidemiologic 
studies have addressed the potential for confounding by copollutants 
and the shape of the concentration-response relationship. To date, 
relatively few studies have evaluated the potential for copollutant 
confounding of the relationship between long-term PM2.5 
exposure and lung cancer mortality or incidence. The small number of 
such studies have generally focused on O3 and report that 
PM2.5 associations remain relatively unchanged in 
copollutant models (U.S. EPA, 2019, section 10.2.5.1.3). However, 
available studies have not systematically evaluated the potential for 
copollutant confounding by other gaseous pollutants or by other 
particle size fractions (U.S. EPA, 2019, section 10.2.5.1.3). Compared 
to total (non-accidental) mortality (discussed above), fewer studies 
have examined the shape of the concentration-response curve for cause-
specific mortality outcomes, including lung cancer. Several studies 
have reported no evidence of deviations from linearity in the shape of 
the concentration-response relationship (Lepeule et al., 2012; 
Raaschou-Nielsen et al., 2013; Puett et al., 2014), though authors 
provided only limited discussions of results (U.S. EPA, 2019, section 
10.2.5.1.4).
    In support of the biological plausibility of an independent effect 
of PM2.5 on cancer, the 2019 ISA notes evidence from recent 
experimental studies demonstrating that PM2.5 exposure can 
lead to a range of effects indicative of mutagenicity, genotoxicity, 
and carcinogenicity, as well as epigenetic effects (U.S. EPA, 2019, 
section 10.2.7). For example, both in vitro and in vivo toxicological 
studies have shown that PM2.5 exposure can result in DNA 
damage (U.S. EPA, 2019, section 10.2.2). Although such effects do not 
necessarily equate to carcinogenicity, the evidence that PM exposure 
can damage DNA, and elicit mutations, provides support for the 
plausibility of epidemiologic associations with lung cancer mortality 
and incidence. Additional supporting studies indicate the occurrence of 
micronuclei formation and chromosomal abnormalities (U.S. EPA, 2019, 
section 10.2.2.3), and differential expression of genes that may be 
relevant to cancer pathogenesis, following PM exposures. Experimental 
and epidemiologic studies that examine epigenetic effects indicate 
changes in DNA methylation, providing some support for PM2.5 
exposure contributing to genomic instability (U.S. EPA, 2019, section 
10.2.3).
    Epidemiologic evidence for associations between PM2.5 
exposure and lung cancer mortality and incidence, together with 
evidence supporting the biological plausibility of such associations, 
contributes to the 2019 ISA's conclusion that the evidence ``is 
sufficient to conclude there is a likely to be causal relationship 
between long-term PM2.5 exposure and cancer'' (U.S. EPA, 
2019, p. 10-77).
    In its letter to the Administrator on the draft ISA, the CASAC 
states that ``the Draft ISA does not present adequate evidence to 
conclude that there is likely to be a causal relationship between long-
term PM2.5 exposure and . . . cancer'' (Cox, 2019a, p. 1 of 
letter). The CASAC specifically states that this causality 
determination ``relies largely on epidemiology studies that . . . do 
not provide exposure time frames that are appropriate for cancer 
causation and that there are no animal studies showing direct effects 
of PM2.5 on cancer formation'' (Cox, 2019a, p. 20 of 
consensus responses).
    With respect to the latency period, it is well recognized that 
``air pollution exposures experienced over an extended historical time 
period are likely more relevant to the etiology of lung cancer than air 
pollution exposures experienced in the more recent past'' (Turner et 
al. 2011). However, many epidemiologic studies conducted within the 
U.S. that examine long-term PM2.5 exposure and lung cancer 
incidence and lung cancer mortality rely on more recent air quality 
data because routine PM2.5 monitoring did not start until 
1999-2000. An exception to this is the American Cancer Society (ACS) 
study that had PM2.5 concentration data from two time 
periods, 1979-1983 and from 1999-2000. Turner et al. (2011), conducted 
a comparison of PM2.5 concentrations between these two time 
periods and found that they were highly correlated (r >0.7), with the 
relative rank order of metropolitan statistical areas (MSAs) by 
PM2.5 concentrations being ``generally retained over time.'' 
Therefore, areas where PM2.5 concentrations were high 
remained high over decades (or low remained low) relative to other 
locations. Long-term exposure epidemiologic studies rely on spatial 
contrasts between locations; therefore, if a location with high 
PM2.5 concentrations continues to have high concentrations 
over decades relative to other locations a relationship between the 
PM2.5 exposure and cancer should persist. This was confirmed 
in a sensitivity analysis conducted by

[[Page 24113]]

Turner et al. (2011), where the authors reported a similar hazard ratio 
(HR) for lung cancer mortality for participants assigned exposure to 
PM2.5 (1979-1983) and PM2.5 (1999-2000) in two 
separate analyses.
    While experimental studies showing a direct effect of 
PM2.5 on cancer formation were limited to an animal model of 
urethane-induced tumor initiation, a large number of experimental 
studies report that PM2.5 exhibits several key 
characteristics of carcinogens, as indicated by genotoxic effects, 
oxidative stress, electrophilicity, and epigenetic alterations, all of 
which provide biological plausibility that PM2.5 exposure 
can contribute to cancer development. The experimental evidence, in 
combination with multiple recent and previously evaluated epidemiologic 
studies examining the relationship between long-term PM2.5 
exposure and both lung cancer incidence and lung cancer mortality that 
reported generally positive associations across different cohorts, 
exposure assignment methods, and in analyses of never smokers further 
addresses uncertainties identified in the 2009 PM ISA. Therefore, upon 
re-evaluating the causality determination for cancer, when considering 
CASAC comments on the Draft PM ISA and applying the causal framework as 
described (U.S. EPA, 2015; U.S. EPA, 2019, section A.3.2.1), the EPA 
continues to conclude in the 2019 Final PM ISA that the evidence for 
long-term PM2.5 exposure and cancer supports a ``likely to 
be causal relationship'' (U.S. EPA, 2019, p. 10-77).
e. Nervous System Effects
    Reflecting the very limited evidence available in the last review, 
the 2009 ISA did not make a causality determination for long-term 
PM2.5 exposures and nervous system effects (U.S. EPA, 
2009c). Since the last review, this body of evidence has grown 
substantially (U.S. EPA, 2019, section 8.2). Recent studies in adult 
animals report that long-term PM2.5 exposures can lead to 
morphologic changes in the hippocampus and to impaired learning and 
memory. This evidence is consistent with epidemiologic studies 
reporting that long-term PM2.5 exposure is associated with 
reduced cognitive function (U.S. EPA, 2019, section 8.2.5). Further, 
while the evidence is limited, early markers of Alzheimer's disease 
pathology have been reported in rodents following long-term exposure to 
PM2.5 CAPs. These findings support reported associations 
with neurodegenerative changes in the brain (i.e., decreased brain 
volume), all-cause dementia, and hospitalization for Alzheimer's 
disease in a small number of epidemiologic studies (U.S. EPA, 2019, 
section 8.2.6). Additionally, loss of dopaminergic neurons in the 
substantia nigra, a hallmark of Parkinson disease, has been reported in 
mice following long-term PM2.5 exposures (U.S. EPA, 2019, 
section 8.2.4), though epidemiologic studies provide only limited 
support for associations with Parkinson's disease (U.S. EPA, 2019, 
section 8.2.6). Overall, the lack of consideration of copollutant 
confounding introduces some uncertainty in the interpretation of 
epidemiologic studies of nervous system effects, but this uncertainty 
is partly addressed by the evidence for an independent effect of 
PM2.5 exposures provided by experimental animal studies.
    In addition to the findings described above, which are most 
relevant to older adults, several recent studies of neurodevelopmental 
effects in children have also been conducted. Positive associations 
between long-term exposure to PM2.5 during the prenatal 
period and autism spectrum disorder (ASD) are observed in multiple 
epidemiologic studies (U.S. EPA, 2019, section 8.2.7.2), while studies 
of cognitive function provide little support for an association (U.S. 
EPA, 2019, section 8.2.5.2). Interpretation of these epidemiologic 
studies is limited due to the small number of studies, their lack of 
control for potential confounding by copollutants, and uncertainty 
regarding the critical exposure windows. Biological plausibility is 
provided for the ASD findings by a study in mice that found 
inflammatory and morphologic changes in the corpus collosum and 
hippocampus, as well as ventriculomegaly (i.e., enlarged lateral 
ventricles) in young mice following prenatal exposure to 
PM2.5 CAPs.
    Taken together, the 2019 ISA concludes that the strongest evidence 
of an effect of long-term exposure to PM2.5 on the nervous 
system is provided by toxicological studies that show inflammation, 
oxidative stress, morphologic changes, and neurodegeneration in 
multiple brain regions following long-term exposure of adult animals to 
PM2.5 CAPs. These findings are coherent with epidemiologic 
studies reporting consistent associations with cognitive decrements and 
with all-cause dementia. There is also initial, and limited, evidence 
for neurodevelopmental effects, particularly ASD. The ISA determines 
that ``[o]verall, the collective evidence is sufficient to conclude a 
likely to be causal relationship between long-term PM2.5 
exposure and nervous system effects'' (U.S. EPA, 2019, p. 8-61).
    In its letter to the Administrator on the draft ISA, the CASAC 
states that ``the Draft ISA does not present adequate evidence to 
conclude that there is likely to be a causal relationship between long-
term PM2.5 exposure and nervous system effects'' (Cox, 
2019a, p. 1 of letter). The CASAC specifically states that ``[f]or a 
likely causal conclusion, there would have to be evidence of health 
effects in studies where results are not explained by chance, 
confounding, and other biases, but uncertainties remain in the overall 
evidence'' (Cox, 2019a, p. 20 of consensus responses). These 
uncertainties in the eyes of CASAC reflect that animal toxicological 
studies ``have largely been done by a single group'' (P.20), and for 
epidemiologic studies that examined brain volume that ``brain volumes 
can vary . . . between normal people'' and the results from studies of 
cognitive function were ``largely non-statistically significant''.
    With these concerns in mind, the EPA re-evaluated the evidence and 
note that animal toxicological studies were conducted in ``multiple 
research groups [and show a range of effects including] inflammation, 
oxidative stress, morphologic changes, and neurodegeneration in 
multiple brain regions following long-term exposure of adult animals to 
PM2.5 CAPs'' (U.S. EPA, 2019, p. 8-61). The results from the 
animal toxicological studies ``are coherent with a number of 
epidemiologic studies reporting consistent associations with cognitive 
decrements and with all-cause dementia'' (U.S. EPA, 2019, p. 8-61). 
Additionally, as discussed in the Preamble to the ISAs (U.S. EPA, 
2015):

    ``. . . the U.S. EPA emphasizes the importance of examining the 
pattern of results across various studies and does not focus solely 
on statistical significance or the magnitude of the direction of the 
association as criteria of study reliability. Statistical 
significance is influenced by a variety of factors including, but 
not limited to, the size of the study, exposure and outcome 
measurement error, and statistical model specifications. Statistical 
significance . . . is just one of the means of evaluating confidence 
in the observed relationship and assessing the probability of chance 
as an explanation. Other indicators of reliability such as the 
consistency and coherence of a body of studies as well as other 
confirming data may be used to justify reliance on the results of a 
body of epidemiologic studies, even if results in individual studies 
lack statistical significance . . . [Therefore, the U.S. EPA] . . . 
does not limit its focus or consideration to statistically 
significant results in epidemiologic studies.''

[[Page 24114]]

    Therefore, upon re-evaluating the causality determination, when 
considering the CASAC comments on the Draft PM ISA and applying the 
causal framework as described (U.S. EPA, 2015; U.S. EPA, 2019, section 
A.3.2.1), the EPA continues to conclude in the 2019 Final PM ISA that 
the evidence for long-term PM2.5 exposure and nervous system 
effects supports a ``likely to be causal relationship'' (U.S. EPA, 
2019, p. 8-61).
2. Populations at Risk of PM2.5-Related Health Effects
    The NAAQS are meant to protect the population as a whole, including 
groups that may be at increased risk for pollutant-related health 
effects. In the last review, based on the evidence assessed in the 2009 
ISA (U.S. EPA, 2009c), the 2011 PA focused on children, older adults, 
people with pre-existing heart and lung diseases, and those of lower 
socioeconomic status as populations that are ``likely to be at 
increased risk of PM-related effects'' (U.S. EPA, 2011, p. 2-31). In 
the current review, the 2019 ISA cites extensive evidence indicating 
that ``both the general population as well as specific populations and 
lifestages are at risk for PM2.5-related health effects'' 
(U.S. EPA, 2019, p. 12-1). For example, in support of its ``causal'' 
and ``likely to be causal'' determinations, the ISA cites substantial 
evidence for:
     PM-related mortality and cardiovascular effects in older 
adults (U.S. EPA, 2019, sections 11.1, 11.2, 6.1, and 6.2);
     PM-related cardiovascular effects in people with pre-
existing cardiovascular disease (U.S. EPA, 2019, section 6.1);
     PM-related respiratory effects in people with pre-existing 
respiratory disease, particularly asthma exacerbations in children 
(U.S. EPA, 2019, section 5.1); and
     PM-related impairments in lung function growth and asthma 
development in children (U.S. EPA, 2019, sections 5.1 and 5.2; 
12.5.1.1).
    The ISA additionally notes that stratified analyses (i.e., analyses 
that directly compare PM-related health effects across groups) provide 
support for racial and ethnic differences in PM2.5 exposures 
and in PM2.5-related health risk (U.S. EPA, 2019, section 
12.5.4). Drawing from such studies, the ISA concludes that ``[t]here is 
strong evidence demonstrating that black and Hispanic populations, in 
particular, have higher PM2.5 exposures than non-Hispanic 
white populations'' and that ``there is consistent evidence across 
multiple studies demonstrating an increase in risk for nonwhite 
populations'' (U.S. EPA, 2019, p. 12-38). Stratified analyses focusing 
on other groups also suggest that populations with pre-existing 
cardiovascular or respiratory disease, populations that are overweight 
or obese, populations that have particular genetic variants, 
populations that are of low socioeconomic status, and current/former 
smokers could be at increased risk for PM2.5-related adverse 
health effects (U.S. EPA, 2019, Chapter 12).
    Thus, the groups at risk of PM2.5-related health effects 
represent a substantial portion of the total U.S. population. In 
evaluating the primary PM2.5 standards, an important 
consideration is the potential PM2.5-related public health 
impacts in these populations.
3. CASAC Advice
    In its review of the draft ISA, the CASAC provided advice on the 
assessment of the scientific evidence for PM-related health and welfare 
effects and on the process under which this review of the PM NAAQS is 
being conducted (Cox, 2019b). With regard to the assessment of the 
evidence, the CASAC recommended that a revised ISA should ``provide a 
clearer and more complete description of the process and criteria for 
study quality assessment'' and that it should include a ``[c]learer 
discussion of causality and causal biological mechanisms and pathways'' 
(Cox, 2019b, p. 1 of letter). The CASAC further advised that the draft 
ISA ``does not present adequate evidence to conclude that there is 
likely to be a causal relationship between long-term PM2.5 
exposure and nervous system effects; between long-term ultrafine 
particulate (UFP) exposure and nervous system effects; or between long-
term PM2.5 exposure and cancer'' (Cox, 2019b, p. 1 of 
letter).
    As discussed above in section I.C.5, and as detailed in the final 
ISA, to address these comments the EPA: (1) Added text to the Preface 
and developed a new Appendix to more clearly articulate the process of 
ISA development; (2) added text to the Preface and to the health 
effects chapters to clarify the discussion of biological plausibility 
and its role in forming causality determinations; and (3) revised the 
determination for long-term UFP exposure and nervous system effects to 
suggestive of, but not sufficient to infer, a causal relationship. The 
EPA's rationales for not revising the other causality determinations 
questioned by the CASAC are discussed above in sections II.B.1.d (i.e., 
for cancer) and II.B.1.e (i.e., for nervous system effects).
    With regard to the process for reviewing the PM NAAQS, the CASAC 
requested the opportunity to review a 2nd draft ISA (Cox, 2019b, p. 1 
of letter) and recommended that ``the EPA reappoint the previous CASAC 
PM panel (or appoint a panel with similar expertise)'' (Cox, 2019b, p. 
2 of letter). As discussed above in section I.C.5, the Agency's 
responses to these recommendations were described in a letter from the 
Administrator to the CASAC chair.\40\
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    In addition to the consensus advice noted above, the CASAC did not 
reach consensus on some issues related to the assessment of the 
PM2.5 health effects evidence. In particular, the CASAC 
members ``had varying opinions on whether there is robust and 
convincing evidence to support the EPA's conclusion that there is a 
causal relationship between PM2.5 exposure and mortality'' 
(Cox, 2019b, p. 3 of letter). ``Some members of the CASAC'' concluded 
that ``the EPA must better justify their determination that short-term 
or long-term exposure to PM2.5 causes mortality'' (Cox, 
2019b, p. 1 of consensus responses). These members recommended that the 
ISA should specifically address the biological action of PM and how 
exposures to low concentrations of PM2.5 could cause 
mortality; the geographic heterogeneity in effect estimates between 
PM2.5 exposure and mortality; concentration concordance 
across epidemiologic, controlled human exposure and animal 
toxicological studies (i.e., how the continuum of effects is impacted 
by the concentrations at which different effects have been observed); 
uncertainties in the shapes of concentration-response functions and in 
the potential for thresholds to exist; how results compare between and 
within studies; and whether PM2.5 exposures result in 
mortality in animal studies (Cox, 2019b, pp. 1-2).
    In contrast, ``[o]ther members of the CASAC are of the opinion 
that, although uncertainties remain, the evidence supporting the causal 
relationship between PM2.5 exposure and mortality is robust, 
diverse, and convincing'' (Cox, 2019b, p. 3 of consensus responses). 
These members noted that epidemiologic observations ``have been 
reproduced around the world in communities with widely varying 
exposures'' and that ``the findings of many of the largest studies have 
been repeatedly reanalyzed, with

[[Page 24115]]

confirmation of the original findings'' (Cox, 2019b, p. 3). These 
committee members additionally stated that the ISA's causality 
determinations consider ``a wide range of evidence from a variety of 
sources, including human clinical exposure and animal toxicology 
studies that have provided rational biological plausibility and 
potential mechanisms'' (Cox, 2019b, p. 3). They highlighted the fact 
that there is new evidence in the current review from epidemiologic 
studies supporting associations between PM2.5 and mortality 
and new evidence from toxicology studies informing the biological 
plausibility of mechanisms that could lead to mortality (Cox, 2019b, p. 
3).

C. Proposed Conclusions on the Current Primary PM2.5 
Standards

    This section describes the Administrator's proposed conclusions 
regarding the adequacy of the current primary PM2.5 
standards. His approach to reaching these proposed conclusions draws 
from the ISA's assessment of the scientific evidence for health effects 
attributable to PM2.5 exposures (U.S. EPA, 2019) and the 
analyses in the PA (U.S. EPA, 2020), including uncertainties in the 
evidence and analyses. Section II.C.1 discusses the evidence- and risk-
based considerations in the PA. Section II.C.2 summarizes CASAC advice 
on the current primary PM2.5 standards, based on its review 
of the draft PA (Cox, 2019a). Section II.C.3 presents the 
Administrator's proposed decision to retain the current primary 
PM2.5 standards.
1. Evidence- and Risk-Based Considerations in the Policy Assessment
    The Administrator's proposed decision in this review draws from his 
consideration of the PM2.5 health evidence assessed in the 
ISA (U.S. EPA, 2019) and the evidence- and risk-based analyses 
presented in the PA (U.S. EPA, 2020), including the uncertainties 
inherent in the evidence and analyses. The sections below summarize the 
consideration of the evidence-based information (II.C.1.a) and risk-
based information (II.C.1.b) in the PA.
a. Evidence-Based Considerations
    The PA considers the degree to which the available scientific 
evidence provides support for the current and potential alternative 
standards in terms of the basic elements of those standards (i.e., 
indicator, averaging time, form, and level). With regard to the current 
indicator, averaging times, and forms, the PA concludes that the 
available evidence continues to support these elements in the current 
review. For indicator, the PA specifically concludes that available 
studies provide strong support for health effects following long- and 
short-term PM2.5 exposures and that the evidence is too 
limited to support potential alternatives (U.S. EPA, 2020, section 
3.5.2.1). For averaging time, the PA notes that epidemiologic studies 
continue to provide strong support for health effects based on annual 
(or multiyear) and 24-hour PM2.5 averaging periods and 
concludes that the evidence does not support considering alternatives 
(U.S. EPA, 2020, section 3.5.2.2). For form, the PA notes that the 
foremost consideration is the adequacy of the public health protection 
provided by the combination of the form and the other elements of the 
standard. It concludes that (1) the form of the current annual standard 
(i.e., arithmetic mean, averaged over three years) remains appropriate 
for targeting protection against the annual and daily PM2.5 
exposures around the middle portion of the PM2.5 air quality 
distribution, and (2) the form of the current 24-hour standard (98th 
percentile, averaged over three years) continues to provide an 
appropriate balance between limiting the occurrence of peak 24-hour 
PM2.5 concentrations and identifying a stable target for 
risk management programs (U.S. EPA, 2020, section 3.5.2.3).
    With regard to level, the considerations in the PA reflect analyses 
of the PM2.5 exposures and ambient concentrations in studies 
reporting PM2.5-related health effects (U.S. EPA, 2020). As 
noted above, the focus is on health outcomes for which the ISA 
concludes the evidence supports a ``causal'' or a ``likely to be 
causal'' relationship with PM exposures.\41\ While the causality 
determinations in the ISA are informed by studies evaluating a wide 
range of PM2.5 concentrations, the PA considers the degree 
to which the evidence supports the occurrence of PM-related effects at 
concentrations relevant to informing conclusions on the primary 
PM2.5 standards. Section II.C.1.a.i below summarizes the 
PA's consideration of exposure concentrations that have been evaluated 
in experimental studies and section II.C.1.a.ii summarizes the PA's 
consideration of ambient concentrations in locations evaluated by 
epidemiologic studies.
---------------------------------------------------------------------------

    \41\ As discussed above in II.A.2, such a focus recognizes that 
standards set to provide protection based on evidence for ``causal'' 
and ``likely to be causal'' health outcomes will also provide some 
measure of protection against the broader range of PM2.5-
associated outcomes, including those for which the evidence is less 
certain.
---------------------------------------------------------------------------

i. PM2.5 Exposure Concentrations Evaluated in Experimental 
Studies
    Evidence for a particular PM2.5-related health outcome 
is strengthened when results from experimental studies demonstrate 
biologically plausible mechanisms through which adverse human health 
outcomes could occur (U.S. EPA, 2015, Preamble p. 20). Two types of 
experimental studies are of particular importance in understanding the 
effects of PM exposures: Controlled human exposure and animal 
toxicology studies. In such studies, investigators expose human 
volunteers or laboratory animals, respectively, to known concentrations 
of air pollutants under carefully regulated environmental conditions 
and activity levels. Thus, controlled human exposure and animal 
toxicology studies can provide information on the health effects of 
experimentally administered pollutant exposures under well-controlled 
laboratory conditions (U.S. EPA, 2015, Preamble, p. 11). The sections 
below summarize the PA's evaluation of the PM2.5 exposure 
concentrations that have been examined in controlled human exposure 
studies and animal toxicology studies.
Controlled Human Exposure Studies
    Controlled human exposure studies have reported that 
PM2.5 exposures lasting from less than one hour up to five 
hours can impact cardiovascular function (U.S. EPA, 2019, section 6.1). 
The most consistent evidence from these studies is for impaired 
vascular function (U.S. EPA, 2019, section 6.1.13.2). In addition, 
although less consistent, the ISA notes that studies examining 
PM2.5 exposures also provide evidence for increased blood 
pressure (U.S. EPA, 2019, section 6.1.6.3), conduction abnormalities/
arrhythmia (U.S. EPA, 2019, section 6.1.4.3), changes in heart rate 
variability (U.S. EPA, 2019, section 6.1.10.2), changes in hemostasis 
that could promote clot formation (U.S. EPA, 2019, section 6.1.12.2), 
and increases in inflammatory cells and markers (U.S. EPA, 2019, 
section 6.1.11.2).
    Table 3-2 in the PA (U.S. EPA, 2020) summarizes information from 
the ISA on available controlled human exposure studies that evaluate 
effects on markers of cardiovascular function following exposures to 
PM2.5. Most of the controlled human exposure studies in 
Table 3-2 of the PA have evaluated average PM2.5 exposure 
concentrations at or above about 100 [micro]g/m\3\, with exposure 
durations typically up to about two hours. Statistically significant 
effects on one or more indicators of

