82684 Federal Register / Vol. 85, No. 244 / Friday, December 18, 2020 / Rules and Regulations 

ENVIRONMENTAL PROTECTION 

AGENCY 

40 CFR Part 50 

[EPA–HQ–OAR–2015–0072; FRL–10018–11– 

OAR] 

RIN 2060–AS50 

Review of the National Ambient Air 

Quality Standards for Particulate 

Matter 

AGENCY: Environmental Protection 

Agency (EPA). 

ACTION: Final action. 

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 final 

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 is retaining the 

current standards, without revision. 

DATES: This final action is effective 

December 18, 2020. 

ADDRESSES: The EPA has established a 

docket for this action under Docket ID 

No. EPA–HQ–OAR–2015–0072. 

Incorporated into this docket is a 

separate docket established for the 

Integrated Science Assessment (Docket 

ID No. EPA–HQ–ORD–2014–0859). All 

documents in the docket are listed in 

https://www.regulations.gov/. Although 

listed in the index, some information is 

not publicly available, e.g., Confidential 

Business Information (CBI) or other 

information whose disclosure is 

restricted by statute. Certain other 

material, such as copyrighted material, 

is not placed on the internet and will be 

publicly available only in hard copy 

form. With the exception of such 

material, publicly available docket 

materials are available electronically 

through https://www.regulations.gov/. 

Out of an abundance of caution for 

members of the public and our staff, the 

EPA Docket Center and Reading Room 

are closed to the public, with limited 

exceptions, to reduce the risk of 

transmitting COVID–19. Our Docket 

Center staff will continue to provide 

remote customer service via email, 

phone, and webform. For further 

information on EPA Docket Center 

services and the current status, please 

visit us online at https://www.epa.gov/ 

dockets. 

FOR FURTHER INFORMATION CONTACT: Dr. 

Lars Perlmutt, Health and 

Environmental Impacts Division, Office 

of Air Quality Planning and Standards, 

U.S. Environmental Protection Agency, 

Mail Code C539–04, Research Triangle 

Park, NC 27711; telephone: (919) 541– 

3037; fax: (919) 541–5315; email: 

perlmutt.lars@epa.gov. 

SUPPLEMENTARY INFORMATION: 

Basis for Immediate Effective Date 

In accordance with section 

307(d)(1)(V), the Administrator has 

designated this action as being subject 

to the rulemaking procedures in section 

307(d) of the Clean Air Act (CAA). 

Section 307(d)(1) of the CAA states that: 

‘‘The provisions of section 553 through 

557 * * * of Title 5 shall not, except as 

expressly provided in this subsection, 

apply to actions to which this 

subsection applies.’’ Thus, section 

553(d) of the Administrative Procedure 

Act (APA), which requires publication 

of a substantive rule to be made ‘‘not 

less than 30 days before its effective 

date’’ subject to limited exceptions, does 

not apply to this action. In the 

alternative, the EPA concludes that it is 

s consistent with APA section 553(d) to 

make this action effective December 18, 

2020. 

Section 553(d)(3) of the APA, 5 U.S.C. 

553(d)(3), provides that final rules shall 

not become effective until 30 days after 

publication in the Federal Register 

‘‘except . . . as otherwise provided by 

the agency for good cause found and 

published with the rule.’’ ‘‘In 

determining whether good cause exists, 

an agency should ‘balance the necessity 

for immediate implementation against 

principles of fundamental fairness 

which require that all affected persons 

be afforded a reasonable amount of time 

to prepare for the effective date of its 

ruling.’’ Omnipoint Corp. v. Fed. 

Commc’n Comm’n, 78 F.3d 620, 630 

(D.C. Cir. 1996) (quoting United States 

v. Gavrilovic, 551 F.2d 1099, 1105 (8th 

Cir. 1977)). The purpose of this 

provision is to ‘‘give affected parties a 

reasonable time to adjust their behavior 

before the final rule takes effect.’’ Id.; 

see also Gavrilovic, 551 F.2d at 1104 

(quoting legislative history). 

The EPA is determining that in light 

of the nature of this action, good cause 

exists to make this final action effective 

immediately because the Agency seeks 

to provide regulatory certainty as soon 

as possible and the Administrator’s 

decision to retain the current NAAQS 

does not change the status quo or 

impose new obligations on any person 

or entity. As a result, there is no need 

to provide parties additional time to 

adjust their behavior, and no person 

will be harmed by making the action 

immediately effective as opposed to 

delaying the effective date by 30 days. 

Accordingly, the EPA is making this 

action effective immediately upon 

publication. 

General Information 

Availability of Information Related to 

This Action 

A number of the documents that are 

relevant to this final decision are 

available through the EPA’s website at 

https://www.epa.gov/naaqs/particulatematter-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-finalintegrated-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, 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-policyassessments-current-review-0, and the 

notice of proposed rulemaking, 

available at https://www.epa.gov/naaqs/ 

particulate-matter-pm-standardsfederal-register-notices-current-review. 

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. History of the PM Air Quality Criteria 

and Standards 

1. Reviews Completed in 1971 and 1987 

2. Review Completed in 1997 

3. Review Completed in 2006 

4. Review Completed in 2012 

D. Current Review of the Air Quality 

Criteria and Standards 

E. Air Quality Information 

1. Distribution of Particle Size in Ambient 

Air 

2. Sources and Emissions Contributing to 

PM in the Ambient Air 

3. Ambient Concentrations and Trends 

a. PM2.5 Mass 

b. PM2.5 Components 

c. PM10

d. PM10–2.5

e. UFP 

4. Background PM 

II. Rationale for Decisions on the Primary 

PM2.5 Standards 


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Federal Register / Vol. 85, No. 244 / Friday, December 18, 2020 / Rules and Regulations 82685 

1The 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 (U.S. EPA, 2016, section 5.2; U.S. EPA, 2020, 

section 5.1.1) in recognition of the linkages between 

oxides of nitrogen, oxides of sulfur, and PM with 

respect to atmospheric deposition and ecological 

effects. Addressing the pollutants together enables 

the EPA to take a comprehensive approach to 

considering the nature and interactions of the 

pollutants, which is important for ensuring that all 

scientific information relevant to ecological effects 

is thoroughly evaluated. 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-qualitystandards. 

A. Introduction 

1. Background on the Current Standards 

2. Overview of Health Effects Evidence 

a. Nature of Effects 

i. Mortality 

ii. Cardiovascular Effects 

iii. Respiratory Effects 

iv. Cancer 

v. Nervous System Effects 

vi. Other Effects 

b. At-Risk Populations 

c. Evidence-Based Considerations 

i. PM2.5 Concentrations Evaluated in 

Experimental Studies 

ii. Ambient Concentrations in Locations of 

Epidemiological Studies 

3. Overview of Risk and Exposure 

Assessment Information 

B. Conclusions on the Primary PM2.5

Standards 

1. CASAC Advice in This Review 

2. Basis for Proposed Decision 

3. Comments on the Proposed Decision 

4. Administrator’s Conclusions 

C. Decision on the Primary PM2.5 Standards 

III. Rationale for Decisions on the Primary 

PM10 Standard 

A. Introduction 

1. Background on the Current Standard 

2. Overview of Health Effects Evidence 

a. Nature of Effects 

i. Mortality 

ii. Cardiovascular Effects 

iii. Respiratory Effects 

iv. Cancer 

v. Metabolic Effects 

vi. Nervous System Effects 

B. Conclusions on the Primary PM10

Standard 

1. CASAC Advice in This Review 

2. Basis for the Proposed Decision 

3. Comments on the Proposed Decision 

4. Administrator’s Conclusions 

C. Decision on the Primary PM10 Standard 

IV. Rationale for Decision on the Secondary 

PM Standards 

A. Introduction 

1. Background on the Current Standards 

2. Overview of Welfare Effects Evidence 

a. Nature of Effects 

i. Visibility 

ii. Climate 

iii. Materials 

3. Overview of Air Quality and 

Quantitative Information 

a. Visibility Effects 

b. Non-Visibility Effects 

B. Conclusions on the Secondary 

Standards 

1. CASAC Advice in This Review 

2. Basis for the Proposed Decision 

3. Comments on the Proposed Decision 

4. Administrator’s Conclusions 

C. Decision on the 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 

Risks 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) 

M. Congressional Review Act (CRA) 

References 

Executive Summary 

This notice presents the 

Administrator’s final decisions to retain 

the current primary (health-based) and 

secondary (welfare-based) National 

Ambient Air Quality Standards 

(NAAQS) for particulate matter (PM), 

without revision. 

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 (mm) 

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 mm 

and PM10 includes particles with 

diameters generally less than or equal to 

10 mm. 

The EPA has established primary 

(health-based) and secondary (welfarebased) NAAQS for PM2.5 and PM10. This 

includes two primary PM2.5 standards, 

an annual average standard with a level 

of 12.0 mg/m3 and a 24-hour standard 

with a 98th percentile form and a level 

of 35 mg/m3. It also includes a primary 

PM10 standard with a 24-hour averaging 

time, a 1-expected exceedance form, and 

a level of 150 mg/m3. Secondary PM 

standards are set equal to the primary 

standards, except that the level of the 

secondary annual PM2.5 standard is 15.0 

mg/m3. In reaching 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), advice from the Clean Air 

Scientific Advisory Committee 

(CASAC), and public comments on the 

proposal. 

For the primary PM2.5 standards, the 

Administrator concludes that there are 

important uncertainties in the evidence 

for adverse health effects below the 

current standards and in the potential 

for additional public health 

improvements from reducing ambient 

PM2.5 concentrations below those 

standards. Based on the available 

evidence, the Administrator has 

concluded that the current primary 

PM2.5 standards are requisite to protect 

public health, with an adequate margin 

of safety, from effects of PM2.5 in 

ambient air and should be retained, 

without revision. Therefore, the EPA is 

retaining those standards (i.e., both the 

annual and 24-hour standards), without 

revision. 

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 concludes that, based on the 

newly available evidence with its 

inherent uncertainties, the current 

primary PM10 standard is requisite to 

protect public health, with an adequate 

margin of safety, from effects of PM10 in 

ambient air, and should be retained, 

without revision. Therefore, the EPA is 

retaining that standard, without 

revision. 

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. Based on his 

consideration of the available evidence 

and quantitative information, he 

concludes that the current secondary 

PM standards are requisite to protect 

public welfare, against visibility effects 

and that there is insufficient 

information to establish distinct 


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82686 Federal Register / Vol. 85, No. 244 / Friday, December 18, 2020 / Rules and Regulations 

2The 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). 

3Under 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 wellbeing.’’ 

secondary PM standards to address 

materials and climate effects. Therefore, 

the EPA is retaining those standards, 

without revision. 

These 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 

provided differing views on the primary 

annual PM2.5 standard, with some 

committee members recommending that 

the EPA retain the current standard and 

other members recommending revision 

of that standard. 

I. Background

A. Legislative Requirements

Two sections of the 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

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 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. 


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4Some 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 the 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. 

5Prior to the review initiated in 2007 (see section 

I.C.4), 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. 

6PM10 refers to particles with a nominal mean 

aerodynamic diameter less than or equal to 10 mm. 

More specifically, 10 mm is the aerodynamic 

diameter for which the efficiency of particle 

collection is 50 percent. 

7The 1997 annual PM2.5 standard was compared 

with measurements made at the communityoriented 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. Areawide 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). 

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 

that are not relevant to standard setting 

may be relevant to implementation of 

the NAAQS once they are established.4

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 

sections 110 and 171–190 of the CAA, 

and related provisions and regulations, 

states are to submit, for the 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. History of the PM Air Quality Criteria 

and Standards 

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 mm (referred to as total suspended 

particulates or TSP). The primary 

standards were set at 260 mg/m3, 24- 

hour average, not to be exceeded more 

than once per year, and 75 mg/m3, 

annual geometric mean. The secondary 

standards were set at 150 mg/m3, 24- 

hour average, not to be exceeded more 

than once per year, and 60 mg/m3, 

annual geometric mean. 

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 PM10,6 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. The level of the 24- 

hour standards (primary and secondary) 

was set at 150 mg/m3, 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 mg/m3, and the 

form was annual arithmetic mean, 

averaged over three years. 

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 mm). The new primary standards 

were as follows: (1) An annual standard 

with a level of 15.0 mg/m3, 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 mg/m3, 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 

mm and less than or equal to 10 mm), 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. 

Following promulgation of the 1997 

p.m. 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 


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8Prior 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 

(PA). 

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 mm 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). 

10The history of the NAAQS review process, 

including revisions to the process, is discussed at 

https://www.epa.gov/naaqs/historical-informationnaaqs-review-process. 

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 

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 holdings on the 

cost and constitutional issues were 

appealed to the U.S. 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 mg/m3, retained the level 

of the annual PM2.5 standards at 15.0 mg/ 

m3, 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 mg/m3, 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. 

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 shortand 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. 

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 


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11The EPA also eliminated the option for spatial 

averaging. 12Consistent with the primary standard, the EPA 

eliminated the option for spatial averaging with the 

annual standard. 

13The CASAC charter is available at: https://

yosemite.epa.gov/sab/sabproduct.nsf/WebCASAC/ 

2019casaccharter/$File/CASAC%202019%20

Renewal%20Charter%203.21.19%20-%20final.pdf. 

The Administrator’s announcement is available at: 

https://archive.epa.gov/epa/newsreleases/actingadministrator-wheeler-announces-science-advisorskey-clean-air-act-committee.html. 

14Based 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 described in detail in 

the proposal in sections II.B.1.d (85 FR 24111, April 

30, 2020) and II.B.1.e (85 FR 24113, April 30, 2020). 

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 

(U.S. EPA, 2010b), 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). 

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 mg/m3 and retained 

the 24-hour PM2.5 standard, with its 

level of 35 mg/m3. 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. 

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). 

D. Current Review of the Air Quality 

Criteria and Standards 

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). From February 9 to February 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 EPA staff in developing a draft 

IRP, which was reviewed by the CASAC 

Particulate Matter Review 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 

chartered CASAC, supplemented by the 

Particulate Matter Review 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

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 (Wheeler, 2019). Rather, the 

EPA has taken steps to address the 

CASAC’s comments in the final 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 ISA. In addition, 

several causality determinations were 

re-examined and, consistent with the 

CASAC advice, the final ISA reflects a 

revised causality determination for longterm 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. 

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 


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15Available at: https://yosemite.epa.gov/sab/ 

sabproduct.nsf/0/ 

6CBCBBC3025E13B4852583D90047B352/$File/ 

EPA-CASAC-19-002_Response.pdf. 

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). 

17Studies identified for the ISA were based on 

the review’s opening ‘‘call for information’’ (79 FR 

71764, December 3, 2014), as well as literature 

searches conducted routinely to identify and 

evaluate ‘‘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/particulatematter. 

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, 

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. 

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 the proposal 

notice (85 FR 24094, April 30, 2020). 

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, as described in 

detail in section I.C.5 of the proposal (85 

FR 24100, April 2020). 

Drawing from his consideration of the 

scientific evidence assessed in the ISA 

and the analyses in the PA, including 

uncertainties in the evidence and 

analyses, and from his consideration of 

advice from the CASAC, on April 14, 

2020 the Administrator proposed to 

retain all of the primary and secondary 

PM standards, without revision. These 

proposed decisions were published in 

the Federal Register on April 30, 2020 

(85 FR 24094, April 30, 2020). The EPA 

held virtual public hearings on the 

proposal on May 20–22, 2020 and May 

27, 2020 (85 FR 26634, May 5, 2020). In 

total, the EPA received more than 

66,000 comments on the proposal from 

members of the public and various 

stakeholder groups by the close of the 

public comment period on June 29, 

2020. Major issues raised in the public 

comments are discussed throughout the 

preamble of this final action. A more 

detailed summary of all significant 

comments, along with the EPA’s 

responses (henceforth ‘‘Response to 

Comments’’), can be found in the docket 

for this rulemaking (Docket No. EPA– 

HQ–OAR–2015–0072). 

As in prior NAAQS reviews, the EPA 

is basing its decision in this review on 

studies and related information 

included in the air quality criteria, 

which have undergone CASAC and 

public review. The studies assessed in 

the ISA 17 and PA, and the integration 

of the scientific evidence presented in 

them, have undergone extensive critical 

review by the EPA, the CASAC, and the 

public. The rigor of that review makes 

these studies, and their integrative 

assessment, the most reliable source of 

scientific information on which to base 

decisions on the NAAQS, decisions that 

all parties recognize as of great import. 

Decisions on the NAAQS can have 

profound impacts on public health and 

welfare, and NAAQS decisions should 

be based on studies that have been 

rigorously assessed in an integrative 

manner not only by the EPA but also by 

the statutorily mandated independent 

scientific advisory committee, as well as 

the public review that accompanies this 

process. Some commenters have 

referred to and discussed individual 

scientific studies on the health effects of 

PM that were not included in the ISA 

(‘‘’new’ studies’’) and that have not gone 

through this comprehensive review 

process. In considering and responding 

to comments for which such ‘‘new’’ 

studies were cited in support, the EPA 

has provisionally considered the cited 

studies in the context of the findings of 

the ISA. The EPA’s provisional 

consideration of these studies did not 

and could not provide the kind of indepth critical review described above, 

but rather was focused on determining 

whether they warranted reopening the 

review of the air quality criteria to 

enable the EPA, the CASAC, and the 

public to consider them further. 

This approach, and the decision to 

rely on studies and related information 

included in the air quality criteria, 

which have undergone CASAC and 

public review, is consistent with the 

EPA’s practice in prior NAAQS reviews 

and its interpretation of the 

requirements of the CAA. Since the 

1970 amendments, the EPA has taken 

the view that NAAQS decisions are to 

be based on scientific studies and 

related information that have been 

assessed as a part of the pertinent air 

quality criteria, and the EPA has 

consistently followed this approach. 

This longstanding interpretation was 

strengthened by new legislative 

requirements enacted in 1977, which 

added section 109(d)(2) of the Act 

concerning CASAC review of air quality 

criteria. See 71 FR 61144, 61148 

(October 17, 2006, final decision on 

review of NAAQS for particulate matter) 

for a detailed discussion of this issue 

and the EPA’s past practice. 

As discussed in the EPA’s 1993 

decision not to revise the O3 NAAQS, 

‘‘new’’ studies may sometimes be of 


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18See https://www.epa.gov/air-trends/particulatematter-pm25-trends and https://www.epa.gov/airtrends/particulate-matter-pm25-trends#pmnat for 

more information. 

19A design value is considered valid if it meets 

the data handling requirements given in 40 CFR 

Appendix N to part 50. 

such significance that it is appropriate 

to delay a decision in a NAAQS review 

and to supplement the pertinent air 

quality criteria so the studies can be 

taken into account (58 FR at 13013– 

13014, March 9, 1993). In the present 

case, the EPA’s provisional 

consideration of ‘‘new’’ studies 

concludes that, taken in context, the 

‘‘new’’ information and findings do not 

materially change any of the broad 

scientific conclusions regarding the 

health and welfare effects of PM in 

ambient air made in the air quality 

criteria. For this reason, reopening the 

air quality criteria review would not be 

warranted. 

Accordingly, the EPA is basing the 

final decisions in this review on the 

studies and related information 

included in the PM air quality criteria 

that have undergone rigorous review by 

the EPA, CASAC and the public. The 

EPA will consider these ‘‘new’’ studies 

for inclusion in the air quality criteria 

for the next PM NAAQS review, which 

the EPA expects to begin soon after the 

conclusion of this review and which 

will provide the opportunity to fully 

assess these studies through a more 

rigorous review process involving the 

EPA, CASAC, and the public. 

E. 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.E.1), sources and 

emissions contributing to PM in the 

ambient air (I.E.2), ambient PM 

concentrations and trends in the U.S. 

(I.E.3), and background PM (I.E.4). 

Additional detail on PM air quality can 

be found in Chapter 2 of the Policy 

Assessment (U.S. EPA, 2020; PA) and 

section I.D of the proposal (85 FR 24100, 

April 30, 2020). 

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) and 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 mm 

in diameter (U.S. EPA, 2019, section 

2.2).The EPA defines PM2.5, also 

referred to as fine particles, as particles 

with aerodynamic diameters generally 

less than or equal to 2.5 mm. 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 mm and less 

than or equal to 10 mm. PM10, which is 

comprised of both fine and coarse 

fractions, includes those particles with 

aerodynamic diameters generally less 

than or equal to 10 mm. In addition, UFP 

are often defined as particles with a 

diameter of less than 0.1 mm based on 

physical size, thermal diffusivity or 

electrical mobility (U.S. EPA, 2019, 

section 2.2). 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, while atmospheric lifetimes 

for UFP and PM10–2.5 are shorter and are 

generally removed from the atmosphere 

within hours, through wet or dry 

deposition (U.S. EPA, 2019, Table 2–1; 

85 FR 24100, April 30, 2020). 

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. Sources and 

emissions of PM are discussed in more 

detail the PA (U.S. EPA, 2020, section 

2.1.1) and in the proposal (85 FR 24101, 

April 30, 2020). 

3. 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.E.3.a and 

I.E.3.b summarize information on PM2.5

mass and components, respectively. 

Section I.E.3.c summarizes information 

on PM10. Sections I.E.3.d and I.E.3.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 

the PA (U.S. EPA, 2020, section 2.3) and 

in the proposal (85 FR 24100, April 30, 

2020). 

a. PM2.5 Mass 

At monitoring sites in the U.S., 

annual PM2.5 concentrations from 2015 

to 2017 averaged 8.0 mg/m3 (and ranged 

from 3.0 to 18.2 mg/m3) and the 98th 

percentiles of 24-hour concentrations 

averaged 20.9 mg/m3 (and ranged from 

9.2 to 111 mg/m3) (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 mg/m3 and 98th percentiles 

of 24-hour concentrations generally at or 

below 30 mg/m3 (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 mg/m3 to 8.0 mg/m3, a 41% 

decrease (U.S. EPA, 2020, section 

2.3.2.1).18 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). 

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,19 52% measured the 

highest annual design value at the nearroad 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 

annual design values at near-road sites, 

those design values were, on average, 

0.7 mg/m3 higher than at the highest 

measuring non-near-road sites (range is 

0.1 to 2.0 mg/m3 higher at near-road 

sites). Although most near-road 

monitoring sites do not have sufficient 

data to evaluate long-term trends in 


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20The 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. 

21The form of the current 24-hour PM10 standard 

is one-expected-exceedance, averaged over three 

years. 

22For more information, see https://

www.epa.gov/air-trends/particulate-matter-pm10- 

trends#pmnat. 

23Sources 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 organic 

aerosols (SOA), and geogenic sources such as 

sulfate formed from volcanic production of SO2 and 

oceanic production of dimethyl-sulfide (U.S. EPA, 

2020, section 2.4). 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). 

near-road PM2.5 concentrations, 

analyses of the data at one near-roadlike site in Elizabeth, NJ,20 show that the 

annual average near-road increment has 

generally decreased between 1999 and 

2017 from about 2.0 mg/m3 to about 1.3 

mg/m3 (U.S. EPA, 2020, section 2.3.2.2). 

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 mg/m3

(ranging from 18 to 173 mg/m3) (U.S. 

EPA, 2020, section 2.3.2.4).21 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. 

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).22 These PM10 concentrations 

have generally declined in the eastern 

U.S., while concentrations in 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. 

d. PM10–2.5

Since the last review, the availability 

of PM10–2.5 ambient concentration data 

has greatly increased because of 

additions to the PM10–2.5 monitoring 

capabilities to the national monitoring 

network. 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 nearroad location. 

Long-term trends in UFP are not 

routinely available at U.S. monitoring 

sites. At one site in Illinois with longterm 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). 

4. 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 the PA (U.S. EPA, 2020, section 

2.4) and in the proposal (85 FR 24102, 

April 30, 2020). At annual and national 

scales, estimated background PM 

concentrations in the U.S. are small 

compared to contributions from 

domestic anthropogenic emissions.23

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 mg/m3 across the sites 

examined. In addition, speciated 

monitoring data from IMPROVE sites 

can provide some insights into how 

contributions from different sources, 

including sources of background PM, 

may have changed over time. Such data 

suggests the estimates of background 

concentrations using speciated 

monitoring data from IMPROVE 

monitors are around 1–3 mg/m3, and 

have not changed significantly since the 

last review. Contributions to 

background PM in the U.S. result 


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24 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. 

25As noted in section I.A above, such protection 

is specified for the sensitive group of individuals 

and not to a single person in the sensitive group 

(see S. Rep. No. 91–1196, 91st Cong., 2d Sess. 10 

[1970]). 

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 Decisions on the 

Primary PM2.5 Standards 

This section presents the rationale for 

the Administrator’s decision to retain 

the current primary PM2.5 standards. 

This decision is based on a thorough 

review in the ISA of the latest scientific 

information, published through 

December 2017,24 on human health 

effects associated with long-and shortterm exposures to PM2.5 in the ambient 

air. This decision also takes into 

account analyses in the PA of policyrelevant information from the ISA, as 

well as information on air quality; the 

analyses of human health risks in the 

PA; CASAC advice; and consideration 

of public comments received on the 

proposal. 

Section II.A provides background on 

the general approach for this review and 

the basis for the existing standard, and 

also presents brief summaries of key 

aspects of the currently available health 

effects and risk information. Section II.B 

summarizes the proposed conclusions 

and CASAC advice, addresses public 

comments received on the proposal and 

presents the Administrator’s 

conclusions on the adequacy of the 

current standard, drawing on 

consideration of the scientific evidence 

and quantitative risk information, 

advice from the CASAC, and comments 

from the public. Section II.C 

summarizes the Administrator’s 

decision on the primary PM2.5

standards. 

A. Introduction 

As in prior reviews, the general 

approach to reviewing the current 

primary PM2.5 standards is based, most 

fundamentally, on using the EPA’s 

assessment of current scientific 

evidence and associated quantitative 

analyses to inform the Administrator’s 

judgment regarding primary PM2.5

standards that protects public health 

with an adequate margin of safety. In 

drawing conclusions with regard to the 

primary PM2.5 standards, the final 

decision on the adequacy of the 

standard is largely a public health 

policy judgment to be made by the 

Administrator. The Administrator’s 

final decision draws upon scientific 

information and analyses about health 

effects, population risks, as well as 

judgments about how to consider the 

range and magnitude of uncertainties 

that are inherent in the scientific 

evidence and risk analyses. The 

approach to informing these judgments, 

discussed more fully below, generally 

reflects a continuum, consisting of 

levels at which scientists generally agree 

that health effects are likely to occur, 

through lower levels at which the 

likelihood and magnitude of the 

response become increasingly uncertain. 

This approach is consistent with the 

requirements of the NAAQS provisions 

of the CAA and with how the EPA and 

the courts have historically interpreted 

the Act. These provisions require the 

Administrator to establish primary 

standards that, in his judgment, are 

requisite to protect public health with 

an adequate margin of safety. In so 

doing, the Administrator seeks to 

establish standards that are neither more 

nor less stringent than necessary for this 

purpose. The Act does not require that 

primary standards be set at a zero-risk 

level, but rather at a level that avoids 

unacceptable risks to public health 

including the health of sensitive 

groups.25 The four basic elements of the 

NAAQS (indicator, averaging time, 

form, and level) are considered 

collectively in evaluating the health 

protection afforded by a standard. 

In evaluating the appropriateness of 

retaining or revising the current primary 

PM2.5 standards, the EPA has adopted 

an approach that builds upon the 

general approach used in the last review 

and reflects the body of evidence of 

information now available. As 

summarized in section II.A.1 below, the 

Administrator’s decisions in the prior 

review were based on an integration of 

information on health effects associated 

with exposure to PM2.5 with information 

on the public health significance of key 

health effects, as well as on policy 

judgments as to when the standard is 

requisite to protect public health with 

an adequate margin of safety and on 

consideration of advice from the CASAC 

and public comments. These decisions 

were also informed by air quality and 

related analyses and quantitative risk 

information. 

Similarly, in this review, as described 

in the PA, the proposal, and elsewhere 

in this document, we draw on the 

current evidence and quantitative 

assessments of public health risk of 

PM2.5 in ambient air. The past and 

current approaches are both based, most 

fundamentally, on the EPA’s 

assessments of the current scientific 

information and associated quantitative 

analyses. The EPA’s assessments are 

primarily documented in the ISA and 

PA, which have received CASAC review 

and public comment (83 FR 53471, 

October 23, 2018; 83 FR 55529, 

November 6, 2018; 84 FR 8523, March 

8, 2019; 84 FR 47944, September 11, 

2019; 84 FR 51555, September 30, 2019; 

84 FR 58713, September 30, 2019). To 

bridge the gap between the scientific 

assessments of the ISA and quantitative 

assessments of the PA and the 

judgments required of the Administrator 

in determining whether the current 

standard remains requisite to protect 

public health with an adequate margin 

of safety, the PA evaluates the policy 

implications of the current evidence in 

the ISA and of the quantitative analyses 

in the PA. 

In considering the scientific and 

technical information, we consider both 

the information available at the time of 

the last review and information newly 

available since the last review, 

including most particularly that which 

has been critically analyzed and 

characterized in the current ISA. We 

additionally consider the quantitative 

risk information described in the PA 

that estimated population-level health 

risks associated with ambient PM2.5

concentrations that have been adjusted 

to simulate air quality scenarios of 

policy interest (e.g., ‘‘just meeting’’ the 

current standards) in multiple study 

areas. The evidence-based discussions 

presented below (and summarized more 

fully in the proposal) draw upon 

evidence from studies evaluating health 

effects related to exposures to PM2.5, as 

discussed in the ISA. The risk-based 

discussions also presented below (and 

summarized more fully in the proposal) 

have been drawn from the quantitative 

analyses for PM2.5, as discussed in the 

PA. Sections II.A.2 and II.A.3 below 

provide an overview for the current 

health effects evidence related to shortand long-term exposures to PM2.5 and 

quantitative risk information with a 

focus on specific policy-relevant 

questions identified for these categories 

of information in the PA. 


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26The Agency also eliminated spatial averaging 

provisions as part of the form of the annual 

standard. 

27 In the last review, the ISA defined ultrafine 

particles (UFP) as generally including particles with 

a mobility diameter less than or equal to 0.1 mm. 

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 mm or smaller 

(U.S. EPA, 2009c, pp. 3–2 to 3–3). 

28The 2011 PA noted the limited body of 

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 

characterized ambient concentrations of UFP (U.S. 

EPA, 2011, section 1.3.2). 

29The 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). 

30 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. 

31The 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 38671–38672, July 18, 1997). 

32Specifically, 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). 

1. Background on the Current Standards 

The last review of the primary PM 

NAAQS was completed in 2012 (78 FR 

3086, January 15, 2013). As noted above 

(section I.C.4), in the last review the 

EPA lowered the level of the primary 

annual PM2.5 standard from 15.0 to 12.0 

mg/m3,26 and retained the existing 24- 

hour PM2.5 standard with its level of 35 

mg/m3. 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, 2012). In particular, she noted the 

‘‘strong and generally robust body of 

evidence of serious health effect 

associated with both long- and shortterm exposures to PM2.5’’ (78 FR 3120, 

January 15, 2013). This included 

epidemiological studies reporting health 

effect associations based on long-term 

average PM2.5 concentrations ranging 

from about 15.0 mg/m3 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. 

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). With regard to 

the indicator, the EPA recognized that 

the health studies available during the 

last review continued to link adverse 

health outcomes (e.g., premature 

mortality, hospital admissions, 

emergency department visits) with longand short-term exposures to PM2.5 (78 

FR 3121, January 15, 2013). In assessing 

the appropriateness of PM2.5 mass as the 

indicator, the EPA also considered the 

available scientific evidence and 

information available related to ultrafine 

particles 27 28 and PM components,29

noting the significant uncertainties and 

limitations associated with the 

evidence, as well as the availability of 

monitoring data. 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). 

With regard to averaging time, in the 

last review, the EPA considered issues 

related to the appropriate averaging time 

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 

2009 ISA, the 2011 PA noted that the 

overwhelming majority of studies 

utilized annual (or multi-year) or 24- 

hour PM averaging periods (U.S. EPA, 

2011, section 2.3.2). Given this 

evidence-base, 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 

existing annual and 24-hour averaging 

times (U.S. EPA, 2011, p. 2–58). The 

CASAC agreed that these conclusions 

were reasonable (Samet, 2010a, p. 2–58). 

The prior Administrator concurred 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 shortterm exposure periods’’ (78 FR 3124, 

January 15, 2013). 

With regard to form, the EPA first 

noted that the form of the annual PM2.5

standard was established in 1997 as an 

annual arithmetic mean, averaged over 

3 years, from single or multiple 

community-oriented monitors.30 That 

is, the level of the annual standard was 

to be compared to measurements made 

at each community-oriented monitoring 

site, or if criteria were met, 

measurements from multiple 

community-oriented monitoring sites 

could be averaged together (i.e., spatial 

averaging) 31 (62 FR 38671–38672, 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 38671–38674, 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).32

At the time of the last review, the EPA 

again considered the form of the 

standard with 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 atrisk populations and populations with 


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33See 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. 

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). 

With regard to the form of the annual 

PM2.5 standard, the prior Administrator 

concluded that public health would not 

be protected with an adequate margin of 

safety in all locations if 

disproportionately higher PM2.5

concentrations in low income and 

minority communities were averaged 

together with lower concentrations 

measured at other sites in a larger 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). 

With regard to the form of the 24-hour 

PM2.5 standard, the EPA recognized that 

the existing 98th percentile form 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.33 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). 

With regard to alternative levels of the 

annual and 24-hour PM2.5 standards, in 

the last review, the EPA considered the 

public health protection provided by the 

standards, taken together, against 

mortality and morbidity effects 

associated with long- 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 standards, 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 longand 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 Administrator considered 

the degree to which epidemiological 

studies indicate confidence in the 

reported health effect associations over 

distributions of PM2.5 concentrations in 

ambient air. She noted that a level of 

12.0 mg/m3 was below the long-term 

mean PM2.5 concentrations reported in 

key epidemiological studies that 

provided evidence of an array of serious 

health effects (78 FR 3161, January 15, 

2013). She further noted that 12.0 mg/m3

generally corresponded to the lower 

portions (i.e., about the 25th percentile) 

of distributions of health events in the 

limited number of epidemiological 

studies for which population-level 

information was available. A level of 

12.0 mg/m3 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). 

Given the uncertainties remaining in 

the scientific evidence, the 

Administrator judged that an annual 

standard level below 12.0 mg/m3 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 error in 

epidemiological studies, and the nature 

and magnitude of estimated risks at 

relatively low ambient PM2.5

concentrations. Furthermore, she noted 

that epidemiological studies had 

reported heterogeneity in effect 

estimates both within and between 

cities and in geographic regions of the 

U.S. She recognized that this 

heterogeneity may be attributed, in part, 

to difference in PM2.5 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 limited number of studies evaluating 

such effects; uncertainties related to 

identifying the relevant exposure time 

periods of concern, and limited 

toxicologic evidence providing 

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 mg/m3 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 mg/m3, the 

prior Administrator concluded that the 

evidence supported retaining the 35 mg/ 

m3 level of the 24-hour PM2.5 standard. 

She noted that the existing 24-hour 

standard, with its 35 mg/m3 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, considered together 

with its inherent uncertainties and 

limitations, supported an annual 

standard with a level of 12.0 mg/m3

combined with a 24-hour standard with 

a level of 35 mg/m3. 

2. Overview of Health Effects Evidence 

In this section, we provide an 

overview of the policy-relevant aspects 

of the health effects evidence available 

for consideration in this review. Section 

II.B of the proposal provides a detailed 

summary of key information contained 

in the ISA (U.S. EPA, 2019) and in the 

PA (U.S. EPA, 2020) on the health 

effects associated with PM2.5 exposures, 

and the related public health 

implications, focusing particularly on 

the information most relevant to 

consideration of effects associated with 

the presence of PM2.5 in ambient air. 

The subsections below briefly 


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34 In this review of the PM 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. 

35The majority of these studies examined nonaccidental mortality outcomes, though some 

Medicare studies lack cause-specific death 

information and, therefore, examine total mortality. 

summarize the information discussed in 

more detail in section II.B of the 

proposal (85 FR 24106 to 24114, April 

30, 2020). 

a. 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 shortterm PM2.5 exposures and mortality 

(II.A.2.a.i), cardiovascular effects 

(II.A.2.a.ii), respiratory effects 

(II.A.2.a.iii), cancer (II.A.2.a.iv), nervous 

system effects (II.A.2.a.v), and other 

effects (II.A.2.a.vi). For these outcomes, 

the ISA concludes that the evidence 

supports either a ‘‘causal’’ or a ‘‘likely 

to be causal’’ relationship with PM2.5

exposures.34

i. Mortality 

Long-Term PM2.5 Exposures 

In the last review, the 2009 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 epidemiological 

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 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 longterm PM2.5 exposures and mortality. 

These studies add support for 

associations with total and nonaccidental 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 and 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. 

A recent series of accountability 

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 mg/m3

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

concentrations 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 mg/m3

decrease in long-term PM2.5

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 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 mg/m3 (Di et al., 2017b) or 10 mg/m3

(Shi et al., 2016). 

Another important consideration in 

characterizing the potential for 

additional public health improvements 

associated with changes in PM2.5

exposure is whether concentrationresponse 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 causespecific 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 (85 

FR 24107, April 30, 2020). 

The biological plausibility of PM2.5- 

attributable mortality is supported by 

the coherence of effects across scientific 

disciplines (i.e., animal toxicologic, 

controlled human exposure studies, and 

epidemiologic). 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 


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36As detailed in the ISA, risk estimates are for a 

10 mg/m3 increase in 24-hour average PM2.5

concentrations, unless otherwise noted (U.S. EPA, 

2019, Preface). 

37Lee et al. (2015) also report that positive and 

statistically significant associations between shortterm PM2.5 exposures and mortality persist in 

analyses restricted to areas with long-term 

concentrations below 12 mg/m3. 

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 

epidemiological studies consistently 

report positive associations with 

mortality across different geographic 

locations, populations, and analytic 

approaches. Recent experimental and 

epidemiological 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). 

Short-Term PM2.5 Exposures 

The 2009 ISA concluded that ‘‘a 

causal relationship exists between shortterm exposure to PM2.5 and mortality’’ 

(U.S. EPA, 2009c). This conclusion was 

based on the evaluation of both multiand single-city epidemiological studies 

that consistently reported positive 

associations between short-term PM2.5

exposure and non-accidental mortality. 

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). 

Some recent studies have expanded 

the examination of potential 

confounders (e.g., U.S. EPA, 2019, 

section 11.1.5.1) to include not only 

copollutants, but also systematic 

evaluations of the potential impact of 

inadequate control from long-term 

temporal trends and weather. 

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 

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 examined 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. 

Positive associations with total 

mortality are further supported by 

analyses reporting positive associations 

with cause-specific mortality, including 

cardiovascular and respiratory mortality 

(U.S. EPA, 2019, section 11.1.3). For 

cause-specific mortality, there has been 

only a 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. 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 for respiratory effects, 

particularly from controlled human 

exposure studies, 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). 

Collectively, these studies indicate that 

the heterogeneity in PM2.5-mortality risk 

estimates cannot be attributed to one 

factor, but instead to 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 few recent studies have 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 (U.S. EPA, 2019, section 

11.1.8.1). 

Recent multicity studies indicate that 

positive and statistically significant 

associations with mortality persist in 

analyses restricted to short-term PM2.5

exposures below 35 mg/m3 (Lee et al., 

2015),37 below 30 mg/m3 (Shi et al., 

2016), and below 25 mg/m3 (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 


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38As 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. 

concentrations (e.g., Di et al., 2017a; 

U.S. EPA, 2019, Figure 11–12). 

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. 

Overall, recent epidemiological 

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 shortterm PM2.5 exposures. The primarily 

positive associations observed across 

studies conducted in diverse geographic 

locations is further supported by the 

results from copollutant 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, p. 11–58). 

ii. Cardiovascular Effects 

Long-Term PM2.5 Exposures 

The scientific evidence reviewed in 

the 2009 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 epidemiological 

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 epidemiological 

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 

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 longterm 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

The available evidence examining the 

relationship between long-term PM2.5

exposure and cardiovascular morbidity 

has greatly expanded since the 2009 

ISA, with positive associations reported 

in several cohorts examining a range of 

cardiovascular outcomes (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, including 

cardiovascular morbidity (e.g., coronary 

heart disease, stroke) and 

atherosclerosis progression (e.g., 

coronary artery calcification) (U.S. EPA, 

2019, sections 6.2.2. to 6.2.9). 

Associations reported in such studies 

are supported by toxicologic 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 

epidemiological 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 

toxicologic 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 toxicologic studies 

demonstrating a relationship between 

long-term exposure to PM2.5 and 

consistent increases in blood pressure in 

rats and mice are coherent with 

epidemiological studies reporting 

positive associations between long-term 

exposure to PM2.5 and hypertension. 

Further, a recent animal toxicologic 

study also demonstrates increased 

plaque progression in mice following 

long-term exposure to PM2.5 and 

provides coherent results with 

epidemiological evidence reporting 

positive associations between long-term 

exposure to PM2.5 and indicators of 

atherosclerosis (U.S. EPA, 2019, section 

6.2.4.2). 

Longitudinal epidemiological 

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 toxicologic 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 epidemiological 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 epidemiological 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 epidemiological 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). 

Short-Term PM2.5 Exposures 

The 2009 ISA concluded that ‘‘a 

causal relationship exists between shortVerDate Sep<11>2014 22:36 Dec 17, 2020 Jkt 253001 PO 00000 Frm 00016 Fmt 4701 Sfmt 4700 E:\FR\FM\18DER2.SGM 18DER2

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39Some animal studies included in the 2009 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. 

term exposure to PM2.5 and 

cardiovascular effects’’ (U.S. EPA, 

2009c). The strongest evidence in the 

2009 ISA was from epidemiological 

studies of emergency department visits 

and hospital admissions for ischemic 

heart disease (IHD) and heart failure 

(HF), with supporting evidence from 

epidemiological studies of 

cardiovascular mortality (U.S. EPA, 

2009c). Animal toxicologic studies 

reported evidence of reduced 

myocardial blood flow during ischemia 

and studies indicating altered vascular 

reactivity (i.e., vascular function), which 

provided coherence and biological 

plausibility for the effects observed in 

epidemiological studies. In addition, 

both animal toxicologic and 

epidemiological panel studies reported 

effects of PM2.5 exposure on ST segment 

depression, an electrocardiogram 

change that potentially indicates 

ischemia.39 Key uncertainties from the 

last review included 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 ISA 

identified a growing body of evidence 

from controlled human exposure and 

animal toxicologic studies, uncertainties 

remained with respect to biological 

plausibility. 

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 

epidemiological 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) report 

positive associations between PM2.5

exposures and hospital admissions for 

HF (U.S. EPA, 2019, section 6.1.3.1). 

Single-city epidemiological studies 

contribute some support, though 

associations reported 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). 

In addition, a number of more recent 

controlled human exposure, animal 

toxicologic, and epidemiological panel 

studies provide evidence that PM2.5

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 (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 of 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 epidemiological studies 

in that they provide evidence that shortterm exposures to PM2.5 may result in 

the types of cardiovascular endpoints 

that could lead to emergency 

department visits and hospital 

admissions for IHD or HF. 

Animal toxicologic 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 epidemiological studies that 

report associations between short-term 

PM2.5 exposure and heart failure (U.S. 

EPA, 2019, section 6.1.3.3). In addition, 

similar to results of controlled human 

exposure studies, there is generally 

consistent evidence in animal 

toxicologic 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. 

Epidemiological studies reporting 

robust associations in copollutant 

models are supported by direct evidence 

from controlled human exposure and 

animal toxicologic 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). Epidemiological 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 epidemiological panel, controlled 

human exposure, and animal 

toxicologic 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 epidemiological 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). 

iii. Respiratory Effects 

Long-Term PM2.5 Exposures 

The 2009 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 

epidemiological 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 toxicologic study 

examining pre- and post-natal exposure 

to PM2.5 CAPs, which found impaired 

lung development. Epidemiological 

evidence for associations between longterm PM2.5 exposure and other 

respiratory outcomes, such as the 


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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 toxicologic 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). Epidemiological studies also 

examined 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 toxicologic 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 

epidemiological 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 

increased chronic bronchitis symptoms 

and long-term PM2.5 exposure (U.S. 

EPA, 2019, section 5.2.11). A common 

uncertainty across the epidemiological 

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 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). 

Short-Term PM2.5 Exposures 

The 2009 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 epidemiological evidence 

demonstrating positive associations 

with various respiratory effects. 

Specifically, the 2009 ISA described 

epidemiological 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 epidemiological 

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 

toxicologic 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 epidemiological 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 respiratoryrelated 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 

epidemiological studies demonstrating 

associations with other respiratoryrelated 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 epidemiological 

evidence for asthma exacerbation is 

more consistent in children than in 

adults. Additionally, epidemiological 

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 epidemiological 

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 toxicologic 

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 

toxicologic evidence for other 

respiratory effects is inconsistent and 

controlled human exposure studies 


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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 shortterm PM2.5 exposure on respiratory 

effects is provided by epidemiological 

studies of asthma and COPD 

exacerbation. While animal toxicologic 

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). 

iv. 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 epidemiological 

studies of lung cancer mortality. The 

few epidemiological 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. 

Toxicologic studies did not focus on 

exposures to specific PM size fractions, 

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), with limited 

evidence of cancer in other organ 

systems. 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 epidemiological 

studies using individual-level data to 

control for smoking status, in studies of 

people who have never smoked), 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). In addition, a subset of the 

studies focusing on lung cancer 

incidence also examined histological 

subtypes, 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). 

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 (nonaccidental) mortality (discussed above), 

fewer studies have examined the shape 

of the concentration-response curve for 

cause-specific mortality outcomes, 

including lung cancer. Several of these 

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 toxicologic 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 epidemiological 

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 

epidemiological 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). 

Epidemiological 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 

epidemiological studies conducted 

within the U.S. that examine long-term 


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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 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 were 

low and remained low) relative to other 

locations. Long-term exposure 

epidemiological 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 

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 

epidemiological 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 ISA. 

Therefore, upon re-evaluating the 

causality determination for cancer, 

when considering CASAC comments on 

the draft 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 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). 

v. 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 epidemiological 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 epidemiological 

studies (U.S. EPA, 2019, section 8.2.6). 

Additionally, loss of dopaminergic 

neurons in the substantia nigra, a 

hallmark of Parkinson’s disease, has 

been reported in mice following longterm PM2.5 exposures (U.S. EPA, 2019, 

section 8.2.4), though epidemiological 

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 

epidemiological 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. 

Epidemiological studies provided 

limited evidence of an association 

between PM2.5 exposure during 

pregnancy and childhood on cognitive 

and motor development (U.S. EPA, 

2019, section 8.2.5.2). While some 

studies report positive associations 

between long-term exposure to PM2.5

during the prenatal period and autism 

spectrum disorder (ASD) (U.S. EPA, 

2019, section 8.2.7.2). Interpretation of 

these epidemiological 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 

toxicologic 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 

epidemiological studies reporting 

consistent associations with cognitive 

decrements and with all-cause 

dementia. 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 toxicologic studies 

‘‘have largely been done by a single 

group,’’ and for epidemiological studies 

that examined brain volume that ‘‘brain 

volumes can vary . . . between normal 

people’’ and the results from studies of 

cognitive function were ‘‘largely nonstatistically significant’’ (Cox, 2019a, p. 

20 of consensus responses). 

With these concerns in mind, and as 

noted in the proposed rule (85 FR 

24114, April 30, 2020), the EPA reevaluated the evidence and note that 

animal toxicologic 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 


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40The other categories evaluated in the ISA 

include nervous system effects and short-term 

exposures; metabolic effects; reproduction and 

fertility; and pregnancy and birth outcomes (U.S. 

EPA, 2019, Table ES–1). 

animal toxicologic studies ‘‘are coherent 

with a number of epidemiological 

studies reporting consistent associations 

with cognitive decrements and with allcause 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.’’ 

Therefore, upon re-evaluating the 

causality determination, when 

considering the CASAC comments on 

the draft 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 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). 

vi. Other Effects 

For other categories of health effects 

and PM2.5 exposures,40 the currently 

available evidence is ‘‘suggestive of, but 

not sufficient to infer, a causal 

relationship,’’ mainly due to 

inconsistent evidence across specific 

outcomes and uncertainties regarding 

exposure measurement error, the 

potential for confounding, and potential 

modes of action (U.S. EPA, 2019, 

sections 7.1.4, 7.2.10, 8.1.6, and 9.1.5). 

These causality determinations are 

revised from ‘‘inadequate to infer a 

causal relationship’’ or not evaluated in 

the 2009 ISA this review; however, the 

‘‘suggestive of, but not sufficient to 

infer, a causal relationship’’ causality 

determinations reflect continued 

uncertainties in the evidence. 

b. At-Risk Populations 

In this review, we use the term ‘‘atrisk populations’’ to describe 

populations with a quality or 

characteristic in common (e.g., a 

specific pre-existing illness or specific 

lifestage) that contributes to them 

having a greater likelihood of 

experiencing PM2.5-related health 

effects. In the current review, consistent 

with the last 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 preexisting 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 nonHispanic 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 greater 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 for 

additional public health improvements 

in these populations. 

c. Evidence-Based Considerations 

The sections below summarize the 

PA’s evaluation of the PM2.5 exposure 

concentrations that have been examined 

in controlled human exposure studies, 

animal toxicology studies, and 

epidemiological studies. 

i. PM2.5 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, p. 20). Two types of 

experimental studies are of particular 

importance in understanding the effects 

of PM exposures: Controlled human 

exposure and animal toxicologic 

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, 

p. 11). 

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). 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 mg/ 

m3, with exposure durations typically 

up to about two hours. Statistically 

significant effects on one or more 

indicators of cardiovascular function are 

often, though not always, reported 

following 2-hour exposures to average 

PM2.5 concentrations at and above about 

120 mg/m3, 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 


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exposures to PM2.5 concentrations at 

and above 149 mg/m3. 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 mg/m3, 

and they almost never exceed 32 mg/m3. 

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 mg/m3

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). 

With respect to 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 shortterm exposures to PM2.5 concentrations 

from 100 to >1,000 mg/m3 and long-term 

exposures to concentrations from 66 to 

>400 mg/m3 (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 mg/m3

(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 mg/m3 (Cangerana 

Pereira et al., 2011). These two studies 

report serious effects following longterm exposures to PM2.5 concentrations 

close to the ambient concentrations 

reported in some PM2.5 epidemiological 

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 Epidemiological Studies 

As summarized above in section 

II.A.2.a, epidemiological 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 

considers what information from these 

epidemiological studies may indicate 

regarding primary PM2.5 standards. The 

use of information from epidemiological 

studies to inform conclusions on the 

primary PM2.5 standards is complicated 

by the fact that such studies evaluate 

associations between distributions of 

ambient PM2.5 and health outcomes, but 

do not identify the specific exposures 

that cause reported effects. Rather, 

health effects can occur over the entire 

distributions of ambient PM2.5

concentrations evaluated, and 

epidemiological studies do not identify 

a population-level threshold below 

which it can be concluded with 

confidence that PM-associated health 

effects do not occur (U.S. EPA, 2020, 

section 3.2.3.2). Therefore, the PA 

evaluates the PM2.5 air quality 

distributions over which 

epidemiological studies support health 

effect associations. As discussed further 

in the PA (U.S. EPA, 2020, section 

3.2.3.2.1), studies of daily PM2.5

exposures examine associations 

between day-to-day variation in PM2.5

concentrations and health outcomes, 

often over several years. While there can 

be considerable variability in daily 

exposures over a multi-year study 

period, most of the estimated exposures 

reflect days with ambient PM2.5

concentrations around the middle of the 

air quality distributions examined (i.e., 

‘‘typical’’ days rather than days with 

extremely high or extremely low 

concentrations). Similarly, for studies of 

annual PM2.5 exposures, most of the 

estimated exposures reflect annual 

average PM2.5 concentrations around the 

middle of the air quality distributions 

examined. In both cases, 

epidemiological studies provide the 

strongest support for reported health 

effect associations for this middle 

portion of the PM2.5 air quality 

distribution, which corresponds to the 

bulk of the underlying data, rather than 

the extreme upper or lower ends of the 

distribution. Consistent with this, and 

as noted in the PA (U.S. EPA, 2020, 

section 3.2.1.1), several epidemiological 

studies report that associations persist 

in analyses that exclude the upper 

portions of the distributions of 

estimated PM2.5 exposures, indicating 

that ‘‘peak’’ PM2.5 exposures are not 

disproportionately responsible for 

reported health effect associations. 

Thus, in considering PM2.5 air quality 

data from epidemiological 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. In Figure 3–7, the PA 

highlights the overall mean (or median) 

PM2.5 concentrations reported in key 

U.S. and Canadian epidemiological 

studies that use ground-based monitors 

alone to estimate long- or short-term 

PM2.5 exposures. In Figure 3–8, the PA 

also considers the emerging body of 

studies that use hybrid modeling 

methods to estimate long- or short-term 

PM2.5 exposures. Hybrid methods 

incorporate data from several sources, 

often including satellites and models, in 

addition to ground-based monitors. 

Epidemiological studies using hybrid 

methods are generally new in this 

review. These modeling methods have 

improved the ability to estimate PM2.5

exposure for populations throughout the 

conterminous U.S. compared with the 

earlier approaches based on monitoring 

data alone. Excellent performance in 

cross-validation tests suggests that 

hybrid methods are reliable for 

estimating PM2.5 exposure in many 

applications. As discussed in Chapter 3 

of the PA, good agreement in health 

study results between monitor- and 

model-based methods for urban areas 

(McGuinn et al., 2017) and general 

consistency in results for the 

conterminous U.S. (Jerrett et al., 2017; 

Di et al., 2016) also suggests that the 

fields are reliable for use in health 

studies. However, there are also 

important limitations associated with 

the modeled fields that should be kept 

in mind. First, performance evaluations 


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41For 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). 

for the methods are weighted toward 

densely monitored urban areas at the 

scales of representation of the 

monitoring networks. Predictions at 

different scales or in sparsely monitored 

areas are relatively untested. Second, 

studies have reported heterogeneity in 

performance with relatively weak 

performance in parts of the western 

U.S., at low concentrations, at greater 

distance to monitors, and under 

conditions where the reliability and 

availability of key input datasets (e.g., 

satellite retrievals and air quality 

modeling) are limited. Lastly, 

differences in predictions among 

different hybrid methods have also been 

reported and tend to be most important 

under conditions with the performance 

issues just noted. Differences in 

predictions can be related to the 

different approaches used to create longterm PM2.5 fields (e.g., averaging daily 

PM2.5 fields vs. developing long-term 

average fields), which can be impacted 

by variability in monitoring schedules, 

and the spatial scale at which these 

fields are created. Future work to further 

characterize the performance of 

modeled fields will be useful in 

informing our understanding of the 

implications of using these fields to 

estimate PM2.5 exposures in health 

studies (U.S. EPA, 2020, section 

2.3.3.1.4). 

In assessing how the overall mean (or 

median) PM2.5 concentrations reported 

in key epidemiological studies can 

inform conclusions on the primary 

PM2.5 standards, there are some 

important considerations. As noted in 

the PA, study-reported PM2.5

concentrations 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 or violate the PM NAAQS (U.S. 

EPA, 2020, section 3.2.3.2.1). 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. 

In the context of epidemiological 

studies that use ground-based monitors, 

analyses of recent air quality in U.S. 

CBSAs indicate that maximum annual 

PM2.5 design values for a given threeyear period are often 10% to 20% higher 

than average monitored concentrations 

(i.e., averaged across multiple monitors 

in the same CBSA) (U.S. EPA, 2020, 

Appendix B, section B.7). This 

comparison is more difficult for 

epidemiological studies that use hybrid 

methods. To try to address this issue, 

the PA also considered a second 

approach to evaluating information from 

epidemiological studies. In this 

approach, the PA calculated 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 considered 

the degree to which such metrics 

indicate that study area air quality 

would likely have met or violated the 

current standards during study periods. 

This approach was generally not well 

received by commenters during the 

review of the PA. 

3. Overview of Risk and Exposure 

Assessment Information 

Beyond the consideration of the 

scientific evidence, discussed above in 

section II.A.2, the EPA also considers 

the extent to which new or updated 

quantitative analyses of PM2.5 air 

quality, exposure, or health risks could 

inform conclusions on the adequacy of 

the public health protection provided by 

the current primary PM2.5 standards. 

Conducting such quantitative analyses, 

if appropriate, could inform judgments 

about the potential for additional public 

health improvements associated with 

PM2.5 exposure and related health 

effects and could help to place the 

evidence for specific effects into a 

broader public health context. 

To this end, the PA includes a risk 

assessment that estimates populationlevel 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 epidemiological studies 

with model-based PM2.5 air quality 

surfaces, baseline health incidence data, 

and population demographics for 47 

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 exposurerelated deaths from ischemic heart 

disease in a single year (i.e., confidence 

intervals range from about 12,000 to 

21,000 deaths).41 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 mg/m3, 14 to 18% for 

a level of 10.0 mg/m3, and 21 to 27% for 

a level of 9.0 mg/m3. 

Uncertainty in risk estimates (e.g., in 

the size of risk estimates) can result 

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 

epidemiological 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). As detailed 

further below in II.B.1, some members 

of CASAC advised that the risk 

assessment estimates did not provide 

useful information about whether the 

current standard is protective, while 

other members thought they were useful 

to understand potential impacts of 

alternative standards. 

B. Conclusions on the Primary PM2.5

Standards 

In drawing conclusions on the 

adequacy of the current primary PM2.5

standards, in view of the advances in 

scientific knowledge and additional 

information now available, the 

Administrator has considered the 

evidence base, information, and policy 

judgments that were the foundation of 

the last review and reflects upon the 

body of evidence and information newly 

available in this review. In so doing, he 

considered the large body of evidence 

presented and assessed in the ISA (U.S. 


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42The CASAC also provided advice on the draft 

ISA’s assessment of the scientific evidence (Cox, 

2019b). That advice, and the resulting changes 

made in the final ISA and final PA, are summarized 

in section II.B.3 of the proposal (85 FR 24114, April 

30, 2020). 

EPA, 2019), the policy-relevant and riskbased conclusions and rationales as 

presented in the PA (U.S. EPA, 2020), 

views expressed by CASAC, and public 

comments. The Administrator has taken 

into account both evidence- and riskbased considerations in developing final 

conclusions on the adequacy of the 

current primary PM2.5 standards. 

Evidence-based considerations include 

the assessment of epidemiological, 

animal toxicologic, and controlled 

human exposure studies evaluating 

long- or short-term exposures to PM2.5

and the integration of evidence across 

each of these disciplines. These 

considerations, as assessed in the ISA 

(U.S. EPA, 2019), focus on the policyrelevant considerations, as discussed in 

II.A.2 above and in the PA (U.S. EPA, 

2020, section 3.2.1). Risk-based 

considerations draw from the results of 

the quantitative analyses and policyrelevant considerations as discussed in 

II.A.3 above and in the PA (U.S. EPA, 

2020, section 3.3.2). 

Section II.B.1 summarizes the advice 

and recommendations of the CASAC. 

Section II.B.2 below summarizes the 

basis for the Administrator’s proposed 

decision, drawing from section II.C.3 of 

the proposal, and section II.B.3 

addresses public comments on the 

proposed decision. The Administrator’s 

conclusions in this review regarding the 

adequacy of the current primary 

standard and whether any revisions are 

appropriate are described in section 

II.B.4. 

1. CASAC Advice in This Review 

With regard to the process for 

reviewing the PM NAAQS, the CASAC 

requested the opportunity to review a 

second 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.D, the 

Agency’s responses to these 

recommendations were described in a 

letter from the Administrator to the 

CASAC chair (Wheeler, 2019). 

As part of its review of the draft PA, 

the CASAC provided advice on the 

adequacy of the public health protection 

afforded by the current primary PM2.5

standards.42 Its advice is documented in 

a letter sent to the EPA Administrator 

(Cox, 2019a). In this letter, the 

committee recommended retaining the 

current 24-hour PM2.5 standard but did 

not reach consensus on whether the 

scientific and technical information 

support retaining or revising the current 

annual standard. In particular, though 

the CASAC agreed that there is a longstanding body of health evidence 

supporting relationships between PM2.5

exposures and various health outcomes, 

including mortality and serious 

morbidity effects, individual CASAC 

members ‘‘differ[ed] 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’’ expressed support 

for retaining the current annual 

standard while ‘‘other members’’ 

expressed support for revising that 

standard in order to increase public 

health protection (Cox, 2019a, p.1 of 

letter). These views are summarized 

below. 

The CASAC members who supported 

retaining the current annual standard 

expressed the view that substantial 

uncertainty remains in the evidence for 

associations between PM2.5 exposures 

and mortality or serious morbidity 

effects. These committee members 

asserted 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 noted that associations 

do not necessarily reflect causal effects, 

and they contended that recent 

epidemiological 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 concluded 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 asserted 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 cited 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 supported 

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 

concluded that the PM2.5 risk 

assessment does not provide a valid 

basis for revising the current standards. 

This conclusion was 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 contended that details 

regarding the derivation of 

concentration-response functions, 

including specification of the beta 

values and functional forms, were 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 concluded 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[d] 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 supported this 

conclusion noted that the body of health 

evidence for PM2.5 not only includes the 

repeated demonstration of associations 

in epidemiological studies, but also 

includes support for biological 


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plausibility established by controlled 

human exposure and animal toxicology 

studies. They pointed 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 concluded 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 

supported revising the current annual 

standard particularly emphasized recent 

findings of associations with PM2.5 in 

areas with average long-term PM2.5

concentrations below the level of the 

annual standard and studies that show 

positive associations even when 

estimated exposures above 12 mg/m3 are 

excluded from analyses. They found 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 

concluded 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 concluded 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). 

2. Basis for Proposed Decision 

On April 14, 2020, the Administrator 

proposed to retain the current primary 

PM2.5 standards. This proposal was 

published in the Federal Register on 

April 30, 2020 (85 FR 24094, April 30, 

2020). In reaching his proposed decision 

to retain the current PM2.5 standards 

(i.e., annual and 24-hour PM2.5

standards), the Administrator 

considered the assessment of the 

available evidence and conclusions 

reached in the ISA (U.S. EPA, 2019); the 

analyses in the PA (U.S. EPA, 2020), 

including uncertainties in the evidence 

and analyses; and the advice and 

recommendations from the CASAC. 

These considerations are summarized 

briefly below and discussed in detail in 

the proposal notice (85 FR 24094, April 

30, 2020). 

As described further in section II.A.2 

of the proposal, the Administrator’s 

consideration of the public health 

protection provided by the current 

primary PM2.5 standards were 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 mg/m3, 35 mg/m3) 

of those standards. 

The Administrator’s proposed 

decision noted that one of the 

methodological limitations highlighted 

by the CASAC members who support 

retaining the annual standard (see 

section II.B.1 above) is that associations 

reported in epidemiological 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). In the proposed 

decision, the Administrator recognized 

that epidemiological 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, as noted in 

the PA (U.S. EPA, 2020, section 3.1.2). 

The Administrator’s proposed decision 

noted that experimental studies do 

provide evidence for health effects 

following particular PM2.5 exposures 

under carefully controlled laboratory 

conditions and further notes that the 

evidence for a given PM2.5-related health 

outcome is strengthened when results 

from experimental studies demonstrate 

biologically plausibility mechanisms 

through which such an outcome could 

occur. In the proposed decision, 

therefore, the Administrator expressed 

greatest confidence 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 epidemiological 

studies. 

In the proposed decision, in light of 

this approach to considering the 

evidence, the Administrator recognized 

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 epidemiological studies 

(including mortality), though he noted 

that 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). The Administrator was 

cautious about placing too much weight 

on reported PM2.5 health effect 

associations for air quality meeting the 

current annual and 24-hour standards. 

He concluded in the proposed decision 

that such associations alone, without 

supporting experimental evidence at 

similar PM2.5 considerations, left 

important questions unanswered 

regarding the degree to which the 

typical PM2.5 exposures likely to occur 

in areas meeting the current standard 

could cause the mortality and morbidity 

outcomes reported in epidemiological 

studies. Given this concern, the 

Administrator noted in the proposal that 

he did not think that recent 

epidemiological 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 

judged that the overall body of 

evidence, including controlled human 

exposure and animal toxicologic 

studies, in addition to epidemiological 

studies, indicated continuing 

uncertainty in the degree to which 

adverse effects could result from PM2.5

exposure in areas meeting the current 

annual and 24-hour standards. 

The Administrator also considered 

the emerging body of evidence from 

accountability studies examining past 

reductions in ambient PM2.5, and the 

degree to which those reductions 

resulted in public health improvements, 

but also recognized that interpreting 

such studies in the context of the 

current primary PM2.5 standards was 

complicated by the fact that some of the 

available accountability studies have 

not evaluated PM2.5 specifically, did not 

show changes in PM2.5 air quality, or 

have not been able to disentangled 

health impacts of the interventions from 

background trends in health. The 

Administrator also recognized 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 standard. Together with the 

Administrator’s concerns regarding the 

lack of experimental studies examining 

PM2.5 exposures typical of areas meeting 

the current standards, the lack of 

demonstrated health improvements in 

areas with air quality meeting the 

current standards led him to conclude, 

at the time of proposal, that there was 

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 also considered the 

potential implications of the risk 

assessment for his proposed decision, 

noting that all risk assessments have 

limitations. He noted that such 

limitations in risk estimates can result 


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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 epidemiological study. The 

Administrator noted agreement with 

some members of the CASAC who 

expressed concerns regarding 

limitations in the epidemiological 

evidence, which provides key inputs to 

the risk assessment. Thus, he judged it 

appropriate to place little weight on 

quantitative estimates of PM2.5- 

associated mortality risk in reaching 

proposed conclusions on the primary 

PM2.5 standards. 

In reaching his proposed decision to 

retain the current primary PM2.5

standards, the Administrator concluded 

that the scientific evidence assessed in 

the ISA (U.S. EPA, 2019), and the 

analyses based on that evidence in the 

PA (U.S. EPA, 2020), do not call into 

question the public health protection 

provided by the current annual and 24- 

hour PM2.5 standards. In particular, the 

Administrator judged that there is 

considerable uncertainty in the 

potential for additional public health 

improvements from reducing 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 judged that 

such standards would be more than 

requisite to protect the public health 

with an adequate margin of safety. This 

judgment reflected his consideration of 

the uncertainties in the potential 

implications of recent epidemiological 

studies due in part to the lack of 

supporting evidence from experimental 

studies and accountability studies 

conducted at PM2.5 concentrations 

meeting the current standards. 