[[Page 24116]]

cardiovascular function are often, though not always, reported 
following 2-hour exposures to average PM2.5 concentrations 
at and above about 120 [micro]g/m\3\, with less consistent evidence for 
effects following exposures to lower concentrations. Impaired vascular 
function, the effect identified in the ISA as the most consistent 
across studies (U.S. EPA, 2019, section 6.1.13.2), is shown following 
2-hour exposures to PM2.5 concentrations at and above 149 
[micro]g/m\3\. Mixed results are reported in the few studies that 
evaluate longer exposure durations (i.e., longer than 2 hours) and 
lower PM2.5 concentrations (U.S. EPA, 2020, section 
3.2.3.1).
    To provide some insight into what these studies may indicate 
regarding the primary PM2.5 standards, analyses in the PA 
examine monitored 2-hour PM2.5 concentrations at sites 
meeting the current standards (U.S. EPA, 2020, section 3.2.3.1). At 
these sites, most 2-hour concentrations are below 11 [mu]g/m\3\, and 
they almost never exceed 32 [mu]g/m\3\. Even the highest 2-hour 
concentrations remain well-below the exposure concentrations 
consistently shown to cause effects in controlled human exposure 
studies (i.e., 99.9th percentile of 2-hour concentrations is 68 [mu]g/
m\3\ during the warm season). Thus, while controlled human exposure 
studies support the plausibility of the serious cardiovascular effects 
that have been linked with ambient PM2.5 exposures (U.S. 
EPA, 2019, Chapter 6), the PA notes that the PM2.5 exposures 
evaluated in most of these studies are well-above the ambient 
concentrations typically measured in locations meeting the current 
primary standards (U.S. EPA, 2020, section 3.2.3.2.1).
Animal Toxicology Studies
    The ISA relies on animal toxicology studies to support the 
plausibility of a wide range of PM2.5-related health 
effects. While animal toxicology studies often examine more severe 
health outcomes and longer exposure durations than controlled human 
exposure studies, there is uncertainty in extrapolating the effects 
seen in animals, and the PM2.5 exposures and doses that 
cause those effects, to human populations.
    As with controlled human exposure studies, most of the animal 
toxicology studies assessed in the ISA have examined effects following 
exposures to PM2.5 concentrations well-above the 
concentrations likely to be allowed by the current PM2.5 
standards. Such studies have generally examined short-term exposures to 
PM2.5 concentrations from 100 to >1,000 [mu]g/m\3\ and long-
term exposures to concentrations from 66 to >400 [mu]g/m\3\ (e.g., see 
U.S. EPA, 2019, Table 1-2). Two exceptions are a study reporting 
impaired lung development following long-term exposures (i.e., 24 hours 
per day for several months prenatally and postnatally) to an average 
PM2.5 concentration of 16.8 [mu]g/m\3\ (Mauad et al., 2008) 
and a study reporting increased carcinogenic potential following long-
term exposures (i.e., 2 months) to an average PM2.5 
concentration of 17.7 [mu]g/m\3\ (Cangerana Pereira et al., 2011). 
These two studies report serious effects following long-term exposures 
to PM2.5 concentrations close to the ambient concentrations 
reported in some PM2.5 epidemiologic studies (U.S. EPA, 
2019, Table 1-2), though still above the ambient concentrations likely 
to occur in areas meeting the current primary standards. Thus, as is 
the case with controlled human exposure studies, animal toxicology 
studies support the plausibility of various adverse effects that have 
been linked to ambient PM2.5 exposures (U.S. EPA, 2019), but 
have not evaluated PM2.5 exposures likely to occur in areas 
meeting the current primary standards.
ii. Ambient Concentrations in Locations of Epidemiologic Studies
    As summarized above in section II.B.1, epidemiologic studies 
examining associations between daily or annual average PM2.5 
exposures and mortality or morbidity represent a large part of the 
evidence base supporting several of the ISA's ``causal'' and ``likely 
to be causal'' determinations for cardiovascular effects, respiratory 
effects, cancer, and mortality. The PA uses two approaches to consider 
what information from epidemiologic studies may indicate regarding 
primary PM2.5 standards (U.S. EPA, 2020, section 3.2.3.2). 
In one approach, the PA evaluates the PM2.5 air quality 
distributions reported by key epidemiologic studies, with a focus on 
overall mean PM2.5 concentrations (i.e., averages over the 
study period of the daily or annual PM2.5 concentrations 
used to estimate exposures) and the concentrations somewhat below these 
overall means (i.e., corresponding to the lower quartiles of exposure 
or health data) (U.S. EPA, 2020, section 3.2.3.2.1). In another 
approach, the PA calculates study area air quality metrics similar to 
PM2.5 design values (i.e., referred to as pseudo-design 
values) and considers the degree to which such metrics indicate that 
study area air quality would likely have met or violated the current 
standards during study periods (U.S. EPA, 2020, section 3.2.3.2.2). 
These approaches are discussed briefly below.
PM2.5 Air Quality Distributions Associated With Mortality or 
Morbidity
    The PA evaluates the PM2.5 air quality distributions 
over which epidemiologic studies support health effect associations and 
the degree to which such distributions are likely to occur in areas 
meeting the current standards. As discussed further in the PA (U.S. 
EPA, 2020, section 3.2.3.2.1), epidemiologic studies generally provide 
the strongest support for reported health effect associations over the 
part of the air quality distribution corresponding to the bulk of the 
underlying data (i.e., estimated exposures and/or health events), often 
falling in the middle part of the distribution (i.e., rather than at 
the extreme upper or lower ends). Thus, in considering PM2.5 
air quality data from epidemiologic studies, the PA evaluates study-
reported means (or medians) of daily and annual average 
PM2.5 concentrations as proxies for the middle portions of 
the air quality distributions that support reported associations. When 
data are available, the PA also considers the broader PM2.5 
air quality distributions around the overall mean concentrations, with 
a focus on the lower quartiles of data to provide insight into the 
concentrations below which data supporting reported associations become 
relatively sparse.
    Based on its evaluation of study-reported PM2.5 
concentrations, the PA notes that key epidemiologic studies conducted 
in the U.S. or Canada report generally positive and statistically 
significant associations between estimated PM2.5 exposures 
(short- or long-term) and mortality or morbidity across a wide range of 
ambient PM2.5 concentrations (U.S. EPA, 2020, section 
3.2.3.2.1). With regard to these study-reported concentrations, the PA 
makes a number of observations, including the following:
     For the large majority of key studies, the 
PM2.5 air quality distributions that support reported 
associations are characterized by overall mean (or median) 
PM2.5 concentrations ranging from just above 8.0 [mu]g/m\3\ 
to just above 16.0 [mu]g/m\3\. Most of these key studies, including all 
but one U.S. study, report overall mean (or median) concentrations at 
or above 9.6 [mu]g/m\3\.
     Several U.S. studies report positive and statistically 
significant health effect associations in analyses restricted to annual 
average PM2.5 concentrations <12 [mu]g/m\3\ (Lee et al. 
(2015); Shi et al. (2016); Di et al., 2017b). Studies also report 
positive and statistically significant health effect associations in 
analyses restricted to days with 24-hour average PM2.5 
concentrations <35 [mu]g/m\3\

[[Page 24117]]

(Lee et al. (2015); Shi et al. (2016); Di et al. (2017a)).
     For some key studies, information on the broader 
distributions of PM2.5 exposure estimates and/or health 
events is available. In these studies, ambient PM2.5 
concentrations corresponding to 25th percentiles of the underlying data 
(i.e., estimated exposures or health events) are generally >6.0 [mu]g/
m\3\.
     A small group of studies report increased life expectancy, 
decreased mortality, and decreased respiratory effects following past 
declines in ambient PM2.5 concentrations. These studies have 
examined ``starting'' annual average PM2.5 concentrations 
(i.e., prior to the reductions being evaluated) ranging from about 13 
to >20 [mu]g/m\3\ (i.e., U.S. EPA, 2020, Table 3-3).
    The PA concludes that the overall mean PM2.5 
concentrations reported by several of these key epidemiologic studies 
are likely below the long-term mean concentrations (i.e., averaged 
across space and over time) in areas just meeting the current annual 
PM2.5 standard (U.S. EPA, 2020, section 3.2.3.3). The PA 
also concludes that there are uncertainties in using study-reported 
concentrations to inform conclusions on the primary PM2.5 
standards (U.S. EPA, 2020, section 3.2.3.2.1). For example, the overall 
mean PM2.5 concentrations reported by key epidemiologic 
studies are not the same as the ambient concentrations used by the EPA 
to determine whether areas meet or violate the PM NAAQS. Overall mean 
PM2.5 concentrations in key studies reflect averaging of 
short- or long-term PM2.5 exposure estimates across 
locations (i.e., across multiple monitors or across modeled grid cells) 
and over time (i.e., over several years). In contrast, to determine 
whether areas meet or violate the NAAQS, the EPA measures air pollution 
concentrations at individual monitors (i.e., concentrations are not 
averaged across monitors) and calculates ``design values'' at monitors 
meeting appropriate data quality and completeness criteria. For the 
annual PM2.5 standard, design values are calculated as the 
annual arithmetic mean PM2.5 concentration, averaged over 3 
years (described in appendix N of 40 CFR part 50). For an area to meet 
the NAAQS, all valid design values in that area, including the highest 
monitored values, must be at or below the level of the standard. 
Additional uncertainties associated with using the PM2.5 
concentrations reported by key epidemiologic studies to inform 
conclusions on the primary PM2.5 standards result from the 
fact that (1) epidemiologic studies do not identify specific 
PM2.5 exposures that result in health effects or exposures 
below which effects do not occur and (2) exposure estimates in some 
recent studies are based on hybrid modeling approaches for which 
performance depends on the availability of monitoring data and varies 
by location. These results and uncertainties are discussed in detail in 
the PA (U.S. EPA, 2020, section 3.2.3.2.1).
PM2.5 Pseudo-Design Values in Epidemiologic Study Locations
    As noted above, a key uncertainty in using study-reported 
PM2.5 concentrations to inform conclusions on the primary 
PM2.5 standards is that they reflect the averages of daily 
or annual PM2.5 air quality concentrations or exposure 
estimates in the study population over the years examined by the study, 
and are not the same as the PM2.5 design values used by the 
EPA to determine whether areas meet the NAAQS. Therefore, the PA also 
considers a second approach to evaluating information from 
epidemiologic studies. In this approach, the PA calculates study area 
air quality metrics similar to PM2.5 design values (i.e., 
referred to in the PA as pseudo-design values; U.S. EPA, 2020, section 
3.2.3.2.2) and considers the degree to which such metrics indicate that 
study area air quality would likely have met or violated the current 
standards during study periods. When pseudo-design values in individual 
study locations are linked with the populations living in those 
locations, or with the number of study-specific health events recorded 
in those locations, these values can provide insight into the degree to 
which reported health effect associations are based on air quality 
likely to have met or violated the current (or alternative) primary 
PM2.5 standards. The results of these analyses are 
summarized below in Table 1 (from U.S. EPA, 2020, Appendix B, Tables B-
5 and B-6).

 Table 1--Summary of Results from Analysis of PM2.5 Pseudo-Design Values
         in Locations of Key U.S. and Canadian Multicity Studies
                    [From U.S. EPA, 2020, Table B-5]
------------------------------------------------------------------------
  Percent of population/health events in     Number of studies  (of the
   locations meeting  current standards             29 evaluated)
------------------------------------------------------------------------
> 25%.....................................  17
> 50%.....................................  9
> 75%.....................................  4
< 25%.....................................  12
------------------------------------------------------------------------

    Given the results of these analyses, the PA concludes that several 
key epidemiologic studies report positive and statistically significant 
PM2.5 health effect associations based largely, or entirely, 
on air quality likely to be allowed by the current primary 
PM2.5 standards (U.S. EPA, 2020, section 3.2.3.3). The PA 
also concludes that there are important uncertainties to consider when 
using this information to inform conclusions on the primary 
PM2.5 standards. For example, for most key multicity 
studies, some study locations would likely have met the current primary 
standards over study periods while others would likely have violated 
one or both standards, complicating the interpretation of these 
analyses. In addition, pseudo-design values are averaged over multiyear 
study periods of varying lengths, rather than reflecting the three-year 
averages of actual design values; analyses necessarily focus on 
locations with at least one PM2.5 monitor, while unmonitored 
areas are not included; and recent changes to PM2.5 
monitoring requirements are not reflected in analyses of pseudo-design 
values. These results and uncertainties are discussed in greater detail 
in the PA (U.S. EPA, 2020, section 3.2.3.2.2).
b. Risk-Based Considerations
    In addition to evaluating PM2.5 concentrations in 
locations of key epidemiologic studies, the PA includes a risk 
assessment that estimates population-level health risks associated with 
PM2.5 air quality that has been adjusted to simulate air 
quality scenarios of policy interest (e.g., ``just meeting'' the 
current standards). The general approach to estimating 
PM2.5-associated health risks combines concentration-
response functions from epidemiologic studies with model-based 
PM2.5 air quality surfaces, baseline health incidence data, 
and population demographics for forty-seven urban study areas (U.S. 
EPA, 2020, section 3.3, Figure 3-10 and Appendix C).
    The risk assessment estimates that the current primary 
PM2.5 standards could allow a substantial number of 
PM2.5-associated deaths in the U.S. For example, when air 
quality in the 47 study areas is adjusted to simulate just meeting the 
current standards, the risk assessment estimates from about 16,000 to 
17,000 long-term PM2.5 exposure-related deaths from ischemic 
heart disease in a single year (i.e., confidence intervals range from 
about 12,000 to

[[Page 24118]]

21,000 deaths).\42\ Compared to the current annual standard, meeting a 
revised annual standard with a lower level is estimated to reduce 
PM2.5-associated health risks by about 7 to 9% for a level 
of 11.0 [micro]g/m\3\, 14 to 18% for a level of 10.0 [micro]g/m\3\, and 
21 to 27% for a level of 9.0 [micro]g/m\3\.
---------------------------------------------------------------------------

    \42\ For the only other cause-specific mortality endpoint 
evaluated (i.e., lung cancer), substantially fewer deaths were 
estimated (U.S. EPA, 2020, section 3.3.2, e.g., Figure 3-5). Risk 
estimates were not generated for other ``likely to be causal'' 
outcome categories (i.e., respiratory effects, nervous system 
effects).
---------------------------------------------------------------------------

    Limitations in the underlying data and risk assessment approaches 
lead to uncertainty in these estimates of PM2.5-associated 
risks (e.g., in the size of risk estimates). Uncertainty in risk 
estimates results from a number of factors, including assumptions about 
the shape of the concentration-response relationship with mortality at 
low ambient PM concentrations, the potential for confounding and/or 
exposure measurement error in the underlying epidemiologic studies, and 
the methods used to adjust PM2.5 air quality. The PA 
characterizes these and other sources of uncertainty in risk estimates 
using a combination of quantitative and qualitative approaches (U.S. 
EPA, 2020, Appendix C, section C.3).
2. CASAC Advice
    As part of its review of the draft PA, the CASAC has provided 
advice on the adequacy of the public health protection afforded by the 
current primary PM2.5 standards.\43\ Its advice is 
documented in a letter sent to the EPA Administrator (Cox, 2019a). In 
this letter, the committee recommends retaining the current 24-hour 
PM2.5 standard but does not reach consensus on whether the 
scientific and technical information support retaining or revising the 
current annual standard. In particular, though the CASAC agrees that 
there is a long-standing body of health evidence supporting 
relationships between PM2.5 exposures and various health 
outcomes, including mortality and serious morbidity effects, individual 
CASAC members ``differ in their assessments of the causal and policy 
significance of these associations'' (Cox, 2019a, p. 8 of consensus 
responses). Drawing from this evidence, ``some CASAC members'' express 
support for retaining the current annual standard while ``other 
members'' express support for revising that standard in order to 
increase public health protection (Cox, 2019a, p.1 of letter). These 
views are summarized below.
---------------------------------------------------------------------------

    \43\ The CASAC also provided advice on the draft ISA's 
assessment of the scientific evidence (Cox, 2019b) and on the 
analyses and information in the draft PA (Cox, 2019a), which drew 
from the draft ISA. That advice, and the resulting changes made in 
the final ISA and final PA, are summarized above in sections I.C.5, 
II.B.1.d, II.B.1.e and II.B.3, and in the final ISA (U.S. EPA, 2019, 
ES-3 to ES-4) and the final PA (U.S. EPA, 2020, section 1.4).
---------------------------------------------------------------------------

    The CASAC members who support retaining the current annual standard 
express the view that substantial uncertainty remains in the evidence 
for associations between PM2.5 exposures and mortality or 
serious morbidity effects. These committee members assert that ``such 
associations can reasonably be explained in light of uncontrolled 
confounding and other potential sources of error and bias'' (Cox, 
2019a, p. 8 of consensus responses). They note that associations do not 
necessarily reflect causal effects, and they contend that recent 
epidemiologic studies reporting positive associations at lower 
estimated exposure concentrations mainly confirm what was anticipated 
or already assumed in setting the 2012 NAAQS. In particular, they 
conclude that such studies have some of the same limitations as prior 
studies and do not provide new information calling into question the 
existing standard. They further assert that ``accountability studies 
provide potentially crucial information about whether and how much 
decreasing PM2.5 causes decreases in future health effects'' 
(Cox, 2019a, p. 10), and they cite recent reviews (i.e., Henneman et 
al., 2017; Burns et al., 2019) to support their position that in such 
studies, ``reductions of PM2.5 concentrations have not 
clearly reduced mortality risks'' (Cox, 2019a, p. 8 of consensus 
responses). Thus, the committee members who support retaining the 
current annual standard advise that, ``while the data on associations 
should certainly be carefully considered, this data should not be 
interpreted more strongly than warranted based on its methodological 
limitations'' (Cox, 2019a, p. 8 of consensus responses).
    These members of the CASAC further conclude that the 
PM2.5 risk assessment does not provide a valid basis for 
revising the current standards. This conclusion is based on concerns 
that (1) ``the risk assessment treats regression coefficients as causal 
coefficients with no justification or validation provided for this 
decision;'' (2) the estimated regression concentration-response 
functions ``have not been adequately adjusted to correct for 
confounding, errors in exposure estimates and other covariates, model 
uncertainty, and heterogeneity in individual biological (causal) 
[concentration-response] functions;'' (3) the estimated concentration-
response functions ``do not contain quantitative uncertainty bands that 
reflect model uncertainty or effects of exposure and covariate 
estimation errors;'' and (4) ``no regression diagnostics are provided 
justifying the use of proportional hazards . . . and other modeling 
assumptions'' (Cox, 2019a, p. 9 of consensus responses). These 
committee members also contend that details regarding the derivation of 
concentration-response functions, including specification of the beta 
values and functional forms, are not well-documented, hampering the 
ability of readers to evaluate these design details. Thus, these 
members ``think that the risk characterization does not provide useful 
information about whether the current standard is protective'' (Cox, 
2019a, p. 11 of consensus responses).
    Drawing from their evaluation of the evidence and the risk 
assessment, these committee members conclude that ``the Draft PM PA 
does not establish that new scientific evidence and data reasonably 
call into question the public health protection afforded by the current 
2012 PM2.5 annual standard'' (Cox, 2019a, p.1 of letter).
    In contrast, ``[o]ther members of CASAC conclude that the weight of 
the evidence, particularly reflecting recent epidemiology studies 
showing positive associations between PM2.5 and health 
effects at estimated annual average PM2.5 concentrations 
below the current standard, does reasonably call into question the 
adequacy of the 2012 annual PM2.5 [standard] to protect 
public health with an adequate margin of safety'' (Cox, 2019a, p.1 of 
letter). The committee members who support this conclusion note that 
the body of health evidence for PM2.5 includes not only the 
repeated demonstration of associations in epidemiologic studies, but 
also includes support for biological plausibility established by 
controlled human exposure and animal toxicology studies. They point to 
recent studies demonstrating that the associations between 
PM2.5 and health effects occur in a diversity of locations, 
in different time periods, with different populations, and using 
different exposure estimation and statistical methods. They conclude 
that ``the entire body of evidence for PM health effects justifies the 
causality determinations made in the Draft PM ISA'' (Cox, 2019a, p. 8 
of consensus responses).
    The members of the CASAC who support revising the current annual 
standard particularly emphasize recent findings of associations with 
PM2.5 in areas with average long-term PM2.5 
concentrations below the level of the