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 judged 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, led the Administrator to 

conclude that standards less stringent 

than the current standards (e.g., with 

higher levels) are also not supported. 

Thus, based on his consideration of 

the available scientific evidence and 

technical information and his 

consideration of advice from the 

CASAC, the Administrator proposed 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 mg/m3, 35 mg/m3), remain requisite 

to protect the public health. As 

discussed in detail in the proposal (85 

FR 24094, April 30, 2020), this proposed 

conclusion reflected his judgment that 

limitations in the science lead to 

considerable uncertainty regarding the 

potential public health implications of 

revising the existing suite of PM2.5

standards. Therefore, the Administrator 

proposed to retain the current 

standards, without revision. 

3. Comments on the Proposed Decision 

Overall, the EPA received a large 

number of unique public comments on 

the proposed decision to retain the 

annual and 24-hour PM2.5 standards. 

These comments generally fall into one 

of two broad groups that expressed 

sharply divergent views. The first group 

is comprised of the many commenters, 

representing industries and industry 

groups, some state and local 

governments, and independent 

organizations, that support the 

Administrator’s proposed decision to 

retain the primary PM2.5 standards. The 

second group of commenters are those 

who asserted that the current primary 

PM2.5 standards are not sufficient to 

protect public health with an adequate 

margin of safety. These commenters 

disagree with the EPA’s proposed 

decision to retain the current PM2.5

standards and generally recommend a 

revised annual standard of between 8– 

10 mg/m3 and a revised 24-hour 

standard between a range of 25–30 mg/ 

m3. Among those calling for revisions to 

the current primary PM2.5 standards 

were commenters representing national 

public health, medical, and 

environmental nongovernmental 

organization, tribes and tribal groups, 

some state and local governments and 

independent organizations and 

individuals. 

We address the key public comments 

received on the proposal (85 FR 24094, 

April 30, 2020) and present the EPA’s 

responses to those comments below. A 

more detailed summary of all significant 

comments, along with the EPA’s 

responses (henceforth ‘‘Response to 

Comments’’), can be found in the docket 

for this rulemaking (Docket No. EPA– 

HQ–OAR–2015–0072). This document 

is available for review in the docket for 

this rulemaking and through the EPA’s 

NAAQS website (https://www.epa.gov/ 

naaqs/particulate-matter-pm-airquality-standards). 

With respect to the various elements 

of the standards, the EPA received very 

few comments related to indicator and 

none advocate for revising the current 

PM2.5 indicator for fine particles. Those 

who express explicit support for 

retaining the current PM2.5 indicator 

generally endorse the rationale put 

forward in the PA. The EPA agrees with 

these commenters, noting that the 

scientific evidence in this review, as in 

the last review, continues to provide 

strong support for health effects 

following short- and long-term PM2.5

exposures and that the available 

information remains too limited to 

support a distinct standard for any 

specific PM2.5 component or group of 

components or to support a distinct 

standard for the ultrafine fraction. 

The EPA also received very few 

comments on averaging time and form. 

Those who did provide comments are 

mostly affiliated with public health 

organizations and environmental 

advocacy groups and generally discuss 

the need for future evaluation of the 

form and averaging time of the current 

24-hour standard (98th percentile, 

averaged over three years). These 

commenters, acknowledging the current 

limitations and uncertainties in the 

available evidence, suggest that in 

future reviews the EPA should evaluate 

how well the current form of the 24- 

hour standard protects against potential 

sub-daily exposures based on new 

epidemiological and experimental 

evidence that considers sub-daily 

exposures, but these commenters 

support retaining the current indicators, 

averaging times, and forms. 

The EPA acknowledges the comments 

related to averaging time and form of the 

24-hour standard and agrees that the 

current information does not support a 

revision to the averaging time or form. 

The EPA will continue to evaluate the 

form and averaging time of the current 

24-hour standard in future reviews 

based on any new relevant information. 

With respect to the level of the 24- 

hour standard, commenters supporting 

revision generally support a revised 

level in the range of 25–30 mg/m3. They 

contend the available scientific 

evidence supports that lower levels 

within this range are required to protect 

public health, including the health of atrisk populations, with an adequate 

margin of safety, and that lower levels 

within this range will provide 

additional margin of safety. The 

commenters cite controlled human 

exposure studies that assess short-term 

exposures (i.e., 2 to 5 hours) and 

epidemiological studies that report 


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associations between adverse health 

effects and concentrations below the 

current standard level as supporting the 

need for this revision. They further add 

that while revising the 24-hour level to 

25 mg/m3 would offer more health 

protection than 30 mg/m3, it would still 

not reduce the risk of adverse health 

outcomes to zero. 

With respect to the level of the annual 

PM2.5 standard, numerous comments 

were received that specifically focus on 

the Administrator’s consideration of 

epidemiological evidence in this review. 

Commenters who support revision 

generally disagree with the 

Administrator’s conclusions and 

judgments about the uncertainties in the 

epidemiological evidence and suggest 

that these studies support revision of 

the PM2.5 annual standard to a level of 

8–10 mg/m3. These commenters state 

that uncertainties in the epidemiological 

studies, alone, do not negate positive 

associations seen in studies using 

diverse study designs and capturing 

large geographic and population 

domains. These commenters note that 

the possibility of confounders and the 

other referenced uncertainties have been 

investigated and found not to be 

material given the overall strength and 

consistency of results from varying 

approaches. The commenters who 

support revising the primary PM2.5

standards generally place substantial 

weight on epidemiologic evidence from 

multi-city U.S. and Canadian studies 

that captured a larger geographic 

domain and population size, and were 

included in the ISA and in the studyrelated analyses in the PA (U.S. EPA, 

2020). Further, they also cite 

epidemiological studies in the ISA (U.S. 

EPA, 2019) that performed restricted/ 

truncated analyses with populations 

living in areas of lower PM2.5

concentrations and contend that 

associations still exist in these studies at 

the concentrations below the levels of 

the current annual and daily standards. 

Moreover, they state that there was no 

evidence for an ambient concentration 

threshold for adverse health effects at 

the lowest observed levels of either 

annual or 24-hour PM2.5 concentrations. 

The EPA disagrees with these 

commenters. First, the EPA notes that, 

consistent with past practices, the 

foremost consideration is the adequacy 

of the public health protection as 

provided by the combination of the 

annual and 24-hour standards together. 

The annual standard limits ‘‘typical’’ 

daily PM2.5 concentrations that make up 

the bulk of the distribution, while the 

24-hour standard adds supplemental 

protection against ‘‘peak’’ daily PM2.5

concentrations. In the judgment of the 

Administrator, therefore, the current 

annual standard (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, while 

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). Further, the 

Administrator notes that changes in 

PM2.5 air quality to meet an annual 

standard would likely result not only in 

lower short- and long-term PM2.5

concentrations near the middle of the 

air quality distribution, but also in fewer 

and lower short-term peak PM2.5

concentrations. Similarly, the 

Administrator recognizes that changes 

in air quality to meet a 24-hour 

standard, would result not only in fewer 

and lower peak 24-hour PM2.5

concentrations, but also in lower annual 

average PM2.5 concentrations. 

Thus, in considering the adequacy of 

the 24-hour standard, an important 

consideration is whether additional 

protection is needed against short-term 

exposures to peak PM2.5 concentrations. 

In examining the scientific evidence, the 

EPA notes that controlled human 

exposure studies do provide evidence 

for health effects following single, shortterm PM2.5 exposures to concentrations. 

These types of exposures correspond 

best to those to ambient exposures that 

might be experienced in the upper end 

of the PM2.5 air quality distribution in 

the U.S. (i.e., ‘‘peak’’ concentrations). 

However, and as noted above in section 

II.A.2.c.i, most of these studies examine 

exposure concentrations considerably 

higher than are typically measured in 

areas meeting the current standards 

(U.S. EPA, 2020, section 3.2.3.1). In 

particular, controlled human exposure 

studies often report statistically 

significant effects on one or more 

indicators of cardiovascular function 

following 2-hour exposures to PM2.5

concentrations at and above 120 mg/m3

(at and above 149 mg/m3 for vascular 

impairment, the effect shown to be most 

consistent across studies). Commenters 

did specifically note one study 

(Hemmingsen et al., 2015b) and contend 

that this study shows significant effects 

on some outcomes at lower 

concentrations, following 5-hour 

exposures to 24 mg/m3. The PA notes 

that this study does not report effects 

consistent with other studies in the ISA 

that evaluate longer exposure durations 

(i.e., longer than 2 hours) and lower 

PM2.5 concentrations (e.g., Bra ̈uner et 

al., 2008 and Hemmingsen et al., 2015a). 

Furthermore, analyses in the PA show 

that the exposure concentrations 

included in this study are not observed 

in areas meeting the current standards 

(U.S. EPA, 2020, Figure A–2), suggesting 

that the current standards provide 

protection against these exposure 

concentrations. To provide insight into 

what these studies may indicate 

regarding the primary PM2.5 standards, 

the PA (U.S. EPA, 2020, p.3–49) notes 

that 2-hour ambient concentrations of 

PM2.5 at monitoring sites meeting the 

current standards almost never exceed 

32 mg/m3. In fact, even the extreme 

upper end of the distribution of 2-hour 

PM2.5 concentrations at sites meeting the 

current standards remains well-below 

the PM2.5 exposure concentrations 

consistently shown in controlled human 

exposure studies to elicit effects (i.e., 

99.9th percentile of 2-hour 

concentrations at these sites is 68 mg/m3

during the warm season). Thus, 

available PM2.5 controlled human 

exposure studies do not indicate the 

need for additional protection against 

exposures to peak PM2.5 concentrations, 

beyond the protection provided by the 

combination of the current 24-hour 

standard and the current annual 

standard (U.S. EPA, 2020, section 

3.2.3.1). With respect to the 

epidemiological evidence and as noted 

above in section II.A.2.c.ii, the 

information from such studies is most 

applicable to examining potential health 

impacts associated with typical (i.e., 

average or mean) exposures and thus are 

most applicable in informing decisions 

on the annual standard (with its 

arithmetic mean form). Furthermore, as 

noted above, the available 

epidemiological studies do not indicate 

that associations in these studies are 

strongly influenced by exposures to 

peak concentrations in the air quality 

distribution, and thus do not indicate 

the need for additional protection 

against short-term exposures to peak 

PM2.5 concentrations. As discussed 

above, the annual standard provides 

protection against the typical 24-hour 

and annual PM2.5 exposures. Thus, in 

the context of a 24-hour standard that is 

meant to provide supplemental 

protection (i.e., beyond that provided by 

the annual standard alone) against 

short-term exposures to peak PM2.5

concentrations, the available evidence 

supports the Administrator’s proposed 

conclusion to retain the current 24-hour 

standard with its level of 35 mg/m3. 

With respect to commenters that 

support revision of the annual standard, 


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43Studies included were multi-city studies in 

Canada and the U.S. that examined health 

endpoints with ‘causal’ or ‘likely to be causal’ 

determinations in the ISA. 

44A design value is a statistic that summarizes 

the air quality data for a given area in terms of the 

indicator, averaging time, and form of the standard. 

Design values can be compared to the level of the 

standard and are typically used to designate areas 

as meeting or not meeting the standard and assess 

progress towards meeting the NAAQS. 

45For 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 the 24-hour 

standard, design values are calculated as the 98th 

percentile of the annual distribution of the 24-hour 

PM2.5 concentrations, averaged over three years. 

46Some epidemiological studies report median 

versus mean air quality concentrations offering that 

the EPA recognizes that there are a large 

number of studies, many of which 

include a variety of study populations 

and geographic locations, that show 

positive associations between mortality 

and morbidity and short-term and longterm PM2.5 exposure. Furthermore, the 

EPA recognizes that while uncertainties 

exist, when the epidemiological 

evidence is viewed together in the 

context of the full body of evidence, the 

scientific information supports that 

exposure to PM2.5 may cause adverse 

health effects (U.S.EPA, 2019, section 

1.7.3, Table 1–4). Therefore, the EPA 

does not dispute commenters that note 

epidemiological studies support the 

conclusion that exposure to PM2.5 is 

associated with morbidity and 

mortality. 

However, while the epidemiological 

evidence when considered together with 

the full body of evidence supports 

health effects associated with PM2.5

exposure, the EPA recognizes that 

important uncertainties and limitations 

in the health effects evidence remain. 

Epidemiological studies evaluating 

health effects associated with long- and 

short-term PM2.5 exposures have 

reported heterogeneity in associations 

between cities and geographic regions 

within the U.S. Heterogeneity in the 

associations observed across PM2.5

epidemiological studies may be due in 

part to exposure error related to 

measurement-related issues, the use of 

central fixed-site monitors to represent 

population exposure to PM2.5, models 

used in lieu of or to supplement 

ambient measurements, limitations in 

hybrid models and our limited 

understanding of factors that may 

influence exposures (e.g., topography, 

the built environment, weather, source 

characteristics, ventilation usage, 

personal activity patterns, 

photochemistry) (U.S. EPA, 2020, p.3– 

25), all of which can introduce bias and/ 

or increased uncertainty is associated 

health effects estimates. Heterogeneity is 

expected when the methods or 

underlying distribution of covariates 

vary across studies (U.S. EPA, 2019, p. 

6–221). In addition, where PM2.5 and 

other pollutants (e.g., ozone, nitrogen 

dioxide, and carbon monoxide) are 

correlated, it can be difficult to 

distinguish whether attenuation of 

effects in some studies results from 

copollutant confounding or collinearity 

with other pollutants in the ambient 

mixture (U.S. EPA, 2019, section 1.5.1). 

The EPA also recognizes that 

methodological study designs to address 

confounding, such as causal inference 

methods, are an emerging field of study 

(U.S. EPA, 2019, section 11.2.2.4 or U.S. 

EPA, 2020, p. 3–24). The Administrator 

weighs these uncertainties in the 

reported associations of PM2.5

concentrations in the studies and 

considers them in the context of the 

entire body of evidence before the 

Agency when reviewing the standards. 

Additionally, while epidemiological 

studies indicate associations between 

exposure to PM2.5 and health effects, 

they do not identify particular PM2.5

exposures that cause effects (section 

II.A.2.c.ii above and U.S. EPA, 2020, 

section 3.1.2). Further, using 

information from epidemiological 

studies to inform decisions on PM2.5

standards is complicated by the 

recognition that no population 

threshold, below which it can be 

concluded with confidence that PM2.5- 

related effects do not occur, can be 

discerned from the available evidence. 

As a result, any general approach to 

reaching decisions on what standards 

are appropriate necessarily requires 

judgments about how to translate the 

information available from the 

epidemiological studies into a basis for 

appropriate standards. This includes 

consideration of how to weigh the 

uncertainties in the reported 

associations in the epidemiological 

studies and the uncertainties in 

quantitative estimates of risk, in the 

context of the entire body of evidence 

before the Agency. Such approaches are 

consistent with setting standards that 

are neither more nor less stringent than 

necessary, recognizing that a zero-risk 

standard is not required by the CAA. 

Commenters who support revising the 

PM2.5 standards further contend that the 

Administrator has arbitrarily rejected an 

established practice of relying on 

epidemiological studies and of setting 

the standard below the long-term mean 

PM2.5 concentrations reported in each of 

the studies that provide evidence of an 

array of serious health effects. The 

commenters state that in declaring that 

the latest epidemiological studies 

cannot justify a decision to strengthen 

the PM NAAQS, the Administrator has 

rejected—without acknowledgment or 

explanation—the EPA’s long history of 

relying on such research as the basis for 

its primary standards. 

As recognized in this and previous 

PM NAAQS reviews, including those 

completed in 2006 and 2012, evidence 

of an association in any epidemiological 

study is ‘‘strongest at and around the 

long-term average where the data in the 

study are most concentrated.’’ In the PA 

(U.S. EPA, 2020, section 3.2.3.2.1), the 

EPA assessed air quality distributions 

reported in key epidemiological studies 

included in the ISA, with a focus on 

characterizing the long-term average or 

mean PM2.5 concentrations. In doing 

this, key studies 43 were identified that 

examined short- and long-term exposure 

and showed positive associations with 

either mortality or morbidity health 

outcomes. The studies either estimated 

PM2.5 exposure using ground-based 

monitored data or using hybrid 

modeling data, which incorporate data 

from several sources, often including 

satellites and models, as well as groundbased monitors (U.S. EPA, 2020, section 

2.3.3). The PA notes some important 

considerations in using study reported 

concentrations to inform conclusions on 

the primary PM2.5 standards. In 

particular, it notes that the overall mean 

PM2.5 concentrations reported by key 

epidemiological studies are not the 

same as the ambient concentrations 

used by the EPA to determine whether 

areas meet or violate the PM NAAQS. 

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 PM NAAQS, the EPA 

measures air pollution concentrations at 

individual monitors (i.e., concentrations 

are not averaged across monitors) and 

calculates design values 44 at monitors 

meeting appropriate data quality and 

completeness criteria.45 For an area to 

meet the NAAQS, all valid design 

values in that area, including the 

highest annual and highest 24-hour 

monitoring values, must be at or below 

the standards. As a result, study 

reported mean concentration values are 

generally lower than the design value of 

the highest monitor in an area, which 

determines compliance. 

The PA first presents results from key 

epidemiological studies that used 

ground-based monitoring data to 

estimate population exposure (U.S. 

EPA, 2020, section 3.2.3.2.1). Study 

reported mean (or medians) 46 were 


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median is a better metric since it is less skewed by 

outlying concentrations. In most studies, the mean 

and median concentrations are very similar and are 

generally used here interchangeably. 

47Some epidemiological studies report median 

versus mean air quality concentrations offering that 

median is a better metric since it is less skewed by 

outlying concentrations. In most studies, the mean 

and median concentrations are very similar and are 

generally used here interchangeably. 

48 Given how air quality monitors in other 

countries differ from the U.S. EPA FRM monitors 

discussed here, a focus on U.S. studies ensures that 

the results most closely compare to the data being 

used for calculating the design values and for 

compliance of the standard. 

49We note that in this study the population was 

divided into regions of the country, with 

statistically significant associations in the Central 

and Eastern Regions and with median long-term 

PM2.5 concentrations of: Central: 10.7 mg/m3; 

Western: 13.1 mg/m3 and Eastern: 14.0 mg/m3. 

50The median of the study reported mean (or 

median) PM2.5 concentrations is 13.3 mg/m3. 

51Recent air quality in U.S. CBSAs in the PA 

indicate that maximum annual PM2.5 design values 

Continued 

examined from the air quality 

distributions reported in key 

epidemiological studies included in the 

ISA exposures (U.S. EPA, 2020, Figure 

3–7). The PA noted that these values are 

most useful in the context of 

considering the level of the primary 

PM2.5 annual standard. This is because 

the mean concentration values from 

these studies, which include studies 

examining both short- and long-term 

exposures, represent ‘‘typical’’ or mean 

exposures, which are most relevant to 

the form and averaging time of the 

annual standard, and not as relevant to 

the daily standard, whose form and 

averaging time focuses on protecting 

against peak concentrations. Further, 

the PA noted that in using these data it 

should be recognized that these mean 

concentrations are generally below the 

design values in the corresponding 

areas. In fact, analyses included in the 

PA of recent air quality in U.S. CBSAs 

indicate that maximum annual PM2.5

design values for a given three-year 

period are often 10% to 20% higher 

than average monitored concentrations 

(i.e., averaged across multiple monitors 

in the same CBSA) (U.S. EPA, 2020, 

Appendix B, section B.7). As noted in 

the PA, the difference between the 

maximum annual design value and the 

average concentrations in an area will 

depend on a number of factors 

including the numbers of monitors, 

monitor citing characteristics, and the 

distribution of ambient PM2.5

concentrations. The PA also recognized 

that the recent requirement for PM2.5

monitoring at near-road locations in 

large urban areas may further increase 

the ratios of maximum annual design 

values to average concentrations in 

some areas (U.S. EPA, 2020, section 

3.2.3.2.1). 

As detailed more in section II.A.2.c.ii, 

the PA next presents data from the 

epidemiological studies that used 

hybrid modeling approaches to estimate 

exposures (U.S. EPA, 2020, Figure 3–8). 

While studies using hybrid modeling 

approaches provide valid methods to 

estimate exposures in epidemiological 

studies and can expand the 

characterization of PM2.5 exposures in 

areas with sparse monitoring networks, 

these exposure estimation methods 

provide additional challenges to 

comparing study reported mean 

concentrations to the annual standard 

level. In these studies, PM2.5

concentrations are typically estimated 

based on a hybrid approach of ‘‘fusing’’ 

data from air quality models, satellites 

and ground-based monitors. As such, 

the reported mean concentrations in an 

area (e.g., county or zip-code) from these 

studies are calculated using the 

estimated concentrations from 

thousands of grid cells across the area. 

Generally, this means a larger number of 

lower concentration grid cells being 

included in the calculation of the mean, 

resulting in a mean concentration even 

further below the design value of the 

highest monitor in the area (which is 

used for determining whether the area is 

meeting the current standard) and even 

further below the mean concentration 

reported in epidemiological studies 

utilizing ground-based monitors to 

estimate exposure. 

It is also important to note that the 

performance of these hybrid modeling 

approaches in estimating PM2.5

concentrations, which are being used as 

surrogates for population exposure in 

the epidemiological study, depends on 

the availability of monitoring data, air 

quality model and the ability of the 

satellite to estimate ground level 

concentration and, thus, varies by 

location. Factors that contribute to 

poorer model performance often 

coincide with relatively low ambient 

PM2.5 concentrations (U.S. EPA 2020, 

2.3.3) Thus, uncertainty in hybrid 

model predictions becomes an 

increasingly important issue as lower 

predicted concentrations are 

considered. This additional source of 

uncertainty is an important 

consideration, particularly when all grid 

cell estimates are being used to calculate 

the study mean concentration, and 

further adds to why using study 

reported mean concentrations from 

epidemiological studies that use hybrid 

approaches to inform conclusions on 

the primary PM2.5 standards is a 

challenge. 

Given all of this, the EPA concludes 

that the overall mean PM2.5

concentrations in hybrid modeling 

studies are more difficult to directly 

compare to design values than groundbased monitoring concentrations in the 

context of setting a standard level. In 

fact, recognizing this challenge, the PA 

tried to assess information from hybrid 

modelling studies by calculating 

‘‘pseudo-design values’’ in locations of 

the key epidemiological studies (U.S. 

EPA, 2020, section 3.2.3.2.2), as noted 

above in section II.A.2.c.ii and detailed 

further in section II.C.1.a.ii of the 

proposal (85 FR 24117, April 30, 2020). 

However, this analysis and the 

associated approach were highly 

criticized by most commenters, with 

none suggesting the methodology be 

carried forward in the review. While the 

EPA believes that the PA’s ‘‘pseudodesign value’’ approach was a step in 

the right direction, the specific 

methodology itself needs further 

development. 

Given these considerations, and in 

light of the comments received, the EPA 

believes it is reasonable to focus on 

study reported mean (or median) 

concentrations 47 from key U.S.48

epidemiological studies that used 

ground-based monitors when 

considering information most 

comparable to the current annual 

standard, while also weighing the 

uncertainties associated with these 

studies and considering support 

provided by other lines of evidence. 

Based on the information shown in 

Figure 3–7 of the PA (U.S. EPA, 2020), 

the mean concentrations in 19 of the 21 

these studies were equal to or greater 

than the level of the current annual 

standard of 12 mg/m3. There were two 

studies, both included in last review, for 

which the mean concentration (11.8 mg/ 

m3; Peng et al., 2009) or median 

concentration (10.7 mg/m3 (Central 

Region); Zeger et al., 2008 49) was 

somewhat below 12 mg/m3. While these 

studies were included in the last review, 

the air quality distributions were not 

used by the prior Administrator in 

making a judgment on the level of the 

standard. The reported study mean 

concentration for one other study was 

12 mg/m3 (Kioumourtzoglou et al., 

2016). The mean 50 of the study reported 

means (or medians) of these 21 studies 

is 13.5 mg/m3, a concentration level 

above the current level of the primary 

annual standard of 12 mg/m3. 

Additionally, based on analyses in the 

PA, it would be expected that most of 

the design values (the metric most 

relevant for comparison to the standard 

level) in the areas included in these 

studies would be greater than 12 mg/ 

m3 51 (section II.A.2.c.ii above and U.S. 


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for a given three-year period are often 10% to 20% 

higher than average monitored concentrations (i.e., 

averaged across multiple monitors in the same 

CBSA) (U.S. EPA, 2020, Appendix B, section B.7). 

EPA 2020, Appendix B, section B.7). 

This is also supported by the pseudodesign value analysis in Figure 3–9 of 

the PA (U.S. EPA, 2020). 

Therefore, although recognizing that 

the proposal identified certain concerns 

about the proper weight to be placed on 

epidemiological studies, the EPA finds 

that its assessment of the mean 

concentrations of the key short-term and 

long-term epidemiological studies in the 

U.S. that use ground-based monitoring 

(i.e., those studies that can provide 

information most directly comparable to 

the current annual standard) is 

fundamentally consistent with the 

assessment in the last review, which 

established the current primary PM2.5

standards. 

Some commenters supporting 

revision of the primary PM2.5 standards 

contend that the quantitative risk 

assessment finds the number of avoided 

deaths resulting from retention of the 

standards will likely number in the 

many thousands, and a substantial 

reduction in these events could be 

achieved by a more stringent PM2.5

standard. While commenters who 

support revising the PM2.5 standards 

support the recommendation of the PA 

to use the evidence-based approach, as 

opposed to the risk-based approach, as 

a basis for ascertaining whether and 

how to revise the primary standards, the 

commenters state that the risk 

assessment does provide qualitative 

support to revise the standards. 

With regard to the quantitative risk 

assessment described by some 

commenters as showing health impacts 

that would be avoided by a more 

stringent standard, the EPA notes that 

these analyses utilize epidemiological 

study effect estimates as concentrationresponse functions to predict the 

occurrence of primarily premature 

mortality under different air quality 

conditions (characterized by the metric 

used in the epidemiological study). 

While the epidemiological studies that 

are inputs to the quantitative risk 

assessment are part of the evidence base 

that supports the conclusion of a 

‘‘causal’’ or ‘‘likely to be causal’’ 

determination in the ISA (U.S. EPA, 

2019), there are uncertainties inherent 

in the derivation of estimates of health 

effects (e.g., total mortality or ischemic 

heart disease mortality) ascribed to 

PM2.5 exposures using effect estimates 

from these studies. For example, the PA 

recognized several important 

uncertainties associated with aspects of 

the quantitative risk assessment 

approach and that the EPA concluded to 

have a medium or greater magnitude on 

risk estimates (U.S. EPA, 2020, section 

C.3.1 and table C–32). These 

uncertainties limit the applicability of 

the risk results for selecting a specific 

standard. Uncertainties 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 epidemiological study all 

limit utility (U.S. EPA, 2020, section 

3.3.2.4). Further, the approach to 

weighing evidence-based and risk-based 

considerations is not a new approach 

and as in previous reviews, the selection 

of a specific approach to reaching final 

decisions on the primary PM2.5

standards will reflect the judgments of 

the Administrator as to what weight to 

place on the various types of 

information available in the current 

review. The EPA notes that in the 

previous review, evidence-based 

considerations were given greater 

weight in the selection of standard 

levels than risk-based approaches (e.g., 

78 FR 3086, 3098–99, January 15, 2013) 

due to a recognition of similar 

limitations. 

Some commenters who support the 

Administrator’s rationale to retain the 

PM2.5 standards contend that, due to 

uncertainties in extrapolating health 

effects observed in animal toxicology 

studies to humans, animal toxicology 

studies are of limited regarding the 

adequacy of the current standard. On 

the other hand, commenters who 

support revisions to the current suite of 

PM2.5 standards generally contend that 

for experimental studies the 

Administrator: (1) Inappropriately tied 

the concept of biological plausibility to 

a specific concentration; (2) incorrectly 

interpreted animal/controlled human 

exposure studies; (3) ignored the 

limitations of experimental studies in 

relation to informing NAAQS levels and 

(4) gave inadequate weight to all of the 

evidence because the Administrator saw 

no absolute corroboration from clinical 

and accountability studies. The 

commenters emphasize their view that 

experimental studies provide important 

information regarding biological 

plausibility of numerous health effects 

(e.g., cardiovascular, respiratory, 

nervous system, and cancer effects) 

associated with PM2.5 exposure. 

Therefore, the commenters contend that 

experimental studies provide biological 

plausibility for human health effects 

linked to PM exposure in 

epidemiological studies and when 

viewed together, support revision of the 

current PM2.5 standards. 

The EPA notes that controlled human 

exposures studies provide crucial 

evidence in assessing whether 

protection is provided for short-term 

exposure concentrations consistently 

shown to elicit effects. In examining the 

controlled human exposure studies, the 

PA notes these studies provide evidence 

for health effects following single, shortterm PM2.5 exposures to concentrations, 

and thus, can be useful to assess 

whether these effects are likely to occur 

in the upper end of the PM2.5 air quality 

distribution in the U.S. (i.e., ‘‘peak’’ 

concentrations) (U.S. EPA, 2020, section 

3.2.3.1). As noted by the commenters, 

most of these studies examine exposure 

concentrations considerably higher than 

are typically measured in areas meeting 

the current standards (U.S. EPA, 2020, 

section 3.2.3.1). As detailed in section 

II.A.2.c.i above, even the extreme upper 

end of the distribution of 2-hour PM2.5

concentrations at sites meeting the 

current standards remains well-below 

the PM2.5 exposure concentrations 

consistently shown to elicit effects. 

Further, human exposure studies have 

not reported health effects at PM2.5 air 

quality concentrations likely to be seen 

in areas meeting the current primary 

PM2.5 standards. As such, these studies 

do not call into question the protection 

provided by the current primary PM2.5

standards. 

Additionally, with respect to the 

experimental evidence, the EPA agrees 

that animal toxicologic studies can be 

useful in understanding and supporting 

the biological plausibility of various 

effects linked to PM2.5 exposures. 

However, it is important to remember 

that for this body of evidence there is 

uncertainty in extrapolating from effects 

in animals to those in human 

populations. As such, animal toxicology 

studies are of limited utility in directly 

informing conclusions on the 

appropriate level of the standard. Thus, 

the available evidence from animal 

toxicologic studies do not call into 

question the protection provided by the 

current primary PM2.5 standards. 

Further, the ISA assesses both human 

exposures studies and animal 

toxicologic studies to evaluate the 

biological plausibility of various effects 

linked to PM2.5 exposures, and thus, we 

agree with the commenters on the 

importance of experimental evidence on 

this account. Within the ISA’s weight of 

evidence evaluation, which is based on 

the integration of findings from various 

lines of evidence, considerations in 

making causality determinations 


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include: ‘‘determining whether 

laboratory studies of humans and 

animals, in combination with 

epidemiological studies, inform the 

biological mechanisms by which PM 

can impart health effects and provide 

evidence demonstrating that PM 

exposure can independently cause a 

health effect’’ (U.S. EPA, 2019, p. ES–8). 

However, the ISA also notes that the 

strength of the PM2.5 exposure-health 

effects relationship varies depending on 

the exposure duration (i.e., short- or 

long-term) and broad health effects 

category (e.g., cardiovascular effects, 

respiratory effects) examined, and that 

across the broad health effects categories 

examined, the evidence supporting 

biological plausibility varies. 

Additionally, while assessing plausible 

biological pathways is an important step 

in evaluating causality determinations, 

the degree of biological plausibility for 

different mechanisms and end points 

can also vary depending on the 

evidence available. As a result, without 

a more clear linkage between 

concentrations below the current 

standard levels and adverse health 

effects, the Administrator noted in the 

proposal that he was ‘‘cautious about 

placing too much weight on reported 

PM2.5 health effect associations’’ 

observed in epidemiological studies (85 

FR 24119, April 30, 2020). As discussed 

in the proposal, the Administrator’s 

proposed decision was based on his 

evaluation of ‘‘the overall body of 

evidence, including controlled human 

exposure and animal toxicologic 

studies, in addition to epidemiological 

studies’’ (85 FR 24120, April 30, 2020). 

Thus, the experimental evidence does 

not suggest that the epidemiological 

evidence must be viewed differently 

than the Administrator has viewed such 

evidence in his proposed decision to 

retain the current primary standards. 

Some commenters who support 

retaining the current primary PM2.5

standards assert that the currently 

available accountability studies do not 

demonstrate that further reduction of 

the PM NAAQS would achieve a 

measurable improvement in public 

health. In contrast, commenters 

opposing the Administrator’s proposed 

decision to retain the PM2.5 standards 

criticize the Administrator’s heavy 

reliance on accountability studies to 

guide his decision, while emphasizing 

that accountability studies are just one 

line of evidence to inform causality. The 

commenters acknowledge the 

importance of well-designed and 

conducted accountability studies but 

warn that accountability studies 

measuring past interventions that are 

highly localized may have actual effects 

too small to be reliably measured. 

Considering the limitations of the 

accountability studies, including 

findings leading to false negative 

results, such studies are not considered 

essential for the proof of evidence 

required by statute, according to these 

commenters. 

The EPA agrees with the commenters 

that well-designed and conducted 

accountability studies can be 

informative and should be considered as 

one line of evidence, recognizing that 

that these studies offer insight into 

examples of how public health has 

responded to implementation of PM2.5

reduction strategies. As discussed in the 

PA (U.S. EPA, 2020, section 3.2.3.2.1) 

and in section III.C.3 of the proposal (85 

FR 24120, April 30, 2020), the EPA 

notes the availability of several such 

accountability studies and other 

retrospective health studies examining 

periods of declining PM2.5

concentrations. As indicated in Table 3– 

3 of the PA (U.S. EPA, 2020), these 

studies conducted in the U.S. indicate 

that declines in ambient PM2.5

concentrations over a period of years 

have been associated with decreases in 

mortality rates and increases in life 

expectancy, improvements in 

respiratory development, and decreased 

incidence of respiratory disease in 

children. When considering the overall 

means in these studies (i.e., the part of 

the air quality distribution over which 

the studies provide the strongest 

support for reported health effect 

associations), we find that ‘‘starting’’ 

annual average PM2.5 concentrations 

(i.e., mean concentration prior to 

reductions being evaluated) range from 

13.2–31.5 mg/m3 and ‘‘ending’’ 

concentrations ranging from 11.6–17.8 

mg/m3. As such, the EPA notes that 

these retrospective studies tend to focus 

on time periods during which ambient 

PM2.5 concentrations were substantially 

higher than those measured more 

recently, as well as ‘‘starting’’ annual 

average PM2.5 concentrations above 

those allowed by the current primary 

PM2.5 standards. As a result, the EPA 

believes that while these studies do 

provide evidence of public health 

improvements as ambient PM2.5 has 

declined over time, no current studies 

have examined public health 

improvements following reductions in 

ambient PM2.5 concentrations in areas 

where the ‘‘starting’’ concentration met 

the current primary standards. Thus, 

while acknowledging that this is an 

emerging field of study for PM2.5-related 

health effects, the available evidence 

supports the Administrator’s 

recognition that currently, there is a lack 

of accountability studies that clearly 

demonstrate that revising the current 

primary PM2.5 standards would result in 

public health improvements. 

Commenters opposed to the 

Administrator’s proposed decision to 

retain the PM2.5 standards contend that 

the EPA’s proposed decision is a 

violation of the CAA because it fails to 

consider sensitive populations and 

contains no margin of safety for them, 

as required under the CAA. In 

particular, these commenters pointed to 

evidence drawn from epidemiological 

studies that included specific at-risk 

groups in their study design and results. 

The EPA disagrees with these 

comments. As discussed above, the 

Administrator’s proposed decision to 

retain the current primary PM2.5

standards followed the same general 

approach used in previous reviews for 

reaching conclusions on what standards 

are appropriate. As such, the 

Administrator recognized that 

judgments of how to translate 

information available from 

epidemiological studies into a basis for 

appropriate standards must be 

considered in conjunction with the 

uncertainties in the epidemiological 

studies and in the context of the entire 

body of evidence before the Agency. 

This approach recognizes that the 

Administrator’s judgment is particularly 

important for a pollutant where a 

population threshold cannot be clearly 

discerned with confidence from the 

evidence and where clinical evidence 

does not demonstrate health effects at 

typical ambient concentrations that 

meet the current standards. This 

approach is also consistent with the 

CAA requirement to set standards that 

are neither more nor less stringent than 

necessary, recognizing that a zero-risk 

standard is not required by the CAA. 

With respect to protection of at-risk 

populations, the EPA has carefully 

evaluated and considered evidence of 

effects in at-risk populations. Unlike 

some of the other NAAQS reviews 

where the epidemiological evidence 

may be less complete, this PM NAAQS 

review has the benefit of having an ISA 

that considered many epidemiological 

studies that assessed impacts for 

populations considered at-risk (e.g., 

populations of older adults, children, or 

those with preexisting conditions, like 

cardiovascular disease). In addition, 

some of the key epidemiology studies 

that the EPA assessed (included in 

Figure 3–7 of the PA) also specifically 

focused on and evaluated at-risk 

populations, including epidemiology 

studies that assessed morbidity and 

mortality associations for age-specific 


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52Analyses of recent air quality in U.S. CBSAs 

indicate that maximum annual PM2.5 design values 

for a given three-year period are often 10% to 20% 

higher than average monitored concentrations (i.e., 

averaged across multiple monitors in the same 

CBSA) (U.S. EPA, 2020, Appendix B, section B.7). 

53As discussed above, the means from these 

studies are most relatable to the level of the annual 

standard. However, because the reported means in 

these studies are based on averaging the monitored 

concentration across an area, they are lower than 

the design value for that same area, since 

attainment of the standard is based on the 

measurements at the highest monitor (and not the 

average across multiple monitors.) 

populations (e.g., Medicare 

populations), as well as epidemiology 

studies that evaluated associations 

between PM2.5 exposure and specific 

health endpoints, like hospital 

admissions for cardiovascular effects in 

populations age 65 and older. The 

Agency takes note that it considered 

these studies to inform its review of the 

primary PM2.5 standards, which include 

at-risk populations, as well as other 

studies in the full body of scientific 

evidence in evaluating effects associated 

with long or short-term PM2.5 exposures 

(i.e., premature mortality, 

cardiovascular effects, cancer, and 

respiratory effects). 

More specifically, in weighing the 

scientific evidence to inform his 

decision on requisite PM2.5 standards 

with an adequate margin of safety, 

including protection for at-risk 

populations, the Administrator’s 

proposed conclusions recognized that 

epidemiological studies, many of which 

by design include at-risk populations, 

examine associations between 

distributions of PM2.5 air quality and 

health outcomes. Further, in noting that 

epidemiological studies do not identify 

particular PM2.5 exposures that cause 

effects, the PA focused on the reported 

mean concentrations from key 

epidemiological studies with the aim of 

providing a potential translation of 

information from epidemiological 

studies into the basis for consideration 

on standard levels (U.S. EPA, 2020, 

section 3.1.2). As discussed in more 

detail above, for the mean 

concentrations of the key 

epidemiological studies in the U.S. that 

use ground-based monitoring (i.e., those 

studies that can provide information 

most directly comparable to the current 

annual standard), the majority of studies 

have long-term mean (or median) 

concentrations above the current 

NAAQS (12.0 mg/m3), with the mean of 

the study reported means or medians 

equal to 13.5 mg/m3, a concentration 

level above the current level of the 

primary annual standard of 12 mg/m3. 

The EPA notes that study reported mean 

(or median) concentration values are 

generally 10–20% lower than the design 

value of the highest monitor in an area, 

which determines compliance, and 

suggesting that that the current level of 

the standard provides even more 

protection than is suggested by the 

reported means.52 In the proposal, the 

Administrator recognized that important 

uncertainties and limitations do remain 

in the epidemiological evidence and the 

Administrator weighed these 

uncertainties, while also considering 

support provided by other lines of 

evidence, in judging whether the 

current standards are requisite with an 

adequate margin of safety. The 

Administrator further considered the 

emerging body of evidence from 

accountability studies examining past 

reductions in ambient PM2.5 and the 

degree to which those reductions have 

resulted in public health improvements. 

As discussed above, such studies have 

focused on time periods during which 

ambient PM2.5 concentrations were 

substantially higher than those 

measured more recently and therefore 

do not demonstrate public health 

improvements attributable to reduction 

in ambient PM2.5 at concentrations 

below the current standard. 

Thus, the Administrator judged that 

the overall body of evidence indicates 

continued uncertainty in the degree to 

which adverse effects could result from 

PM2.5 exposures in areas meeting the 

current annual and 24-hour standards. 

Additionally, the current annual 

standard is below the lowest ‘‘starting’’ 

concentration in the available 

accountability studies (i.e., 13.2 mg/m3) 

and below the reported mean 

concentration in the majority of the key 

U.S. epidemiological studies using 

ground-based monitoring data 53 (i.e., 

mean of the reported means was 13.5 

mg/m3). In addition, concentrations in 

areas meeting the current 24-hour and 

annual standards remain well-below the 

PM2.5 exposure concentrations 

consistently shown to elicit effects in 

controlled human exposure studies. In 

specifically assessing his proposed 

decision, the Administrator noted that 

more stringent standards would be more 

than requisite to protect public health 

with an adequate margin of safety. 

4. Administrator’s Conclusions 

This section summarizes the 

Administrator’s conclusions and final 

decisions related to the current primary 

PM2.5 standards and presents his 

decision to retain those standards, 

without revision. As described above 

(section I.D) and in section II.A.2 of the 

proposal (85 FR 24105, April 30, 2020), 

the Administrator’s 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 mg/m3, 35 mg/m3) 

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 is 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 is 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 is informed 

by the Administrator’s consideration of 

advice from the CASAC and public 

comments received on the proposal 

notice. 

As an initial matter, the Administrator 

recognizes that, with regard to effects 

classified as having evidence of a causal 


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or likely causal relationship with long 

or short-term PM2.5 exposures (i.e., 

premature mortality, cardiovascular 

effects, cancer, and respiratory effects), 

the EPA considered the full range of 

studies evaluating these effects, 

including studies of at-risk populations, 

to inform its review of the primary PM2.5

standards. Thus, the Administrator 

notes that his judgment in this final 

decision reflects placing the greatest 

weight on evidence of effects for which 

the ISA determined there is a causal or 

likely causal relationship with long- and 

short-term PM2.5 exposures. 

With respect to the indicator, the 

Administrator recognizes that the 

scientific evidence in this review, as in 

the last review, continues to provide 

strong support for health effects 

following short- and long-term PM2.5

exposures. He notes the PA conclusion 

that the available information continues 

to support the PM2.5 mass-based 

indicator and remains too limited to 

support a distinct standard for any 

specific PM2.5 component or group of 

components, and too limited to support 

a distinct standard for the ultrafine 

fraction. Further, the Administrator 

notes that the EPA received very few 

comments on the indicator, with no 

commenters advocating for revising the 

current PM2.5 indicator for fine particles. 

Thus, as proposed, the Administrator 

concludes that it is appropriate to retain 

PM2.5 as the indicator for the primary 

standards for fine particulates. 

With respect to averaging time and 

form, the Administrator notes that the 

scientific evidence continues to provide 

strong support for health effects 

associations with both long-term (e.g., 

annual or multi-year) and short-term 

(e.g., mostly 24-hour) exposures to PM2.5

and, consistent with the conclusions in 

the PA, judges that the current evidence 

does not support considering 

alternatives (U.S. EPA, 2020, section 

3.5.2). The Administrator also notes that 

very few comments were received 

related to averaging time and form and 

none directly advocated for changing 

the form or averaging time. In the 

current review, epidemiological and 

controlled human exposure studies have 

examined a variety of PM2.5 exposure 

durations. Epidemiological studies 

continue to provide strong support for 

health effects associated with short-term 

PM2.5 exposures based on 24-hour PM2.5

averaging periods, and the EPA notes 

that associations with sub-daily 

estimates are less consistent and, in 

some cases, smaller in magnitude (U.S. 

EPA, 2019, section 1.5.2.1; U.S. EPA, 

2020, section 3.5.2.2). In addition, 

controlled human exposure and panelbased studies of sub-daily exposures 

typically examine subclinical effects, as 

the commenters acknowledge, rather 

than the more serious population-level 

effects that have been reported to be 

associated with 24-hour exposures (e.g., 

mortality, hospitalizations). Taken 

together, the ISA concludes that 

epidemiological studies do not indicate 

that sub-daily averaging periods are 

more closely associated with health 

effects than the 24-hour average 

exposure metric (U.S. EPA, 2019, 

section 1.5.2.1). Additionally, while 

recent controlled human exposure 

studies provide consistent evidence for 

cardiovascular effects following PM2.5

exposures for less than 24 hours (i.e., 

<30 minutes to 5 hours), exposure 

concentrations in these studies are wellabove the ambient concentrations 

typically measured in locations meeting 

the current standards (U.S. EPA, 2020, 

section 3.2.3.1). Thus, these studies also 

do not suggest the need for additional 

protection against sub-daily PM2.5

exposures, beyond that provided by the 

current primary standards. Therefore, 

the Administrator’s judgment is that the 

current 24-hour averaging time remains 

appropriate. 

In relation to the form of the 24-hour 

standard (98th percentile, averaged over 

three years), the Administrator notes 

that epidemiological studies continue to 

provide strong support for health effect 

associations with short-term (e.g., 

mostly 24-hour) PM2.5 exposures (U.S. 

EPA, 2020, section 3.5.2.3) and that 

controlled human exposure studies 

provide evidence for health effects 

following single short-term ‘‘peak’’ 

PM2.5 exposures. Thus, the evidence 

supports retaining a standard focused 

on providing supplemental protection 

against short-term peak exposures and 

supports a 98th percentile form for a 24- 

hour standard. The Administrator 

further notes that this form also 

provides 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). As such, the 

Administrator concludes, as proposed, 

to retain the form and averaging time of 

the current 24-hour standard (98th 

percentile, averaged over three years) 

and annual standard (annual average, 

averaged over three years). 

The Administrator also proposed to 

retain the current levels of the 24-hour 

standard (98th percentile, averaged over 

three years) at 35 mg/m3 and annual 

standard (annual average, averaged over 

3 years) at 12 mg/m3. The majority of the 

comments received focused on this 

proposed decision to retain the current 

levels of both standards. In reaching his 

final decision regarding the level of the 

standards, the Administrator considered 

the large body of evidence presented 

and assessed in the ISA (U.S. EPA, 

2019), the policy-relevant and riskbased conclusions and rationales as 

presented in the PA (U.S. EPA, 2020), 

views expressed by the CASAC, and 

public comments. In particular, in 

considering the ISA and PA, he 

considers key epidemiological studies 

that evaluate associations between PM2.5

air quality distributions and mortality 

and morbidity, including key 

‘‘accountability studies’’; the availability 

of experimental studies to support 

biological plausibility; controlled 

human exposure studies examining 

effects following short-term PM2.5

exposures; air quality analyses; and the 

important uncertainties and limitations 

associated with this information. 

As an initial matter, the Administrator 

recognizes that the current annual 

standard is most effective in controlling 

PM2.5 concentrations near the middle of 

the air quality distribution (i.e., around 

the mean of the distribution), but can 

also provide some control over shortterm peak PM2.5 concentrations. On the 

other hand, the 24-hour standard, with 

its 98th percentile form, is most 

effective at limiting peak 24-hour PM2.5

concentrations, but in doing so will also 

have an effect on annual average PM2.5

concentrations. Thus, while either 

standard could be viewed as providing 

some measure of protection against both 

average exposures and peak exposures, 

the 24-hour and annual standards are 

not expected to be equally effective at 

limiting both types of exposures. Thus, 

consistent with previous reviews, 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. Specifically, he recognizes 

that the annual standard is more likely 

to appropriately limit the ‘‘typical’’ 

daily and annual exposures that are 

most strongly associated with the health 

effects observed in epidemiological 

studies. The Administrator concludes 

that an annual standard (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. Further, recognizing that 

the 24-hour standard (with its 98th 

percentile form) is more directly tied to 

short-term peak PM2.5 concentrations, 

and thus more likely to appropriately 

limit exposures to such concentrations, 

the Administrator concludes that the 


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current 24-hour standard (98th 

percentile, averaged over three years) 

remains appropriate to provide a 

balance between limiting the occurrence 

of peak 24-hour PM2.5 concentrations 

and identifying a stable target for risk 

management programs. However, the 

Administrator recognizes that changes 

in PM2.5 air quality to meet an annual 

standard would likely result not only in 

lower short- and long-term PM2.5

concentrations near the middle of the 

air quality distribution, but also in fewer 

and lower short-term peak PM2.5

concentrations. The Administrator 

further recognizes that changes in air 

quality 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, in considering the adequacy of 

the 24-hour standard, the Administrator 

notes the importance of considering 

whether additional protection is needed 

against short-term exposures to peak 

PM2.5 concentrations. In examining the 

scientific evidence, he notes that 

controlled human exposure studies 

provide evidence for health effects 

following single, short-term PM2.5

exposures to concentrations. These 

types of exposures correspond best to 

those to ambient exposures that might 

be experienced in the upper end of the 

PM2.5 air quality distribution in the U.S. 

(i.e., ‘‘peak’’ concentrations). However, 

most of these studies examine exposure 

concentrations considerably higher than 

are typically measured in areas meeting 

the current standards (U.S. EPA, 2020, 

section 3.2.3.1). In particular, controlled 

human exposure studies often report 

statistically significant effects on one or 

more indicators of cardiovascular 

function following 2-hour exposures to 

PM2.5 concentrations at and above 120 

mg/m3 (at and above 149 mg/m3 for 

vascular impairment, the effect shown 

to be most consistent across studies). To 

provide insight into what these studies 

may indicate regarding the primary 

PM2.5 standards, the PA (U.S. EPA, 

2020, p.3–49) notes that 2-hour ambient 

concentrations of PM2.5 at monitoring 

sites meeting the current standards 

almost never exceed 32 mg/m3. In fact, 

even the extreme upper end of the 

distribution of 2-hour PM2.5

concentrations at sites meeting the 

current standards remains well-below 

the PM2.5 exposure concentrations 

consistently shown in controlled human 

exposure studies to elicit effects (i.e., 

99.9th percentile of 2-hour 

concentrations at these sites is 68 mg/m3

during the warm season). Additionally, 

the Administrator notes the limited 

utility of the animal toxicologic studies 

in directly informing conclusions on the 

appropriate level of the standard given 

the uncertainty in extrapolating from 

effects in animals to those in human 

populations. Thus, the available 

experimental evidence does not indicate 

the need for additional protection 

against exposures to peak PM2.5

concentrations, beyond the protection 

provided by the combination of the 

current 24-hour standard and the 

current annual standard (U.S. EPA, 

2020, section 3.2.3.1). 

With respect to the epidemiological 

evidence, the Administrator notes that 

the available epidemiological studies do 

not indicate that associations in those 

studies are strongly influenced by 

exposures to peak concentrations in the 

air quality distribution and thus do not 

indicate the need for additional 

protection against short-term exposures 

to peak PM2.5 concentrations (U.S. EPA 

2020, section 3.5.1). Lastly, the 

Administrator notes CASAC consensus 

support for retaining the current 24- 

hour standard. Thus, the Administrator 

concludes that the 24-hour standard 

with its level of 35 mg/m3 is adequate to 

provide supplemental protection (i.e., 

beyond that provided by the annual 

standard alone) against short-term 

exposures to peak PM2.5 concentrations. 

In reviewing the level of the annual 

standard, the Administrator recognizes 

that the annual standard, with its form 

based on the arithmetic mean 

concentration, is most appropriately 

meant to limit the ‘‘typical’’ daily and 

annual exposures that are most strongly 

associated with the health effects 

observed in epidemiological studies. 

However, the Administrator also 

recognizes that while epidemiological 

studies examine associations between 

distributions of PM2.5 air quality and 

health outcomes, they do not identify 

particular PM2.5 exposures that cause 

effects and thus, they cannot alone 

identify a specific level at which the 

standard should be set, as such a 

determination necessarily requires the 

Administrator’s judgment. Thus, any 

approach that uses epidemiological 

information in reaching decisions on 

what standards are appropriate 

necessarily requires judgments about 

how to translate the information 

available from the epidemiological 

studies into a basis for appropriate 

standards. This includes consideration 

of how to weigh the uncertainties in the 

reported associations between daily or 

annual average PM2.5 exposures and 

mortality or morbidity in the 

epidemiological studies. Such an 

approach is consistent with setting 

standards that are neither more nor less 

stringent than necessary, recognizing 

that a zero-risk standard is not required 

by the CAA. 

The Administrator recognizes that 

important uncertainties and limitations 

that were present in epidemiological 

studies in previous reviews, remain in 

the current review. As discussed above, 

these uncertainties include exposure 

measurement error; potential 

confounding by copollutants; increasing 

uncertainty of associations at lower 

PM2.5 concentrations; and heterogeneity 

of effects across different cities or 

regions. The Administrator also 

recognizes the advice given by the 

CASAC on this matter. As discussed 

above (section II.B.1), the CASAC 

members who support retaining the 

annual standard expressed their 

concerns with available PM2.5

epidemiological studies. They assert 

that recent epidemiological studies do 

not provide a sufficient basis for 

revising the current standards. They 

also identify several key concerns 

regarding the associations reported in 

PM2.5 epidemiological 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). 

Taking into consideration the views 

expressed by these CASAC members, 

the Administrator recognizes that 

epidemiological 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). While the 

Administrator remains concerned about 

placing too much weight on 

epidemiological studies to inform 

conclusions on the adequacy of the 

current primary standards, he notes that 

several commenters advocated for using 

the epidemiological studies in a manner 

they characterized as similar to the last 

review, to determine the level of the 

annual standard. The previous PM 

NAAQS review completed in 2012 

noted that the evidence of an 

association in any epidemiological 

study is ‘‘strongest at and around the 

long-term average where the data in the 

study are most concentrated’’ (78 FR 

3140, January 15, 2013). Accordingly, 

the Administrator notes the 

characterization of study reported shortterm and long-term mean PM2.5

concentrations (section II.A.2.c.ii). As 

discussed in more detail above in 

section II.B.3 in responding to 

comments, when assessing the mean 

concentrations of the key short-term and 


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54There were two studies, both included in the 

last review, for which the mean concentration (11.8 

mg/m3; Peng et al., 2009) or median concentration 

(10.7 mg/m3 (Central Region); Zeger et al., 2008) was 

somewhat below 12 mg/m3. 

55The median of the study reported mean (or 

median) PM2.5 concentrations is 13.3 mg/m3, which 

is also above the level of the current standard. 

long-term epidemiological studies in the 

U.S. that use ground-based monitoring 

(i.e., those studies that can provide 

information most directly comparable to 

the current annual standard), the 

majority of studies (i.e., 19 out of 21) 

have mean concentrations at or above 

the level of the current annual standard 

(12.0 mg/m3), with the mean of the study 

reported means or medians equal to 13.5 

mg/m3, a concentration level above the 

current level of the primary annual 

standard of 12 mg/m3.54 The 

Administrator further notes his caution 

in directly comparing the reported study 

mean values to the standard level given 

that, as discussed in more detail above, 

study-reported mean concentrations, by 

design, are generally lower than the 

design value of the highest monitor in 

an area, which determines compliance. 

In fact, analyses of recent air quality in 

U.S. CBSAs indicate that maximum 

annual PM2.5 design values for a given 

three-year period are often 10% to 20% 

higher than average monitored 

concentrations (i.e., averaged across 

multiple monitors in the same CBSA) 

(U.S. EPA, 2020, Appendix B, section 

B.7). He further notes his concern in 

placing too much weight on any one 

epidemiological study but instead feels 

that it is more appropriate to focus on 

the body of studies together and 

therefore takes note of the calculation of 

the mean of study-reported means (or 

medians). Thus, in summary, while the 

Administrator is cautious about placing 

too much weight on the epidemiological 

evidence on its own, he notes: (1) The 

reported mean concentration in the 

majority of the key U.S. epidemiological 

studies using ground-based monitoring 

data are above the level of the current 

annual standard; (2) the mean of the 

reported study means (or medians) (i.e., 

13.5 mg/m3) is above the level of the 

current standard; 55 (3) air quality 

analyses show the study means to be 

lower than their corresponding design 

values by 10–20%; and (4) that these 

analyses must be considered in light of 

uncertainties inherent in the 

epidemiological evidence. When taken 

together, the Administrator judges that, 

even if he were to place greater weight 

on the epidemiological evidence, this 

information would not call into 

question the adequacy of the current 

standards. 

In addition to the evidence, the 

Administrator also considers the 

potential implications of the risk 

assessment. He notes that all risk 

assessments have limitations and that 

he remains concerned about the 

uncertainties in the underlying 

epidemiological data used in the risk 

assessment. The Administrator also 

notes that in previous reviews, these 

uncertainties and 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 3086, 3098–99, January 15, 

2013). These uncertainties and 

limitations have included 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 

epidemiological study (U.S. EPA, 2020, 

section 3.3.2.4). Additionally, the 

Administrator notes similar concern 

expressed by members of the CASAC 

who support retaining the current 

standards; they highlighted similar 

uncertainties and limitations in the risk 

assessment (Cox, 2019a). In light of all 

of this, the Administrator judges it 

appropriate to place little weight on 

quantitative estimates of PM2.5- 

associated mortality risk in reaching 

conclusions about the level of the 

primary PM2.5 standards. 

The Administrator additionally 

considers the emerging body of 

evidence from accountability studies 

examining past reductions in ambient 

PM2.5, and the degree to which those 

reductions have resulted in public 

health improvements. The 

Administrator agrees with public 

commenters who note that welldesigned and conducted accountability 

studies can be informative. However, 

the Administrator also recognizes that 

interpreting such studies in the context 

of the current primary PM2.5 standards 

is 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 13.2 to >20 mg/m3 (i.e., U.S. 

EPA, 2020, Table 3–3). Given the lack of 

available accountability 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. 

When the above considerations are 

taken together, the Administrator 

concludes 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 and consideration of CASAC 

advice and public comments, does not 

call into question the adequacy of 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 for additional public health 

improvements from reducing ambient 

PM2.5 concentrations 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. This judgment reflects the 

Administrator’s consideration of the 

uncertainties in the potential 

implications of the lower end of the air 

quality distributions from the 

epidemiological 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. 

In reaching this conclusion, the 

Administrator notes that the current 

standards provide an adequate margin 

of safety. With respect to the annual 

standard, the level of 12 mg/m3 is below 

the lowest ‘‘starting’’ concentration (i.e., 


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56As discussed above, the means from these 

studies are most relatable to the level of the annual 

standard. However, because the reported means in 

these studies are based on averaging the monitored 

concentration across an area, they tend to be lower 

than the design value for that same area, since 

attainment of the standard is based on the 

measurements at the highest monitor (and not the 

average across multiple monitors.) 

57 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. 

13.2 mg/m3) in the available 

accountability studies that show public 

health improvements attributable to 

reductions in ambient PM2.5. In 

addition, while the Administrator 

places less weight on the 

epidemiological evidence for the 

purposes of selecting a standard, he 

notes that the current level of the annual 

standard is below the reported mean 

(and median) concentrations in the 

majority of the key U.S. epidemiological 

studies using ground-based monitoring 

data 56 (noting that these means tend to 

be 10–20% lower than their 

corresponding area design values which 

is the more relevant metric when 

considering the level of the standard) 

and below the mean of the reported 

means (or medians) of these studies (i.e., 

13.5 mg/m3). In addition, the 

Administrator recognizes that 

concentrations in areas meeting the 

current 24-hour and annual standards 

remain well-below the PM2.5 exposure 

concentrations consistently shown to 

elicit effects in human exposure studies. 

In addition, based on the 

Administrator’s review of the science, 

including controlled human exposure 

studies examining effects following 

short-term PM2.5 exposures, the 

epidemiological studies described 

above, 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 than the current 

standards (e.g., with higher levels) are 

also not supported. 

When the above information is taken 

together, the Administrator concludes 

that the available scientific evidence 

and technical information continue to 

support the current annual and 24-hour 

PM2.5 standards. This conclusion 

reflects the fact that important 

limitations in the evidence remain. The 

Administrator concludes 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 

concludes 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 mg/m3, 35 mg/m3), when taken 

together, remain requisite to protect the 

public health. Therefore, the 

Administrator reaches the final 

conclusion that the current suite of 

primary PM2.5 standards is requisite to 

protect public health from fine particles 

with an adequate margin of safety, 

including the health of at-risk 

populations, and is retaining the 

standards, without revision. 

C. Decision on the Primary PM2.5

Standards 

For the reasons discussed above and 

taking into account information and 

assessments presented in the ISA and 

PA, the advice from the CASAC, and 

consideration of public comments, the 

Administrator concludes that the 

current annual and 24-hour primary 

PM2.5 standards are requisite to protect 

public health from fine particles with an 

adequate margin of safety, including the 

health of at-risk populations, and is 

retaining the current standards without 

revision. 

III. Rationale for Decisions on the 

Primary PM10 Standard 

This section presents the rationale for 

the Administrator’s decision to retain 

the existing primary PM10 standard. 

This decision is based on a thorough 

review of the latest scientific 

information, published through 

December 2017,57 and assessed in the 

ISA, on human health effects associated 

with PM10–2.5 in ambient air. This 

decision also accounts for 

considerations in the PA of the policyrelevant information, CASAC advice, 

and consideration of public comments 

received on the proposal. 

Section III.A provides background on 

the general approach for this review and 

the basis for the existing standard, and 

also presents a brief summary of key 

aspects of the currently available health 

effects information. Section III.B 

summarizes the CASAC advice and the 

Administrator’s proposed decision to 

retain the existing primary PM10

standard, addresses public comments 

received on the proposal, and presents 

the Administrator’s conclusions on the 

adequacy of the current standard, 

drawing on consideration of information 

in the ISA and the PA information, 

advice from the CASAC, and comments 

from the public. Section III.C 

summarizes the Administrator’s 

decision on the primary PM10 standard. 

A. Introduction 

As in prior reviews, the general 

approach to reviewing the current 

primary PM10 standard is based, most 

fundamentally, on using the EPA’s 

assessment of the current scientific 

evidence to inform the Administrator’s 

judgment regarding a primary PM10

standard that protects public health 

with an adequate margin of safety. In 

drawing conclusions with regard to the 

primary PM10 standard, the final 

decision on the adequacy of the current 

standard is largely a public health 

policy judgment to be made by the 

Administrator. The Administrator’s 

final decision draws upon the scientific 

information about health effects, as well 

as judgments about how to consider the 

range and magnitude of uncertainties 

that are inherent in the scientific 

evidence. The approach to informing 

these judgments, discussed more fully 

below, is based on the recognition that 

the available health effects evidence 

generally reflects a continuum, 

consisting of levels at which scientists 

generally agree that health effects are 

likely to occur, through lower levels at 

which the likelihood and magnitude of 

the response become increasingly 

uncertain. This approach is consistent 

with the requirements of the NAAQS 

provisions in the CAA and with how the 

EPA and the courts have interpreted the 

Act. These provisions require the 

Administrator to establish primary 

standards that, in his judgment, are 

requisite to protect public health with 

an adequate margin of safety. In so 

doing, the Administrator seeks to 

establish standards that are neither more 

nor less stringent for this purpose. The 

Act does not require that primary 

standards be set at a zero-risk level, but 

rather at a level that avoids 

unacceptable risks to public health 

including the health of sensitive groups. 

The four basic elements of the NAAQS 

(indicator, averaging time, form, and 

level) are considered collectively in 

evaluating the health protection 

afforded by a standard. 

In evaluating the appropriateness of 

retaining or revising the current primary 


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PM10 standard, the EPA has adopted an 

approach which is similar to that used 

in the last review and which reflects the 

body of evidence and information now 

available. As summarized in section 

III.A.1 below, the Administrator’s 

decisions in the prior review were based 

on an integration of information on 

health effects associated with exposure 

to PM10–2.5, on the public health 

significance of key health effects, on 

policy judgments as to whether the 

standard is requisite to protect public 

health with an adequate margin of 

safety, and on consideration of the 

CASAC advice and public comments. 

Similarly, in this review, as described 

in the PA, the proposal, and elsewhere 

in this document, we draw on the 

current evidence pertaining to the 

public health risk of PM10–2.5 in ambient 

air. The past and current approaches are 

both based, most fundamentally, on the 

EPA’s assessment of the current 

scientific and technical information. 

The EPA’s assessments are primarily 

documented in the ISA and the PA, 

which have received CASAC review and 

public comment (83 FR 53471, October 

23, 2018; 84 FR 47944, September 11, 

2019). To bridge the gap between the 

scientific assessment of the ISA and the 

judgments required of the Administrator 

in determining whether the current 

standard is requisite to protect public 

health with an adequate margin of 

safety, the PA evaluates the policy 

implications of the current evidence in 

the ISA. 

In considering the scientific and 

technical information, we consider both 

the information available at the time of 

the last review and information newly 

available since the last review, 

including most particularly that which 

has been critically analyzed and 

characterized in the current ISA. The 

evidence-based discussions presented 

below in section III.A.2 (and 

summarized more fully in the proposal) 

draw upon evidence from studies 

evaluating health effects related to 

exposures to PM10–2.5, as discussed in 

the ISA. 

1. Background on the Current Standard 

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 mg/m3 and its one-expectedexceedance 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 

retain a standard that provides some 

measure of protection against all 

PM10–2.5 (regardless of composition or 

source of origin), (2) the extent to which 

a standard with a PM10 indicator can 

provide protection against exposures to 

PM10–2.5, and (3) the degree of public 

protection provided by the existing 

PM10 standard. 

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 the epidemiological 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 

were expected to vary widely. Though 

most of the available studies examined 

associations in urban areas, the 

Administrator noted that some studies 

had also found associations between 

mortality and morbidity and relatively 

high ambient concentrations of particles 

of non-urban crustal origin. In the last 

review, in considering this body of 

evidence, and consistent with the 

CASAC’s advice, the 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 composition, 

location, or source of origin (78 FR 

3176, January 15, 2013). 

With regard to the appropriateness of 

retaining a PM10 indicator for a standard 

meant to protect against exposures to 

PM10–2.5 in ambient air, the prior 

Administrator noted that PM10 mass 

included both coarse PM (PM10–2.5) and 

fine PM (PM2.5). As a result, the 

concentration of thoracic coarse 

particles (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 in 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 

that contaminants in urban areas would 

increase PM10–2.5 particle toxicity. 