[[Page 24119]]

annual standard and studies that show positive associations even when 
estimated exposures above 12 [mu]g/m\3\ are excluded from analyses. 
They find it ``highly unlikely'' that the extensive body of evidence 
indicating positive associations at low estimated exposures could be 
fully explained by confounding or by other non-causal explanations 
(Cox, 2019a, p. 8 of consensus responses). They additionally conclude 
that ``the risk characterization does provide a useful attempt to 
understand the potential impacts of alternate standards on public 
health risks'' (Cox, 2019a, p. 11 of consensus responses). These 
committee members conclude that the evidence available in this review 
reasonably calls into question the protection provided by the current 
primary PM2.5 standards and supports revising the annual 
standard to increase that protection (Cox, 2019a).
3. Administrator's Proposed Decision on the Current Primary 
PM2.5 Standards
    This section summarizes the Administrator's considerations and 
conclusions related to the current primary PM2.5 standards 
and presents his proposed decision to retain those standards, without 
revision. As described above (section II.A.2), his approach to 
considering the adequacy of the current standards focuses on evaluating 
the public health protection afforded by the annual and 24-hour 
standards, taken together, against mortality and morbidity associated 
with long- or short-term PM2.5 exposures. This approach 
recognizes that changes in PM2.5 air quality designed to 
meet either the annual or the 24-hour standard would likely result in 
changes to both long-term average and short-term peak PM2.5 
concentrations and that the protection provided by the suite of 
standards results from the combination of all of the elements of those 
standards (i.e., indicator, averaging time, form, level). Thus, the 
Administrator's consideration of the public health protection provided 
by the current primary PM2.5 standards is based on his 
consideration of the combination of the annual and 24-hour standards, 
including the indicators (PM2.5), averaging times, forms 
(arithmetic mean and 98th percentile, averaged over three years), and 
levels (12.0 [mu]g/m\3\, 35 [mu]g/m\3\) of those standards.
    In establishing primary standards under the Act that are 
``requisite'' to protect public health with an adequate margin of 
safety, the Administrator is seeking to establish standards that are 
neither more nor less stringent than necessary for this purpose. He 
recognizes that the requirement to provide an adequate margin of safety 
was intended to address uncertainties associated with inconclusive 
scientific and technical information and to provide a reasonable degree 
of protection against hazards that research has not yet identified. 
However, the Act does not require that primary standards be set at a 
zero-risk level; rather, the NAAQS must be sufficiently protective, but 
not more stringent than necessary.
    Given these requirements, the Administrator's final decision in 
this review will be a public health policy judgment drawing upon 
scientific and technical information examining the health effects of 
PM2.5 exposures, including how to consider the range and 
magnitude of uncertainties inherent in that information. This public 
health policy judgment will be based on an interpretation of the 
scientific and technical information that neither overstates nor 
understates its strengths and limitations, nor the appropriate 
inferences to be drawn, and will be informed by the Administrator's 
consideration of advice from the CASAC and public comments received on 
this proposal document.
    With regard to the CASAC, the Administrator recognizes that while 
the committee supports retaining the current 24-hour PM2.5 
standard, it does not reach consensus on the annual standard (Cox, 
2019a, pp. 1-3 of letter). In particular, some members of the CASAC 
conclude that the new scientific evidence and data do not reasonably 
call into question the public health protection afforded by the current 
annual standard, while other members conclude that the weight of the 
evidence does reasonably call into question the adequacy of that 
standard (Cox, 2019a, p. 1 of letter).
    As discussed above (II.C.2), the CASAC members who support 
retaining the annual standard emphasize their concerns with available 
PM2.5 epidemiologic studies. They assert that recent studies 
``mainly confirmed what had already been anticipated or assumed in 
setting the 2012 NAAQS'' (Cox, 2019a, p. 8 consensus responses) and do 
not provide a basis for revising the current standards. They also 
identify several key concerns regarding the associations reported in 
PM2.5 epidemiologic studies and conclude that ``while the 
data on associations should certainly be carefully considered, this 
data should not be interpreted more strongly than warranted based on 
its methodological limitations'' (Cox, 2019a, p. 8 consensus 
responses).
    One of the methodological limitations highlighted by these 
committee members is that associations reported in epidemiologic 
studies are not necessarily indicative of causal relationships and such 
associations ``can reasonably be explained in light of uncontrolled 
confounding and other potential sources of error and bias'' (Cox, 
2019a, p. 8). Thus, these committee members do not think that recent 
epidemiologic studies reporting health effect associations at 
PM2.5 air quality concentrations likely to have met the 
current primary standards support revising those standards.
    Consistent with the views expressed by these CASAC members, the 
Administrator recognizes that epidemiologic studies examine 
associations between distributions of PM2.5 air quality and 
health outcomes, and they do not identify particular PM2.5 
exposures that cause effects (U.S. EPA, 2020, section 3.1.2). In 
contrast, he notes that experimental studies (i.e., controlled human 
exposure, animal toxicology) do provide evidence for health effects 
following particular PM2.5 exposures under carefully 
controlled laboratory conditions (e.g., U.S. EPA, 2015, Preamble 
Chapters 5 and 6). He further notes that the evidence for a given 
PM2.5-related health outcome is strengthened when results 
from experimental studies demonstrate biologically plausible mechanisms 
through which such an outcome could occur (e.g., U.S. EPA, 2015, 
Preamble p. 20). Thus, when using the PM2.5 health evidence 
to inform conclusions on the adequacy of the current primary standards, 
the Administrator is most confident in the potential for 
PM2.5 exposures to cause adverse effects at concentrations 
supported by multiple types of studies, including experimental studies 
as well as epidemiologic studies.
    In light of this approach to considering the evidence, the 
Administrator recognizes that controlled human exposure and animal 
toxicology studies report a wide range of effects, many of which are 
plausibly linked to the serious cardiovascular and respiratory outcomes 
reported in epidemiologic studies (including mortality), though the 
PM2.5 exposures examined in these studies are above the 
concentrations typically measured in areas meeting the current annual 
and 24-hour standards (U.S. EPA, 2020, section 3.2.3.1). In the absence 
of evidence from experimental studies that PM2.5 exposures 
typical of areas meeting the current annual and 24-hour standards can 
activate biological pathways that plausibly contribute to serious 
health outcomes, the Administrator is cautious about placing too much 
weight on reported PM2.5 health effect associations for air 
quality

[[Page 24120]]

meeting those standards. He concludes that such associations alone, 
without supporting experimental evidence at similar PM2.5 
concentrations, leave important questions unanswered regarding the 
degree to which the typical PM2.5 exposures likely to occur 
in areas meeting the current standards can cause the mortality or 
morbidity outcomes reported in epidemiologic studies. Given this 
concern, the Administrator does not think that recent epidemiologic 
studies reporting health effect associations at PM2.5 air 
quality concentrations likely to have met the current primary standards 
support revising those standards. Rather, he judges that the overall 
body of evidence, including controlled human exposure and animal 
toxicological studies, in addition to epidemiologic studies, indicates 
continuing uncertainty in the degree to which adverse effects could 
result from PM2.5 exposures in areas meeting the current 
annual and 24-hour standards.
    The Administrator additionally considers the emerging body of 
evidence from studies examining past reductions in ambient 
PM2.5, and the degree to which those reductions have 
resulted in public health improvements. As an initial matter, he notes 
the observation from some CASAC members (i.e., those who support 
retaining the current annual standard) that in accountability studies, 
``reductions of PM2.5 concentrations have not clearly 
reduced mortality risks, especially when confounding was tightly 
controlled'' (Cox, 2019a, p. 8). The Administrator recognizes that 
interpreting such studies in the context of the current primary 
PM2.5 standards is also complicated by the fact that some of 
the available studies have not evaluated PM2.5 specifically 
(e.g., as opposed to PM10 or total suspended particulates), 
did not show changes in PM2.5 air quality, or have not been 
able to disentangle health impacts of the interventions from background 
trends in health (U.S. EPA, 2020, section 3.5.1). He further recognizes 
that the small number of available studies that do report public health 
improvements following past declines in ambient PM2.5 have 
not examined air quality meeting the current standards (U.S. EPA, 2020, 
Table 3-3). This includes recent U.S. studies that report increased 
life expectancy, decreased mortality, and decreased respiratory effects 
following past declines in ambient PM2.5 concentrations. 
Such studies have examined ``starting'' annual average PM2.5 
concentrations (i.e., prior to the reductions being evaluated) ranging 
from about 13 to > 20 [mu]g/m\3\ (i.e., U.S. EPA, 2020, Table 3-3). It 
also includes a recent study conducted in Japan that reports reduced 
mortality following reductions in ambient PM2.5 due to the 
introduction of diesel emission controls (Yorifuji et al., 2016). As in 
the U.S. studies, ambient PM2.5 concentrations in this study 
were above those allowed by the current primary PM2.5 
standards. Given the lack of studies reporting public health 
improvements attributable to reductions in ambient PM2.5 in 
locations meeting the current standards, together with his broader 
concerns regarding the lack of experimental studies examining 
PM2.5 exposures typical of areas meeting the current 
standards (discussed above), the Administrator judges that there is 
considerable uncertainty in the potential for increased public health 
protection from further reductions in ambient PM2.5 
concentrations beyond those achieved under the current primary 
PM2.5 standards.
    In addition to the evidence, the Administrator considers the 
potential implications of the risk assessment for his proposed 
decision. In doing so, he notes that all risk assessments have 
limitations and that, in previous reviews, these limitations have often 
resulted in less weight being placed on quantitative estimates of risk 
than on the underlying scientific evidence itself (e.g., 78 FR 3128, 
January 15, 2013). Such limitations in risk estimates can result from 
uncertainty in the shapes of concentration-response functions, 
particularly at low concentrations; uncertainties in the methods used 
to adjust air quality; and uncertainty in estimating risks for 
populations, locations and air quality distributions different from 
those examined in the underlying epidemiologic study (U.S. EPA, 2020, 
section 3.3.2.4).
    In addition to these general uncertainties with risk assessments, 
the Administrator notes the concerns expressed by members of the CASAC 
who support retaining the current standards. Their concerns largely 
reflect their overall views on the limitations in the PM2.5 
epidemiologic evidence, which provides key inputs to the risk 
assessment. These committee members assert that ``the conclusions from 
the risk assessment do not comprise valid empirical evidence or grounds 
for revising the current NAAQS'' (Cox, 2019a, p. 9 consensus 
responses). As discussed above, the Administrator agrees with the broad 
concerns expressed by these members of the CASAC regarding associations 
at PM2.5 concentrations meeting the current standards. He 
further notes their concerns regarding the characterization of 
uncertainty in the risk assessment and the evaluation of modeling 
assumptions (Cox, 2019a). In light of these concerns, together with the 
more general uncertainty in risk estimates summarized above, the 
Administrator judges it appropriate to place little weight on 
quantitative estimates of PM2.5-associated mortality risk in 
reaching conclusions on the primary PM2.5 standards.
    When the above considerations are taken together, the Administrator 
proposes to conclude that the scientific evidence that has become 
available since the last review of the PM NAAQS, together with the 
analyses in the PA based on that evidence, does not call into question 
the public health protection provided by the current annual and 24-hour 
PM2.5 standards. In particular, the Administrator judges 
that there is considerable uncertainty in the potential public health 
impacts of reductions in ambient PM2.5 below the 
concentrations achieved under the current primary standards and, 
therefore, that standards more stringent than the current standards 
(e.g., with lower levels) are not supported. That is, he judges that 
such standards would be more than requisite to protect the public 
health with an adequate margin of safety. As described above, this 
judgment reflects his consideration of the uncertainties in the 
potential implications of recent epidemiologic studies due in part to 
the lack of supporting evidence from experimental studies and 
retrospective accountability studies conducted at PM2.5 
concentrations meeting the current standards.
    For the 24-hour standard, he notes that this judgment is consistent 
with the consensus advice of the CASAC (Cox, 2019). For the annual 
standard, this judgment is consistent with the advice of some CASAC 
members and reflects the Administrator's disagreement with the 
``[o]ther members of CASAC'' who recommend revising the current annual 
standard based largely on evidence from recent epidemiology studies 
(Cox, 2019a, p. 1 of letter).
    In addition, based on the Administrator's review of the science, 
including experimental and accountability studies conducted at levels 
just above the current standard, he judges that the degree of public 
health protection provided by the current standard is not greater than 
warranted. This judgment, together with the fact that no CASAC member 
expressed support for a less stringent standard, leads the 
Administrator to conclude that standards less stringent

[[Page 24121]]

than the current standards (e.g., with higher levels) are also not 
supported.
    When the above information is taken together, the Administrator 
proposes to conclude that the available scientific evidence and 
technical information continue to support the current annual and 24-
hour PM2.5 standards. This proposed conclusion reflects the 
fact that important limitations in the evidence remain. The 
Administrator proposes to conclude that these limitations lead to 
considerable uncertainty regarding the potential public health 
implications of revising the existing suite of PM2.5 
standards. Given this uncertainty, and the advice from some CASAC 
members, he proposes to conclude that the current suite of primary 
standards, including the current indicators (PM2.5), 
averaging times (annual and 24-hour), forms (arithmetic mean and 98th 
percentile, averaged over three years) and levels (12.0 [mu]g/m\3\, 35 
[mu]g/m\3\), when taken together, remain requisite to protect the 
public health. Therefore, the Administrator proposes to retain the 
current suite of primary PM2.5 standards, without revision, 
in this review. He solicits comment on this proposed decision and on 
the supporting rationale described above.

III. Rationale for Proposed Decisions on the Primary PM10 Standard

    The current primary PM10 standard is intended to protect 
the public health against exposures to PM10-2.5 (78 FR 3164, 
January 15, 2013). This section provides the rationale supporting the 
Administrator's proposed decision to retain the current primary 
PM10 standard. Section III.A summarizes the Agency's 
approach to reaching a decision on the primary PM10 standard 
in the last review and presents the general approach to reaching a 
proposed decision in this review. Section III.B summarizes the 
scientific evidence for PM10-2.5-related health effects. 
Section III.C presents the Administrator's proposed conclusions 
regarding the adequacy of the current primary PM10 standard.

A. General Approach

1. Approach Used in the Last Review
    The last review of the PM NAAQS was completed in 2012 (78 FR 3086, 
January 15, 2013). In that review the EPA retained the existing primary 
24-hour PM10 standard, with its level of 150 [mu]g/m\3\ and 
its one-expected-exceedance form on average over three years, to 
continue to provide public health protection against exposures to 
PM10-2.5. In support of this decision, the prior 
Administrator emphasized her consideration of three issues: (1) The 
extent to which it was appropriate to maintain a standard that provides 
some measure of protection against all PM10-2.5 (regardless 
of composition or source or origin), (2) the extent to which a standard 
with a PM10 indicator can provide protection against 
exposures to PM10-2.5, and (3) the degree of public health 
protection provided by the existing PM10 standard. Her 
consideration of each of these issues is summarized below.
    First, the prior Administrator judged that the evidence provided 
``ample support for a standard that protects against exposures to all 
thoracic coarse particles, regardless of their location or source of 
origin'' (78 FR 3176, January 15, 2013). In support of this, she noted 
that epidemiologic studies had reported positive associations between 
PM10-2.5 and mortality or morbidity in a large number of 
cities across North America, Europe, and Asia, encompassing a variety 
of environments where PM10-2.5 sources and composition are 
expected to vary widely. Though most of the available studies examined 
associations in urban areas, she noted that some studies had also 
linked mortality and morbidity with relatively high ambient 
concentrations of particles of non-urban crustal origin. In light of 
this body of available evidence, and consistent with the CASAC's 
advice, the prior Administrator concluded that it was appropriate to 
maintain a standard that provides some measure of protection against 
exposures to all thoracic coarse particles, regardless of their 
location, source of origin, or composition (78 FR 3176, January 15, 
2013).
    In reaching the conclusion that it was appropriate to retain a 
PM10 indicator for a standard meant to protect against 
exposures to ambient PM10-2.5, the prior Administrator noted 
that PM10 mass includes both coarse PM (PM10-2.5) 
and fine PM (PM2.5). As a result, the concentration of 
PM10-2.5 allowed by a PM10 standard set at a 
single level declines as the concentration of PM2.5 
increases. Because PM2.5 concentrations tend to be higher in 
urban areas than rural areas, she observed that a PM10 
standard would generally allow lower PM10-2.5 concentrations 
in urban areas than in rural areas. She judged it appropriate to 
maintain such a standard given that much of the evidence for 
PM10-2.5 toxicity, particularly at relatively low particle 
concentrations, came from study locations where thoracic coarse 
particles were of urban origin, and given the possibility that 
PM10-2.5 contaminants in urban areas could increase particle 
toxicity. Thus, in the last review the prior Administrator concluded 
that it remained appropriate to maintain a standard that allows lower 
ambient concentrations of PM10-2.5 in urban areas, where the 
evidence was strongest that exposure to thoracic coarse particles was 
associated with morbidity and mortality, and higher concentrations in 
non-urban areas, where the public health concerns were less certain. 
The prior Administrator concluded that the varying concentrations of 
coarse particles that would be permitted in urban versus non-urban 
areas under the 24-hour PM10 standard, based on the varying 
levels of PM2.5 present, appropriately reflected the 
differences in the strength of evidence regarding coarse particle 
health effects.
    Finally, in specifically evaluating the degree of public health 
protection provided by the primary PM10 standard, with its 
level of 150 [mu]g/m\3\ and its one-expected-exceedance form on average 
over three years, the prior Administrator recognized that the available 
health evidence and air quality information was much more limited for 
PM10-2.5 than for PM2.5. In particular, the 
strongest evidence for health effects attributable to 
PM10-2.5 exposure was for cardiovascular effects, 
respiratory effects, and/or premature mortality following short-term 
exposures. For each of these categories of effects, the 2009 ISA 
concluded that the evidence was ``suggestive of a causal relationship'' 
(U.S. EPA, 2009c, section 2.3.3). These determinations contrasted with 
those for PM2.5, as described in Chapter 3 above, which were 
determined in the ISA to be either ``causal'' or ``likely to be 
causal'' for mortality, cardiovascular effects, and respiratory effects 
(U.S. EPA, 2009c, Tables 2-1 and 2-2).
    The prior Administrator judged that the important uncertainties and 
limitations associated with the PM10-2.5 evidence and 
information raised questions as to whether additional public health 
improvements would be achieved by revising the existing PM10 
standard. She specifically noted several uncertainties and limitations, 
including the following:
     The number of epidemiologic studies that have employed 
copollutant models to address the potential for confounding, 
particularly by PM2.5, was limited. Therefore, the extent to 
which PM10-2.5 itself, rather than one or more copollutants, 
contributes to reported health effects remained uncertain.
     Only a limited number of experimental studies provided 
support for the associations reported in epidemiologic studies, 
resulting in

[[Page 24122]]

further uncertainty regarding the plausibility of the associations 
between PM10-2.5 and mortality and morbidity reported in 
epidemiologic studies.
     Limitations in PM10-2.5 monitoring data (i.e., 
limited data available from FRM/FEM sampling methods) and the different 
approaches used to estimate PM10-2.5 concentrations across 
epidemiologic studies resulted in uncertainty in the ambient 
PM10-2.5 concentrations at which the reported effects occur, 
increasing uncertainty in estimates of the extent to which changes in 
ambient PM10-2.5 concentrations would likely impact public 
health.
     While PM10-2.5 effect estimates reported for 
mortality and morbidity were generally positive, most were not 
statistically significant, even in single-pollutant models. This 
included effect estimates reported in some study locations with 
PM10 concentrations above those allowed by the current 24-
hour PM10 standard.
     The composition of PM10-2.5, and the effects 
associated with various components, were also key uncertainties in the 
available evidence. Without more information on the chemical speciation 
of PM10-2.5, the apparent variability in associations across 
locations was difficult to characterize.
    In considering these uncertainties and limitations, the prior 
Administrator particularly emphasized the considerable degree of 
uncertainty in the extent to which health effects reported in 
epidemiologic studies are due to PM10-2.5 itself, as opposed 
to one or more co-occurring pollutants. This uncertainty reflected the 
relatively small number of PM10-2.5 studies that had 
evaluated copollutant models, particularly copollutant models that 
included PM2.5, and the very limited body of controlled 
human exposure evidence supporting the plausibility of 
PM10-2.5-attributable adverse effects at ambient 
concentrations.
    When considering the evidence as a whole, the prior Administrator 
concluded that the degree of public health protection provided by the 
current PM10 standard against exposures to 
PM10-2.5 should be maintained (i.e., neither increased nor 
decreased). Her judgment that protection did not need to be increased 
was supported by her consideration of uncertainties in the overall body 
of evidence. Her judgment that the degree of public health protection 
provided by the current standard is not greater than warranted was 
supported by the observation that positive and statistically 
significant associations with mortality were reported in some single-
city U.S. study locations likely to have violated the current 
PM10 standard. Thus, the prior Administrator concluded that 
the existing 24-hour PM10 standard, with its one-expected 
exceedance form on average over three years and a level of 150 [mu]g/
m\3\, was requisite to protect public health with an adequate margin of 
safety against effects that have been associated with 
PM10-2.5. In light of this conclusion, the EPA retained the 
existing PM10 standard.
2. Approach in the Current Review
    The approach for this review builds on the last review, taking into 
account the more recent scientific information now available. The 
approach summarized below draws from the approach taken in the PA (U.S. 
EPA, 2020) and is most fundamentally based on using the ISA's 
assessment of the current scientific evidence for health effects of 
PM10-2.5 exposures (U.S. EPA, 2019).
    As discussed above for PM2.5 (II.A.2), the approach in 
the PA places the greatest weight on effects for which the evidence has 
been determined to demonstrate a ``causal'' or a ``likely to be 
causal'' relationship with PM exposures (U.S. EPA, 2019). This approach 
focuses policy considerations and conclusions on health outcomes for 
which the evidence is strongest. Unlike for PM2.5, the ISA 
does not identify any PM10-2.5-related health outcomes for 
which the evidence supports either a ``causal'' or a ``likely to be 
causal'' relationship. Thus, for PM10-2.5 the PA considers 
the evidence determined to be ``suggestive of, but not sufficient to 
infer, a causal relationship,'' recognizing the greater uncertainty in 
such evidence.
    The preamble to the ISA states that ``suggestive'' evidence is 
``limited, and chance, confounding, and other biases cannot be ruled 
out'' (U.S. EPA, 2015, Preamble Table II). In light of the additional 
uncertainty in the evidence for PM10-2.5-related health 
outcomes, compared to the evidence supporting ``causal'' or ``likely to 
be causal'' relationships for PM2.5, the approach to 
evaluating the primary PM10 standard in this review is more 
limited than the approach to evaluating the primary PM2.5 
standards (discussed in II.A.2). Specifically, the approach for 
PM10 does not include evaluations of air quality 
distributions in locations of individual epidemiologic studies, 
comparisons of experimental exposures with ambient air quality, or the 
quantitative assessment of PM10-2.5 health risks. The 
substantial uncertainty in such analyses, if they were to be conducted 
based on the currently available PM10-2.5 health studies, 
would limit their utility for informing conclusions on the primary 
PM10 standard. Therefore, as discussed further below, the 
focus of the evaluation of the primary PM10 standard is on 
the overall body of evidence for PM10-2.5-related health 
effects. This includes consideration of the degree to which 
uncertainties in the evidence from the last review have been reduced 
and the degree to which new uncertainties have been identified.