Therefore, in the last review, the 

Administrator concluded that it 

remained appropriate to maintain a 

standard that requires lower 

concentrations of PM10–2.5 in ambient air 

in urban areas, where the strongest 

evidence was for associations between 

mortality and morbidity, and allows 

higher concentrations of PM10–2.5 in 

non-urban areas, where the evidence of 

public health concerns was less certain. 

The 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 the 

varying levels of PM2.5 present, 

appropriately reflected the differences 

in the strength of evidence regarding the 

health effects of coarse particles. 

With regard to evaluating the degree 

of public health protection provided by 

the current primary PM10 standard, with 

its level of 150 mg/m3 and its oneexpected-exceedance form on average 

over three years, the Administrator 

recognized that the available scientific 

evidence and air quality information 

was much more limited for PM10–2.5

than for PM2.5. In particular, the 

strongest evidence for PM10–2.5-related 

health effects was for cardiovascular 

effects, respiratory effects, and 

premature mortality following shortterm 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). The 

Administrator noted the significant 

uncertainties and limitations associated 

with the PM10–2.5 scientific evidence 

leading to these causal determinations 

and questioned whether additional 

public health improvements would be 

achieved by revising the existing 

primary PM10 standard. She specifically 

took note of several uncertainties and 

limitations, including the following: 

• There were a limited number of 

epidemiological studies that employed 

copollutant models to address the 

potential for confounding, particularly 

by PM2.5, that would further the 

understanding of the extent to which 

PM10–2.5 itself, rather than copollutants, 

contributed to the reported health 

effects. 

• The plausibility of the associations 

between PM10–2.5 and mortality and 

morbidity reported in epidemiological 

studies was uncertain given the limited 

number of experimental studies 

providing support for these associations. 

• 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 

epidemiological studies resulted in 

uncertainties in the ambient PM10–2.5

concentrations at which the reported 


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58As noted in the Preamble to the ISA, 

‘‘suggestive’’ evidence is ‘‘limited, and chance, 

confounding, and other biases cannot be ruled out’’ 

(U.S. EPA, 2015, Table II). 

59Compared 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 

PM10–2.5 exposures in animal studies. 

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 where the ambient PM10

concentrations were above those 

allowed by the current 24-hour PM10

standard. 

• The composition of PM10–2.5, and 

the effects associated with specific 

components, were also key uncertainties 

in the evidence. With a lack of 

information on the chemical speciation 

of PM10–2.5, the apparent variability in 

associations across study locations was 

difficult to characterize. 

In considering these uncertainties and 

limitations, the prior Administrator 

particularly took note of degree of 

uncertainty associated with the extent to 

which health effects reported in the 

epidemiological studies are due to 

PM10–2.5 itself, as opposed to one or 

more copollutants, especially PM2.5. 

This uncertainty reflects the relatively 

small number of studies available for 

PM10–2.5 in ambient air that had 

evaluated copollutant models, and the 

very limited evidence from controlled 

human exposure studies supporting the 

plausibility of adverse health effects 

attributable to PM10–2.5 at ambient 

concentrations. 

When considering the available 

evidence overall, 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 a more 

stringent standard to provide additional 

protection was not necessary was 

supported by her consideration of the 

uncertainties in the overall body of 

evidence. Her judgment that a less 

stringent standard was not needed and 

that the degree of public health 

protection provided by the current 

standard was not greater than warranted 

was supported by the positive and 

statistically significant associations with 

mortality observed in some single-city 

study locations that were likely to have 

violated the current PM10 standard. 

Therefore, the prior Administrator 

concluded that the existing 24-hour 

standard, with its one-expected 

exceedance form on average over three 

years and a level of 150 mg/m3, 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 primary PM10

standard. 

2. Overview of Health Effects Evidence 

In this section, we provide an 

overview of the policy-relevant aspects 

of the PM10–2.5-related health effects 

evidence available for consideration in 

this review. Section III.B of the proposal 

provides a detailed summary of key 

information contained in the ISA and 

the PA on the health effects associated 

with PM10–2.5 exposures, and the related 

public health implications. As described 

in the proposal, the ISA does not 

identify any PM10–2.5-related health 

outcomes for which the evidence 

supports either a ‘‘causal’’ or ‘‘likely to 

be causal relationship’’ (85 FR 24122, 

April 30, 2020). Therefore, for PM10–2.5, 

we consider the evidence determined to 

be ‘‘suggestive of, but not sufficient to 

infer, a causal relationship,’’ recognizing 

the greater uncertainty in such 

evidence.58

While studies conducted since the 

time of the last review have 

strengthened support for relationships 

between PM10–2.5 exposures and some 

key health outcomes, several key 

uncertainties from the last review have, 

to date, ‘‘still not been addressed’’ (U.S. 

EPA, 2019, section 1.4.2, p. 1–41). For 

example, in the last review, 

epidemiological studies relied on a 

number of methods to estimate PM10–2.5

exposures, but the methods had not 

been systematically compared to 

evaluate spatial and temporal 

correlations in exposure estimates. 

Methods employed by these studies 

included: (1) Calculating the difference 

between PM10 and PM2.5 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 colocated, and (3) direct measurement of 

PM10–2.5 using a dichotomous sampler 

(U.S. EPA, 2019, section 1.4.2). More 

recent epidemiological studies, 

available since the last review, continue 

to use these approaches to estimate 

PM10–2.5 concentrations. 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 methods used to estimate 

PM10–2.5 concentrations have not been 

systematically evaluated (U.S. EPA, 

2019, section 3.3.1.1), contributing to 

the uncertainty regarding spatial and 

temporal correlations in PM10–2.5

concentrations across methods and in 

PM10–2.5 exposure estimates used in 

epidemiological studies (U.S. EPA, 

2019, sections 2.5.1.2.3 and 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, this 

uncertainty is particularly important for 

the coarse size fraction. 

In addition to the uncertainty 

associated with PM10–2.5 exposure 

estimates in the epidemiological 

studies, information in the current 

review remains limited with regard to 

the potential for confounding by 

copollutants and provides limited 

support for the biological plausibility of 

serious effects following PM10–2.5

exposures; both of these limitations 

continue to contribute broadly to 

uncertainty in the PM10–2.5 health 

evidence. Uncertainty related to 

potential confounding is related to the 

relatively few epidemiological studies 

that have evaluated PM10–2.5 health 

effect associations in copollutant 

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 limited 

number of controlled human exposure 

and animal toxicology 59 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 and health 

effects, including the uncertainties in 

the evidence, are summarized in the 

sections below. 

a. Nature of Effects 

i. Mortality 

With regard to long-term PM10–2.5

exposure and mortality, very few 

studies were available at the time of the 

last review. As such, the 2009 ISA 

concluded that the evidence was 

‘‘inadequate to determine if a causal 

relationship exists’’ (U.S. EPA, 2009c). 

Since the time of the last review, there 

is limited new evidence and many of 

the limitations noted in the 2012 review 

persist. In the current review, some 

recent cohort studies conducted in the 

U.S. and Europe reported positive 

associations between long-term PM10–2.5

exposure and total (nonaccidental) 

mortality, though results are 


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inconsistent across studies (U.S. EPA, 

2019, Table 11–11). The examination of 

copollutant models in these studies 

remains limited, and when copollutants 

are included, PM10–2.5 effect estimates 

are often attenuated after adjusting for 

PM2.5 (U.S. EPA, 2019, Table 11–11). 

These studies employed a number of 

approaches for estimating PM10–2.5

exposures, including direct 

measurements from dichotomous 

samplers, calculating the difference 

between PM10 and PM2.5 measured at 

co-located monitors, and calculating the 

difference of area-wide PM10 and PM2.5

concentrations. As discussed above as a 

limitation in the last review, temporal 

and spatial correlations between these 

approaches still have not been 

evaluated, contributing to uncertainty 

regarding the potential for exposure 

measurement error (U.S. EPA, 2019, 

section 3.3.1.1, Table 11–11). The 2019 

ISA concludes that this uncertainty 

‘‘reduces the confidence in the 

associations observed across studies’’ 

(U.S. EPA, 2020, p. 11–125) and 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). 

With regard to short-term PM10–2.5

exposures and mortality, 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 last review, multicity 

epidemiological 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). These studies 

contribute to increasing confidence in 

the relationship between the short-term 

PM10–2.5 exposures and mortality, 

however, the use of varying approaches 

to estimate PM10–2.5 exposures continue 

to contribute uncertainty to the 

associations observed. Additionally, the 

2019 ISA notes than an analysis by Adar 

et al. (2014) indicates ‘‘possible 

evidence of publications bias, which 

was not observed for PM2.5’’ (U.S. EPA, 

2019, section 11.3.2, p. 11–106). Studies 

newly available in this review expand 

the assessment of potential copollutant 

confounding of the short-term PM10–2.5- 

mortality relationship and provide 

evidence that PM10–2.5 associations 

generally remain positive in copollutant 

models, although 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 by a 

lack of information on the correlation 

between PM10–2.5 and gaseous pollutants 

and the small number of locations 

where copollutant analyses have been 

conducted. Associations with causespecific 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). As 

discussed further below, the ISA 

concludes that 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 cause-specific 

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). 

ii. Cardiovascular Effects 

With regard to long-term exposures, 

the evidence available in the last review 

describing the relationship between 

long-term exposure to PM10–2.5 and 

cardiovascular effects was characterized 

in the 2009 ISA as ‘‘inadequate to infer 

the presence or absence of a causal 

relationship.’’ The limited number of 

epidemiological studies available at that 

time 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 of long-term PM10–2.5

exposures and cardiovascular mortality 

remains limited, with no consistent 

pattern of associations across studies, 

and as discussed above, uncertainty 

from the use of various approaches for 

estimating PM10–2.5 concentrations (U.S. 

EPA, 2019, Table 6–70). The evidence 

for associations between PM10–2.5 and 

cardiovascular morbidity has grown 

and, while results across studies are not 

entirely consistent, some 

epidemiological 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). With 

respect to copollutant confounding, the 

effect estimates for PM10–2.5- 

cardiovascular mortality are often 

attenuated, but remain positive, in 

copollutant models adjusted for PM2.5. 

For cardiovascular 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 2019 ISA 

concluded that ‘‘evidence from 

experimental animal studies is of 

insufficient quantity to establish 

biological plausibility’’ (U.S. EPA, 2019, 

p. 6–277). Despite this substantial data 

gap in the toxicologic evidence for longterm PM10–2.5 exposures and based 

largely on the observation of positive 

associations in some high-quality 

epidemiological 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). 

With regard to short-term PM10–2.5

exposures and cardiovascular effects, 

the 2009 ISA found the available 

evidence was ‘‘suggestive of a causal 

relationship,’’ based primarily on 

several epidemiological studies 

reporting associations between shortterm PM10–2.5 exposure and 

cardiovascular effects, including IHD 

hospitalizations, supraventricular 

ectopy, and changes in heart rate 

variability (HRV). In addition, studies 

found increases in cardiovascular 

disease emergency department visits 

and hospital admissions linked to dust 

storm events resulting in high 

concentrations of crustal material. 

However, the 2009 ISA noted the 

potential for exposure measurement 

error and copollutant confounding in 

these studies. Moreover, there was only 

limited evidence of cardiovascular 

effects from a small number of 

controlled human exposure and animal 

toxicologic studies that examined 

PM10–2.5 exposures (U.S. EPA, 2009c, 

section 6.2.12.2). Therefore, the 

potential for exposure measurement 

error and copollutant confounding, 

along with the limited evidence of 

biological plausibility for cardiovascular 

effects following inhalation exposure, 

contributed uncertainty to the scientific 

evidence available at the time of the last 

review (U.S. EPA, 2009c, section 

6.3.13). 

The evidence related to short-term 

PM10–2.5 exposure and cardiovascular 


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effects has somewhat expanded since 

the last review, but a number of 

important uncertainties persist. The 

2019 ISA notes that there are a small 

number of epidemiological studies 

reporting positive associations between 

short-term PM10–2.5 exposures and 

cardiovascular morbidity. There 

continues to be limited evidence, 

however, to suggest that these 

associations are biologically plausible, 

or independent of copollutant 

confounding. Additionally, the ISA 

concludes that it remains unclear how 

the approaches used to estimate PM10–2.5

concentrations in epidemiological 

studies may impact exposure 

measurement error. The 2019 ISA 

concludes that overall ‘‘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). 

iii. Respiratory Effects 

With regard to short-term PM10–2.5

exposures and respiratory effects, the 

2009 ISA concluded that, based on a 

small number of epidemiological 

studies observing some respiratory 

effects and limited evidence to support 

biological plausibility, the relationship 

is ‘‘suggestive of a causal relationship.’’ 

Epidemiological 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 of pulmonary inflammation. 

Animal toxicologic studies were limited 

to those that used non-inhalation (e.g., 

intra-tracheal instillation) routes of 

PM10–2.5 exposure. 

Recently available epidemiological 

studies link short-term PM10–2.5

exposure with asthma exacerbation and 

respiratory mortality. Some associations 

remained positive in copollutant models 

including PM2.5 or gaseous pollutants, 

although associations were attenuated 

in some studies of mortality. Limited 

evidence is available that observes 

positive associations with other 

respiratory outcomes, including COPD 

exacerbation, respiratory infection, and 

combined respiratory-related diseases 

(U.S. EPA, 2019, Table 5–36). The lack 

of systematic evaluation of the various 

methods used to estimate PM10–2.5

concentrations and the resulting spatial 

and temporal variability in PM10–2.5

concentrations compared to PM2.5

continues to be an uncertainty in this 

evidence (U.S. EPA, 2019, sections 

2.5.1.2.3 and 3.3.1.1). Based on the 

overall evidence, the 2019 ISA 

concludes that the ‘‘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). 

iv. Cancer 

In the last review, little information 

was available from studies of cancer 

following inhalation exposures to 

PM10–2.5. Thus, the 2009 ISA concluded 

that the evidence was ‘‘inadequate to 

assess the relationship between longterm PM10–2.5 exposures and cancer’’ 

(U.S. EPA, 2009c). Since the last review, 

the available studies of long-term 

PM10–2.5 exposure and cancer remain 

limited, with a few recent 

epidemiological studies that report 

positive, but imprecise, associations 

with lung cancer incidence. Uncertainty 

remains in these studies due to 

exposure measurement error from 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). Very few 

experimental studies of PM10–2.5

exposures have been conducted, 

although the available studies indicate 

that PM10–2.5 exhibits genotoxicity and 

oxidative stress, two key characteristics 

of carcinogens. While limited, these 

studies provide some evidence of 

biological plausibility for the findings in 

a small number of epidemiological 

studies (U.S. EPA, 2019, section 10.3.4). 

Taken together, the small number of 

available epidemiological and 

experimental studies, along with 

uncertainty related to exposure 

measurement error, contribute to the 

2019 ISA conclusion 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). 

v. Metabolic Effects 

The 2009 ISA did not make a 

causality determination for PM10–2.5- 

related metabolic effects. Since the last 

review, one epidemiological 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). Uncertainties 

with this evidence include the potential 

for copollutant confounding and 

exposure measurement error (U.S. EPA, 

2019, Tables 7–14 7–15). There is 

limited evidence to support biological 

plausibility of metabolic effects, 

although 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 somewhat expanded evidence 

available in this review, the 2019 ISA 

concludes that ‘‘the evidence is 

suggestive of, but not sufficient to infer, 

a causal relationship between [long]- 

term PM10–2.5 exposures and metabolic 

effects’’ (U.S. EPA, 2019, p. 7–56). 

vi. Nervous System Effects 

The 2009 ISA did not make a causal 

determination for PM10–2.5 exposures 

and nervous system effects. Newly 

available evidence since that time 

includes epidemiological studies that 

report associations between long-term 

PM10–2.5 exposures 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 PM10–2.5 exposure with 

neurodevelopmental effects are not 

consistently reported in children (U.S. 

EPA, 2019, section 8.4.4 and 8.4.5). 

Uncertainties in these studies include 

the potential for copollutant 

confounding, given that no studies 

examined copollutant models (U.S. 

EPA, 2019, section 8.4.5), and exposure 

measurement error based on the various 

methods used across studies to estimate 

PM10–2.5 concentrations (U.S. EPA, 2019, 

Table 8–25). Additionally, there is very 

limited animal toxicologic evidence to 

provide support for biological 

plausibility of nervous system effects 

(U.S. EPA, 2019, sections 8.4.1 and 

8.4.5). Considering the available studies 

and associated limitations, 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). 

B. Conclusions on the Primary PM10

Standard 

In drawing conclusions on the 

adequacy of the current primary PM10

standard, in view of the advances in 

scientific knowledge and additional 

information now available, the 

Administrator has considered the 

evidence base, information, and policy 

judgments that were the foundation of 

the last review and reflects upon the 

body of evidence and information newly 

available in this review. In so doing, the 

Administrator has taken into account 

the evidence-based considerations, as 

well as advice from the CASAC and 

public comments. Evidence-based 

considerations draw upon the EPA’s 

assessment and integrated synthesis of 

the scientific evidence from animal 


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toxicologic, controlled human exposure 

studies, and epidemiological studies 

evaluating health effects related to 

exposures to PM10–2.5 as presented in the 

ISA and discussed in section III.A.2. In 

addition to the evidence, the 

Administrator has weighed a range of 

policy-relevant considerations as 

discussed in the PA and summarized in 

sections III.B and III.C of the proposal 

and summarized in section III.B.2 

below. These considerations, along with 

the advice from the CASAC (section 

III.B.1) and public comments (section 

III.B.3), are discussed below. A more 

detailed summary of all significant 

comments, along with the EPA’s 

responses (henceforth ‘‘Response to 

Comments’’), can be found in the docket 

for this rulemaking (Docket No. EPA– 

HQ–OAR–2015–0072). This document 

is available for review in the docket for 

this rulemaking and through the EPA’s 

NAAQS website (https://www.epa.gov/ 

naaqs/particulate-matter-pm-airquality-standards). The Administrator’s 

conclusions in this review regarding the 

adequacy of the current primary PM10

standard and whether any revisions are 

appropriate are described in section 

III.B.4. 

1. CASAC Advice in This Review 

As a part of the 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.B.1 

above), 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 agrees with the 

draft PA ‘‘that key uncertainties 

identified in the last review remain’’ 

(Cox, 2019a, p. 13 of consensus 

responses) and that ‘‘none of the 

identified health outcomes linked to 

PM10–2.5’’ were judged to be causal or 

likely causal. (Cox, 2019a, p. 12 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 controlled human exposure 

and animal toxicology studies of the 

PM10–2.5 fraction to improve the 

understanding of biological causal 

mechanisms and pathway (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 consideration of 

retaining the current standard in this 

review’’ (Cox, 2019a, p. 3 of letter). 

2. Basis for the Proposed Decision 

At the time of the proposal, the 

Administrator carefully considered the 

assessment of the current evidence and 

conclusions reached in the ISA, 

considerations and staff conclusions 

and associated rationales presented in 

the PA, and the advice and 

recommendations of the CASAC (85 FR 

24125, April 30, 2020). In reaching his 

proposed decision on the primary PM10

standard, the Administrator first noted 

the decision to retain the primary PM10

standard in the last review recognized 

that epidemiological studies had 

reported positive associations between 

PM10–2.5 and mortality and morbidity in 

cities across North America, Europe, 

and Asia. The studies encompassed a 

variety of environments where PM10–2.5

sources and composition were expected 

to vary widely. Although many of the 

studies examined associations between 

PM10–2.5 and health effects in urban 

areas, some of the studies also linked 

mortality and morbidity with relatively 

high ambient concentrations of particles 

of non-urban crustal origin. Drawing on 

this information, the EPA judged that it 

was 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 Administrator noted that the 

evidence for several PM10–2.5-related 

health effects, particularly for long-term 

exposures, has expanded since the time 

of the last review. Recently available 

epidemiological studies conducted in 

North America, Europe, and Asia 

continue to report positive associations 

with mortality and morbidity in cities 

where PM10–2.5 sources and composition 

are expected to vary widely, but 

uncertainties remain with respect to the 

methods used to assign exposure in the 

studies. While the Administrator 

recognized that important uncertainties 

persist in the scientific evidence, as 

described below and in section III.A.2 

above, he also recognized that PM10–2.5

exposures may be associated with a 

broader range of health effects that have 

been linked with PM10–2.5 exposures. 

These studies provide an important part 

of the body of evidence supporting the 

ISA’s revised causality determinations, 

including new determinations, for longterm 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 on this information, the 

Administrator proposed to conclude 

that the scientific studies available since 

the last review continue to support 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 the uncertainties in the 

scientific evidence, the Administrator 

noted that the decision in the last 

review highlighted limitations in the 

estimates of ambient PM10–2.5

concentrations used in epidemiological 

studies, the limited evaluation of 

copollutant models to address potential 

confounding, and the limited number of 

experimental studies to support 

biologically plausible pathways for 

PM10–2.5-related health effects. These 

and other limitations raised questions as 

to whether additional public health 

improvements would be achieved by 

revising the existing PM10 standard. 

Despite some additional new 

evidence available in this review, the 

Administrator recognized that, similar 

to the last review, uncertainties remain 

in the scientific evidence for PM10–2.5- 

related health effects. As summarized 

above (section III.A.2), these include 

uncertainties in the PM10–2.5 exposure 

estimates used in epidemiological 

studies, in the independence of PM10–2.5

health effect associations, and in 

support for the biologic plausibility of 

PM10–2.5-related effects from controlled 

human exposure and animal toxicologic 

studies (U.S. EPA, 2020, section 4.2). 

These uncertainties contributed to the 

conclusions in the 2019 ISA that the 

evidence for key PM10–2.5 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’’ or ‘‘likely 

to be causal’’ relationships in the 

current review, the Administrator 

judged that the evidence of health 

effects associated with PM10–2.5 in 

ambient air provides an uncertain 

scientific foundation for making 

decisions for standard setting. As such, 

he further judged that, consistent with 

the last review, limitations in the 

evidence raise questions as to whether 

additional public health protections 

would be achieved by revising the 

existing PM10 standard. 

In reaching his proposed conclusions 

on the primary PM10 standard, the 

Administrator additionally considered 

the advice and recommendations from 

the CASAC. As described above (section 

III.B.1), the CASAC recognized the 


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60See generally Whitman v. American Trucking 

Associations, 531 U.S. 457, 465–472, 475–76 (2001). 

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 agreed with 

the PA conclusion that the evidence 

available in this review ‘‘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 recommended that this evidence 

‘‘supports consideration of retaining the 

current standard in this review’’ (Cox, 

2019a, p. 3 of letter). 

In considering the information above, 

the Administrator proposed 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 reflected the expanded 

evidence available in this review for 

health effects from PM10–2.5 exposures. 

However, important uncertainties and 

limitations in the evidence remain. 

Consistent with the decision in the last 

review, the Administrator proposed to 

conclude that these limitations 

contribute to considerable uncertainty 

regarding the potential public health 

implications of revising the existing 

PM10 standard. Given this uncertainty, 

and consistent with the advice from the 

CASAC, the Administrator proposed 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 proposed to retain the 

primary PM10 standard, without 

revision. 

3. Comments on the Proposed Decision 

Of the public comments received on 

the proposal, very few commenters 

provided comments on the primary 

PM10 standard. Of those commenters 

who did provide comments on the 

primary PM10 standard, the majority 

supported the Administrator’s proposed 

decision to retain the current primary 

PM10 standard, without revision. This 

group includes primarily industries and 

industry groups. All of these 

commenters generally note their 

agreements with the rationale provided 

in the proposal and the CASAC 

concurrence with the PA conclusion 

that the current evidence does not 

support revision to the standard. Most 

also cite the EPA and CASAC 

statements that the newly available 

information in this review does not call 

into question the adequacy of the 

current standard. The EPA agrees with 

these comments and with the CASAC 

advice regarding the adequacy of the 

current primary standard and the lack of 

support for revision of the standard. 

Some commenters disagreed with the 

Administrator’s proposed conclusion to 

retain the current primary PM10

standard, primarily focusing their 

comments on the need for revisions to 

the form of the standard or the level of 

the standard. With regard to comments 

on the form of the standard, some 

commenters assert that the EPA should 

revise the standard by adopting a 

separate form (or a ‘‘compliance 

threshold’’ in their words)—the 99th 

percentile, averaged over three years— 

for the primary PM10 standard for 

continuous monitors, which provide 

data every day, while maintaining the 

current form of the standard (one 

exceedance, averaged over three years) 

for 1-in-6 samplers, given the 

widespread use of continuous 

monitoring and to ease the burden of 

demonstrating exceptional events. 

These commenters, in support of their 

comment, contend that the 99th 

percentile would effectively change the 

form from the 2nd high to the 4th high 

and would allow no more than three 

exceedances per year, averaged over 

three years. These commenters 

additionally highlight the EPA’s 

decision in the 1997 review to adopt a 

99th percentile form, averaged over 

three years, citing to advantages of a 

percentile-based form in the 

Administrator’s rationale in that review. 

The comments further assert that a 99th 

percentile form for the primary PM10

standard is still more conservative than 

the form for other short-term NAAQS 

(e.g., PM2.5 and NO2). 

First, the EPA has long recognized 

that the form is an integral part of the 

NAAQS and must be selected together 

with the other elements of the NAAQS 

to ensure the appropriate stringency and 

requisite degree of public health 

protection. Thus, if the EPA were to 

change the form according to the 

monitoring method it would be 

establishing two different NAAQS, 

varying based on the monitoring 

method. The EPA has not done this to 

date, did not propose such an approach, 

and declines to adopt it for the final 

rule, as we believe such a decision in 

this final rule is beyond the scope of the 

proposal, and that each PM standard 

should have a single form, indicator, 

level and averaging time, chosen by the 

Administrator as necessary and 

appropriate. While certain continuous 

monitors may be established and 

approved as a Federal Equivalent 

Method (FEM) for PM10, as an 

alternative to a Federal Reference 

Method (FRM), the use of an FEM is 

intended as an alternative means of 

determining compliance with the 

NAAQS, not as authorizing a different 

NAAQS. 

Even if the commenters had asked 

that the change in form be made without 

regard to monitoring method, the EPA 

does not believe such a change would 

be warranted. The change in form for 

continuous monitors suggested by the 

commenters, without also lowering the 

level of such a standard, would allow 

more exceedances and thereby markedly 

reduce the public health protection 

provided against exposures to PM10–2.5

in ambient air. These commenters have 

not provided new evidence or analyses 

to support their conclusion that an 

appropriate degree of public health 

protection could be achieved by 

allowing the use of an alternative form 

(i.e., 99th percentile), while retaining 

the other elements of the standard. 

With regard to the commenters’ 

assertion that an alternate form of the 

standard would ease the burden of 

demonstrating exceptional events, the 

EPA first recognizes, consistent with the 

CAA, that it may be appropriate to 

exclude monitoring data influenced by 

‘‘exceptional’’ events when making 

certain regulatory determinations. 

However, the EPA notes that the cost of 

implementation of the standards may 

not be considered by the EPA in 

reviewing the standards 60 and further 

the EPA believes it is unnecessary to 

alter the standard for the purpose of 

reducing the burden of demonstrating 

exceptional events. The EPA continues 

to update and develop documentation 

and tools to facilitate the 

implementation of the 2016 Exceptional 

Events Rule, including new documents 

intended to assist air agencies with the 

development of demonstrations for 

specific types of exceptional events. 

Moreover, with regard to the 

commenters’ specific concerns for 

wildfires or high winds, the EPA 

released updated guidance documents 

on the preparation of exceptional event 

demonstrations related to wildfires in 

September 2016, high wind dust events 

in April 2019, and prescribed fires in 

August 2019. These guidance 

documents outline the regulatory 

requirements and provide examples for 

air agencies preparing demonstrations 

for wildfires, high wind dust, and 

prescribed fire events. 

For all of the reasons discussed above, 

the EPA does not agree with the 

commenters that the form of the primary 

PM10 standard should be revised to a 

99th percentile for continuous monitors. 


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61PM10 concentrations presented as the annual 

second maximum 24-hour concentration (in mg/m3) 

at 262 sites in the U.S. For more information, see: 

https://www.epa.gov/air-trends/particulate-matterpm10-trends. 

62PM2.5 concentrations presented as the 

seasonally-weighted annual average concentration 

(in mg/m3) at 406 sites in the U.S. For more 

information, see: https://www.epa.gov/air-trends/ 

particulate-matter-pm25-trends. 

Some commenters who disagreed 

with the proposal to retain the current 

standard advocate for revision to the 

primary PM10 standard to protect public 

health with an adequate margin of 

safety. In their recommendations for 

revising the standard, some commenters 

contend that the current standard, with 

its indicator of PM10 to target exposures 

to PM10–2.5, has become less protective 

as ambient concentrations of PM2.5 have 

been reduced with revisions to that 

standard. These commenters assert that 

the current primary PM10 standard 

allows increased exposure to PM10–2.5 in 

ambient air because retaining the 

primary PM10 would allow 

proportionately more PM10–2.5 mass as 

the PM2.5 standard has been revised 

downward. Moreover, in support of 

their recommendations, the commenters 

note that the available evidence of 

PM10–2.5-related health effects has been 

expanded and strengthened since the 

time of the last review. Taken together, 

the commenters contend that the 

primary PM10 standard should be 

revised and failure to do so would be 

arbitrary and capricious. 

We disagree with the commenters that 

the primary PM10 standard should be 

revised because reductions in ambient 

concentrations of PM2.5 result in a less 

protective PM10 standard. As an initial 

matter, we note that overall, ambient 

concentrations of both PM10 and PM2.5

have declined significantly over time. 

Ambient concentrations of PM10 have 

declined by 46% across the U.S. from 

2000 to 2019,61 while PM2.5

concentrations in ambient air have 

declined by 43% during this same time 

period.62 While trends data is not 

currently available for PM10–2.5

concentrations in ambient air, the 

expanded availability of monitoring 

data from the NCore network in this 

review can provide insight into the 

relative contributions of fine and coarse 

PM to total PM10 concentrations. 

The 2019 ISA provides a comparison 

of the relative contribution of PM2.5 and 

PM10–2.5 to PM10 concentrations by 

region and season using the more 

comprehensive monitoring data from 

the NCore network available in this 

review (U.S. EPA, 2019, section 

2.5.1.1.4). The data indicate that, for 

urban areas, there are roughly 

equivalent amounts of PM2.5 and 

PM10–2.5 contributing to PM10 in ambient 

air, while rural locations have a slightly 

higher contribution of PM10–2.5

contributing to PM10 concentrations 

than PM2.5 (U.S. EPA, 2019, section 

2.5.1.1.4, Table 2–7). There is generally 

a greater contribution from the PM2.5

fraction in the East and a greater 

contribution from the PM10–2.5 fraction 

in the West and Midwest. However, as 

described in the 2019 ISA, PM10 has 

become considerably coarser across the 

U.S. compared to similar observations 

in the 2009 ISA (U.S. EPA, 2019, section 

2.5.1.1.4; U.S. EPA, 2009c). 

The EPA recognizes that when the 

primary annual PM2.5 standard was 

revised from 15 mg/m3 to 12 mg/m3

while leaving the 24-hour PM2.5

standards unchanged at 35 mg/m3 and 

the 24-hour PM10 standard unchanged at 

150 mg/m3, the PM10–2.5 fraction of PM10

could increase in some areas as the 

PM2.5 fraction decreases. Moreover, the 

EPA recognizes that in most areas of the 

country PM2.5 and PM10 concentrations 

have declined and are well below their 

respective 24-hour standards, which 

may also allow the relative ratio of PM2.5

to PM10–2.5 to vary. In considering the 

available health effects evidence in this 

review, there continue to be significant 

uncertainties and limitations that make 

it difficult to fully assess the public 

health implications of revising the 

primary PM10 standard even considering 

the possibility for additional variability 

in the relative ratio of PM2.5 to PM10–2.5

in current PM10 air quality across the 

U.S. As described in detail above in 

section III.A.2 and in the proposal (85 

FR 24125, April 30, 2020), these 

uncertainties contribute to the 

determinations in the 2019 ISA that the 

evidence for key PM10–2.5 health effects 

is ‘‘suggestive of, but not sufficient to 

infer, a causal relationship’’ (U.S. EPA, 

2019). Beyond these uncertainties, the 

EPA also notes that, while the NCore 

monitoring network has been expanded 

since the time of the last review, 

epidemiological studies available in this 

review do not use PM10–2.5 NCore data 

in evaluating associations between 

PM10–2.5 in ambient air and long- or 

short-term exposures. In the absence of 

such evidence, the public health 

implications of changes in ambient 

PM10 concentrations as PM2.5

concentrations decrease remain unclear. 

Therefore, the EPA continues to 

recognize this as an area for future 

research, to address the existing 

uncertainties (U.S. EPA, 2020, section 

4.5), and inform future reviews of the 

PM NAAQS. 

Taken together, at the time of 

proposal, the Administrator concluded 

that 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. 

Therefore, the EPA does not agree with 

the commenters that the currently 

available air quality information or 

scientific evidence support revisions to 

the primary PM10 standard in this 

review. 

4. Administrator’s Conclusions 

Having carefully considered advice 

from the CASAC and the public 

comments, as discussed above, the 

Administrator believes that the 

fundamental scientific conclusions on 

health effects of PM10–2.5 in ambient air 

that were reached in the ISA and 

summarized in the PA remain valid. 