B. Health Effects Related to Thoracic Coarse Particle Exposures

    This section briefly outlines the key evidence for health effects 
associated with PM10-2.5 exposures. This evidence is 
discussed more fully in the ISA (U.S. EPA, 2019) and the PA (U.S. EPA, 
2020, Chapter 4).
    While studies conducted since the last review have strengthened 
support for relationships between PM10-2.5 exposures and 
some health outcomes (discussed below), several key uncertainties in 
the evidence from the last review have, to date, ``still not been 
addressed'' (U.S. EPA, 2019, section 1.4.2, p. 1-41). For example, 
epidemiologic studies available in the last review relied on various 
methods to estimate PM10-2.5 exposures, and these methods 
had not been systematically compared to evaluate spatial and temporal 
correlations in exposure estimates. Methods included (1) calculating 
the difference between PM10 and PM2.5 
concentrations at co-located monitors, (2) calculating the difference 
between county-wide averages of monitored PM10 and 
PM2.5 based on monitors that are not necessarily co-located, 
and (3) direct measurement of PM10-2.5 using a dichotomous 
sampler (U.S. EPA, 2019, section 1.4.2). In the current review, more 
recent epidemiologic studies continue to use these approaches to 
estimate PM10-2.5 concentrations. Additionally, some recent 
studies estimate long-term PM10-2.5 exposures as the 
difference between PM10 and PM2.5 concentrations 
based on information from spatiotemporal or land use regression (LUR) 
models, in addition to monitors. As in the last review, the various 
methods used to estimate PM10-2.5 concentrations have not 
been systematically evaluated (U.S. EPA, 2019, section 3.3.1.1), 
contributing to uncertainty regarding the spatial and temporal 
correlations in PM10-2.5 concentrations across methods and 
in the PM10-2.5 exposure estimates used in epidemiologic 
studies (U.S. EPA, 2019, section 2.5.1.2.3 and section 2.5.2.2.3). 
Given the greater spatial and temporal variability of 
PM10-2.5 and fewer PM10-2.5 monitoring sites, 
compared to PM2.5,

[[Page 24123]]

this uncertainty is particularly important for the coarse size 
fraction.
    Beyond uncertainty associated with PM10-2.5 exposure 
estimates in epidemiologic studies, the limited information on the 
potential for confounding by copollutants and the limited support 
available for the biological plausibility of serious effects following 
PM10-2.5 exposures also continue to contribute broadly to 
uncertainty in the PM10-2.5 health evidence. Uncertainty 
related to potential confounding stems from the relatively small number 
of epidemiologic studies that have evaluated PM10-2.5 health 
effect associations in copollutants models with both gaseous pollutants 
and other PM size fractions. Uncertainty related to the biological 
plausibility of serious effects caused by PM10-2.5 exposures 
results from the small number of controlled human exposure and animal 
toxicology \44\ studies that have evaluated the health effects of 
experimental PM10-2.5 inhalation exposures. The evidence 
supporting the ISA's ``suggestive'' causality determinations for 
PM10-2.5, including uncertainties in this evidence, is 
summarized below in sections III.B.1 to III.B.7.
---------------------------------------------------------------------------

    \44\ Compared to humans, smaller fractions of inhaled 
PM10-2.5 penetrate into the thoracic regions of rats and 
mice (U.S. EPA, 2019, section 4.1.6), contributing to the relatively 
limited evaluation of PM10-2.5 exposures in animal 
studies.
---------------------------------------------------------------------------

1. Mortality
a. Long-Term Exposures
    Due to the dearth of studies examining the association between 
long-term PM10-2.5 exposure and mortality, the 2009 PM ISA 
concluded that the evidence was ``inadequate to determine if a causal 
relationship exists'' (U.S. EPA, 2009c). Since the completion of the 
2009 ISA, some recent cohort studies conducted in the U.S. and Europe 
report positive associations between long-term PM10-2.5 
exposure and total (nonaccidental) mortality, though results are 
inconsistent across studies (U.S. EPA, 2019, Table 11-11). The 
examination of copollutant models in these studies remains limited and, 
when included, PM10-2.5 effect estimates are often 
attenuated after adjusting for PM2.5 (U.S. EPA, 2019, Table 
11-11). Across studies, PM10-2.5 exposure concentrations are 
estimated using a variety of approaches, including direct measurements 
from dichotomous samplers, calculating the difference between 
PM10 and PM2.5 concentrations measured at 
collocated monitors, and calculating difference of area-wide 
concentrations of PM10 and PM2.5. As discussed 
above, temporal and spatial correlations between these approaches have 
not been evaluated, contributing to uncertainty regarding the potential 
for exposure measurement error (U.S. EPA, 2019, section 3.3.1.1 and 
Table 11-11). The 2019 ISA concludes that this uncertainty ``reduces 
the confidence in the associations observed across studies'' (U.S. EPA, 
2019, p. 11-125). The ISA additionally concludes that the evidence for 
long-term PM10-2.5 exposures and cardiovascular effects, 
respiratory morbidity, and metabolic disease provide limited biological 
plausibility for PM10-2.5-related mortality (U.S. EPA, 2019, 
sections 11.4.1 and 11.4). Taken together, the 2019 ISA concludes that, 
``this body of evidence is suggestive, but not sufficient to infer, 
that a causal relationship exists between long-term PM10-2.5 
exposure and total mortality'' (U.S. EPA, 2019, p. 11-125).
b. Short-Term Exposures
    The 2009 ISA concluded that the evidence is ``suggestive of a 
causal relationship between short-term exposure to PM10-2.5 
and mortality'' (U.S. EPA, 2009c). Since the completion of the 2009 
ISA, multicity epidemiologic studies conducted primarily in Europe and 
Asia continue to provide consistent evidence of positive associations 
between short-term PM10-2.5 exposure and total 
(nonaccidental) mortality (U.S. EPA, 2019, Table 11-9). Although these 
studies contribute to increasing confidence in the PM10-2.5-
mortality relationship, the use of a variety of approaches to estimate 
PM10-2.5 exposures continues to contribute uncertainty to 
the associations observed. In addition, the 2019 ISA notes that an 
analysis by Adar et al. (2014) indicates ``possible evidence of 
publication bias, which was not observed for PM2.5'' (U.S. 
EPA, 2019, section 11.3.2, p. 11-106). Recent studies expand the 
assessment of potential copollutant confounding of the 
PM10-2.5-mortality relationship and provide evidence that 
PM10-2.5 associations generally remain positive in 
copollutant models, though associations are attenuated in some 
instances (U.S. EPA, 2019, section 11.3.4.1, Figure 11-28, Table 11-
10). The 2019 ISA concludes that, overall, the assessment of potential 
copollutant confounding is limited due to the lack of information on 
the correlation between PM10-2.5 and gaseous pollutants and 
the small number of locations in which copollutant analyses have been 
conducted. Associations with cause-specific mortality provide some 
support for associations with total (nonaccidental) mortality, though 
associations with cause-specific mortality, particularly respiratory 
mortality, are more uncertain (i.e., wider confidence intervals) and 
less consistent (U.S. EPA, 2019, section 11.3.7). The ISA concludes 
that the evidence for PM10-2.5-related cardiovascular and 
respiratory effects provides only limited support for the biological 
plausibility of a relationship between short-term PM10-2.5 
exposure and cardiovascular mortality (U.S. EPA, 2019, Section 11.3.7). 
Based on the overall evidence, the 2019 ISA concludes that, ``this body 
of evidence is suggestive, but not sufficient to infer, that a causal 
relationship exists between short-term PM10-2.5 exposure and 
total mortality'' (U.S. EPA, 2019, p. 11-120).
2. Cardiovascular Effects
a. Long-term Exposures
    In the 2009 PM ISA, the evidence describing the relationship 
between long-term exposure to PM10-2.5 and cardiovascular 
effects was characterized as ``inadequate to infer the presence or 
absence of a causal relationship.'' The limited number of epidemiologic 
studies reported contradictory results and experimental evidence 
demonstrating an effect of PM10-2.5 on the cardiovascular 
system was lacking (U.S. EPA, 2019, section 6.4).
    The evidence relating long-term PM10-2.5 exposures to 
cardiovascular mortality remains limited, with no consistent pattern of 
associations across studies and, as discussed above, uncertainty 
stemming from the use of various approaches to estimate 
PM10-2.5 concentrations (U.S. EPA, 2019, Table 6-70). The 
evidence for associations with cardiovascular morbidity has grown and, 
while results across studies are not entirely consistent, some 
epidemiologic studies report positive associations with IHD and 
myocardial infarction (MI) (U.S. EPA, 2019, Figure 6-34); stroke (U.S. 
EPA, 2019, Figure 6-35); atherosclerosis (U.S. EPA, 2019, section 
6.4.5); venous thromboembolism (VTE) (U.S. EPA, 2019, section 6.4.7); 
and blood pressure and hypertension (U.S. EPA, 2019, Section 6.4.6). 
PM10-2.5 cardiovascular mortality effect estimates are often 
attenuated, but remain positive, in copollutants models that adjust for 
PM2.5. For morbidity outcomes, associations are inconsistent 
in copollutant models that adjust for PM2.5, NO2, 
and chronic noise pollution (U.S. EPA, 2019, p. 6-276). The lack of 
toxicological evidence for long-term PM10-2.5 exposures 
represents a substantial data gap (U.S. EPA, 2019, section 6.4.10), 
resulting in the 2019

[[Page 24124]]

ISA conclusion that ``evidence from experimental animal studies is of 
insufficient quantity to establish biological plausibility'' (U.S. EPA, 
2019, p. 6-277). Based largely on the observation of positive 
associations in some high-quality epidemiologic studies, the ISA 
concludes that ``evidence is suggestive of, but not sufficient to 
infer, a causal relationship between long-term PM10-2.5 
exposure and cardiovascular effects'' (U.S. EPA, 2019, p. 6-277).
b. Short-Term Exposures
    The 2009 ISA found that the available evidence for short-term 
PM10-2.5 exposure and cardiovascular effects was 
``suggestive of a causal relationship.'' This conclusion was based on 
several epidemiologic studies reporting associations between short-term 
PM10-2.5 exposure and cardiovascular effects, including IHD 
hospitalizations, supraventricular ectopy, and changes in heart rate 
variability (HRV). In addition, dust storm events resulting in high 
concentrations of crustal material were linked to increases in total 
cardiovascular disease emergency department visits and hospital 
admissions. However, the 2009 ISA noted the potential for exposure 
measurement error and copollutant confounding in these epidemiologic 
studies. In addition, there was only limited evidence of cardiovascular 
effects from a small number of experimental studies (e.g. animal 
toxicological studies and controlled human exposure studies) that 
examined short-term PM10-2.5 exposures (U.S. EPA, 2009c, 
section 6.2.12.2). In the last review, key uncertainties included the 
potential for exposure measurement error, copollutant confounding, and 
limited evidence of biological plausibility for cardiovascular effects 
following inhalation exposure (U.S. EPA, 2019, section 6.3.13).
    The evidence for short-term PM10-2.5 exposure and 
cardiovascular outcomes has expanded since the last review, though 
important uncertainties remain. The 2019 ISA notes that there are a 
small number of epidemiologic studies reporting positive associations 
between short-term exposure to PM10-2.5 and cardiovascular-
related morbidity outcomes. However, there is limited evidence to 
suggest that these associations are biologically plausible, or 
independent of copollutant confounding. The ISA also concludes that it 
remains unclear how the approaches used to estimate PM10-2.5 
concentrations in epidemiologic studies may impact exposure measurement 
error. Taken together, the 2019 ISA concludes that ``the evidence is 
suggestive of, but not sufficient to infer, a causal relationship 
between short-term PM10-2.5 exposures and cardiovascular 
effects'' (U.S. EPA, 2019, p. 6-254).
3. Respiratory Effects--Short-Term Exposures
    Based on a small number of epidemiologic studies observing 
associations with some respiratory effects and limited evidence from 
experimental studies to support biological plausibility, the 2009 ISA 
(U.S. EPA, 2009c) concluded that the relationship between short-term 
exposure to PM10-2.5 and respiratory effects is ``suggestive 
of a causal relationship.'' Epidemiologic findings were consistent for 
respiratory infection and combined respiratory-related diseases, but 
not for COPD. Studies were characterized by overall uncertainty in the 
exposure assignment approach and limited information regarding 
potential copollutant confounding. Controlled human exposure studies of 
short-term PM10-2.5 exposures found no lung function 
decrements and inconsistent evidence for pulmonary inflammation. Animal 
toxicological studies were limited to those using non-inhalation (e.g., 
intra-tracheal instillation) routes of PM10-2.5 exposure.
    Recent epidemiologic findings consistently link PM10-2.5 
exposure to asthma exacerbation and respiratory mortality, with some 
evidence that associations remain positive (though attenuated in some 
studies of mortality) in copollutant models that include 
PM2.5 or gaseous pollutants. Studies provide limited 
evidence for positive associations with other respiratory outcomes, 
including COPD exacerbation, respiratory infection, and combined 
respiratory-related diseases (U.S. EPA, 2019, Table 5-36). As noted 
above for other endpoints, an uncertainty in these epidemiologic 
studies is the lack of a systematic evaluation of the various methods 
used to estimate PM10-2.5 concentrations and the resulting 
uncertainty in the spatial and temporal variability in 
PM10-2.5 concentrations compared to PM2.5 (U.S. 
EPA, 2019, sections 2.5.1.2.3 and 3.3.1.1). Taken together, the 2019 
ISA concludes that ``the collective evidence is suggestive of, but not 
sufficient to infer, a causal relationship between short-term 
PM10-2.5 exposure and respiratory effects'' (U.S. EPA, 2019, 
p. 5-270).
4. Cancer--Long-Term Exposures
    In the last review, little information was available from studies 
of cancer following inhalation exposures to PM10-2.5. Thus, 
the 2009 ISA determined the evidence was ``inadequate to assess the 
relationship between long-term PM10-2.5 exposures and 
cancer'' (U.S. EPA, 2009c). Since the 2009 ISA, the assessment of long-
term PM10-2.5 exposure and cancer remains limited, with a 
few recent epidemiologic studies reporting positive, but imprecise, 
associations with lung cancer incidence. Uncertainty remains in these 
studies with respect to exposure measurement error due to the use of 
PM10-2.5 predictions that have not been validated by 
monitored PM10-2.5 concentrations (U.S. EPA, 2019, sections 
3.3.2.3 and 10.3.4). Relatively few experimental studies of 
PM10-2.5 have been conducted, though available studies 
indicate that PM10-2.5 exhibits two key characteristics of 
carcinogens: Genotoxicity and oxidative stress. While limited, such 
experimental studies provide some evidence of biological plausibility 
for the findings in a small number of epidemiologic studies (U.S. EPA, 
2019, section 10.3.4).
    Taken together, the small number of epidemiologic and experimental 
studies, along with uncertainty with respect to exposure measurement 
error, contribute to the determination in the 2019 ISA that, ``the 
evidence is suggestive of, but not sufficient to infer, a causal 
relationship between long-term PM10-2.5 exposure and 
cancer'' (U.S. EPA, 2019, p. 10-87).
5. Metabolic Effects--Long-Term Exposures
    The 2009 ISA did not make a causality determination for 
PM10-2.5-related metabolic effects. Since the last review, 
one epidemiologic study shows an association between long-term 
PM10-2.5 exposure and incident diabetes, while additional 
cross-sectional studies report associations with effects on glucose or 
insulin homeostasis (U.S. EPA, 2019, section 7.4). As discussed above 
for other outcomes, uncertainties with the epidemiologic evidence 
include the potential for copollutant confounding and exposure 
measurement error (U.S. EPA, 2019, Tables 7-15 and 7-15). The evidence 
base to support the biological plausibility of metabolic effects 
following PM10-2.5 exposures is limited, but a cross-
sectional study that investigated biomarkers of insulin resistance and 
systemic and peripheral inflammation may support a pathway leading to 
type 2 diabetes (U.S. EPA, 2019, sections 7.4.1 and 7.4.3). Based on 
the expanded, though still limited evidence base, the 2019 ISA 
concludes that, ``[o]verall, the evidence is

[[Page 24125]]

suggestive of, but not sufficient to infer, a causal relationship 
between [long]-term PM10-2.5 exposure and metabolic 
effects'' (U.S. EPA, 2019, p. 7-56).
6. Nervous System Effects--Long-Term Exposures
    The 2009 ISA did not make a causality determination for 
PM10-2.5-related nervous system effects. In the current 
review, newly available epidemiologic studies report associations 
between PM10-2.5 and impaired cognition and anxiety in 
adults in longitudinal analyses (U.S. EPA, 2019, Table 8-25, section 
8.4.5). Associations of long-term exposure with neurodevelopmental 
effects are not consistently reported in children (U.S. EPA, 2019, 
sections 8.4.4 and 8.4.5). Uncertainties in these studies include the 
potential for copollutant confounding, as no studies examined 
copollutants models (U.S. EPA, 2019, section 8.4.5), and for exposure 
measurement error, given the use of various model-based subtraction 
methods to estimate PM10-2.5 concentrations (U.S. EPA, 2019, 
Table 8-25). In addition, there is only limited animal toxicological 
evidence supporting the biological plausibility of nervous system 
effects (U.S. EPA, 2019, sections 8.4.1 and 8.4.5). Overall, the 2019 
ISA concludes that, ``the evidence is suggestive of, but not sufficient 
to infer, a causal relationship between long-term PM10-2.5 
exposure and nervous system effects (U.S. EPA, 2019, p. 8-75).

C. Proposed Conclusions on the Current Primary PM10 Standard

    This section describes the Administrator's proposed conclusions 
regarding the adequacy of the current primary PM10 standard. 
The approach to reaching these proposed conclusions draws from the 
ISA's assessment of the scientific evidence for health effects 
attributable to PM10-2.5 exposures (U.S. EPA, 2019). Section 
III.C.1 discusses the evidence-based considerations from the PA. 
Section III.C.2 summarizes CASAC advice on the current primary 
PM10 standard, based on its review of the draft PA. Section 
III.C.3 presents the Administrator's proposed conclusions on the 
current primary PM10 standard.
1. Evidence-Based Considerations in the Policy Assessment
    In the last review, the strongest evidence for PM10-2.5-
related health effects was for cardiovascular effects, respiratory 
effects, and premature mortality following short-term exposures. For 
each of these categories of effects, the ISA concluded that the 
evidence was ``suggestive of a causal relationship'' (U.S. EPA, 2009c, 
section 2.3.3). As summarized in the sections above, key uncertainties 
in the evidence resulted from limitations in the approaches used to 
estimate ambient PM10-2.5 concentrations in epidemiologic 
studies, limited examination of the potential for confounding by co-
occurring pollutants, and limited support for the biological 
plausibility of the serious effects reported in many epidemiologic 
studies. Since 2009, the evidence base for several PM10-2.5-
related health effects has expanded, broadening our understanding of 
the range of health effects linked to PM10-2.5 exposures 
(U.S. EPA, 2020, Chapter 4). This includes expanded evidence for the 
relationships between long-term exposures and cardiovascular effects, 
metabolic effects, nervous system effects, cancer, and mortality. 
However, key limitations in the evidence that were identified in the 
2009 ISA persist in studies that have become available since the last 
review. As discussed in the PA, these limitations include the 
following:
     The use of a variety of methods to estimate 
PM10-2.5 exposures in epidemiologic studies and the lack of 
systematic evaluation of these methods, together with the relatively 
high spatial and temporal variability in ambient PM10-2.5 
concentrations and the small number of monitoring sites, results in 
uncertainty in exposure estimates;
     The limited number of studies that evaluate 
PM10-2.5 health effect associations in copollutant models, 
together with evidence from some studies for attenuation of 
associations in such models, results in uncertainty in the independence 
of PM10-2.5 health effect associations from co-occurring 
pollutants;
     The limited number of controlled human exposure and animal 
toxicology studies of PM10-2.5 inhalation contributes to 
uncertainty in the biological plausibility of the PM10-2.5-
related effects reported in epidemiologic studies.
    Thus, while new evidence is available for a broader range of health 
outcomes in the current review, including an increase in the number of 
studies that report effects related to long-term PM10-2.5 
exposure, that evidence is subject to the same types of uncertainties 
that were identified in the last review of the PM NAAQS. As in the last 
review, these uncertainties contribute to the conclusions in the 2019 
ISA that the evidence for the PM10-2.5-related health 
effects discussed in this section is ``suggestive of, but not 
sufficient to infer'' causal relationships.
2. CASAC Advice
    As part of its review of the draft PA, the CASAC has provided 
advice on the adequacy of the public health protection afforded by the 
current primary PM10 standard. As for PM2.5 
(section II.C.2), the CASAC's advice is documented in a letter sent to 
the EPA Administrator (Cox, 2019a).
    In its comments on the draft PA, the CASAC concurs with the draft 
PA's overall preliminary conclusions that it is appropriate to consider 
retaining the current primary PM10 standard without 
revision. The CASAC finds the more limited approach taken for 
PM10, compared with the approach taken for PM2.5, 
to be ``reasonable and appropriate'' given the less certain evidence 
and the conclusion that ``key uncertainties identified in the last 
review remain'' (Cox, 2019a, p. 13 of consensus responses). To reduce 
these uncertainties in future reviews, the CASAC recommends 
improvements to PM10-2.5 exposure assessment, including a 
more extensive network for direct monitoring of the PM10-2.5 
fraction (Cox, 2019a, p. 13 of consensus responses). The CASAC also 
recommends additional human clinical and animal toxicology studies of 
the PM10-2.5 fraction to improve the understanding of 
biological causal mechanisms and pathways (Cox, 2019a, p. 13 of 
consensus responses). Overall, the CASAC agrees with the EPA that ``. . 
. the available evidence does not call into question the adequacy of 
the public health protection afforded by the current primary 
PM10 standard and that evidence supports considering of 
retaining the current standard in this review'' (Cox, 2019a, p. 3 of 
letter).
3. Administrator's Proposed Decision on the Current Primary 
PM10 Standard
    This section summarizes the Administrator's considerations and 
proposed conclusions related to the current primary PM10 
standard and presents his proposed decision to retain that standard, 
without revision. As discussed above for PM2.5 (II.C.3), in 
establishing primary standards under the Act that are ``requisite'' to 
protect the public health with an adequate margin of safety, the 
Administrator is seeking to establish standards that are neither more 
nor less stringent than necessary for this purpose. He recognizes that 
the Act does not require that primary standards be set at a zero-risk 
level; rather, the NAAQS must be sufficiently protective, but not more 
stringent than necessary.
    Given these requirements, and consistent with the primary 
PM2.5

[[Page 24126]]

standards discussed above (II.C.3), the Administrator's final decision 
in this review will be a public health policy judgment that draws upon 
the scientific information examining the health effects of 
PM10-2.5 exposures, including how to consider the range and 
magnitude of uncertainties inherent in that information. His decision 
will require judgments based on an interpretation of the science that 
neither overstates nor understates its strengths and limitations, nor 
the appropriate inferences to be drawn.
    As an initial matter, the Administrator notes that the decision to 
retain the primary PM10 standard in the last review 
recognized that epidemiologic studies had reported positive 
associations between PM10-2.5 and mortality or morbidity in 
cities across North America, Europe, and Asia. These studies 
encompassed a variety of environments where PM10-2.5 sources 
and composition were expected to vary widely. Although most of these 
studies examined PM10-2.5 health effect associations in 
urban areas, some studies had also linked mortality and morbidity with 
relatively high ambient concentrations of particles of non-urban 
crustal origin. Drawing from this evidence, the EPA judged it 
appropriate to maintain a standard that provides some measure of 
protection against exposures to PM10-2.5, regardless of 
location, source of origin, or particle composition (78 FR 3176, 
January 15, 2013). The Agency further judged it appropriate to retain a 
PM10 standard to provide such protection given that the 
varying concentrations of PM10-2.5 permitted in urban versus 
non-urban areas under a PM10 standard, based on the varying 
levels of PM2.5 present (i.e., lower PM10-2.5 
concentrations allowed in urban areas, where PM2.5 
concentrations tend to be higher), appropriately reflected differences 
in the strength of PM10-2.5 health effects evidence.
    Since the last review, the Administrator notes that the evidence 
for several PM10-2.5-related health effects has expanded, 
particularly for long-term exposures. Recent epidemiologic studies 
continue to report positive associations with mortality and morbidity 
in cities across North America, Europe, and Asia, where 
PM10-2.5 sources and composition are expected to vary 
widely. While the Administrator recognizes that important uncertainties 
remain, as described below, he also recognizes that the expansion in 
the evidence since the last review has broadened the range of effects 
that have been linked with PM10-2.5 exposures. Such studies 
provide an important part of the body of evidence supporting the ISA's 
strengthened causality determinations (and new determinations) for 
long-term PM10-2.5 exposures and mortality, cardiovascular 
effects, metabolic effects, nervous system effects and cancer (U.S. 
EPA, 2019; U.S. EPA, 2020, section 4.2). Drawing from his consideration 
of this evidence, the Administrator proposes to conclude that the 
scientific studies that have become available since the last review do 
not call into question the decision to maintain a primary 
PM10 standard that provides some measure of public health 
protection against PM10-2.5 exposures, regardless of 
location, source of origin, or particle composition.
    With regard to uncertainties in the evidence, the Administrator 
notes that the decision in the last review highlighted limitations in 
estimates of ambient PM10-2.5 concentrations used in 
epidemiologic studies, the limited evaluation of copollutant models to 
address the potential for confounding, and the limited number of 
experimental studies supporting biologically plausible pathways for 
PM10-2.5-related effects. These and other limitations in the 
PM10-2.5 evidence raised questions as to whether additional 
public health improvements would be achieved by revising the existing 
PM10 standard.
    In the current review, despite the expanded body of evidence for 
PM10-2.5-related health effects, the Administrator 
recognizes that similar uncertainties remain. As summarized above 
(III.B), these include uncertainties in the PM10-2.5 
exposure estimates used in epidemiologic studies, in the independence 
of PM10-2.5 health effect associations, and in support for 
the biological plausibility of PM10-2.5-related effects 
(e.g., from controlled human exposure and animal toxicology studies) 
(U.S. EPA, 2020, section 4.2). These uncertainties contribute to the 
determinations in the 2019 ISA that the evidence for key 
PM10-2.5-related health effects is ``suggestive of, but not 
sufficient to infer'' causal relationships (U.S. EPA, 2019). In light 
of his emphasis on evidence supporting ``causal'' and ``likely to be 
causal'' relationships (II.A.2, III.A.2), the Administrator judges that 
the PM10-2.5-related health effects evidence provides an 
uncertain scientific foundation for making standard-setting decisions. 
He further judges that, as in the last review, limitations in this 
evidence raise questions as to whether additional public health 
improvements would be achieved by revising the existing PM10 
standard.
    In reaching conclusions on the primary PM10 standard, 
the Administrator also considers advice from the CASAC. As noted above, 
the CASAC recognizes the uncertainties in the evidence for 
PM10-2.5-related health effects, stating that ``key 
uncertainties identified in the last review remain'' (Cox, 2019a, p. 13 
of consensus responses). Given these uncertainties, the CASAC agrees 
with the PA conclusion that the evidence ``does not call into question 
the adequacy of the public health protection afforded by the current 
primary PM10 standard'' (Cox, 2019a, p. 3 of letter). The 
CASAC further recommends that this evidence ``supports consideration of 
retaining the current standard in this review'' (Cox, 2019a, p. 3 of 
letter).
    When the above information is taken together, the Administrator 
proposes to conclude that the available scientific evidence continues 
to support a PM10 standard to provide some measure of 
protection against PM10-2.5 exposures. This conclusion 
reflects the expanded evidence for PM10-2.5-related health 
effects in the current review. However, important limitations in the 
evidence remain. Consistent with the decision in the last review, the 
Administrator proposes to conclude that these limitations lead to 
considerable uncertainty regarding the potential public health 
implications of revising the existing PM10 standard. Given 
this uncertainty, and consistent with the CASAC's advice, the 
Administrator proposes to conclude that the available evidence does not 
call into question the adequacy of the public health protection 
afforded by the current primary PM10 standard. Therefore, he 
proposes to retain the primary PM10 standard, without 
revision, in the current review. The Administrator solicits comment on 
this proposed decision and on the supporting rationale described above.