Additionally, the Administrator believes 

the judgments he proposed (85 FR 

24125, April 30, 2020) with regard to 

the evidence remain appropriate. 

Further, in considering the adequacy of 

the current primary PM10 standard in 

this review, the Administrator has 

carefully considered the policy-relevant 

evidence and conclusions contained in 

the ISA; the rationale and conclusions 

presented in the PA; the advice and 

recommendations from the CASAC; and 

public comments, as addressed in 

section III.B.3 above. In the discussion 

below, the Administrator gives weight 

to the PA conclusions, with which the 

CASAC has concurred, as summarized 

in section III.D of the proposal, and 

takes note of the key aspects of the 

rationale for those conclusions that 

contribute to his decision in this review. 

After giving careful consideration to all 

of this information, the Administrator 

believes that the conclusions and policy 

judgments supporting his proposed 

decision remain valid, and that the 

current primary PM10 standard provides 

requisite protection of public health 

with an adequate margin of safety and 

should be retained. 

In considering the PA evaluations and 

conclusions, the Administrator 

specifically notes that, while the health 

effects evidence is somewhat expanded 

since the last review, the overall 

conclusions are generally consistent 

with what was considered in the last 

review (U.S. EPA, 2020, section 4.4). In 

so doing, he additionally notes that the 

CASAC supports retaining the current 

standard, agreeing 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’’ (Cox, 

2019a, p. 3 of letter). As noted below, 

the newly available evidence for several 

PM10–2.5-related health effects has 


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expanded since the last review, in 

particular for long-term exposures. The 

Administrator recognizes, however, that 

there are a number of uncertainties and 

limitations associated with the available 

information, as described in the 

proposal (85 FR 24125, April 30, 2020) 

and below. 

With regard to the current evidence 

on PM10–2.5-related health effects, the 

Administrator takes note of recent 

epidemiological studies that 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 

significant uncertainties remain, as 

described below, the Administrator 

recognizes that this expanded body of 

evidence has broadened the range of 

effects that have been linked with 

PM10–2.5 exposures. These studies 

provide an important part of the 

scientific foundation supporting the 

ISA’s revised 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 concludes 

that the scientific information available 

since the time of the last review 

supports a decision to maintain a 

primary PM10 standard to provide 

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 first notes 

that a number of limitations were 

identified in the last review related to: 

(1) Estimates of ambient PM10–2.5

concentrations used in epidemiological 

studies; (2) limited evaluation of 

copollutant models to address the 

potential for confounding; and (3) 

limited experimental studies supporting 

biological plausibility for PM10–2.5- 

related effects. In the current review, 

despite the expanded body of evidence 

for PM10–2.5 exposures and health 

effects, the Administrator recognizes 

that similar uncertainties remain. As 

summarized in section III.B.1 above and 

in responding to public comments, 

uncertainties in the current review 

continue to include those associated 

with the exposure estimates used in 

epidemiological studies, the 

independence of the PM10–2.5 health 

effect associations, and the biologically 

plausible pathways for PM10–2.5 health 

effects (U.S. EPA, 2020, section 4.2). 

These uncertainties contribute to the 

2019 ISA determinations that the 

evidence is ‘‘suggestive of, but not 

sufficient to infer’’ causal relationships 

(U.S. EPA, 2019). In light of his 

emphasis on evidence supporting 

‘‘causal’’ or ‘‘likely to be causal’’ 

relationships (sections II.A.2 and III.A.2 

above), recognizing that the NAAQS 

should allow for a margin of safety but 

finding that there is too much 

uncertainty that a more stringent 

standard would improve public health, 

the Administrator judges that the 

available evidence provides support for 

his conclusion that the current standard 

provides the requisite level of protection 

from the effects of PM10–2.5. 

In making this judgment, the 

Administrator considers whether this 

level of protection is more than what is 

requisite and whether a less stringent 

standard would be appropriate to 

consider. He notes that there continues 

to be uncertainty associated with the 

evidence, for example exposure 

measurement error, as reflected by the 

‘‘suggestive of, but not sufficient to 

infer’’ causal determinations. The 

Administrator recognizes that the CAA 

requirement that primary standards 

provide an adequate margin of safety, as 

summarized in section I.A above, is 

intended to address uncertainties 

associated with inconclusive scientific 

evidence and technical information, as 

well as to provide a reasonable degree 

of protection against hazards that 

research has not yet identified. Based on 

all of the considerations noted here, and 

considering the current body of 

evidence, including uncertainties and 

limitations, the Administrator 

concludes that a less stringent standard 

would not provide the requisite 

protection of public health, including an 

adequate margin of safety. 

The Administrator also considers 

whether the level of protection 

associated with the current standard is 

less than what is requisite and whether 

a more stringent standard would be 

appropriate to consider. In so doing, the 

Administrator considers, as discussed 

above, the level of protection offered 

from exposures for which public health 

implications are less clear. In so doing, 

he again notes the significant 

uncertainties and limitations that persist 

in the scientific evidence in this review. 

In particular, he notes limitations in the 

approaches used to estimate ambient 

PM10–2.5 concentrations in 

epidemiological 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 epidemiological 

studies that are reflected by the 

‘‘suggestive of, but not sufficient to 

infer’’ causal determinations. Thus, in 

light of the currently available 

information, including the uncertainties 

and limitations of the evidence base 

available to inform his judgments 

regarding protection against PM10–2.5- 

related effects, the Administrator does 

not find it appropriate to increase the 

stringency of the standard in order to 

provide the requisite public health 

protection. Rather, he judges it 

appropriate to maintain the level of 

protection provided by the current PM10

standard for PM10–2.5 exposures and he 

does not judge the available information 

and the associated uncertainties to 

indicate the need for a greater level of 

public health protection. 

In reaching his conclusions on the 

primary PM10 standard, the 

Administrator also considers advice 

from the CASAC, including that 

regarding uncertainties that remain in 

this review (summarized in section 

III.B.1 above). In their comments, the 

CASAC noted that uncertainties persist 

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). In considering 

these comments, the Administrator 

takes note of the CASAC consideration 

of the uncertainties related to the 

evidence and its conclusion that 

‘‘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 Administrator further 

notes the CASAC overall conclusion in 

this review that the current evidence 

‘‘supports consideration of retaining the 

current standard in this review’’ (Cox, 

2019a, p. 3 of letter). 

Thus, in light of the currently 

available information, including 

uncertainties and limitations in the 

evidence base available to inform his 

judgments regarding public health 

protection, as well as CASAC advice, 

the Administrator does not find it 

appropriate to revise the standard. 

Rather, he judges it appropriate to retain 

the primary PM10 standard to provide 

the requisite degree of public health 

protection against PM10–2.5 exposures, 

regardless of location, source of origin, 

or particle composition. 

With regard to the uncertainties 

identified above, the Administrator 

notes that his final decision in this 

review is a public health policy 

judgment that draws upon scientific 

information, as well as judgments about 

how to consider the range and 

magnitude of uncertainties that are 

inherent in the information. 

Accordingly, he recognizes that his 

decision requires judgments based on 


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63 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. 

64The final ISA was released in October 2020: 

https://www.epa.gov/isa/integrated-scienceassessment-isa-oxides-nitrogen-oxides-sulfur-andparticulate-matter. 

the interpretation of the evidence that 

neither overstates nor understates the 

strength or limitations of the evidence 

nor the appropriate inferences to be 

drawn. He recognizes, as described in 

section I.A above, that the Act does not 

require that primary standards be set at 

a zero-risk level; rather, the NAAQS 

must be sufficient but not more 

stringent than necessary to protect 

public health, including the health of 

sensitive groups with an adequate 

margin of safety. 

Recognizing and building upon all of 

the above considerations and 

judgments, the Administrator has 

reached his conclusion in the current 

review. As an initial matter, he 

recognizes the control exerted by the 

current primary PM10 standard against 

exposures to PM10–2.5 in ambient air. 

With regard to key aspects of the 

specific elements of a standard, the 

Administrator recognizes continued 

support in the current evidence base for 

PM10 as the indicator for the standard. 

In so doing, he notes that such an 

indicator provides protection from 

exposure to all coarse PM, regardless of 

location, source of origin, or particle 

composition. Similarly, with regard to 

averaging time, form, and level of the 

standard, the Administrator takes note 

of uncertainties in the available 

evidence and information and continues 

to find that the current standard, as 

defined by its current elements, is 

requisite. He has additionally 

considered the public comments 

regarding revisions to these elements of 

the standard and continues to judge that 

the existing level and the existing form, 

in all its aspects, together with the other 

elements of the existing standard 

provide an appropriate level of public 

health protection. 

For all of the reasons discussed above, 

and recognizing the CASAC conclusion 

that the current evidence provides 

support for retaining the current 

standard, the Administrator concludes 

that the current primary PM10 standard 

(in all of its elements) is requisite to 

protect public health with an adequate 

margin of safety from effects of PM10–2.5

in ambient air, and should be retained 

without revision. 

C. Decision on the Primary PM10

Standard 

For the reasons discussed above and 

taking into account information and 

assessments presented in the ISA and 

PA, the advice from the CASAC, and 

consideration of public comments, the 

Administrator concludes that the 

current primary PM10 standard is 

requisite to protect public health with 

an adequate margin of safety, including 

the health of at-risk populations, and is 

retaining the current standard without 

revision. 

IV. Rationale for the Decision on the 

Secondary PM Standards 

This section presents the rationale for 

the Administrator’s decision to retain 

the current secondary PM standards, 

without revision. This decision is based 

on a thorough review of the latest 

scientific information generally 

published through December 2017,63 as 

presented in the ISA, on non-ecological 

public welfare effects associated with 

PM and pertaining to the presence of 

PM in ambient air, specifically 

visibility, climate, and materials effects. 

This decision also accounts for analyses 

in the PA of policy-relevant information 

from the ISA and quantitative analyses 

of air quality related to visibility 

impairment; CASAC advice; and 

consideration of public comments 

received on the proposal. 

The EPA is separately reviewing the 

ecological effects associated with PM in 

conjunction with reviews of other 

pollutants that, along with PM, 

contribute jointly to atmospheric 

deposition. As explained in both the PM 

IRP (U.S. EPA, 2016, p. 1–17) and the 

IRP for review of the secondary NAAQS 

for oxides of nitrogen, oxides of sulfur 

and PM (U.S. EPA, 2017, p. 1–1), and 

discussed in the proposal for this review 

(85 FR 24127, April 30, 2020), in 

recognition of the linkages between 

oxides of nitrogen, oxides of sulfur, and 

PM with respect to atmospheric 

deposition, and with respect to the 

ecological effects, the reviews of the 

ecological effects evidence and the 

secondary standards for these pollutants 

are being conducted together. 

Addressing the pollutants together 

enables the EPA to take a 

comprehensive approach to considering 

the nature and interactions of the 

pollutants, which is important for 

ensuring that all scientific information 

relevant to ecological effects is 

thoroughly evaluated. This combined 

review of the ecological criteria for 

oxides of nitrogen, oxides of sulfur, and 

particulate matter is ongoing.64

Section IV.A provides background on 

the general approach for this review and 

the basis for the existing secondary PM 

standards, and also presents brief 

summaries of key aspects of the 

currently available welfare effects 

evidence and quantitative information. 

Section IV.B summarizes the proposed 

conclusions and CASAC advice, 

addresses public comments received on 

the proposal, and presents the 

Administrator’s conclusions on the 

adequacy of the current standards, 

drawing on consideration of this 

information, advice from the CASAC, 

and comments from the public. Section 

IV.C summarizes the Administrator’s 

decision on the secondary PM 

standards. 

A. Introduction 

As in prior reviews, the general 

approach to reviewing the current 

secondary standards is based, most 

fundamentally, on using the EPA’s 

assessment of the current scientific 

evidence and associated quantitative 

analyses to inform the Administrator’s 

judgment regarding secondary standards 

for PM that are requisite to protect the 

public welfare from known or 

anticipated adverse effects associated 

with the presence of PM in the ambient 

air. The EPA’s assessments are primarily 

documented in the ISA and PA, both of 

which have received CASAC review and 

public comment (83 FR 53471, October 

23, 2018; 84 FR 47944, September 11, 

2019). To bridge the gap between the 

scientific assessments of the ISA and 

judgments required of the Administrator 

in determining whether the current 

standards provide the requisite welfare 

protection, the PA evaluates the policy 

implications of the assessment of the 

current evidence in the ISA and of the 

quantitative air quality information 

documented in the PA. In evaluating the 

public welfare protection afforded by 

the current standards, the four basic 

elements of the NAAQS (indicator, 

averaging time, level, and form) are 

considered collectively. 

The secondary standard is to ‘‘specify 

a level of air quality the attainment and 

maintenance of which in the judgment 

of the Administrator . . . is requisite to 

protect the public welfare from any 

known or anticipated adverse effects 

associated with the presence of such air 

pollutant in the ambient air’’ (CAA, 

section 109(b)(2)). The secondary 


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65The 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. 

66 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 suspended 

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). 

standard is not meant to protect against 

all known or anticipated PM-related 

effects, but rather those that are judged 

to be adverse to the public welfare, and 

a bright-line determination of adversity 

is not required in judging what is 

requisite (78 FR 3212, January 15, 2013; 

80 FR 65376, October 26, 2015). Thus, 

the level of protection from known or 

anticipated adverse effects to public 

welfare that is requisite for the 

secondary standard is a public welfare 

policy judgment to be made by the 

Administrator. In exercising that 

judgment, the Administrator seeks to 

establish standards that are neither more 

nor less stringent than necessary for this 

purpose. The Act does not require that 

the standards be set at a zero-risk level, 

but rather at a level that reduces risk to 

protect the public welfare from known 

or anticipated adverse effects. In 

reaching conclusions on the standards, 

the Administrator’s final decision draws 

upon the scientific information and 

analyses about welfare effects, 

environmental exposure and risks, and 

associated public welfare significance, 

as well as judgment about how to 

consider the range and magnitude of 

uncertainties that are inherent in the 

scientific evidence and quantitative 

analyses. The approach to informing 

these judgments is based on the 

recognition that the available evidence 

generally reflects a continuum, 

consisting of levels at which scientists 

generally agree that effects are likely to 

occur, through lower levels at which the 

likelihood and magnitude of the 

responses become increasingly 

uncertain. This approach is consistent 

with the requirements of the CAA and 

with how the EPA and the courts have 

historically interpreted the Act. 

In considering the scientific and 

technical information, we consider both 

the information available at the time of 

the last review and information newly 

available since the last review, 

including most particularly that which 

has been critically analyzed and 

characterized in the current ISA. We 

additionally consider the quantitative 

information described in the PA that 

estimated visibility impairment 

associated with current air quality 

conditions in areas with monitoring 

data that met completeness criteria (U.S. 

EPA, 2020, chapter 5). The evidencebased discussions presented below (and 

summarized more fully in the proposal) 

draw upon evidence from studies 

evaluating visibility, climate, and 

materials effects related to PM in 

ambient air, as discussed in the ISA. 

The quantitative-based discussions also 

presented below (and summarized more 

fully in the proposal) have been drawn 

from the quantitative analyses for PMrelated visibility impairment, as 

discussed in the PA. 

1. Background on the Current Standards 

In the last review, completed in 

2012,65 the EPA retained the secondary 

24-hour PM2.5 standard, with its level of 

35 mg/m3, and the 24-hour PM10

standard, with its level of 150 mg/m3 (78 

FR 3228, January 15, 2013). The EPA 

also retained the level, set at 15 mg/m3, 

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 in the 

last review for non-visibility effects and 

visibility effects are described below. In 

the previous PM NAAQS review, the 

prior Administrator concluded 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 

(78 FR 3225–3226, January 15, 2013; 

U.S. EPA, 2011, section 5.2.3). In 

reaching this decision, the prior 

Administrator considered the scientific 

evidence, noting the 2009 ISA 

conclusion ‘‘that a causal relationship 

exists between PM and effects on 

climate’’ and that aerosols 66 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). She also noted that 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). The prior 

Administrator recognized the strong 

evidence indicating that aerosols affect 

climate and further considered what the 

available information indicated 

regarding the adequacy of protection 

provided by the secondary PM 

standards. In particular, she noted that 

a number of uncertainties in the 

scientific information (i.e., 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) 

affected our ability to conduct a 

quantitative analysis to determine a 

distinct secondary standard based on 

climate. 

In the last review, the prior 

Administrator concluded that 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 PM-related materials effects (78 

FR 3225–3226, January 15, 2013; U.S. 

EPA, 2011, p. 5–29). In reaching this 

conclusion, she considered materials 

effects associated with the deposition of 

PM (i.e., dry and wet deposition), 

including both physical damage 

(materials effects) and aesthetic qualities 

(soiling effects). She noted the 2009 ISA 

conclusion 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), but also 

recognized that 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). 

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 


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67Preference 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. 

68The 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. 

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 

to support revisions of the current 

standards to protect against these other 

welfare effects’’ (Samet, 2010a, p. 5). 

Thus, in considering non-visibility 

welfare effects, 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 

mg/m3 while revising only the form of 

the standard to remove the option for 

spatial averaging (78 FR 3225–3226, 

January 15, 2013). 

Having reached the conclusion 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, 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 mg/ 

m3. These considerations, and the prior 

Administrator’s conclusions regarding 

visibility are summarized below and 

discussed in more detail in the proposal 

(85 FR 24128–24129, April 30, 2020). 

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). 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.67

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., PM 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. 

The prior Administrator concluded 

that it was appropriate to use an 

adjusted version of the original 

IMPROVE algorithm,68 in conjunction 

with monthly average relative humidity 

data based on long-term climatological 

means, as the basis for deriving a 

visibility index. 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, and she recognized 

that the mass monitoring methods 

available at that time were unable to 

measure the full water content of 

ambient PM2.5 and did not provide 

information on the composition of 

PM2.5, both of which contribute to 

visibility impacts (77 FR 38980, June 29, 

2012). As noted at the time of the 

proposal, the prior Administrator 

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). 

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 

average PM2.5 light extinction, as 

evidenced by reasonably high cityspecific 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. 

The statistical form of the index, 3- 

year average of annual 90th percentile 

values, was based on the prior 

Administrator’s consideration of the 

analyses conducted in the 2011 UFVA 

of three different statistics and 

consistency of this statistical form with 

the Regional Haze Program, which 

targets the 20 percent most impaired 

days for improvements in visual air 

quality in Federal Class I areas. 

Moreover, the prior Administrator 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, 


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69The 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. 

70For comparison, 20 dv, 25 dv, and 30 dv are 

equivalent to 64, 112, and 191 megameters (Mm¥1), 

respectively. 

71 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. 

72Uncertainties and limitations in the public 

preference studies included 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. 

2011, p. 4–58).69 The Administrator also 

noted 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). 

In selecting a level for the index, the 

prior Administrator considered the 

‘‘candidate protection levels’’ (CPLs) 70

identified in the 2011 PA based on the 

visibility preference studies, ranging 

from 20 to 30 deciviews (dv),71 while 

noting the uncertainties and limitations 

in these public preference studies.72 She 

concluded that that the current 

substantial degrees of variability and 

uncertainty inherent in the public 

preference studies should be reflected in 

a higher target protection level than 

would be appropriate if the underlying 

information were more consistent and 

certain. 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). 

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 mg/m3, 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. Overview of Welfare Effects Evidence 

In this section, we provide an 

overview of the policy-relevant aspects 

of the welfare effects evidence available 

for consideration in this review. 

Sections IV.B and IV.C of the proposal 

provide a detailed summary of key 

information contained in the ISA and in 

the PA on the visibility and nonvisibility welfare effects associated with 

PM in ambient air, and the related 

public welfare implications (85 FR 

24129, April 30, 2020). The subsections 

below briefly summarize the nature of 

PM-related visibility and non-visibility 

effects. 

a. Nature of Effects 

The evidence base available in the 

current review includes decades of 

research on visibility impairment, 

climate effects, and materials effects 

associated with PM (U.S. EPA, 2004, 

2009c, 2019). 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). The strongest 

evidence for PM-related visibility 

impairment comes from the 

fundamental relationship between light 

extinction and PM mass (U.S. EPA, 

2009c), as well as studies of the public 

perception of visibility impairment 

(U.S. EPA, 2010b), which confirm a 

well-established ‘‘causal relationship 

exists between PM and visibility 

impairment’’ (U.S. EPA, 2009c, p. 2–28). 

Beyond its effects on visibility, the 2009 

ISA also identified a causal relationship 

‘‘between PM and climate effects, 

including both direct effects of radiative 

forcing and indirect effects that involve 

cloud and feedbacks that influence 

precipitation formation and cloud 

lifetimes’’ (U.S. EPA, 2009, p. 2–29). 

The evidence also supports a causal 

relationship between PM and effects on 

materials, including soiling effects and 

materials damage (U.S. EPA, 2009, p. 2– 

31). 

The evidence newly available in this 

review is consistent with the evidence 

available at the time of the last review 

and supports the conclusions of causal 

relationships between PM and visibility, 

climate, and materials effects (U.S. EPA, 

2019, chapter 13). Evidence newly 

available in this review augments the 

previously available evidence of the 

relationship between PM and visibility 

impairment (U.S. EPA, 2019, section 

13.2), climate effects (U.S. EPA, 2019, 

section 13.3), and materials effects (U.S. 

EPA, 2019, section 13.4). 

i. Visibility 

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. Consistent with conclusions of 

causality in the last review, the 2019 

ISA concludes that ‘‘the evidence is 

sufficient to conclude that a causal 

relationship exists between PM and 

visibility impairment’’ (U.S. EPA, 2019, 

section 13.2.6). These conclusions are 

based on the 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). 

The fundamental relationship 

between light extinction and PM mass, 

and the EPA’s understanding of this 

relationship, has changed little since the 


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73The 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). 

74Preference 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, 2011; Smith and Howell, 2009). 

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). 

As at the time of the last review, 

direct measurements of PM light 

extinction, scattering, and absorption 

continue to be considered more accurate 

for quantifying visibility than PM massbased 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 number of measurement 

methods have been used for visibility 

impairment (e.g., transmissometers, 

integrating nephelometers, 

teleradiometers, telephotometers, and 

photography and photographic 

modeling), although each of these 

methods has its own strengths and 

limitations (U.S. EPA, 2019, Table 13– 

1). As recognized in the last review, 

there are no common performancebased 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. 

The theoretical relationship between 

light extinction and PM characteristics, 

as 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). This estimation of light 

extinction is consistent with the method 

used in the last review. The algorithm 

used to estimate light extinction, known 

as the IMPROVE algorithm,73 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). 

At the time of the last review, two 

versions of the IMPROVE algorithm 

were available in the literature—the 

original IMPROVE algorithm (Malm and 

Hand, 2007; Ryan et al., 2005; 

Lowenthal and Kumar, 2004) and the 

revised IMPROVE algorithm (Pitchford 

et al., 2007). As described in detail in 

the proposal (85 FR 24130, April 30, 

2020) and the ISA (U.S. EPA, 2019, 

section 13.2.3), the algorithm has been 

further evaluated and refined since the 

time of the last review (Lowenthal and 

Kumar, 2016), particularly for PM 

characteristics and relative humidity in 

remote areas. All three versions of the 

IMPROVE algorithm were considered in 

evaluating visibility impairment in this 

review. 

Consistent with the evidence 

available at the time of the last review, 

our understanding of public perception 

of visibility impairment comes from 

visibility preference studies conducted 

in four areas in North America.74 The 

detailed methodology for these studies 

are described in the proposal (85 FR 

24131, April 30, 2020), the 2019 ISA 

(U.S. EPA, 2019), and the 2009 ISA 

(U.S. EPA, 2009c). In summary, the 

study participants were queried 

regarding multiple images that 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 the 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. 

Based on the results of the four cities, 

a range encompassing the PM2.5

visibility index values from images that 

were judged to be acceptable by at least 

50 percent of study participants across 

all four of the urban preference studies 

was identified (U.S. EPA, 2010b, p. 4– 

24; U.S. EPA, 2020, 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 level of 

impairment that was determined to be 

‘‘acceptable’’ by at least 50 percent of 

study participants (78 FR 3226–3227, 

January 15, 2013). As noted in the 

proposal (85 FR 24131, April 30, 2020), 

the evidence base for public preferences 

of visibility impairment has not been 

augmented since the last review. There 

are no new visibility preference studies 

that have been conducted in the U.S. 

since the time of the last review and 

there is very little new information 

available regarding acceptable levels of 

visibility impairment in the U.S. 

ii. Climate 

The current evidence continues to 

support the conclusion of a causal 

relationship between PM and climate 

effects (U.S. EPA, 2019, section 13.3.9). 

Since the last review, climate impacts 

and 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 


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75Radiative 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). 

76As 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. 

77 Given the lack of new information to inform a 

different visibility metric, the metric used in the 

updated analyses 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 years, 

and a level of 30 dv (U.S. EPA, 2020, section 

5.2.1.2). 

78While 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. As detailed in the PA, there 

are uncertainties associated with the precision and 

forcing effects 75 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 in the proposal (85 FR 

24131, April 30, 2020), the general 

conclusions are similar between the 

IPCC AR4 and AR5 reports with regard 

to effects of PM on global climate. 

Consistent with the evidence available 

in the last review, the key components, 

including sulfate, nitrate, organic carbon 

(OC), black carbon (BC), and dust, that 

contribute to climate processes vary in 

their reflectivity, forcing efficiencies, 

and direction of forcing. 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 

uncertainties that continue to exist 

related these mechanisms. 

As described in the proposal (85 FR 

24133, April 30, 2020), PM has a very 

heterogeneous distribution globally and 

patterns of forcing 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 variable with 

possible differences in the direction of 

the forcing 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, which 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 (U.S. EPA, 

2019, section 13.3.7). 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. 

iii. Materials 

Consistent with the last review, the 

current evidence continues to support 

the conclusion 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, 

generally involving soiling and/or 

corrosion (U.S. EPA, 2019, section 

13.4.2; 85 FR 24133, April 30, 2020). 

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.76 There is a limited 

amount of new data for consideration in 

this review from studies primarily 

conducted outside of the U.S. on 

buildings and other items of cultural 

heritage. However, these studies 

involved concentrations PM in ambient 

air greater than those typically observed 

in the U.S. (U.S. EPA, 2019, section 

13.4). 

Building on the evidence available in 

the 2009 ISA, and as described in detail 

in the proposal (85 FR 24134, April 30, 

2020) and in the 2019 ISA (U.S. EPA, 

2019, section 13.4), research has 

progressed on: (1) The theoretical 

understanding of soiling of items of 

cultural heritage; (2) the quantification 

of degradation rates and further 

characterization of factors that influence 

damage of stone materials; (3) materials 

damage from PM components besides 

sulfate and black carbon and 

atmospheric gases besides SO2; (4) 

methods for evaluating soiling of 

materials by PM mixtures; (5) PMattributable damage to other materials, 

including glass and photovoltaic panels; 

(6) development of dose-response 

relationships for soiling of building 

materials; and (7) damage functions to 

quantify material decay as a function of 

pollutant type and load. While the 

evidence of PM-related materials effects 

has expanded somewhat since the last 

review, there remains insufficient 

evidence to relate soiling or damage to 

specific PM levels in ambient air or to 

establish a quantitative relationship 

between PM and materials degradation. 

The current evidence is generally 

similar to the evidence available in the 

last review, including associated 

limitations and uncertainties and a lack 

of evidence to inform quantitative 

relationships between PM and materials 

effects, therefore leading to similar 

conclusions about the PM-related effects 

on materials. 

3. Overview of Air Quality and 

Quantitative Information 

a. Visibility Effects 

In the current review, quantitative 

analyses were conducted to further our 

understanding of the relationship 

between recent air quality and 

calculated light extinction. As at the 

time of the last review, these analyses 

explored this relationship as an estimate 

of visibility impairment in terms of the 

24-hour PM2.5 standard and the 

visibility index. Generally, the results of 

the updated analyses are similar to 

those based on the data available at the 

time of the last review (U.S. EPA, 2020, 

section 5.2.1.1). Compared to the last 

review, updated analyses incorporate 

several refinements, including: (1) The 

evaluation of three versions of the 

IMPROVE equation 77 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; 78 (2) the 


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bias of 24-hour PM2.5 measurements (U.S. EPA, 

2020, p. 2–18), as well as to the fractional 

uncertainty associated with 24-hour PM component 

measurements (U.S. EPA, 2020, p. 2–21). Given the 

uncertainties present when evaluating data quality 

on a 24-hour basis, the uncertainty associated with 

sub-daily measurements may be even greater. 

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. 

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 

analyses in the current review are 

updated from the last review and 

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). 

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 

the one location that exceeds the current 

24-hour PM2.5 standard, light extinction 

estimates are 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 (Kelly et al., 2012b; 78 FR 

3201, January 15, 2013). 

Using the recently modified 

IMPROVE equation from Lowenthal and 

Kumar (2016), new in this review, the 

resulting 3-year visibility index is 

slightly higher at all of the sites 

compared to the original and revised 

IMPROVE equation estimates (U.S. EPA, 

2020, Figure 5–4). These higher 

estimates are to be expected, given the 

higher OC multiplier included in the 

IMPROVE equation from Lowenthal and 

Kumar (2016), which reflects the use of 

data from remote areas with higher 

concentrations of organic PM when 

validating the equation. As such, it is 

important to note that the Lowenthal 

and Kumar (2016) version of the 

equation may overestimate light 

extinction in non-remote areas, 

including the urban areas in the 

updated analyses in this review. 

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 2015–2017 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 

IMPROVE equations) (U.S. EPA, 2020, 

Appendix D, Table D–3). The unique 

conditions at this urban site (e.g., higher 

OC concentrations, much lower 

temperatures, and the complete lack of 

sunlight for long periods) that affect 

quantitative relationships between OC, 

OM and visibility (e.g., Hand et al., 

2012; Hand et al., 2013) may differ 

considerably from those under which 

the Lowenthal and Kumar (2016) 

equation has been evaluated, making the 

most appropriate approach for 

characterizing light extinction in this 

area unclear. 

At the time of 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 was a minor contributor to 

visibility impairment (U.S. EPA, 2010b), 

although this fraction may be 

responsible for 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 very little 

data available from PM10–2.5 monitors to 

quantify the contribution of coarse PM 

to calculated light extinction. 

Since the last review, the expansion 

of PM10–2.5 monitoring efforts has 

increased the availability of data for use 

in estimating light extinction. As such, 

both PM2.5 and PM10–2.5 concentrations 

can be included as inputs in the 

equations in the updated analyses in 

this review. For 2015–2017, 20 of the 67 

PM2.5 sites analyzed have collocated 

PM10–2.5 monitoring data available. 

These 20 sites meet both the 24-hour 

PM2.5 and 24-hour PM10 standards. 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 coarse fraction 

contribution to light extinction is 

minimal, contributing less than 1 dv to 

the 3-year visibility metric. The 20 

locations with collocated PM2.5 and 

PM10–2.5 monitoring data available in 

this review would be expected to have 

relatively low concentrations of coarse 

PM. In areas with higher concentrations 

of coarse PM, such as the southwestern 

U.S., the coarse fraction may be a more 

important contributor to light extinction 

and visibility impairment than in the 

locations included in the updated 

analyses in this review. 

Overall, the results of the updated 

analyses in this review 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) results in higher light 

extinction values, which were expected 

given the higher OC multiplier in the 

equation and its validation using data 

from remote areas far away from 

emission sources. There are only small 

differences in estimates of light 

extinction when the coarse fraction is 

included in the equation, although a 

somewhat larger coarse fraction 

contribution to light extinction would 

be expected in areas with higher 

concentrations of coarse PM. Overall, 

the updated 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, 

in terms of a 3-year visibility index. 

b. Non-Visibility Effects 

Consistent with the evidence 

available at the time of the last review, 

and as described in detail in the PA 

(U.S. EPA, 2020, section 5.2.2.2), the 

data remain insufficient to conduct 

quantitative analyses for PM effects on 

climate and materials. For PM-related 

climate effects, as explained in more 

detail in the proposal (85 FR 24131– 

24133, 24136, April 30, 2020), our 

understanding of PM-related climate 

effects is still limited by significant key 

uncertainties. The newly available 

evidence does not appreciably improve 

our 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). Significant 

uncertainties also persist related to 

quantifying the contributions of PM and 

PM components to the direct and 

indirect effects on climate forcing, such 

as changes to the pattern of rainfall, 

changes to wind patterns, and effects on 

vertical mixing in the atmosphere (U.S. 

EPA, 2020, sections 5.2.2.1.1 and 5.4). 

Additionally, while improvements have 

been made to climate models since the 

time of the last review, the 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 our understanding of 


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climate forcing on a global scale is 

somewhat expanded since the last 

review, significant limitations remain to 

quantifying potential adverse PMrelated climate effects in the U.S. and 

how they would vary in response to 

incremental changes in PM 

concentrations across the U.S. As such, 

while new research is available on 

climate forcing on a global scale, the 

remaining limitations and uncertainties 

are significant, and the new global scale 

research does not translate directly for 

use at regional spatial scales. Therefore, 

the evidence does not provide a clear 

understanding at the necessary spatial 

scales for quantifying the relationship 

between PM mass in ambient air and the 

associated climate-related effects in the 

U.S. that would be most relevant to 

informing consideration of a national 

PM standard on climate in this review 

(U.S. EPA, 2020, section 5.2.2.2.1; U.S. 

EPA, 2019, section 13.3). 