IV. Rationale for Proposed Decisions on the Secondary PM Standards

    This section presents the rationale for the Administrator's 
proposed decision to retain the current secondary PM standards, without 
revision. This rationale is based on a thorough review of the latest 
scientific information generally published through December 2017,\45\ 
as presented in the ISA, on non-ecological public welfare effects

[[Page 24127]]

associated with PM and pertaining to the presence of PM in ambient air. 
The Administrator's rationale also takes into account the PA's 
evaluation of the policy-relevant information in the ISA and 
quantitative analyses of air quality related to visibility impairment 
and the CASAC's advice and recommendations, as reflected in discussions 
of the drafts of the ISA and PA at public meetings and in the CASAC's 
letters to the Administrator.
---------------------------------------------------------------------------

    \45\ In addition to the review's opening ``call for 
information'' (79 FR 71764, December 3, 2014), ``the current ISA 
identified and evaluated studies and reports that have undergone 
scientific peer review and were published or accepted for 
publication between January 1, 2009 and March 31, 2017. A limited 
literature update identified some additional studies that were 
published before December 31, 2017'' (U.S. EPA, 2019, Appendix, p. 
A-3). References that are cited in the ISA, the references that were 
considered for inclusion but not cited, and electronic links to 
bibliographic information and abstracts can be found at: https://hero.epa.gov/hero/particulate-matter.
---------------------------------------------------------------------------

    In presenting the rationale for the Administrator's proposed 
decision and its foundations, section IV.A provides background on the 
general approach for review of the secondary PM standards, including a 
summary of the approach used in the last review (section IV.A.1) and 
the general approach for the current review (section IV.A.2). Section 
IV.B summarizes the currently available evidence for PM-related 
visibility impairment and section IV.C summarizes the available 
information for other PM-related welfare effects. Section IV.D presents 
the Administrator's proposed conclusions on the current secondary PM 
standards.

A. General Approach

    In the last review of the PM NAAQS, completed in 2012, the EPA 
retained the secondary 24-hour PM2.5 standard, with its 
level of 35 [mu]g/m\3\, and the 24-hour PM10 standard, with 
its level of 150 [micro]g/m\3\ (78 FR 3228, January 15, 2013). The EPA 
also retained the level, set at 15 [micro]g/m\3\, and averaging time of 
the secondary annual PM2.5 standard, while revising the 
form. With regard to the form of the annual PM2.5 standard, 
the EPA removed the option for spatial averaging (78 FR 3228, January 
15, 2013). Key aspects of the Administrator's decisions on the 
secondary PM standards for non-visibility effects and visibility 
effects are described below in section IV.A.1.
1. Approach Used in the Last Review
    The 2012 decision on the adequacy of the secondary PM standards was 
based on consideration of the protection provided by those standards 
for visibility and for the non-visibility effects of materials damage, 
climate effects and ecological effects. As noted earlier, the current 
review of the public welfare protection provided by the secondary PM 
standards against ecological effects is occurring in the separate, on-
going review of the secondary NAAQS for oxides of nitrogen and oxides 
of sulfur (U.S. EPA, 2016, Chapter 1, section 5.2; U.S. EPA, 2020, 
Chapter 1, section 5.1.1). Thus, the consideration of ecological 
effects in the 2012 review is not discussed here. Rather, the sections 
below focus on the prior Administrator's consideration of climate and 
materials effects (section IV.A.1.a) and visibility effects (section 
IV.A.1.b).
a. Non-Visibility Effects
    With regard to the role of PM in climate, the prior Administrator 
considered whether it was appropriate to establish any distinct 
secondary PM standards to address welfare effects associated with 
climate impacts. In considering the scientific evidence, she noted the 
2009 ISA conclusion ``that a causal relationship exists between PM and 
effects on climate'' and that aerosols \46\ alter climate processes 
directly through radiative forcing and by indirect effects on cloud 
brightness, changes in precipitation, and possible changes in cloud 
lifetimes (U.S. EPA, 2009c, section 9.3.10). Additionally, the major 
aerosol components with the potential to affect climate processes 
(i.e., black carbon (BC), organic carbon (OC), sulfates, nitrates and 
mineral dusts) vary in their reflectivity, forcing efficiencies, and 
direction of climate forcing (U.S. EPA, 2009c, section 9.3.10).
---------------------------------------------------------------------------

    \46\ In the climate sciences research community, PM is 
encompassed by what is typically referred to as aerosol. An aerosol 
is defined as a solid or liquid suspended in a gas, but PM refers to 
the solid or liquid phase of an aerosol. In this review of the 
secondary PM NAAQS the discussion on climate effects of PM uses the 
term PM throughout for consistency with the ISA (U.S. EPA, 2019) as 
well as to emphasize that the climate processes altered by aerosols 
are generally altered by the PM portion of the aerosol. Exceptions 
to this practice include the discussion of climate effects in the 
last review, when aerosol was used when discussing suspending 
aerosol particles, and for certain acronyms that are widely used by 
the climate community that include the term aerosol (e.g., aerosol 
optical depth, or AOD).
---------------------------------------------------------------------------

    Noting the strong evidence indicating that aerosols affect climate, 
the prior Administrator further considered what the available 
information indicated regarding the adequacy of protection provided by 
the secondary PM standards. She noted that a number of uncertainties in 
the scientific information affected our ability to quantitatively 
evaluate the standards in this regard. For example, the ISA and PA 
noted the spatial and temporal heterogeneity of PM components that 
contribute to climate forcing, uncertainties in the measurement of 
aerosol components, inadequate consideration of aerosol impacts in 
climate modeling, insufficient data on local and regional microclimate 
variations and heterogeneity of cloud formations. In light of these 
uncertainties and the lack of sufficient data, the 2011 PA concluded 
that it was not feasible in the last review ``to conduct a quantitative 
analysis for the purpose of informing revisions [to the secondary PM 
NAAQS] based on climate'' (U.S. EPA, 2011, pp. 5-11 to 5-12) and that 
there was insufficient information available to base a national ambient 
air quality standard on climate impacts associated with ambient air 
concentrations of PM or its constituents (U.S. EPA, 2011, section 
5.2.3). The prior Administrator agreed with this conclusion (78 FR 
3225-3226, January 15, 2013).
    With regard to materials effects, the she also considered effects 
associated with the deposition of PM (i.e., dry and wet deposition), 
including both physical damage (materials effects) and aesthetic 
qualities (soiling effects). The deposition of PM can physically affect 
materials, adding to the effects of natural weathering processes, by 
promoting or accelerating the corrosion of metals; by degrading paints; 
and by deteriorating building materials such as stone, concrete, and 
marble (U.S. EPA, 2009c, section 9.5). Additionally, the deposition of 
PM from ambient air can reduce the aesthetic appeal of buildings and 
objects through soiling. The ISA concluded that evidence was 
``sufficient to conclude that a causal relationship exists between PM 
and effects on materials'' (U.S. EPA, 2009c, sections 2.5.4 and 9.5.4). 
However, the 2011 PA noted that quantitative relationships were lacking 
between particle size, concentrations, and frequency of repainting and 
repair of surfaces and that considerable uncertainty exists in the 
contributions of co-occurring pollutants to materials damage and 
soiling processes (U.S. EPA, 2011, p. 5-29). The 2011 PA concluded that 
none of the evidence available in the last review called into question 
the adequacy of the existing secondary PM standards to protect against 
material effects (U.S. EPA, 2011, p. 5-29). The prior Administrator 
agreed with this conclusion (78 FR 3225-3226, January 15, 2013).
    In considering non-visibility welfare effects in the last review, 
as discussed above, the prior Administrator concluded that, while it is 
important to maintain an appropriate degree of control of fine and 
coarse particles to address non-visibility welfare effects, ``[i]n the 
absence of information that would support any different standards . . . 
it is appropriate to retain the existing suite of secondary standards'' 
(78 FR 3225-3226, January 15, 2013). Her decision was consistent with 
the CASAC advice related to non-visibility effects. Specifically, the 
CASAC agreed with the 2011 PA conclusions that, while these effects are 
important, ``there is not currently a strong technical basis

[[Page 24128]]

to support revisions of the current standards to protect against these 
other welfare effects'' (Samet, 2010a, p. 5). Thus, the prior 
Administrator concluded that it was appropriate to retain all aspects 
of the existing 24-hour PM2.5 and PM10 secondary 
standards. With regard to the secondary annual PM2.5 
standard, she concluded that it was appropriate to retain a level of 
15.0 [mu]g/m\3\ while revising only the form of the standard to remove 
the option for spatial averaging (78 FR 3225-3226, January 15, 2013).
b. Visibility Effects
    Having reached the conclusion to retain the existing secondary PM 
standards to protect against non-visibility welfare effects, the prior 
Administrator next considered the level of protection that would be 
requisite to protect public welfare against PM-related visibility 
impairment and whether to adopt a distinct secondary standard to 
achieve this level of protection. In reaching her final decision that 
the existing 24-hour PM2.5 standard provides sufficient 
protection against PM-related visibility impairment (78 FR 3228, 
January 15, 2013), she considered the evidence assessed in the 2009 ISA 
(U.S. EPA, 2009c) and the analyses included in the Urban-Focused 
Visibility Assessment (2010 UFVA; U.S. EPA, 2010b) and the 2011 PA 
(U.S. EPA, 2011). She also considered the degree of protection for 
visibility that would be provided by the existing secondary standard, 
focusing specifically on the secondary 24-hour PM2.5 
standard with its level of 35 [micro]g/m\3\. These considerations, and 
the prior Administrator's conclusions regarding visibility are 
discussed in more detail below.
    In the last review, the ISA concluded that, ``collectively, the 
evidence is sufficient to conclude that a causal relationship exists 
between PM and visibility impairment'' (U.S. EPA, 2009c, p. 2-28). 
Visibility impairment is caused by light scattering and absorption by 
suspended particles and gases, including water content of aerosols.\47\ 
The available evidence in the last review indicated that specific 
components of PM have been shown to contribute to visibility 
impairment. For example, at sufficiently high relative humidity values, 
sulfate and nitrate are the PM components that scatter more light and 
thus contribute most efficiently to visibility impairment. Elemental 
carbon (EC) and organic carbon (OC) are also important contributors, 
especially in the northwestern U.S. where their contribution to 
PM2.5 mass is higher. Crustal materials can be significant 
contributors to visibility impairment, particularly for remote areas in 
the arid southwestern U.S. (U.S. EPA, 2009c, section 2.5.1).
---------------------------------------------------------------------------

    \47\ All particles scatter light and, although a larger particle 
scatters more light than a similarly shaped smaller particle of the 
same composition, the light scattered per unit of mass is greatest 
for particles with diameters from ~0.3-1.0 [micro]m (U.S. EPA, 
2009c, section 2.5.1). Particles with hygroscopic components (e.g., 
particulate sulfate and nitrate) contribute more to light extinction 
at higher relative humidity than at lower relative humidity because 
they change size in the atmosphere in response to relative humidity.
---------------------------------------------------------------------------

    Visibility impairment can have implications for people's enjoyment 
of daily activities and for their overall sense of well-being (U.S. 
EPA, 2009c, section 9.2). In consideration of the potential public 
welfare implication of various degrees of PM-related visibility 
impairment, the prior Administrator considered the available visibility 
preference studies that were part of the overall body of evidence in 
the 2009 ISA and reviewed as a part of the 2010 UFVA. These preference 
studies provided information about the potential public welfare 
implications of visibility impairment from surveys in which 
participants were asked questions about their preferences or the values 
they placed on various visibility conditions, as displayed to them in 
scenic photographs or in images with a range of known light extinction 
levels.\48\
---------------------------------------------------------------------------

    \48\ Preference studies were available in four urban areas in 
the last review. Three western preference studies were available, 
including one in Denver, Colorado (Ely et al., 1991), one in the 
lower Fraser River valley near Vancouver, British Columbia, Canada 
(Pryor, 1996), and one in Phoenix, Arizona (BBC Research & 
Consulting, 2003). A pilot focus group study was also conducted for 
Washington, DC (Abt Associates, 2001), and a replicate study with 26 
participants was also conducted for Washington, DC (Smith and 
Howell, 2009). More details about these studies are available in 
Appendix D of the PA.
---------------------------------------------------------------------------

    In noting the relationship between PM concentrations and PM-related 
light extinction, the prior Administrator focused on identifying an 
adequate level of protection against visibility-related welfare 
effects. She first concluded that a standard in terms of a 
PM2.5 visibility index would provide a measure of protection 
against PM-related light extinction that directly takes into account 
the factors (i.e., species composition and relative humidity) that 
influence the relationship between PM2.5 in ambient air and 
PM-related visibility impairment. A PM2.5 visibility index 
standard would afford a relatively high degree of uniformity of visual 
air quality protection in areas across the country by directly 
incorporating the effects of differences of PM2.5 
composition and relative humidity. In defining a target level of 
protection in terms of a PM2.5 visibility index, as 
discussed below, she considered specific elements of the index, 
including the basis for its derivation, as well as an appropriate 
averaging time, level, and form.
    With regard to the basis for derivation of a visibility index, the 
prior Administrator concluded that it was appropriate to use an 
adjusted version of the original IMPROVE algorithm,\49\ in conjunction 
with monthly average relative humidity data based on long-term 
climatological means. In so concluding, she noted the CASAC conclusion 
on the reasonableness of reliance on a PM2.5 light 
extinction indicator calculated from PM2.5 chemical 
composition and relative humidity. In considering alternative 
approaches for a focus on visibility, she recognized that the available 
mass monitoring methods did not include measurement of the full water 
content of ambient PM2.5, nor did they provide information 
on the composition of PM2.5, both of which contribute to 
visibility impacts (77 FR 38980, June 29, 2012). In addition, at the 
time of the proposal, she recognized that suitable equipment and 
performance-based verification procedures did not then exist for direct 
measurement of light extinction and could not be developed within the 
time frame of the review (77 FR 38980-38981, June 29, 2012).
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    \49\ The revised IMPROVE algorithm (Pitchford et al., 2007) uses 
major PM chemical composition measurements and relative humidity 
estimates to calculate light extinction. For more information about 
the derivation of and input data required for the original and 
revised IMPROVE algorithms, see 78 FR 3168-3177, January 15, 2013.
---------------------------------------------------------------------------

    With regard to the averaging time of the index, the prior 
Administrator concluded that a 24-hour averaging time would be 
appropriate for a visibility index (78 FR 3226, January 15, 2013). 
Although she recognized that hourly or sub-daily (4- to 6-hour) 
averaging times, within daylight hours and excluding hours with 
relatively high humidity, are more directly related to the short-term 
nature of the perception of PM-related visibility impairment and 
relevant exposure periods for segments of the viewing public than a 24-
hour averaging time, she also noted that there were data quality 
uncertainties associated with the instruments used to provide the 
hourly PM2.5 mass measurements required for an averaging 
time shorter than 24 hours. She also considered the results of analyses 
that compared 24-hour and 4-hour averaging times for calculating the 
index. These analyses showed good correlation between 24-hour and 4-
hour

[[Page 24129]]

average PM2.5 light extinction, as evidenced by reasonably 
high city-specific and pooled R-squared values, generally in the range 
of over 0.6 to over 0.8. Based on these analyses and the 2011 PA 
conclusions regarding them, the prior Administrator concluded that a 
24-hour averaging time would be a reasonable and appropriate surrogate 
for a sub-daily averaging time.
    With regard to the statistical form of the index, the prior 
Administrator settled on a 3-year average of annual 90th percentile 
values. In so doing, she noted that a 3-year average form provided 
stability from the occasional effect of inter-annual meteorological 
variability that can result in unusually high pollution levels for a 
particular year (78 FR 3198, January 15, 2013; U.S. EPA, 2011, p. 4-
58).\50\ Regarding the annual statistic to be averaged, the 2010 UFVA 
evaluated three different statistics: 90th, 95th, and 98th percentiles 
(U.S. EPA, 2010b, chapter 4). In considering these alternative 
percentiles, the 2011 PA noted that the Regional Haze Program targets 
the 20 percent most impaired days for improvements in visual air 
quality in Federal Class I areas and that the median of the 
distribution of these 20 percent worst days would be the 90th 
percentile. The 2011 PA further noted that strategies that are 
implemented so that 90 percent of days would have visual air quality 
that is at or below the level of the standard would reasonably be 
expected to lead to improvements in visual air quality for the 20 
percent most impaired days. Lastly, the 2011 PA recognized that the 
available studies on people's preferences did not address frequency of 
occurrence of different levels of visibility and did not identify a 
basis for a different target for urban areas than that for Class I 
areas (U.S. EPA, 2011, p. 4-59). These considerations led the prior 
Administrator to conclude that 90th percentile form was the most 
appropriate annual statistic to be averaged across three years (78 FR 
3226, January 15, 2013).
---------------------------------------------------------------------------

    \50\ The EPA recognized that a percentile form averaged over 
multiple years offers greater stability to the air quality 
management process by reducing the possibility that statistically 
unusual indicator values will lead to transient violations of the 
standard, thus reducing the potential for disruption of programs 
implementing the standard and reducing the potential for disruption 
of the protections provided by those programs.
---------------------------------------------------------------------------

    With regard to the level of the index, she considered the 
visibility preferences studies conducted in four urban areas (U.S. EPA, 
2011, p. 4-61). Based on these studies, the PA identified a range of 
levels from 20 to 30 deciviews (dv) \51\ as being a reasonable range of 
``candidate protection levels'' (CPLs).\52\ In considering this range 
of CPLs, she noted the uncertainties and limitations in public 
preference studies, including the small number of stated preference 
studies available; the relatively small number of study participants 
and the extent to which the study participants may not be 
representative of the broader study area population in some of the 
studies; and the variations in the specific materials and methods used 
in each study. She concluded that the substantial degree of variability 
and uncertainty in the public preference studies should be reflected in 
a target protection level at the upper end of the range of CPLs. 
Therefore, she concluded that it was appropriate to set a target level 
of protection in terms of a 24-hour PM2.5 visibility index 
at 30 dv (78 FR 3226-3227, January 15, 2013).
---------------------------------------------------------------------------

    \51\ Deciview (dv) refers to a scale for characterizing 
visibility that is defined directly in terms of light extinction. 
The deciview scale is frequently used in the scientific and 
regulatory literature on visibility.
    \52\ For comparison, 20 dv, 25 dv, and 30 dv are equivalent to 
64, 112, and 191 megameters (Mm-\.1\), respectively.
---------------------------------------------------------------------------

    Based on her considerations and conclusions summarized above, the 
prior Administrator concluded that the protection provided by a 
secondary standard based on a 3-year visibility metric, defined in 
terms of a PM2.5 visibility index with a 24-hour averaging 
time, a 90th percentile form averaged over 3 years, and a level of 30 
dv, would be requisite to protect public welfare with regard to visual 
air quality (78 FR 3227, January 15, 2013). Having reached this 
conclusion, she next determined whether an additional distinct 
secondary standard in terms of a visibility index was needed given the 
degree of protection from visibility impairment afforded by the 
existing secondary standards. Specifically, she noted that the air 
quality analyses showed that all areas meeting the existing 24-hour 
PM2.5 standard, with its level of 35 [micro]g/m\3\, had 
visual air quality at least as good as 30 dv, based on the visibility 
index defined above (Kelly et al., 2012b, Kelly et al., 2012a). Thus, 
the secondary 24-hour PM2.5 standard would likely be 
controlling relative to a 24-hour visibility index set at a level of 30 
dv. Additionally, areas would be unlikely to exceed the target level of 
protection for visibility of 30 dv without also exceeding the existing 
secondary 24-hour standard. Thus, the prior Administrator judged that 
the 24-hour PM2.5 standard ``provides sufficient protection 
in all areas against the effects of visibility impairment--i.e., that 
the existing 24-hour PM2.5 standard would provide at least 
the target level of protection for visual air quality of 30 dv which 
[she] judges appropriate'' (78 FR 3227, January 15, 2013). She further 
judged that ``[s]ince sufficient protection from visibility impairment 
would be provided for all areas of the country without adoption of a 
distinct secondary standard, and adoption of a distinct secondary 
standard will not change the degree of over-protection for some areas 
of the country. . . adoption of such a distinct secondary standard is 
not needed to provide requisite protection for both visibility and 
nonvisibility related welfare effects'' (78 FR 3228, January 15, 2013).
2. Approach for the Current Review
    To evaluate whether it is appropriate to consider retaining the 
current secondary PM standards, or whether consideration of revision is 
appropriate, the EPA has adopted an approach in this review that builds 
upon the general approach used in the last review and reflects the body 
of evidence and information now available. As summarized above, past 
approaches have been based most fundamentally on using information from 
studies of PM-related visibility effects, quantitative analyses of PM-
related visibility impairment, information from studies of non-
visibility welfare effects, advice from the CASAC, and public comments 
to inform the selection of secondary PM standards that, in the 
Administrator's judgment, protect the public welfare from any known or 
anticipated effects.
    Similarly, in this review, the EPA draws on the available evidence 
and quantitative assessments pertaining to the public welfare impacts 
of PM in ambient air. In considering the scientific and technical 
information, the Agency considers both the information available at the 
time of the last review and the information that is newly available in 
this review. This includes information on PM-related visibility and 
non-visibility effects. Consistent with the approach in the last 
review, the quantitative air quality analyses for PM-related visibility 
effects provide a context for interpreting the evidence of visibility 
impairment and the potential public welfare significance of PM 
concentrations in ambient air associated with recent air quality 
conditions.

B. PM-Related Visibility Impairment

    The information summarized here is based on the EPA's scientific 
assessment of the latest evidence on visibility effects associated with 
PM; this assessment is documented in the ISA

[[Page 24130]]

and its policy implications are further discussed in the PA. In 
considering the scientific and technical information, the PA reflects 
upon both the information available in the last review and information 
that is newly available since the last review. Policy implications of 
the currently available evidence are discussed in the PA (as summarized 
in section IV.D.1). The subsections below briefly summarize the 
following aspects of the evidence: The nature of PM-related visibility 
impairment (section IV.B.1), the relationship between ambient PM and 
visibility (section IV.B.2), and public perception of visibility 
impairment (section IV.B.3).
1. Nature of PM-Related Visibility Impairment
    Visibility refers to the visual quality of a human's view with 
respect to color rendition and contrast definition. It is the ability 
to perceive landscape form, colors, and textures. Visibility involves 
optical and psychophysical properties involving human perception, 
judgment, and interpretation. Light between the observer and the object 
can be scattered into or out of the sight path and absorbed by PM or 
gases in the sight path. The conclusions of the ISA that ``the evidence 
is sufficient to conclude that a causal relationship exists between PM 
and visibility impairment'' is consistent with conclusions of causality 
in the last review (U.S. EPA, 2019, section 13.2.6). These conclusions 
are based on strong and consistent evidence that ambient PM can impair 
visibility in both urban and remote areas (U.S. EPA, 2019, section 
13.1; U.S. EPA, 2009c, section 9.2.5).
2. Relationship Between Ambient PM and Visibility
    The fundamental relationship between light extinction and PM mass, 
and the EPA's understanding of this relationship, has changed little 
since the 2009 ISA (U.S. EPA, 2009c). The combined effect of light 
scattering and absorption by particles and gases is characterized as 
light extinction, i.e., the fraction of light that is scattered or 
absorbed per unit of distance in the atmosphere. Light extinction is 
measured in units of 1/distance, which is often expressed in the 
technical literature as visibility per megameter (abbreviated 
Mm-\1\). Higher values of light extinction (usually given in 
units of Mm-\1\ or dv) correspond to lower visibility. When 
PM is present in the air, its contribution to light extinction is 
typically much greater than that of gases (U.S. EPA, 2019, section 
13.2.1). The impact of PM on light scattering depends on particle size 
and composition, as well as relative humidity. All particles scatter 
light, as described by the Mie theory, which relates light scattering 
to particle size, shape, and index of refraction (U.S. EPA, 2019, 
section 13.2.3; Van de Hulst, 1981; Mie, 1908). Fine particles scatter 
more light than coarse particles on a per unit mass basis and include 
sulfates, nitrates, organics, light-absorbing carbon, and soil (Malm et 
al., 1994). Hygroscopic particles like ammonium sulfate, ammonium 
nitrate, and sea salt increase in size as relative humidity increases, 
leading to increased light scattering (U.S. EPA, 2019, section 13.2.3).
    Direct measurements of PM light extinction, scattering, and 
absorption are considered more accurate for quantifying visibility than 
PM mass-based estimates because measurements do not depend on 
assumptions about particle characteristics (e.g., size, shape, density, 
component mixture, etc.) (U.S. EPA, 2019, section 13.2.2.2). 
Measurements of light extinction can be made with high time resolution, 
allowing for characterization of subdaily temporal patterns of 
visibility impairment. A variety of measurement methods have been used 
(e.g., transmissometers, integrating nephelometers, teleradiometers, 
telephotometers, and photography and photographic modeling), each with 
its own strengths and limitations (U.S. EPA, 2019, Table 13-1). 
However, there are no common performance-based criteria to evaluate 
these methods and none have been deployed broadly across the U.S. for 
routine measurement of visibility impairment.
    In the absence of a robust monitoring network for the routine 
measurement of light extinction across the U.S., estimation of light 
extinction based on existing PM monitoring can be used. A theoretical 
relationship between light extinction and PM characteristics has been 
derived from Mie theory (U.S. EPA, 2019, Equation 13.5) and can be used 
to estimate light extinction by combining mass scattering efficiencies 
of particles with particle concentrations (U.S. EPA, 2019, section 
13.2.3; U.S. EPA, 2009c, sections 9.2.2.2 and 9.2.3.1). However, 
routine ambient air monitoring rarely includes measurements of particle 
size and composition information with sufficient detail for these 
calculations. Accordingly, a much simpler algorithm has been developed 
to make estimating light extinction more practical.
    This algorithm, known as the IMPROVE algorithm,\53\ provides for 
the estimation of light extinction (bext), in units of 
Mm-\1\, using routinely monitored components of fine 
(PM2.5) and coarse (PM10-2.5) PM. Relative 
humidity data are also needed to estimate the contribution by liquid 
water that is in solution with the hygroscopic components of PM. To 
estimate each component's contribution to light extinction, their 
concentrations are multiplied by extinction coefficients and are 
additionally multiplied by a water growth factor that accounts for 
their expansion with moisture. Both the extinction efficiency 
coefficients and water growth factors of the IMPROVE algorithm have 
been developed by a combination of empirical assessment and theoretical 
calculation using particle size distributions associated with each of 
the major aerosol components (U.S. EPA, 2019, section 13.2.3.1, section 
13.2.3.3).
---------------------------------------------------------------------------

    \53\ The algorithm is referred to as the IMPROVE algorithm as it 
was developed specifically to use monitoring data generated at 
IMPROVE network sites and with equipment specifically designed to 
support the IMPROVE program and was evaluated using IMPROVE optical 
measurements at the subset of monitoring sites that make those 
measurements (Malm et al., 1994).
---------------------------------------------------------------------------

    The original IMPROVE algorithm, so referenced here to distinguish 
it from subsequent variations developed later, was found to 
underestimate the highest light scattering values and overestimate the 
lowest values at IMPROVE monitors throughout the U.S. (Malm and Hand, 
2007; Ryan et al., 2005; Lowenthal and Kumar, 2004) and at sites in 
China (U.S. EPA, 2019, section 13.2.3.3). To resolve these biases, a 
revised IMPROVE equation was developed (Pitchford et al., 2007). Since 
the last review, Lowenthal and Kumar (2016) further offered a number of 
modifications to the revised IMPROVE equation, with a focus of the 
application of the IMPROVE equation in remote sites. In particular, one 
of the modifications was to increase the multiplier to estimate the 
concentration of organic matter, [OM], from the concentration of 
organic carbon, [OC]. This modification was based on their evaluations 
of monitoring data from remote IMPROVE sites, which showed that in 
areas further away from PM sources, PM mass is often more oxygenated 
and contains a larger amount of organic PM. (U.S. EPA, 2019, section 
13.2.3.3). As discussed below in section IV.D.1, analyses conducted in 
the current review estimate PM-related visibility impairment using each 
of these versions of the IMPROVE equation.

[[Page 24131]]

3. Public Perception of Visibility Impairment
    In the last review, visibility preference studies were available 
from four areas in North America.\54\ Study participants were queried 
regarding multiple images that, depending on the study, were either 
photographs of the same location and scenery that had been taken on 
different days on which measured extinction data were available or 
digitized photographs onto which a uniform ``haze'' had been 
superimposed. Results of those studies indicated a wide range of 
judgments on what study participants considered to be acceptable 
visibility across the different study areas, depending on the setting 
depicted in each photograph. As a part of the 2010 UFVA, each study was 
evaluated separately, and figures were developed to display the 
percentage of participants that rated the visual air quality depicted 
as ``acceptable'' (U.S. EPA, 2010b). Based on the results of the 
studies in the four cities, a range encompassing the PM2.5 
visibility index values from images that were judged to be acceptable 
by at least 50% of study participants across all four of the urban 
preference studies was identified (U.S. EPA, 2010b, p. 4-24; PA, Figure 
5-2). Much lower visibility (considerably more haze resulting in higher 
values of light extinction) was considered acceptable in Washington, 
DC, than was in Denver, and 30 dv reflected the highest degree of 
visibility impairment judged to be acceptable by at least 50 percent of 
study participants (78 FR 3226-3227, January 15, 2013).
---------------------------------------------------------------------------

    \54\ Preference studies were available in four urban areas in 
the last review: Denver, Colorado (Ely et al., 1991), Vancouver, 
British Columbia, Canada (Pryor, 1996), Phoenix, Arizona (BBC 
Research & Consulting, 2003), and Washington, DC (Abt Associates, 
2001; Smith and Howell, 2009).
---------------------------------------------------------------------------

    Since the time of the last review, no new visibility preference 
studies have been conducted in the U.S. Similarly, there is little 
newly available information regarding acceptable levels of visibility 
impairment in the U.S.

C. Other PM-Related Welfare Effects

    The information summarized here is based on the EPA's scientific 
assessment of the latest evidence on the non-visibility welfare effects 
associated with PM. This assessment is documented in the ISA and its 
policy implications are further discussed in the PA. In considering the 
scientific and technical information, the PA reflects consideration of 
both the information available in the last review and information that 
is newly available since the last review. The subsections below briefly 
summarize the evidence related to climate effects (section IV.C.1) and 
materials effects (section IV.C.2).
1. Climate
    In this review, as in the last review, the ISA concludes that 
``overall the evidence is sufficient to conclude that a causal 
relationship exists between PM and climate effects'' (U.S. EPA, 2019, 
section 13.3.9). Since the last review, climate impacts have been 
extensively studied and recent research reinforces and strengthens the 
evidence evaluated in the 2009 ISA. New evidence provides greater 
specificity about the details of radiative forcing effects \55\ and 
increases the understanding of additional climate impacts driven by PM 
radiative effects. The Intergovernmental Panel on Climate Change (IPCC) 
assesses the role of anthropogenic activity in past and future climate 
change, and since the last review, has issued the Fifth IPCC Assessment 
Report (AR5; IPCC, 2013) which summarizes any key scientific advances 
in understanding the climate effects of PM since the previous report. 
As in the last review, the ISA draws substantially on the IPCC report 
to summarize climate effects. As discussed in more detail below, the 
general conclusions are similar between the IPCC AR4 and AR5 reports 
with regard to effects of PM on global climate.
---------------------------------------------------------------------------

    \55\ Radiative forcing (RF) for a given atmospheric constituent 
is defined as the perturbation in net radiative flux, at the 
tropopause (or the top of the atmosphere) caused by that 
constituent, in watts per square meter (Wm-2), after allowing for 
temperatures in the stratosphere to adjust to the perturbation but 
holding all other climate responses constant, including surface and 
tropospheric temperatures (Fiore et al., 2015; Myhre et al., 2013). 
A positive forcing indicates net energy trapped in the Earth system 
and suggests warming of the Earth's surface, whereas a negative 
forcing indicates net loss of energy and suggests cooling (U.S. EPA, 
2019, section 13.3.2.2).
---------------------------------------------------------------------------

    Atmospheric PM has the potential to affect climate in multiple 
ways, including absorbing and scattering of incoming solar radiation, 
alterations in terrestrial radiation, effects on the hydrological 
cycle, and changes in cloud properties (U.S. EPA, 2019, section 
13.3.1). Atmospheric PM interacts with incoming solar radiation. Many 
species of PM (e.g., sulfate and nitrate) efficiently scatter solar 
energy. By enhancing reflection of solar energy back to space, 
scattering PM exerts a cooling effects on the surface below. Certain 
species of PM such as black carbon (BC), brown carbon (BrC), or dust 
can also absorb incoming sunlight. A recent study found that whether 
absorbing PM warms or cools the underlying surface depends on several 
factors, including the altitude of the PM layer relative to cloud cover 
and the albedo (i.e., reflectance) of the surface (Ban-Weiss et al., 
2014). PM also perturbs incoming solar radiation by influencing cloud 
cover and cloud lifetime. For example, PM provides nuclei upon which 
water vapor condenses, forming cloud droplets. Finally, absorbing PM 
deposited on snow and ice can diminish surface albedo and lead to 
regional warming (U.S. EPA, 2019, section 13.3.2).
    PM has direct and indirect effects on climate processes. PM 
interactions with solar radiation through scattering and absorption, 
collectively referred to as aerosol-radiation interactions (ARI), are 
also known as the direct effects on climate, as opposed to the indirect 
effects that involve aerosol-cloud interactions (ACI). The direct 
effects of PM on climate result primarily from particles scattering 
light away from Earth and sending a fraction of solar energy back into 
space, decreasing the transmission of visible radiation to the surface 
of the Earth and resulting in a decrease in the heating rate of the 
surface and the lower atmosphere. The IPCC AR5, taking into account 
both model simulations and satellite observations, reports a radiative 
forcing from aerosol-radiation interactions (RFari) from anthropogenic 
PM of -0.35  0.5 watts per square meter (Wm-2) 
(Boucher, 2013), which is comparable to AR4 (-0.5  0.4 
Wm-2). Estimates of effective radiative forcing \56\ from 
aerosol-radiation interactions (ERFari), which include the rapid 
feedback effects of temperature and cloud cover, rely mainly on model 
simulations, as this forcing is complex and difficult to observe (U.S. 
EPA, 2019, section 13.3.4.1). The IPCC AR5 best estimate for ERFari is 
-0.45  0.5 Wm-2, which reflects this uncertainty 
(Boucher, 2013).
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    \56\ Effective radiative forcing (ERF), new in the IPCC AR5, 
takes into account not just the instantaneous forcing but also a set 
of climate feedbacks, involving atmospheric temperature, cloud 
cover, and water vapor, that occur naturally in response to the 
initial radiative perturbation (U.S. EPA, 2019, section 13.3.2.2).
---------------------------------------------------------------------------

    By providing cloud condensation nuclei, PM increases cloud droplet 
number, thereby increasing cloud droplet surface area and albedo 
(Twomey, 1977). The climate effects of these perturbations are more 
difficult to quantify than the direct effects of aerosols with RF but 
likely enhance the cooling influence of clouds by increasing cloud 
reflectivity (traditionally referred to as the first indirect effect) 
and lengthening cloud lifetime (second indirect effect). These effects 
are reported as the radiative

[[Page 24132]]

forcing from aerosol-cloud interaction (ERFaci) (U.S. EPA, 2019, 
section 13.3.3.2).\57\ IPCC AR5 estimates ERFaci at -0.45 
Wm-2, with a 90% confidence interval of -1.2 to 0 
Wm-2 (U.S. EPA, 2019, section 13.3.4.2). Studies have also 
calculated the combined effective radiative forcing from aerosol-
radiation and aerosol-cloud interactions (ERFari+aci) (U.S. EPA, 2019, 
section 13.3.4.3). IPCC AR5 reports a best estimate of ERFari+aci of -
0.90 (-1.9 to -0.1) Wm-2, consistent with these estimates 
(Boucher, 2013).
---------------------------------------------------------------------------

    \57\ While the ISA includes estimates of RFaci and ERFaci from a 
number of studies (U.S. EPA, 2019, sections 13.3.4.2, 13.3.4.3, 
13.3.3.3), this discussion focuses on the single best estimate with 
a range of uncertainty, as reported in the IPCC AR5 (Boucher, 2013).
---------------------------------------------------------------------------

    PM can also strongly reflect incoming solar radiation in areas of 
high albedo, such as snow- and ice-covered surfaces. The transport and 
subsequent deposition of absorbing PM such as BC to snow- and ice-
covered regions can decrease the local surface albedo, leading to 
surface heating. The absorbed energy can then melt the snow and ice 
cover and further depress the albedo, resulting in a positive feedback 
loop (U.S. EPA, 2019, section 13.3.3.3; Bond et al., 2013; U.S. EPA, 
2012b). Deposition of absorbing PM, such as BC, may also affect surface 
temperatures over glacial regions (U.S. EPA, 2019, section 13.3.3.3). 
The IPCC AR5 best estimate of RF from the albedo effects is +0.04 
Wm-2, with an uncertainty range of +0.02 to +0.09 
Wm-2 (Boucher, 2013).
    A number of new studies are available since the last review that 
have exampled the individual climate effects associated with key PM 
components, including sulfate, nitrate, OC, BC, and dust, along with 
updated quantitative estimate of the radiative forcing with the 
individual species. Sulfate particles form through oxidation of 
SO2 by OH in the gas phase and in the aqueous phase by a 
number of pathways, including in particular those involving ozone and 
H2O2 (U.S. EPA, 2019, section 13.3.5.1). The main 
source of anthropogenic sulfate is from coal-fired power plants, and 
global trends in the anthropogenic SO2 emissions are 
estimated to have increased dramatically during the 20th and early 21st 
centuries, although the recent implementation of more stringent air 
pollution controls on sources has led to a reversal in such trends in 
many places (U.S. EPA, 2019, section 13.3.5.1; U.S. EPA, 2020, section 
2.3.1). Sulfate particles are highly reflective. Consistent with other 
recent estimates (Takemura, 2012, Zelinka et al., 2014, Adams et al., 
2001, described below), on a global scale, the IPCC AR5 estimates that 
sulfate contributes more than other PM types to RF, with RFari of -0.4 
(-0.6 to -0.2) Wm-2, where the 5% and 95% uncertainty range 
is represented by the numbers in the parentheses (Myhre et al., 2013), 
which is the same estimate from AR4. Sulfate is also a major 
contributor to the influence of PM on clouds (Takemura, 2012). A total 
effective radiative forcing (ERFari+aci) for anthropogenic sulfate has 
been estimated to be nearly -1.0 Wm-2 (Zelinka et al., 2014, 
Adams et al., 2001).
    Nitrate particles form through the oxidation of nitrogen oxides and 
occur mainly in the form of ammonium nitrate. Ammonium preferentially 
associates with sulfate rather than nitrate, leading to formation of 
ammonium sulfate at the expense of ammonium nitrate (Adams et al., 
2001). As anthropogenic emissions of SO2 decline, more 
ammonium will be available to react with nitrate, potentially leading 
to future increases in ammonium nitrate particles in the atmosphere 
(U.S. EPA, 2019, section 13.3.5.2; Hauglustaine et al., 2014; Lee et 
al., 2013; Shindell et al., 2013). Warmer global temperatures, however, 
may decrease nitrate abundance given that it is highly volatile at 
higher temperatures (Tai et al., 2010). The IPCC AR5 estimates RFari of 
nitrate of -0.11 (-0.3 to -0.03) Wm-2 (Boucher, 2013), which 
is one-fourth of the RFari of sulfate.
    Primary organic carbonaceous PM, including BrC, are emitted from 
wildfires, agricultural fires, and fossil fuel and biofuel combustion. 
SOA form when anthropogenic or biogenic nonmethane hydrocarbons are 
oxidized in the atmosphere, leading to less volatile products that may 
partition into PM (U.S. EPA, 2019, section 13.3.5.3). Organic particles 
are generally reflective, but in the case of BrC, a portion is 
significantly absorbing at shorter wavelengths (<400 nm). The IPCC AR5 
estimates an RFari for primary organic PM from fossil fuel combustion 
and biofuel use of -0.09 (-0.16 to -0.03) Wm-\2\ and an 
RFari estimate for SOA from these sources of -0.03 (-0.27 to +0.20) 
Wm-\2\ (Myhre et al., 2013). Changes in the RFari estimates 
for individual PM components since AR4 have generally been modest, with 
one exception for the estimate for primary organic PM from fossil fuel 
combustion and biofuel use (Myhre et al., 2013).\58\ The wide range in 
these estimates, including inconsistent signs for forcing, reflect 
uncertainties in the optical properties of organic PM and its 
atmospheric budgets, including the production pathways of anthropogenic 
SOA (Scott et al., 2014; Myhre et al., 2013; McNeill et al., 2012; 
Heald et al., 2010). The IPCC AR5 also estimates an RFari of -0.2 
Wm-\2\ for primary organic PM arising from biomass burning 
(Boucher, 2013).
---------------------------------------------------------------------------

    \58\ The estimate of RFari for SOA is new in AR5 and was not 
included in AR4 (Myhre et al., 2013).
---------------------------------------------------------------------------

    Black carbon (BC) particles occur as a result of inefficient 
combustion of carbon-containing fuels. Like directly emitted organic 
PM, BC is emitted from biofuel and fossil fuel combustion and by 
biomass burning. BC is absorbing at all wavelengths and likely has a 
large impact on the Earth's energy budget (Bond et al., 2013). The IPCC 
AR5 estimates a RFari from anthropogenic fossil fuel and biofuel use of 
+0.4 (+0.5 to +0.8) Wm-\2\ (Myhre et al., 2013). Biomass 
burning contributes an additional +0.2 (+0.03 to +0.4) 
Wm-\2\ to BC RFari, while the albedo effect of BC on snow 
and ice adds another +0.04 (+0.02 to +0.09) Wm-\2\ (Myhre et 
al., 2013; U.S. EPA, 2019, section 13.3.5.4, section 13.3.4.4).
    Dust, or mineral dust, is mobilized from dry or disturbed soils as 
a result of both meteorological and anthropogenic activities. Dust has 
traditionally been classified as scattering, but a recent study found 
that dust may be substantially coarser than currently represented in 
climate models, and thus more light-absorbing (Kok et al., 2017). The 
IPCC AR5 estimates RFari as -0.1  0.2 Wm-\2\ 
(Boucher, 2013), although the results of the study by Kok et al. (2017) 
would suggest that in some regions dust may have led to warming, not 
cooling (U.S. EPA, 2019, section 13.3.5.5).
    The new research available in this review expands upon the evidence 
available at the time of the last review. Consistent with the evidence 
available in the last review, the key PM components, including sulfate, 
nitrate, OC, BC, and dust, that contribute to climate processes vary in 
their reflectivity, forcing efficiencies, and direction of forcing.
    Radiative forcing due to PM elicits a number of responses in the 
climate system that can lead to significant effects on weather and 
climate over a range of spatial and temporal scales, mediated by a 
number of feedbacks that link PM and climate. Since the last review, 
the evidence base has expanded with respect to the mechanisms of 
climate responses and feedbacks to PM radiative forcing. However, the 
new literature published since the last review does not reduce the 
considerable

[[Page 24133]]

uncertainties that continue to exist related to these mechanisms.
    Unlike well-mixed, long-lived greenhouse gases in the atmosphere, 
PM has a very heterogenous distribution across the Earth. As such, 
patterns of RFari and RFaci tend to correlate with PM loading, with the 
greatest forcings centralized over continental regions. The climate 
response to this PM forcing, however, is more complicated since the 
perturbation to one climate variable (e.g., temperature, cloud cover, 
precipitation) can lead to a cascade of effects on other variables. 
While the initial PM radiative forcing may be concentrated regionally, 
the eventual climate response can be much broader spatially or be 
concentrated in remote regions, and may be quite complex, affecting 
multiple climate variables with possible differences in the sign of the 
response in different regions or for different variables (U.S. EPA, 
2019, section 13.3.6). The complex climate system interactions lead to 
variation among climate models, with some studies showing relatively 
close correlation between forcing and surface response temperatures 
(e.g., Leibensperger et al., 2012), while other studies show much less 
correlation (e.g., Levy et al., 2013). Many studies have examined 
observed trends in PM and temperature in the U.S. Climate models have 
suggested a range of factors which can influence large-scale 
meteorological processes and may affect temperature, including local 
feedback effects involving soil moisture and cloud cover, changes in 
the hygroscopicity of the PM, and interactions with clouds alone (U.S. 
EPA, 2019, section 13.3.7). While evidence in this review suggests that 
PM influenced temperature trends across the southern and eastern U.S. 
in the 20th century, this evidence is not conclusive and significant 
uncertainties continue to exist. Further research is needed to better 
characterize the effects of PM on regional climate in the U.S. before 
PM climate effects can be quantified.
    While expanded since the last review, the evidence of PM-related 
climate effects is still limited by significant uncertainties, 
particularly for understanding effects at regional scales. Large 
spatial and temporal heterogeneities in direct and indirect PM 
radiative forcing, and associated climate effects, can occur for a 
number of reasons, including the frequency and distribution of 
emissions of key PM components contributing to climate forcing, the 
chemical and microphysical processing that occurs in the atmosphere, 
and the atmospheric lifetime of PM relative to other pollutants 
contributing to radiative forcing (U.S. EPA, 2019, section 13.3). In 
addition to the uncertainty in characterizing radiative forcing, large 
uncertainty exists in quantifying changes in specific climate variables 
associated with PM-related radiative forcing. Moreover, studies have 
shown that predicting climate variables for regions within the U.S. 
(which is of particular interest for the review of the PM NAAQS) is 
more uncertain than predicting climate variables globally due to 
natural climate variability (e.g., Deser et al., 2012) and 
uncertainties in the representation of key atmospheric processes in 
state-of-the-art climate models. Furthermore, quantifying the influence 
of incremental changes in U.S. anthropogenic emissions on regional 
climate is subject to even greater uncertainty because the signal of 
U.S. anthropogenic emissions is relatively small compared with the 
global emissions considered in the studies cited above. Overall, these 
limitations and uncertainties make it difficult to quantify how 
incremental changes in the level of PM mass in ambient air in the U.S. 
would result in changes to climate in the U.S. Thus, as in the last 
review, the PA concludes that the data remain insufficient to conduct 
quantitative analyses for PM effects on climate in the current review 
(U.S. EPA, 2020, section 5.2.2.2.1).
2. Materials
    In considering the evidence available in the current review of PM-
related materials effects, the current evidence continues to support 
the conclusion from the last review that there is a causal relationship 
between PM deposition and materials effects. Effects of deposited PM, 
particularly sulfates and nitrates, to materials include both physical 
damage and impaired aesthetic qualities. Because of their electrolytic, 
hygroscopic, and acidic properties and their ability to sorb corrosive 
gases, particles contribute to materials damage by adding to the 
effects of natural weathering processes, by potentially promoting or 
accelerating the corrosion of metals, degradation of painted surfaces, 
deterioration of building materials, and weakening of material 
components.\59\ The newly available evidence on materials effects of PM 
in this review are primarily from studies conducted outside of the U.S. 
on buildings and other items of cultural heritage and at concentrations 
greater than those typically observed in the U.S.; however, they 
provide limited new data for consideration in this review (U.S. EPA, 
2019, section 13.4).
---------------------------------------------------------------------------