For PM-related materials effects, as 

explained in more detail in the proposal 

(85 FR 24133–24134, 24137, April 30, 

2020), the available evidence has been 

somewhat expanded to include 

additional information about the soiling 

process and the types of materials 

impacted by PM. This evidence 

provides some limited information to 

inform dose-response relationships and 

damage functions associated with PM, 

although most of these studies were 

conducted outside of the U.S. where PM 

concentrations in ambient air are 

typically above those observed in the 

U.S. (U.S. EPA, 2020, section 5.2.2.1.2; 

U.S. EPA, 2019, section 13.4). The 

evidence available in this review also 

includes studies examining effects of 

PM on the energy efficiency of solar 

panels and passive cooling building 

materials, although the evidence 

remains insufficient 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 the available 

evidence is somewhat expanded since 

the time of the last review, quantitative 

relationships have not been established 

for PM-related soiling and corrosion and 

frequency of cleaning or repair that 

would help inform our understanding of 

the public welfare implications of 

materials effects (U.S. EPA, 2020, 

section 5.2.2.2.2; U.S. EPA, 2019, 

section 13.4). Therefore, there is 

insufficient information to inform 

quantitative analyses assessing materials 

effects to inform a consideration of a 

national PM standard on materials in 

this review (U.S. EPA, 2020, section 

5.2.2.2.2; U.S. EPA, 2019, section 13.4). 

B. Conclusions on the Secondary 

Standards 

In drawing conclusions on the 

adequacy of the current secondary PM 

standards, in view of the advances in 

scientific knowledge and additional 

information now available, the 

Administrator has considered the 

evidence base, information, and policy 

judgments that were the foundation of 

the last review and reflects upon the 

body of information and evidence 

available in this review. In so doing, the 

Administrator has taken into account 

both evidence-based and quantitative 

information-based considerations, as 

well as advice from the CASAC and 

public comments. Evidence-based 

considerations draw upon the EPA’s 

assessment and integrated synthesis of 

the scientific evidence from studies 

evaluating welfare effects related to 

visibility, climate, and materials 

associated with PM in ambient air as 

discussed in the PA (summarized in 

sections IV..B, V.C, and IV.D.1 of the 

proposal, and section IV.A.2 above). The 

quantitative information-based 

considerations draw from the results of 

the quantitative analyses of visibility 

impairment presented in the PA (as 

summarized in section IV.D.1 of the 

proposal and section IV.A.3 above) and 

consideration of these results in the PA. 

Consideration of the evidence and 

quantitative information in the PA and 

by the Administrator is framed by 

consideration of a series of policyrelevant questions. Section IV.B.2 below 

summarizes the rationale for the 

Administrator’s proposed decision, 

drawing from section IV.D.3 of the 

proposal. The advice and 

recommendations of the CASAC and 

public comments on the proposed 

decision are addressed below in 

sections IV.D.2 and IV.D.3, respectively. 

The Administrator’s conclusions in this 

review regarding the adequacy of the 

secondary PM standards and whether 

any revisions are appropriate are 

described in section IV.D.4. 

1. CASAC Advice in This Review 

In comments on the draft PA, the 

CASAC concurred with the staff’s 

overall preliminary conclusions that it 

is appropriate to consider retaining the 

current secondary 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 agreed that research on PMrelated effects has expanded since the 

last review, it also concluded that ‘‘there 

are still significant uncertainties 

associated with the accurate 

measurement of PM to the direct and 

indirect effects of PM on climate’’ (Cox, 

2019a, pp. 13–14 of consensus 

responses). The committee 

recommended that the EPA summarize 

the ‘‘current scientific knowledge and 

quantitative modeling results for effects 

of reducing PM2.5’’ on several climaterelated 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, climate, and 

materials, the CASAC agreed 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). 

2. Basis for the Proposed Decision 

At the time of the proposal, the 

Administrator carefully considered the 

assessment of the current evidence and 

conclusions reached in the ISA; the 

currently available quantitative 

information, including associated 

limitations and uncertainties, described 

in detail and characterized in the PA; 

considerations and staff conclusions 

and associated rationales presented in 

the PA; and the advice and 

recommendations from the CASAC (85 

FR 24137, April 30, 2020). 

In reaching his proposed decision on 

the secondary PM standards, the 

Administrator first recognized the 

longstanding body of evidence for PMrelated visibility impairment. The 

Administrator recognized that visibility 

impairment can have implications for 

people’s enjoyment of daily activities 

and for their overall sense of well-being. 

In so doing, and consistent with the 

approach used in the last review 

(section IV.A.1 above), the 

Administrator first defined a target level 

of protection in terms of a PM visibility 

index that accounts for the factors that 

influence the relationship between PM 

in ambient air and visibility (i.e., size 

fraction, species composition, and 

relative humidity). He then considered 

air quality analyses examining the 

relationship between this PM visibility 

index and the current 24-hour PM2.5 and 

24-hour PM10 standards in areas that 


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79As described in detail in section IV.A.3.a 

above, the EPA’s updated quantitative analyses in 

this review included 67 areas that met data 

completeness criteria for inclusion in the analyses 

(see U.S. EPA, 2020, Appendix D for details of the 

criteria). Of those monitoring locations that met the 

data completeness criteria, all but one location met 

the current secondary PM2.5 standard (U.S. EPA, 

2020, Table D–7). 

80 In the last review, the focus was on PM2.5

components given their prominent role in PMrelated visibility impairment in urban areas and the 

limited data available for PM10–2.5 (77 FR 38980, 

June 29, 2010; U.S. EPA, 2020, section 5.2.1.2). 

81While 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. As detailed in the PA, there 

are uncertainties associated with the precision and 

bias of 24-hour PM2.5 measurements (U.S. EPA, 

2020, p. 2–18), as well as to the fractional 

uncertainty associated with 24-hour PM component 

measurements (U.S. EPA, 2020, p. 2–21). Given the 

uncertainties present when evaluating data quality 

on a 24-hour basis, the uncertainty associated with 

sub-daily measurements may be even greater. 

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. 82Based on the preference studies, the 2011 PA 

identified a range of levels from 20 to 30 deciviews 

(dv) as being a reasonable range of ‘‘candidate 

protection levels’’ or ‘‘CPLs’’ for a visibility index 

(U.S. EPA, 2011, p. 4–61; U.S. EPA, 2020, section 

5.2.1.1). 

83As noted above, in the last review, the 

Administrator explained that the current substantial 

degrees of variability and uncertainty inherent in 

the public preference studies should be reflected in 

a higher target protection level than would be 

appropriate if the underlying information were 

more consistent and certain (78 FR 3216, January 

15, 2013). 

met data completeness criteria for 

inclusion in the analyses.79

To identify a target level of protection, 

the Administrator first defined the 

specific characteristics of the visibility 

index, noting that in the last review, the 

EPA used an index based on estimates 

of light extinction by PM2.5 components 

calculated using the IMPROVE 

algorithm. As described in section 

IV.A.2 above, the IMPROVE algorithm 

estimates light extinction using 

routinely monitored components of 

PM2.5 and PM10–2.5,80 along with 

estimates of relative humidity. The 

Administrator recognized that, despite 

revisions to the IMPROVE algorithm 

since the last review (U.S. EPA, 2020, 

section 5.2.1.1), our fundamental 

understanding of the relationship 

between PM in ambient air 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 measure 

light extinction (85 FR 24130, 24135, 

April 30, 2020), the Administrator 

judged that estimated light extinction, 

as calculated using the IMPROVE 

algorithms, continued to provide a 

reasonable basis for defining a target 

level of protection against PM-related 

visibility impairment in the current 

review. 

In further defining the characteristics 

of a visibility index based on estimates 

of light extinction, the Administrator 

considered the appropriate averaging 

time, form, and level of the index. The 

Administrator judged that the decisions 

made in the last review with regard to 

averaging time and form remain 

reasonable. In the last review, a 24-hour 

averaging time was judged to be an 

appropriate surrogate for the sub-daily 

periods relevant for visual perception,81

recognizing the relatively strong 

correlations between 24-hour and subdaily (i.e., 4-hour) average PM2.5 light 

extinction and that this longer averaging 

time may be less influenced by atypical 

conditions and/or atypical instrument 

performance (78 FR 3226, January 15, 

2013). In the decision to set the form as 

the 3-year average of annual 90th 

percentile values in the last review, it 

was noted that: (1) A 3-year average 

provided stability from the occasional 

effect of interannual meteorological 

variability (78 FR 3198, January 15, 

2013); (2) the 90th percentile 

corresponds to the 20 percent worst 

days for visibility, which are targeted in 

Class I areas by the Regional Haze 

program; 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). Recognizing that 

the information available in the current 

review is similar to that available in the 

last review, at the time of proposal the 

Administrator judged that these 

decisions remain reasonable, and it 

remains appropriate to define a 

visibility index based on estimated light 

extinction in terms of a 24-hour 

averaging time and a form based on the 

3-year average of annual 90th percentile 

values. 

At the time of the last review, the 

level of the visibility index was set at 30 

dv, based on the upper end of the range 

of levels of visibility impairment judged 

to be acceptable by at least 50% of study 

participants in the available visibility 

preference studies (U.S. EPA, 2020, 

section 5.2.1.1). (78 FR 3226–27, 

January 15, 2013; 85 FR 24131 April 30, 

2020).82 In the last review, the 

Administrator 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 

20 dv to 30 dv 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 3226–27, January 15, 2013). 

In considering the preference studies 

in this review, the Administrator first 

noted that, as a part of the last review, 

a range of levels was identified for the 

PM2.5 visibility index based on an 

aggregated evaluation of the results of 

these studies that reflected variability in 

levels of visibility that were considered 

acceptable in the four study areas (U.S. 

EPA, 2010b). Because no visibility 

preference studies have been conducted 

in the U.S. since the last review, and 

given the general lack of new preference 

studies over the last several reviews, the 

Administrator proposed to conclude 

that the range considered in the last 

review remained appropriate to 

consider in the current review. 

The Administrator highlighted the 

following uncertainties and limitations 

in the underlying public preference 

studies (U.S. EPA, 2020, section 5.2.1.1), 

consistent with those identified in the 

last review: 

• 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 visibility preferences in 

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. 

After considering these preference 

studies, along with their inherent 

uncertainties and limitations, the 

Administrator judged in the proposal 

that a level of 30 dv continued to be an 

appropriate target level of protection for 

the visibility index in the current 

review.83

Having defined a target level of 

protection in terms of a visibility index 

based on the elements described above, 

(i.e., with a 24-hour averaging time; a 3- 

year average of the annual 90th 


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84As discussed above and 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. 

percentile form; and a level of 30 dv), 

the Administrator next considered the 

degree of protection from visibility 

impairment afforded by the existing 

secondary standards. In so doing, he 

considered the updated analyses of PMrelated visibility impairment (U.S. EPA, 

2020, section 5.2.1.2), specifically 

noting the improvements over the 

analyses in the last review, in particular 

the use of multiple versions of the 

IMPROVE algorithm, including the 

version incorporating revisions since 

the last review (85 FR 24135–24136, 

April 30, 2020). The analyses in this 

review expand upon our understanding 

of how variation in equation inputs 

impacts calculated light extinction (U.S. 

EPA, 2020, Appendix D) and also better 

characterizes the influence of the coarse 

fraction on light extinction for the 

subset of sites with available PM10–2.5

monitoring data (U.S. EPA, 2020, 

section 5.2.1.2). 

The Administrator noted that the 

results of the updated analyses are 

consistent with the results from the last 

review, regardless of the IMPROVE 

equation used. The results of the 

analyses demonstrated that, in areas 

meeting the 24-hour PM2.5 standard, the 

3-year visibility metric is at or below 

about 30 dv,84 and is below 25 dv in 

most of the areas. In those locations 

with PM10–2.5 monitoring data available, 

which met both the current 24-hour 

PM2.5 and 24-hour PM10 standards, 3- 

year visibility metrics were at or below 

30 dv regardless of if the coarse fraction 

was included in the calculation (U.S. 

EPA, 2020, section 5.2.1.2). In 

considering these updated analyses, the 

Administrator proposed to conclude 

that the scientific and quantitative 

information available in this review 

support the adequacy of the current 

secondary PM2.5 and PM10 standards to 

protect against PM-related visibility 

impairment. 

With respect to non-visibility welfare 

effects, the Administrator considered 

the evidence related to climate and 

materials effects and proposed to 

conclude 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 PMrelated welfare effects. With regard to 

climate, the Administrator recognized 

that a number of improvements and 

refinements have been made to climate 

models since the last review, while also 

noting that significant limitations 

continue to exist in quantifying the 

contributions of the direct and indirect 

effects of PM and PM components on 

climate forcing (85 FR 24139, April 30, 

2020; U.S. EPA, 2020, sections 5.2.2.1.1 

and 5.4). The Administrator also 

recognized that climate models continue 

to exhibit considerable variability in 

estimates of PM-related climate impacts 

at regional scales (e.g., ∼100 km) 

compared to simulations at global scales 

(85 FR 24139, April 30, 2020; U.S. EPA, 

2020, section 5.2.2.1.1 and 5.4). In 

considering this uncertainty, the 

Administrator proposed to conclude 

that the scientific information available 

in the current review remains 

insufficient to quantify the impacts of 

ambient PM on climate in the U.S. with 

confidence (85 FR 34139, April 30, 

2020; U.S. EPA, 2020, sections 5.2.2.1.1 

and 5.4) and that there is insufficient 

information available in this review to 

base a national ambient air quality 

standard on climate impacts. 

With respect to materials effects, the 

Administrator recognized that 

deposition of fine or coarse particles can 

result in physical damage and/or 

impaired aesthetic qualities. Particles 

can contribute to materials damage by 

adding to the effects of 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. The 

Administrator, while recognizing that 

some new evidence of PM-related 

materials effects is available in this 

review, noted that this evidence is 

primarily from studies conducted 

outside of the U.S. with PM 

concentrations that are higher than 

those typically observed in ambient air 

in the U.S. (U.S. EPA, 2019, section 

13.4). Consistent with the information 

available at the time of the last review, 

the Administrator recognized a limited 

amount of information available 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 public 

welfare. Therefore, at the time of 

proposal, the Administrator judged that 

the scientific information available in 

this review remains insufficient to 

quantify the public welfare impacts of 

PM in ambient air on materials with 

confidence and that there is insufficient 

information available in this review to 

support a distinct national ambient 

standard based on materials effects. 

Thus, based on consideration of the 

scientific and quantitative information 

available in this review, with its 

uncertainties and limitations, and 

information that might inform his 

public welfare judgments, as well as 

consideration of advice from the 

CASAC, including their concurrence 

with the PA conclusions that the current 

evidence does not support revision of 

the secondary PM standards (discussed 

in section IV.B.1 above). The 

Administrator proposed to conclude 

that it is appropriate to retain the 

current secondary PM standards 

without revision based on his judgment 

that the current secondary PM standards 

are requisite to protect against PMrelated effects on visibility and that 

there is insufficient information 

available in this review to base a 

national ambient air quality standard for 

PM on climate and materials impacts. 

3. Comments on the Proposed Decision 

Of the public comments received on 

the proposal, very few were specific to 

the secondary PM standards. Of those 

commenters who did provide comments 

on the secondary PM standards, the 

majority support the Administrator’s 

proposed decision to retain the current 

standards. Some commenters disagree 

with the Administrator’s proposed 

conclusion to retain the current 

secondary standards, primarily focusing 

their comments on the need for a 

revised standard to protect against 

visibility impairment. In addition to the 

comments addressed in this notice, the 

EPA has prepared a Response to 

Comments document that addresses 

other specific comments related to 

setting the secondary PM standards. 

This document is available for review in 

the docket for this rulemaking and 

through the EPA’s NAAQS website 

(https://www.epa.gov/naaqs/particulatematter-pm-air-quality-standards). 

Of the comments addressing the 

proposed decision, many of the 

commenters support the Administrator’s 

proposed decision to retain the current 

secondary PM standards, without 

revision. This group includes industries 

and industry groups and state and local 

governments and organizations. All of 

these commenters generally note their 

agreement with the rationale provided 

in the proposal and with the views 

expressed by the CASAC that the 

current evidence does not support 

revision to the standards. Most also cite 

the EPA and CASAC statements that the 

scientific evidence and quantitative 

information in this review has not 

substantially altered our previous 


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85As noted earlier in section IV, ‘‘the current ISA 

identified and evaluated studies and reports that 

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). 

86As discussed in section I.D, the EPA has 

provisionally considered studies that were 

highlighted by commenters and that were published 

after the ISA. These studies are generally consistent 

with the evidence assessed in the ISA, and they do 

not materially alter our understanding of the 

scientific evidence or the Agency’s conclusions 

based on that evidence or warrant reopening of the 

air quality criteria. 

understanding of the effects of PM on 

visibility, climate, and materials beyond 

what was previously examined and does 

not call into question the adequacy of 

the current standards. They all find the 

proposed decision to retain the current 

standards to be well supported and a 

reasonable exercise of the 

Administrator’s public welfare policy 

judgment under the CAA. The EPA 

agrees with these comments and with 

the CASAC advice regarding the 

adequacy of the current secondary PM 

standards and the lack of support for 

revision of these standards. 

Of the commenters who disagree with 

the proposal to retain the current 

standards, nearly all of these 

commenters recommend more stringent 

standards, primarily to protect against 

visibility impairment. These comments 

were submitted primarily by national 

public health, medical, and 

environmental nongovernmental 

organizations, and some individuals. 

The commenters who recommend 

strengthening the standards state their 

support for revisions to provide greater 

public welfare protection, generally 

claiming that the current standards are 

inadequate and do not provide the 

requisite protection against known or 

anticipated welfare effects. 

Additionally, some of the commenters 

who disagree with the proposal did not 

specifically recommend revising the 

current standards, but instead 

recommend additional research to 

address key uncertainties and 

limitations in the available scientific 

and quantitative information that would 

inform decisions regarding a national 

standard to protect against PM-related 

non-visibility and visibility effects. 

The EPA received relatively few 

comments on the proposed decision that 

it is not appropriate to establish any 

distinct secondary PM standards to 

address PM-related climate effects. The 

majority of the comments that were 

received agree with the EPA that the 

currently available information is not 

sufficient for supporting quantitative 

analyses for the climate effects of PM in 

ambient air. These commenters support 

the Administrator’s proposed decision 

not to set a distinct standard for climate. 

Several commenters note, however, that 

the EPA should frequently reconsider 

the available evidence and quantitative 

information and should revise the 

standard as necessary to provide 

requisite protection against PM-related 

climate effects. The EPA agrees with the 

commenters that quantitative analyses 

of the relationship between PM and 

climate effects are not supported by the 

available information in this review, 

and new information about PM-related 

welfare effects, including climate, will 

be assessed consistent with CAA 

requirements in the next review of the 

PM NAAQS. 

There were also very few commenters 

who commented on the proposed 

decision that it is not appropriate to 

establish any distinct secondary PM 

standards to address PM-related 

material effects. As with comments on 

climate effects, commenters generally 

agree with the EPA that the evidence is 

not sufficient to support quantitative 

analyses for PM-related materials 

effects. However, some commenters 

contend that the EPA failed to consider 

the following information: (1) Studies 

conducted outside of the U.S. on the 

cost of soiling of materials that are also 

found in the U.S.; (2) recent work 

related to soiling of photovoltaic 

modules and other surfaces, and; (3) 

quantitative relationships between PM 

in ambient air and materials effects used 

in several studies. These commenters 

further assert that the EPA failed to 

specify a level of air quality that 

protects against adverse effects of PM on 

materials and failed to propose a 

standard that provides requisite 

protection against materials effects 

attributable to PM. 

We disagree with the commenters that 

the EPA failed to consider the relevant 

scientific information about materials 

effects available in this review. As an 

initial matter, the ISA considered and 

included studies related to materials 

effects of PM, including studies 

conducted in and outside of the U.S., on 

newly studied materials including 

photovoltaic modules that were 

published prior to the cutoff date for the 

literature search.85 These include the 

Besson et al. (2017) study referenced by 

the commenters (U.S. EPA, 2019, 

section 13.4.2). The Gr<ntoft et al. 

(2019) study referenced by the same 

commenters was published after the 

cutoff date for the literature search. 

However, the EPA has provisionally 

considered new studies, including the 

new studies highlighted by the 

commenters, in the context of the 

findings of the ISA (see Appendix in 

Response to Comments document).86

Based on this provisional consideration, 

the EPA concludes that the new studies 

are not sufficient to alter the 

conclusions reached in the ISA 

regarding PM and materials effects. 

Moreover, we disagree with the 

commenters that the EPA failed to 

consider quantitative information from 

studies available in this review. As 

detailed in section 5.2.2.1.2 of the PA, 

a number of new studies are available 

that apply new methods to characterize 

PM-related effects on previously studied 

materials; however, the evidence 

remains insufficient to relate soiling or 

damage to specific levels of PM in 

ambient air or to establish quantitative 

relationships between PM and materials 

degradation. The uncertainties in the 

evidence identified in the last review 

persist in the evidence in the current 

review, with significant uncertainties 

and limitations to establishing 

quantitative relationships between 

particle size, concentration, chemical 

components, and frequency of painting 

or repair of materials. While some new 

evidence is available in this review, 

overall, the data are insufficient to 

conduct quantitative analyses for PMrelated materials effects. Quantitative 

relationships have not been established 

between characteristics of PM and 

frequency of repainting or cleaning of 

materials, including photovoltaic panels 

and other energy-efficient materials, that 

would help inform our understanding of 

the public welfare implications of 

soiling (U.S. EPA, 2020, section 

5.2.2.2.2; U.S. EPA, 2019, section 13.4). 

Similarly, the information does not 

support quantitative analyses between 

microbial deterioration of surfaces and 

the contribution of carbonaceous PM to 

the formation of black crusts that 

contribute to soiling (U.S. EPA, 2020, 

section 5.2.2.2.2; U.S. EPA, 2019, 

section 13.4). We also note that 

quantitative relationships are difficult to 

assess, in particular those characterized 

using damage functions as these 

approaches depend on human 

perception of the level of soiling 

deemed to be acceptable and evidence 

in this area remains limited in the 

current review (U.S. EPA, 2020, section 

5.2.2.1.2). Additionally, we note the 

CASAC’s concurrence with conclusions 

in the PA that uncertainties remain 

about the best way to evaluate materials 

effects of PM in ambient air (Cox, 2019a, 

p. 13 of consensus responses). Further, 

no new studies are available in this 

review to link human perception of 

reduced aesthetic appeal of buildings 


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and other objects to materials effects 

and PM in ambient air. Finally, 

uncertainties remain about deposition 

rates of PM in ambient air to surfaces 

and the interaction of PM with 

copollutants on these surfaces (U.S. 

EPA, 2020, p. 5–34). 

As summarized above and in the 

proposal, the evidence in this review for 

PM effects on materials is not 

substantively changed from that in the 

last review. There continues to be a lack 

of evidence related to materials effects 

that establishes quantitative 

relationships and supports quantitative 

analyses of PM-related materials soiling 

or damage. While the information 

available in this review continues to 

support a causal relationship between 

PM in ambient air and materials effects 

(U.S. EPA, 2019, section 13.4), the EPA 

is unable to relate soiling or damage to 

specific levels of PM in ambient air and 

is unable to evaluate or consider a level 

of air quality to protect against such 

materials effects. Although the EPA did 

not propose a distinct level of air quality 

or a national standard based on air 

quality impacts (85 FR 24139, April 30, 

2020), we did identify data gaps that 

prevented us from doing so. The EPA 

identified a number of key uncertainties 

and areas of future research (U.S. EPA, 

2020, p. 5–42) that may inform 

consideration of the materials effects of 

PM in ambient air in future reviews of 

the PM NAAQS. 

Commenters who disagreed with the 

Administrator’s proposed decision to 

retain the current secondary PM 

standards provided a number of 

comments on the scientific evidence 

and quantitative analyses for visibility 

impairment. These commenters criticize 

various aspects of the EPA’s proposal to 

retain the standards, including specific 

aspects of the visibility index, the target 

level of protection identified by the 

Administrator, and the appropriateness 

of a single national standard for 

purposes of protecting against PMrelated visibility impairment. In general, 

these comments indicated support for a 

more stringent standard for visibility 

impairment, although the commenters 

did not necessarily specify the 

alternative standard that would, in their 

judgment, address the concerns raised. 

Rather, most of these commenters 

focused on particular aspects of the 

visibility metric underlying the current 

secondary standard, including the form, 

averaging time, and target level of 

protection necessary to protect against 

visibility impairment. 

Several commenters argue that the 

evidence does not support a single level 

of ‘‘acceptable’’ visibility. Commenters 

expressed the view that the public 

preference studies present important 

evidence related to the importance of 

visibility, but that they do not provide 

enough information to set a national 

standard for visibility impairment 

because the results show that visibility 

preferences vary regionally and/or 

locally for a variety of reasons. 

Commenters additionally state that the 

EPA failed to explain and analyze the 

uncertainties associated with the public 

preference studies, including: (1) The 

different methods used in the studies 

and their influence on the responses; (2) 

the impact of different scenes being 

viewed on the full range of public 

preferences; and (3) factors that were 

not considered in the study methods 

that could impact judgments in the 

studies. These commenters suggest that 

the secondary standards should account 

for regional variability, although they 

did not provide specific 

recommendations regarding how to 

accomplish this. 

The EPA agrees with commenters that 

the available scientific evidence 

indicates that public preferences for 

‘‘acceptable’’ visibility and air quality 

depends in large part on the 

characteristics of the scene being 

viewed. The EPA understands that there 

is a wide range of urban and rural 

scenes within the U.S. and included in 

the public preference studies, including 

natural vistas such as the Rocky 

Mountains in Colorado and man-made 

urban structures such as the Washington 

Monument. However, the EPA disagrees 

with commenters that the available 

evidence cannot support a national 

standard to protect against PM-related 

visibility impairment. As at the time of 

the last review, the EPA believes that 

the scenes presented in the public 

preference studies include important 

types of valued scenic views, and 

therefore, when considered together, 

can inform consideration of an 

acceptable level of visual air quality at 

the national scale, taking into account 

variation across the U.S. as evidenced in 

the studies. 

With regard to the comments that 

these studies do not provide enough 

information to account for regional 

variability that is important to consider 

when setting a national standard for 

visibility protection, the EPA recognizes 

that there may be regional variability in 

the available evidence but believes that 

these studies provide significant 

information that is useful for the 

Administrator to consider in his 

judgments on the public welfare 

implications of PM-related visibility 

effects. While the EPA acknowledges 

that there may be regional differences in 

the stated preferences for visibility, the 

EPA finds there is not enough 

information available at this time to take 

such regional differences into account. 

The commenter did not provide specific 

recommendations for the EPA’s 

consideration of such information even 

if such information were available, and 

the EPA finds the question of how, or 

if, to account for regional preferences in 

setting a national standard is a 

substantial question that should be 

addressed when it is presented by the 

available information. 

With regard to the commenters’ 

assertion that the current secondary 

standards are inadequate to protect the 

public welfare from PM-related 

visibility impairment, the EPA disagrees 

that the currently available information 

is sufficient to suggest that a more 

stringent standard is warranted. The 

EPA identified and addressed in great 

detail the limitations and uncertainties 

associated with the public preference 

studies as a part of the last review (78 

FR 3210, January 15, 2013). Given that 

the evidence related to public 

preferences is the same in this review as 

it was at the time of the last review, the 

EPA reiterated the limitations and 

uncertainties inherent in this evidence 

as a part of the PA (U.S. EPA, 2020, 

section 5.5). The PA highlights key 

uncertainties associated with public 

perception of visibility impairment and 

identifies areas for future research to 

inform future PM NAAQS reviews, 

including those raised by the 

commenters (U.S. EPA, 2020, p. 5–41). 

For example, the PA notes the critical 

need for information to further our 

understanding of human perception of 

visibility impairment in public 

preference studies in order to address 

uncertainties and limitations in the 

evidence, including an expansion of the 

number and geographic coverage of 

preference studies in urban, rural, and 

Class I areas to account for the potential 

for people to have different preferences 

based on the conditions that they 

commonly encounter and potential 

differences in preferences based on the 

scene types (U.S. EPA, 2020, p. 5–41). 

These same commenters further argue 

that the EPA omitted recent studies that 

could further inform our understanding 

of the public welfare implications of 

visibility impairment. Commenters 

specifically point to a recent metaanalysis of available preference studies 

(Malm et al., 2019) and also cites to 

several related studies (Malm et al., 

2011; Malm, 2013, 2016; Molenar and 

Malm, 2012). Commenters additionally 

contend that studies of the economic 

effects of impaired visibility were 

omitted from the ISA and PA and were 


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87As noted earlier in section IV, ‘‘the current ISA 

identified and evaluated studies and reports that 

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). 

88A valid filter-based 24-hour concentration 

measurement is one collected via FRM, and that has 

undergone laboratory equilibration (at least 24 

hours at standardized conditions of 20–23 °C and 

30–40% relative humidity) prior to analysis (see 

Appendix L of 40 CFR part 50 for the 2012 NAAQS 

for PM). 

89For coarse PM and PM2.5 components, data 

completeness criteria were selected for the 

quantitative analyses consistent with those in 

Appendix N of 40 CFR part 50 for the 2012 NAAQS 

for PM. 

not considered in the EPA’s approach 

for evaluating visibility. 

The EPA disagrees with the 

commenters that studies related to 

visibility were inappropriately omitted 

from the ISA in this review. As an 

initial matter, the ISA considered and 

included studies related to PM-related 

visibility impairment and public 

preferences that were published prior to 

the cutoff date for the literature 

search.87 As described in the Preamble 

to the ISA, ‘‘studies and reports that 

have undergone scientific peer review 

and have been published (or accepted 

for publication) are considered for 

inclusion in the ISA’’ (U.S. EPA, 2015, 

p. 6). The meta-analysis by Malm et al. 

(2019) was published after the cutoff 

date for the literature search for the ISA, 

and therefore, was not included in the 

ISA. Malm et al. (2019) was 

provisionally considered, along with 

other studies published after the cut-off 

date, and the EPA concluded that these 

studies did not materially change the 

broad scientific conclusions of the ISA 

regarding welfare effects, including 

visibility impairment. Moreover, the 

other citations provided by the 

commenters (Malm et al., 2011; Malm, 

2013, 2016; Molenar and Malm, 2012) 

are not peer-reviewed publications and 

as such do not meet the criteria for 

inclusion in the ISA. With regard to 

studies of economic effects, these 

studies were not considered to be 

within the scope of the ISA, and 

therefore were not included in this 

review (U.S. EPA, 2019, p. P–16). The 

studies submitted by the commenters, 

together with other new evidence, will 

be assessed consistent with CAA 

requirements in the next review of the 

PM NAAQS. 

Some commenters contend that the 

EPA’s visibility analyses only focused 

on locations that met the current 

standards. These commenters argue that 

the EPA concluded at the beginning of 

the analysis that the current standards 

do not need to be revised and that the 

EPA’s approach ignores information 

available since the last review, leading 

to the Administrator to propose no 

revisions to the standards based on this 

flawed approach. 

We disagree with commenters that the 

updated analyses of visibility 

impairment in this review only 

considered air quality in areas that meet 

the current standards. As described in 

detail in the PA, locations included in 

the analyses were those that met 

specific data completeness criteria for 

the monitoring data required as inputs 

to the IMPROVE equations for 

estimating light extinction (U.S. EPA, 

2020, Appendix D). The data set used 

for the updated analyses is comprised of 

sites with data for the 2015–2017 period 

that supported a valid 24-hour PM2.5

design value and met strict criteria for 

PM species. For PM2.5 concentrations, 

data were screened so that all days 

either had a valid filter-based 24-hour 

concentration measurement or at least 

18 valid hourly concentration 

measurements (U.S. EPA, 2020, section 

D.2.1.2).88 For coarse PM 

concentrations, data were included for 

sites with ≥11 valid days for each 

quarter of 2015–2017. For PM2.5

component concentrations, data were 

included for days with valid data for all 

chemical components listed in Table D– 

1 in the PA and for sites with ≥11 valid 

days for each quarter of 2015–2017.89 Of 

all of the PM monitoring locations in the 

U.S., 67 monitoring sites met the data 

completeness criteria and light 

extinction was calculated without the 

coarse fraction in the IMPROVE 

equations. Of these 67 monitoring sites, 

20 locations met the data completeness 

criteria for coarse PM, and as such, light 

extinction was also estimated with the 

coarse fraction as an input to the 

IMPROVE equation at these sites (U.S. 

EPA, 2020, section 5.2.1.2, Appendix 

D). For the sites that met the data 

completeness criteria for inclusion in 

the analyses, all of the sites met the 

annual PM2.5 and 24-hour PM10

standards, and all but one site (located 

in southern California) met the 24-hour 

PM2.5 standard. Therefore, we disagree 

with the commenters that the analysis 

was designed to consider only locations 

that met the current standards and did 

not consider locations that did not meet 

the current secondary PM standards. 

Moreover, the EPA notes that data from 

areas exceeding the current standard are 

generally of limited use in deciding 

whether to retain the standard, or lower 

it, because it is not representative or 

informative of circumstances and effects 

that would be expected to be seen upon 

attainment of the standard. 