    \59\ As discussed in the ISA (U.S. EPA, 2019, section 13.4.1), 
corrosion typically involves reactions of acidic PM (i.e., acidic 
sulfate or nitrate) with material surfaces, but gases like 
SO2 and nitric acid (HNO3) also contribute. 
Because ``the impacts of gaseous and particulate N and S wet 
deposition cannot be clearly distinguished'' (U.S. EPA, 2019, p. 13-
1), the assessment of the evidence in the ISA considers the combined 
impacts.
---------------------------------------------------------------------------

    Materials damage from PM generally involves one or both of two 
processes: soiling and corrosion (U.S. EPA, 2019, section 13.4.2). 
Soiling and corrosion are complex, interdependent processes, typically 
beginning with deposition of atmospheric PM or SO2 to 
exposed surfaces. Constituents of deposited PM can interact directly 
with materials or undergo further chemical and/or physical 
transformation to cause soiling, corrosion, and physical damage. 
Weathering, including exposure to moisture, ultraviolet (UV) radiation 
and temperature fluctuations, affects the rate and degree of damage 
(U.S. EPA, 2019, section 13.4.2).
    Soiling is the result of PM accumulation on an object that alters 
its optical characteristics or appearance. These soiling effects can 
impact the aesthetic value of a structure or result in reversible or 
irreversible damage to the surface. The presence of air pollution can 
increase the frequency and duration of cleaning and can enhance 
biodeterioration processes on the surface of materials. For example, 
deposition of carbonaceous components of PM can lead to the formation 
of black crusts on surfaces, and the buildup of microbial biofilms \60\ 
can discolor surfaces by trapping PM more efficiently (U.S. EPA, 2009c, 
p. 9-195; U.S. EPA, 2019, section 13.4.2). The presence of PM may alter 
light transmission or change the reflectivity of a surface. 
Additionally, the organic and nutrient content of deposited PM may 
enhance microbial growth on surfaces.
---------------------------------------------------------------------------

    \60\ Microbial biofilms are communities of microorganisms, which 
may include bacteria, algae, fungi and lichens, that colonize an 
inert surface. Microbial biofilms can contribute to biodeterioration 
of materials via modification of the chemical environment.
---------------------------------------------------------------------------

    Since the last review, very little new evidence has become 
available related to deposition of SO2 to materials such as 
limestone, granite, and metal. Deposition of SO2 onto 
limestone can transform the limestone into gypsum, resulting in a 
rougher surface, which allows for increased surface area for 
accumulation of deposited PM (Camuffo and Bernardi, 1993; U.S. EPA, 
2019, section 13.4.2). Oxidation of deposited SO2 that 
contributes to the transformation of limestone to gypsum can be 
enhanced by the formation of surface coatings from deposited

[[Page 24134]]

carbonaceous PM (both elemental and organic carbon) (McAlister et al., 
2008, Grossi et al., 2007). Ozga et al. (2011) characterized damage to 
two concrete buildings in Poland and Italy. Gypsum was the main damage 
product on surfaces of these buildings that were sheltered from rain 
runoff, while PM embedded in the concrete, particularly carbonaceous 
particles, were responsible for darkening of the building walls (Ozga 
et al., 2011).
    Building on the evidence available in the 2009 ISA, research has 
progressed on the theoretical understanding of soiling of cultural 
heritage in a number of studies. Barca et al. (2010) developed and 
tested a new methodological approach for characterizing trace elements 
and heavy metals in black crusts on stone monuments to identify the 
origin of the chemicals and the relationship between the concentrations 
of elements in the black crusts and local environmental conditions. 
Recent research has also used isotope tracers to distinguish between 
contributions from local sources versus atmospheric pollution to black 
crusts on historical monuments in France (Kloppmann et al., 2011). A 
study in Portugal found that biological activity played a major role in 
soiling, specifically in the development of colored layers and in the 
detachment process (de Oliveira et al., 2011). Another study found 
damage to cement renders, often used for restoration, consolidation, 
and decorative purposes on buildings, following exposure to sulfuric 
acid, resulting in the formation of gypsum (Lanzon and Garcia-Ruiz, 
2010).
    Corrosion of stone and the decay of stone building materials by 
acid deposition and sulfate salts were described in the 2009 ISA (U.S. 
EPA, 2009c, section 9.5.3). Since that time, advances have been made on 
the quantification of degradation rates and further characterization of 
the factors that influence damage of stone materials (U.S. EPA, 2019, 
section 13.4.2). Decay rates of marble grave stones were found to be 
greater in heavily polluted areas compared to a relatively pristine 
area (Mooers et al., 2016). The time of wetness and the number of 
dissolution/crystallization cycles were identified as hazard indicators 
for stone materials, with greater hazard during the spring and fall 
when these indicators are relatively high (Casati et al., 2015).
    A study examining the corrosion of steel as a function of PM 
composition and particle size found that changes in the composition of 
resulting rust gradually changed with particle size (Lau et al., 2008). 
In a study of damage to metal materials under in Hong Kong, which 
generally has much higher PM concentrations than those observed in the 
U.S., Liu et al. (2015) found that iron and steel were corroded by both 
PM and gaseous pollutants (SO2 and NO2), while 
copper and copper alloys were mainly corroded by gaseous pollutants 
(SO2 and O3) and aluminum and aluminum alloy 
corrosion was mainly attributed to PM and NO2.
    A number of studies have also found materials damage from PM 
components besides sulfate and black carbon and atmospheric gases 
besides SO2. Studies have characterized impacts of nitrates, 
NOX, and organic compounds on direct materials damage or on 
chemical reactions that enhance materials damage (U.S. EPA, 2019, 
section 13.4.2). Other studies have found that soiling of building 
materials can be attributed to enhanced biological processes and 
colonization, including the development and thickening of biofilms, 
resulting from the deposition of PM components and atmospheric gases 
(U.S. EPA, 2019, section 13.4.2).
    Since the last review, other materials have been studied for damage 
attributable to PM, including glass and photovoltaic panels. Soiling of 
glass can impact its optical and thermal properties and can lead to 
increased cleaning costs and frequency. The development of haze \61\ on 
modern glass has been measured and modeled, with a strong correlation 
between the size distribution of particles and the evolution of the 
mass deposited on the surface of the glass. Measurements showed that, 
under sheltered conditions, mass deposition accelerated regularly with 
time in areas closest to sources of PM (i.e., near roadways) and coarse 
mineral particles were more prevalent compared to other sites (Alfaro 
et al., 2012). Model predictions were found to correctly simulate the 
development of haze at site locations when compared with measurements 
(Alfaro et al., 2012).
---------------------------------------------------------------------------

    \61\ In this discussion of non-visibility welfare effects, haze 
is used as it has been defined in the scientific literature on 
soiling of glass, i.e., the ratio of diffuse transmitted light to 
direct transmitted light (Lombardo et al., 2010). This differs from 
the definition of haze as used in the discussion of visibility 
welfare effects in section V.B above, where it is used as a 
qualitative description of the blockage of sunlight by dust, smoke, 
and pollution.
---------------------------------------------------------------------------

    Soiling of photovoltaic panels can lead to decreased energy 
efficiency. For example, soiling by carbonaceous PM decreased solar 
efficiency by nearly 38%, while soil particles reduced efficiency by 
almost 70% (Radonjic et al., 2017). The rate of photovoltaic power 
output can also be degraded by soiling and has been found to be related 
to the rate of dust accumulation. In five sites in the U.S. 
representing different meteorological and climatological 
conditions,\62\ photovoltaic module power transmission was reduced by 
approximately 3% for every g/m\2\ of PM deposited on the cover plate of 
the photovoltaic panel, independent of geographical location (Boyle et 
al., 2017). Another study found that photovoltaic module power output 
was reduced by 40% after 10 months of exposure without cleaning, 
although a number of anti-reflective coatings can generally mitigate 
power reduction resulting from dust deposition (Walwil et al., 2017). 
Energy efficiency can also be impacted by the soiling of building 
materials, such as light-colored marble panels on building exteriors, 
that are used to reflect a large portion of solar radiation for passive 
cooling and to counter the urban heat island effect. Exposure to acidic 
pollutants in urban environments have been found to reduce the solar 
reflectance of marble, decreasing the cooling effect (Rosso et al., 
2016). Highly reflective roofs, or cool roofs, have been designed and 
constructed to increase reflectance from buildings in urban areas, to 
both decrease air conditioning needs and urban heat island effects, but 
these efforts can be impeded by soiling of materials used for 
constructing cool roofs. Methods have been developed for accelerating 
the aging process of roofing materials to better characterize the 
impact of soiling and natural weather on materials used in constructing 
cool roofs (Sleiman et al., 2014).
---------------------------------------------------------------------------

    \62\ Of the five sites studied, three were in rural, suburban, 
and urban areas representing a semi-arid environment (Front Range of 
Colorado), one site represented a hot and humid environment (Cocoa, 
Florida), and one represented a hot and arid environment 
(Albuquerque, New Mexico) (U.S. EPA, 2019, section 13.4.2; Boyle et 
al., 2017).
---------------------------------------------------------------------------

    Some progress has been made since the last review in the 
development of dose-response relationships for soiling of building 
materials, yet some key relationships remain poorly characterized. The 
first general dose-response relationships for soiling of materials were 
generated by measuring contrast reflectance of a soiled surface to the 
reflectance of the unsoiled substrate for different materials, 
including acrylic house paint, cedar siding, concrete, brick, 
limestone, asphalt shingles, and window glass with varying total 
suspended particulate (TSP) concentrations (Beloin and Haynie, 1975; 
U.S. EPA, 2019, section 13.4.3). Continued efforts to develop dose-
response curves for soiling have led to some advancements for modern 
materials, but these relationships

[[Page 24135]]

remain poorly characterized for limestone. One study quantified the 
dose-response relationships between PM10 and soiling for 
painted steel, white plastic, and polycarbonate filter material, but 
there was too much scatter in the data to produce a dose-response 
relationship for limestone (Watt et al., 2008). A dose-response 
relationship for silica-soda-lime window glass soiling by 
PM10, NO2, and SO2 was quantified 
based on 31 different locations (Lombardo et al., 2010; U.S. EPA, 2019, 
section 13.4.3, Figure 13-32, Equation 13-8). The development of this 
dose-response relationship required several years of observation time 
and had inconsistent data reporting across the locations.
    Since the time of the last review, there has also been progress in 
developing methods to more rapidly evaluate soiling of different 
materials by PM mixtures. Modern buildings typically have simpler 
lines, less detailed surfaces, and a greater use of glass, tile, and 
metal, which are easier to clean than stone. There have also been major 
changes in the types of materials used for buildings, including a 
variety of polymers available for use as coatings and sealants. New 
economic and environmental considerations beyond aesthetic appeal and 
structural damage are emerging (U.S. EPA, 2019, section 13.4.3). 
Changes in building materials and design, coupled with new approaches 
in quantifying the dose-response relationship between PM and materials 
effects, may reduce the amount of time needed for observations to 
support the development of material-specific dose-response 
relationships.
    In addition to dose-response functions, damage functions have also 
been used to quantify material decay as a function of pollutant type 
and load. Damage can be determined from sample surveys or inspection of 
actual damage and a damage function can be developed to link the rate 
of material damage to time of replacement or maintenance. A cost 
function can then link the time for replacement and maintenance to a 
monetary cost, and an economic function links cost to the dose of 
pollution based on the dose-response relationship (U.S. EPA, 2019, 
section 13.4.3). Damage functions are difficult to assess because it 
depends on human perception of the level of soiling deemed to be 
acceptable and evidence in this area remains limited in the current 
review. Since the last review, damage functions for a wide range of 
building materials (i.e., stone, aluminum, zinc, copper, plastic, 
paint, rubber, stone) have been developed and reviewed (Brimblecombe 
and Grossi, 2010). One study estimated long-term deterioration of 
building materials and found that damage to durable building material 
(such as limestone, iron, copper, and discoloration of stone) is no 
longer controlled by pollution as was historically documented but 
rather that natural weathering is a more important influence on these 
materials in modern times (Brimblecombe and Grossi, 2009). Even as PM-
attributable damage to stone and metals has decreased over time, it has 
been predicted that there will be potentially higher degradation rates 
for polymeric materials, plastic, paint, and rubber due to increased 
oxidant concentrations and solar radiation (Brimblecombe and Grossi, 
2009).
    As at the time of the last review and described just above, 
sufficient evidence is not available to conduct a quantitative 
assessment of PM mass or component-related soiling and corrosion 
effects. While soiling associated with PM can lead to increased 
cleaning frequency and repainting of surfaces, no quantitative 
relationships have been established between characteristics of PM or 
the frequency of cleaning or repainting that would help to inform the 
EPA's understanding of the public welfare implications of soiling (U.S. 
EPA, 2019, section 13.4). Similarly, while some information is 
available with regard to microbial deterioration of surfaces and the 
contribution of carbonaceous PM to the formation of black crusts that 
contribute to soiling, the available evidence does not support 
quantitative analyses (U.S. EPA, 2019, section 13.4). While some new 
evidence is available with respect to PM-attributable materials 
effects, the data are insufficient to conduct quantitative analyses for 
PM effects on materials in the current review.

D. Proposed Conclusions on the Current Secondary PM Standards

    In reaching proposed conclusions on the current secondary PM 
standards, the Administrator takes into account policy-relevant 
evidence-based and quantitative information-based considerations, as 
well as advice from the CASAC. Evidence-based considerations draw from 
the EPA's assessment and integrated synthesis of the scientific 
evidence of PM-related welfare effects in the ISA (U.S. EPA, 2019, 
section 13.2). Quantitative information-based considerations draw from 
the EPA's assessment of recent air quality and associated PM-related 
visibility impairment in the PA (U.S. EPA, 2020, Chapter 5). Section 
IV.D.1 below summarizes evidence- and quantitative information-based 
considerations and the associated conclusions reached in the PA. 
Section IV.D.2 describes advice received from the CASAC on the 
secondary standards. Section IV.D.3 presents the Administrator's 
proposed decision on the current secondary PM standards.
1. Evidence- and Quantitative Information-Based Considerations in the 
Policy Assessment
    The PA considers the degree to which the available scientific 
evidence and quantitative information supports or calls into question 
the adequacy of the protection afforded by the current secondary PM 
standards. In doing so, the PA considers the evidence assessed in the 
ISA, including the extent to which the new evidence for PM-related 
visibility impairment, climate effects, or materials effects alters key 
conclusions from the last review. The PA also considers quantitative 
analyses of visibility impairment and the extent to which they may 
indicate different conclusions from those in the last review regarding 
the degree of protection from adverse effects provided by the current 
secondary standards.
    With regard to visibility impairment, the PA presents updated 
analyses based on recent air quality information, with a focus on 
locations meeting the current 24-hour PM2.5 and 
PM10 standards. In the absence of advances in the monitoring 
methods for directly measuring light extinction, and given the lack of 
a robust monitoring network for the routine measurement of light 
extinction across the U.S. (section IV.B.2), as in the last review, the 
PA analyses use calculated light extinction to estimate PM-related 
visibility impairment (U.S. EPA, 2020, section 5.2.1.1). Compared to 
the last review, updated analyses incorporate several refinements. 
These include (1) the evaluation of three versions of the IMPROVE 
equation \63\ to calculate light extinction (U.S. EPA, 2020, Appendix 
D, Equations D-1 through D-3) in order to better understand the 
influence of variability in equation inputs; \64\ (2) the

[[Page 24136]]

use of 24-hour relative humidity data, rather than monthly average 
relative humidity as was used in the last review (U.S. EPA, 2020, 
section 5.2.1.2, Appendix D); and (3) the inclusion of the coarse 
fraction in the estimation of light extinction in the subset of areas 
with PM10-2.5 monitoring data available for the time period 
of interest (U.S. EPA, 2020, section 5.2.1.2, Appendix D). The PA's 
updated analyses include 67 monitoring sites that measure 
PM2.5, including 20 sites that measure both PM10 
and PM2.5, that are geographically distributed across the 
U.S. in both urban and rural areas (U.S. EPA, 2020, Appendix D, Figure 
D-1).\65\
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    \63\ Given the lack of new information to inform a different 
visibility metric, the metric used in the PA is that defined by the 
EPA in the last review as the target level of protection for 
visibility (discussed above in section IV.A.1): A PM2.5 
visibility index with a 24-hour averaging time, a 90th percentile 
form averaged over 3 year, and a level of 30 dv (U.S. EPA, 2020, 
section 5.2.1.2).
    \64\ While the PM2.5 monitoring network has an 
increasing number of continuous FEM monitors reporting hourly 
PM2.5 mass concentrations, there continue to be data 
quality uncertainties associated with providing hourly 
PM2.5 mass and component measurements that could be input 
into IMPROVE equation calculations for sub-daily visibility 
impairment estimates. Therefore, the inputs to these light 
extinction calculations are based on 24-hour average measurements of 
PM2.5 mass and components, rather than sub-daily 
information.
    \65\ These sites are those that have a valid 24-hour 
PM2.5 design value for the 2015-2017 period and met 
strict criteria for PM species for this analysis, based on 24-hour 
average PM2.5 mass and component data that were available 
from monitors in the IMPROVE network, CSN, and NCore Multipollutant 
Monitoring Network (U.S. EPA, 2020, Appendix D). PM10-2.5 
monitoring data is available for 20 of the 67 sites examined.
---------------------------------------------------------------------------

    In areas that meet the current 24-hour PM2.5 standard 
for the 2015-2017 time period, all sites have light extinction 
estimates at or below 27 dv using the original and revised IMPROVE 
equations (and most areas are below 25 dv; U.S. EPA, 2020, section 
5.2.1.2). In addition, the one location that exceeds the current 24-
hour PM2.5 standard also has light extinction estimates at 
or below 27 dv (U.S. EPA, 2020, Figure 5-3). These findings are 
consistent with the findings of the analysis in the last review with 
older air quality data from 102 sites (Kelly et al., 2012b; 78 FR 3201, 
January 15, 2013).
    When light extinction is calculated using the updated IMPROVE 
equation from Lowenthal and Kumar (2016), the resulting 3-year 
visibility metrics are slightly higher at all sites compared to light 
extinction calculated using the IMPROVE equations used in previous 
reviews (U.S. EPA, 2020, Figure 5-4). These results are consistent with 
the higher OC multiplier included in the IMPROVE equation from 
Lowenthal and Kumar (2016), reflecting the use of data from remote 
areas with higher concentrations of organic PM when validating that 
equation. As such, it is important to note that the Lowenthal and Kumar 
(2016) version of the IMPROVE equation may overestimate light 
extinction in non-remote areas, including in the urban areas in the 
PA's analyses.
    Nevertheless, when light extinction is calculated using the 
Lowenthal and Kumar (2016) equation for those sites that meet the 
current 24-hour PM2.5 standard, the 3-year visibility metric 
is generally at or below 30 dv. The one exception to this is a site in 
Fairbanks, Alaska that just meets the current 24-hour PM2.5 
standard in 2015-17 and has a 3-year visibility index value just above 
30 dv, rounding to 31 dv (compared to 27 dv when light extinction is 
calculated with the original and revised IMPROVE equations) (U.S. EPA, 
2020, Appendix D, Table D-3). However, the unique conditions at this 
urban site (e.g., higher OC concentrations, much lower temperatures, 
and the complete lack of sunlight for long periods) affect quantitative 
relationships between OC, OM and visibility (e.g., Hand et al., 2012; 
Hand et al., 2013), making the most appropriate approach for 
characterizing light extinction in this area unclear.
    In the last review, the EPA noted that PM2.5 is the size 
fraction of PM responsible for most of the visibility impairment in 
urban areas (77 FR 38980, June 29, 2012). Data available at the time of 
the last review suggested that PM10-2.5 is often a minor 
contributor to visibility impairment (U.S. EPA, 2010b), though it may 
make a larger contribution in some areas in the desert southwestern 
region of the U.S. However, at the time of the last review, there was 
little data available from PM10-2.5 monitors to quantify the 
contribution of coarse PM to calculated light extinction.
    Since the last review, an expansion of PM10-2.5 
monitoring efforts has increased the availability of data for use in 
estimating light extinction with both PM2.5 and 
PM10-2.5 concentrations included as inputs in the equations. 
For 2015-2017, 20 of the 67 PM2.5 sites analyzed in the PA 
have collocated PM10-2.5 monitoring data available. These 20 
sites meet both the 24-hour PM2.5 standard and 24-hour 
PM10 standard. All of these sites have 3-year visibility 
metrics at or below 30 dv regardless of whether light extinction is 
calculated with or without the coarse fraction, and for all three 
versions of the IMPROVE equation. Generally, the contribution of the 
coarse fraction to light extinction at these sites is minimal, 
contributing less than 1 dv to the 3-year visibility metric. However, 
these 20 locations would be expected to have relatively low 
concentrations of coarse PM. If PM10 and PM10-2.5 
data were available in locations with higher concentrations of coarse 
PM, such as in the southwestern U.S., the coarse fraction may be a more 
important contributor to light extinction and visibility impairment 
than in the locations examined in the PA analyses.
    In summary, the findings of these updated quantitative analyses are 
consistent with those in the last review. The 3-year visibility metric 
is generally at or below 27 dv in areas that meet the current secondary 
standards, with only small differences observed for the three versions 
of the IMPROVE equation. Though such differences are modest, the 
IMPROVE equation from Lowenthal and Kumar (2016) always results in 
higher light extinction values, which is expected given the higher OC 
multiplier included in the equation and its validation using data from 
remote areas far away from emissions sources. There is very little 
difference in estimates of light extinction when PM10-2.5 is 
included in the equation, although a somewhat larger coarse fraction 
contribution to light extinction would be expected in areas with higher 
coarse particle concentrations. Overall, the PA finds that updated 
quantitative analyses indicate that the current secondary PM standards 
provide a degree of protection against visibility impairment similar to 
the target level of protection identified in the last review, defined 
in terms of a PM visibility index.
    With regard to PM-related climate effects, the PA recognizes that 
while the evidence base has expanded since the last review, the new 
evidence has not appreciably improved the understanding of the spatial 
and temporal heterogeneity of PM components that contribute to climate 
forcing (U.S. EPA, 2020, sections 5.2.2.1.1 and 5.4). Despite 
continuing research, there are still significant limitations in 
quantifying the contributions of PM and PM components to the direct and 
indirect effects on climate forcing (e.g., changes to the pattern of 
rainfall, changes to wind patterns, effects on vertical mixing in the 
atmosphere) (U.S. EPA, 2020, sections 5.2.2.1.1 and 5.4). In addition, 
while a number of improvements and refinements have been made to 
climate models since the last review, these models continue to exhibit 
variability in estimates of the PM-related climate effects on regional 
scales (e.g., ~100 km) compared to simulations at the global scale 
(U.S. EPA, 2020, sections 5.2.2.1.1 and 5.4). While new research has 
added to the understanding of climate forcing on a global scale, there 
remain significant limitations to quantifying potential adverse effects 
from PM on climate in the U.S. and how they would vary in response to 
incremental changes in PM concentrations in the U.S. Overall, the PA 
recognizes that while new research is available on climate forcing on a 
global scale, the remaining uncertainties and limitations are 
significant, and the new global scale