Furthermore, it is unclear what 

additional information the commenters 

contend that the EPA omitted from its 

consideration in this review. All 

scientific information available in this 

review has been considered and 

integrated as a part of the ISA. The 

Administrator, in considering the 

adequacy of the current secondary PM 

standards, considered the available 

scientific evidence and quantitative 

information in this review, along with 

CASAC advice and public comments, 

and concluded that the current 

secondary PM standards provide 

requisite protection against visibility 

impairment. 

Some commenters additionally 

contend that the EPA’s evaluation of 

public welfare effects of PM in the 

proposal solely focuses on fine PM and 

ignores coarse PM. These commenters 

assert that trends data show that coarse 

PM is increasing, which they believe to 

be a concern to public welfare. 

We disagree with the commenters that 

the EPA’s proposal failed to consider 

the public welfare implications of 

coarse PM. First, we note that there is 

limited new scientific evidence 

available in this review on climate- and 

materials-related effects of coarse PM 

beyond that of the last review (85 FR 

24131, April 30, 2020). With regard to 

the contribution of coarse PM to 

visibility impairment, we first note that 

at the time of 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 (U.S. EPA, 

2020, p. 5–22). Data available for 

PM10–2.5 was very limited in the last 

review and was not used in quantitative 

analyses of estimated PM2.5 light 

extinction (U.S. EPA, 2020, Appendix 

D, section D–1). Since the time of 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 fine and 

coarse fractions of PM. As described in 

the PA, the analyses of visibility 

impairment were updated in this review 

to include consideration of the coarse 

fraction of PM in estimating 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 

updated analyses in this review 

included 20 sites that measured both 

PM10 and PM2.5 (U.S. EPA, 2020, section 

5.2.1.2, Appendix D), all of which meet 

the current 24-hour PM2.5 and PM10

standards. All of these sites have 3-year 

visibility at or below 30 dv regardless of 

whether light extinction is calculated 


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with or without the coarse fraction, and 

for all three versions of the IMPROVE 

equation used in this review. Generally, 

the contribution of the coarse fraction of 

PM to light extinction in these locations 

was minimal, contributing less than 1 

dv to the 3-year visibility metric (U.S. 

EPA, 2020, section 5.2.1.2, Appendix 

D). While there were not monitoring 

data available to evaluate the impact of 

coarse PM on estimates of light 

extinction in locations expected to have 

higher concentrations of coarse PM, the 

coarse fraction may be a more important 

contributor to light extinction and 

visibility impairment than in those areas 

included in the PA analyses in this 

review. As additional information and 

monitoring data become available to 

further evaluate the impact of coarse PM 

on estimates of light extinction in more 

locations, including geographical 

locations expected to have high 

concentrations of coarse PM, such 

information will be considered in a 

future PM NAAQS review. 

Several commenters in support of 

revising the secondary PM standards to 

protect against visibility impairment, 

generally recommend revisions to 

elements of the secondary standard and 

visibility index (indicator, averaging 

time, form, and level) consistent with 

those supported by the CASAC and 

public comments in previous PM 

reviews. We address comments on the 

elements of a visibility index and a 

revised standard for visibility effects 

below. 

With regard to an indicator for the 

secondary standards to protect against 

visibility impairment, a number of 

commenters suggest that the EPA failed 

to explain why the current indicator is 

adequate and pointed to 

recommendations from the CASAC in 

the PM reviews completed in 2012 and 

2006 with regard to alternate indicators. 

As noted by the commenters, in the 

2012 review, the CASAC recommended 

three alternate indicators for a 

secondary standard to protect against 

visibility impairment: (1) Using direct, 

continuous measurement of PM light 

extinction to support hourly or multihour daylight-only averaging time(s); (2) 

using PM speciation data to calculate 

seasonal (or monthly) regional species 

and relative humidity values to combine 

with the denser continuous PM2.5

monitoring network to calculate hourly 

PM light extinction; or, (3) using hourly 

PM2.5 as a basis for a sub-daily (hourly 

or multi-hour) daylight-only indicator, 

which would intentionally remove the 

variable influence of water from the 

regulatory metric. In the 2006 review, as 

noted by the commenters, the CASAC 

recommended a PM2.5 mass indicator, 

coupled with revisions to the averaging 

time, form, and level of the standard, to 

protect against visibility impairment. 

The EPA generally agrees with 

commenters that an indicator based on 

directly measured light extinction 

would provide the most direct link 

between PM in ambient air and PMrelated visibility impairment. However, 

as noted in the proposal (85 FR 24138, 

April 30, 2020, sections IV.B.1 and 

IV.D.1), the Administrator concluded 

that in the absence of a monitoring 

network to directly measure light 

extinction, he judged that estimated 

light extinction, as calculated using the 

IMPROVE algorithms, continues to 

provide a reasonable basis for defining 

a target level of protection against PMrelated visibility impairment in the 

current review. There has been little 

progress in development of such a 

monitoring network since the time of 

the last review when CASAC concluded 

that, in the absence of such a monitoring 

network, relying on a calculated PM2.5

light extinction indicator based on PM2.5

components and relative humidity 

represented a reasonable approach and 

that the inputs for calculating light 

extinction were readily available 

through existing monitoring networks 

and approved monitoring protocols (78 

FR 3205, January 15, 2013). Further, in 

this review, the CASAC generally agreed 

with the EPA 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. 

With regard to the elements of the 

visibility index, in considering the 

adequacy of the current secondary PM 

standards to protect against visibility 

impairment, as described in the 

proposal (85 FR 24135, April 30, 2020), 

the Administrator first defined an 

appropriate target level of protection in 

terms of a PM visibility index. In 

defining this target level of protection, 

the Administrator first considered the 

indicator of such an index. He noted 

that, given the lack of availability of 

methods and an established network for 

directly measuring light extinction, a 

visibility index based on estimates of 

light extinction by PM2.5 components 

derived from an adjusted version of the 

original IMPROVE algorithm would be 

most appropriate, consistent with the 

last review. As described in the 

proposal and above (section IV.A.2.a.i), 

the IMPROVE algorithm estimates light 

extinction using routinely monitored 

components of PM2.5 and PM10–2.5, along 

with estimates of relative humidity. The 

Administrator, while recognizing that 

some revisions to the IMPROVE 

algorithm have been made since the 

time of the last review, noted that the 

fundamental relationship between 

ambient PM and light extinction has 

changed very little and the different 

versions of the IMPROVE algorithms 

can appropriately reflect this 

relationship across the U.S. (85 FR 

24138, April 30, 2020). As such, he 

judged that defining a target level of 

protection in terms of estimated light 

extinction continues to be a reasonable 

approach in the current review. 

With regard to averaging time, 

commenters were critical of the 24-hour 

averaging time to protect against 

visibility impairment and argue for a 

sub-daily averaging time. While some 

comments clearly focused on the 

averaging time of the current secondary 

PM2.5 standard, other comments were 

unclear as to whether they 

recommended a sub-daily averaging 

time for the secondary PM2.5 standard or 

for the visibility index used in defining 

a target level of the protection. 

Nonetheless, all of these commenters 

contend that people do not perceive 

visibility impairment over a 24-hour 

period, but rather their perception of 

impairment ranges from minutes to 

multiday, and that daylight hours are 

much more important in terms of 

visibility impairment, particularly in 

urban areas. As with comments on the 

indicator of the standard, some 

commenters also point to previous 

CASAC advice on the need for a subdaily standard. 

In defining the characteristics of a 

visibility index, the EPA continues to 

believe that a 24-hour averaging time is 

reasonable. This is in part based on 

analyses conducted in the last review 

that showed relatively strong 

correlations between 24-hour and subdaily (i.e., 4-hour average) PM2.5 light 

extinction from the analyses conducted 

in the last review (85 FR 24138, April 

30, 2020; 78 FR 3226, January 15, 2013), 

indicating that a 24-hour averaging time 

is an appropriate surrogate for the subdaily time periods relevant for visual 

perception. The EPA believes that these 

analyses continue to provide support for 

consideration of a 24-hour averaging 

time for the visibility index in this 

review. The EPA also recognizes that 

the longer averaging time may be less 

influenced by atypical conditions and/ 

or atypical instrument performance (85 

FR 24138, April 30, 2020; 78 FR 3226, 

January 15, 2013). When taken together, 

the available scientific information and 

updated analyses of calculated light 

extinction available in this review 

continue to support that a 24-hour 

averaging time is appropriate when 

defining a target level of protection 


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against visibility impairment in terms of 

a visibility index. 

Moreover, the EPA disagrees with 

commenters that a secondary PM2.5

standard with a 24-hour averaging time 

does not provide requisite protection 

against the public welfare impacts of 

visibility impairment. At the time of the 

last review, the EPA recognized that 

hourly or sub-daily (i.e., 4- to 6-hour) 

averaging times, within daylight hours 

and excluding hours with high relative 

humidity, are more directly related to 

the short-term nature of visibility 

impairment and the relevant viewing 

periods for segments of the viewing 

public than a 24-hour averaging time. At 

that time, the EPA agreed that a subdaily averaging time would generally be 

preferable. However, the Agency noted 

significant data quality uncertainties 

associated with the instruments that 

would provide hourly PM2.5 mass 

concentrations necessary to inform a 

sub-daily averaging time. These 

uncertainties, as described in the last 

review, included short-term variability 

in hourly data from available 

continuous monitoring methods, which 

would prohibit establishing a sub-daily 

averaging time (78 FR 3209, January 15, 

2013). For all of these reasons, the EPA 

continues to believe that a sub-daily 

averaging time is not supported by the 

information available in this review. 

With regard to the form of the 

visibility index, many of the 

commenters contend that the form used 

in evaluating visibility impairment is 

not appropriate. First, commenters 

contend that a 90th percentile form is 

too low and excludes too many days 

that could have visibility impairment. 

These same commenters also suggest 

that a 3-year average form is not 

justified and does not protect visibility 

and public welfare. These commenters 

also argue that the EPA failed to 

consider the 98th percentile form for the 

visibility index as a part of the proposal. 

Second, some commenters recommend 

a form for the visibility index within the 

range of 95th to 98th percentile, coupled 

with a multi-hour sub-daily averaging 

time, consistent with the CASAC advice 

in the 2006 review. 

The EPA disagrees with these 

commenters on both points. With regard 

to the form of the visibility index, the 

EPA continues to conclude that a 3-year 

average of annual 90th percentile values 

is appropriate. In so doing, the EPA 

notes that a 3-year average form 

provides stability from the occasional 

effect of inter-annual meteorological 

variability that can result in unusually 

high pollution levels for a particular 

year, consistent with the decision in the 

last review (78 FR 3198, January 15, 

2013; U.S. EPA, 2011, p. 4–58). With 

regard to the annual statistical form to 

be averaged over 3-years, the EPA 

considers the evaluation in the 2010 

UFVA of three different statistics: 90th, 

95th, and 98th percentiles (U.S. EPA, 

2010b, chapter 4). In considering the 

alternative statistical forms, 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. Finally, the 2011 PA 

recognized that the public preference 

studies available at the time of the last 

review 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 for 

Federal Class I areas (U.S. EPA, 2011, p. 

4–59). The analyses and considerations 

for the form of a visibility index from 

the 2011 PA continue to provide 

support for a 90th percentile form, 

averaged across three years, in defining 

the characteristics of a visibility index 

in this review. 

Some commenters contend that the 

EPA’s proposal to retain the level of 30 

dv for a visibility index is arbitrary, 

capricious, and not technically sound. 

These commenters assert that the EPA 

failed to consider recent research 

studies that provide a meta-analysis of 

visibility preference studies that suggest 

that a level of 30 dv is unacceptable to 

study participants included in the metaanalysis. 

As an initial matter, as described 

above, the studies cited by the 

commenters in support of their rationale 

were either published after the cutoff 

date for the literature search for the ISA 

(Malm et al., 2019) or were not peerreviewed studies that met the inclusion 

criteria for the ISA (Malm et al., 2011; 

Malm, 2013, 2016; Molenar and Malm, 

2012). The EPA provisionally 

considered the Malm et al. (2019) study 

and concludes that this study does not 

sufficiently alter the conclusions 

reached in the ISA regarding PM and 

visibility effects. 

With regard to a level of 30 dv for the 

visibility index, the EPA believes that it 

is appropriate to establish a target level 

of protection based on the upper end of 

the range of levels of visibility 

impairment judged to be acceptable by 

at least 50% of study participants in the 

available visibility preference studies 

(U.S. EPA, 2020, section 5.2.1.1). The 

2011 PA identified a range of levels 

from 20 to 30 dv based on the responses 

in the public preference studies 

available at that time. Given the lack of 

new preferences studies available in this 

review, the EPA again relies on the same 

studies and the range of levels identified 

in those studies in the current review. 

As described in detail in the PA (U.S. 

EPA, 2020, sections 5.2.1.1 and 5.5), 

there are a number of uncertainties and 

limitations associated with the public 

preference studies, including those 

described in section IV.B.2 above. 

Recognizing these uncertainties and 

limitations, the EPA concludes that 

substantial degrees of variability and 

uncertainty in the public preference 

studies should be reflected in a target 

level of protection at the upper end of 

the range than if the information was 

more consistent and certain. Therefore, 

the EPA believes that 30 dv is an 

appropriate level for a visibility index in 

this review. 

A number of commenters advocate for 

a more stringent standard, 

recommending that the level of the 

secondary PM2.5 standards be lowered. 

Some commenters reference the 

recommendations of previous CASAC 

panels for revisions to the secondary 24- 

hour PM2.5 standard. Additionally, some 

commenters contend that the secondary 

PM2.5 standards should be set equal to 

the primary PM2.5 standards, with some 

of the commenters aligning their 

support for their position with their 

recommendations for revisions to the 

primary PM2.5 standards in this review. 

We disagree with the commenters that 

the secondary PM2.5 standard should be 

revised to provide additional public 

welfare protection beyond that achieved 

under the current standard. Based on 

the available scientific and quantitative 

information, and for the reasons 

discussed above, the EPA concludes 

that it is appropriate to define a target 

level of protection in terms of a 

visibility index based on estimated light 

extinction with a 24-hour averaging 

time, a 3-year 90th percentile form, and 

a level of 30 dv. In having concluded 

that this visibility index is appropriate, 

the EPA then considers the degree of 

protection from visibility impairment 

afforded by the existing standard. In so 

doing, we consider results of updated 

analyses of calculated light extinction 

that demonstrate that, in areas meeting 

the current PM mass-based standards, 

the target level of protection in terms of 

a visibility index is also achieved (85 FR 

24135, April 30, 2020; U.S. EPA, 2020, 

section 5.2.1.2). The results of these 

analyses (as described in detail in 


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section IV.A.3.a above and in section 

5.2.1.2 of the PA) demonstrate that the 

3-year visibility metric is at or below 

about 30 dv in all areas meeting the 

current PM2.5 standard, and below 25 dv 

in most areas. For those areas with 

available PM10–2.5 monitoring data, 

which met both the current 24-hour 

PM2.5 and PM10 standards, 3-year 

visibility metrics were at or below 30 dv 

regardless of if 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 

concluded at the time of proposal that 

the updated scientific evidence and 

quantitative information support the 

adequacy of the current secondary PM2.5

and PM10 standards to protect against 

PM-related visibility impairment (85 FR 

24138–24139, April 30, 2020). 

With regard to comments 

recommending to set the secondary 

PM2.5 standards equal to the current 

primary PM2.5 standards, these 

commenters do not provide a basis for 

their recommendation, nor do they 

provide a rationale for revising the 

secondary PM2.5 standards to their 

recommended revised levels of the 

primary PM2.5 standards. However, we 

note that the primary annual PM2.5

standard, with its lower level, would be 

the controlling standard. The EPA 

disagrees that such revisions would be 

appropriate, for all of the reasons 

discussed above. 

4. Administrator’s Conclusions 

In considering the adequacy of the 

current secondary PM standards in this 

review, the Administrator has carefully 

considered the: (1) Policy-relevant 

evidence and conclusions contained in 

the ISA; (2) the quantitative information 

presented and assessed in the PA; (3) 

the evaluation of this evidence, the 

quantitative information, and the 

rationale and conclusions presented in 

the PA; (4) the advice and 

recommendations from the CASAC; and 

(5) public comments, as addressed in 

section IV.B.3 above. In the discussion 

below, the Administrator gives weight 

to the PA conclusions, with which the 

CASAC concurred, as summarized in 

section IV.D of the proposal, and takes 

note of key aspects of the rationale for 

those conclusions that contribute to his 

decision in this review. After giving 

careful consideration to all of this 

information, the Administrator believes 

that the conclusions and policy 

judgments supporting his proposed 

decision remain valid and the secondary 

PM standards should be retained. 

In considering the PA evaluations and 

conclusions, the Administrator 

specifically takes note of the overall 

conclusions that the welfare effects 

evidence and quantitative information 

are generally consistent with what was 

considered in the last review (U.S. EPA, 

2020, section 5.4). In so doing, he 

additionally notes that the CASAC 

supports retaining the current standard 

agreeing with the EPA ‘‘that the 

available evidence does not call into 

question the protection afforded by the 

current secondary PM standards’’ (Cox, 

2019a, p. 3 of letter). As noted below, 

the newly available welfare effects 

evidence, critically assessed in the ISA 

as part of the full body of current 

evidence, reaffirms conclusions on the 

visibility, climate, and materials effects 

recognized in the last review, including 

key conclusions on which the current 

standard is based. Further, as discussed 

in more detail above, the updated 

quantitative analyses of visibility 

impairment for areas meeting the 

current standards support the adequacy 

of the current secondary PM2.5 and PM10

standards to protect against PM-related 

visibility impairment. The 

Administrator also recognizes 

limitations and uncertainties continue 

to be associated with the available 

information. 

With regard to the current evidence 

on visibility effects, as summarized in 

the PA and discussed in detail in the 

ISA, the Administrator takes note of the 

long-standing body of evidence for PMrelated visibility impairment. This 

evidence, which is based on the 

fundamental relationship between light 

extinction and PM mass, demonstrates 

that ambient PM can impair visibility in 

both urban and remote areas, and has 

changed very little since the last review 

(U.S. EPA, 2019, section 13.1; U.S. EPA, 

2009a, section 9.2.5). The evidence 

related to public perception of visibility 

impairment comes from studies from 

four areas in North America. These 

studies provide information to inform 

our understanding of levels of visibility 

impairment that the public judged to be 

‘‘acceptable’’ (U.S. EPA, 2010b; 85 FR 

24131, April 30, 2020). In considering 

these public preference studies, the 

Administrator notes that, as described 

in the ISA, no new visibility studies 

have been conducted in the U.S. and 

there is little newly available 

information with regard to acceptable 

levels of visibility impairment in the 

U.S. The Administrator recognizes that 

visibility impairment can have 

implications for people’s enjoyment of 

daily activities and their overall wellbeing, and therefore, considers the 

degree to which the current secondary 

standards protect against PM-related 

visibility impairment. 

Based on the considerations discussed 

above in sections IV.B.2 and IV.B.3, the 

Administrator first concludes, 

consistent with the last review, that a 

target level of protection for a secondary 

PM standard is most appropriately 

defined in terms of a visibility index 

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. In defining a target level of 

protection, the Administrator has 

considered the specific aspects of such 

an index, including the appropriate 

indicator, averaging time, form, and 

level. 

First, with regard to indicator, the 

Administrator notes that in the last 

review, the EPA used an index based on 

estimates of light extinction by PM2.5

components calculated using an 

adjusted version of the IMPROVE 

algorithm. As described above (section 

IV.A.3), this algorithm allows the 

estimation of light extinction using 

routinely monitored components of 

PM2.5 and PM10–2.5, along with estimates 

of relative humidity. The Administrator 

recognizes that, while there have been 

some revisions to the IMPROVE 

algorithm since the time of the last 

review, our fundamental understanding 

of the relationship between PM in 

ambient air and light extinction has 

changed little and the various IMPROVE 

algorithms can appropriately reflect this 

relationship across the U.S. In the 

absence of a monitoring network for 

direct measurement of light extinction 

(section IV.A.3), he concludes that 

calculated light extinction indicator that 

utilizes 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. 

In further defining the characteristics 

of a visibility index, the Administrator 

next considers the appropriate averaging 

time, form, and level of the index. Given 

the available scientific information in 

this review, and in considering the 

CASAC’s advice and public comments, 

the Administrator concludes that, 

consistent with the decision in the last 

review, a visibility index with a 24-hour 

averaging time and a form based on the 

3-year average of annual 90th percentile 

values remains reasonable in this 

review. With regard to the averaging 

time and form of such an index, the 

Administrator takes note of analyses 

conducted in the last review that 

demonstrated relatively strong 

correlations between 24-hour and subdaily (i.e., 4-hour average) PM2.5 light 

extinction (78 FR 3226, January 15, 


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2013), indicating that a 24-hour 

averaging time is an appropriate 

surrogate for the sub-daily time periods 

of the perception of PM-related 

visibility impairment and the relevant 

exposure periods for segments of the 

viewing public. This decision also 

recognized that a 24-hour averaging 

time may be less influenced by atypical 

conditions and/or atypical instrument 

performance (78 FR 3226, January 15, 

2013). The Administrator recognizes 

that there is no new information in the 

current review to support updated 

analyses of this nature, and therefore, he 

believes these analyses continue to 

provide support for consideration of a 

24-hour averaging time for a visibility 

index in this review. With regard to the 

statistical form of the index, the 

Administrator notes that, consistent 

with the last review: (1) A multi-year 

percentile form offers greater stability 

from the occasional effect of interannual meteorological variability (78 FR 

3198, January 15, 2013; U.S. EPA, 2011, 

p. 4–58); (2) a 90th percentile represents 

the median of the distribution of the 20 

percent worst visibility days, which are 

targeted in Federal Class I areas by the 

Regional Haze Program; and (3) public 

preference studies did not provide 

information to identify a different target 

than that identified for Federal Class I 

areas (U.S. EPA, 2011, p. 4–59). 

Therefore, the Administrator judges that 

a visibility index based on estimates of 

light extinction, with a 24-hour 

averaging time and a 90th percentile 

form, averaged over three years, remains 

appropriate. 

With regard to the level of a visibility 

index, the Administrator judges that it 

is appropriate to establish a target level 

of protection of 30 dv, reflecting the 

upper end of the range of visibility 

impairment judged to be acceptable by 

at least 50% of study participants in the 

available public preference studies (78 

FR 3226, January 15, 2013). The 2011 

PA identified a range of levels from 20 

to 30 dv based on the responses in the 

public preference studies available at 

that time. At the time of the last review, 

the Administrator noted a number of 

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. In 

considering the available preference 

studies, with their inherent 

uncertainties and limitations, the prior 

Administrator concluded that the 

substantial degree of variability and 

uncertainty in the public preference 

studies should be reflected in a target 

level of protection based on the upper 

end of the range of CPLs. 

Given that there are no new 

preference studies available in this 

review, the Administrator notes that his 

judgments are based on the same 

studies, with the same range of levels, 

available in the last review. The 

Administrator recognizes a number of 

limitations and uncertainties associated 

with these studies, as identified in the 

PA (U.S. EPA, 2020, section 5.5), 

including the following: (1) Available 

studies may not represent the full range 

of preferences for visibility in the U.S. 

population, particularly given the 

potential variability in preferences 

based on the conditions commonly 

encountered and the scenes being 

viewed; (2) available preference studies 

were conducted 15 to 30 years ago and 

may not accurately represent the current 

day preferences of people in the U.S.; 

(3) the variety of methods used in the 

preference studies may potentially 

influence the responses as to what level 

of impairment is deemed acceptable; 

and (4) factors that are not captured in 

the methods of the preference studies, 

such as the time of day when light 

extinction is the greatest or the 

frequency of impairment episodes, may 

influence people’s judgment on 

acceptable visibility (U.S. EPA, 2020, 

section 5.2.1.1). Therefore, in 

considering the scientific information, 

with its uncertainties and limitations, as 

well as public comments on the level of 

the target level of protection against 

visibility impairment, the Administrator 

concludes that it is appropriate to again 

use a level of 30 dv for the visibility 

index. 

Having concluded that the protection 

provided by a standard defined in terms 

of a PM2.5 visibility index, with a 24- 

hour averaging time, and a 90th 

percentile form, averaged over 3 years, 

set at a level of 30 dv, is requisite to 

protect public welfare with regard to 

visual air quality, the Administrator 

next considers the degree of protection 

from visibility impairment afforded by 

the existing secondary PM standards. 

This determination requires considering 

such protection not in isolation but in 

the context of the full suite of secondary 

standards. 

In this context, the Administrator has 

considered the degree of protection from 

visibility afforded by the existing 

secondary PM2.5 standards. The 

Administrator has considered both 

whether the existing 24-hour PM2.5

standard of 35 mg/m3 is sufficient (i.e., 

not under-protective) and whether it is 

not more stringent than necessary (i.e., 

not over-protective). 

As discussed in section IV.A.3 above, 

the Administrator considers the updated 

analyses of visibility impairment 

presented in the PA (U.S. EPA, 2020, 

section 5.2.1.2), which reflect a number 

of improvements since the last review. 

Specifically, the updated analyses 

examine multiple versions of the 

IMPROVE equation, including the 

version incorporating revisions since 

the time of the last review (section 

IV.A.3.a above). These updated analyses 

provide a further understanding of how 

variation in the inputs to the algorithms 

impact the estimates of light extinction 

(U.S. EPA, 2020, Appendix D). 

Additionally, for a subset of monitoring 

sites with available PM10–2.5 data, the 

updated analyses better characterize the 

influence of coarse PM on light 

extinction than in the last review (U.S. 

EPA, 2020, section 5.2.1.2). 

As discussed above in section 

IV.A.3.a, the results of the updated 

analyses are consistent with those from 

the last review. Regardless of which 

version of the IMPROVE equation is 

used, the analyses demonstrate that, 

based on 2015–2017 data, the 3-year 

visibility metric is at or below about 30 

dv in all areas meeting the current 24- 

hour PM2.5 standard, and below 25 dv 

in most of those areas. In locations with 

available PM10–2.5 monitoring, which 

met both the current 24-hour secondary 

PM2.5 and PM10 standards, 3-year 

visibility index metrics were at or below 

30 dv regardless of whether the coarse 

fraction was included as an input to the 

algorithm for estimating light extinction 

(U.S. EPA, 2020, section 5.2.1.2). While 

the inclusion of the coarse fraction had 

a relatively modest impact on the 

estimates of light extinction, as noted in 

responding to comments in section 

IV.B.3 above, the Administrator 

recognizes the continued importance of 

the PM10 standard given the potential 

for larger impacts on light extinction in 

areas with higher coarse particle 

concentrations, which were not 

included in the PA’s analyses due to a 

lack of available data (U.S. EPA, 2019, 

section 13.2.4.1; U.S. EPA, 2020, section 

5.2.1.2). He notes that the air quality 

analyses showed that all areas meeting 

the existing 24-hour PM2.5 standard, 

with its level of 35 mg/m3, had visual air 

quality at least as good as 30 dv, based 

on the visibility index. 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 


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also exceeding the existing secondary 

24-hour standard. Thus, the 

Administrator judges 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 he judges appropriate. 

With respect to the non-visibility 

welfare effects of PM in ambient air, the 

Administrator concludes that it is 

generally appropriate to retain the 

existing standards and that there is 

insufficient information to establish any 

distinct secondary PM standards to 

address climate and materials effects of 

PM. With regard to climate, he 

recognizes that there have been a 

number of improvements and 

refinements to climate models since the 

last review. However, as discussed in 

sections IV.A.3.b and IV.B.3 above, 

while the evidence continues to support 

a causal relationship between PM and 

climate effects (U.S. EPA, 2019, section 

13.3.9), the Administrator notes that 

significant limitations continue to exist 

related to quantifying the contributions 

of 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 that models 

continue to exhibit considerable 

variability in estimates of PM-related 

climate impacts at regional scales (e.g., 

∼100 km) as compared to simulations at 

global scales. Therefore, the resulting 

uncertainty leads the Administrator to 

conclude that the available scientific 

information in this review remains 

insufficient to quantify climate impacts 

associated with particular 

concentrations of PM in ambient air 

(U.S. EPA, 2020, section 5.2.2.2.1) or to 

evaluate or consider a level of PM air 

quality in the U.S. to protect against 

climate effects and that there is 

insufficient information available at this 

time to base a national ambient standard 

on climate impacts. 

With regard to materials effects, the 

Administrator notes that the evidence 

available in this review continues to 

support a causal relationship between 

materials effects and PM deposition 

(U.S. EPA, 2019, section 13.4). He 

recognizes that the deposition of fine 

and coarse particles to materials can 

lead to physical damage and/or 

impaired aesthetic qualities. Particles 

can contribute to materials damage by 

adding to the 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 information is available in this 

review, as discussed in sections IV.A.3.b 

and IV.B.3 above, this information is 

primarily conducted outside the U.S. in 

areas where PM concentrations in 

ambient air are typically higher than 

those observed in the U.S. (U.S. EPA, 

2020, section 13.4). Additionally, the 

newly available information in this 

review does not support quantitative 

analyses of PM-related materials effects 

in this review (U.S. EPA, 2020, section 

5.2.2.2.2). Given the limited amount of 

information available and its inherent 

uncertainties and limitations, the 

Administrator concludes that he is 

unable to relate soiling or damage to 

specific levels of PM in ambient air or 

to evaluate or consider a level of air 

quality to protect against such materials 

effects, and that there is insufficient 

information available in this review to 

support a distinct national ambient 

standard based on materials effects. 

With regard to the secondary PM 

standards, the Administrator concludes 

that it is appropriate to retain the 

existing secondary PM standards, 

without revision. This conclusion is 

based on the considerations discussed 

above in sections IV.A.3.b and IV.B.2, 

including the latest scientific 

information and the advice of the 

CASAC, and the public comments 

received on the proposal, as discussed 

above in section IV.B.3. For visibility 

effects, this decision also reflects his 

consideration of the evidence for PMrelated light extinction, together with 

his consideration of the updated 

analyses of the protection provided 

against visibility impairment 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 shortterm concentrations of both fine and 

coarse particles, there is insufficient 

information to establish distinct 

secondary PM standards to address nonvisibility PM-related welfare effects. The 

Administrator concurs with the advice 

of 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). This is also 

consistent with the conclusions at the 

time of the proposal (IV.B.2) and with 

the majority of public comments 

received on the proposed decision 

(section IV.B.3). 

In addition, the Administrator judges 

that, based on his review of the science 

and his judgment that air quality should 

be maintained to provide the target level 

of protection for visual air quality of 30 

dv (as discussed in more detail above), 

the degree of public welfare protection 

provided by the current secondary 

standards 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 than the current 

secondary standards (e.g., with higher 

levels) are also not supported. 

Thus, based on his consideration of 

the evidence and analyses for welfare 

effects, his consideration of the 

CASAC’s advice and public comments 

on the secondary standards, and in the 

absence of information that would 

support establishment of any different 

standards, the Administrator concludes 

that it is appropriate to retain the 

current 24-hour and annual PM2.5

standards and the 24-hour PM10

standard, without revision. 

D. Decision on the Secondary PM 

Standards 

For the reasons discussed above and 

taking into account information and 

assessments presented in the ISA and 

PA, advice from the CASAC, and 

consideration of public comments, the 

Administrator concludes that the 

current secondary PM standards are 

requisite to protect public welfare from 

known or anticipated adverse effects 

and is retaining the standards, without 

revision. 

V. Statutory and Executive Order 

Reviews 

Additional information about these 

statutes and Executive Orders can be 

found at http://www2.epa.gov/lawsregulations/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. Changes 

made during Executive Order 12866 

review have been documented in the 

docket. Because this action does not 

change the existing PM NAAQS, 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. 


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B. Executive Order 13771: Reducing 

Regulations and Controlling Regulatory 

Costs 

This action is not an Executive Order 

13771 regulatory action. There are no 

costs or cost savings compared to the 

current baseline for this action because 

EPA is retaining 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 

retains 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 retains, 

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 

regulations upon small entities), 

reviewed 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 retains 

the existing PM NAAQS, 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 a ‘‘significant 

energy action’’ as defined by Executive 

Order 13211 (66 FR 28355, May 22, 

2001) because it is not likely to have a 

significant adverse effect on the supply, 

distribution, or use of energy and has 

not otherwise been designated as a 

significant energy action by the 

Administrator of the Office of 

Information and Regulatory Affairs 

(OIRA). 

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 

action described in this document is to 

retain without revision the existing PM 

NAAQS based on the Administrator’s 

conclusions that the existing primary 

standards protect public health, 

including the health of sensitive groups, 

with an adequate margin of safety, and 

the existing secondary standards protect 

public welfare from known or 

anticipated adverse effects. As 

discussed in section II, the EPA 

expressly considered the available 

information regarding health effects 

among at-risk populations in reaching 

the 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). 

M. Congressional Review Act (CRA) 

This action is subject to the CRA, and 

the EPA will submit a rule report to 

each House of the Congress and to the 

Comptroller General of the United 

States. The Administrator of OIRA has 

not determined that this action is a 

‘‘major rule’’ as defined by 5 U.S.C. 

804(2). 

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List of Subjects in 40 CFR Part 50 

Environmental protection, Air 

pollution control, Carbon monoxide, 

Lead, Nitrogen dioxide, Ozone, 

Particulate matter, Sulfur oxides. 

Dated: December 4, 2020. 

Andrew Wheeler, 

Administrator. 

[FR Doc. 2020–27125 Filed 12–17–20; 8:45 am] 

BILLING CODE 6560–50–P 


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