[[Page 24137]]

research does not translate directly to use at regional spatial scales. 
Thus, the evidence does not provide a clear understanding at the 
spatial scales needed for the NAAQS of a quantitative relationship 
between concentrations of PM mass in ambient air and the associated 
climate-related effects (U.S. EPA, 2020, sections 5.2.2.2.1 and 5.4). 
The PA concludes that the evidence does not call into question the 
adequacy of the current secondary PM standards for climate effects.
    With regard to materials effects, the PA notes the availability of 
new evidence in this review related to the soiling process and the 
types of materials that are affected. Such evidence provides some 
limited information to inform dose-response relationships and damage 
functions associated with PM, though most recent studies have been 
conducted outside the U.S. (U.S. EPA, 2020, section 5.2.2.1.2; U.S. 
EPA, 2019, section 13.4). The recent evidence includes studies 
examining PM-related effects on the energy efficiency of solar panels 
and passive cooling building materials, though there remains 
insufficient evidence to establish quantitative relationships between 
PM in ambient air and these or other materials effects (U.S. EPA, 2020, 
section 5.2.2.1.2). While new research has expanded the body of 
evidence for PM-related materials effects, the PA recognizes the lack 
of information to inform quantitative analyses assessing materials 
effects or the potential public welfare implications of such effects. 
Thus, the PA concludes that the evidence does not call into question 
the adequacy of the current secondary PM standards for materials 
effects.
    Overall, the PA recognizes that the newly available welfare effects 
evidence, critically assessed in the ISA as part of the full body of 
evidence, reaffirms the conclusions on the visibility, climate, and 
materials effects of PM as recognized in the last review (U.S. EPA, 
2020, sections 5.2.1.1., 5.2.2.1, and 5.4). Further, there is a general 
consistency of the currently available evidence with the evidence that 
was available in the last review, including with regard to key aspects 
of the decision to retain the standards in the last review (U.S. EPA, 
2020, sections 5.2.1.1, 5.2.2.1, and 5.4). The quantitative analyses 
for visibility impairment for recent air quality conditions indicate a 
similar level of protection against visibility effects considered to be 
adverse in the last review (U.S. EPA, 2020, sections 5.2.1.2 and 5.4). 
Collectively, the PA finds that the evidence and quantitative 
information-based considerations support consideration of retaining the 
current secondary PM standards, without revision (U.S. EPA, 2020, 
section 5.4).
2. CASAC Advice
    As part of its review of the draft PA, the CASAC has provided 
advice on the adequacy of the current secondary PM standards. In its 
comments on the draft PA, the CASAC concurs with staff's overall 
preliminary conclusions that it is appropriate to consider retaining 
the current secondary PM standards without revision (Cox, 2019a). The 
CASAC ``finds much of the information . . . on visibility and materials 
effects of PM2.5 to be useful, while recognizing that 
uncertainties and controversies remain about the best ways to evaluate 
these effects'' (Cox, 2019a, p. 13 of consensus responses). Regarding 
climate, while the CASAC agrees that research on PM-related effects has 
expanded since the last review, it also concludes that ``there are 
still significant uncertainties associated with the accurate 
measurement of PM contributions to the direct and indirect effects of 
PM on climate'' (Cox, 2019a, pp. 13-14 of consensus responses). The 
committee recommends that the EPA summarize the ``current scientific 
knowledge and quantitative modeling results for effects of reducing 
PM2.5'' on several climate-related outcomes (Cox, 2019a, p. 
14 of consensus responses), while also recognizing that ``it is 
appropriate to acknowledge uncertainties in climate change impacts and 
resulting welfare impacts in the United States of reductions in 
PM2.5 levels'' (Cox, 2019a, p. 14 of consensus responses). 
When considering the overall body of scientific information for PM-
related effects on visibility, materials, and climate, the CASAC agrees 
that ``the available evidence does not call into question the 
protection afforded by the current secondary PM standards and concurs 
that they should be retained'' (Cox, 2019a, p. 3 of letter).
3. Administrator's Proposed Decision on the Current Secondary PM 
Standards
    This section summarizes the Administrator's considerations and 
conclusions related to the current secondary PM2.5 and 
PM10 standards and presents his proposed decision to retain 
those standards, without revision. In establishing secondary standards 
under the Act that are ``requisite'' to protect the public welfare from 
any known or anticipated adverse effects, the Administrator is seeking 
to establish standards that are neither more nor less stringent than 
necessary for this purpose. He notes that secondary standards are not 
meant to protect against all known or anticipated effects, but rather 
those that are judged to be adverse to the public welfare. Consistent 
with the primary standards discussed above (sections II.C.3 and 
III.C.3), the Act does not require standards to be set at a zero-risk 
level; but rather at a level that limits risk sufficiently so as to 
protect the public welfare, but not more stringent than necessary to do 
so.
    Given these requirements, the Administrator's final decision in 
this review will be a public welfare policy judgment that draws upon 
the scientific and technical information examining PM-related 
visibility impairment, climate effects and materials effects, including 
how to consider the range and magnitude of uncertainties inherent in 
that information. The Administrator recognizes that his final decision 
will be based on an interpretation of the scientific evidence and 
technical analyses that neither overstates nor understates their 
strengths and limitations, nor the appropriate inferences to be drawn.
    As an initial matter in considering the secondary standards, the 
Administrator notes the longstanding body of evidence for PM-related 
visibility impairment. As in the last review, this evidence continues 
to demonstrate a causal relationship between ambient PM and effects on 
visibility (U.S. EPA, 2019, section 13.2). The Administrator recognizes 
that visibility impairment can have implications for people's enjoyment 
of daily activities and for their overall sense of well-being. 
Therefore, as in previous reviews, he considers the degree to which the 
current secondary standards protect against PM-related visibility 
impairment.
    In doing so, the Administrator adopts an approach consistent with 
the approach used in the last review (section IV.A.1). That is, he 
first defines an appropriate target level of protection in terms of a 
PM visibility index that accounts for the factors that influence the 
relationship between particles in the ambient air and visibility (i.e., 
size fraction, species composition, and relative humidity). He then 
considers air quality analyses examining this PM visibility index in 
locations meeting the current 24-hour PM2.5 and 
PM10 standards (U.S. EPA, 2020, section 5.2.1.2).
    To identify a target level of protection, the Administrator first 
defines the specific characteristics of the visibility index. He notes 
that in the last review, the EPA used an index based on estimates of 
light extinction by PM2.5

[[Page 24138]]

components calculated using an adjusted version of the original IMPROVE 
algorithm. As described above (sections IV.B and IV.D.1), this 
algorithm allows the estimation of light extinction using routinely 
monitored components of PM2.5 and PM10-2.5,\66\ 
along with estimates of relative humidity. While revisions have been 
made to the IMPROVE algorithm since the last review (U.S. EPA, 2020, 
section 5.2.1.1), the Administrator recognizes that our fundamental 
understanding of the relationship between ambient PM and light 
extinction has changed little and that the various IMPROVE algorithms 
can appropriately reflect this relationship across the U.S. In the 
absence of a robust monitoring network to directly measure light 
extinction (sections IV.B.2 and IV.D.1), he judges that estimated light 
extinction, as calculated using the IMPROVE algorithms, continues to 
provide a reasonable basis for defining a target level of protection 
against PM-related visibility impairment in the current review.
---------------------------------------------------------------------------

    \66\ In the last review, the focus was on PM2.5 
components given their prominent role in PM-related visibility 
impairment in urban areas and the limited data available for 
PM10-2.5 (77 FR 38980, June 29, 2012; U.S. EPA, 2020, 
section 5.2.1.2).
---------------------------------------------------------------------------

    In further defining the characteristics of a visibility index based 
on estimates of light extinction, the Administrator considers the 
appropriate averaging time, form, and level of the index. With regard 
to the averaging time and form, the Administrator judges that the 
decisions made in the last review remain reasonable. In that review, a 
24-hour averaging time was selected and the form was defined as the 3-
year average of annual 90th percentile values. The decision on 
averaging time recognized the relatively strong correlations between 
24-hour and sub-daily (i.e., 4-hour average) PM2.5 light 
extinction (78 FR 3226, January 15, 2013), indicating that a 24-hour 
averaging time is an appropriate surrogate for the sub-daily time 
periods relevant for visual perception. This decision also recognized 
that the longer averaging time may be less influenced by atypical 
conditions and/or atypical instrument performance (78 FR 3226, January 
15, 2013). The decision to set the form as the 3-year average of annual 
90th percentile values noted that (1) a 3-year average provides 
stability from the occasional effect of inter-annual meteorological 
variability (78 FR 3198, January 15, 2013; U.S. EPA, 2011, p. 4-58); 
(2) the 90th percentile corresponds to the median of the distribution 
of the 20 percent worst days for visibility, which are targeted in 
Class I areas by the Regional Haze Program; \67\ and (3) available 
studies on people's visibility preferences did not identify a basis for 
a different target than that identified for Class I areas (U.S. EPA, 
2011, p. 4-59). Given the similar information available in the current 
review, the Administrator judges that these decisions remain reasonable 
and, therefore, that it remains appropriate to define a visibility 
index in terms of a 24-hour averaging time and a form based on the 3-
year average of annual 90th percentile values.
---------------------------------------------------------------------------

    \67\ In the last review, 90th, 95th, and 98th percentile forms 
were evaluated (U.S. EPA, 2010b, section 4.3.3; 78 FR 3198, January 
15, 2013), and a standard with a 90th percentile form was reasonably 
expected to limit the occurrence of days with peak PM-related light 
extinction (78 FR 3198, January 15, 2013).
---------------------------------------------------------------------------

    The level of the index was set at 30 dv in the last review, 
reflecting the highest degree of visibility impairment judged to be 
acceptable by at least 50% of study participants in the available 
visibility preference studies (78 FR 3226-3227, January 15, 2013). The 
focus on 30 dv, rather than a lower level, was supported in light of 
the important uncertainties and limitations in the underlying public 
preference studies. Consistent with the last review, the Administrator 
notes the following uncertainties and limitations in these studies 
(U.S. EPA, 2020, section 5.2.1.1):
     The available studies may not capture the full range of 
visibility preferences in the U.S. population, particularly given the 
potential for preferences to vary based on the visibility conditions 
commonly encountered and the types of scenes being viewed.
     The available preference studies were conducted 15 to 30 
years ago and may not reflect the visibility preferences of the U.S. 
population today.
     The available preference studies have used a variety of 
methods, potentially influencing responses as to what level of 
visibility impairment is deemed acceptable.
     Factors that are not captured by the methods used in 
available preference studies may influence people's judgments on 
acceptable visibility, including the duration of visibility impairment, 
the time of day during which light extinction is greatest, and the 
frequency of episodes of visibility impairment.
    Because no visibility preference studies have been conducted in the 
U.S. since the last review, the Administrator recognizes that these 
uncertainties and limitations persist. Therefore, in the current review 
his consideration of the degree of visibility impairment constituting 
an adverse public welfare impact is based on the same preference 
studies, with the same uncertainties and limitations, that were 
available in the last review. Drawing from this information, the 
Administrator judges it appropriate to again use 30 dv as the level of 
the visibility index.
    Having concluded that it remains appropriate in this review to 
define the target level of protection in terms of a visibility index 
based on estimated light extinction as described above (i.e., with a 
24-hour averaging time; a 3-year, 90th percentile form; and a level of 
30 dv), the Administrator next considers the degree of protection from 
visibility impairment afforded by the existing secondary standards. He 
considers the updated analyses of PM-related visibility impairment 
presented in the PA (U.S. EPA, 2020, section 5.2.1.2), which reflect 
several improvements over the previous review. Specifically, the 
updated analyses examine multiple versions of the IMPROVE algorithm, 
including the version incorporating revisions since the last review 
(section IV.D.1). This approach provides an improved understanding of 
how variation in equation inputs impacts calculated light extinction 
(U.S. EPA, 2020, Appendix D). In addition, for a subset of monitoring 
sites with available PM10-2.5 data, updated analyses better 
characterize the influence of the coarse fraction on light extinction 
(U.S. EPA, 2020, section 5.2.1.2).
    The Administrator notes that the results of these updated analyses 
are consistent with the results from the last review. Regardless of the 
IMPROVE equation used, they demonstrate that the 3-year visibility 
metric is at or below about 30 dv in all areas meeting the current 24-
hour PM2.5 standard,\68\ and below 25 dv in most of those 
areas (section IV.D.1). In the locations with available 
PM10-2.5 monitoring, which met both the current 24-hour 
PM2.5 and PM10 standards, 3-year visibility 
metrics were at or below 30 dv regardless of whether the coarse 
fraction was included in the calculation (U.S. EPA, 2020, section 
5.2.1.2). Given the results of these analyses, the Administrator 
concludes that the updated scientific

[[Page 24139]]

evidence and technical information support the adequacy of the current 
secondary PM2.5 and PM10 standards to protect 
against PM-related visibility impairment. While the inclusion of the 
coarse fraction had a relatively modest impact on calculated light 
extinction in these analyses, he nevertheless recognizes the continued 
importance of the PM10 standard given the potential for 
larger impacts in locations with higher coarse particle concentrations, 
such as in the southwestern U.S., which were not included in the PA's 
analyses due to insufficient coarse particle data (U.S. EPA, 2019, 
section 13.2.4.1; U.S. EPA, 2020, section 5.2.1.2).
---------------------------------------------------------------------------

    \68\ As discussed in the PA (U.S. EPA, 2020, section 5.2.1.2), 
one site in Fairbanks, Alaska just meets the current 24-hour 
PM2.5 standard and has a 3-year visibility index value of 
27 dv based on the original IMPROVE equation and 31 dv based on the 
Lowenthal and Kumar (2016) equation. At this site, use of the 
Lowenthal and Kumar (2016) equation may not be appropriate given 
that PM composition and meteorological conditions may differ 
considerably from those under which revisions to the equation have 
been validated (U.S. EPA, 2020, section 5.2.1.2).
---------------------------------------------------------------------------

    With respect to non-visibility welfare effects, the Administrator 
considers the evidence for PM-related impacts on climate and on 
materials and concludes that it is generally appropriate to retain the 
existing secondary standards and that it is not appropriate to 
establish any distinct secondary PM standards to address non-visibility 
PM-related welfare effects. With regard to climate, he recognizes that 
a number of improvements and refinements have been made to climate 
models since the time of the last review. However, despite continuing 
research and the strong evidence supporting a causal relationship with 
climate effects (U.S. EPA, 2019, section 13.3.9), the Administrator 
notes that there are still significant limitations in quantifying the 
contributions of the direct and indirect effects of PM and PM 
components on climate forcing (U.S. EPA, 2020, sections 5.2.2.1.1 and 
5.4). He also recognizes that models continue to exhibit considerable 
variability in estimates of PM-related climate impacts at regional 
scales (e.g., ~100 km), compared to simulations at the global scale 
(U.S. EPA, 2020, sections 5.2.2.1.1 and 5.4). The resulting uncertainty 
leads the Administrator to conclude that the scientific information 
available in the current review remains insufficient to quantify, with 
confidence, the impacts of ambient PM on climate in the U.S. (U.S. EPA, 
2020, section 5.2.2.2.1) and that there is insufficient information at 
this time to base a national ambient standard on climate impacts.
    With respect to materials effects, the Administrator notes that the 
evidence available in the current review continues to support the 
conclusion that there is a causal relationship with PM deposition (U.S. 
EPA, 2019, section 13.4). He recognizes that deposition of particles in 
the fine or coarse fractions can result in physical damage and/or 
impaired aesthetic qualities. Particles can contribute to materials 
damage by adding to the effects of natural weathering processes and by 
promoting the corrosion of metals, the degradation of painted surfaces, 
the deterioration of building materials, and the weakening of material 
components. While some new evidence on materials effects of PM is 
available in this review, the Administrator notes that this evidence is 
primarily from studies conducted outside of the U.S. (U.S. EPA, 2019, 
section 13.4). Given the more limited amount of information on the 
quantitative relationships between PM and materials effects in the 
U.S., and uncertainties in the degree to which those effects could be 
adverse to the public welfare, the Administrator judges that the 
scientific information available in the current review remains 
insufficient to quantify, with confidence, the public welfare impacts 
of ambient PM on materials and that there is insufficient information 
at this time to support a distinct national ambient standard based on 
materials impacts.
    Taken together, the Administrator concludes that the scientific and 
technical information for PM-related visibility impairment, climate 
impacts, and materials effects, with its attendant uncertainties and 
limitations, supports the current level of protection provided by the 
secondary PM standards as being requisite to protect against known and 
anticipated adverse effects on public welfare. For visibility 
impairment, this conclusion reflects his consideration of the evidence 
for PM-related light extinction, together with his consideration of 
updated analyses of the protection provided by the current secondary 
PM2.5 and PM10 standards. For climate and 
materials effects, this conclusion reflects his judgment that, although 
it remains important to maintain secondary PM2.5 and 
PM10 standards to provide some degree of control over long- 
and short-term concentrations of both fine and coarse particles, it is 
generally appropriate to retain the existing secondary standards and 
that it is not appropriate to establish any distinct secondary PM 
standards to address non-visibility PM-related welfare effects. His 
conclusions on the secondary standards are consistent with advice from 
the CASAC, which agrees ``that the available evidence does not call 
into question the protection afforded by the current secondary PM 
standards'' and recommends that the secondary standards ``should be 
retained'' (Cox, 2019a, p. 3 of letter). Thus, based on his 
consideration of the evidence and analyses for PM-related welfare 
effects, as described above, and his consideration of CASAC advice on 
the secondary standards, the Administrator proposes to retain those 
standards (i.e., the current 24-hour and annual PM2.5 
standards, 24-hour PM10 standard), without revision.

V. Statutory and Executive Order Reviews

    Additional information about these statutes and Executive Orders 
can be found at http://www2.epa.gov/laws-regulations/laws-and-executive-orders.

A. Executive Order 12866: Regulatory Planning and Review and Executive 
Order 13563: Improving Regulation and Regulatory Review

    The Office of Management and Budget (OMB) determined that this 
action is a significant regulatory action and it was submitted to OMB 
for review. Any changes made in response to OMB recommendations have 
been documented in the docket. Because this action does not propose to 
change the existing NAAQS for PM, it does not impose costs or benefits 
relative to the baseline of continuing with the current NAAQS in 
effect. Thus, the EPA has not prepared a Regulatory Impact Analysis for 
this action.

B. Executive Order 13771: Reducing Regulations and Controlling 
Regulatory Costs

    This action is not expected to be an Executive Order 13771 
regulatory action. There are no quantified cost estimates for this 
proposed action because EPA is proposing to retain the current 
standards.

C. Paperwork Reduction Act (PRA)

    This action does not impose an information collection burden under 
the PRA. There are no information collection requirements directly 
associated with a decision to retain a NAAQS without any revision under 
section 109 of the CAA and this action proposes to retain the current 
PM NAAQS without any revisions.

D. Regulatory Flexibility Act (RFA)

    I certify that this action will not have a significant economic 
impact on a substantial number of small entities under the RFA. This 
action will not impose any requirements on small entities. Rather, this 
action proposes to retain, without revision, existing national 
standards for allowable concentrations of PM in ambient air as required 
by section 109 of the CAA. See also American Trucking Associations v. 
EPA, 175 F.3d 1027, 1044-45 (D.C. Cir. 1999) (NAAQS do not have 
significant impacts upon small entities because NAAQS themselves impose 
no

[[Page 24140]]

regulations upon small entities), rev'd in part on other grounds, 
Whitman v. American Trucking Associations, 531 U.S. 457 (2001).

E. Unfunded Mandates Reform Act (UMRA)

    This action does not contain any unfunded mandate as described in 
the UMRA, 2 U.S.C. 1531-1538, and does not significantly or uniquely 
affect small governments. This action imposes no enforceable duty on 
any state, local, or tribal governments or the private sector.

F. Executive Order 13132: Federalism

    This action does not have federalism implications. It will not have 
substantial direct effects on the states, on the relationship between 
the national government and the states, or on the distribution of power 
and responsibilities among the various levels of government.

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

    This action does not have tribal implications, as specified in 
Executive Order 13175. It does not have a substantial direct effect on 
one or more Indian Tribes. This action does not change existing 
regulations; it proposes to retain the current primary NAAQS for PM, 
without revision. Executive Order 13175 does not apply to this action.

H. Executive Order 13045: Protection of Children From Environmental 
Health Risks and Safety Risks

    This action is not subject to Executive Order 13045 because it is 
not economically significant as defined in Executive Order 12866. The 
health effects evidence for this action, which includes evidence for 
effects in children, is summarized in section II.B above and is 
described in the ISA and PA, copies of which are in the public docket 
for this action.

I. Executive Order 13211: Actions Concerning Regulations That 
Significantly Affect Energy Supply, Distribution or Use

    This action is not subject to Executive Order 13211, because it is 
not likely to have a significant adverse effect on the supply, 
distribution, or use of energy. The purpose of this document is to 
propose to retain the current PM NAAQS. This proposal does not change 
existing requirements. Thus, the EPA concludes that this proposal does 
not constitute a significant energy action as defined in Executive 
Order 13211.

J. National Technology Transfer and Advancement Act (NTTAA)

    This action does not involve technical standards.

K. Executive Order 12898: Federal Actions To Address Environmental 
Justice in Minority Populations and Low-Income Populations

    The EPA believes that this action does not have disproportionately 
high and adverse human health or environmental effects on minority, 
low-income populations and/or indigenous peoples, as specified in 
Executive Order 12898 (59 FR 7629, February 16, 1994). The 
documentation related to this is contained in sections II through IV 
above. The action proposed in this document is to retain, without 
revision, the existing NAAQS for PM based on the Administrator's 
conclusion that the existing standards protect public health, including 
the health of sensitive groups, with an adequate margin of safety and 
protect the public welfare. As discussed in section II, the EPA 
expressly considered the available information regarding health effects 
among at-risk populations in reaching the proposed decision that the 
existing standard is requisite.

L. Determination Under Section 307(d)

    Section 307(d)(1)(V) of the CAA provides that the provisions of 
section 307(d) apply to ``such other actions as the Administrator may 
determine.'' Pursuant to section 307(d)(1)(V), the Administrator 
determines that this action is subject to the provisions of section 
307(d).

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List of Subjects in 40 CFR Part 50

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

Andrew Wheeler,
Administrator.
[FR Doc. 2020-08143 Filed 4-29-20; 8:45 am]
 BILLING CODE 6560-50-P